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, the performance of organic solar cells is correlated 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 the dissociation of excitons and transport of free charges to avoid recombination, leading to high photovoltaic performance. A new method is therefore proposed to improve the photovoltaic performance by simultaneously tuning the film-depth-dependent optical and electronic distributions.