Improved morphology and bias stress study of a naphthalenetetracarboxylic diimide bottom contact field effect transistor

Abstract

Organic thin film field effect transistors (OFETs) have attracted considerable interest for use in a number of applications such as flexible active matrix displays, chemical sensors, radio frequency identification tags and labels, smart cards, and large-area logic circuits. OFETs have been studied in one of two configurations- top contact and bottom contact. Historically, many reports have illustrated the characteristics of bottom contact OFETs based on p-channel materials such as pentacene, copper phthalocyanine (CuPc) and sexthiophene in which holes are the majority carriers. However, very few studies have investigated n-channel organic semiconductor growth on substrates with prepatterned OFET metal contacts, relevant to bottom-contact devices. The investigation and development of materials that can be used in n-channel organic transistors, particularly those that can be operated in air, is crucial for the development of practical organic electrics, such as the most power-efficient families of logic elements called “complementary” circuits, in which both hole — carrying (p- channel) and electron carrying (n-channel) semiconductors are required8–10. In this study, bottom contact OFETs with various surface treatments based on 1, 4, 5, 8-naphthalene-teracarboxylic di-imide (NTCDI) derivatives with three different fluorinated N-substituents, systematically investigated with a particular emphasis on the interplay between the morphology of the organic semiconductor films and the electrical device properties. The topography of the NTCDI bottom contact device without any surface treatment was first characterized by AFM (Fig. 1). It can be observed that the growth of NTCDI films on Si/SiO 2 substrates is dominated by crystalline grain structures, however their growth on bare gold is dominated by a dewetting resulting in a rough and amorphous film. A clear “gap” is formed at the interface between Si/SiO 2 and Au substrates. In order to overcome the morphology limitations, different methods have been studied: 1) surface chemical modifications of Au electrode and Si/SiO 2 substrates are applied to improve the morphology in the OFET channel close to the electrode edge. Because SAM preparation normally is time consuming and some SAM-forming thiols have unpleasant odors, we also investigated an alternative means of improving this near-contact NTCDI morphology without SAM modification, namely, spin coating a thin layer of insulating polymer on the gate-gate dielectric substrate. 2) semiconductor-contact thickness ratios are optimized to allow charge injection through larger interface areas. AFM images of the NTCDI bottom contact devices with surface treatment are shown in Fig. 2. It can be observed that the relatively unstructured film is still grown on top of the gold electrodes and the terrace crystal structure was formed on the silicon substrate. However, compared to the AFM image of the untreated F15-NTCDI device, it is very difficult to detect any “gap” area at the interface between SiO 2 and the Au electrode. Based on a series of treatments, a large range of performances of bottom contact side-chain-fluorinated NTCDI OFETs (mobility from 1×10-6 to 8×10-2 cm2/Vs, on/off ratio from 102 to 105) were obtained. The surface treatments enabled systems that showed essentially OFET activity to perform nearly as well as top contact devices. In addition, for the fresh bottom contact NTCDI device, the effect of gate bias stress on the tens-of-minutes time scale, during which the threshold voltage (Vt) shifted and relaxed with similar time constants, was observed.