Towards Electrolyte-Gated Organic Light-Emitting Transistors: Advances and Challenges Jonathan Sayago,1 Sareh Bayatpour, 1 Fabio Cicoira2,* and Clara Santato1,* 1
Département de génie physique, École Polytechnique de Montréal
2500 Chemin de Polytechnique, H3T 1J7, Montréal, Québec, Canada 2
Département de génie chimique, École Polytechnique de Montréal
2500 Chemin de Polytechnique, H3T 1J7, Montréal, Québec, Canada *
[email protected],
[email protected]
1
Introduction 1.
Electrolyte-gated organic transistors
2. Electrolytes employed in electrolyte-gated organic transistors 3. Preliminary results and challenges in electrolyte-gated organic light-emitting transistors
2
Introduction The light-emitting properties of organic semiconductors, together with their large-area processability and mechanical flexibility, are among the most attractive characteristics of this class of materials [1– 6]. Considering that light-emission, e.g. the color of the emitted light, can be modified by changing their molecular structure, organic semiconductors are interesting for technological applications such as flexible displays [7], solid-state lighting [8,9], organic lasers [10–12], and organic electroluminescent sensors [13]. Organic light-emitting diodes (OLEDs) have already entered the market as components for flat-panel displays and light sources [14]. Other light-emitting devices have been demonstrated, such as lightemitting electrochemical eells (LEECs) [15–26] and organic light-emitting transistors (OLETs) [27– 40]. Typically, LEECs are based on thin films of light-emitting polymers blended with an electrolyte, sandwiched between two electrodes. Upon application of an electrical bias, ions redistribute within the light-emitting film facilitating charge carrier injection. Indeed, charge injection in LEECs can be independent of the workfunction of the electrodes, contrary to what is commonly observed in OLEDs, where low workfunction metal electrodes, such as Ca, are commonly used to facilitate injection of electrons in the organic semiconductor. The detailed working principle of LEECs, making use of different light-emitting materials and different device structures, e.g. vertical versus planar, has been the object of an interesting scientific debate [41,42] and it is still under investigation. OLETs are optoelectronic devices that couple the light-emitting function of OLEDs with the switching and amplifying functions of organic transistors [43–45]. Electrons and holes injected from the drain and the source electrodes, upon application of a suitable gate bias, form excitons in the transistor channel, whose radiative recombination generates light. The fundamentals of OLETs together with recent, exciting developments in the field are discussed by J. Zaumseil et al. elsewhere in this book. Despite the impressive progress experienced in the field of OLETs, their practical application requires improvements in their performance, in terms of operating voltage and electroluminescence efficiency. This chapter focuses on one strategy to achieve high performance OLETs, namely the coupling of light-emitting organic semiconductors and ionic species [46]. The experimental 3
configuration considered is the electrolyte-gated (EG) organic transistor, which is introduced in the first part of the chapter. The second part of the chapter deals with a variety of electrolytes that can be used as gating media in EG organic transistors. In the last section, key results and challenges of electrolyte-gated organic light-emitting transistors (EG-OLETs) are discussed.
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1. Electrolyte-Gated Organic Transistors The principle of electrolyte-gating has been known since almost sixty years, having been used in the early works of Shockley, Bardeen and Brattain [47,48]. Later on, Wrighton and coworkers deeply investigated microelectrochemical transistors, based on this principle [49–53]. Electrolyte-gated organic transistors consist of source and drain electrodes and a channel containing the organic active material (an organic semiconductor or an organic conducting polymer) in contact with a gate electrode via an electrolyte (Fig. 1a-b). The electrolyte replaces the gate dielectric (e.g. SiO2) used in more conventional transistor structures [54–56].
a)
-
Drain
-
b)
Gate Electrolyte
PDMS Well
Organic channel
Source
Substrate Fig. 1: Electrolyte-gated organic transistor: a) schematic device structure and b) image (top view) of a device where are shown the transistor channel, the square-shaped source and drain electrodes, and the electrolyte, confined by a polydimethylsiloxane (PDMS) well. The region of the organic semiconductor delimited by the source (S) and the drain (D) electrodes defines the transistor channel, whose geometry is characterized by the interelectrode distance (channel length, L) and the electrode width (channel width, W). Upon application of an appropriate gate–source bias (Vgs), charge carriers are injected from the S and D electrodes into the transistor channel, where they move under the action of a drain–source bias (Vds). The current flowing between S and D (Ids) is modulated by Vgs. In electrolyte-gated organic transistors, the application of a Vgs induces the formation of an electrical double layer (EDL) at the electrolyte/organic semiconductor interface [57]. To illustrate the concept of EDL, we refer to electrode/electrolyte interfaces in electrochemical cells. When an electrical bias is applied between two electrodes immersed into an electrolyte, electrolyte ions move toward electrodes of opposite charge, driven by the electric field. Eventually, ions form a charged layer at 5
the electrode, known as the Helmholtz plane. The Helmholtz plane and the electrode surface form two parallel oppositely charged layers, the EDL. If we replace one of the two electrodes with an organic semiconducting channel connected to source and drain electrodes, we can obtain an organic field-effect transistor, if the EDL, at the electrolyte/organic semiconductor interface, induces an electrostatic doping in the organic semiconductor (Fig. 2a) [58, 59]. It is worth to point out that, in organic field effect transistors, the organic semiconducting channel is not permeable to the ions of the electrolyte. Because of their nanoscale thickness (of the order of a few nm), EDLs are able to accumulate a high density of charge carriers upon application of a relatively low gate bias. Electrolyte-gating is indeed used to fabricate transistors operating at low electrical biases (~1 V). The capacitances per unit areas of EDLs are in the order of 10 μF cm-2 [57], whereas the typical capacitance of a 200 nm-thick SiO2 dielectric is in the order of tenths of nF cm-2. The EDL approach, with electrolytes such as LiClO4 solutions or ionic liquids, has allowed the fabrication of low-voltage organic transistors using a wide range of semiconductor materials, such as organic single crystals of pentacene and rubrene [60,61], polymer films of poly(3-hexylthiophene) (P3HT) [56]. Electrolyte-gating is a promising alternative to other approaches such as high-k dielectrics [62] or ultrathin gate dielectrics [63] to reduce the operation voltage of transistors. A different scenario is obtained if the transistor channel material is electrochemically active, permeable to ions [64–67]. Here, the application of a gate bias induces a redistribution of ions within the transistor channel and the electrolyte (Fig. 2b) that, together with charge injection from source and drain, results in the electrochemical doping/dedoping of the channel. This mechanism governs devices known as organic electrochemical transistors. It is important to note that the two above doping mechanisms represent two models to describe electrolyte-gated transistors. Actually, the mechanism of operation of electrolyte-gated transistors, might involve both electrostatic and electrochemical doping, simultaneously.
6
a)
b) Electrostatic doping
Electrochemical doping Gate electrode
Gate electrode
- - - - - -
+
+
+ -
D
+
+
+
+
-
+
-
+
-
- - - - - -
+
+
-
-
+
+
+
+
S
D
+
+
+
-
-
- - - - - -
+++++++ Semiconductor
+
-
Substrate
+
-
+
+
-
-
+
Cation
-
Anion
+ Hole
-
+
-
+
+
+
Electron
-
+
S
Substrate
Fig. 2: a) Electrostatic and b) electrochemical doping mechanism governing the operation of an electrolyte-gated transistor (example for a p-type). Upon application of a gate electrical bias, ion incorporation in the transistor channel material only takes place in the electrochemical mechanism. 2. Electrolytes employed in electrolyte-gated organic transistors An electrolyte has to satisfy different criteria to be used in electrolyte-gated organic transistors. It has to be chemically stable when in contact with the organic semiconductor and it must have an electrochemical stability window compatible with the electrical biases applied to the electrodes of the transistor. The ionic conductivity of the electrolyte to be considered too, in particular to determine the response time of the transistor. Several classes of electrolytes have been employed to gate organic transistors. Among them, electrolytic solutions, ionic liquids, ion gels, polyelectrolytes and polymer electrolytes (Fig. 3).
Fig. 3: Schematic structures of different types of electrolytes used in electrolyte-gated organic transistors, according to their physical phase [68]. Electrolytic solutions are obtained by dissolving a salt in a polar solvent (e.g. water, polyethylene oxide). The nature of the solvent affects the characteristics of the electrolytic solution in terms of 7
density, viscosity, permittivity, and thermal stability. The dissociation of the salt results in the formation of cations and anions that, as a function of their mobility, participate more or less effectively in the ionic conduction [69]. Electrolytic solutions have been widely used in electrolyte-gated organic transistors. Commonly used salts are LiClO4 [70] and LiCF3SO3 [71]. Water has been clearly the solvent of choice for applications in bioelectronics [55]. Ionic liquids (ILs), i.e. molten salts at room temperature, are substances containing only ions and whose melting point is below 100 °C [72]. The versatility, in terms of possible technological and practical applications, of ILs is due to the fact that it is possible to tailor their molecular structures by chemical synthesis. This gave rise to the term: task specific ILs [73]. Ionic liquids can show high thermal stability [74], non-flammability [75] and non-toxicity [76] that make them attractive candidates as electrolyte-gating media. Commonly used ILs are quaternary ammonium salts or cyclic amines that can be aromatic (e.g. pyridium, imidazolium) or saturated (e.g. piperidinium, pyrrolidinium) [73]. The cations of ILs employed in electrolyte-gated organic transistors are generally bulky and asymmetric, with more than one heteroatom (Fig. 4) [77]. ILs based on dialkylimidazolium cations have gained popularity for their stability in water and oxygen and their non-toxic characteristics [78]. The anions of ILs are basically weakly coordinating compounds such as PF6-, BF4-, SbF6- and bis(trifluoromethanesulfonyl)imide ([TFSI-]). Therefore the resulting ILs is a highly polar but noncoordinating solvent [78]. ILs based on [TFSI-] have a limited degree of ion interaction and higher ionic conductivity [79]. The hydrophobic properties of [TFSI-] facilitate its drying process, for example [EMIM][TFSI] can reach a water content of less than 20 ppm, after vacuum drying (10 Torr) at 100 °C [72].
8
Fig. 4: Molecular structures of cations commonly used in ionic liquids [77]. The ionic conductivity, at room temperature, of ILs used as electrolytes ranges between 0.1-10 mS cm-1, which is lower than conventional aqueous electrolyte solution used in electrochemistry (a few mS cm-1), but might be comparable to the conductivity of lithium salt-based organic electrolytes; for instance LiPF6 (1 mol/dm3) in a mixture of ethylene carbonate with l,2-dimethoxyethane has a conductivity of ~15 mS cm-1 [73]. Relatively high conductive ILs can be obtained using [EMIM] as the cation, while relatively low conductive ILs make use of tetraalkylammonium, piperidinium, pyrrolidinium or pyridinium cations [73]. The conductivity of classic electrolytes is proportional to the number of charge carriers and inversely proportional to the medium viscosity [80]. In ionic liquids, complex ion-ion interactions may result in ionic stable aggregates that may be regarded as charge neutral and do not participate in ionic conduction [81]. The viscosity of an IL is related to a combination of electrostatic, van der Waals interactions, hydrogen bonding and ion size and polarizability [82]. Table 1 shows some fundamental physicochemical properties and the electrochemical stability windows of ILs of interest for electrolyte-gated transistors such as: [EMIM][TFSI] (1-ethyl-3methylimidazolium
bis(trifluoromethylsulfonyl)imide),
propylpiperidinium
bis(trifluoromethylsulfonyl)imide),
methylimidazolium
bis(trifluoromethylsulfonyl)imide),
[PMPip][TFSI] [BMIM][TFSI] [BMIM][PF6]
(1-methyl-1(1-butyl-3(1-butyl-3-
methylimidazolium hexafluorophosphate) and [BMPyrr][TFSI] (1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide). The physical properties of a number of ILs are available in the literature [73, 75, 83]. 9
Table 1: Physicochemical properties and electrochemical stability windows of ionic liquids of interest for electrolyte-gated organic transistors [84]. [EMIM][TFSI]
[PMPip][TFSI]
[BMIm][TFSI]
[BMIM][PF6]
[BMPyrr] [TFSI]
Conductivity (mS/cm)
6.63 (20 °C)
2.124 (30 °C)
3.41 (20 °C)
1.37 (20 °C)
2.12 (20 °C)
Melting point (°C)
-3
8
-4
-8
-18
Density (g/cm3)
1.52 (20 °C)
1.413 (23 °C)
1.44 (19 °C)
1.37 (25 °C)
1.40 (23 °C)
39.4 (20 °C)
175.5 (25 °C)
49 (25 °C)
310 (25 °C)
94 (20 °C)
2.6; -2.1
2.7; -3.2
2.5; -2.1
2.2; -1.8
2.8; -2.5
Viscosity (mP s)
ECSW* (V)
*ECSW: Electrochemical stability window in terms of anodic limit and cathodic limits measured with platinum working electrode, glassy carbon as the counter electrode and Ag/AgCl as reference electrode.
Ion gels, polyelectrolytes and polymer electrolytes contain a polymer backbone that makes their physical structure more solid and easy to handle for device applications. However, as a general rule, the ionic conductivity is higher in liquid than in solid electrolytes [85]. An ionic liquid can be blended with a suitable polymer, possibly with repeating units that match the molecular structure of the ionic liquid, to form an ion-gel [68]. Ion-gels are characterized by a small amount of polymer (~ 4 wt %) and present a good compromise between mechanical stability and ionic conductivity. Interestingly, certain kinds of ionic liquids (e.g. vinyl-imidazolium) can be polymerized to form polymer IL electrolyte where only anions are able to move [86,87]. Polyelectrolytes are polymers that have an electrolyte group bonded covalently to the polymer backbone repeated unit. When the polyelectrolyte is in a polar solvent, the counter ion is solvated. A dissociated polyelectrolyte results in mobile counterions and charged polymer chains with immobile ions. Thus polyelectrolytes effectively transport only one type of ion. Polyelectrolytes show relatively low ionic conductivities, in the range 10-3 to 1 mS cm-1 [88]. A polymer electrolyte is a salt dissolved in a solvating polymer matrix. The most commonly used solvating polymer matrix is poly(ethylene oxide) (PEO). Polymer electrolytes, for example based on alkali metal salts mixed with PEO or polyvinyl alcohol, result in mechanical stable structures but 10
their ionic conductivity is lower than that of liquid electrolytes [89]. The ionic conductivity of polymer electrolytes is in the interval of 10-5 to 1 mS cm-1 [90]. 3. Preliminary results and challenges in electrolyte-gated organic light-emitting transistors
Since their appearance in 2003, OLETs experienced a significant progress due to improved organic channel materials, gate dielectrics and device architectures. OLETs open a new perspective in the study of fundamental physical processes such as charge carrier injection-, transport, and exciton recombination in organic thin films and single crystals. Therefore OLETs are, besides their intrinsic technological interest, relevant tools to characterize the charge transport and light-emitting properties of organic materials. As an example, OLETs have been used as the experimental platform to investigate simultaneous charge carrier transport and solid-state light-emission in organic materials [91]. The technological interest for low-power organic transistors naturally extends to OLETs, such that several groups are exploring the electrolyte-gating approach to simultaneously modulate the transistor current and the light generated within an electroluminescent organic transistor channel. Besides that, electrolyte-gating represents an exciting opportunity for investigating the properties of organic materials under high charge carrier and exciton density as well as high current density conditions, to unveil the interrelationships between charge carrier density, charge carrier mobility, and light emission in organic electroluminescent materials. Interestingly, high current density (~33 kA cm-2) has been observed [92] in single crystal OLETs, where there is no detrimental effect on the emission efficiency, in contrast to organic light-emitting diodes [93, 94]. Electrochemical and electrostatic operation modes have been reported for EG-OLETs. In the electrochemical doping mode, the photoluminescence of the organic semiconductor is quenched in the bulk and the charge carrier injection at the drain and source electrodes is enhanced [95, 96]. Liu et al. reported electrolyte-gated organic light-emitting electrochemical transistors where a gate was applied to a bilayer electrochemical light-emitting cell [64]. The transistor made use of a thin film of poly[2-methoxy-5-(2’-ethylhex-yloxy)-1,4-phenylenevinylene] (MEH-PPV) as the lightemitting polymer and of a blend of PEO and KCF 3SO3 as the gating electrolyte. Bottom Au contacts 11
served as the cathode (electron injecting) and anode (hole injecting) electrodes (L=500 m), and a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) thin film, laminated on top of the PEO-KCF3SO3 blend, as the top gate electrode (Fig. 5).
a)
b)
Top view:
After VG= 4 V
c)
After VG= -4 V
d)
Fig. 5: Organic light-emitting electrochemical transistor: a) device structure based on a PEDOT:PSS gate electrode, a KCF3SO3-PEO electrolyte, a MEH-PPV light-emitting polymer semiconductor, and Au source and drain bottom electrodes; b) top view of the transistor channel, upon application of a cathode-anode voltage of 4 V a gate bias (V G) of 4 V (left) and -4 V (right); c) and d) proposed working principle for the light-emitting transistor upon application of positive (c) and negative d) gate bias. Adapted from [64]. The device was operated in three different modes: (I) emission, where an electrical bias larger than the energy gap of the electroluminescent polymer was applied between the anode and the cathode (4 V) such that electronic charge carrier injection took place, leading to the formation of p- and n-doped regions and light emission; (II) n-doping, where a positive bias was applied between the gate and the cathode (4 V) with, as a consequence, migration of anions from the electrolyte and diffusion of cations from the electrolyte into the MEH-PPV to extend the n-doping of the polymer; (III) p-doping, where a negative bias was applied between the gate and the anode (-4 V) such that anions from the electrolyte diffused into the MEH-PPV to produce its p-type doping. The position of the light-emitting region within the EG-OLET transistor channel could be controlled by the polarity and magnitude of the applied gate bias that induced a preferential penetration of 12
cations(anions) into the light-emitting polymer, affecting the length of the n-(p-)type doped regions. The on/off ratio in the transistor ranged from 10 to 100 and the gate threshold bias was -2.3 V. A comparison with previous works on transistors based on electroluminescent organic polymers blended with mobile ions points to the key role played by the electrolyte-gating medium and the PEDOT:PSS gate electrode in establishing the OLET performance. In 2004, Edman et al. reported on transistors where the channel was a mixture of the light-emitting polymer Superyellow, a crown ether (ionic solvent) and a LiCF3SO3 salt, the gate dielectric was SiO2 on doped-Si, which served as the gate, and the source and drain bottom-contact electrodes were made of Au [97]. In such transistors, the authors observed an improvement of the hole injection properties, attributed to the electrochemical doping of the polymer. No electron injection was observed. This suggests that electrostatic effects are not sufficient to modulate the electrochemical reactions that are the underpinning to the spatial control of the p-n junction (i.e. the light-emitting element). In 2010, Yumusak et al. reported an organic electrochemical light-emitting field-effect transistor. In this case no spatial control of the light-emitting region was reported [98]. The transistor (Fig. 6a) made use of a thin film of LiCF 3SO3 dissolved in PEO blended with the light-emitting polymer poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-1,4-phenylenevinylene) (MDMO-PPV). The thin film was deposited on the gate dielectric, poly(vinyl alcohol) (PVA), which was overgrown on conductive glass (SnO2:In/glass) serving as the gate electrode and the substrate. Gold bottom contacts for the drain and source electrodes completed the device. Only p-type output and transfer transistor characteristics were reported (Fig. 6b). More recently, Yumusak et al. reported [99] on the optical and electrical characterization of electrochemically-doped organic field-effect transistors making use of a thin-film of MDMO–PPV mixed with PEO as the light-emitting polymer channel and a mixture of LiCF3SO3 with benzocyclobutene, as the gate electrolyte. Gate-modulated electroluminescence was observed even if the emission was localized close to the drain (electron-injecting) electrode, i.e. no spatial control of the light-emitting region was achieved. Initially light-emission was observed only in the saturation regime. After further sweeping cycles, light-emission was also observed in the linear regime.
13
Fig. 6: a) Device structure and b) p-type output characteristics of the organic electrochemical lightemitting field-effect transistor reported in [98]. Investigating ion-gel gated transistors making use of a (light-emitting) polymer, Bhat et al. discussed [100] the factors affecting charge carrier transport and electroluminescence in organic materials as induced by electrochemical vs. electrostatic doping. The ion-gel was based on 1-ethyl-3methylimidazoliumbis
(trifluoromethylsulfonyl)
imide/poly
(styrene-block-ethylene
oxide-
blockstyrene) and the light-emitting polymer was poly(9,9′-dioctylfluorene-co-benzothiadiazole), (F8BT). Au contacts photolithographically patterned on glass served as the source and drain bottom electrodes whereas drop cast PEDOT:PSS, offset with respect to the transistor channel, served as the top gate electrode. The transfer characteristics showed considerable hysteresis when the gate-source voltage (V gs) was swept (at 5 mV/s) within the range -3 ≤ V gs ≤ 1. The onset voltage was about -1.2 V and the on-off ratio was 105. Output characteristics showed well-defined linear and saturation behavior. The devices emitted light in proximity to the electron-injecting drain electrode when the drain-source voltage (Vds) exceeded the energy gap of the polymer (approximately 2.6 eV). No movement of the recombination region across the channel was observed, differently from ambipolar F8BT lightemitting transistors making use of the PMMA gate dielectric
[101].
The shape of the transistor current measured against Vds at high Vgs showed that the current is higher during the reverse sweep compared to the forward one. This behavior was attributed to the p-type electrochemical doping of F8BT. This hypothesis was in agreement with a capacitance versus frequency study, performed at various Vgs. In the low frequency regime, below 100 Hz, the absolute capacitance for Vgs = -2 V was higher than for Vgs = -2.5 and -3 V, suggesting the presence of a
14
pseudocapacitance due to ion incorporation into the F8BT (as opposed to the electrical double layer capacitance measured at the F8BT/ion-gel interface). The presence of electrochemical doping was also suggested by the extremely high values of the differential transmission observed for both the charge-induced absorption and bleaching signals in optical spectra obtained from biased transistors by Charge Accumulation Spectroscopy. Indeed, such high values are not compatible with a purely electrostatic operation mode of the transistor, thus pointing to the presence of a high polaron concentration in the semiconducting polymer due to electrochemical doping. In agreement with the exclusive p-type behavior of the transistor characteristics, no signs of electron accumulation at the polymer semiconductor/ion-gel interface were obtained from Charge Accumulation Spectroscopy upon application of gate biases up to 4 V. Fundamental studies on the charge carrier transport properties of thin films of MEH-PPV and F8BT, two of the light-emitting polymers employed in EG-OLETs, were carried out by Paulsen et al. [102]. In particular, conductivity and electrochemistry were investigated at high charge carrier density and anodic potentials. In agreement with previous studies [103, 104], as the charge carrier density is increased, an increase in mobility is first observed, followed by a peak and eventual decrease in mobility and conductivity upon further increase of the charge density. Overall, the peak in conductivity versus charge carrier density was confirmed to be a general phenomenon for polymer semiconductors gated with ionic liquids. The experimental configuration employed by Paulsen et al. was a microelectrochemical transistor (Fig. 7), previously proposed by Wrighton et al. [49]. Transistor transfer curves and cyclic voltammograms were simultaneously obtained using such a configuration. A Pt mesh electrode immersed in the electrolyte served as the gate and the counter electrode. Ag quasi reference electrode was also immersed in the electrolyte. Source and drain electrodes, together with the open transistor channel, served as the working electrode. The three-dimensional charge density was obtained by dividing the total gate-induced charge (obtained after cyclic voltammetry measurements, i.e. gate current versus gate voltage during the forward scan) by the area of the polymer film in contact with electrolyte, the thickness of the film, and the unit charge [105, 106].
15
Fig. 7: Device structure of a microelectrochemical transistor including: source and drain electrodes, which, taken together with the open transistor channel, behave as the working electrode of the electrochemical cell; gate electrode, which behaves as the counter electrode in the electrochemical cell. The microelectrochemical transistor includes a reference electrode immersed in the electrolyte-gating medium [102]. The polymer thin film was spin coated on a substrate pre-patterned with Au source and drain electrodes (interelectrode distance, L, and electrode width, W, of 250 μm and 7.5 mm, respectively). The ionic liquid, 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM] [FAP]), was deposited on the polymer thin film and served as the gate electrolyte. The peak mobility and the peak conductivity were: 0.08 cm2 V-1 s-1 and 6.9 S cm-1, for MEH-PPV transistors, and 0.07 cm2 V-1 s-1 and 11.7 S cm-1 for F8BT transistors. Braga et al. built [107] organic electrochemical transistors to switch and drive red, green and blue OLEDs. The transistor channel was deposited by aerosol jet printing from a chloroform solution of P3HT on Au drain and source pre-patterned substrates where, sequentially, the ion gel electrolyte that served as the gating medium, and a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) PEDOT:PSS gate electrode were overgrown. The ion gel consisted of a gelating triblock copolymer poly(styrene-b-methylmethacrylate-b–styrene) (PS-PMMA-PS) swollen with the ionic liquid [EMIM][TFSI]. OLEDs were made using a graded emissive layer (G-EML) sandwiched between ITO/glass substrate and Al/LiF electrodes, finished through shadow mask evaporation. Green GEML OLEDs used 4,4′,4′′-tris(carbazol-9-yl) triphenylamine (TCTA) and 4,7-diphenyl-1,10phenanthroline (BPhen) as hole- and electron-transport host materials and the green phosphorescent 16
emitter fac-tris(2-phenylpyridine) iridium (III). Analogously, red G-EML OLEDs used TCTA and BPhen as hole- and electron-transport host materials and the red phosphorescent emitter bis(1phenylisoquinoline)-(acetylacetonate) iridium (III). Blue G-EML OLEDs used TCTA and 2,2′,2′′(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) as hole- and electron-transport host materials
and
the
blue
phosphorescent
emitter
bis(3,5-difluoro-2-(2-pyridyl)phenyl(2-
carboxypyridyl)iridium(III). The organic electrochemical transistors/OLED devices showed high brightness up to 900 cd m -2, with low supply and gate voltages of -4.2 V and 0.6 V for green G-EML. Red and blue devices had a maximum luminance of 20 cd m-2 and 400 cd m-2, respectively, for the same supply voltage. Green G-EML OLEDs showed a turn-on voltage of -2.6 V, which is compatible with the organic electrochemical transistors requirements since higher turn-on voltages could be beyond the electrochemical stability window of the electrolyte. The current density provided by the organic electrochemical transistors was orders of magnitude higher than that required by the OLEDs, so that even devices with low electrode width (W) to length (L) ratio (e.g., W/L = 1) could properly drive OLEDs up to 900 cd m-2 with sub -1 V gate voltages. This factor was key to achieve OLED:organic electrochemical transistor footprint area ratios 100:1, which is important to facilitate the display architecture and overall display brightness. The device proved to be relatively stable up to 2 hours of dynamic operation with a square wave-function (0 < VGATE < –0.64 V) at 10 Hz. During this stressing period, a 20% decrease of the maximum OLED luminescence was observed. The maximum on-off switching rate of the organic electrochemical transistors was 100 Hz, which represents a limitation for video rate displays, however further improvements in switching speed can be made e.g. by controlling the thickness of the polymer semiconductor and the electrolyte layer. The results mentioned above demonstrate that the all-organic OET/OLED pixels are promising candidates to integrate Active-Matrix Organic Light-Emitting Diode (AM-OLED) displays. Their high brightness, low operation voltage, adequate footprint ratio and device stability are all desirable characteristics in AM-OLED technology.
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Relevant questions and perspectives in the field of EG-OLETs Considering the preliminary results in EG-OLETs, a number of interesting questions can be formulated. One of the first questions concerns the effect of the proximity between the electrolyte and the light-emitting organic semiconductor: how does this proximity, which is affected by the device structure, affect the electroluminescence process? Hodgkiss et al. made relevant observations by time-resolved spectroscopy [95] on conjugated polyelectrolytes with low-density ionic side chains. The experiments were carried on a conjugated polyelectrolyte derived from F8BT, specifically a copolymer containing tetra-alkyl ammonium moieties and BF4- counter anions attached to a ~7% density of polymer alkyl side chains (indicated in what follows as FN-BF4-7%). The photoluminescence quantum efficiency of FN-BF 4-7% thin films was low (~6%) compared with F8BT (~60%) thin films. Interestingly, time-resolved optical spectroscopy studies indicated that ions induce the formation of long-lived, weakly emissive and immobile charge states. The assessment of the effect of the proximity of the electrolyte on the light-emission properties could benefit from photoluminescence experiments performed on thin films of light-emitting polymers exposed to electrolytes where a biased (gate) electrode is immersed. Another open question concerns the relationship between the quantum efficiency of the emitted light with the charge carrier density. In principle, high-mobility light-emitting polymers, limiting excitonpolaron quenching, permit high steady state exciton density, leading to good electroluminescence efficiency. It is worth to notice that other sources of light-quenching such as electric field- and metalquenching can be limited in OLETs, compared to OLEDs. In particular, metal quenching is limited since, in OLETs, the light emission region can be moved a few μm from the metal electrodes by tuning the relative values of the gate and drain biases. A number of other questions are still open, such as how the specific physicochemical characteristics of the electrolyte, e.g. in terms of ionic conductivity, viscosity, and ion size and shape, affect the nand p-type doping of the transistor channel. Within the same context, since ambipolar injection is needed in OLETs, the fundamentals of charge carrier injection in electrolyte-gated transistors should be carefully investigated [108,109]. To promote n-injection in the light emitting organic 18
semiconductor, device structures including a double gate as well as electrodes based on carbon nanotubes [110] seem promising routes to high performance EG-OLETs. Acknowledgements We acknowledge Prof. D. Rochefort (Université de Montréal), Dr M. Anouti (Université de Tours), Prof. J. Leger (Western Washington University), Prof. J. Tagüeña (Centro de Investigación en Energía, Universidad Nacional Autónoma de México) for fruitful discussions. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
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