Device performance and density of trap states of organic and inorganic field-effect transistors

Trap states, present in any semiconductor, have a large influence on charge transport as well as various other physical processes relevant for device performance. Therefore, quantifying the density of trap states (trap DOS) in a semiconductor is a crucial step towards understanding and improving field-effect transistors (FET), organic photovoltaic cells (OPV) and organic light-emitting diodes (OLED). We present a method to determine the free vs. total charge carrier density in FETs, and therefore the trap DOS, through full normalization of the transfer curve. We apply this method to many different materials, prepared under a wide range of processing conditions, e.g. organic single crystals, thermally evaporated and inkjet printed thin-films, leading to various degrees of order/disorder. They are compared to inorganic thin-films. A quantitative analysis of the spectral density of the trap DOS reveals the trap DOS in p- and n-type semiconductors to be very similar provided they have a similar morphology. The variation by 3 orders of magnitude is dominated by the degree of crystalline order. Further, the trap DOS in organic materials is essentially the same as in inorganic materials, again, provided they have a similar morphology. Surprisingly, ink-jet printed organic polymers have a relatively low trap DOS, which is in between the one of organic single crystals and thin-films.