Active layer limitations and non-geminate recombination in polymer-fullerene bulk heterojunction solar cells
A possible route to 15% power conversion efficiency from donor-acceptor bulk heterojunction solar cells is to optimize the energy levels of the donor and acceptor materials for maximum light harvesting while maintaining a high open circuit voltage. In order to attain this high efficiency, the ability to coat thick active layers is required. Thick active layers are necessary to achieve high external quantum efficiencies (90%) for all photon energies larger than the optical bandgap, while maintaining a high fill factor (FF>0.7). However, as demonstrated-based on an extensive literature review of more than 30 high performing copolymer donor-acceptor heterojunctions-achieving a high fill factor using relatively thick (> 300 nm) active layers is very challenging. Reduced bimolecular recombination as a fundamentally important characteristic of high fill factor bulk heterojunction solar cells with large active layer thicknesses is discussed using transient charge extraction measurements. Twelve different models aimed at explaining this reduced bimolecular recombination in bulk heterojunction solar cells are discussed and compared.