Charge carrier mobility in conjugated organic polymers – Case studies using multi-step computational approach

Organic solar cells, because of their lightweight, low cost and processing flexibility, are attracting considerable attention in the field of photovoltaic cells. Recently, multiscale computational methods have been proposed to study mobilities of organic conjugated polymers that are used in solar cells. In this study, we use a similar multi-step method to investigate charge transport in ordered and orientationally disordered conjugated fluorene and carbazole based organic polymers. The multi-step approach employed in this work involves the use of the density functional theory (DFT), semiempirical (ZINDO) and Monte Carlo (MC) theoretical methods. We compare the one-dimensional (1D) and the three-dimensional (3D) mobility simulation results with experimental values when possible. We find that the simplified multi-step approach used in ordered systems can predict mobility trends that correlate closely with their inter-molecular distances as determined by X-ray experiments. However, the calculated magnitudes of mobilities are two (or more) orders of magnitude higher than the corresponding experimental values for the pristine films. 3D orientationally disordered simulations give decreased mobilities that are roughly of the same order of magnitude as the experimental values. It is known that, currently, numerous approximations are involved even in the most sophisticated multiscale mobility simulations. Our results suggest that, for the purpose of obtaining expediently reasonable mobility values and trends for many conjugated polymers, a simplified multi-step approach can be adequate.