Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
The free carrier generation mechanism in organic photovoltaic cells is not understood and actively debated. Free energy gradient from interfacial to bulk charged states and carrier delocalisation in higher-energy charge transfer states have been suggested as the dominant driving force for charge separation but their contributions have not been quantified yet.
Here we combine broadband transient absorption spectroscopy with optical probing of ultrafast carrier drift in an electric field to gain a quantitative measure of charge separation distances in high performance PTB7:PC71BM solar cells. We find that interfacial charge transfer states and spatially separated charge pairs show distinct absorption spectra. We observe that charge pairs with spatial separation of about 2 nm are generated in blends with optimised morphology in 200 fs after excitation but their dissociation into free carriers is much slower and takes tens-to-hundreds of picoseconds. We observe a linear correlation between the pair dissociation rate and carrier mobility in the blend both showing a decrease by two orders of magnitude in a time span from 1 ps to 3 ns. The rate and efficiency of pair dissociation strongly depends on the blend ratio and nanoscale morphology of the active layer. Our results show a crucial role of high carrier mobility and blend morphology in free carrier generation.
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International Conference on Hybrid and Organic Photovoltaics
International Conference on Hybrid and Organic Photovoltaics (HOPV) is celebrated yearly in May. The main topics are the development, function and modeling of materials and devices for hybrid and organic solar cells. The field is now dominated by perovskite solar cells but also other hybrid technologies, as organic solar cells, quantum dot solar cells, and dye-sensitized solar cells and their integration into devices for photoelectrochemical solar fuel production.
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