Proceedings of Online International Conference on Hybrid and Organic Photovoltaics (OnlineHOPV20)
DOI: https://doi.org/10.29363/nanoge.onlinehopv.2020.012
Publication date: 22nd May 2020
Management of the Coulomb interaction in organic LEDs and solar cells
The physics of organic semiconductors is often controlled by large electron-hole Coulomb interactions and by large spin exchange energies. I will discuss recent strategies that allow these interactions to be harnessed for efficient device operation. For LEDs, 3:1 statistical formation of triplet:singlet excitons through electron-hole recombination limits efficiency if the triplet state is non-emissive, but is recovered for systems with reduced exchange energy and strong spin-orbit coupling in organo-metallic systems or for system where triplet-triplet collisions produce singlet excitons efficiently. We have recently demonstrated that π-conjugated radical materials with doublet ground states can operate with high efficiency in the doublet manifold. For organic PV systems, long-range charge separation from the donor-acceptor heterojunction must overcome a substantial Coulomb barrier, which we find always sets up a measurable optical Stark shift that we measure to be 200 meV or more. Whether this separation is ultrafast (sub-picosecond) or slow (>10 psec) depends on the ‘excess’ energy provided by the photogenerated exciton to the separating electron-hole pair, both for fullerene acceptor systems and those with non-fullerene acceptors. Longer time bimolecular recombination should show similar spin statistics to LED operation, causing significant non-radiative recombination for triplet formation where there is easy access to a low-lying localized triplet states. I will present evidence that though fullerene systems generally show rapid bimolecular triplet formation, this can process can be very strongly reduced in some non-fullerene acceptor systems.
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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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