Correlations between performance of organic solar cells and film-depth-dependent optical and electronic variations

In donor: acceptor bulk heterojunction organic solar cells, the chemical miscibility between different components and phase evolution dynamics within thin films often induce phase segregation and molecular aggregation/orientation, both of which are film-depth-dependent. This leads to strong variations of molecular energy levels, photon absorption, exciton generation, charge transfer and transport along film-depth direction. However, currently there is a lack of comprehensive investigation of film-depth-dependent optical and electronic variations on the photovoltaic performance. In this work, using the recently developed film-depth-dependent light absorption spectroscopy which simultaneously reveals vertical optical and electronic variations, we correlate the performance of organic solar cells with film-depth-dependent profiles of photon absorption and charge transport energy levels, which is subsequently compared with experimentally observed open circuit voltage, short circuit current, and efficiency. Because both light interference and vertical material variations contribute to film-depth-dependent exciton generation profiles, the local gradient of transport energy levels which provides extra built-in electric force could accelerate dissociation of excitons and transport of free charges to avoid recombination, leading to high photovoltaic performance. We therefore propose a new method to improve the photovoltaic performance by simultaneously tuning the film-depth-dependent optical and electronic distributions