Calculating the trap density of states in organic field-effect transistors from experiment: A comparison of different methods

The spectral density of localized states in the band gap of pentacene (trap\nDOS) was determined with a pentacene-based thin-film transistor from\nmeasurements of the temperature dependence and gate-voltage dependence of the\ncontact-corrected field-effect conductivity. Several analytical methods to\ncalculate the trap DOS from the measured data were used to clarify, if the\ndifferent methods lead to comparable results. We also used computer simulations\nto further test the results from the analytical methods. Most methods predict a\ntrap DOS close to the valence band edge that can be very well approximated by a\nsingle exponential function with a slope in the range of 50-60 meV and a trap\ndensity at the valence band edge of approx. 2x10E21 eV-1cm-3. Interestingly,\nthe trap DOS is always slightly steeper than exponential. An important finding\nis that the choice of the method to calculate the trap DOS from the measured\ndata can have a considerable effect on the final result. We identify two\nspecific simplifying assumptions that lead to significant errors in the trap\nDOS. The temperature-dependence of the band mobility should generally not be\nneglected. Moreover, the assumption of a constant effective accumulation layer\nthickness leads to a significant underestimation of the slope of the trap DOS.\n