Exploring charge pair generation in single-component macromolecular materials for photovoltaic energy conversion

The discovery of several new classes of electron-accepting molecule that work well as electron acceptors at molecular heterojunctions had led to a strong increase in the power-conversion efficiency in organic solar cells, from 11 to 19% in the last 5 years. The impressive performance of the new materials and the range of chemical structures has led to interest in whether alternatives to the traditional ‘bulk heterojunction’ architecture for an organic solar cell can be found.<br/>One approach concerns structures where, through chemical design, electron-rich and electron-poor components of a single macromolecular material are constrained to have a particular geometry relative to each other, being electronically coupled either through space or through bond. In a second intriguing development, devices based on a single molecular material have been reported deliver relatively high charge-generation efficiency in the absence of a donor-acceptor interface. This latter example challenges the current understanding of how free charges are generated in organic semiconductors. Achieving efficient energy conversion with a single macromolecular component would be both interesting and practically useful.<br/>In this work, we report on studies of the design, characterisation and modelling of both types of structure, including a variety of non-fullerene acceptors, and macromolecular compounds with tethered donor and acceptor type domains. By combining experimental characterisation under varying conditions (field, temperature, excitation) with molecular and device-level calculations, we endeavour to relate exciton and charge dissociation efficiency in single-component devices to molecular parameters.