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Reduced Recombination in High Efficiency Molecular Nematic Liquid Crystalline: Fullerene Solar Cells
Advanced Energy Materials, Volume: 6, Issue: 22, Start page: 1600939
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Bimolecular recombination in bulk heterojunction organic solar cells is theprocess by which nongeminate photogenerated free carriers encounter eachother, and combine to form a charge transfer (CT) state which subsequentlyrelaxes to the ground state. It is governed by the diffusion of the slower andf...
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Bimolecular recombination in bulk heterojunction organic solar cells is theprocess by which nongeminate photogenerated free carriers encounter eachother, and combine to form a charge transfer (CT) state which subsequentlyrelaxes to the ground state. It is governed by the diffusion of the slower andfaster carriers toward the electron donor–acceptor interface. In an increasingnumber of systems, the recombination rate constant is measured to be lowerthan that predicted by Langevin’s model for relative Brownian motion and thecapture of opposite charges. This study investigates the dynamics of chargegeneration, transport, and recombination in a nematic liquid crystallinedonor:fullerene acceptor system that gives solar cells with initial power conversionefficiencies of >9.5%. Unusually, and advantageously from a manufacturingperspective, these efficiencies are maintained in junctions thickerthan 300 nm. Despite finding imbalanced and moderate carrier mobilitiesin this blend, strongly suppressed bimolecular recombination is observed,which is ≈150 times less than predicted by Langevin theory, or indeed, morerecent and advanced models that take into account the domain size andthe spatial separation of electrons and holes. The suppressed bimolecularrecombination arises from the fact that ground-state decay of the CT state issignificantly slower than dissociation.
organic solar cells, reduced bimolecular recombination, nematic liquid crystal donor
Faculty of Science and Engineering