High-mobility organic thin-film transistors with photolithographically patterned top contacts

Due to its large-area capability and high resolution, photolithography is the preferred patterning method for pentacene thin-film transistors (TFTs) for display and circuit applications [1,2]. Since the morphology of thin pentacene films is very sensitive to solvents and heat [3,4], the photolithographic patterning of the source/drain contacts is ideally performed prior to the pentacene deposition, which explains the general preference for the bottom-contact (coplanar) TFT structure. However, as experiments [5] and simulations [6,7] have shown, the bottom-contact TFT structure is associated with substantially larger contact resistance than the top-contact (staggered) structure, which means that for the same channel length, top-contact TFTs are expected to provide larger transconductance and higher cutoff frequency than bottom-contact TFTs. Here we report on organic TFTs with Au top contacts patterned by ordinary photolithography and wet etching (using common solvents, photoresists, and etchants) having field-effect mobilities (0.4 cm²/Vs) and on/off current ratios (10⁷) similar to those of optimized bottom-contact pentacene TFTs [1,2,5].