Integrating theory, synthesis, spectroscopy and device efficiency to design and characterize donor materials for organic photovoltaics: a case study including 12 donors
There have been remarkable improvements in the power conversion efficiency of solution-processable Organic Photovoltaics (OPV) have largely been driven by the development of novel narrow bandgap copolymer donors comprising an electron-donating (D) and an electron-withdrawing (A) group within the repeat unit. The large pool of potential D and A units and the laborious processes of chemical synthesis and device optimization, has made progress on new high efficiency materials slow with a few new efficient copolymers reported every year despite the large number of groups pursuing these materials. In our paper we present an integrated approach toward new narrow bandgap copolymers that uses theory to guide the selection of materials to be synthesized based on their predicted energy levels, and time-resolved microwave conductivity (TRMC) to select the best-performing copolymer–fullerene bulk heterojunction to be incorporated into complete OPV devices. We validate our methodology by using a diverse group of 12 copolymers, including new and literature materials, to demonstrate good correlation between (a) theoretically determined energy levels of polymers and experimentally determined ionization energies and electron affinities and (b) photoconductance, measured by TRMC, and OPV device performance. The materials used here also allow us to explore whether further copolymer design rules need to be incorporated into our methodology for materials selection. For example, we explore the effect of the enthalpy change (ΔH) during exciton dissociation on the efficiency of free charge carrier generation and device efficiency and find that ΔH of -0.4 eV is sufficient for efficient charge generation.