IEEE ELECTRON DEVICE LETTERS, VOL. 33, NO. 11, NOVEMBER 2012 1571

High-Performance Flexible TFT Circuits Using TIPSPentacene and Polymer Blend on Plastic

Min Hee Choi, Byung Soon Kim, and Jin Jang, Member, IEEE

Abstract—We report the high-performance circuit on 25-μmpolyimide substrate with organic thin-film transistors (TFTs)(OTFTs) using 6,13-bis(triisopropyl-silylethynyl) pentacene andpolymer blending. Because of high mobility (0.64 cm2/V · s) andlow threshold voltage (< |1 V|) of the TFTs, an 11-stage ringoscillator (RO) made of these TFTs exhibits the switching speedof 31.4 kHz at the supply voltage of 20 V. The output waveformsof the RO change little after 5000 times bending with a radius ofcurvature of 2 mm, and thus, the OTFT circuits can be applied tovery flexible electronics.

Index Terms—Electronic circuits, flexible substrate, organicthin-film transistor (TFT) (OTFT), solution process.

I. INTRODUCTION

TO FABRICATE fully flexible displays, integration ofdisplay drive circuitry on plastic is needed. Therefore,

organic thin-film transistor (TFT) (OTFT)-based circuits onplastic substrate have been studied [1]–[3]. Most of the circuitsreported operate under high supply voltage, and the output isnot perfect. For example, the ring oscillator (RO) operates upto 10 kHz at a supply voltage of 40 V, and the shift registershows low output voltage (∼10 V) [1]. Therefore, furtherimprovement of OTFT circuits is necessary to use them forflexible electronics on plastic with integrated drivers.

High-performance p-channel OTFTs using small-moleculesemiconductor/polymer semiconductor blend and small-molecule semiconductor/polymer insulator blend have beenreported [4], [5]. The OTFTs show the improved performanceand enhanced stability due to the improved crystallinityof the semiconductor with self-passivation by insulatingpolymer. This is mainly due to the phase separation betweensemiconductor and polymer insulator materials [6], [7]. Thesmall-molecule and polymer insulator blended solution,however, cannot be used for inkjet printing because of its highviscosity [5], [8]. Most of the organic circuits reported so farare using the OTFTs without polymer blending [9].

In this letter, we studied the performance improvementof OTFT circuits based on small-molecule semiconductor

Manuscript received June 15, 2012; accepted August 9, 2012. Date of publi-cation September 24, 2012; date of current version October 19, 2012. This workwas supported by the Ministry of Knowledge Economy (Korea) through theIndustrial Strategic Technology Development Program under Grant KI002182(thin-film transistor backplane technology for next-generation display). Thereview of this letter was arranged by Editor C. Jagadish.

The authors are with the Advanced Display Research Center and Departmentof Information Display, Kyung Hee University, Seoul 130-701, Korea (e-mail:jjang@khu.ac.kr).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2012.2213294

Fig. 1. Process flow and structure of the TIPS PEN:PS TFT studied in thepresent work. (a) PI coating, (b) Mo gate formation, (c) S-ALO gate insulatorcoating, (d) Au S/D electrode formation, (e) active layer coating, (f) active layerpatterning using parylene encapsulation layer, and (g) detachment of TFT on PIfrom glass.

and insulating polymer blending. The 6,13-bis(triisopropyl-silylethynyl)pentacene (TIPS PEN) was synthesized by usingthe technology reported in the literature [10], and amor-phous polystyrene (PS, Mw = 280 kDa) from Aldrich wasused as received without further purification. Moreover, 1,2,4-trimethylbenzene (mesitylene, purchased from Aldrich) wasused as the solvent for 1.0-wt% TIPS PEN:PS blending.

II. DEVICE FABRICATION

Fig. 1 shows the fabrication process of TIPS PEN:PS TFTon plastic substrate. The polyimide (PI) was spin coated on a150 × 150 mm2 glass as shown in Fig. 1(a). Then, 125-nm-thick SiO2/SiNx/SiO2/SiNx/SiO2 was deposited by PECVDon PI as a gas barrier, and molybdenum (Mo, 40 nm) wasdeposited for gate electrodes by sputtering and patterned byphotolithography [Fig. 1(b)]. Then, the gate dielectric layer(S-ALO, organic–inorganic hybrid layer, 350 nm thick, anddielectric constant of 4.37) was spin coated and cured at250 ◦C for 2 h in a vacuum oven [Fig. 1(c)]. Then, 50-nm-thick Au was deposited by sputtering on the via-patterned gatedielectric and patterned for source/drain electrodes as shown inFig. 1(d).

To reduce the contact resistance between the TIPS PENand Au, the self-assembled monolayer was formed on theAu electrodes by immersing the sample in a 1-m methanolicsolution of pentafluorothiophenol [11]. The substrate was thenannealed at 110 ◦C in an oven, and the blend solution, TIPSPEN and PS solution, was spin coated on it and annealedagain at 110 ◦C for 2 min in an oven to eliminate the solventresidue [Fig. 1(e)]. Parylene C was deposited on the organic

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1572 IEEE ELECTRON DEVICE LETTERS, VOL. 33, NO. 11, NOVEMBER 2012

Fig. 2. Performance of the TIPS pentacene TFT. (a) Transfer. (b) Outputcharacteristics. The performance parameters for the 37 TFTs fabricated withL(= 4 μm) with various W ’s(= 20−6000 μm). (c) μsat, (d) μlin at VD =−0.1 V, (e) VT , and (f) S.

semiconductor layer as an encapsulation layer and patternedby photolithography using dry etching [Fig. 1(f)]. The printingprocess and measurement of OTFT characteristics were per-formed in ambient air. The TFT performances were measuredby a semiconductor parameter analyzer (Agilent 4156C), andthen the PI layer with TFTs was detached, resulting in free-standing TFTs on PI substrate as shown in Fig. 1(g). The TFTperformances were again measured to check the change in theTFT performance during detaching process.

III. RESULTS AND DISCUSSION

The channel width (W ) and length (L) of TIPS PEN:PSTFT on PI/glass are 50 and 4 μm, respectively. The deviceshows good ohmic contact and thus exhibits the average field-effect mobility (μsat) of 0.61 cm2/V · s in the saturation re-gion, on/off current ratio (ION/IOFF) of 107, and subthresholdswing (S) of ∼0.2 V/decade, with a threshold voltage (VT ) of−0.27 V as shown in Fig. 2(a) and (b).

Fig. 2 shows the distribution of (c) μsat, (d) μlin, (e) VT , and(f) S measured for the 37 OTFTs on PI/glass with an L of 4 μmwith W ’s of 20, 40, 50, 100, 200, 500, 1000, 2000, 3000, and6000 μm. The average μsat is (0.64 ± 0.28) cm2/V · s, μlin is(0.26 ± 0.150) cm2/V · s, S is (19 ± 38) mV/decade, and theaverage VT is (−0.13 ± 0.29) V.

A ratioed inverter consisting of p-channel load and driveOTFTs was fabricated. The inverter property depends on theratio between the widths of load and drive TFTs. The load TFTis always turned on through the diode connection, where themeasured transient characteristics for a supply voltage (V DD)ranging from 0.5 to 2.0 V are given. The W ′s of the load anddriving TFTs are 300 and 3000 μm, respectively, with an L of5 μm. The voltage transfer characteristics of the inverters showthe low output level due to the small S and small VT of < |1 V|for the TFTs.

In order to check the switching speed of the invertersbased on the TIPS PEN:PS TFTs, a RO consisting of 11stages (W/L of load TFT = 300 μm/5 μm and that ofdriving TFT = 2400 μm/5 μm) with a buffer was fabricated.Fig. 3 shows the output waveforms for the RO operated at three

Fig. 3. Output waveforms for the RO operating from supply voltages of(a) 3, (b) 5, and (c) 10 V. Moreover, (d) shows a plot of oscillation frequencyand simulated power consumption as functions of supply voltage.

different supply voltages: (a) V DD = 3 V, (b) V DD = 5 V,and (c) V DD = 10 V. Fig. 3(d) shows a plot of oscillationfrequency (fosc) and the simulated power consumption(Pavg)as functions of V DD ranging from 3 to 20 V, showing the in-crease of fosc with V DD [12]. The tp decreases with increasingV DD, and the single-stage propagation delay time (tp) of lessthan 1 μs is achieved for high V DD. The RO operates with foscof ∼7.8 kHz at V DD = 3 V, confirming that the TIPS PEN:PSTFTs could be used in low-voltage-driven electronic system.

The fosc can be achieved from the following:

fosc =1

N(tPHL + tPLH). (1)

Here, tPHL and tPLH are the delay times for the output tochange from high to low and from low to high, respectively,and N is the number of delay cells (stages) in the loop. Thefosc is 31.4 kHz at V DD = 20 V as shown in Fig. 3(d). The tp,which is the average time needed for the output responding to achange in the input logic state, is given by the following:

tp =tPHL + tPLH

2(2)

and it is found to be 1.5 μs at V DD = 20 V.Mechanical stability is an important factor for flexible de-

vices. We measured the performances of the TIPS PEN:PS TFTafter various bending times, i.e., convex bend, to confirm thepotential use of the OTFTs to flexible electronic system.

Fig. 4 shows the output waveforms of a RO on PI substrate.Fig. 4(a) shows the output waveforms before bending andafter 5000 times bending with a radius of curvature of 2 mm,corresponding to 0.52%. Fig. 4(b) shows the image of thesetup of the frontward and backward bending tests and thenumber of bending times. The initial frequency is 31.6 kHz atV DD = 20 V and remains almost the same even after bendingthe oscillator (TFTs) up to 5000 times (31.4 kHz). The ROperformance changes little after the bending test; for example,the frequency remains almost the same as the unstressed RO.This means that the circuit has higher flexibility and is thussuitable for rollable/flexible display, due to highly flexibleOTFT circuits on PI substrate.

CHOI et al.: HIGH-PERFORMANCE FLEXIBLE TFT CIRCUITS 1573

Fig. 4. Bending test for the TIPS PEN:PS TFT-based RO on PI substrate.(a) Output signal with bending time at the curvature radius of 2 mm. (b) Imageof the apparatus of frontward and backward bending tests and the number ofbending times.

IV. CONCLUSION

High-performance OTFT circuits based on TIPS PEN:PSblending have been demonstrated on 25-μm-thick PI substrate.The TFT circuits can be driven even at 3 V because of highμFE, low value of VT (< |1 V|), and low S (<0.2 V/decade).Because of high μFE and low VT of the OTFTs, an 11-stage ROexhibits the switching speed of 31.4 kHz at V DD of 20 V, andits performance is found to be almost the same even after 5000times bending at the radius of 2 mm. Therefore, highly flexibleOTFT circuits can be built on PI by using TIPS PEN and PSblend semiconductor.

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