Gate-field dependent photosensitivity of soluble 1,2,4,5-tetra(5′- hexyl- [2, 2′] terthiophenyl-5-vinyl)-benzene based organic thin film transistors

The photoresponsive current-voltage characteristics of organic thin film transistors (OTFTs) have been studied as a function of gate-bias. For the active layer of the OTFTs, soluble 1,2,4,5-tetra(5′ -hexyl- [2, 2′] terthiophenyl-5-vinyl)-benzene materials have been used, and the thickness of the active layers varied. The photosensitivity of the OTFTs was controlled through both gate-bias (V_g) and incident light power. With increasing incident light power, the photosensitivity decreased during the source-drain current actively varied with V_g [i.e., at on-state with accumulated hole channel (V_g < V_onset)], while it increased for V_g≥V_onset (at off-state without accumulated hole channel). These variations are caused by two kinds of photocurrent mechanisms: One based on the photovoltaic effect for V_g < V_onset and another based on the photoconductive effect for V_g V _onset. The maximum photosensitivity of OTFTs was found to be approximately 40 times higher in the on-state than in the off-state due to the contribution of photovoltaic effect in the on-state.