Numerical modeling of time-resolved photocurrent in organic semiconductor films

Abstract

We performed numerical simulations of transient photocurrents in organic thin films, in conjunction with experiments. This enabled us to quantify the contribution of multiple charge generation pathways to charge carrier photogeneration, as well as extract parameters that characterize charge transport, in functionalized anthradithiophene (ADT-TES-F) films prepared using two different deposition methods: drop casting on an untreated substrate and spin casting on a pentauorobenzenethiol (PFBT)-treated substrate. These deposition methods yielded polycrystalline lms with considerably larger grain sizes in the case of the spin cast lm. In both drop cast and spin cast films, simulations revealed two competing charge photogeneration pathways: fast charge generation on a picosecond (ps) or sub-ps time scale with efficiencies below 10%, and slow charge generation, on the time scale of tens of nanoseconds, with efficiencies of 11-12% in drop cast and 50-60% in spin cast films, depending on the applied electric field. The total charge photogeneration efficiency in the spin cast sample was 59-67% compared to 14-20% in the drop cast sample, whereas the remaining 33-41% and 80-86%, respectively, of the absorbed photon density did not contribute to charge carrier generation on these time scales. The spin cast film also exhibited higher hole mobilities, lower trap densities, shallower traps, and lower charge carrier recombination, as compared to the drop cast lm. As a result, the spin cast lm exhibited higher photocurrents despite a considerably lower lm thickness (and thus reduced optical absorption and cross section of the current flow).