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Reduced Recombination in High Efficiency Molecular Nematic Liquid Crystalline: Fullerene Solar Cells
Author(s) -
Armin Ardalan,
Subbiah Jegadesan,
Stolterfoht Martin,
Shoaee Safa,
Xiao Zeyun,
Lu Shirong,
Jones David J.,
Meredith Paul
Publication year - 2016
Publication title -
advanced energy materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 10.08
H-Index - 220
eISSN - 1614-6840
pISSN - 1614-6832
DOI - 10.1002/aenm.201600939
Subject(s) - recombination , chemical physics , materials science , acceptor , charge carrier , ground state , fullerene , electron , dissociation (chemistry) , organic solar cell , polymer solar cell , molecular physics , brownian motion , diffusion , energy conversion efficiency , condensed matter physics , atomic physics , optoelectronics , polymer , physics , thermodynamics , chemistry , biochemistry , quantum mechanics , composite material , gene
Bimolecular recombination in bulk heterojunction organic solar cells is the process by which nongeminate photogenerated free carriers encounter each other, and combine to form a charge transfer (CT) state which subsequently relaxes to the ground state. It is governed by the diffusion of the slower and faster carriers toward the electron donor–acceptor interface. In an increasing number of systems, the recombination rate constant is measured to be lower than that predicted by Langevin's model for relative Brownian motion and the capture of opposite charges. This study investigates the dynamics of charge generation, transport, and recombination in a nematic liquid crystalline donor:fullerene acceptor system that gives solar cells with initial power conversion efficiencies of >9.5%. Unusually, and advantageously from a manufacturing perspective, these efficiencies are maintained in junctions thicker than 300 nm. Despite finding imbalanced and moderate carrier mobilities in this blend, strongly suppressed bimolecular recombination is observed, which is ≈150 times less than predicted by Langevin theory, or indeed, more recent and advanced models that take into account the domain size and the spatial separation of electrons and holes. The suppressed bimolecular recombination arises from the fact that ground‐state decay of the CT state is significantly slower than dissociation.