An analysis of the difference in behavior of top and bottom contact organic thin film transistors using device simulation

This paper presents a systematic analysis of an already reported phenomenon, namely, the difference in device performance of top and bottom contact organic thin film transistors (OTFT) by combining experiments and two-dimensional device simulations. The mobility of the as measured devices in the bottom contact OTFT is found to be lower by two orders of magnitude than the top contact structure, which is generally attributed to the higher metal-semiconductor contact resistance in the bottom contact devices due to lower contact area. However, we found that this large mobility difference exists even after correcting for the metal-semiconductor contact resistance through transfer line method (TLM). This result suggests that structural differences are playing a dominant role in lowering down the performance of bottom contact devices. This effect is then systematically investigated through two-dimensional physics-based numerical simulations by considering several structural inhomogenities around the contacts. The main reason for such an occurrence is attributed to the poor morphology (or comparatively low mobility) of pentacene films around the source/drain electrodes in the bottom contact devices. Finally, we also show a reasonable match between the simulated and experimental device characteristics, enabling calibration of the simulator for further use in design of OTFTs.