Tuning the Threshold Voltage in Organic Thin-Film Transistors by Local Channel Doping Using Photoreactive Interfacial Layers

In recent years, organic thin-fi lm transistors (OTFTs) have attracted a great deal of attention due to their potential applications in low cost sensors, [ 1 ] memory cards, [ 2 ] and integrated circuits. [ 3 ] Great efforts are under way to design OTFTs with high performance, high stability, high reproducibility, and low cost. [ 4 ] Two of the most crucial device parameters are the charge carrier mobility and the threshold voltage ( V Th ). Concerning the mobility, the main goals for most applications is its maximization. [ 5 ] For V Th , the situation is more complex: for example, for integrated circuits it would be desirable to tune V Th over a broad range, [ 6 ] e.g., for inverter applications. In silicon technology, complementary circuits that consist of p-channel and n-channel transistors are typically used. [ 7 ] There have been many attempts to adapt this technology to OTFTs and fabricate organic complementary inverters. [ 2 , 8 , 9 ] They, however, suffer from poor n-type transistor performance and/or air instability of n-type semiconductor materials. An alternative approach is the adaptation of unipolar depletion-load inverters enabling simplifi ed processing, even if they do not provide the low power consumption and the simple circuit design intrinsic to complementary logic. [ 10 , 11 ] Depletion-load inverters consist of an enhancement-mode driver transistor and a depletionmode load transistor and can be realized using only p-type OTFTs. So far there have been attempts to achieve this target by using a level shifter [ 12 , 13 ] or a dual gate structure. [ 14 ] The main objective is to fi nd a reproducible method to realize driver and