Quantitative analysis of charge-carrier trapping in organic thin-film transistors from transfer characteristics

A dynamic method for quantifying the amount and mechanism of trapping in organic field effect transistors (OFETs) is proposed. It exploits transfer characteristics acquired upon application of a triangular waveform gate sweep V (G). The analysis of the transfer characteristics at the turning point V (G)=-V (max) between forward and backward gate sweeps, viz. around the maximum gate voltage V (max) applied, provides a differential slope Delta m which depends exclusively on trapping. Upon a systematic change of V (max) it is possible to extract the initial threshold voltage, equivalent to one of the observables of conventional stress measurements, and assess the mechanism of trapping via the functional dependence on the current. The analysis of the differential logarithmic derivative at the turning point yields the parameters of trapping, as the exponent beta and the time scale of trapping tau. In the case of an ultra-thin pentacene OFET we extract beta=1 and tau=10(2)-10(3) s, in agreement with an exponential distribution of traps. The analysis of the hysteresis parameter Delta m is completely general and explores time scales much shorter than those involved in bias stress measurements, thus avoiding irreversible damage to the device.