Modeling Efficient Organic Photovoltaic Cells
The quest for higher efficiency organic photovoltaic cells (OPVs) is dependent on furthering the current understanding of fundamental device physics. At the heart of OPV operation is the diffusion of bound electron-hole pairs, or excitons, and their dissociation into free charge carriers at a donor-acceptor interface. The characteristic length an exciton can diffuse is called the diffusion length (LD). Knowledge of LD for OPV active materials of interest allows for intelligent device design where no excitons are generated more than a diffusion length away from a donor-acceptor interface. There are currently no methods available for the direct measurement of LD, instead one-dimensional models describing exciton motion with the diffusion equation are used to connect a value of LD to experimental data. Further complicating matters, not all device designs can be understood by the simple 1-D diffusion equation. This project seeks to create three-dimensional models of exciton diffusion at the molecular instead of device level. By employing the Monte Carlo method, exciton diffusion at the molecular level can be modeled by a series of energy transfer events between molecules instead of as diffusion across an active layer. The rates of energy transfer events are described by photophysical parameters which are easily obtainable for the OPV active materials of interest. The better understanding of exciton diffusion enabled by this work will guide the development of future high-efficiency OPVs.