Organic Field-Effect Transistors for Sensing Applications - CiteSeerX

Organic Field-Effect Transistors for Sensing Applications

Francesco Maddalena

Organic Field-Effect Transistors for Sensing Applications Francesco Maddalena PhD thesis University of Groningen

The Zernike Institute for Advanced Materials PhD Thesis series 2011-25 ISSN: 1570-1530 ISBN: 978-90-367-5145-2 ISBN: 978-90-367-5146-9 (digital version)

The research described in this thesis was performed in the research group of Molecular Electronics: Physics of Organic Semiconductors of the Zernike Institute for Advanced Materials at the University of Groningen, The Netherlands. The project was financially supported by the Zernike Institute for Advanced Materials.

Rijksuniversiteit Groningen

Organic Field-Effect Transistors for Sensing Applications Proefschrift

ter verkrijging van het doctoraat in de Wiskunde en Natuurwetenschappen aan de Rijksuniversiteit Groningen op gezag van de Rector Magnificus, dr. E. Sterken, in het openbaar te verdedigen op vrijdag 25 november 2011 om 16:15 uur door

Francesco Maddalena geboren op 6 mei 1979 te Udine, Italië

Promotores:

Prof. dr. D. M. de Leeuw Prof. dr. ir. P. W. M. Blom

Beoordelingscommissie:

Prof. dr. J. C. Hummelen Prof. dr. B. Poolman Prof. dr. M. A. Loi

In loving memory of my uncle Vergilio Gamboso, OFM, historian, writer and poet.

Contents

1

2

3

Introduction

1

1.1 Organic Electronics

2

1.2 Conduction in Semiconducting polymers

3

1.3 Organic Field-Effect Transistors

6

1.4 Organic diodes

8

1.5 Sensors

10

1.6 Motivation and outline of the thesis

13

References

14

Materials and Experimental Techniques

19

2.1 Semiconductor and gate dielectric materials

20

2.2 Devices

22

2.3 Experimental

26

2.4 Biochemical experimental procedures

28

References

31

Device Characteristics of Dual-Gate Field-Effect Transistors

33

3.1 Introduction

34

3.2 Results and Discussion

36

3.3 Conclusions

46

References

47

4

5

6

Sulphate-detecting biosensor based upon organic field-effect transistors

49

4.1. Introduction

50

4.2 Results and discussion

54

4.3 Environmental conditions and role of the protein dipole

60

4.4 Conclusions

61

References

61

Doping Kinetics of Organic Semiconductors Investigated by Field-Effect Transistors

63

5.1. Introduction

64


67

5.3 Conclusions

72

References

73

Carrier Density Dependence of the Hole Mobility 75 in Doped and Undoped Organic Semiconductors 6.1 Introduction

76


77

6.3 Anomalous behavior

81

6.4 Conclusions

84

References

84

List of Publications

87

Summary

89

Samenvatting

93

Acknowledgements

97

CHAPTER 1

Chapter 1

Introduction

Abstract In the last two decades organic semiconductors have quickly become a prominent field in physics, which has opened several possibilities for new and innovative technologies. This chapter gives short overview of conduction in organic semiconductors and briefly describes the fundamental operation of the organic electronic devices investigated in this thesis and a brief introduction on sensors and biosensor applications in (organic) electronics. Finally, the motivation and a short outline of the thesis are given.

1

INTRODUCTION

1.1 Organic electronics From the serendipitous occasion when Shirakawa, Mc. Darmid and Heeger discovered conduction in organic polymers [1] to recent years, the field of organic electronics has quickly developed and the behavior of organic devices has been thoroughly studied. The great advantage of organic materials is the way they can be processed in an easy and much cheaper way than inorganic semiconductors. Organic semiconductors also allow the possibility of fabrication of flexible and transparent devices, features that might be attractive in the competitive microelectronic market.

Figure 1. Fully processed 150-mm wafer foil containing all-polymer transistors and integrated circuits (photograph: Philips).

Conjugated polymers have the potential to be processed from solution at low temperatures. High throughput processing techniques such as spin-coating, inkjet printing or roll-to-roll printing are easy and inexpensive ways of device fabrication, in contrast to the more complicated and expensive evaporation methods for inorganic materials. Inkjet-printing and roll-to-roll processing of organic semiconductors and conductors for instance have been successfully applied in the fabrication of recently 13.56 MHz transponders [2].

2

CHAPTER 1

Two of the challenges in the field of organic electronic are the relative low mobility of the charge carriers in conjugated materials and stability in ambient conditions. Most organic semiconductors therefore present poor device performances that impede the way toward commercialization. Recently however, several spin-coatable organic materials have been developed with charge carrier mobilities comparable to that of amorphous silicon [3] and good air-stability [4]. The organic semiconductors have already found several applications such as organic light-emitting diodes [5, 6], small screens for mobile phones [7] and organic solar cells [8, 9], which are appearing in new products in the market. Organic field-effect transistors (OFETs) are one of the main building blocks of electronic circuits. OFETs have great potential for use in low-cost and flexible lowend electronics. OFETs have successfully been applied in active matrix displays [10, 11] to address and drive individual pixels. Furthermore, OFETs have found applications in radio frequency identification (RFID) tags [2, 12].

1.2 Conduction in Semiconducting polymers The basic structure of semiconducting polymers and organic semiconductors in general, is defined by the presence of a conjugated system in their molecular structure, which is an extended π-bond system along the backbone of the polymer molecule [13]. Organic semiconductors in general are usually amorphous materials (small-molecule crystals like pentacene being a notable exception), presenting strong electron-phonon coupling [14], causing the charge carriers to be more localized than their inorganic counterparts. Moreover, a semiconducting polymer is not a perfect conjugated system. Due to kinks and twists of the polymer chains and chemical defects the conjugation is often broken, resulting in conjugated segments of different lengths. Because of the variation in the conjugation length and the interaction energies, a semiconducting polymer cannot have two well defined and delocalized energy bands. Instead a charge carrier, electron or hole, will be localized within conjugated segments with varying energy levels. The distribution of the energy levels is usually approximated by a Gaussian function (see Figure 2), which is also observed in the absorption spectra of conjugated polymers [15].

3

INTRODUCTION

Figure 2. Schematic representation of the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in an organic semiconductor. The levels are not well defined, but form a Gaussian distribution of localized states.

Figure 3. Energy levels versus distance. In hopping transport the charge carrier is localized due to disorder, defects or self-localization and hops from site to site with the essential help of lattice vibrations (phonons). The mobility of the carriers for this type of transport increases with the increase of temperature and increase of charge carrier density.

4

CHAPTER 1

1.2.1 Hopping Transport The main conduction mechanism through conjugated polymers is hopping transport: phonon-assisted tunneling of a charge carrier from an occupied localized state to a nearby unoccupied localized state [16, 17, 18]. This is graphically represented in Figure 3. The transition rate (Wij) from an occupied localized state i with an energy εi to an unoccupied state j with an energy εj was described by Miller and Abrams, assuming a single-phonon jump from site to site and a low rate of doping [19]:

  ε j −εi  Exp − Wij = υ 0 Exp(−2γRij )  k BT  1 

  

εi < ε j

(1.1)

εi ≥ ε j

where ν0 is the attempt-to-jump rate, γ is the inverse localization length, T is

absolute temperature, kB the Boltzmann constant and Rij is the distance between states i and j. The tunneling probability between each state is determined by the first exponential term and depends on the spatial distance between the two states. The second exponential term describes the temperature dependence of the tunneling rate when an energy barrier is present for the tunneling jump, equal to εjεi, if εj>εi. When this model is applied to polymer semiconductors it is assumed that the conjugated segments play the role of nearly isolated localized states. Hence, unlike in band transport, where the charge carriers are scattered by the phonons present in the system, in hopping transport phonons aid the charge carriers to overcome the energetic barriers between different sites. This is clearly observed in the decrease in charge carrier mobility with decreasing temperature. The hopping model by Miller and Abrams was further extended into the ‘variable range hopping’ (VRH) model [20], which assumes that the localized states are spread over the entire energy gap.

1.2.2 Charge carrier density dependence of the mobility Comparative studies between organic field-effect transistors and organic lightemitting diodes (OLEDs) [21] have shown a significant difference in charge carrier mobility between the two types of device. The mobility in OLEDs is nearly constant, except at high fields, when it becomes field-dependent. On the other hand, the mobility in OFETs depends superlinearly on the gate voltage and is sometimes a few orders of magnitude higher than in diodes. The discrepancy in 5

INTRODUCTION

charge carrier mobility between OFETs and OLEDs is explained by a charge carrier density dependence of the mobility. Vissenberg and Matters [22] further developed the VRH model in order to explain charge density-dependent transport in OFETs. The Vissenberg and Matters model predicts that conduction at low temperatures and low carrier densities occurs in the tail of the Gaussian DOS, which can be approximated by an exponential function [22]. From the charge density dependent conductivity described by the Vissenberg and Matters model an empirical expression for the charge carrier mobility µ as function of carrier density p and temperature T was subsequently derived by Tanase et al., using percolation theory [21]: T0

µ ( p, T ) = µ ( F = 0 )

  T0  4  T   T    Sin π   T0 σ 0  T   T0   p T −1 + e  (2α )3 BC    

(1.2)

where µ(F=0) is the zero-field mobility, σ0 is a conductivity prefactor, e the elementary charge T0 the isokinetic temperature, α–1 the effective overlap parameter between localized states and BC is the critical number for onset of percolation [23]. Eq. 1.2 describes the mobility dependence on charge carrier density and temperature, unifying the description of the mobility for both OLEDs and OFETs, for temperatures lower than T0. One must note that conjugated polymers that show high regioregularity and form polycrystalline or nanocrystalline structures, may present slightly different characteristics than amorphous and fully disordered polymers, due to a morphology that allows for elongated conjugated paths and improved hopping transport.

1.3 Organic Field-Effect Transistors The idea of a field-effect transistor (FET) was first conceived in 1930 [24] and further developed by Shockley and Pearson in 1948 [25]. However, the first practical applications began only in 1960 [26]. The most widely used FET is the metal-insulator-semiconductor FET (MIS-FET), in which the gate electrode is electrically insulated from the conducting channel by an insulating layer. The most typically used configuration is the metal-oxide-semiconductor FET (MOSFET), 6

CHAPTER 1

where the insulator is an oxide. The fundamental function of the FET is the modulation of the current flow between two Ohmic contacts i.e. the source and the drain electrodes, by applying a bias to the gate electrode. Hence name ‘transistor’, which is a portmanteau of ‘transverse modulated resistor’. By varying the voltage on the gate electrode accumulation or depletion of charge carriers can be induced in the semiconductor layer close to the semiconductor/insulator interface. The FET can be treated analogously to a plane capacitor, where the two plates are, the conductive channel in the semiconducting layer and the gate electrode respectively. To understand how a MIS-FET operates it is important to keep focus on what happens at the semiconductor-insulator interface when different biases are applied to the metal, respect to a ground. We will discuss the case for a p-type semiconductor, since most conjugated polymers are of this type. There are three possible cases. The first case is when the bias on the gate electrode (VGS) is equal to the flat-band bias (VFB). Then we have a flat-band condition, where, as the name says it, there is no band bending and the Fermi levels of the semiconductor and the gate electrode are aligned. The only charge carriers present in the semiconductor are the carriers that occur by ‘natural’ means such as thermal excitation or doping (Figure 4a.). The second case is when we apply a negative bias on the gate electrode (more negative than VFB), then its Fermi energy level will be raised. Consequently the bands of the semiconductor will bend upwards in energy causing an accumulation of positive charges in the valence band at the interface to compensate for the negative charges on the gate (keeping in mind that the device is similar to a capacitor). This is the accumulation condition (or regime) (see Figure 4b.). In this regime the accumulation of charges present at the interface with the insulator (the S/I-interface) forms a channel between the source- and drain electrodes, allowing a hole current to flow between them. This is called the oncurrent. When we apply a positive bias (more positive of VFB, Figure 4c.) to the gate electrode the Fermi level of the electrode will be lowered. This is the reverse situation of the previous case: the bands of the semiconductor will bend downwards causing a depletion of positive charges in the valence band and a slight accumulation of negative charges in the conduction band. This is called the depletion condition or regime. In this regime there will be a very low charge density, hence the current flow, called the off-current, between the source- and the drain electrodes will be very low, usually several orders of magnitude lower than the on-current. Unlike in the inorganic semiconductors, an inversion regime does not occur in OFETs. There are two main reasons; first injection of minority carriers (electrons in the case of p-type semiconductors as depicted in Figure 4) is blocked 7

INTRODUCTION

due to the large injection barrier present at the source electrode. Second, the minority carriers that are present in the organic semiconductor will not be mobile due to trapping in the material. Hence the off-current is the result of bulkconduction of majority carriers, mostly due to unintentional doping, or leakage. The majority of the charge carriers are located close to the semiconductor/dielectric interface, in the first 1-3 nm, where the charge carriers also have the highest mobility [27]. This means that, for an undoped semiconductor, there will be a negligible current in the bulk, and only the charges accumulated in the first few nanometers from the interface yield conduction. If the semiconductor is doped however, there will be also a non-negligible charge carrier density in the bulk of the semiconductor, giving rise to bulk conduction.

Figure 4. Ideal energy band diagram at the metal-insulator-semiconductor interfaces of an OFET. The figure depicts three situations for a p-type semiconductor: a.) The flat-band condition where there is no applied bias, hence no band-bending. b.) Accumulation regime where a negative bias is applied on the metal causing the bands of the semiconductor to bend ‘upwards’ which causes accumulation of holes at the semiconductor-insulator interface. c.) Depletion regime where a positive bias is applied to the metal, which causes a depletion of holes in the semiconductor.

1.4 Organic diodes Organic diodes are two terminal devices that have been extensively studied in the past, especially in the context of organic light-emitting diodes since the discovery of electroluminescence in poly(p-phenylene vinylene) [28]. Organic diodes comprise an organic semiconductor sandwiched between a bottom and top electrode. By carefully choosing the electrodes it is possible to fabricate diodes that 8

CHAPTER 1

conduct both electrons and holes (such as in light-emitting diodes) or single carrier diodes that conduct either electrons or holes as shown in Figure 5. The built-in electrical field of the diode depends on the electrodes, which is approximately equal to the difference in the work functions of the electrodes [29]. The current flow in organic diodes is limited by the bulk transport and not by charge injection as long as the injection barriers, the energy offset between the work function of the contact the respective energy band of the polymer, are smaller than ~0.2 eV [30, 31]. Due to the low charge carrier mobility in organic semiconductors, the charges build up in the device and space-charge effects are observed in polymer diodes [30].

Figure 5. a.) Dual carrier diode. The work function of the anode matches the HOMO of the semiconductor and allows injection of holes and the work function of the cathode matches the LUMO of the semiconductor allowing for injection of electrons. b.) Hole-only diode. The work function of the anode matches the HOMO of the semiconductor, allowing injection of holes, but the work function of the cathode does not match the LUMO of the semiconductor, forming an injection barrier that prevents injection of electrons. c.) Electron-only diode. The work function of the anode does not match the HOMO of the semiconductor, forming an injection barrier that prevents injection of holes, while the work function of the cathode does match the LUMO of the semiconductor allowing injection of electrons.

9

INTRODUCTION

1.5 Sensors A sensor is an analytical device, which has a response to a physical entity (such as a chlorine molecule, a photon or an electron) and converts the response into a signal that can be analyzed. A sensor is composed by three main components: the detection unit, the transducer and the processing unit [32] as shown schematically in Figure 6. The detection unit is the part of the sensor which interacts and detects the physical entity one desires to measure, which is defined here as analyte. The interaction can be either physical, such as photo-excitation in silicon or chemical, such as doping of an organic semiconductor. The transducer is the component that translates the changes in the detection unit into measurable signals, usually electric signals. The processing unit is the surrounding hardware and software necessary to convert the signal from the transducer into an analyzable output. The two most important features for any sensor are selectivity and sensitivity. Selectivity is the property of a sensor to selectively detect the desired analyte without interference from other species present in the system. Sensitivity determines the strength of the sensor response when it detects the analyte.

Figure 6. Schematic diagram of a sensor showing the three main components. Under the cartoons of the detection unit and the transducer several detection and transducing mechanisms are respectively listed as possible examples.

A straightforward application of organic electronics for sensing purposes is the chemical doping of organic semiconductors in a FET. The detection unit will be the semiconducting polymer itself, which will be doped by a particular analyte. Since the FET itself will act as a transducer, the detection unit will be integrated with the transducer itself, greatly simplifying the sensor structure. The doping process will then introduce additional charge carriers in the material, causing the current to increase. The change of the current can be monitored in real-time, as it has been 10

CHAPTER 1

shown by Kao et al. for regioregular poly(3-hexylthiophene) and poly(2,5-bis(3tetradecyl-thiophen-2-yl)thieno[3,2-b]thiophene) OFETs doped with fluoroalkyl trichloro-silane [33].

1.5.1 Biosensors A particular type of sensor is the biosensor, which is characterized by the fact that the detection unit is composed by one or several biological molecules, such as enzymes or DNA, or even cells, that can detect different (bio)chemical compounds ranging from simple salts to complex bio-molecules. Biosensors have the advantage that most biological molecules are very selective in their interaction with possible analytes. Enzymes and receptors, for example, will interact only with a limited variety of chemical species. In recent years biosensors have been a topic of increasing interest both in academic and in industrial research. Several reviews have summarized biosensor development [34, 35, 36]. Biosensor have great potential for practical applications, from medical applications, such as quantitative analysis of chemicals in human blood, monitoring, in real time the glucose concentration in blood of diabetes patients [37] or label free DNA detection [38], to industrial applications such as in the food industry [39] or in the chemical analysis [40]. The research and development of biosensors has focused primarily on redox enzymes, which reduce or oxidize a substrate. The reason is that one can take advantage of the flux of electrons created in the redox reaction or keep track of the reducing or oxidizing agents. Non-redox enzymes can also be used in this setup if one of the analyte reaction products can subsequently undergo a redox reaction with an electrode. Biosensors are divided in three generations. The first generation biosensors were proposed by Clark and Lyons [41] and implemented by Updike and Hicks half a century ago [42]. These sensors use a biological component in solution or immobilized behind a dialysis or ion-selective membrane on the surface of an electrode. The most widely used biological components are redox enzymes that oxidize the analyte, usually by reduction of oxygen into hydrogen peroxide. The sensing mechanism is based on the measurement of the production or disappearance of certain species, usually hydrogen peroxide, through change in the redox potential. The second generation biosensors use an artificial electron mediator, which replaces O2 as the electron shuttle [43]. Ferrocene, quinones, quinoid-like dyes, 11

INTRODUCTION

organic conducting salts, and viologens have been used as mediators. The artificial mediators resolve the problem of the low solubility of O2 in water, enhancing the performance of the biosensors. Artificial electron mediators also allow for the exploitation of redox enzymes, which do use molecular oxygen as an oxidizing agent.

Figure 7. Schematic view of a biosensor in a FET-like configuration with a liquid electrochemical cell separated by an insulating layer.

Finally, we have the third generation biosensors, where the reaction itself causes the response, and no product or mediator diffusion is directly involved. Several examples of third generation biosensors have been constructed such as a superoxide bismutase [44] biosensor or a glucose biosensor. First, second and third generation biosensors can also use (organic) MIS-FETs as transducers, using a liquid electrochemical cell with a reference electrode instead of a gate electrode, as is shown in Figure 7. Detection of the analyte occurs through the monitoring of the change in the gating potential of the solution by the reference electrode, while keeping the source-drain current constant [45]. An example of a MIS-FET-based biosensor is the enzymatically selective FET (ENFET) described by Wilson et al., where a chemically sensitive enzyme layer forms part of the insulator layer of the FET structure. An interesting alternative for the use of a liquid electrochemical cell is a solidstate OFET-based biosensor (Bio-FET), where the biomolecule is immobilized on the surface of the OFET itself. This is depicted schematically in Figure 8. Then, applying drops of liquid on the surface of the Bio-FET will allow for detection of the analytes. The Bio-FET offers the possibility to be integrated with ‘lab-on-achip’ technology [46], which would allow putting a drop of fluid on the chip and get direct qualitative and quantitative analysis of the sample for different analytes. The possibility to integrate multiple different Bio-FETs can open several new 12

CHAPTER 1

possibilities, such as a Bio-FET-based ‘blood analyzer’, where a drop of blood would be put on the chip and directly different analytes in the blood can be detected and quantified, such as sugar levels, antibodies or pathogens.

Figure 8. Third generation biosensor based thin-film MIS-FET technology.

1.6 Motivation and outline of the thesis The main focus of this thesis is on organic field effect transistors and their application for (bio)sensing. The research is focused more on fundamental understanding than on realization of the sensors. A brief outline of each chapter is presented in the following: Chapter 2 describes the semiconductors and dielectrics used throughout the thesis and gives an overview of the device preparation and characterization. Chapter 3 discusses the device operation of dual-gate organic field-effect transistors. It is demonstrated that the change in the threshold voltage of the bottom gate depends on the top gate bias with two linear relations for two different regimes. The insight obtained from the dual-gate organic field-effect transistor is used to fabricate and analyze biosensors based organic transistors in the next chapter. Chapter 4 presents fabrication, basic operation and testing of a Bio-FET as biosensor for detecting sulfate ions. The sensor is based on a the dual-gate organic field-effect transistor as described in the previous chapter. Chapter 5 deals with the kinetics of acid doping of the semiconductor regioregular poly-3-hexylthiophene with vaporized chloroalkylsilane in field-effect transistors. The dopant density has been derived as a function of temperature and 13

INTRODUCTION

exposure time from the shift of the pinch-off voltage, being the gate bias where current starts to flow. The doping kinetics is described by empirical stretched exponential time dependence with a thermally activated relaxation time. The study of the doping kinetics in organic semiconductors is the basis for fabrication of more reliable gas sensors. Chapter 6 explores the charge carrier mobility dependence on the charge carrier density in conjugated polymers. We investigated the mobility of poly(3hexylthiophene) and other polymers over a carrier density range from 1015 cm–3 to 1020 cm–3. Hole-only diodes were used for low densities and field-effect transistors were used for the high carrier densities. Intermediate densities were probed using chemically doped Schottky diodes and transistors. We demonstrate that the mobility is constant for carrier densities below 1016 cm–3 and follows a power law dependence for carrier densities higher than 1018 cm–3. We also make note of possible anomalies rising from trapping effects or morphology.

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CHAPTER 1

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INTRODUCTION

18

CHAPTER 2

Chapter 2 Materials and Experimental Techniques

Abstract In this chapter we present the materials used throughout this Thesis. Here we also describe the design of the devices and the experimental procedures for fabrication and characterization.

19

MATERIALS AND EXPERIMENTAL TECHNIQUES

2.1 Semiconductor and gate dielectric materials The semiconductors used throughout this thesis were regio-regular poly-3hexylthiophene (rr-P3HT), poly(4,4’-didecylbithiophene-co-2,5-thieno[2,3b]thiophene) (PDTT) and poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). These are all disordered semiconductors with the exception of rr-P3HT which can have polycrystalline morphology. The organic dielectrics used were polystyrene (PS), polystyrene with pendant maleimide groups (PSMI) and polymethylmethacrylate (PMMA). The chemical structure of these materials is shown in Figure 1. Regio-regular poly-3-hexylthiophene was purchased from Rieke chemicals and was subsequentely purified. rr-P3HT was dissolved in distilled toluene, dedoped with hydrazine at 60 ºC and precipitated in methanol. The fraction collected was Soxhlet extracted for at least 64 h with methanol, n-hexane, dichloromethane and finally with chloroform. The chloroform fraction was precipitated in methanol, dried under vacuum and stored in a glove box under N2-atmosphere. Regio-regular poly-3-hexylthiophene used in chapter 6 of this thesis, however, was obtained from Imperial College London and used without further purification. The molecular weight was 33,000 g/mol as measured by GPC and the regioregularity was > 97%, as measured by NMR. PDTT and PSMI were synthetized by F. Brouwer and J.C. Hummelen [1]. For the synthesis of PDTT 2,5-bis(trimethylstannyl)-thieno[2,3b]thiophene (600 mg, 1.0 mmol), 5,5`-dibromo-4,4`-didecyl-2,2`-bithiophene (774 mg, 1.0 mmol) and Pd(PPh3)4 (75 mg, 0.06 mmol) were reacted in a mixture of 25 mL dry DMF and 75 mL dry toluene according to the general Stille polymerization procedure. Pure PDTT (659 mg, 87%) was obtained as a dark orange powder. The monomers were synthetized according to literature procedures [2]. PSMI was functionalized from commercial polystyrene (Dow Styron 683) according to the literature procedure [3]. The amount of maleimide groups was estimated between 20% with gel permeation chromatography and 29% with proton nuclear magnetic resonance spectroscopy.

20

CHAPTER 2

Figure 1. Chemical structure of semiconductors and dielectrics used throughout this thesis. a.) poly(4,4’-didecylbithiophene-co-2,5-thieno[2,3-b]thiophene)

(PDTT)

b.)

regioregular

poly(3-

hexylthiophene) (rr-P3HT), c.) poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEHPPV), d.) polystyrene (PS), e.) polystyrene with pendant maleimide group (PSMI) and f.) polymethyl methacrylate (PMMA).

21


2.2 Devices 2.2.1 Organic Field-Effect Transistor Organic field-effect transistors (OFETs) were processed using heavily n-doped silicon wafers as bottom gate electrode, with a 200 nm thick thermally grown silicon oxide layer. Gold source and drain electrodes were patterned on top of the oxide layer by conventional lithography [4]. Two types of source-drain patterns were used. The first is an interpenetrating finger structure was used with channel lengths varying from 5 to 40 µm and a constant channel width of 10,000 µm (see Figure 2 d). This pattern had long leads, with the advantage of addressing the transistor relatively far away from the channel. The second is a ring structure with variable channel length from 5 to 40 µm and width of either 1000 µm or 2500 µm. This structure is preferred when parasitic leakage needs to be avoided [5] (Figure 2 e). The oxide layer was treated with the primer hexamethyldisilazane (HMDS) in order to make the surface hydrophobic to improve wetting and prevent charge trapping at the interface [6]. Prior to spin-coating the polymer semiconductors were dissolved and stirred overnight at room temperature, except for MEH-PPV which was stirred at 70 ºC. The solvents used were chloroform for rr-P3HT, 1,2dichlorobenzene for PDTT and toluene for MEH-PPV. Afterwards the polymers were spin-coated onto the substrates from solution at room temperature under inert atmosphere. The thickness of the layers was between 20 and 200 nm, depending on concentration of the solution and spin-coating speed. After spin-coating the film was optionally annealed. rr-P3HT was annealed in a vacuum oven at 150 ºC for 2 hours and PDTT was annealed at 125 ºC on a hot plate under inert atmosphere for 30 minutes. A schematic view of a (single-gate) OFET is shown in Figure 2 a.

2.2.2 Dual-gate organic field-effect transistors The fabrication of dual-gate organic field-effect transistors (DG-OFETs, Figure 2 b) was similar to the fabrication of single-gate OFETs with the addition of an extra insulating layer on top of the semiconductor and a top gate. The dielectrics used were polystyrene (PS, Dow Styron 683) and polymethylmethacrylate (PMMA, Merck), as shown in Figure 2 d and 2 f. Both polymers were dissolved in

22

CHAPTER 2

Figure 2. Structure of organic field-effect transistors used throughout this thesis. a.) Single-gate OFET. b.) Dual-gate OFET. c.) The Bio-FET. d.) Interdigitated drain-source electrodes structure. e.) Ring drain-source electrodes structure [5].

23


methyl ethyl ketone (MEK). The dielectrics were spin-coated at different speeds in order to achieve different layer thicknesses. The top gate electrode consisted of 60 nm Ag and was evaporated through a shadow mask.

2.2.3 The Biological Field-Effect Transistor The fabrication of the biological organic field-effect transistor or Bio-FET (Figure 2 c) is for most part similar to the fabrication of a DG-OFET. The semiconductor, PDTT, was spin-coated under inert atmosphere from a 1% w/v solution in 1, 2-dichlorobenzene, obtaining a film with a thickness of about 50 nm. The semiconductor layer was annealed at 125 ºC on a hot plate under inert atmosphere and cooled to room temperature for about one hour. On top of the semiconductor an insulating layer of PSMI was deposited by spin-coating from a 3% w/v in MEK, forming a 50 nm film, with a capacitance of 4.3 nF/cm2. The final step of the Bio-FET fabrication is the attachment of the singlecysteine mutant sulphate binding protein (SBP, see section 2.3) on the polystyrene maleimide layer. The mutated SBP (SBP-G289C) was dissolved at a concentration of 10 µM in 2mM tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl) buffer at pH 7.5 with addition of 10 µM dithiothreitol (DTT) to prevent the formation of intermolecular disulfide bonds. Then the protein solution was incubated upon the PSMI surface of the transistor at different times, no longer than 15 minutes, to allow the SBP-G289C mutant to bind to the surface of the polymer. Afterwards the surface was gently washed with de-ionized water and spin-dried before measurement. As control a device was incubated with only buffer and DTT without protein present for 15 minutes. The same was repeated after exposure of a 1 mM Na2SO4 solution for 15 minutes, but after this step the surfaces where not rinsed with de-ionized water but only spin-dried.

2.2.4 Diodes Hole-only diodes (HODs) were prepared on 3×3 cm square glass substrates covered with a transparent patterned layer of indium tin oxide (ITO) [7]. Prior to spin-coating of the organic layers, the glass-ITO surface was subjected to chemical and physical treatments in order to remove contaminants, smoothen the surface and improve the ITO work function [8]. The cleaning procedure consisted, in order, in

24

CHAPTER 2

the following steps: rubbing of the surface with de-ionized water/soap solution, rinsing with de-ionized water, ultrasonic treatment in acetone, rinsing with deionized water, ultrasonic treatment in 2-propanol, spin-drying, drying in oven at 140 ºC for 10 minutes, and finally UV-ozone treatment for 20 minutes. In order to improve the wetting of the semiconducting polymer upon the substrate and to increase the stability of the ITO electrode [9] and the device performance, a conductive transparent layer of 60 nm low-ohmic PEDOT:PSS, consisting of a mixture of polyethylene dioxythiophene (PEDOT) and polystyrene sulfonate (PSS), was spin-coated on top of the ITO [10]. After spin-coating, the sample was baked at 140 ºC for 10 minutes in order to remove the remaining water that can cause degradation of the device [11].

Figure 3. Schematic representation of the hole-only diode. a.) Top view. Light gray: bottom electrode; dark gray: top electrode. b.) Side view.

Schottky diodes (SDs) were prepared in a similar manner as HODs. However glass substrates with no ITO pattern were used. The glass substrates were cleaned in the same manner as described above and afterwards 60 nm gold bottom electrodes were evaporated upon the glass with 1 nm chromium adhesion layer. No PEDOT:PSS was used for the fabrication of SDs. The organic semiconductor layer was spin-coated from solution under inert atmosphere and annealed in the same manner as for OFETs. The polymer thickness varied between 100 nm and 200 nm. After deposition of the organic semiconductor the top electrode was thermally evaporated using a shadow mask in a vacuum deposition system at ~10-6 mbar. The top electrode consisted in 20 nm of palladium and 80 nm of gold for HODs and 90 nm of aluminum for SDs. The devices prepared had different areas ranging from 1 to 100 mm2.

25


2.3 Experimental 2.3.1 Electrical Measurements Single- and double-gate field-effect transistors were measured in a custom made probe station. Transfer characteristics (IDrain vs. VGate) and output characteristics (IDrain vs. VDrain) of the devices were recorded using a Keithley 4200 semiconductor parameter analyzer. The measurements were performed in the dark and at high vacuum (10–6-10–7 mbar) unless specified otherwise. The output characteristics were measured sweeping VDrain from +2 V to –50 V, keeping VGate constant. This was done at different gate biases, usually from VGate from +10 V to – 50 V with steps of 10 V. The transfer characteristics were measured by sweeping the gate, usually from +50 V to –50 V at constant drain bias. In order to perform the temperature measurements, the sample table, consisting of a copper block, was either heated with an electrical resistance or cooled using liquid nitrogen and a temperature controller was used. Dual-gate OFETs were measured in similar manner as single-gate OFETs. The bottom gate bias was swept from +40 V to –50 V while the top gate bias was held constant. The sweeps were repeated for different top gate biases, varied with steps of 10 V from +40 V to –60 V. The Bio-FET devices were measured before and after attachment of the protein on the PSMI surface and after exposure to the sulphate solution. The pristine devices were measured both in vacuum and in air, in the dark. After incubation with the protein the devices were measured in air, in the dark. Hole-only and Schottky diodes were measured under inert atmosphere in the dark. The sample holder was designed in such manner that the four devices on the substrate could be electrically addressed individually without the need of moving the sample. The current-voltage measurements for hole- only and Schottky diodes were performed in dark using a Keithley 2400 Source Meter Unit. Capacitance-Voltage measurements were performed on Schottky diodes. The impedance and phase were recorded at different frequencies (from 100 Hz to 1 MHz) at different DC-bias using a Solartron SI 1260 impedance/gain-phase analyzer.

26

CHAPTER 2

2.3.2 Atomic force microscopy measurements Atomic force microscopy was used to investigate the binding of SBP-G289C on top of the PSMI surface. Substrates were prepared by evaporating 1 nm chromium and 100 nm of gold on a monitor silicon wafer. After cutting in square pieces of 1×1 cm PSMI was spin-coated. Then a solution of the mutated protein was dropcasted. After a given incubation period the surfaces were rinsed with deionized water and investigated by tapping–mode atomic force spectroscopy (TMAFM), which allows, unlike normal AFM, a safer and more reliable way to investigate the surface of soft materials such as polymers and biomolecules. The height and phase profiles of the surface were recorded with a Veeco MultimodeTM SPM, with an X/Y-sensitivity of 10 nm and a Z-range sensitivity of 0.2 nm. The X/Y-range of the TM-AFM was 500×500 nm, while the Z-range was only 10 nm.

Figure 4. Schematic representation of the measurement set-up for the doping of organic semiconductors in OFETs.

27


2.3.3 Measurements of devices with exposure to dopant A schematic representation of the set-up used to measure the doping of organic semiconductors in OFETs is shown in Figure 3. The OFETs were first measured in vacuum as described in section 2.4. Afterwards the turbo pump valve was closed setting the chamber to a pressure of less than 10–4 mbar. Then 20 µL trichloro perfluorooctylsilane (TCFOS) was injected into an antechamber in order to be vaporized. The valve to the measurement chamber was opened yielding a TCFOS partial pressure from ~2.5×10–2 mbar to ~2.5×10–1 mbar and the transistors were measured as a function of temperature and exposure time as described in the previous section.

2.3.4 Other experimental procedures The synthesis and purification of the polymer made used of several spectroscopy techniques. 1H-NMR spectra were measured using a Varian VXR300 (300 MHz) at 25 ˚C. FT-IR spectra were recorded on a Nicolet Nexus FT-IR spectrometer. GPC was performed on a Spectra Physics AS 1000 series machine equipped with a Viskotek H-502 viscometer and a Shodex RI-71 refractive index detector. The layer thickness of the organic films was determined using a Dektak profile analyzer. All reagents and solvents were purchased from Acros Organics or Sigma Aldrich and used without further purification or treatment unless otherwise indicated.

2.4 Biochemical experimental procedures [12, 13] The experimental details provided below offer a layman description intended for those who are not familiar with the terminology and the procedures of the biochemical experimental work. The gene encoding the wild type and the single-cysteine mutant SBP-G289C of the sulphate-binding protein (SBP), used in Chapter 4 of this thesis, were cloned, expressed, purified as described previously in: Kuiper et al. (2009) [13]. The

28

CHAPTER 2

sulphate-binding protein with the mutation position and the sulphate binding site is shown in Figure 5.

Figure 5. Structure of the 1SBP Sulphate-Binding Protein with the mutation and the sulphate-binding site highlighted. The mutation changes the 289th amino acid in the peptide chain from a glycine to a cysteine. The mutation was specifically made on the external part of the peptide chain, in a loop between two alpha helices, to avoid loss of structure and activity of the protein.

To obtain a protein that would be suitable chemical immobilisation upon a surface with maleimide anchoring groups, the wild type SBP needed to be mutated in order to include a cysteine side-tail that could be bound to PSMI. The first step to obtain the desired mutant protein is to create a mutation at the DNA level that can be later inserted into a living cell so that the proteins can be expressed (i.e. produced by the cell). For this purpose a standard circular piece of DNA (plasmid), named plasmid pBADMycHisB [14], was digested with restriction enzymes that

29


cut specific sequences of DNA bases (restriction sites) [15]. These yielded a major fragment, called vector, which is the DNA fragment used as a vehicle to transfer foreign genetic material into another (bacterial) cell and a shorter DNA fragment. An extra DNA fragment, called the ‘linker fragment’ was then attached to the vector DNA. The linker DNA sequence contained a ‘cleavage site’ that codes for a sequence of amino acids that can be recognized and cut specifically by the tobacco etch virus (TEV) protease enzyme, so it is possible to selectively cut the protein tail after purification. Another restriction site was added, followed by a histidine tagencoding sequence that encodes a tail of 10 histidine amino acids to facilitate protein purification. The resulting vector after the attachment of the linker fragment was named pERG1. The gene that encodes for the Sulphate-Binding Protein (SBP) from Salmonella typhimurium LT2 was selected and amplified using the polymerase chain reaction (PCR) technique [15]. Afterwards the pERG1 vector was digested with the restriction enzymes that cut at the two ends of the gene amplified by PCR and the sequence for the SBP gene was ligated into the vector sequence. The new vector containing the SBP gene was renamed pJMK1. For convenience, the numbering of the amino acids coded by the gene is the same as used in the crystal structure [16]. Site directed mutagenesis [15] was then used to mutate the desired amino acids in SBP to cysteine residues. This technique applies PCR, where the oligonucleotide primers used are sequences that partially encode the original fragment of DNA but also contain the desired mutation in the sequence. In this way it is possible to amplify the desired mutated DNA strand. The point mutation that has been made to obtain the desired mutant was: G289C (the amino acid glycine, G, at the 289th position is changed into a cysteine, C) by the mutation GGC → TGC (a guanine nucleotide, G, was changed into a thymine, T) of the amino acid encoding sequence. Both the mutant and the wild type SBP were produced in E. coli bacteria after the vector with the (mutated) SBP gene was added to their genetic material. The cells were cultivated and brought to expression (forced to produces the target protein). Afterwards the bacterial cells were cooled at 4 ºC and centrifuged. The sediment was diluted in suspension again and after addition of DNAse and RNAse (enzymes that break down DNA and RNA) was passed through a French Pressure Cell at 10,000 psi to destroy the cells and release the proteins. The resulting suspension was centrifuged and the supernatant was then purified by using highperformance liquid chromatography and passed through purification columns to remove all negative ions, especially sulphate. The histidine tail used for

30

CHAPTER 2

purification was cut with the TEV protease enzyme. All purification steps were performed at 4 ºC. The protein was finally stored at a concentration of 30-50 µM in 2 mM tris(hydroxymethyl)aminomethane hydrochloride buffer (pH= 8.0) and frozen in liquid nitrogen and conserved at –80 ºC.

References 1

The synthesis of the semiconductor and dielectric describes were done by F. Brouwer and J. C. Hummelen, Stratingh Institute for Chemistry, Zernike Institute for Advanced Materials, University of Groningen. 2

M. Heeney, C. Bailey, K. Genevicious, M. Shkunov, D. Sparrowe and I. McCulloch, J. Am. Chem. Soc. 127, 1078 (2005). 3

M. P. Stevens and A. D. Jenkins, J. Pol. Sci.-Pol. Chem. Ed. 17, 3675 (1979).

4

The field-effect transistor substrates with patterned gold electrodes were provided by Philips Research Laboratories Eindhoven.

5

E. J. Meijer, C. Detcheverry, P. J. Baesjou, E. van Veenendaal, D. M. de Leeuw and T. M. Klapwijk, J. Appl. Phys. 93, 4831(2003). 6

H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielsen, K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering, R. A. J. Janssen, E. W. Meijer, P. Herwig and D. M. de Leeuw, Nature 401, 685 (1999). 7

The glass substrates with patterned ITO electrodes were provided by Philips Research Laboratories Eindhoven. 8

N. R. Armstrong, C. Carter, C. Donley, A. Simmonds, P. Lee, M. Brumbach, B. Kippelen, B. Domercq and S. Yoo, Thin Solid Films 445, 342 (2003). 9

J. C. Scott, J. H. Kaufman, P. J. Brock, R. DiPietro, J, Salem and J. A. Goitia, J. Appl. Phys. 79, 2745 (1996). 10

L. Groenendaal, G. Zotti, P. H. Aubert, S. M. Waybright and J. R. Reynolds, Adv. Mat. 15, 855 (2003).

31


11

X. Crispim, S. Marciniak, W. Osikowicz, G. Zotti, A. W. Denier van der Gon, F. Louwet, M. Fahlman, L. Groenendaal, F. de Schryver and W. R. Salaneck, J. Pol. Sci. B 41, 2561 (2003). 12

The biochemical experimental procedures described in section 2.4 were performed by J. M. Kuiper and B. Poolman, from the Department of Biochemistry, Groningen Biomolecular Science and Biotechnology Institute & Zernike Institute for Advanced Materials, University of Groningen. 13

J. M. Kuiper, R. Pluta, W. H. C. Huibers, F. Fusetti, E. R. Geertsma and B. Poolman, Protein Sci. 18, 1033 (2009). 14

http://www.invitrogen.com/site/us/en/home.html.

15

J. D. Watson, M. Gilman, J. Witkowski and M. Zoller Recombinant DNA, Scientific American Books (2001).

16

32

J. W. Pflugrath and F. A. Quiocho, J. Mol. Biol. 200, 163 (1988).

CHAPTER 3

Chapter 3 Device Characteristics of Dual-Gate Field-Effect Transistors

Abstract Dual-gate organic field-effect transistors (DG-OFETs) were fabricated by solution processing using different p-type polymer semiconductors and polymer top-dielectric. The DG-OFETs were characterized by sweeping the bottom gate bias while fixing the top gate potential, and vice versa. We demonstrate that the change in the threshold voltage of the bottom gate depends on the top gate bias with two linear relationships for two different regimes. The transition regime between both linear regimes is marked by a drop in the transconductance. The transition regime results from the fact that the charges accumulated in the conduction channel of the sweeping gate will start screening the influence of the sweeping gate potential on the conduction channel of the fixed gate.

* Part of this chapter was published as: F. Maddalena, M. Spijkman, J.J. Brondijk, P. Fonteijn, F. Brouwer, J. C. Hummelen, D. M. de Leeuw, P.W.M. Blom and B. de Boer, Organic Electronics 9, 839 (2008).

33

DEVICE CHARACTERISTICS OF DUAL-GATE FIELD-EFFECT TRANSISTORS

3.1 Introduction In recent years, the field of organic electronics has developed quickly and the behaviour of organic devices has been thoroughly studied mainly due to the discovery of novel polymeric materials. Especially solution-processable organic semiconductors with high mobilities and air-stability allow not only a better performance of OFETs but also the possibility of processing and operating allpolymer devices under ambient conditions [1, 2]. Different structures for OFETs have been proposed, such as vertical channel thin film transistors (TFT) [3], yet the standard planar structure has remained virtually unchanged and still is the most widely used device geometry. Planar OFETs can have either a top gate or bottom gate configuration, yet processing an OFET with both top and bottom gate present enables the study of a whole new device structure. Moreover, the dual gate OFET (DG-OFET) could further improve the performance of organic TFTs, without adding a significantly higher complexity to the processing of devices. One of the main bottlenecks for the practical applications of OFETs in the electronic industry is the instability of most organic semiconductors during operation under environmental conditions and the limited control of the electrical parameters of the device. A very important issue is the control of the threshold voltage (VTH), which becomes crucial for proper operation, low power consumption and increasing the noise margin, especially in complicated organic circuitry. The first issue is often solved either by using an environmentally stable organic semiconductor [4, 5] or by coating the device with a passivating material. The second issue is often more difficult and can be achieved, to a limited extend, by modifying the surface of the dielectric with self-assembled monolayers [6]. A dualgate organic field-effect transistor (DG-OFET) solves both problems at once: the top insulator acts as a passivating layer by protecting the semiconductor from ambient, and the top gate can be used to accurately control the threshold voltage. The use of DG-OFET devices has already been exploited to significantly improve the noise margin in organic circuitry [7]. Furthermore, the DG-OFET has the potential to expand the possibilities of what has already been achieved and thoroughly studied with common organic TFT devices.

34

CHAPTER 3

Figure 1. Schematic structure of the dual-gate organic field-effect transistor.

Dual-gate transistors, and even triple gate FETs, have already been researched extensively in the field of inorganic, e.g. silicon, electronics [8]. Modeling and analysis of organic dual-gate transistors however has only been recently developed [9, 10, 11, 12, 13], mostly on devices with evaporated pentacene as semiconductor rather than polymer semiconductors cast from solution. Recently, a linear dependence between the threshold voltage of the bottom channel, VTH, and the applied top gate (VG,Top) was proposed and observed, with the slope of the linear fit equal to the ratio between the capacitances of both dielectric layers [11]. In this work, DG-OFETs (Figure 1) processed from solution with p-type polymeric semiconductors and polymeric top dielectrics are investigated. We demonstrate that a single linear relationship between VTH and VG,Top is incomplete for a DG-OFET with both the top and bottom channels active, since the influence of the top gate potential on the bottom channel is screened by the charge carriers in the top channel and vice versa. Furthermore, the transfer characteristics of the DG-OFET will be discussed, where a transition region can be observed when both channels are in accumulation.

35


3.2 Results and Discussion The fundamental principles of the operation of the DG-OFET do not differ from that of a common organic TFT. In a single gate OFET the charges are induced at the semiconductor/insulator interface forming a conducting channel. The induced charges are mostly confined in the first 2-4 nm from the interface [14], hence the conduction is dominated by this thin channel rather than by the bulk. This implies that the two channels in DG-OFETs with semiconductor layers thicker than ~15 nm will thus remain independent and will not merge. The formation of the channel, in addition to creating a conducting path between the source and drain electrodes, also screens the gate potential similar to the operation of a plate capacitor. This means that when a channel is in the accumulation regime, the gate that causes the accumulation is fully screened by the charges within the first 2-4 nm and the second channel will not feel the presence of this gate. Sweeping single gate potentials in a dual-gate device, leaving the other gate floating, will therefore give separate and independent characteristics for the two channels. The results for the single-gate measurements of a typical DG-OFET with PDTT (25 nm) as the semiconductor and polystyrene (430 nm) as top dielectric are shown in Figure 2. The figure shows the output and transfer characteristics of the top and bottom channels of the DG-OFET measured independently. The currents of both channels are of the same order of magnitude, hence the conduction in the channels is similar. The mobilities calculated from the transfer characteristics in the linear regime are also similar since the capacitances of both dielectrics are comparable. The mobilities found (for VDrain= –5 V and VGate–VTH= –20 V) are 7.3 × 10–3 cm2 V–1s–1 for the bottom channel and 2.2 × 10–3 cm2 V–1s–1 for the top channel. The threshold voltage of the bottom channel is –5.3 V at VDrain= –5 V and –6.0 V at VDrain= –45 V. The threshold voltage of the top channel is +3.0 V at VDrain= –5 V and +3.9 V at VDrain= –45 V. Crucial for analyzing the true properties of a dual-gate device is that a DG-OFET is obtained in which both channels exhibit similar mobilities. We also note that in Figures 2 a and 2 b the saturation of the output characteristics occurs earlier than expected: for VGate= –40 V the source-drain bias that marks the onset of saturation is around –30 V rather than –40 V. This is explained by the fact that one gate is not connected. Consequently, this floating gate will have an effective potential which lies between the source and drain bias, depending on the leakage resistances between sourcegate and drain-gate. Hence the potential of the floating gate results in an additional electrostatic coupling. Figure 3 shows the forward and backward scans of the

36

CHAPTER 3

transfer characteristics of two DG-OFETs with different materials and layer thicknesses, when the bottom gate potential is swept while keeping the top gate potential constant at different top gate potentials. There is almost no hysteresis present in the scans of both of the devices.

Figure 2. Device characteristics of the bottom and top channel in a DG-OFET, measured separately. a.) and c.) are the output and transfer characteristics of the bottom channel, respectively. b.) and d.) are the output and transfer characteristics of the top channel. The device is a finger FET (L/W= 20/10000) with PDTT as semiconductor (thickness 25 nm) and polystyrene as the top dielectric (thickness 430 nm). The mobilities obtained (for VGate–VTH = –20 V) are 7.3×10-3 cm2V-1s-1 for the bottom channel and 2.2×10-3 cm2V-1s-1 for the top channel.

37


Figure 3. Forward and backward scans of the transfer curves for two DG-OFET with different capacitance ratios and materials in dual-gate mode. The devices shown are: a.) a finger transistor (L/W= 20/10000) with PDTT as semiconductor (thickness 25 nm) and polystyrene (thickness 430 nm) as the top dielectric, and b.) a finger transistor (L/W= 20/10000) with rr-P3HT as semiconductor (thickness 75 nm) and polystyrene (thickness 450 nm) as the top dielectric.

38

CHAPTER 3

3.2.1 Threshold voltage shift in dual-gate OFETs We extract the bottom channel threshold voltage from the transfer curves shown in Figure 3. The threshold voltage is defined as the onset of strong inversion [15]. Although most organic transistors only operate in accumulation mode and show no current in inversion, the classical metal-oxide-semiconductor field-effect theory is used to extract VTH from the transfer characteristics of the transistor in accumulation mode, where the current depends quadratically on the gate voltage: IDrain≈ (VGate – VTH)2. The square root of the saturation current is then plotted against the gate voltage. This curve is fitted linearly and the intercept on the VGaxis is defined as the VTH of the transistor. At the threshold voltage the amount of accumulated charges in organic semiconductor, QG, is equal to zero and this point defines the onset of the charge accumulation. The threshold voltage, for the device depicted in fidure 3 b is plotted as a function of the top gate voltage in Figure 4 and clearly demonstrates the shift in threshold voltage. The linear relationship previously reported [10, 11, 12] is also found here, yet a more detailed analysis of the data demonstrates two linear relationships to be present. The intercept of the two linear fits is close to zero top gate bias. This double linear relationship can be readily explained from the working principle of the DG-FET having two active channels. The current in the channel is dependent on the amount of charges, QG, which are induced by the gate potential. If one of the channels is depleted (or ‘OFF’), screening of the field does not occur and, consequently, the other channel will depend on both gates via:

QG = C 2VG ,Top + C1VG , Bottom

(3.1)

where C2 is the capacitance of the layer between the channel and the top gate, C1 the capacitance of the layer between the channel and the bottom gate, and VG,Top and VG,Bottom are the top and bottom bias, respectively. Note that C1 or C2 can consist of the capacitance of a dielectric layer in series with the capacitance of the semiconducting layer. In these devices, the bottom gate potential is swept while the top gate voltage is held constant. If the bottom gate voltage, is more positive than the threshold voltage VTH, no appreciable accumulation of charges in the channels is expected. The point where VG,Bottom = VTH marks the onset for charges to accumulate at the semiconductor-insulator interface. At this point no charges have accumulated and QG = 0. Then, from rearranging Eq. 3.1, the threshold voltage, when sweeping VG,Bottom, will depend on VG,Top in the following way: 39


VG ,Bottom = VTH = −

C2 VG ,Top = −∆ ⋅ VG ,Top C1

(3.2)

where –∆ is the slope of the line obtained by plotting VTH versus VG Top. As shown in Figure 4, we obtain two cases by sweeping the bottom gate bias and stepping the top gate bias at fixed values. We note that p-type semiconductors are used in these devices. Now several scenarios are possible, depending on the different regimes of screening within the DG-OFET: 1-

When the top gate potential is held negative, the top channel is in the accumulation regime. However, by setting the bottom gate at sufficiently positive values we can fully deplete the top channel. This is observed in Figure 3 for bottom gate voltages larger than +20 V. Thus when the bottom gate is sufficiently positive both channels are depleted and the top channel is sensitive for both gate potentials since no charge in the bottom channel is present to screen the bottom gate. According to Eq. 3.1 sweeping VG,Bottom to less positive values will lead to the point where VG,Bottom = VTH and charges will start to accumulate in the top channel while the bottom channel will still be depleted. The accumulation of charges will occur in the top channel first due to a constant negative top gate potential is applied.

Since the accumulation occurs in the top channel from Eq. 3.1 it is found that the capacitance C1, between the bottom gate electrode and the top channel, will be equal to the capacitances of the bottom insulator (CB) and the semiconductor layers (CS) in series: C1 = (1/CB + 1/CS)–1, while the capacitance C2, between the top gate electrode and the top channel, will simply be the capacitance of the top insulator layer (CT): C2 = CT. At the onset where VG,Bottom = VTH, no charge carriers have accumulated at the bottom interface (QG = 0), and from Eq. 3.2 follows that:

∆= 2-

40

CT (C B + C S ) C B CS

(3.3)

On the other hand, if the top gate bias is held positive, for a p-type semiconductor, no accumulation of charges occurs in the top channel close to the semiconductor/dielectric interface and, consequently, no top channel is formed. The bottom channel will switch on when VG,Bottom = VTH and charges start to accumulate according to Eq. 3.1. Then the capacitance C1, between the bottom gate electrode and the bottom channel is simply the

CHAPTER 3

capacitance of the bottom insulator layer: C1 = CB, while the capacitance C2, between the top gate electrode and the bottom channel is equal to the capacitance of the top insulator and semiconductor layers: C2 = (1/CT + 1/CS)–1. Hence from Eq. 3.2:

∆* =

CT C S C B (CT + C S )

(3.4)

Figure 4. Plot of the bottom channel threshold voltage versus the top gate bias for a finger transistor (L/W = 20/10,000) with rr-P3HT (75 nm) as semiconductor and polystyrene (450 nm) as the top dielectric (see Figure 3 b). The drain-source voltage was equal to –20 V. The plot can be fitted with two linear relationships with slopes ∆ = 0.39 for linear fit 1 and ∆* = 0.25 for linear fit 2. The intercept of the two fits indicates a transition in the slope and is positioned around top gate voltage between –10 V and 0 V, which corresponds to the threshold voltage of the single gate bottom channel

41


The values calculated with Eq. 3.3 and 3.4 are in good agreement with the slopes extracted from the plot of VTH against VG,Top. The transition point where the slope changes from ∆ to ∆* is the point where, due to the top gate potential, the top channel starts to accumulate charge carriers before the bottom channel. This onset of charge accumulation is around a top gate bias of approximately zero volts. . Table 1: Comparison between the calculated ∆ and ∆* and the corresponding experimentally found linear fits for the relationship between VTH and VG Top. The bottom capacitance (CB) is fixed by using 200 nm thermally grown SiOx at 1.7×10–8 F.cm–2.

Device Semiconductor/ Top dielectric PDTT( 25 nm)/ PS (375 nm) P3HT( 22 nm)/ PS ( 1250 nm) rr-P3HT( 22 nm)/ PS ( 800 nm) rr-P3HT( 25 nm)/ PS (460 nm) rr-P3HT(25 nm)/ PS (410 nm) rr-P3HT( 25 nm)/ PS (395 nm) rr-P3HT( 75 nm)/ PS ( 605 nm) rr-P3HT( 75 nm)/ PS ( 560 nm) rr-P3HT( 75 nm)/ PS ( 450 nm) MEH-PPV(86 nm)/PMMA (354 nm)

42

CS (F·cm-2)

CT (F·cm-2)

∆

∆*

Fit 2

1.1×10-7

5.9×10-9

0.40 0.33±0.03

0.33

0.23±0.03

1.1×10-7

1.8×10-9

0.13 0.14±0.01

0.10

0.09±0.02

1.1×10-7

2.7×10-9

0.19 0.17±0.02

0.16

0.15±0.08

1.1×10-7

4.8×10-9

0.33 0.24±0.02

0.27

0.20±0.03

1.1×10-7

5.4×10-9

0.37 0.39±0.05

0.30

0.31±0.08

1.1×10-7

5.6×10-9

0.38 0.36±0.05

0.31

0.35±0.06

3.4×10-8

3.6×10-9

0.33 0.29±0.05

0.19

0.20±0.04

3.4×10-8

4.0×10-9

0.35 0.34±0.02

0.21

0.22±0.03

3.4×10-8

4.9×10-9

0.43 0.41±0.03

0.25

0.26±0.04

3.1×10-8

9.0×10-9

1.07 1.08±0.01

0.61

0.64±0.02

Fit 1

CHAPTER 3

Table 1 summarizes the obtained ∆-values for a series of dielectric materials, which are in good agreement with the capacitances calculated from the thickness of the layers. If the capacitance of the semiconductor, CS, is very high with respect to CT and CB, we can approximate that: ∆ ≈ ∆* ≈ CT/CB, which is the relationship that was previously found [10, 11, 12]. We also conclude from Table 1 that for a top dielectric layer thicker than 1 µm, hence a very low value of CT, the difference between ∆ and ∆* becomes negligible and falls within the experimental error of the measurements. The same analysis can be applied when sweeping the top gate potential at a fixed bottom gate bias. Furthermore, Figure 3 demonstrates that the drain current after the switch-on voltage, depends very strongly on the change of the bottom gate bias when the fixed top gate bias is set to values more negative than the threshold voltage. Compared to the curves of single gate devices (Figure 2) and the curves where the top gate is positive, the increase (or decrease) in current occurs at much faster rate. This increased change in current can be explained by the penetration of the unscreened field of the (positive) bottom gate. The local field in the top channel, controlled by the change in the bottom gate bias, will vary to a larger extend than that for a single gate devices since the contribution of the negative top gate field is also present, leading to a faster increase or decrease in the accumulated charges in the channel, hence a faster increase or decrease in the current. On the other hand when the top gate is fixed to values more positive than the threshold voltage it will never induce any accumulation in the channels and the change in the field in the bottom channel, since the top channel will not reach accumulation, will be the same as in a single gate device.

3.2.2 Transition region and charge screening in dual-gate OFETs In addition, from the transfer curves in Figure 3, a flattening can be observed around the point where the bottom gate bias is zero and the top gate bias is negative. This feature is depicted more clearly by replotting the data in Figure 5 as a decrease of the differential of the current against the bottom gate bias, δID/δVG. For the replotted curves at a negative top gate bias, the differential δID/δVG starts to decrease around a bottom gate bias of zero Volts and continues to decrease until a bottom gate bias of –15 V. This feature of the DG-OFET transfer curves marks the transition region. This feature is similar to the transfer characteristics of an OFET with a doped semiconductor, where there is a field-effect conductive channel at the

43


interface and a bulk current resulting from dopant density present in the semiconductor [16]. Despite the similarities in transfer characteristics, the system here analyzed is not significantly doped, since no intentional dopants were added and the measurements were performed in high vacuum and dark, so the second channel arises from a second interface field-effect channel and not from bulk conduction. The transition region in the transfer curves is present only when the top

Figure 5. Transconductance δID/δVG of the transfer characteristics shown in Figure 3 a. For bottom gate bias between 0 and –15 V, a clear decrease in the transconductance is observed for a fixed negative top gate bias.

channel is accumulating charge carriers, and is caused by the screening of the bottom gate potential by the charges accumulating in the bottom channel. When a negative top gate bias and a positive bottom gate bias are applied, there will be no accumulation in the bottom channel; hence no screening of the bottom gate and the top channel will depend on both gate potentials. Sweeping the bottom gate bias from high positive values towards less positive values will lead to accumulation in the top channel. As explained above, this happens when VG, Bottom = VTH, the point where the sum of the fields influencing the top channel, which depend on the gate

44

CHAPTER 3

potentials and the capacitances of the layers between the gates and the channel, will start accumulating charges. Current will then start to flow between the source and the drain electrodes. Keeping the top gate bias constant, the current will increase according to the change in bottom gate bias and will depend on the capacitance of the layers between the top channel and the bottom gate (CS and CB). The bottom channel will be insensitive for the top gate potential since the charges accumulated in the top channel screen the top gate potential. However, when a negative VG,Bottom bias is reached for –VG,Bottom>–VTH, the bottom channel will start to accumulate charge carriers and switch on. These charges will partially screen the influence of the bottom gate potential on the top channel, causing a decrease in the transconductance of the top channel. This is marked by the transition region where the bottom channel depends on the bottom gate only and the top channel will depend on both gates. Hence we can observe, experimentally, the screening of the gate potential by the accumulated charges, confirming the theoretical models described in literature [14]. Eventually when VG,Bottom is negative and sufficiently large, the charges accumulated in the bottom channel will completely screen the influence of the bottom gate potential on the top channel and the change in overall drain current will depend only on the change of the current of the bottom channel. The current in the top channel will not be modified since VG,Top is held constant. If the top gate bias is positive, no accumulation in the top channel is feasible; hence no transition region will appear since the device operation will depend on the bottom channel only. The bottom channel-only dependence for very negative VG,Bottom is clearly visible from Figure 3 and Figure 5 where for both positive and negative VG,Top, the change in the current and its differential converges for VG,Bottom < –15 V. We note that the considerations stated above hold only for a DG-OFET for two working channels of comparable conductance, i.e., the mobility in one channel is within two orders of magnitude of the other channel. If only one channel is active or if one of the channel is far worse performing than the other (for example a difference in mobilities of four orders of magnitude), then the presence of the transition region in the transfer characteristics of the DG-OFET will disappear, and only one ∆-factor for the relationship between VTH and VG,Top will be found. Obviously, when only one channel is active and influenced by the gates, the presence of the second channel is negligible.

45


3.3 Conclusions In conclusion, we have demonstrated that the dependence of the bottom threshold voltage on the top gate bias presents two linear relationships depending on which channel of the DG-OFET is switching on first. Furthermore, a decrease in transconductance marks a transition region caused by the screening of the second channel that switches on.

Figure 6. Summary of the device operation of the DG-OFET with a p-type semiconductor. The cases depicted are for a device where the bottom gate is swept and the top gate is held constant as seen in figure 3. The semi-transparent arrows in the cartoons represent the penetration of the electric field. a.) Both gate biases are positive, no charge is accumulated in the semiconductor. The electric field from both gate electrodes penetrates through the film. b.) The top gate bias is negative but no accumulation occurs, since the strongly positive bottom gate bias causes depletion throughout the semiconductor. c.) The top gate bias is negative and accumulation occurs in the semiconductor at the top insulatorsemiconductor interface. The accumulation depends also on the positive bottom gate bias, since the electric field penetrates through the semiconductor. d.) Both gate biases are negative, but the bottom gate bias close to zero bias. The field from the bottom gate is only partially screened by the accumulated charges and still can influence the accumulation at the top insulator-semiconductor interface. This partial screening can be detected as a drop in transconductance (Figure 5). e.) The top gate bias is positive and the bottom gate bias is negative. Accumulation occurs at the bottom insulator-semiconductor interface and depends also on the top gate bias. f.) Both gate biases are negative. Accumulation occurs at both insulator-semiconductor interfaces. Since both gate potentials are screened by the respective accumulated charges at the interfaces, each channel depends only on its respective gate bias.

46

CHAPTER 3

We demonstrate that the change in the threshold voltage depends on the top gate bias with two linear relationships for two different regimes. If one of the gate potentials is positive and the channel is in depletion, while the other channel is in accumulation, then both gate potentials will influence the active channel. If both channels are in accumulation, the gate potentials are screened by the accumulated charge carriers closest to that gate and both channels operate individually: no mutual influences are observed. For a dual-gate OFET with its top channel in accumulation, we demonstrate a drop in the transconductance when the bottom gate potential becomes negative. This transition regime between both linear regimes is marked by a drop in the transconductance, where the bottom channel depends on the bottom gate only and the top channel will depend on both gates. The transition regime results from the fact that the charges accumulated in the bottom channel will start to screen the influence of the bottom gate potential on the top channel and the change in overall drain current will depend only on the change of the current of the bottom channel. The transition region in the transconductance is a direct experimental observation of the screening of the gate potential caused by the accumulated charge at the semiconductor-insulator interface by the gate bias. A summary of the device operation of the DG-OFET is depicted in Figure 6.

References 1

H. Rost, J. Ficker, J.S. Alonso, L. Leenders and I. McCulloch, Synth. Met. 145, 83 (2004).

2

G.H. Gelinck, A.W. Marsman, F.J. Touwslager, S. Setayesh, D.M. de Leeuw, R.C.G. Naber and P.W.M. Blom, Appl. Phys. Lett. 87, 92903 (2005).

3

N. Stutzmann, R.H. Friend and H. Sirringhaus, Science 299, 1881 (2003).

4

T. D. Anthopoulos, G. C. Anyfantis, G. C. Papavassiliou and D. M. de Leeuw, Appl. Phys. Lett. 90, 122105 (2007). 5

A. J. J. M. van Breemen, P. T. Herwig, C. H. T. Chlon, J. Sweelssen, H. F. M. Schoo, E. M. Benito, D. M. de Leeuw, C. Tanase, J. Wildeman and P. W. M. Blom, Adv. Funct. Mater. 15, 872 (2005).

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6

K. Suemori, S. Uemura, M. Yoshida, S. Hocino, N. Takada, T. Kodzasa and T. Kamata, Appl. Phys. Lett. 91, 192112 (2007). 7

M. Spijkman, E. C. P. Smits, P. W. M. Blom, D. M. de Leeuw, Y. Bon Saint Côme, S. Setayesh and E. Cantatore, Appl. Phys. Lett. 92, 143304 (2008). 8

A. Kranti and G. A. Armstrong, Semicond. Sci. Technol. 21, 409 (2006).

9

L. L. Chua, R. H. Friend and P. K. H. Ho, Appl. Phys. Lett. 87, 253512 (2005).

10

S. Iba, T. Sekitani, Y. Kato, T. Someya, H. Kawaguchi, M. Takamiya, T. Sakurai and S. Takagi, Appl. Phys. Lett. 87, 23509 (2005). 11

G. H. Gelinck, E. van Veenendaal and R. Coehoorn, Appl. Phys. Lett. 87, 73508 (2005).

12

M. Morana, G. Bret and C. Brabec, Appl. Phys. Lett. 87, 153511 (2005).

13

J. B. Koo, K. S. Suh, I. K. Youand and S. H. Kim, Jpn. J. Appl. Phys. Part 1 46, 5062 (2007).

14

C. Tanase, E. J. Meijer, P. W. M. Blom and D. M. de Leeuw, Org. Electron. 4, 33 (2003).

15 16

S. M. Sze, Physics of Semiconductor Devices, Wiley, New York (1981).

E. J. Meijer, C. Detcheverry, P. J. Baesjou, E. van Veenendaal, D. M. de Leeuw and T. M. Klapwijk, J. Appl. Phys. 93, 4831 (2003).

48

CHAPTER 4

Chapter 4 A sulphate-detecting biosensor based upon an organic field-effect transistors

Abstract An organic field-effect transistor with integrated proteins for sensing of sulphate ions is presented. A sulphate receptor was engineered to contain a thiol group for surface-anchoring without affecting its binding activity. The modified receptor was covalently coupled to a maleimide-functionalized polystyrene layer, and integrated as gate dielectric in a dual-gate transducer. The binding of sulphate ions in dry conditions was detected by a shift in the threshold voltage. Combined with surface density measurements by AFM, an effective charge of –1.7q per protein was found, as expected from the Bio-FET operation model.

* Part of this work was published in F. Maddalena, M. J. Kuiper, B. Poolman, F. Brouwer, J.C. Hummelen, D. M. de Leeuw, B. De Boer and P. W. M. Blom, J. Appl. Phys. 108, 124501 (2010).

49

SULPHATE-DETECTING BIOSENSOR BASED UPON ORGANIC FIELD-EFFECT TRANSISTORS

4.1. Introduction A standard biosensor is composed of three parts [1]: the biological detection component, which can be a biological molecule or even a cell, the transducer which transforms the signal detected by the biological component into an electrical or luminescence change, and the processing unit which amplifies and/or filters the signal. A crucial property for any sensor is selectivity [2]. Usually the target analyte is present as a minor component in a mixture with many other species. A sensor with poor selectivity will not only detect the target analyte but also other compounds, greatly reducing the efficacy. In nature most biochemical processes use biomolecules such as nucleic acids or proteins that are selective. By incorporating these biomolecules in a transducer as a thin film transistor a selective biosensor (Bio-FET) can be achieved. Transistors based on organic materials are especially suited for this purpose, since these materials can be tailored and functionalized to bind specific biomolecules. Here, we present a prototype BioFET sensor with integrated proteins as schematically depicted in Figure 1. The bottom part is the transducer, an organic field-effect transistor consisting of a source and drain electrode, an organic semiconductor, a gate dielectric and gate electrode.

Figure 1. Schematic view of the Bio-FET. The bottom part is the transducer, that operated as a dualgate organic field-effect transistor.

The semiconductor is protected by a thin insulating top layer, onto which a protein is attached. The purpose of this layer is two-fold. Firstly, it acts as a protection layer for the organic semiconductor. Secondly, the insulator is tailored with anchor groups to attach the receptor protein. In order to arrive at a selective

50

CHAPTER 4

sensor, we focused on a sulphate-binding protein (SBP) that is associated with ATP-binding cassette transporters [3, 4]. These proteins confer high-affinity and selectivity on the transport process due to their dissociation constants (Kd) in the (sub)micromolar range and have also been used in biosensor devices [5]. Moreover, their structure has been extensively studied due to their well defined behaviour during overexpression, purification and crystallization trials [6].

Figure 2. Operation principle of the Bio-FET, including the binding of sulphate ions to the receptor and charge compensation in the semiconductor. a.) Cross-section of the Bio-FET shown in Figure 1 set in accumulation. b.) A negative charged analyte binds to the protein receptor and the trapped charges are compensated by positive charges at the top insulator-semiconductor interface. c.) Chemical structure of PDTT, the semiconductor used in the Bio-FET. d.) Chemical structure of PSMI, the top insulator used in the Bio-FET.

4.1.1 The operation principle of the Bio-FET The operation principle of the Bio-FET is depicted in Figure 2. As a first order approximation, the Bio-FET can be regarded as a dual-gate field effect transistor [7] where the top gate has been replaced by the bioreceptors. As shown in Figure 51


2a, upon application of a negative bias on the bottom gate the transistor accumulates positive charge at the bottom insulator-semiconductor interface. The green spheres on top of the device represent the protein receptors, which capture sulphate ions, hence trapping negative charges at the interface. The top insulator of the Bio-FET will act as a capacitor, where the charges trapped at the interface by the protein receptors will be compensated in the semiconductor, as seen in Figure 2b. If charges have been trapped by the bio-receptors the threshold voltage will be shifted. The change in threshold voltage, will depend on the amount of charge trapped at the interface: ∆VTHCB=QREC. The total (effective) charge at the interface can be written as: QREC=Z·q·NREC, where Z·q is the effective charge of a bound analyte, being q is the elementary charge and NREC is the number proteins with bound analyte per unit area [8, 9, 10]. It is to be noted that Z can be either an integer or a fractionary number, depending on the effective charge of one single bio-receptor-analyte complex. The relation between the shift in the threshold voltage and NREC is equal to:

∆VTH =

1 Z ⋅ q ⋅ N REC CB

(4.1)

4.1.2 Basics of the fabrication of the Bio-FET. The challenge for the realization of the Bio-FET is the attachment of the analyte receptor to the surface of the top insulator. For this purpose, we functionalized the insulator with maleimide side-chains, that can chemically bind to thiol groups. Since the wild type SBP does not contain any thiol group, it is functionalized with a thiol-containing cysteine group. In this way a covalent link between the thiol of the cysteine and the maleimide side-chains of the insulator can be established. Since many proteins contain cysteines in their amino acid sequence, it might seem superfluous to engineer an additional cysteine into their structure. However, several (bacterial) receptor proteins, like SBP, do not contain cysteines in their amino-acid sequence [11, 12]. We note that even proteins that do contain cysteines will probably need to be engineered with an extra cysteine as well, since the cysteines in their original structure might be positioned in the inside of the protein structure or form di-sulfide cysteine-cysteine bridges as part of the structure. This would render the thiol group of the cysteine useless as a possible anchor for chemical coupling. Furthermore, the position of the cysteine should not 52

CHAPTER 4

be too close to the reactive center of the protein, since the binding of the thiol group might interfere with the biochemical activity of the protein [13]. For these reasons, modification of the protein is often desirable or even required to anchor the protein to a surface.

Figure 3. 3-Dimensional structure of SBP with Cys-289 and analyte-binding site highlighted [12, 14].

Guided by the 3D-structure of the protein (Figure 3) [10], we engineered a surface-exposed cysteine at position 289 of the protein, replacing a glycine [15]. The modification, G289C, was chosen in such manner not to disrupt the overall structure of the protein and relatively far away from the sulphate binding site. The modification did not affect the functionality of the protein as the modified SBP(G289C) binds sulphate with a Kd of 0.2 µM, similar to the Kd of the wild type protein. We demonstrate that the modified SBP binds to the top surface of the PSMI-insulating layer and that sulphate ions can be detected under dry conditions.

53


4.2 Results and discussion 4.2.1 Fluorescence binding-assay To confirm the accessibility of the thiol group in the cysteine group at position 289 of the protein for maleimide the fluorophore 2-(4'-maleimidylanilino)naphthalene-6-sulfonate (MAL-ANS), was used to probe the reactivity of Cys-289. The assay is schematically shown in Figures 4a and 4b Mal-ANS is a non-fluorescent compound at the chosen excitation wavelength, but if the maleimide group of Mal-ANS is bound to a thiol group, such as the thiol of cysteine, the complex becomes highly fluorescent. SBP-G289C was used at a concentration of 10 µM solution in 2 mM Tris-HCl buffer, pH 8.0 and Mal-ANS was used in a concentration twice as large. As a control the same experiment was performed using the wild-type SBP, which contains no cysteines. The fluorescence of the Mal-ANS-protein complex in buffer is characterized by an excitation maximum and an emission maximum at 328 and 445 nm, respectively. Figure 4c shows the fluorescence intensity versus time, reflecting the chemical ligation of the fluorophore to the protein. The extent of labeling of SBP(G289C) was estimated from the protein concentration and calibration of the fluorescence upon ligation of Mal-ANS to 2-mercaptoethanol, a small thiol-containing molecule. The labeling of SBP(G289C) was complete after 15 min and the extent of labeling was close to 100%. Control experiments with wild type SBP showed that the maleimide reacts specifically to Cys-289 (see Figure 3).

54

CHAPTER 4

Figure 4. Mal-ANS fluorescence test. a.) Mal-ANS itself is non–fluorescent. b.) When the maleimide group of the Mal-ANS molecule binds to a thiol the molecule becomes fluorescent c.) Fluorescence intensity versus time upon incubation of Mal-ANS with 2-mercaptoethanol, wild type SBP and SBP(G289C). The inset shows the structure of 2-mercaptoethanol. The excitation wavelength was 328 nm.

55


Figure 5. TM-AFM measurements of the PSMI surface; X/Y-range=500×500 nm, Z-range=10 nm. a.) Pristine surface. b.) Surface after exposure to buffer, 2 mM Tris-HCl, pH 7.5. c.) Surface after exposure to the SBP solution for 5 minutes. d.) Surface after exposure to the SBP solution for 10 minutes.

4.2.2. AFM Measurements The binding of SBP(G289C) to the maleimide-functionalized top surface of PSMI was investigated by tapping mode AFM. The height profiles are shown in Figure 5. Images 5 a and 5 b present the surface of PSMI before and after exposure to the buffer used to dilute the protein, for 15 minutes, respectively. The images are

56

CHAPTER 4

practically identical showing that the buffer leaves the PSMI surface unmodified. Figures 5 c and 5 d show PSMI exposed to the protein solution (10 µM) in buffer for 5 and 10 minutes. The surface now shows globular corpuscles. The objects have diameters that range from 5 to 20 nm with a height of 3 to 6 nm. These dimensions agree reasonably well with those of a single SBP molecule, within the resolution limit of the AFM tip. The reported dimensions of this protein are 3.5×3.5×6.5 nm [12]. Figures 5 c and 5 d therefore indicate that the proteins probably retain their shape and structure on the surface even after the solution was removed from the surface by spin-drying. We extracted an areal density, NREC, of about 7×1015 proteins per m2. We note that some objects are too big to be single polypeptides, which could indicate formation of aggregates.

Figure 6. Dissociation constants of the wild type (WT) and mutated SBP(G289C), both for the pristine protein and the dried and subsequently rehydrated protein.

57


4.2.3. Stability of the protein to drying A critical issue is the stability of a protein upon a surface, especially under dry conditions. To determine the stability of the protein we measured the dissociation constant [16] for the pristine SBP and for SBP subjected to drying on a polymer and afterwards rehydrated with buffer. SBP can bind chromate ions, albeit with lesser affinity than sulphate ions and when this binding occurs, a quenching of the fluorescence of the protein can be observed. The dissociation constant for chromate ions (Kd[CrO42–]) was determined by measuring the decrease in fluorescence during the titration with chromate ions. The excitation and emission wavelengths used were 285 nm and 325 nm respectively. The dissociation constants for the wild type SBP and SBP(G289C) are shown in Figure 6. We see that there is an increase of the dissociation constant of chromate for the dried protein with respect to the pristine protein, indicating that the affinity of the protein towards chromate ions is diminished by the drying of SBP. However the data show that the proteins are still active even after drying and rehydration. This indicates that the wild type SBP and SBP(G289C), are both stable even under dry conditions. The drying process does not affect the affinity of the protein towards the analyte; the protein remains active even after being dried.

4.2.4. Electrical Measurements of the Bio-FET The transfer characteristics of the Bio-FET prototype are presented in Figure 7. The black curve (square symbol) presents the pristine PDTT film in vacuum and dark. The mobility is 7.2×10-4 cm2V–1s–1 at a gate bias of –20 V. The dark gray curve (triangle symbol) shows the current transport of the Bio-FET, that is, with SBP(G289C) bound to the PDTT film. The exposure to buffer caused a small shift in the transfer characteristics, which could be due to trapped ions in the protein layer. The light gray curve (diamond symbol) shows the transfer characteristics of the device with protein coating after exposure to sulphate ions from a 1 mM Na2SO4 solution. A significant shift in both pinch-off and threshold voltage was observed. The shift of the threshold voltage was –11 V. The sign is consistent with the presence of a negative charge at the surface of the PSMI layer. Because the Bio-FET was incubated under 1 mM sulphate, well above Kd, it may be assumed that all SBP receptors have captured a sulphate ion [15].

58

CHAPTER 4

Figure 7. Transfer characteristics of Bio-FET transducers with L/W= 20/10000 in the saturated regime, with PDTT as semiconductor coated with a 50 nm PSMI Layer.

Rearranging Eq. (4.1) to Z = (∆VTH CB ) / (q ⋅ N REC ) and applying the shift found experimentally from the transfer curves shown in Figure 7 we can calculate the effective charge that is trapped by the receptor proteins at the interface. NREC was extracted from the AFM data. We estimate that the charge per receptor as Z = –1.7, which is close to the charge of –2 of a single sulphate ion. Finding a value that is less than the total charge of the ion trapped is not surprising. Most proteins tend to compensate the charge of an analyte in the interior of their threedimensional structure by protonation or de-protonation of the amino acid side tails.

59


4.3 Environmental conditions and role of the protein dipole 4.3.2 The Bio-FET in environmental conditions There are many factors present when dealing with organic field-effect transistors in the presence of water and ions in solution even when the problem of electrolysis is taken out as it is done here by drying the devices before measurement. These were ignored in the quantitative analysis of the Bio-FET for the sake of simplicity. The most important one was the assumption that a single monolayer of protein receptors was present on the surface of PSMI, while it is possible that there are many layers of protein receptors and tightly packed SBPaggregates present on the surface of PSMI. It is also important to note that the effects of ionic solutions on organic fieldeffect transistors are still largely unknown and much more investigation about this subject needs to be done in the future before coming to a full understanding of BioFET devices and, more important, building an efficient Bio-FET. Many issues need to be resolved before a more reliable device can be produced, such as the stability of the transistor transducer and complete shielding of the transistor from water and ions, especially is one desires to use a Bio-FET device in wet conditions.

4.3.2 Impact of the protein dipole It is well known that proteins, although they might be neutral as a whole, have a strong dipole. In the polypeptide structure several amino acids can be charged, which can give rise to strong dipole moments [17, 18, 19]. The literature reports that such a dipole in the protein might have strong impact on the transfer curve of a Bio-FET and that the shift in dipole when an analyte binds can be used as a detection mechanism through the “Cooperative Molecular Field Effect” (COMFEffect) [20]. This would give rise to a field equal to ∆Φ = (µ ⊥ ⋅ N REC ) / ε , where

µ⊥ is the value of the component of the dipole vector perpendicular to the surface and ε is the dielectric constant. The dipole moment can be roughly calculated through an internet server program [21] using the amino acid sequence and crystallographic structure of wild type SBP [12, 14]. The result gives an estimate of 365 Debye (1 Debye ≈ 3.33×10-30 C·m), which is at least one order of magnitude larger than the dipole moments of small molecules and salts in gaseous form [22]. The program used can only calculate the

60

CHAPTER 4

dipole roughly from the amino acid sequence and special configuration as presented in the pdb-files and ignoring possible (fictitious) ligands. The sequence and the crystal structure of the wild type SBP receptor, coded as 1SBP, can be found on-line at the RCSB Protein Data Bank [14]. We calculate the field formed through the COMEF-Effect on our device, assuming that all proteins are arranged perpendicular to the surface and with the value of NREC obtained from the AFM measurements. We find a field of ∆Φ= 0.32 V·m–1. This is a very weak field in spite of the very strong dipole of the protein. Hence the dipole of the protein could account at most for a shift in the order of a few mV. We can safely conclude that the effect of the dipole of the proteins can be disregarded.

4.4. Conclusions In summary, we have demonstrated an organic Bio-FET prototype transducer with an integrated sulphate binding protein. Fluorescence spectroscopy and tapping mode AFM were used to confirm the covalent coupling of the SBP receptor to the surface of a maleimide functionalized polystyrene layer. Measurement of the dissociation constant of SBP after drying and rehydration showed that the protein remains active even after being dried. In the Bio-FET transducer the sulphate ions could be detected by a shift in the threshold voltage. The effective charge per protein is derived as –1.7q per protein.

References 1

H. Nakamura and I. Karube, Anal. Bioanal. Chem. 377 , 446 (2003).

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J. T. Mabeck and G. G. Malliaras, Anal. Bioanal. Chem. 384 , 343 (2006).

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H. Biemans-Oldehinkel, M. K. Doeven and B. Poolman, FEBS Lett. 580, 1023 (2006).

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S. Shrestha, L. L. E. Salins, C. M. Ensor and S. Daunert, Biotechnol. Bioeng. 78, 517 (2002).

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R. Berntsson, M. K. Doeven, F. Fusetti, H. H. Duurkens, D. Sengupta, S. J. Marrink, A. M. Thunnissen, B. Poolman and D. J. Slotboom, EMBO Journal 28, 1332 (2009). 7

G. M. Gelinck, E. van Veenendaal and R. Coehoorn, Appl. Phys. Lett. 87, 73508 (2005).

8

S. H. Han, Y. H. Kim, S. H. Lee, M. H. Choi, J. Jang and D. J. Choo, Org. Electron. 9, 1040 (2008). 9

M. Spijkman, E. C. P. Smits, P. W. M. Blom, D. M. de Leeuw, Y. B. Saint Come, S. Setayesh and E. Cantatore, Appl. Phys. Lett. 92, 143304 (2008).

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F. Maddalena, M. Spijkman, J. J. Brondijk, P. Fonteijn, F. Brouwer, J. C. Hummelen, D. M. de Leeuw, P. W. M. Blom and B. de Boer, Org. Electron. 9, 839 (2008). 11

A. R. Glenn, Ann. Rev. Microbiol. 30, 41 (1976).

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J. W . Pflugrath, and F. A. Quiocho, J. Mol. Biol. 200, 163 (1988).

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U. Langel, B. F. Cravatt, A. Graslund, N. G. H. von Heijne, M. Zorko, T. Land and S. Niessen, Introduction to peptides and proteins, 1st ed., CRC Press (2009). 14

http://www.rcsb.org/pdb/home/home.do, file: 1SBP.

15

J. M. Kuiper, R. Pluta, W. H. C. Huibers, F. Fusetti, E. R. Geertsma and B. Poolman, Protein Sci. 18, 1033 (2009). 16

A. Cornish-Bowden, Fundamentals of enzyme kinetics, 3rd ed., London Portland (2004). 17

C. E. Felder, J. Prilusky, I. Silman and J. L. Sussman, Nucleic Acids Res. 35, W512 (2007).

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S. Takashima, J. Non-Cryst. Solids 305, 303 (2002).

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J. Antosiewicz, Biophys. J. 69, 1344 (1995).

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D. Cahen, R. Naaman and Z. Vager, Adv. Func. Mat. 15, 1571 (2005).

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22

Various Authors, CRC Handbook of Chemistry and Physics, 85th Ed., CRC Press (2004).

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CHAPTER 5

Chapter 5 Doping Kinetics of Organic Semiconductors Investigated by Field-Effect Transistors

Abstract

The kinetics of acid doping of the semiconductor regio-regular poly3-hexylthiophene with vaporized chlorosilane has been investigated using field-effect transistors. The dopant density has been derived as a function of temperature and exposure time from the shift of the pinch-off voltage, being the gate bias where current starts to flow. The doping kinetics are perfectly described by empirical stretched exponential time dependence with a saturation dopant density of 1 ± 0.5 ×1026 m–3 and a thermally activated relaxation time. We show that a similar relationship holds for previously reported kinetics of poly-thienylene-vinylene doped with molecular oxygen

* Part of this work was published as F. Maddalena, E. J. Meijer, K. Asadi, D. M. de Leeuw and P. W. M. Blom, Applied Physics Letters 97, 043302 (2010).

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KINETICS OF ORGANIC SEMICONDUCTORS INVESTIGATED BY FIELD-EFFECT TRANSISTORS

5.1 Introduction Organic electronics is well-established in the fields of light-emitting diodes, displays and solar cells [1, 2, 3, 4]. Presently, organic field-effect transistors (OFETs) are being developed for low-end high-volume applications such as smart labels and electronic barcodes, [5] and non-volatile memories [6, 7, 8]. Furthermore, OFETs are being investigated as possible gas sensors [9, 10, 11]. Upon exposure to gasses changes have been observed in key transistor parameters such as threshold voltage and field-effect mobility [12, 13]. The most dominant change however is a change in the conductance due to chemical doping of the semiconductor. A gas sensor could be envisaged by monitoring the increase in conductance upon exposure to the target analyte [14].

Figure 1. Transfer characteristics of a field effect transistor (L/W= 10/10000, VDRAIN= -2 V) in vacuum and after 1 minute exposure to HCl vapour.

To fabricate a reliable gas sensor it is also important to understand the kinetics of the doping process. However, the number of studies on the doping kinetics is limited. Most studies focus on the conductivity of organic semiconductors as a function of dopant density [15, 16]. The increase of the conductivity with time has only been investigated for regio-regular poly-3-hexylthiophene (rr-P3HT) and

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poly-thienylenevinylene (PTV) exposed to molecular oxygen [17]. From analysis of the transfer curves the dopant density and bulk mobility could be derived as a function of exposure time. The study was performed to analyze the shelf-life of organic transistors. In order to understand the kinetics of the doping of organic semiconductors, we doped regio-regular poly-3-heylthiophene (rr-P3HT) in field-effect transistors with gaseous HCl. At the partial vapor pressures used however, the doping process was much too fast to reliably determine the kinetics. Figure 1 shows that the polymer was fully doped in a matter of a minutes. To expand the time scale and have a more controlled doping of the semiconductor, instead of HCl we used vaporized trichloro-(1H, 1H, 2H, 2H)perfluorooctylsilane (TCFOS). This molecule is reported to yield acid doping of conjugated molecules [14]. The doping process of rr-P3HT with TCFOS is sufficiently slow to measure the doping kinetics. The dopant density is derived from the transfer curves as a function of temperature and exposure time. We show that the kinetics can phenomenologically be described with stretched exponential time dependence and that the same relationship holds for previously reported doping studies [17].

5.1.1 Doping in organic semiconductors Applying a gate bias more positive than the threshold voltage to a p-type transistor will deplete any mobile holes present in the semiconductor layer, turning the transistor in the off–state. This can be best observed from the transfer characteristics of the transistor. Through the logarithm of the current against the gate voltage one can determine the exact point when the channel current starts to flow rising from the leakage current. Such point it called the ‘pinch–off voltage’ VPINCH. In an undoped or negligibly doped semiconductor VPINCH is equal to the switch–on voltage, VSO, the bias point at which the transistor intrinsically switches on if no significant dopants or traps are present in the material. Introducing chemical dopants into the semiconducting layers will shift the pinch–off voltage higher values, since a higher bias is needed to deplete the semiconductor. The cartoons in Figure 2 show schematically the process of depletion in a doped p-type material. A doped semiconductor will have two forms of conduction. First the common field-effect conduction through the channel formed by the gate through accumulation of charges at the semiconductor-insulator

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interface. Second the bulk conduction through the mobile charges in the bulk of the semiconductor. The latter are formed not by field-effect but by doping of the material. If the transistor is in accumulation (Figure 2 a) both conduction processes will occur. However, since the concentration of the charges is usually far higher in the channel than in the bulk, hence also the mobility, the contribution of the bulk conduction is minimal. To really observe doping and bulk conduction one must go into the depletion region of the transistor, as seen in Figure 2 b, where the channel conduction is switched off and only bulk conduction is present. The effect of the dopant will appear as an extra ‘shoulder’ in the transfer characteristics of the OFET (see Figure 3). Increasing the bias to sufficiently high positive biases will eventually also deplete the charge carriers in the bulk, turning the device completely off (Figure 2 c).

Figure 2. Schematic view of the charge depletion in a doped p-type OFET. In this device holes are present in the bulk, caused by the dopands. a.) When the gate bias is negative (VGATE < VSO), then the gate will accumulate positive charges at the insulator-semiconductor interface forming a conductive channel. The current, between source and drain, flowing through the channel will add to the current flowing through the doped bulk. b.) By applying a positive gate bias (VGATE > VSO) no accumulation will occur at the semiconductor-insulator interface. Only conduction through the doped bulk will occur. c.) If the gate bias is positive enough (VGATE > VPO) then also the mobile charges caused by doping will be depleted and no current will flow between the source and the drain.

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5.2 Results and Discussion To determine the doping kinetics of our semiconductor, the transfer curves of OFETs were measured as a function of temperature and exposure time. The measurement chamber was evacuated to less than 10–4 mbar and vaporized TCFOS was injected into the measurement chamber (p ~2.5·10–2 mbar) at t= 0 s as described in the experimental section of Chapter 2. Without TCFOS the transfer curve does not change with time; stress can be disregarded on the time scale of the experiment.

Figure 3. a.) Transfer curves in the linear regime of a rr-P3HT organic field-effect transistor in vacuum and after exposure to TCFOS vapor at room temperature for different exposure times. The channel length and width are 10 µm and 10000 µm respectively. B.) The transfer curves corrected for the threshold voltage shift with respect to the pristine undoped transistor at t=0.

The transfer curves at different times of exposure to TCFOS at room temperature are presented in Figure 3 a. The curves shift towards positive gate bias with exposure time. Furthermore a shoulder appears and the on-current in accumulation slightly increases. We note that the off-current in depletion increases as well, which is due to increased parasitic leakage currents outside of the defined transistor area caused by the use of unshielded drain electrodes [17]. The analysis of the shift of the transfer curves towards positive gate bias with exposure time is complicated by the fact that two effects have to be disentangled. First, the doping process can lead to the formation of charged states at the interface between the insulator and the semiconductor. This will lead to a shift of the threshold voltage. Furthermore, upon exposure to TCFOS the rr-P3HT gets chemically doped and a higher positive bias is needed to deplete the bulk semiconductor. To get a

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DOPING


quantitative result for the threshold voltage shift between the measurements we linearly extrapolated the current as a function of the gate bias for each measurement on a linear scale and determined from the intercept the threshold voltage. The transfer curves corrected for the threshold voltage shift are presented in Figure 3 b and show a cross-over from an accumulation mode into a bulk depletion mode transistor. The current in accumulation, at negative bias, is dominated by the channel current while in depletion, at positive bias, the current is mainly flowing through the bulk semiconductor. The remaining shift of the transfer curves now arises from the doping and is characterized by a shift in the pinch-off voltage. At the pinch-off voltage, defined as the gate bias where current starts to flow, the bulk semiconductor is fully depleted. When the gate bias is sufficient to enable charge accumulation in the channel the current increases superlinearly with gate bias, which is manifested by the shoulder in the transfer curve. This superlinear increase is due to the fact that the charge carrier mobility in the channel is much larger than in the bulk. This difference in charge carrier mobility originates from the fact that the mobility is charge density dependent [18] and the charge density in the channel is much larger than in the bulk of the semiconductor. The dopant density can be determined from the pinch-off voltage. We assume that the dopants are homogeneously distributed in the bulk semiconductor. Furthermore for a small source drain bias we assume that the depletion of the semiconductor takes place uniformly over the entire channel length. The width of the depletion layer is then given by [17, 19]:

W Depl =

ε 0 ε S  Ci

 2C i2 (VGate − VTh ) 1+ − 1   qN Aε 0 ε S  

(5.1)

where ε0 is the permittivity in vacuum, εS the relative dielectric constant of the semiconductor, q the elementary charge, Ci the capacitance of the gate dielectric per unit area, and where NA is the dopant density. Exactly at the pinch-off voltage the whole film is just depleted from charge carriers. The width of the depletion layer then is equal to the thickness of the semiconductor layer dS [18]. The

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Figure 4. a.) Dopant density versus exposure time at different temperatures for rr-P3HT doped with silane (TCFOS) vapor. The solid lines are a fit to the data with a stretched exponential time and temperature dependence. b.) Relaxation τ as a function of reciprocal temperature, extracted from the fit. As expected for the relaxation time of a stretched exponential relationship, τ has an exponential dependence with the inverse absolute temperature.

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insulator and semiconductor capacitances per unit area are given by Ci = ε0εi/di, and Cs= ε0εS/ds where di and ds are the thicknesses of the gate dielectric and the semiconductor respectively, and where εi is the relative dielectric constant of the gate dielectric. The dopant density can now be recalculated from Eq. 5.1 as [17]:

NA =

2VPO ε 0  d S2 2d S d I q + εI  εS

  

(5.2)

The dopant densities calculated with Eq. 5.2 using the pinch-off voltages extracted from Figure 3 are presented in Figure 4 a on a double logarithmic scale as a function of time. Values for the dopant densities derived at other temperatures are included as well. Figure 4 a shows that the dopant density increases with time as a power law. The exponent however, increases with temperature. This dependence suggests that the time and temperature dependence can phenomenologically be described by a single stretched exponential:

   t β    N A (t ) = N A (t → ∞) 1 − exp −        τ    

(5.3)

where NA(t→∞) is the saturation dopant density, β is a dispersion parameter typically equal to T/T0, with T being the absolute temperature and T0 a characteristic temperature, and τ being the average relaxation time. The stretchedexponential dependence is observed in a wide class of disordered systems, such as relaxation of glasses toward equilibrium, dielectric relaxation in a charge-densitywave system and dispersive transport in photoconductors [20]. It occurs when there is an anomalously large distribution of characteristic times involved, and it holds for all dispersive transport processes in an exponential distribution of trap states [21]. The solid lines in Figure 4 a represent a simultaneous fit to all the data points. A good agreement is obtained with NA(t→∞) = 1 ± 0.5 ×1026 m–3 and T0 = 690 ± 40 K. The value for the saturation dopant density derived indicates that about 10% of the thiophene units can be chemically oxidized. This number is in good agreement with the maximum doping level derived from electrochemical studies; in polythiophene about one out of every four monomeric units can be oxidised [22]. Figure 4 b shows that the relaxation time, τ, is thermally activated by [23]:

 E ACT    k BT 

τ = υ 0−1 Exp 

70

(5.4)

CHAPTER 5

where ν0 is a frequency prefactor, EACT is the activation energy of the doping process (or the slowest step in the process) and kB is the Boltzmann constant. The activation energy of 0.6 eV, or about 58 kJ·mol–1, agrees with reported values for protonation reactions of organic molecules [24, 25, 26, 27], suggesting acid doping of rr-P3HT by TCFOS. Reported values for ν span many orders of magnitude [28]. Here we derived a value for the prefactor of about 1 Hz, which cannot yet be explained. The doping of rr-P3HT was investigated at different partial pressures of TCOFS, viz. 2.5·10–2 and 2.5·10–1 mbar. The main effect is a decrease of the relaxation time with increase of TCOFS partial vapor pressure, indicating that increasing the pressure of the dopant increases the speed of the doping of the semiconductor.

Figure 5. Dopant density versus time at different temperatures for PTV doped with molecular oxygen at different pressures.17 The solid lines are the fit with a stretched exponential function.

Doping of poly-thienylene-vinylene (PTV) exposed in field-effect transistors to molecular oxygen has previously been reported [17]. Similar to this study the dopant density has been derived as a function of the partial oxygen pressure and exposure time, but the doping kinetics were not further analyzed. Here, we replot the reported values in Figure 5; the fully drawn curves are a stretched exponential fit to the data using Eq. 5.3. Also for the oxygen doping of PTV a good agreement is obtained. For the fit we used the same maximum dopant density, NA(t→∞), as derived above for rr-P3HT. The characteristic temperature T0 follows directly from

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DOPING


the slope of dopant density versus time and amounts to T0 = 1700 ± 30 K. The dopant density has not been reported as a function of temperature in the PTV study. Hence we can determine the average relaxation time τ but not the activation energy. We find that τ is inversely proportional to the oxygen pressure, τ ~ p O−12.4 . When we assume that the activation energy does not depend on oxygen pressure, the relation between τ and pressure can then be rationalized by a prefactor that increases with oxygen pressure. The electrical transport in organic semiconductors is dominated by thermally activated hopping between localized states at the Fermi level [29, 30]. When the effect of doping on the charge transport is only governed by filling up of transport states with the charge carriers that are released from the dopants, it can be expected that the characteristic temperature derived from the doping study would be similar to the isokinetic temperature characterizing the electrical transport states. However, the differences obtained, viz. 690 K and 425 K for rr-P3HT and 1700 K and 380 K [31] for PTV, indicates that chemical doping creates new states in the band gap, as confirmed by numerous optical absorption investigations.

5.3 Conclusions In summary, we have investigated acid doping of rr-P3HT with silane TCFOS vapor in field-effect transistors. The transfer curves shift towards positive gate bias, into depletion, upon exposure time. The dopant density has been derived from the shift in pinch-off voltage as a function of temperature and exposure time. The dopant density kinetics can phenomenologically be described by stretched exponential time dependence. The doping is thermally activated with activation energy of about 0.6 eV, which agrees with reported values for protonation reactions of organic molecules. Reinterpretation of reported doping densities as a function of time for PTV doped with molecular oxygen showed the same relationship. The good agreements obtained indicate that the doping kinetics of organic semiconductors follows stretched exponential time dependence.

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B. P. Rand, C. Girotto, A. Mityashin, A. Hadipour, J. Genoe, and P. Heremans, Appl. Phys. Lett. 95, 173304 (2009). 3

M. Suzuki, H. Fukagawa, Y. Nakajima, T. Tsuzuki, T. Takei, T. Yamamoto and S. Tokito, J. Soc. Inf. Display 17, 1037 (2009).

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M. Pagliaro, R. Ciriminna and G. Palmisano, ChemSusChem 1, 880 (2008).

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E. Cantatore, T.C. T. Geuns, G. H. Gelinck, E. van Veenendal, A. F. A. Gruijthuijsen, L. Schrijnemakers, S. Drews and D. M. de Leeuw , J . Solid-St. Circ. 42, 84 (2007). 6

K. Asadi, D. M. de Leeuw, B. de Boer and P. W. M. Blom, Nat. Mater. 7, 547 (2008).

7

M. Debucquoy, M. Rockele, J. Genoe, G. H. Gelinck and P. Heremans, Org. Electron. 10, 1252 (2009). 8

D. M. de Leeuw and E. Cantatore, Mat. Sci. Semicon. Proc. 11, 199 (2008).

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A. Caboni, E. Orgiu, M. Barbaro and A. Bonfiglio, IEEE Sens. J. 9, 1963 (2009).

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A. K. Diallo, J. Tardy, Z. Q. Zhang, F. Bessueille, N. Jaffrezic-Renault and M. Lemiti, Appl. Phys. Lett. 94, 263302 (2009). 11

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A. Das, R. Dost, T. H. Richardson, M. Grell, D. C. Wedge, D. B. Kell, J.J. Morrison and M. L. Turner, Sensor Actuat. B-Chem. 137, 586 (2009). 13

T. Toccoli, A. Pallaoro, M. Tonezzer, N. Coppede and S. Iannotta, Solid State Electron. 52, 417 (2008). 14

C. Y. Kao, B. Lee, L. S. Wielunski, M. Heeney, I. McCulloch, E. Garfunkel, L. C. Feldman, and V. Podzorov, Adv. Funct. Mater. 19, 1906 (2009). 15

C. K. Chan, W. Zhao, S. Barlow, S. Marder and A. Kahn, Org. Electron. 9, 575 (2008).

16

L. Li, G. Meller and H. Kosina, J. Appl. Phys. 101, 033716 (2007).

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17

E. J. Meijer, C. Detcheverry, P. J. Baesjou, E. van Veenendaal, D. M. de Leeuw and T. M. Klapwijk, J. Appl. Phys. 93, 4831 (2003). 18

W. F. Pasveer, J. Cottaar, C. Tanase, R. Coehoorn, P. A. Bobbert, P. W. M. Blom, D. M. de Leeuw and M. A. J. Michels, Phys. Rew. Lett. 94, 206601 (2005). 19

S.M. Sze, Physics of Semiconductor Devices, Wiley, New York (1981).

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J. Kakalios, R. A. Street and W. B. Jackson, Phys. Rev. Lett. 59, 1037 (1987).

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Y. F. Chen and S. F. Huang, Phys. Rev. B 44, 13775 (1991).

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G. Tourillon, Polyhtiophene and its derivatives, page 293, in Handbook of Conducting Polymers, Volume 1, edited by T.A. Skotheim, Marcel Dekker (1986).

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R. S. Crandall, Phys. Rev. B 43, 4057 (1991).

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R. D. Bach, C. Canepa, J. E. Winter and P. E. Blanchette, J. Org. Chem. 62, 5191 (1997).

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P. Nama, R. Flammang, H. T. Lea, P. Gerbaux and M. T. Nguyena, Int. J. Mass Spectrom. 228, 151 (2003). 26

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CHAPTER 6

Chapter 6

Carrier Density Dependence of the Hole Mobility in Doped and Undoped Organic Semiconductors

Abstract The charge carrier mobility in conjugated polymers depends on the charge carrier density. Here we investigate the mobility of poly(3-hexylthiophene) over a carrier densities range from 1015 cm–3 to 1020 cm–3 in order to experimentally establish the relation between mobility and carrier density. Hole-only diodes were used for low densities and field-effect transistors were used for the high carrier densities. Intermediate densities were probed using chemically doped Schottky diodes and transistors. We demonstrate that the mobility is constant for carrier densities below 1016 cm–3 and follows a power law dependence for carrier densities higher than 1018 cm–3. We also make note of possible anomalies rising from trapping effects or morphology.

* Part of this work was published as J. J. Brondijk, F. Maddalena, K. Asadi, H. J. van Leijen, M. Heeney, P. W. M. Blom and D. M. de Leeuw, Physica Status Solidi B, accepted (2011). 75

CARRIER DENSITY DEPENDENCE OF THE HOLE MOBILITY IN ORGANIC SEMICONDUCTORS

6.1 Introduction Solution-processable conjugated polymers such as polythiophene derivatives are attractive candidates for application in low-cost and flexible microelectronic devices. The electrical transport in semiconducting polymers is dominated by thermally assisted intermolecular hopping of the charge carriers in a Gaussian density of states [1]. The transport depends on carrier density, temperature and electric field [2, 3, 4, 5, 6]. At room temperature and at relatively small electric field the transport is dominated by the carrier-density and the field dependence plays a negligible role [6, 7]. Experimentally capturing the full extent of the relation between mobility and carrier density is necessary for understanding and improvement of device performance. For poly(p-phenylene vinylene) derivatives, the mobility extracted from diodes is relatively low, around 10-7 cm2/Vs, and independent of carrier density. The mobility extracted from field-effect transistors however increases with charge carrier density up to typically 10-3 cm2/Vs. The difference originates from the charge carrier density, which in diodes is typically 1015 cm–3 to 1016 cm–3 and in transistors from 1018 cm–3 to 1020 cm–3 [8]. However, a full mobility carrier-density relation is hindered by a gap in the carrier density between 1016 cm–3 to 1018 cm–3 [8, 9]. Polythiophenes have been extensively applied in organic field-effect transistors (OFETs) [10, 11] and solar cells [12]. Recently polythiophene and its derivatives have found applications in the field of chemical sensors and biosensors [13, 14]. The benchmark polythiophene is regioregular poly(3-hexyl-thiophene) (rr-P3HT). Doping in organic semiconductors has been an important topic since the introduction of these semiconductors [15, 16]. More recently achievements have been made in the field of stable n-type doping and solution-processed doping [17, 18, 19]. To probe the charge carrier mobility in the region between 1016 cm–3 to 1018 –3 cm , we deliberately doped rr-P3HT [20]. In this work we investigated hole-only diodes, (doped) Schottky diodes and (doped) transistors. In this way over the whole range of charge carrier densities, the mobility can be unified by a zero-field mobility with a density dependent term based on hopping in an exponential density of states (DOS) [8, 21].

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6.2 Results and discussion To chemically dope rr-P3HT (the polymer was obtained from M. Heeney, Imperial College London), we exposed the devices to vaporized trichloro-(1H, 1H, 2H, 2H)-perfluorooctylsilane (TCFOS; Sigma Aldrich) at a partial pressure of ~2.5·10–1 mbar (see chapter 2, section 2.3.3). The transfer characteristics of the transistors were measured as a function of exposure time [22]. We note that in the case of Schottky diodes the evaporation of the top electrode was performed after the doping process to guarantee a uniformly doped semiconductor layer.

Figure 1. (a) Current density versus voltage for a rr-P3HT hole-only diode with a thickness of 135 nm measured as a function of temperature. The solid lines represent the fit according to the carrier density-dependent and field-dependent mobility model. (b) Current density versus voltage of an undoped and a doped Schottky diode with a thickness of 195 nm. The solid lines represent the fit according to the model.

The current density of a rr-P3HT hole-only diode as a function of applied voltage is presented in Figure 1 a. The transport was measured as a function of temperature. At low bias the current density scales at low bias with the voltage squared, indicating space-charge-limited current (SCLC) with a constant mobility. The injecting contact was PEDOT:PSS, which is an Ohmic contact to rr-P3HT, because the work function matches the energy of the highest occupied molecular orbital (HOMO) of rr-P3HT [23]. The transport in the diode is then bulk limited. At high bias the current density is enhanced. Charge transport is due to thermally activated hopping between localized states at the Fermi level. Device simulations were performed by using a numerical drift-diffusion model [24], incorporating a hopping mobility that depends on both charge-carrier density and electric field [21, 77


25, 26]. Figure 1 a shows that for all temperatures a perfect agreement between measured and calculated current densities is obtained. As fit parameters we used a room temperature zero-field mobility of 1.5×10–4 cm2 V–1s–1 with an activation energy of about 0.2 eV, a zero-field conductivity of 5×106 Sm–1 a characteristic temperature for the exponential DOS, T0, of 475 K and overlap parameter α-–1 of 3 Å. The numbers agree well with reported values for rr-P3HT [Error! Bookmark not defined.]. We note that at room temperature and for the low applied bias the density dependence dominates and the electric field dependence is negligible. To extract from the fit the charge carrier density mobility relation we used the procedure reported by Craciun et al. [9]. The mobility is presented later (page 80) as a function of average charge density in Figure 4. In order to increase the charge carrier density in diodes we fabricated doped Schottky diodes. As a reference, however, first undoped diodes were investigated. The Schottky diodes exhibit a rectification of more than 6 orders of magnitude. The current in forward bias is presented in Figure 1 b. The current density is calculated with the same numerical model as for the hole-only diodes using identical fit parameters. The solid line in Figure 1 b shows that a good agreement between calculated and measured current densities is obtained. To increase the carrier density Schottky diodes were doped. Exposing rr-P3HT to vaporized TCFOS yields p-type doping with additional mobile holes [20, 22]. Figure 1 b shows that the current density in forward bias increases. The origin is an increase in mobility due to an increased charge density. To quantify the relation we determined the charge carrier density independently from CV measurements in reverse bias. In order to determine the acceptor density of the dopant in the semiconductor we used impedance spectrometry. An AC voltage of 100 mV was superimposed to the applied reverse DC bias. A frequency scan from 10 Hz to 1 MHz was made at each bias and the equivalent parallel capacitance (CP) was extracted. Figure 2 shows CP–2 versus reverse VDC for a diode exposed 20 minutes to TCFOS vapor. A straight line is obtained. The acceptor density, NA, was calculated with the MottSchottky relation, from the slope of the variation of the CP–2 with DC bias VDC:

  1  −  (6.1) 2 qε 0ε S  ∂ (1 / C P ) / ∂V DC  where q is the elementary charge, ε0 is the dielectric constant in vacuum and εS is NA =

2

the relative dielectric constant of the semiconductor. The extracted value was then used as an input to calculate the forward current density. The solid line in Figure 1 b shows that a good agreement is obtained. From 78

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Figure 2. Inverse of the capacitance squared versus DC bias of the doped Schottky diode from Figure 1 b. The solid line represents the linear fit.

the calculation, the corresponding average mobility was extracted. For three doped Schottky diodes the mobility is presented as a function of charge carrier density in Figure 4 in the next page. To probe the mobility at high carrier density field-effect transistors were investigated. The carrier density quadratically decreases with the distance from the semiconductor gate-dielectric interface. The density at the interface dominates the transport and was calculated as reported previously [27]. The field-effect mobility µFE was approximated at each gate bias using the relation [28]:

µ FE =

L ∂I d WC I Vd ∂V g

(6.2)

where L and W are the length and width of the channels of the OFET respectively, CI is the capacitance per unit are of the gate insulator, Vd is the drain current and Vg is the gate bias. The extracted mobility versus carrier density values for an undoped rr-P3HT transistor are presented in Figure 4. Exposing the transistors to TCFOS vapor leads to doping of rr-P3HT. The doping density can be varied by changing the exposure time. Transfer curves were recorded in-situ at different exposure times are presented in Figure 3. A shoulder appears in the transfer curve; a higher positive bias is needed to deplete the doped bulk semiconductor and pinch-off the channel. Also the on-current at negative gate

79


Figure 3. Transfer curves in the linear regime of a rr-P3HT organic field-effect transistor in vacuum before and after exposure to TCFOS vapor at room temperature for different exposure times. The channel length and width are 10 µm and 2500 µm respectively. The transfer curves are corrected for The switch-on voltage shift with respect to the pristine undoped transistor at t=0. [8] The threshold voltages range from 5.8 V for the undoped transistor to 19 Vafter 138 min.

Figure 4. Charge carrier mobility versus carrier density as derived for rr-P3HT from undoped holeonly diodes, doped Schottky diodes, and undoped and doped field-effect transistors.

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bias, in accumulation, slightly increases due to a shift in threshold voltage. The threshold voltage, defined as the onset of the channel current at flatband [29], shifts to positive values upon doping. Each transfer curve in Figure 3 is corrected for its threshold voltage indicating that the accumulation currents are identical within experimental error. The corrected transfer curves show a cross-over from an accumulation mode into a bulk depletion mode transistor [22, 30]. The current in accumulation is dominated by the channel current, while in depletion, at positive gate bias, the current is mainly flowing through the bulk semiconductor. The doping density can be calculated from the pinch-off voltage [22, 30] and the mobility can be calculated from the current at flat band conditions at zero gate bias normalized for the shift in threshold voltage [22, 30]. The extracted mobilities and carrier densities are presented in Figure 4 as well. The extracted mobility and charge carrier density values from all devices investigated are presented in Figure 4. The gap between the undoped diodes and undoped transistors is probed with the doped diodes and the doped transistors Moreover Figure 4 shows that the hole mobility is flattening for charge carrier densities below 1016 cm 3 and increases with a power law for higher charge densities. The power law dependence is due to hopping transport in disordered semiconductors [8, 21]. Slight deviations from the power law at intermediate density might be due to anisotropy in the charge transport caused by the nanocrystalline nature of rr-P3HT.

6.3 Anomalous behavior In some cases, however, we find that the dependence of mobility on charge density deviates from the power law trend expected from the Vissenberg and Matters and Tanase et al. (VMT) transport model [8, 21]. Figure 5 shows the charge carrier mobility versus carrier density derived from undoped hole-only diodes, undoped and doped field-effect transistors for rr-P3HT (Rieke Chemicals) and poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene viny-lene) (MEH-PPV, Merck). MEH-PPV, shows that the trend according to the VMT model of charge carrier mobility versus charge density observed above still holds for other materials other than the polythiophene studied above. On the other hand the data from rrP3HT (Rieke) shows a clear anomaly from the trend expected from the VMT model. Although the mobility versus charge carrier density relationship between 81


the two different batches of rr-P3HT are quite similar for undoped diodes and OFETs, there is a clear difference in the relationship obtained from doped OFETs in the gap between low and high charge carrier densities. The mobility increases sharply, with a power law relationship higher than the one expected from the VMT model. Such anomaly has been observed before for other batches of rr-P3HT and for poly(2,5-thienylene vinylene) (PTV) [30]. The reason for such anomaly can be threefold. First the anomaly could rise from a difference in morphology. Different batches of rr-P3HT, might have been synthesized in different conditions or by different synthetic route. Moreover, even if the synthetic process for two different batches of semiconducting polymers is kept constant, some small differences in the end products might still arise. Such differences might cause a change in the overall regioregularity and average chain length of the polymers, hence in the overall morphology of spin-coated films [31]. Moreover, in a solid thin film of a single material there may be a significant difference in morphology of the polymer between the semiconductor/insulator interface region where field-effect conduction in OFETs occurs and the bulk of the material, where bulk conduction occurs in diodes and doped OFETs. Since the hopping rate of charge carriers, which determines the mobility and conduction in semiconducting polymers, strongly depends on the morphology [32], the differences in the morphology will strongly influence conduction. Hence the sharp increase in mobility with charge carrier density due to chemical doping, might be the result of different morphologies between different materials or within the spincoated thin films, as it was previously explained for similar behavior in rrP3HT and PTV [30, 33]. Therefore, the study of the interface and bulk morphology of disordered organic semiconductors is of great importance in OFETs, especially if the bulk conduction becomes relevant such as in the case of doped transistors.

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Figure 5. Charge carrier mobility versus carrier density as derived for rr-P3HT (Rieke) and MEHPPV from undoped (Und.) hole-only diodes, and undoped and doped (Dop.) field-effect transistors.

A second explanation is that differences in conduction might also arise from anisotropy effect due to the nano-crystalline nature of rr-P3HT and other regioregular semiconducting polymers. The anisotropy arises not only from the regioregular or self-organizing nature of polymer materials, but it is also strongly influenced by the solvent, concentration and spincoating conditions [34, 35]. The anisotropy will cause the hopping transport in the material to differ in different directions of conduction. Diodes conduct charges in the perpendicular direction respect to the plane of the spincoated film, while in OFETs there is conduction parallel to the plane. It has already been observed that the packing of rr-P3HT polymer chains is more favorable for the charge transport in the directions parallel rather than perpendicular to the plane of the film, leading to an increased difference in charge carrier mobility between rr-P3HT diodes and OFETs [36]. Hence the stronger the anisotropic effects in a polymer film the stronger the deviation will be from the trend expected from the VMT model. Although anisotropy alone cannot explain the sharp increase in the charge carrier mobility for the doped OFET in Figure 5, it might be the cause of the difference between the hole-only diodes measurements between the two different rr-P3HT batches shown in Figures 4 and 5. A third possible explanation for the deviation might be the presence of a nonnegligible amount of traps in the material. Defects in the polymer chains, chemical impurities and disorder can form trap energy states. If a charge carrier hops unto a 83


trap state it will become trapped, i.e. immobile, impeding conduction. The release of trapped charge carriers depends on how energetically deep the trap is. The effect of traps is more pronounced at a low carrier density regime as in diodes. In the high charge carrier density regime, as in OFETs, most traps become filled and conduction is not greatly affected. If chemical dopands are introduced into the semiconducting polymer, the extra charge carriers formed by the doping process will fill the traps present in the material, greatly improving the charge carrier mobility at lower charge carrier densities [37], which might explain the sharp increase of the mobility observed in Figure 5.

6.4 Conclusion In summary, we have experimentally probed the charge carrier mobility as a function of carrier density for rr-P3HT over a wide density range. The mobility at low 1015 - 1016 cm–3 and high 1018 - 1020 cm–3 carrier density was extracted from undoped hole-only diodes and field-effect transistors, respectively. The mobility at intermediate density has been probed by chemically doped Schottky diodes and transistors. We demonstrate that the room temperature mobility is nearly constant at densities below 1016 cm–3, whereas the mobility follows a power law for densities higher than 1018 cm-3. The mobility at intermediate densities has been probed by chemically doped Schottky diodes and transistors and unites the lowand high density regimes. We also observe that in some materials there can be a deviation from the power law, which can originate either from morphology differences between materials or in the spincoated film, anisotropy effect or significant presence of traps in the material.

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N. I. Craciun, J. J. Brondijk, and P. W. M. Blom, Phys. Rev. B 77, 035206 (2008).

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List of Publications

1. J. Broos, F. Maddalena and B. H. Hesp, “In vivo synthesized proteins with monoexponential fluorescence decay kinetics”, J. Am. Chem. Soc. 126, 2223 (2004).

2. F. Maddalena, M. Spijkman, J. J. Brondijk, P. Fonteijn, F. Brouwer, J. C. Hummelen, D. M. de Leeuw, P. W. M. Blom and B. de Boer, “Device characteristics of polymer dual-gate field-effect transistors”, Org. Electron. 9, 839-846 (2008).

3. F. Maddalena, E. J. Meijer, K. Asadi, D. M. de Leeuw and P. W. M. Blom, “Doping kinetics of organic semiconductors investigated by field-effect transistors”, Appl. Phys. Lett. 97, 043302 (2010)

4. F. Maddalena, M. J. Kuiper, B. Poolman, F. Brouwer, J. C. Hummelen, D. M. de Leeuw, B. de Boer and P. W. M. Blom, “Organic field-effect transistor-based biosensors functionalized with protein receptors”, J. Appl. Phys. 108, 124501 (2010).

5. J. J. Brondijk, F. Maddalena, K. Asadi, H. J. van Leijen, M. Heeney, P. W. M. Blom and D. M. de Leeuw, “Carrier-density dependence of the hole mobility in doped and undoped regioregular poly(3-hexylthiophene)”, Physi. Status Solidi B, accepted (2011).

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88

Summary The discovery that organic polymers have the ability to function as conductors and semiconductors has opened a new field of possibilities in electronics. Although organic electronics will probably never be able to compete with inorganic semiconductor electronics in terms of charge carrier mobility, operation speed and miniaturization, it can provide cheap, easy processable, flexible devices for lowend applications in electronics, such as white light illumination, flexible displays, solar cells, RFID-tags and sensors. Semiconducting polymers are a special group of plastics, which differentiates itself from ‘common’ non-conducting plastics by the fact that they possess a conjugated backbone, i.e. alternating single and double bonds. The conjugation allows electrons and holes in the π-molecular orbitals to be delocalized along the conjugated segments of the molecule. However, since the conjugation is often broken by defects and twisting of the polymer chain, the charge carriers are localized within the conjugated segments. From this disorder in conjugation length, the HOMO and LUMO levels in organic semiconductors are not well defined but they have a Gaussian spread. The main transport mechanism in semiconducting polymers is dominated by hopping transport, which is strongly dependent on the energetic and structural disorder in the polymer. Sensors, devices which have a response to a physical entity and convert the response into an analyzable signal, are becoming steadily one of the most important and investigated applications in organic electronics. Of special interest is the use of organic field-effect transistors (OFETs) as sensors or as core component thereof. A particular type of sensor is the biosensor, a sensor that contains a biological molecule or even cell as part of the detection mechanism. The use of biological molecules or cells as detection unit has the great advantage of enhancing the selectivity of the sensor, since biological molecules have so evolved to react very specifically only with few selected compounds. This thesis focuses on the study of organic field effect transistors and their usefulness for (bio)sensing applications. The research is more focused toward fundamental understanding of the use of the OFET as a sensor or biosensor rather than realization of the sensors itself, although a prototype biosensor based upon dual-gate structure has been shown. In Chapter 2 the materials used throughout this Thesis have been discussed and the design of the devices and the experimental procedures for fabrication and characterization were described. 89

SUMMARY

In Chapter 3 the device operation of organic dual-gate field-effect transistors (DG-OFET), fabricated by solution processing, is characterized. It is shown that by sweeping the bottom gate bias of the devices and keeping the top gate bias constant, the threshold voltage of the bottom gate of the devices depends on the top gate bias with two linear relationships for two different regimes, assuming that the mobilities of the top and bottom channels have similar magnitudes, and that the capacitance of the semiconductor layer is not negligible. The first regime is observed when one gate is positive, hence the respective channel is in depletion and no gate screening will occur, while the other gate is in accumulation regime, the field of the positive gate will penetrate to such a large extent that the channel in accumulation will be affected by both gates. The second regime is observed when both channels are in accumulation, the charges present in the channels will screen the respective gate potentials, hence both channels will operate individually and no mutual influences are observed. Moreover, it is observed that a direct effect of the shielding of the gate bias by the accumulated charge carriers at the interface is clearly marked by a drop in the transconductance of the dual-gate transistors between the two regimes. The DG-OFET also stands as theoretical model for the operation of the biosensor described in Chapter 4, the Bio-FET. The Bio-FET is an organic FET transducer with a DG-OFET structure where an integrated sulphate-binding protein (SBP) layer replaces the top gate electrode. SBP was genetically engineered to contain a thiol group for surface-anchoring without affecting its binding activity. The modified receptor was covalently coupled to a maleimide-functionalized polystyrene layer spincoated on top of the transistor’s surface. Fluorescence spectroscopy and tapping-mode atomic force microscopy were used to confirm the covalent coupling of SBP to the surface of the functionalized polystyrene layer. Measurement of the dissociation constant of SBP after drying and rehydration showed that the protein remains active even after being dried, making it suitable and versatile for sensing purposes. The binding of sulphate ions in dry conditions was detected by a shift in the threshold voltage. Combined with surface density measurements by AFM, an effective charge of –1.7q per protein was found, as expected from the Bio-FET operation model, based on the operation characteristics of the DG-OFET. Organic field-effect transistors can be also used directly as sensor devices. The most straightforward sensing mechanism is chemical doping of the organic semiconductor itself in the OFET. To build a proper sensor, however one must understand the process and kinetics of doping. In Chapter 5 the kinetics of 90

acid doping of regio-regular poly-3-hexylthiophene in field-effect transistors were investigated. The dopand used was a vaporized perfluorinated chloroalkylsilane. The dopant density has been derived from the shift of the pinch-off voltage, as a function of temperature and exposure time. The change in dopand density can phenomenologically be described by stretched exponential time dependence, with a saturation dopant density of 1 ± 0.5 ×1026 m–3 and a thermally activated relaxation time with an energy barrier of 0.6 eV, which agrees with reported values for protonation reactions of organic molecules. It was shown that a similar relationship holds for previously reported kinetics of poly-thienylene-vinylene doped with molecular oxygen. The good agreements obtained indicate that the doping kinetics of disordered organic semiconductors follows indeed a stretched exponential time dependence. An important parameter in organic electronics, as in electronics in general, is the charge carrier mobility in semiconductors. For the response in sensors based upon organic semiconductors it is very important to understand the relationship between the charge carrier density in the device and the mobility of the charge carriers. In Chapter 6 the charge carrier mobility dependence on the charge carrier density in semiconducting polymers was investigated. Here the mobility of the organic semiconductors was investigated over a carrier densities range from 1015 cm–3 to 1020 cm–3 in order to experimentally establish the relation between mobility and carrier density. The mobility at low (1015 -1016 cm–3) and high (1018 - 1020 cm–3) carrier density was extracted from undoped hole-only diodes and field-effect transistors, respectively. Intermediate densities were probed using chemically doped Schottky diodes and transistors. It was shown that the mobility is almost constant for carrier densities below 1016 cm–3 and follows a power law dependence for carrier densities higher than 1018 cm–3. In some materials however an anomaly in the intermediate range was observed, characterized by a sharp increase in mobility with the charge carrier density. The anomaly is probably caused by a significant presence of trapping levels, caused either by impurities or disorder, which are filled by the extra charge carriers introduced by doping, resulting in the strong increase of the mobility of the material. Finally it is important to note that organic electronics, and in particular organic field-effect transistors, is a promising technology for low-cost sensing applications without necessarily sacrificing the selectivity and sensitivity of the sensing devices. The use of OFETs as sensors can be both direct, as through the chemical doping mechanism, or can be integrated with biological molecules, to give rise to biosensors, which would combine the selectivity of the biological molecules with the low-cost fabrication and flexibility of semiconducting polymers. 91

SUMMARY

92

Samenvatting De ontdekking dat organische polymeren het vermogen hebben om te functioneren als geleiders en halfgeleiders, heeft een nieuw veld van mogelijkheden in de elektronica geopend. Alhoewel de organische elektronica waarschijnlijk nooit in staat zal zijn te concurreren met de elektronica van anorganische halfgeleider in termen van ladingsdragers mobiliteit, snelheid en miniaturisatie, kan wel organische elektronica goedkope, makkelijk te verwerken, flexibele apparaten bieden voor ‘low-end’ toepassingen, zoals wit licht verlichting, flexibele displays, zonnecellen, RFID-tags en sensoren. Halfgeleidende polymeren zijn een speciale groep van kunststoffen, die zich onderscheiden van 'gewone' niet-geleidende kunststoffen door het feit dat ze een geconjugeerd structuur bezitten, dat wil zeggen afwisselend enkele en dubbele bindingen. De conjugatie maakt het mogelijk dat de elektronen en de gaten in de π-moleculaire orbitalen worden gedelokaliseerd langs de geconjugeerde segmenten van het molecuul. Echter, de conjugatie wordt vaak gebroken door gebreken en/of door het draaien van de polymeerketen, dus zijn de ladingsdragers gelokaliseerd binnen de geconjugeerde segmenten. Door deze verspreiding in conjugatielengte, zijn de HOMO en LUMO energieniveaus in organische halfgeleiders niet goed gedefinieerd, maar ze hebben een Gaussische spreiding. Het belangrijkste transportmechanisme in halfgeleidende polymeren wordt dan gedomineerd door ‘hopping transport’, dat sterk afhankelijk is van de energetische en de structurele wanorde in het polymeer. Sensoren, apparaten die een reactie op een fysieke entiteit hebben en in een analyseerbaar signaal omzetten, zijn tot een van de belangrijkste en meest onderzochte toepassingen in organische elektronica geworden. Van bijzonder belang is het gebruik van organische veldeffect transistoren (OFETs) als sensoren of hoofdcomponent daarvan. Een bepaald type sensor is de biosensor, een sensor die een biologisch molecuul of zelfs een cel als onderdeel van het detectiemechanisme bevat. Het gebruik van biologische moleculen of cellen als detectie-eenheid heeft het grote voordeel dat het de selectiviteit van de sensor verhoogt, omdat de biologische moleculen zo geëvolueerd zijn om zeer specifiek te reageren met slechts weinig geselecteerde verbindingen. In dit proefschrift worden organische veldeffect transistors en hun nut voor mogelijke toepassingen als (bio-)sensoren onderzocht. Het onderzoek is meer gericht op het fundamenteel begrip van het gebruik van de OFET als sensor of 93

SAMENVATTING

biosensor, in plaats van de realisatie van de sensoren zelf. Alhoewel een prototype biosensor gebaseerd op dual-gate structuur wel wordt gepresenteerd. In hoofdstuk 2 worden de materialen die gebruikt zijn beschreven, het ontwerp van de apparaten en de experimentele procedures voor de fabricage en de karakterisering daarvan. In hoofdstuk 3 wordt de werking van de organische dual-gate veldeffect transistor (DG-OFET) gekarakteriseerd. Door het veranderen van de onderste gatepotentiaal en het constant houden van de bovenste gate-potentiaal van DG-OFET, wordt aangetoond dat de drempelspanning van de onderste gate-elektrode van de DG-OFET van de bovenste gate-potentiaal afhangt met twee lineaire relaties voor twee verschillende regimes. Aangenomen wordt dat de mobiliteiten van de bovenste en onderste geleidende kanalen ongeveer even groot zijn en dat de capaciteit van de halfgeleiderlaag niet te verwaarlozen is. De eerste lineaire afhankelijkheid is waargenomen wanneer een gate-potentiaal positief is, omdat het desbetreffende kanaal in uitputtingsregime is en dus geen afscherming van de potentiaal door de lading zal plaatsvinden, terwijl de andere gate-elektrode in accumulatie regime is. Het tweede regime is waargenomen wanneer beide kanalen in accumulatie zijn. De ladingdragers die in de kanalen aanwezig zijn, zullen de beide gate-potentialen afschermen en dus zullen de kanalen onafhankelijk van elkaar werken. Bovendien wordt een direct effect gezien van de afscherming van de gate-potentiaal door de geaccumuleerde ladingsdragers. Dat wordt gekenmerkt door een daling in de transconductantie van de dual-gate transistor tussen de twee regimes. De DG-OFET staat ook als theoretische model voor de werking van de biosensor beschreven in hoofdstuk 4: de Bio-FET. De Bio-FET is een OFET transducer met een DG-OFET structuur waar een geïntegreerde sulfaat-bindend eiwit (SBP) laag vervangt de top gate-elektrode. SBP werd genetisch gemanipuleerd om een thiolgroep voor oppervlakte-verankering te bevatten, zonder dat dit zijn bindende activiteit beinvloedt. De gemuteerde receptor werd covalent gebonden aan een maleïmide-gefunctionaliseerde polystyreen laag gespincoat op de top van het oppervlak van de transistor. Fluorescentiespectroscopie en ‘tappingmode’ atoomkrachtmicroscopie (AFM) werden gebruikt om de covalente koppeling van SBP aan het oppervlak van de gefunctionaliseerde polystyreen laag te bevestigen. Meting van de dissociatie constante van SBP na het drogen en rehydratatie toonde aan dat het eiwit actief blijft, zelfs na het drogen, waardoor het geschikt is als detectie eenheid voor de Bio-FET. De binding van sulfaationen in droge omstandigheden werd gedetecteerd door een verschuiving in de 94

drempelspanning. In combinatie met oppervlaktedichtheid metingen door AFM, een daadwerkelijke lading van -1.7q per eiwit werd gevonden, zoals verwacht uit de Bio-FET werking model, gebaseerd op de werking kenmerken van de DGOFET. Organische veldeffect transistoren kunnen ook rechtstreeks gebruikt worden als sensoren. Het meest eenvoudige sensor mechanisme is de chemische doping van de organische halfgeleider in de OFET. Voor het bouwen van een goede sensor moet men goed het proces en de kinetiek van doping begrijpen. In hoofdstuk 5 werd de kinetiek van doping van de regio-reguliere poly-3-hexylthiophene door zuur in veld-effect transistors onderzocht. De gebruikte dopand was een verdampte geperfluoreerde chloroalkylsilaan. De doteringsdichtheid is afgeleid van de verschuiving van de ‘pinch-off’ spanning, als een functie van de temperatuur en de doteringstijd. De verandering in doteringsdichtheid kan feno-menologisch worden beschreven door een uitgerekt exponentiële tijdsafhankelijkheid, met een verzadiging van de doteringsdichtheid van 1 ± 0.5 x 1026 m-3 en een thermisch geactiveerde relaxatietijd met een energiebarriere van 0.6 eV, die met gerapporteerde waarden voor de protoneringreacties van organische moleculen overeenkomt. Het wordt geobserveerd dat een soortgelijke relatie geldt voor eerder gemelde kinetiek van polythienyleen vinyleen gedoteerd met moleculaire zuurstof. De verkregen goede overeenkomsten geven aan dat de doping kinetiek van wanordelijke organische halfgeleiders inderdaad volgt een uitgestrekt exponentiële tijdsafhankelijkheid. Een belangrijke parameter in organische elektronica, zoals in de elektronica in het algemeen, is de mobiliteit van ladingsdragers in halfgeleiders. Voor de reactie van de sensoren op basis van organische halfgeleiders is het heel belangrijk om te begrijpen wat de relatie tussen de ladingsdichtheid in het apparaat en de mobiliteit van de ladingsdragers is. In hoofdstuk 6 wordt de ladingsdragers mobiliteit afhankelijkheid van de ladingsdichtheid in halfgeleidende polymeren onderzocht. Hier werd de mobiliteit van de organische halfgeleiders op een drager dichtheden variëren van 1015-1020 cm-3 onderzocht, om experimenteel vast te stellen wat de relatie is tussen mobiliteit en dragerdichtheid. De mobiliteit bij lage waarden (1015 1016 cm-3) en hoge waarden (1018-1020 cm-3) van de ladingsdragersdichtheid wordt berekend uit respectievelijk ongedoopt hole-only diodes en veldeffect transistors. Tussenliggende dichtheden werden bestudeerd met behulp van chemisch gedoteerde Schottky diodes en transistors. Er wordt aangetoond dat de mobiliteit bijna constant is voor carrier dichtheden onder 1016 cm-3 en volgt een machtsverband voor carrier dichtheden hoger dan 1018 cm-3. In sommige materialen wordt er echter een anomalie in het tussenliggende bereik geobserveerd, 95

SAMENVATTING

gekenmerkt door een sterke toename van de mobiliteit met de ladingsdichtheid. De anomalie wordt waarschijnlijk veroorzaakt door een significante aanwezigheid van ‘trapping’ energie niveaus, veroorzaakt door onzuiverheden of wanorde, die gevuld worden door de extra ladingsdragers geïntroduceerd door doping, wat resulteert in de sterke toename van de mobiliteit van het materiaal. Het is belangrijk om op te merken dat organische elektronica, en in het bijzonder organische veldeffect transistors, een veelbelovende technologie is voor sensortoepassingen met lage kosten zonder dat het noodzakelijkerwijs ten koste gaat van de selectiviteit en gevoeligheid van de sensoren. Het gebruik van OFETs als sensoren kan zowel direct, door chemische doping mechanisme, of door ze te integreren met biologische moleculen. De selectiviteit van de biologische moleculen zou dan gecombineerd zijn met de low-cost fabricage en flexibiliteit van halfgeleidende polymeren.

96

Acknowledgements

The work described in this thesis, would not have been possible without the aid and support of many people, who all deserve my thanks. First of all I want to thank my promotor, Dago de Leeuw, who took over the supervision of my project after Bert de Boer past away. One could not have had asked for a better supervisor. His guidance was crucial to the realization of this thesis. Dago, thank you for your patience and for encouraging and pushing me these last few years. I wish to thank my other promotor, Paul Blom, for giving me the great opportunity to pursue a Ph.D. in his group and for all the help and advice he gave me during the course of my Ph.D. I thank the reading committee, Kees Hummelen, Bert Poolman and Maria Antonietta Loi for their valuable suggestions and comments on my thesis. Kees and Bert also deserve special thanks for the help and guidance they provided, especially during the Bio-FET meetings. Maria you also have my gratitude for your counsel and encouragements and pointing the way towards my new job. I must add that I feel honoured to have worked with all these people, who have always been kind, friendly and helpful, but also are scientists of outstanding quality who made and still make significant contributions to science. Although he passed away before the completion of this thesis, Bert de Boer also deserves my special thanks. He was the one who convinced me to do my master and Ph.D theses in the MEPOS group. He was a great supervisor and a great scientist and his untimely death left a void in all of his PhD. Students and the whole MEPOS group. Naturally one cannot forget to give thanks and acknowledgements to the technicians of the Physics of Organic Semiconductors laboratory: Minte Mulder, Jan Harkema and Frans van der Horst. Their technical assistance was essential to

97

ACKNOWLEDGEMENTS

the practical work that is presented in this thesis and for always keeping the laboratories and the clean room in top shape. I must also thank Renate as well for all her help during the years, especially in finding the way in the labythints of bureaucracy. Frank Brouwer and Marjon Kuiper you also deserve my thanks for your help and collaboration. Frank, thank you for providing me with many of the polymers that are featured in this thesis, you have done outstanding work. Marjon, thank you for providing me with the protein and for the biochemical work you did. You have done excellent work as well. A special thanks to Edward Meijer for his aid on the work of doped semiconductors. Also your thesis has been a very valuable source of information. Irina, you have been a good friend and we had good times together. We shared the office (and gossips and jokes) for more than 5 years I could not have asked for a better ‘office mate’. Thank you for the suggestion you gave me on making LEDs. Ilias you also have been a great friend and a companion of both laughter and serious business during these last years and, of course, also a fellow AVGN aficionado. I still wear the AVGN t-shirt you gave me, which seems to confuse Italian people a lot. Kamal, of course I cannot forget you either. You have been a good colleague and friend and you always been helpful. Thank you for everything. Johan, thank you as well for your help and your collaboration during the past years. Jan Kotlarski, you have been a great friend as well. I will miss watching absurd anime with you. ありがとう！ I want to thank also the rest of the MEPOS group for the great time I had during my master thesis and my Ph.D.: Ronald, Teunis, Cristina (your thesis has also been a very useful source of information to me), Afshin, Jur, Hylke, Auke, Claudia, Zheng, Yuan (“it’s too small!”), Jia (Jia-Jia), Mark-Jan (thank you for your help as well), Edsger, Dennis, Valy, Lacra, Paul be B., Gert-Jan, Date, JanAnton, Fatemeh, Krisztina, Dorota, Marianna, Simone, Oleksandr, Reza, Herman, Martij K., Martin L., Milo, Anne-Marije, Xiao-Nan, Hennie and Jolt. I wish to thank my dear good friends: Magda (thanks for agreeing to be my paranimph!), Marta, Markella, Jamila, Mark & Kitty, Daphne, Aliya, Jeroen & Martin, Hilke, Michael, Agnieszka, Małgorzata (Gosha), Marianne, Rachel, Hannah, Fr. Wagenaar, Roberto, Mervil & Minne for the help on the Dutch ‘Samenvatting’, Luciano & Rosellina, Sylvain & Belen, Zheng, Jarek, Noreen and little Justin. 98

I must also thank my family, especially my mother Vania, for all their moral support, which was very important in the past few years, especially in patches of very rough times. Last but not least, I wish to thank Guglielmo Lanzani, my new boss, for letting me be part of the IIT family and for agreeing to be part of the ‘opposition’ during my Ph.D. defence.

Francesco Maddalena November 2011

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Organic Field-Effect Transistors for Sensing Applications - CiteSeerX

Organic Field-Effect Transistors for Sensing Applications - CiteSeerX

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