Recycling the Energy of Indoor Light: Highly Efficient Organic Photovoltaics via a Ternary Strategy

A ternary approach in organic photovoltaics (OPVs) is simple and reliable to effectively tune the optical, electrical, and morphological properties of the photoactive layer for high efficiency as well as long-term stability under 1 sun. However, there have been few papers reporting the benefits of ternary systems for recycling the energy of indoor light, which has narrow emission spectra and weak illumination compared to outdoor light. In this study, by using two compatible donor polymers (PM7 and PM7 D1) with slightly different band gaps and similar chemical structures, we demonstrate a simultaneous modulation of light absorption and molecular packing under ambient conditions, which resulted in efficient indoor OPVs exhibiting power conversion efficiency (PCE) over 20% under 1000 lx of warm white light-emitting diode (2900 K). From morphological analysis, we infer that PM7 serves to seed the nucleation of PM7 D1 in the ternary blend, templating its crystallization and alignment along the PM7 backbone. Such templating effect leads to increased domain spacing and relative degree of crystallinity (rDoC) compared to those of each binary system. We further show that higher rDoC helps suppress both bimolecular and trap-assisted recombination of photogenerated charges in the ternary devices. As a result, the complementary absorption and synergistic molecular assembly of the two donor polymers enhance the short-circuit current density as to increase the average PCEs from 9.6 to 10.3% under 1 sun and from 18.7 to 20.0% under 1000 lx. We envision that our strategy of incorporating both a planar and a flexible donor polymer with similar chemical structures can be generally applicable to attain a high-performance ternary OPV under both 1 sun and indoor light.