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Japanese Journal of Applied Physics 52 (2013) 05DC12 http://dx.doi.org/10.7567/JJAP.52.05DC12
Light-Exposure Effects on Electrical Characteristics of 6,13-Bis(triisopropylsilylethynyl)Pentacene/CdTe Composite Thin-Film Transistors Jaehoon Park1 y, Dong Wook Kim2 y , Bong Kuk Lee3 , Ye-Sul Jeong4 , Michael Petty4 , Jong Sun Choi2 , and Lee-Mi Do3 1
Department of Electronic Engineering, Hallym University, Chuncheon 200-702, Korea Department of Electrical Information and Control Engineering, Hongik University, Seoul 121-791, Korea 3 IT Convergence Technology Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-700, Korea 4 School of Engineering and Computing Sciences, Durham University, Durham DH1 3LE, United Kingdom E-mail:
[email protected];
[email protected] 2
Received October 24, 2012; revised December 21, 2012; accepted December 25, 2012; published online May 20, 2013 We report the light-exposure effects on solution-processed organic thin-film transistors (TFTs) based on a 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) semiconductor. Under light exposure, the increase in drain current and the positive shift of threshold voltage are observed, which are more remarkable for the TIPS-pentacene/cadmium telluride composite TFT. Herein the photosensitivity in these TFTs is explained in terms of photovoltaic and photoconductive effects. Composite semiconductors blended with sensitizers are suggested to enhance the quantum efficiency of organic phototransistors. # 2013 The Japan Society of Applied Physics
1. Introduction
Solution-processed organic thin-film transistors (TFTs) have received much attention due to their advantages such as low-cost manufacture and compatibility with plastic substrates for flexible electronics,1–4) thereby achieving encouraging electrical performances which are comparable or even surpassing to those of vacuum-deposited organic transistors;5–7) there are generally many factors concerning the performance of TFTs, such as field-effect mobility, on/off current ratio in the semiconducting layer, gate-leakage currents related to the gate dielectric property, and geometrical or extrinsic factors such as the channel dimension and semiconductor morphology.8–12) And pioneering works have demonstrated a wide range of applications, for example as the driving elements of flexible displays, radio-frequency identification tags and various sensors.13–15) Among the compelling applications in this research area, photosensitive devices, such as phototransistor and photomemory,16) promise a large potential of market for flexible displays functionalized with built-in touch screen panels as well as flexible image sensors. To date, several studies have reported the light illumination effects on the electrical characteristics of organic TFTs.17,18) Noteworthy is that the off-state drain current of organic TFTs substantially increases upon light exposure but the on-state drain current is relatively less influenced. This indicates that photogenerated charges in an organic semiconductor layer are far below electric-fieldinduced charges when these transistors are turned on. Hence it is essential to improve the photoinduced charge generation in an organic semiconductor layer for developing highly photosensitive TFTs. In the present paper, we investigated the light-exposure effects on 6,13-bis(triisopropylsilylethynyl)pentacene (TIPSpentacene)-based TFTs. It is shown that the photosensitivity in the TIPS-pentacene/cadmium telluride (CdTe) composite TFT can be further enhanced in comparison with the pristine y
These authors contributed equally to this work.
Fig. 1. (Color online) Schematic representation of the TIPS-pentacenebased organic TFT.
TIPS-pentacene case. The result demonstrates that CdTe is a useful sensitizer for photosensitive organic TFTs. 2. Experimental Procedure
The TIPS-pentacene-based organic TFTs were fabricated on glass substrates (Marienfeld); the device architecture is depicted in Fig. 1. A stripe-patterned 50-nm-thick Al gate electrode was thermally evaporated through a first shadow mask onto the substrate. For a gate insulator, a 400-nm-thick cross-linked poly(4-vinyl phenol) (PVP) film was formed by spin-coating from a solution (Dongjin Seichem); the crosslinking of thin film was processed by thermal curing for 5 min at 100 C and then for 20 min at 200 C in a dry oven. 40-nm-thick Au source and drain electrodes were patterned by conventional photolithography processes. The channel length and width were 20 and 400 m. TIPS-pentacene solutions (EM Index, 3 wt % dissolved in anisole solvent) were prepared by stirring for a week. The binary blend of TIPS-pentacene with CdTe particles (Sigma Aldrich, diameter size:
