Proceedings of nanoGe Spring Meeting 2022 (NSM22)
DOI: https://doi.org/10.29363/nanoge.nsm.2022.134
Publication date: 7th February 2022
Over the past five years, the solar power conversion efficiencies of organic solar cells (OSC) have greatly improved from 11% to 18%, closing the gap with their inorganic and hybrid counterparts. The major breakthrough behind the rapid efficiency improvement is the development of non-fullerene acceptor molecules, replacing the traditional fullerene molecules as electron-accepting materials. Understanding the photophysical processes underlying these high-performance materials is crucial to OSC research. In this talk, I will present transient optical spectroscopy results on non-fullerene OSC blends with small interfacial energy offsets. By optically probing the time evolution of excited states, we show that free charges are generated via thermal activation of interfacial charge-transfer (CT) states on a hundred picosecond timescale. Reduced charge separation rate is observed at lower temperatures, leading to increasing charge recombination either directly at the donor-acceptor interface or via the emissive singlet exciton state. A kinetic model is used to rationalize the results, showing that although photogenerated charges have to overcome a significant Coulomb potential to generate free carriers, OSC blends can achieve high photocurrent generation yields even at reduced temperatures given that the thermal dissociation rate of charges outcompetes the recombination rate.
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