Decoupling Photocurrent Loss Mechanisms in Photovoltaic Cells Using Complementary Measurements of Exciton Diffusion

摘要

Significant work has been directed at measuring the exciton diffusion length (L-D) in organic semiconductors due to its significance in determining the performance of photovoltaic cells. Several techniques have been developed to measure L-D, often probing photoluminescence or charge carrier generation. Interestingly, in this study it is shown that when different techniques are compared, both the diffusive behavior of the exciton and active carrier recombination loss pathways can be decoupled. Here, a planar heterojunction device based on the donor-acceptor pairing of boron subphthalocyanine chloride-C-60 is examined using photoluminescence quenching, photovoltage-, and photocurrent-based L-D measurement techniques. Photovoltage yields the device relevant L-D of both active materials as a function of forward bias subject to geminate recombination losses. These values are used to accurately predict the photocurrent as a function of voltage, suggesting geminate recombination is the dominant mechanism responsible for photocurrent loss. By combining these measurements with photocurrent and photoluminescence quenching, the intrinsic L-D, as well as the voltage-dependent charge transfer state dissociation and charge collection efficiencies are quantitatively determined. The results of this work provide a method to decouple all relevant loss pathways during photoconversion, and establish the factors that can limit the performance of excitonic photovoltaic cells.