Chinese Chemical Letters  2016, Vol. 27 Issue (12): 1781-1787   PDF    
Electron transport in solution-grown TIPS-pentacene single crystals: Effects of gate dielectrics and polar impurities
Zhuo-Ting Huanga, Guo-Biao Xuea, Jia-Ke Wua, Shuang Liua, Huan-Bin Lia, Yu-Hui Yanga, Feng Yanb, Paddy K. L. Chanc, Hong-Zheng Chena, Han-Ying Lia     
a MOE Key Laboratory of Macromolecular Synthesis and Functionalization, State Key Laboratory of Silicon Materials, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ;
b Department of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University, Hong Kong, China ;
c Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China
Abstract: The n-channel behavior has been occasionally reported in the organic field-effect transistors (OFETs) that usually exhibit p-channel transport only. Reconfirmation and further examination of these unusual device performances should deepen the understanding on the electron transport in organic semiconductors. 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene), a widely examined p-channel material as Au is used for source-drain electrodes, has recently been reported to exhibit electron transport when grown from non-polar solvent on divinyltetramethyldisiloxanebis (benzocyclobutene) (BCB) dielectric, spurring the study on this unusual electron transport. This paper describes FET characteristics of solution-grown TIPS-pentacene single crystals on five polymer gate dielectrics including polystyrene (PS), poly(methyl methacrylate) (PMMA), poly(4-vinyl phenol) (PVP), poly(vinyl alcohol) (PVA) and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE-CFE)). In addition to the p-channel behavior, electron transport occurs in the crystals on PMMA, PS, thick PVA (40 nm) and a bilayer dielectric of PMMA on P(VDF-TrFE-CFE), while does not on PVP and thin PVA (2 nm). The two distinct FET characteristics are consistent with the previous reported trap effect of hydroxyl groups (in PVP and PVA) and reduced injection barrier by Na+ ions (as impurity in PVA). The highest electron mobility of 0.48 cm2 V-1 s-1 has been achieved in the crystals on PMMA. Furthermore, the electron transport is greatly attenuated after the crystals are exposed to the vapor of a variety of polar solvents and the attenuated electron transport partially recovers if the crystals are heated, indicating the adverse effect of polar impurities on electron transport. By reconfirming the n-channel behavior in the OFETs based on TIPS-pentacene, this work has implications for the design of n-channel and ambipolar OFETs.
Key words: TIPS-pentacene single crystal     Electron transport     Polymer dielectric     Polar solvent     Organic field-effect transistor    
1. Introduction

Organic field-effect transistors (OFETs) have attracted tremendous interest for their low-cost,flexible and large-area electronic applications [1, 2]. Nowadays great achievements have been obtained in organic hole transporting semiconductors. Experimentally,the π-channel organic semiconductors have exhibited remarkable hole mobility approaching 100 cm2 V-1 s-1 [3-15]. In sharp contrast,development of the n-channel organic semiconductors is much slower. Up to now,electron mobility closeto 10 cm2 V-1 s-1 isonlydocumented inverylimited reports [16-25]. The uneven development in π-channel and n-channel organic semiconductors spurs investigations on the strategies to improve the n-channel materials because hole and electron transports are equally important for complementary circuits and other double-carrier devices [26-28].

The well-known reasons for the low mobility in the n-channel OFETs have close relations with high electron injection barriers [29, 30],electron traps [31-33] and dipolar disorder [34-36]. Accordingly,efforts have been made to avoid the negative effects of these factors on the performance of n-channel OFETs [37-39]. In addition to the n-channel OFETs,"unexpected" n-channel behavior has been occasionally found in typical π-channel OFETs [40-43]. For example,in 2005,Singh et al. found the significant electron

transport in pentacene on poly(vinyl alcohol) organic gate dielectric with Au as the source-drain electrodes,and the electron mobility up to 0.2 cm2 V-1 s-1 was obtained [41, 42]. Lately,Xu et al. reported that as a small amount (~14 mol%) of a typical n- channel molecule was added into a typical π-channel molecule,electron transport was observed [43]. Interestingly,the two molecules formed solid solutions instead of either phase separated blends [44, 45] and bi-layers [46-48] or co-crystals [49]. Further examination of these unusual device performances should provide implications for understanding and enhancing the electron transport in organic semiconductors. Very recently,we have found [50] a similar unusual electron transport in solution-grown single crystals of 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-pentacene) [51]. Interestingly,although TIPS-pentacene is recognized as a typical π-channel semiconductor as Au is used for the source-drain electrodes [9, 52-56],electron transport emerges as TIPS-pentacene crystallizes on divinyl-tetramethyldisiloxane- bis(benzocyclobutene) (BCB) dielectric from non-polar solvents but does not in the case of polar solvents. In this work,we aim to reconfirm and further examine this unusual electron transport. Considering that gate dielectrics [33, 36, 57-60] and polar solvents have been shown to affect the electron transport dramatically,we investigate the effects of varied gate dielectrics and polar impurities on the electron transport in TIPS-pentacene single crystals.

2. Experimental

Crystal growth and characterization: Poly(methyl methacrylate) (PMMA,Aldrich,Mw = 120kD),polystyrene (PS,Aldrich,Mw = 280 kD),poly(4-vinyl phenol) (PVP,Aldrich,Mw = 25 kD),poly(vinyl alcohol) (PVA,Aldrich,Mw= 130 kD) and poly(vinyli- dene fluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF- TrFE-CFE)) were used as polymer dielectrics. P(VDF-TrFE-CFE) was synthesized according to the literature procedure [61, 62]. PMMA (1,5 mg/mL),PS (1 mg/mL) and PVP (1 mg/mL) were dissolved in n-butyl acetate,PVA (1,10 mg/mL) was dissolved in deionized water,and P(VDF-TrFE-CFE) (3 mg/mL) was dissolved in butanone,then the solutions were spin-coated onto highly doped silicon substrates with 300 nm SiO2. After dielectric layers coating,they were baked at 80 ℃ for about 2 h,except that the P(VDF-TrFE- CFE) and PVA (1,10 mg/mL) dielectrics were baked at 60 ℃ for 3 h and 24 h,respectively. The thicknesses of polymer dielectrics are around 2 nm (1 mg/mL),6 nm (3 mg/mL),10 nm (5 mg/mL) and 40 nm (10 mg/mL). TIPS-pentacene (Sigma-Aldrich) crystals were grown in ambient condition using the droplet-pinned crystallization (DPC) method onto the polymer dielectric covered highly doped silicon substrates with 300 nm SiO2. A TIPS- pentacene solution (15 mL,0.4mg/mL) dissolved in hexane (TCI,HPLC) was dropped onto a silicon substrate (1 cm × 1 cm) with a smaller piece of silicon wafer (0.3 cm × 0.3 cm,pinner) to pin the solution droplet. The silicon substrate was placed on a Teflon slide inside a Petri-dish (35 mm × 10 mm),allowing the solvent to slowly evaporate on a hotplate of 25 ± 1 ℃. Solution dried within three minutes and aligned crystals formed. For the treatment of polar solvent vapor,dichloromethane (CH2Cl2) (Aladdin,HPLC),chloroform (CHCl3) (Labor,HPLC),tetrahydrofuran (THF) (Aladdin,HPLC) and 1,2-dichloroethane (1,2-DCE) (Aladdin,HPLC) were used to treat the TIPS-pentacene single crystals grown on PMMA dielectric,as shown in Fig. 7a. For heat treatment,vapor-treated FETs were heated at 80 ℃ for 2 h and then cooled down naturally. The morphology of the crystals was characterized by optical microscopy (OM,Nikon LV100 POL) and Atomic force microscopy (AFM,Veeco 3D).

Figure 1. (a) Molecular structures of the PS,PMMA,PVP,PVA and P(VDF-TrFE-CFE). (b) A schematic representation of the FET configuration,where S is the source,D is the drain and G is the gate.

Figure 2. Morphology of TIPS-pentacene single crystals grown on PMMA (a,f,k,p),PS (b,g,l,q),PMMA&PDVF-TrFE-CFE (c,h,m,r),PVP (d,i,n,s) and PVA (1 mg/mL) (e,j,o,t). (a-e) Optical microscopy (OM) images of the crystals. (f-j) OM images of the crystals under crossed-polarizers. (k-o) AFM images of surface morphology of crystals. (π-t) AFM images of surface steps on crystals.

FETs fabrication and measurement: FETs were constructed in a toπ-contact,bottom-gate configuration by depositing source and drain electrodes (90 nm Au),using a shadow mask with channel length (L) of 50 mm and width (W) of 1 mm. The real W/L value was measured (Fig. 3b inset) to calculate the mobility values. The devices were characterized in a N2 glovebox using a Keithley 4200- SCS semiconductor parameter analyzer. The capacitance was measured by Keysight E4980AL Precision LCR Meter and the values are listed in Table 1. For statistics,data of charge mobility are presented as mean ± SD.

Figure 3. FET characteristics of crystals grown on PMMA (a-d),PS (e-h) and PMMA&P(VDF-TrFE-CFE) (i-l). Typical transfer (a,b,e,f,i,j) and output (c,d,g,h,k,l) characteristics of the FETs in operation modes of π-channel (a,c,e,g,i,k) and n-channel (b,d,f,h,j,l) have been shown as well. Inset image in b: An OM image showing the method of W/L measurement. L was measured from the real channel length and W was measured from the contacting area of the crystals that cross the S and D electrodes. W was calculated by the equation: W = S(W1 + W2)/2.

Table 1
FET parameters of the TIPS-pentacene OFETs on various polymer dielectric layers coated on 300 nm SiO2.

3. Results and discussion 3.1. The effect of gate dielectrics on electron transport

Five polymer dielectrics with different dielectric constants,PS (k 〜2.6),PMMA (k 〜3.5),(PVP) (k 〜4.2),(PVA) (k 〜7.8) and (P(VDF-TrFE-CFE) (k 〜50) [62],were selected to study their influence on the electron transport in TIPS-pentacene single crystals. Instead of pure polymer dielectrics,bilayer dielectrics with polymer films on 300 nm SiO2 were used for FETs,with the thick SiO2 layers to diminish the leakage currents and to maintain similar capacitance values. The molecular structures of the polymers and the polymer films surface morphology have been shown in Figs. 1a and S1 in Supporting Information,respectively. Hexane,a nonpolar solvent,was selected to grow the crystals in order to exclude the adverse effect of polar solvents on electron transport [24, 50]. TIPS-pentacene single crystals were grown using droplet-pinned crystallization (DPC) method [16, 63-65] directly on polymer dielectrics. Well-aligned single crystals were obtained (Figs. 2 and S2 in Supporting information). For the ribbons grown on PMMA,PS,PVP and PVA,the same color of individual ribbons under crossed-polarizers indicates their single-crystallinity (Fig. 2f-j). For those grown on P(VDF-TrFE-CFE),poor morphology of crystals was obtained due to the poor wettability of the solvent on the fluorinated surface (Fig. S2e-h in Supporting information). In order to avoid the negative effect of the poor morphology on the electron transport,a PMMA film around 10 nm thick,was covered onto the P(VDF-TrFE-CFE) surface through spin-coating to prepare a PMMA&P(VDF-TrFE-CFE) surface. As a result,the morphology of crystals grown on the PMMA&P(VDF-TrFE-CFE) was greatly improved and smooth single-crystalline ribbons were obtained (Fig. 2c,h,m,r).

Based on these aligned ribbon crystals,we proceeded to fabricate FETs by depositing Au source and drain electrodes through a shadow mask in a toπ-contact,bottom-gate configuration (Fig. 1b). Since the crystals did not fully cover the channel,the active W/L value was measured to calculate the mobility (Fig. 3b,inset). All the devices were tested in the N2 glovebox,and the saturation regime mobility was calculated from 50 devices for each polymer dielectric.

The typical transfer and output characteristics of TIPS- pentacene on PMMA,PS and PMMA&P(VDF-TrFE-CFE) dielectrics have been shown in Fig. 3,exhibiting obvious electron transport. For every device,n-channel behavior was observed in addition to their π-channel behavior,with the FET parameters summarized in Table 1and Fig. 4. For PS(Fig. 3e-h),an average electron mobility(me) of 0.036 ±0.025 cm2 V-h-1 (range: 0.00073-0.087 cm2 V-1s-1),on-to-off current ratios (Ion/Ioff)〜102,and threshold voltages (VT) between 78 and 98 V were obtained. For PMMA (Fig. 3a-d),an average electronmobility(me)of0.23±0.13cm2V-1s-1(range:0.0035- 0.48 cm2 V-1 s-1),on-to-off current ratios (Ion/Ioff)〜101,and threshold voltages (VT) between 72 V and 94 V were obtained as well. The slightly higher electron mobility on PMMA is attributed to the more smooth morphology of the crystals than that on PS dielectric,as shown by AFM (Fig. 2k,l,p,q). In comparison,crystals grown on PMMA&P(VDF-TrFE-CFE) surfaces show worse n-channel behavior (Fig. 3i-l),with only 37 devices (out of 50) exhibiting electron transport. And the achieved electron mobility is lower,with an average value of 0.0059 ± 0.0075 cm2 V-1 s-1 (range: 0.00044-0.0034 cm2 V-1 s-1). The lower mobility in the case of PMMA&P(VDF-TrFE-CFE) surfaces is attributed to the strong surface polarization effects originated from the high-k dielectric of P(VDF-TrFE-CFE) [36, 66].

In sharp contrast,the crystals grown on PVP dielectric show no electron transport (Fig. 5b,red arrow) but only hole transport (Fig. 5a). The suppressed electron transport is attributed to the hydroxyl groups on the PVP that has been demonstrated to heavily trap electrons [33]. Consistently,the crystals grown on PVA (1 mg/mL) dielectric that contains hydroxyl groups as well exhibit unipolar behavior without electron transport (Fig. 6a and b,red arrow). Interestingly,as we increased the thickness of the PVA dielectric films from 2 nm to 40 nm,we observed electron transport (Fig. 6c-f). An average electron mobility (me) of 0.038 ±0.041 cm2 V-1s-1 (range: 0.00085-0.14 cm2 V-1s-1), on-to-off current ratios (Ion/Ioff) ~102,and threshold voltages (VT) between 63 V and 92 V were obtained. This unipolar to ambipolar transition was previously reported in another typical π-channel molecule,pentacene [41, 42, 67, 68]. Similarly,hole transport was only observed for pentacene on PVA dielectric of 100 nm thick,while additional electron transport emerged for pentacene on thick PVA films (600 nm). As proved by Takebayashi et al.,the emerging electron transport is due to the Na+ ions in PVA,which is introduced by synthesis process. The Na+ ions change the character of the injection barrier at the Au/semiconductor junction and enable the n-type operation. This phenomenon reveals that electron transport is possible even on the hydroxyl-rich surfaces.

Figure 4. Hole (a) and electron (b) mobility of TIPS-pentacene single crystals on various polymer dielectrics.

Figure 5. FET characteristics of TIPS-pentacene single-crystals grown on PVP. Typical transfer of the FETs in p-channel operation (a) and n-channel operation (b)[1TD$DIF].

Figure 6. FET characteristics of TIPS-pentacene single-crystals grown on PVA (1 mg/ mL) (a,b) and PVA (10mg/mL) (c-f),respectively. (a-d) Typical transfer characteristics of FET in π- (a,c) and n- (b,d) channel operation mode. (e,f) Output characteristics of FET in π- (e) and n- (f) channel operation mode.

3.2. The effect of polar impurities on electron transport

Next,we examined the effect of polar impurities to the electron transport. 15 devices based on TIPS-pentacene single crystals grown on PMMA were exposed for 2 min (Fig. 7a) to the vapor of a variety of polar molecules including CHCl3,CH2Cl2,1,2-DCE and THF,and the FET performance before and after the vapor exposure was compared. After being exposed to the CHCl3 vapor,the electron mobility sharply drops (Fig. 7b,green line) from 0.15 cm2 V-1 s-1 to 0.0074 cm2 V-1 s-1. Because the morphology of the PMMA dielectric and single crystals were little changed after treated in CHCl3 vapor (Fig. 7c and d),the reduced electron mobility is not due to the erosion of the PMMA and TIPS-pentacene single crystals. Instead,the lowered mobility is attributed to the incorporation of trace amount of CHCl3 near the conduction channels. Similar to the surface polarization effects of the dielectrics [33, 57-59],the incorporated CHCl3 molecules,as polarizable sources [69],will introduce dipolar disorder to the FET channels and lead to reduced electron mobility. As a supporting evidence for the CHCl3 incorporation,the reduced electron transport partially recovered to 0.03 cm2 V-1 s-1 after the devices were heated for 2 h at 80 ℃ in the N2 glovebox (Fig. 7b,blue line). Consistently,exposure to other polar vapors of CH2Cl2,1,2-DCE and THF did not change the morphology of the PMMA dielectric and single crystals (Fig. S3 in Supporting Information),but greatly reduced electron mobility (Fig. 8 and Fig. S4 in Supporting information). Also,the reduced mobility partially recovered after heat treatment to remove the polar impurities.

Figure 7. (a) Schematic representations of the method to treat the crystals with polar solvent vapor. (b) Sqrt (IDS) vs. VG curves of the as-prepared, CHCl3-treated and heat-treated TIPS-pentacene single crystals, and the electron mobility calculated from each curve has been marked. Inset: Comparison of transfer characteristics among the as-prepared, CHCl3-treated and heat-treated crystals. (c, d) AFM images of PMMA dielectric and TIPS-pentacene treated by CHCl3 vapor, respectively.

Figure 8. Statistical charts of electron mobility treated by polar solvent, including CHCl3 (a), CH2Cl2 (b), 1,2-DCE (c) and THF (d), and heating.

4. Conclusion

In summary,we have demonstrated the effect of gate dielectrics and polar impurities on the electron transport in TIPS-pentacene single crystals. Electron transport emerges in the crystals on PMMA,PS,and a bilayer dielectric of PMMA on P(VDF-TrFE-CFE),confirming the recently reported unusual electron transport of TIPS-pentacene single crystals on BCB dielectric. In contrast,electron transport is suppressed in the crystals on PVP,in consistent with the known trap effect of hydroxyl groups (in PVP). Electron transport of the crystals on PVA depends on the PVA thickness and,on thick one of 40 nm,the electron mobility reaches as high as 0.14 cm2 V-1 s-1. Among these five dielectrics,the highest electron mobility of 0.48 cm2 V-1 s-1 has been achieved in the crystals on PMMA,which is attributed to hydroxyl-free and relatively low-k value of the PMMA dielectric as well as the smooth crystal morphology. In addition,the electron transport is sensitive to the polar impurities. Exposure to the vapor of a variety of polar solvents including CHCl3,CH2Cl2,1,2-DCE and THF results in dramatically reduced electron mobility. These results have the potential to lead to design criteria for n-channel and ambipolar OFETs.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at

Acknowledgments This work was supported by the 973 Program (No. 2014CB643503),National Natural Science Foundation of China (Nos. 51373150,51461165301),and Zhejiang Province Natural Science Foundation (No. LZ13E030002).
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