Charge Carrier Transport Modeling in Organic Photovoltaic Devices with Active Layers of P3HT/PCBM Self-Assembled Nanoparticles

Conjugate semiconducting polymer nanoparticles are fabricated in aqueous dispersions to reduce the usage of chloroarene solvents for health and safety reasons and have potentially significant benefits in organic solar cell manufacturing.  In this presentation, we study the performance of organic photovoltaic (OPV) devices with active layers consisting of P3HT and PCBM self-assembled nanoparticles.  The two types of active layer morphologies examined are P3HT/PCBM blend nanoparticle assemblies, where individual nanoparticles contain both donor and acceptor polymer materials, and separate nanoparticle assemblies, where individual nanoparticles contain either the donor or the acceptor material only.

We present a systematic analysis of charge carrier transport in these OPV devices based on deterministic charge carrier transport models.   The models account for transient drift and diffusional transport of electrons and holes together with charge carrier trapping and detrapping kinetics in the active layer, coupled self-consistently with Poisson's equation for the electric field in the layer.  Dynamics of free charge carrier bimolecular recombination, as well as charge pair dissociation and geminate recombination processes also are accounted for.  Photocurrent-voltage (I-V) relations at steady state are generated for the analysis of OPV device performance.  We demonstrate the improvement in power conversion efficiency (PCE) by the introduction of interlayers between the active layer and the electrodes and by the reduction of the energy barriers for charge extraction.  Fitting the modeling predictions to experimental measurements of I-V device characteristics determines key kinetic parameters such as exciton generation rate, zero-field electron and hole mobilities, electron-hole distance in bounded charge pairs, as well as geminate and bimolecular recombination rates.  We analyze the effects on PCE of nanoparticle size in both types of P3HT/PCBM nanoparticle assemblies and of P3HT:PCBM nanoparticle ratio in separate nanoparticle assemblies.  The model predictions provide valuable input toward developing strategies for synthesis of active layers with prescribed morphology that optimizes OPV device performance.