Organic field effect transistor based on a novel ... - Science Direct

Microelectronic Engineering 80 (2005) 394–397 www.elsevier.com/locate/mee

Organic field effect transistor based on a novel soluble pentacene precursor and operating at low voltages Damien Zandera, Norbert Hoffmannb, Kamal Lmimounic, Stephane Lenfantc, Christian Petita and Dominique Vuillaumec a) Centre de recherche en STIC, UFR Sciences, Moulin de la Housse, BP 1039, 51687 Reims cedex2, France. Laboratoire des Réactions Sélectives et Applications, UMR CNRS 6519, UFR Sciences, Moulin de la Housse, BP 1039, 51687 Reims cedex2, France. c) Institut d'Electronique, de Micro-electronique et de Nanotechnologie IEMN UMR CNRS 8520, « Molecular Nanostructures & Devices » group, Avenue Poincaré, BP60069,59652 Villeneuve d'Ascq cedex, France. Tel: 33 (0) 3 26 91 84 21 email: [email protected]. b)

Abstract: We report the characterization of organic field effect transistors fabricated with a novel soluble pentacene precursor. The detail to obtain this novel precursor and the chemical characterization of this precursor are given. The soluble precursor has been deposited on a thin oxide film with lithographically defined electrodes. The determined carrier mobility in organic film 2 -1 -1 is of order of 0.01-0.05 cm V s . Keywords: organic field effect transistor, soluble pentacene precursor.

1. Introduction Low-cost electronic devices, such as organic thin film transistors have attracted a growing interest for different applications: in large-area electronic, flap panel displays, smart tags, or circuits on flexible substrates [1-5]. In the last decade, different organic semiconductors were studied to determine their crystalline structures and electronics properties, especially their electric field carrier mobility [3,6-8]. Pentacene is the 0167-9317/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.mee.2005.04.096

organic semi-conductor that offers high mobility and large Ion/Ioff ratio. However, pentacene is not soluble and pentacene films were obtained by vapor deposition [3,6-8]. This method is not suitable for low cost and large area applications. The solubility of organic semiconductors is vital for their use in low-cost electronic devices since the desired processing techniques include solution-based methods like spin coating, dip coating or printing techniques. However, practically all the small molecules used in OFETs (Organic Field Effect

D. Zander et al. / Microelectronic Engineering 80 (2005) 394–397

Transistors) are insoluble, and need to undergo vapor deposition to form thin films. Solution processing has been reported with oligothiophenes end-substituted with alkyl groups [9-11]. An alternative strategy consists of using a soluble precursor that would convert into the desired molecule through a thermal post-processing step. Recently, this strategy has been reported and it was shown the possibility to synthesize a soluble pentacene precursor in chlorinated solvents [12,13]. This precursor is obtained by using an efficient Lewis acid-catalyzed Diels-Alder reaction and the pentacene film is formed after a thermal retro Diels-Alder reaction. In this work, we report the synthesis of a novel soluble pentacene precursor and we demonstrate OFET working at low voltages. In the first part we will describe the experimental procedure and the chemical reaction used to synthesize the novel pentacene precursor. In the second part, we will discuss the OFET performances. 2. Experimental procedure, chemical reaction and characterization To synthesize the pentacene precursor, we used a Diels Alder reaction between pentacene (1) and sulfinylbenzamide (2), in presence of a catalyst (SnCl4), as shown in figure 1. The obtained pentacene precursor (3) was purified by column chromatography (for this step, we have employed a silica gel and an eluent: CHCl3/CH2Cl2). The structure of product (3) has been characterized and deduced by 1H NMR (Nuclear Magnetic Resonance), 13C NMR, MS (Masse Spectrometry) and IR (Infrared Spectroscopy). The 13C NMR spectral of product 3 shows peaks situated between 120ppm and 140ppm, these peaks are attributed to the different aromatic carbons and three characteristic peaks. These characteristic peaks are situated at 70.7ppm, 58.9ppm, and 173.4ppm, and correspond respectively to, carbon linked to nitrogen atom, carbon linked to sulfur atom and carbon linked to oxygen atom, as shown in figure 1. Moreover, the masse spectroscopy indicates that the molar mass of (3) is equal to 445 g. mol-1, which corresponds to the theoretical mass of product 3. The OFET was made using a Si(P+) substrate covered with a 9nm thick dry thermal oxide. The OFET has bottom source and drain contacts made by electron-beam lithography. The electrodes are made of a Ti adhesion layer (5 nm) and a Pt layer (25 nm). We

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used channel width (W)/length (L) ratios of 200µm/1µm and 20µm/0.2µm. All devices have been made on the same substrate. The product (3) was dissolved in chloroform at a concentration of ~1wt%. The obtained solution was deposed by spin coating at 500rpm in an inert ambient. The deposited film was annealed at 160°C during 5min. During heating the pentacene precursor (3) returns to the pentacene form making the organic pentacene film between electrodes. O S N + O

1

SnCl4

2

∆T and ∆t

O O

N S

3 Fig.1 Diels-Alder and Retro Diels-Alder reactions between pentacene and sulfinylbenzamide. ∆T and ∆t indicate heating temperature and heating time.

These devices were electrically characterized in ambient conditions using a probe station and HP4145B parameter analysis. 3. Experimental results Figures 2 and 3 present drain current Ids as a function of the drain-source voltage Vds for different gate-source voltages Vgs and Ids as a function of Vgs at Vds = -7.5V, which corresponds to the saturation regime. In the figure 3, we plotted the Ids current in a logarithm scale to determine the ION/IOFF ratio. The carrier mobility in the semiconductor film is calculated in saturation régime. In this region, the drain current is modeled by the equation I ds = µC 0 W (V gs − VT ) 2 , where 2L µ, C0, W, L and VT represent respectively the carrier mobility, the capacitance of the gate, the


width and length of the channel and the subthreshold voltage. In this regime, the mobility can be calculated from the slope of the plot of I ds = f(Vgs) .

As shown figure 2 and 3, after our annealing conditions, the organic film made with the soluble pentacene precursor has field effect properties. The AFM image presented in figure 4 shows the layered structure of the pentacene film. The height of each layer of pentacene is equal to 1.4nm, which corresponds to the length on the pentacene molecule. This structure is similar to the one observed for a vacuum evaporated pentacene film [3].

Fig.2 Ids(Vds) characteristic for a W/L=200/1µm device.

We have obtained a carrier mobility of 0.05 cm2V-1s-1 (best case) with an ION/IOFF ratio of 104. These performances are reproducible, other devices with the same W/L present similar results with carrier motilities close to 10-2 cm2V-1s-1. These mobility values are one order of magnitude lower than those reported in the reference [13], on the other hand, they are of the same order of magnitude than those reported in the reference [14]. In the vaccum sublimated film, it is found that the charge carrier mobility is now of the order of few cm2V-1s-1 [3,15-16] especialy with a small oligomer as a pentacene. Further works will be mandatory to improved these results, however, the present results with this low-cost and soluble approach better than those obtained usually with soluble polymers and on a par with the best cases ever reported for region-regular P3HT polymers [17].

Z (nm)

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1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 0

50

100

150

200

250

X (nm)

Fig.4 AFM image of a pentacene island (250nm×250nm), after heating.

Fig.3 Ids(Vgs) characteristic for a W/L=200/1µm device.

In our case, we observed in the Ids(Vds) characteristic a shift of ~-1.5V with Vds, which can be attributed to a not optimized metal/organic contact. This effect can be also contributed to limite charge carrier mobility in the semi conductor film. Figure 5 represents a Ids(Vds) curve for a device with W/L equal to 20/0.2µm. This device is made on the same substrate than the larger one. For this device, we obtained lower mobility values and a weak ION/IOFF ratio. The mobility values are ranging


from 10-4 to 4.2×10-4 cm2V-1s-1, for a low Vgs (≤3V) and high Vgs (≥3.5V), respectively. The ION/IOFF ratio is equal to 250. It is then important to note that in these both cases the mobility value depends on Vgs. This effect is not observed when L is equal to 1µm. This mobility variation may be related to change in the film morphology in case of deposition between smaller channel lengths and/or width (smaller than 200µm).

Fig.5 Ids(Vds) characteristics for a transistors with a ratio W/L=20/0.2µm.

4. Conclusion In summary, we have demonstrated a field effect transistor working at low voltages and based on a novel soluble pentacene precursor route. We give the chemical characterization. An AFM analysis shows the formation of a layered film after heating, similar to the structure for vacuum evaporated films. After our annealing conditions, the organic film made with the soluble pentacene precursor has shown field effect properties. The highest carrier mobility obtained is 0.05 cm2V-1s-1. The Organic field effect transistor has been demonstrated down to 200 nm channel length. References: [1] A. Dodabalapur, Z. Bao, A Makhija, J.G. Laquindanum, V.R. Raju, Y. Freng, H.E. Katz, J. Rogers, Appl. Phys. Let., 73 (1998) 142-144. [2] H.E. Katz and Z. Bao, J. Phys. Chem. B, 104 (2000) 671-678. [3] C.D. Dimitrakopuolos and P.R.L. Malenfant, Adv. Mater. 14 (2002) 99-117.

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