Origin of mobility degeneration at high gate bias in organic thin film transistors based on carriers´ freeze to surface charges

Traditionally, mobility of the bulk organic semiconductor materials was thought to increase with the increase of carriers´ concentration. However, the experimental results show that the field effect mobility in OTFTs degenerate with the increasing gate voltage at high gate bias. Recently, even the source/drain current was found to decrease at high gate voltage bias. To interpret this phenomenon, we proposed a model based on carriers´ freeze to surface charges. According to this model, the charge carriers´ concentration in semiconductor layer as well as surface charges, would increase with the increase of the gate voltage, and combination of them results in the mobility firstly to increase then to decrease with the increasing the gate voltage.