Air-stable and high-performance organic field-effect transistors based on ordered, large-domain phthalocyanine copper thin film
Introduction
Organic field-effect transistors (OFETs) based on polycrystalline small molecules have attracted considerable interest from the scientific community owing to its applications in low-cost, flexible, large-area electronics such as organic active matrix displays, sensors and radio-frequency identification card [1], [2], [3], [4], [5], [6]. In recent years, tremendous progress has been achieved in both the design of organic semiconductors and the fabrication techniques to optimize the performance of OFET devices. There are several organic small molecule materials exhibiting excellent field-effect mobility, which can rival to that of hydrogenated amorphous silicon [7], [8]. However, the stability of OFETs should be considered for practical applications besides the charge mobility [9], [10]. The family of phthalocyanines represents one of the most promising candidates for organic electronics. Particularly, phthalocyanine copper (CuPc) was widely studied as one of good small molecular semiconductors because of their large π-conjugated system, excellent film growth and unique electronic characteristics [11]. A large number of studies have been devoted to improve the carrier mobility by optimizing the thin film morphology and engineering the interface of devices [12], [13], [14], [15]. Nevertheless, because of the inherent anisotropy of organic small molecules and the van der Waals nature of the intermolecular interactions, it is difficult to obtain large-size domains of ordered crystallites and continuous CuPc thin film, and the deposited films are usually polycrystalline with high-density grain boundaries (GBs). Because of the abrupt collapse of the π-π stack and large disorder in films, the present of the GBs is the main transport barriers for charge carriers, which result in inferior device performances. Furthermore, some studies have also showed that the high-density GBs are responsible for the instability of the OFETs. Mariucci et al. have demonstrated that higher density of GBs contributes to a higher sensitivity to air ambient [16]. Chosh et al. have also found that OFETs with larger grain size could lead to higher stability [17]. Weak epitaxial growth (WEG) is a very effective technique to fabricate highly ordered organic semiconductor thin films with large crystalline domains (>10 μm2) for high-performance OFETs [18], [19]. The highly oriented and continuous CuPc thin film with standing-up molecular orientation on para-sexiphenyl (p-6P) layer fabricated by the WEG technique leads to a significant improvement in the carrier transportation in OFET devices. However, it seldom studies the air stability of OFET fabricated by WEG.
In present research we studied how the domain size and boundary influence the electrical properties and the stability of CuPc thin films fabricated by the WEG method. The mobility of CuPc OFETs fabricated by WEG was strongly domain-size dependent. A hole field-effect mobility as high as 0.18 cm2/Vs was obtained in CuPc devices with a large crystalline domain size of about 60 μm2, which can be comparable to CuPc single crystals [20], [21]. In addition, the CuPc/p-6P FETs with large domains of ordered crystallites show high stability as exposed to air ambient for 20 days. But the devices with randomly aligned crystallites stored in atmosphere with the same temperature and humidity for 20 days exhibited the large changes of electric characteristics including positive-shifted threshold voltage, much lower Ion/Ioff and mobility. Stored under the same conditions, the oxygen content of the CuPc sample with large domains of ordered crystallites is much less measured by XPS. It is because that more oxygen and water can diffuse into CuPc thin film with randomly aligned crystallites, which has more number of boundaries and higher misorientation degree than the film with large domains of ordered crystallites. The study provides a guide for rational optimization of film morphology for high performance and air stable OFETs.
Section snippets
Experimental
The oriented CuPc-based FETs were fabricated on a heavily doped n-type silicon wafer serving as the gate electrode and substrate. About 200 nm-thick SiO2 layer with a capacitance of Ci = 17 nF/cm2 was deposited on the Si wafer as gate insulator. The SiO2/Si substrate (Si-Mat, silicon materials) was ultrasonically cleaned in acetone, alcohol and distilled water for 15 min respectively, then dried by N2 flow and treated by ozone for 15 min. The CuPc and p-6P materials were purchased from Sigma-Aldrich
Results and discussion
Fig. 1b–d shows the AFM morphologies of 30 nm CuPc film with different size of domains. The CuPc film grown on bare SiO2/Si substrate, consisting of regular crystals of homogeneous size, shows completely disorderly arrangement with large number of boundaries (Fig. 1b). And the boundary is characterized with abrupt collapse of the π-conduction network. Compared with CuPc crystalline films grown on bare SiO2, the morphologies were consisted of oriented domains with different sizes as grown on p-6P
Conclusion
In summary, the device performances and air-stability of CuPc-based OFETs have been studied. The field-effect properties of highly oriented CuPc/p-6p OFETs prepared by WEG has a strong dependence on the domain size, which has been demonstrated based on the analysis of film morphology, electrical properties, and KPFM. The high field-effect mobility of 0.18 cm2/Vs was obtained in CuPc/p-6p OFETs with a large oriented crystalline domain size of 60 μm2. The CuPc/p-6P thin film is very stable even
Acknowledgements
This project was supported in part by the National Natural Science Foundation of China (11334014, 51173205, 51203192, 61306085), the Program for New Century Excellent Talents in University (NCET-13-0598), the China Postdoctoral Science Foundation (2015T80881), Hunan Provincial Natural Science Foundation of China (2015JJ1015) and the Fundamental Research Funds for the Central Universities of Central South University (2015zzts013). Y. G. acknowledges support by National Science Foundation
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