(579b) Analysis of Charge Carrier Transport in Organic Photovoltaic Thin Films and Nanoparticle Assemblies

Active layers consisting of organic polymers have generated significant interest in photovoltaic device fabrication.  Organic photovoltaic (OPV) devices are much more cost-effective to fabricate than conventional inorganic photovoltaic (IPV) devices because of the easier synthesis of the organic active layers.  However, the low carrier mobilities in the polymeric layers limit the efficiency of OPV devices.  Charge carrier mobilities are typically estimated experimentally based on time-of-flight (TOF) methods, where a transient current is produced due to the excess charge carrier transport through the OPV active layer.

We present a systematic analysis of charge carrier transport in OPV devices based on phenomenological charge carrier transport models that we have developed for predicting transient currents in devices with active layers composed of P3HT and PCBM polymers, as well as their blends.  These are transient drift-diffusion models for electron and hole transport in the active layer, coupled self-consistently with Poisson’s equation for the electric field in the layer.  Charge trapping and detrapping processes also are accounted for. Comparisons between the model predictions and experimental measurements of transient photocurrents are used to validate our transport models.  Fitting the modeling predictions to the experimental data of photocurrent evolution is used to examine the propensity of the material to generate charge, determine the value of charge mobility constants (in field-dependent mobility expressions), and analyze the kinetics of trapping and detrapping processes.  We have investigated effects of materials morphology by fitting transient photocurrent data in active layers consisting of both thin films and nanoparticle assemblies and comparing the fitting outcomes.  Moreover, we have analyzed the effect on charge carrier transport of nanoparticle surface characteristics, as well as of thermal annealing of both thin-film and nanoparticle-assembly active layers.  The model predictions provide valuable input toward synthesis of new nanoparticle assemblies that lead to improved OPV device performance.