Simple source/drain contact structure for solution-processed n-channel fullerene thin-film transistors

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Simple source/drain contact structure for solution-processed n-channel

fullerene thin-ﬁlm transistors

Fang-Chung Chen

⇑

, Tzung-Han Tsai, Shang-Chieh Chien

Department of Photonic and Display Institute, National Chiao Tung University, Hsinchu 30010, Taiwan

a r t i c l e

i n f o

Article history:

Received 20 September 2011

Received in revised form 25 November 2011 Accepted 31 December 2011

Available online 17 January 2012 Keywords: Polymer Field effect Transistor Interface

a b s t r a c t

This paper describes a simple approach for reducing the contact resistances at the source/ drain (S/D) contacts in solution-processed n-channel organic thin-film transistors (OTFTs). Blending poly(ethylene glycol) (PEG) into the fullerene semiconducting layer significantly improved the device performance. The PEG molecules in the blends underwent chemical reactions with the Al atoms of the electrodes, thereby forming a better organic-metal inter-face. Further, the rougher surface obtained after the addition of PEG could also increase the effective contact area, thereby reducing the resistance. As a result, the electrical properties of the devices were significantly improved. Unlike conventional bilayer structures, this approach allows the ready preparation of OTFTs with a low electron injection barrier at the S/D contacts.

1. Introduction

Organic thin film transistors (OTFTs) are attracting increasing attention due to their potential applications in low-cost, flexible electronics, such as smart cards, radio frequency identification tags, and electronic paper displays [1–4]. Currently, hole mobilities higher than 1 cm2V 1s 1 have been widely demonstrated for p-channel OTFTs[3,4]. On the other hand, although high-performance n-channel devices have been also reported, achieving comparable performance to p-channel ones still remains challenging. This could be attributed to several reasons; for example, the electrons are easily trapped by the surface states, such as hydroxyl groups, at the semiconductor/dielectric inter-faces, thereby decreasing the electron mobilities [5–7]. Besides, low-work-function metals, such as calcium, are usually required to reduce the electron injection barrier at the source/drain (S/D) contacts[8]. These materials are relatively difficult to handle because of their susceptibility to atmospheric moisture and oxygen. Recently, many methods for constructing effective contacts between the

n-channel semiconductors and the S/D electrodes have been reported[9,10]. For example, Cho et al. demonstrated high-performance n-channel OTFTs by inserting a layer of titanium sub-oxide (TiOx) at the electrode/semiconductor

interfaces[9]. Chu et al. also used a nanoscale Cs2CO3

inter-facial layer to reduce the S/D contact resistances [10]. However, the bilayer structures proposed in these works required one additional step to the process, which might increase the fabrication cost. In this work, we presented a simple approach for preparing high-performance n-chan-nel OTFTs. We found that the device performance of solu-tion-processable OTFTs made of [6,6]-phenyl-C61-butyric

acid methyl ester (PCBM) could be enhanced after the addition of poly(ethylene glycol) (PEG) into the semicon-ducting layer. The PEG molecules reacted with the elec-trode material, Al atoms, during the fabrication process of the S/D contacts; the resulting interface exhibited low contact resistances, thereby enhancing the device perfor-mance signiﬁcantly[11–15].

2. Experimental

Fig. 1(a) displays the chemical structures of the materi-als used in this study. The OTFTs were fabricated on

⇑Corresponding author. Tel.: +886 3 5131484; fax: +886 3 5735601. E-mail address:[email protected](F.-C. Chen).

Organic Electronics 13 (2012) 599–603

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Organic Electronics

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indium tin oxide (ITO)-coated glass substrates. The cleaned ITO substrates were then covered with a layer of 760-nm cross-linked poly(4-vinylphenol) (PVP) as the dielectric layer, which was prepared through spin-coating a solution consisting of PVP and poly(melamine-co-formaldehyde) methylated, a cross-linking agent, in propylene glycol monomethyl ether acetate [16,17]. The PVP films were firstly baked at 120 °C for 5 min and then thermally cross-linked at 200 °C for 30 min. A semiconducting layer of PCBM was subsequently deposited from a chloroform solution inside a N2-filled glove box. The devices were then

thermally annealed at 80 °C for 15 min. For the preparation of devices containing PEG, the polymer was blended into the PCBM layer at different weight ratios. Finally, 100-nm thick Al was thermally evaporated through a shadow mask to form the S/D electrodes. The channel length (L) and width (W) were 100 and 2000

l

m, respectively. The current–voltage (I–V) curves were measured using the

Keithley 4200 measurement system in a nitrogen environ-ment inside a glove box. The surface morphology of the thin ﬁlms was visualized using a Digital Instruments Dimension 3100 atomic force microscope (AFM).

3. Results and discussion

Fig. 1(b) shows the output characteristic of the OTFTs prepared with and without PEG. We can see that the PEG device exhibited higher output current. Apparently, the de-vice performance was improved after the addition of PEG. To further investigate the effect of PEG, we blended PEG molecules with different molecule weights into the PCBM layer; Fig. 1(c) displays the effect of the PEG molecule weight (Mw) on the transfer characteristic of the devices. For the device made of pristine PCBM, the electron mobil-ity, calculated from the square root of the drain current

Fig. 1. (a) The chemical structures of the materials used in the work. (b) The output characteristics of the OTFTs prepared with and without PEG. The Mw of the PEG was 20,000. The optimized PCBM:PEG weight ratio was 1:0.05. (c) The transfer characteristics of the OTFTs containing PEG molecules with different molecules weights. The drain voltage (Vd) was maintained at 60 V. All the PCBM:PEG weight ratio was 1:0.05.

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versus the gate voltage, was 0.014 cm2_V 1_s 1_{. The on–off}

ratio was only 103. On the other hand, after the addition of PEG, the devices not only exhibited higher ﬁeld-effect current but also showed lower threshold voltages. While the PEG Mw was 20,000, the device exhibited the highest mobility (0.044 cm2_V 1_s 1_{); the on–off ratio and the}

threshold voltage were also improved to 4.1 104 _and

2.61 V, respectively. The device performance clearly has been improved after the use of PEG. The electrical charac-teristic parameters of the OTFTs were summarized inTable 1.

In order to study the mechanism responsible for the de-vice improvement, transfer line method was further adopted to analysis the I–V characteristics of the devices; the results were displayed in Fig. 2 [18,19]. The device resistance was obtained from the inverse slope of the Id–

Vdcurves in the linear regime. As shown in the inset of

Fig. 2, the total parasitic resistance (Rp) consists two

com-ponents, namely, the contact resistance (Rc) and the bulk

resistance (Rb), for the top-contact device conﬁguration.

The Rp could be obtained from the L = 0 intersection of

the linear ﬁt of device resistances as a function of the chan-nel length at different gate voltages. The ﬁtting curves at different gate voltages met at an intersection point, which was the minimum effective contact resistance (Rc). The

dif-ference between Rpand Rcwas equal to Rb. From the results

shown in Fig. 2, we can see that the Rc was decreased

signiﬁcantly from 22.4 to 1.7 MX after the addition of PEG, suggesting that the electron injection efﬁciency was indeed improved. On the other hand, the Rbwas relatively

unchanged (3 MX). Therefore, we inferred that chemical reactions between PEG molecules and Al atoms modiﬁed the nature of the S/D contacts, thereby decreasing the elec-tron injection barrier [11–15]. More interestingly, the channel resistance (Rch), which can be extracted from the

slope of the ﬁtting curves, was also decreased from 0.18 to 0.09 MX/

l

m once PEG molecules were added. Because PEG itself is insulating, we suspected that the blending of PEG also inﬂuenced the morphology of PCBM, thereby improving the charge transporting in the channel.Table 2 summarizes more detailed analysis of the device resis-tances for OTFTs prepared with different conditions.

To understand the origin of the device enhancement, we investigated the surface morphologies of the PCBM thin ﬁlms prepared with different Mws of PEG on PVP dielec-trics; the height-mode images were displayed in Fig. 3. The ﬁlm containing no PEG exhibited a rather smooth

sur-face. On the other hand, we could clearly observe phase separation on the surfaces after PEG was blended into the PCBM layer. The average diameter of the holes on top of the thin ﬁlm was 25 nm when the PEG Mw was 400 (Fig. 3(b)). In addition, we can also see that the dimension of the holes was decreased with the increasing PEG Mw, but the number of the holes was increased. These results could be explained in terms of the different diffusivities of the PEG molecules. Because PEG with a lower Mw had a higher diffusivity in the blend, more complete degree of phase separation could be achieved, thereby resulting in a larger dimension of the holes.

FromFig. 3, we inferred that the PEG molecules proba-bly did not cover the entire surface. However, the interfa-cial reactions should be still sufﬁcient to decrease the charge injection barrier. Further, the AFM images appar-ently indicated that the thin ﬁlms containing high-Mw PEG had rougher surface. Therefore, the interface should have larger effective contact area, thereby more effectively decreasing the contact resistance. As a result, the device fabricated with high-Mw PEG exhibited better device performance.

To probe possible chemical reactions at the electrode contacts, we used X-ray photoelectron spectroscopy (XPS) to study the surface of the PCBM thin films. A thin layer of Al was deposited on the thin films.Fig. 4presents the C 1s spectra of the thin films. We can observe a main peak at a binding energy of 284.5 eV, which became boar-der after PEG was blended into the thin film. We assign the shoulder at 286.0 eV to the extra C–O bonds of PEG molecules [14]. Another peak around 289.5 eV appeared for the sample containing PEG. This additional peak was associated with the presence of higher oxidized carbon species[14]. Further, in the lower-energy regime, we could also observe some signals appeared, suggesting the

forma-Table 1

Characteristics of OTFTs containing PEG molecules with different molecular weights. Molecular Weight (g/mole) Mobility (cm2 V1 s 1 ) Threshold voltage (V) On/off ratio No PEGa _0.014 _17.02 _{6.0 10}3 200 0.015 4.78 1.2 104 400 0.022 0.84 1.4 104 600 0.025 0.56 1.1 104 1000 0.026 0.74 1.2 104 3000 0.029 0.20 1.1 104 8000 0.038 0.90 2.0 104 20,000 0.044 2.61 4.1 104 a

Only neat PCBM was used as the semiconducting layer.

Fig. 2. The device resistance at different gate voltages as a function a channel length (L). The inset shows the equivalent circuit of the model. Note that RcS, RcD, RbS, RbDrepresent the contact resistance at the source

and drain contact, and the bulk resistance at the source and drain contact, respectively.

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Fig. 3. (a) The height-mode AFM image of the neat PCBM ﬁlm prepared on a PVP dielectric layer. (b–f) The AFM images of the PCBM blends containing PEG with different Mws. The Mw was (b) 400, (c) 1000, (d) 3000, (e) 8000 and (f) 20,000. All the image sizes are 3 3lm2

. All the PCBM:PEG weight ratio was 1:0.05.

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tion of carbide-like/Al complex junction[14,20]. Thus, the XPS data conﬁrmed the chemical reactions between PEG molecules and Al atoms, which could change the nature of the interfaces and decrease the device contact resis-tances[20].

Finally, according to the literature[5,7], electrons are easily trapped by the hydroxyl groups on the dielectric surface. In this work, we adopted PVP as the dielectric materials. The unreacted hydroxyl groups of the cross-linked PVP might behave as electron traps, thereby leading to a lower electron mobility. To further improve the device performance, hydroxyl-free gate dielectric, such as divinyl-teamethyl-disiloxanebis(benzocyclobutene) (BCB), could be used to decrease the density of electron traps on the dielectric surface[5,21]; such studies are ongoing. 4. Conclusion

We have developed a simple approach toward high-performance solution-processed n-channel OTFTs. Doping

PEG molecules into the PCBM layer signiﬁcantly improved the device performance. The PEG molecules in the blends underwent a chemical reaction with Al atoms, thereby forming a better organic-metal contact. As a result, the electrical properties of the devices were improved. The ef-fect of PEG Mws was also investigated; we found that the use of high-Mw PEG resulted in a rougher surface in the semiconducting layer, thereby increasing the effective contact area and decreasing the contact resistances. This approach allows the ready preparation of OTFTs with low carrier injection barrier at the S/D contacts. Moreover, unlike convention buffer layer methods, in which the elec-trodes are usually modiﬁed through vacuum deposition, this solution-processed approach allows printing technol-ogy to be used for device fabrication.

Acknowledgments

We thank the National Science Council of Taiwan (NSC 100-2221-E-009-082) and the Ministry of Education of Tai-wan (through the ATU program) for ﬁnancial support. References

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Table 2

Extracted resistances of the OTFTs investigated in this study following the Transfer Line-Method. The weight ratio of PCBM:PEG was kept at 1:0.05.

Molecular weight (g/ mole) Contact resistance (MX) Bulk resistance (MX) Parasitic resistance (MX) Channel resistance (MX/lm) No PEGa _22.4 _3.4 _25.8 _0.18 400 6.2 2.8 9.0 0.08 1000 5.8 3.1 8.9 0.09 3000 5.2 2.4 7.6 0.07 8000 3.2 2.9 6.1 0.08 20,000 1.7 3.3 5.0 0.09

a_{Only neat PCBM was used as the semiconducting layer.}

295 290 285 280 0.2 0.4 0.6 0.8 1.0

289.5

286.0

284.5 Normalized Counts (a.u.)

Binding energy (eV)

PCBM/Al

PCBM:PEG/Al

Fig. 4. The C 1s spectra of the PCBM thin ﬁlms prepared with and without PEG (Mw = 20,000). A thin layer (5 nm) of Al was deposited on the thin ﬁlms. The PCBM:PEG weight ratio was 1:0.05.