Investigation of Energy Transfer in Organic Photovoltaic Cells and Impact on Exciton Diffusion Length Measurements

This work demonstrates an approach for measuring the Forster radius of energy transfer between electron donating and accepting materials commonly used in organic photovoltaic cells (OPVs). While energy transfer processes are surprisingly common in OPVs, they are often incorrectly ignored in measurements of the exciton diffusion length and in models of device performance. Here, the efficiency of energy transfer between an emissive donor and an absorptive acceptor is investigated through complementary experimental and theoretical techniques. This is accomplished by spatially separating the donor and acceptor materials using a wide-energy-gap spacer layer to suppress direct charge transfer and tracking donor photoluminescence as a function of spacer layer thickness. Fitting experimental data obtained for a variety of donor materials allows for the extraction of Forster radii that are in good agreement with predicted values. The impact of donor-acceptor excitonic energy transfer on device performance and on measurements of the exciton diffusion length is also investigated using the archetypical small molecule donor material boron subphthalocyanine chloride (SubPc). An average exciton diffusion length of 7.7 nm is extracted from photoluminescence quenching experiments using SubPc. This value is independent of the quenching material when the role of energy transfer is properly modeled.