Going beyond polaronic theories in describing charge transport in rubrene single crystals

We show that the charge transport properties of single crystals of rubrene can be well described without using polaron theories along the two high-mobility axes. The charge carriers can be considered as holes with coupling to the lattice but not to the degree, which requires the use of polaron theories. It is possible to use the Boltzmann Transport Equation (BTE) in the relaxation time approximation (RTA) to evaluate mobilities due to various scattering mechanisms after introducing a transport reduction factor (PTRF). TRF takes into account the fraction of charge carriers, which have path lengths that are larger than the lattice constant, and permits the use of the BTE in the RTA even when the magnitude of the overall mobility is lower than the value typically required for the use of the BTE. We are then able to calculate mobilities due to various scattering mechanisms. We calculate the effective electron-phonon coupling constant from the published values for various phonon modes. The values of the effective mass from calculations and measurements reported in the literature vary slightly; we assume an intermediate value for the effective mass. With no fitting parameters needed for calculating temperature-dependent mobilities for trap-free crystals, we are able to get excellent agreement with the measured values along the two high-mobility crystallographic directions. In samples, with some trapping, a small density of exponentially distributed trap states is assumed and gives a very good fit to the measured data. Our work provides strong evidence that it is not necessary to invoke polaronic effects to understand charge transport in rubrene crystals. Published under license by AIP Publishing.