J. Phys. Chem. B 2008, 112, 10405–10410
10405
Molecular Orientation and Interface Compatibility for High Performance Organic Thin Film Transistor Based on Vanadyl Phthalocyanine Liqiang Li,†,‡ Qingxin Tang,† Hongxiang Li,*,† and Wenping Hu*,† Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 and Graduate School of Chinese Academy of Sciences, Beijing 100039, China ReceiVed: January 25, 2008; ReVised Manuscript ReceiVed: June 23, 2008
Organic thin film field-effect transistors (OTFTs) with mobility up to 1.0 cm2 V-1 s-1 and on/off ratio of 106-108 as well as good environmental stability were demonstrated by using vanadyl phthalocyanine (VOPc), a pyramid-like compound with an ultra closely π-stacked structure. The high performance, remarkable stability, low price, easy availability and nontoxicity of VOPc enabled it to be a promising candidate for OTFTs. Furthermore, we found that the mobility of the devices on OTS-modified Si/SiO2 substrates was 2 orders of magnitude higher than that of devices on Si/SiO2 substrates. Significantly, the relationship between field effect property and insulator surface property was explained from two new aspects of distribution of molecular orientation and interface compatibility, which might provide not only a useful model to explain why the surface modification with OTS could largely improve the field-effect performance but also a guide for rational optimization of device structure for higher performance. In addition, the field effect property of VOPc devices under vacuum, i.e., the oxygen doping effect on the VOPc devices, was measured. We found that the hole mobility decreased by several orders of magnitude with decreasing pressure. At a pressure below 10-2 Pa, the device on OTS-modified substrates exhibited ambipolar conduction. These results indicated that the oxygen doping exerted essential effect on the field-effect property of VOPc, which was clearly distinct from that observed for pentacene-based OFETs. 1. Introduction It is particularly attractive to construct high performance organic thin-film field-effect transistors (OTFTs) with high mobility and long environmental stability.1-4 The packing of organic molecules in films definitely plays important roles for high performance OTFTs.5-8 Generally, π-stacked structure is favorable to obtain high mobility due to large intermolecular π-orbit overlap and short intermolecular distance.5-8 However, most reported organic semiconductors with high-performance, e.g., pentacene and rubrene,3,9,10 usually have herringbone structure, and many of them are rather sensitive in air so that their devices easily degrade when operated in ambient conditions. Few OTFTs have been addressed with both high mobility and air stability,11,12 especially of π-stacked organic semiconductors. Moreover, it is also important to understand the relationship between the field-effect property and devices structure in OTFTs, besides developing high performance and stable materials. It has been commonly reported that the fieldeffect property showed substantial dependence on the insulator surface property.13-20 Many groups have observed that the performance can be significantly improved by treating the insulator surface with self-assembling monolayer (SAM) such as octadecyltrichlorosilane (OTS),13-18 but the detailed reason for this improvement is still under scrutiny. Therefore, clarification of the reasons is unequivocally favorable to rational optimization of device structure for high performance devices. * Correspondiong authors. E-mail: (H.L.)
[email protected]; (W.H.) huwp@ iccas.ac.cn. † Institute of Chemistry. ‡ Graduate School of Chinese Academy of Sciences.
Phthalocyanines have attracted attentions in OTFTs for some years after the pioneering works of Bao and Kloc et al.21-23 due to their remarkably chemical and thermal stabilities, nontoxicity and good field-effect performance. The representative candidates such as copper phthalocyanine (CuPc) and copper hexadecafluoro-phthalocyanine (F16CuPc) etc.21-25 have planar molecular geometry and herringbone packing structure. Here, we will introduce vanadyl phthalocyanine (VOPc, Figure 1a), which is different from those planar phthalocyanines, and has a nonplanar, pyramid-like molecular geometry. Its molecules form a face-to-face π-stacked structure (Figure 1b) in its phase II, in which neighboring molecules have very large π-orbit overlap, and the shortest intermolecular π atomic contact is 3.208 Å in the convex pair and 3.212 Å in the concave pair (Figure 1b), respectively.26 Such short intermolecular π atomic contacts are substantially shorter than most of π-stacked compounds (which usually have intermolecular distance at 3.4-3.6 Å)7,8,27-30 and many other organic semiconductors.31-34 This feature (large π-orbit overlap and short intermolecular distance) is encouraging for high performance OTFTs. Ohta et al.35 pioneered in investigating field-effect activity of VOPc in OTFTs (the mobility was calculated at ∼5 × 10-3 cm2 V-1 s-1). After that, Wang et al.36 demonstrated the potential high performance of VOPc by device structure optimization. Evoked by the above considerations and taking the great environmental stability of VOPc into consideration, here we will first demonstrate the high mobility, stability, and reproducibility of the nonplanar, pyramid-like molecular material in OTFTs. Then, the relationship between field-effect property and insulator surface property, i.e., the origin for performance improvement by surface treatment with OTS, will be further examined carefully. It will be demonstrated that the close packing of VOPc
10.1021/jp800879g CCC: $40.75 2008 American Chemical Society Published on Web 08/01/2008
10406 J. Phys. Chem. B, Vol. 112, No. 34, 2008
Figure 1. (a) Molecular structure of VOPc. (b) Molecular stacking of VOPc in phase II as viewed down b-axis. VOPc forms face-to-face π-stacked structure with significant molecular overlap and very short intermolecular distances. (c) Schematic top-contact, bottom-gate device geometry on OTS-modified Si/SiO2 substrates.
molecules, the high interface compatibility between the gate insulator and VOPc layer, and the high molecular orientation in the VOPc active layer coresulted in the high performance of the OTFTs of VOPc on OTS-modified substrates.
Li et al.
Figure 2. (a) Output and (b) transfer characteristics of VOPc OTFTs fabricated on OTS-modified Si/SiO2 substrates at the substrates temperature of 90 °C. (c) Output and (d) Transfer characteristics of VOPc OTFTs fabricated on Si/SiO2 substrates at 90 °C.. The fieldeffect mobility in the saturation region, on/off ratio and threshold voltage (VT) on OTS modified substrates were estimated at around 0.86 cm2 V-1 s-1, 3.8 × 106 and -8.5 V. The corresponding values on Si/SiO2 substrates were around 0.006 cm2 V-1 s-1, 2.5 × 105 and -14.7 V.
visible-near-infrared (UV-NIR) absorption spectra between 200 and 1500 nm were obtained on a Jasco V-570 instrument. The contact angle (CA) was obtained on a Dataphysics OCA20 contact-angle system under ambient conditions.
2. Experimental Section The heavily doped, p-type Si wafer containing a 300 nmthick SiO2 layer as gate dielectric was used as gate. The wafer was successively washed by water, Piranha solution, water, alcohol, chloroform. Modification of Si/SiO2 wafer (or quartz) with octadecyltrichlorosilane (OTS) was carried out by vapor deposition method.37-39 The clean wafers were dried under vacuum at 100 °C for 0.5 h in order to eliminate the influence of the moisture. After cooling to room temperature, a little drop of OTS was placed near the wafers. Subsequently, this system was heated to 120 °C and maintained for 2 h under vacuum. The modification of SiO2 by (3-chloro)propyltriehoxysilane (CPTS) and vinyltriethoxysilane (VTS) was carried out by immersing silicon wafer in 2% (v/v) toluene for 2 h. Vanadyl phthalocyanine (VOPc) was purchased from Alfa Aesar China (Tianjin) Co. Ltd. and further purified three times by gradient sublimation. Films of VOPc were deposited on the substrates by thermal evaporation under a pressure of (4-6) × 10-4 Pa at a substrate temperature of 90 °C. The deposition rate was held at 0.1-0.3 Å/s to give a total thickness of 60 nm. The deposition rate and film thickness were monitored by ULVAC CRTM-6000. Gold (99.99%) was used as the source, and drain electrodes and were deposited via an interdigital shadow mask (50 nm thick). The channel length and width were 0.11 mm and 5.30 mm, respectively. To minimize the influence of heat (radiation), a small (0.4 mm in diameter) and short (25 mm in length) tungsten wire was used as thermal evaporation boat, and the deposition rate was controlled at around 0.2 Å/s. The FET characteristics were measured with a Keithley 4200 SCS, a Micromanipulator 6150 probe station in a clean and shielded box at room temperature in air. AFM measurement was carried out with Nanoscopy IIIa (USA) using tapping model. X-ray diffraction (XRD) was measured on D/max2500 by the usual θ-2θ method with 40 KV voltage and a Cu KR source (λ ) 1.541 Å). X-ray rocking curves analysis were performed on D/max 2500 looking at the (010) peak. Ultraviolet-
3. Results and Discussion A series of bottom gate, top contact devices were fabricated by using Si/SiO2 and OTS-modified Si/SiO2 substrates. Both devices exhibited good field-effect modulation characteristics (Figure 2). It was also apparent that the devices based on OTS substrates exhibited much better field-effect characteristics (Figure 2a,b) than those devices on Si/SiO2 substrates (Figure 2c,d). For example, the mobility of the devices on Si/SiO2 was calculated at 0.003-0.006 cm2 V-1 s-1 and on/off ratio was at 105-106, while the devices on OTS-modified Si/SiO2 substrates exhibited mobility at 0.3-1.0 cm2 V-1 s-1 and on/off ratio at 106-108. The mobility of the devices on OTS-modified Si/SiO2 substrates was 2 orders of magnitude higher than that of devices on pure Si/SiO2, and the performance parameters of the devices on OTS-modified Si/SiO2 substrates were comparable to that of devices of pentacene (it was a benchmark in OTFTs, showing maximum mobility at 0.3-1.5 cm2 V-1 s-1 and on/off ratio of 105-108 on OTS-modified Si/SiO2 substrates40-43). Moreover, we measured the field-effect property of VOPc under vacuum. The hole and electron mobility of device on OTS substrates versus vacuum pressure were shown in Figure 3a. The hole mobility decreased by several orders of magnitude with decreasing pressure. Furthermore, their threshold voltage became very large (|VT| > 50 V). At the pressure below ∼10-2 Pa, obvious n-type conduction could be observed and increased with decreasing pressure. At the pressure from 10-2 Pa to 3.2 × 10-3 Pa, the device exhibited ambipolar conduction. Figure 3b-d showed the ambipolar transfer and output curves measured at the pressure of 3.2 × 10-3 Pa, respectively, from which the mobility in the saturation region, on/off ratio, and threshold voltage can be calculated to 1.4 × 10-4 cm2 V-1 s-1, 1.1 × 104 and -79.5 V for p-type conduction, and 5.5 × 10-5 cm2 V-1 s-1, 1.2 × 103 and 74.3 V for n-type conduction, respectively. These results indicated that doping of VOPc by trapped oxygen not only changed conduction type,44 but also
Organic Thin Film Transistor
Figure 3. Field-effect property of VOPc device on OTS substrates under vacuum, (a) hole and electron mobility versus vacuum pressure, (b) ambipolar transfer and (c,d) ambipolar output characteristics of VOP devices measured at pressure of 3.2 × 10-3 Pa.
increased the hole transporting ability. The large decrease of hole mobility in vacuum probably was as follows: oxygen doping could change the energy level of VOPc. With the decrease of oxygen concentration, the conduction type of VOPc was converted into n-type, and the hole injection barrier increased gradually, which decreased the hole transporting ability of VOPc.45-48 As for the device on bare substrate, the p-type performance also decreased by several orders of magnitude. However, even at the pressure down to 3.2 × 10-3 Pa, the device only exhibited very weak n-type conduction (
