Optimizing the open-circuit voltage and overall performance for indoor organic photovoltaics via wide-gap polymeric donor materials

Abstract

Due to the far-reaching digitization of our society, a huge market opens up for small photovoltaic devices as efficient and sustainable power sources for indoor electronics. Harvesting indoor light presents unique challenges and requirements. Interestingly, organic photovoltaics can be adapted to operate optimally under artificial illumination and reduced light intensity by dedicated structural changes to the organic absorbers. State-of-the-art indoor organic photovoltaics use workhorse push-pull polymeric donor materials such as PM6, optimized for outdoor use. To push these devices into a higher efficiency regime, novel wide-gap donor materials with deeper highest occupied molecular orbital (HOMO) energies are mandatory to achieve a maximum open-circuit voltage (Voc). In this study, the synthesis and characterization of the wide-gap donor polymer P-BDTT(F)-Tc-TT-Tc and two novel derivatives thereof – P-BDTT(Cl)-Tc-TT-Tc and P-BDTT(CN)-Tc-TT-Tc – are described. All materials show the envisaged wide-gap absorption tuned to the indoor illumination spectrum and deep HOMO energy levels. Photovoltaic performances of in total six novel binary blends and two ternary blends are evaluated under both solar and indoor illumination. Overall, the devices based on chlorinated and cyanated polymers exhibit lower performances. The novel P-BDTT(F)-Tc-TT-Tc:IT-4F blend is characterized by a high Voc and affords a power conversion efficiency of 18.8% under indoor LED illumination, outperforming the reference combination PM6:IT-4F. Ternary devices with IO-4Cl as the third component push the efficiency further up to 19.7% due to an increased Voc without sacrificing too much on the fill factor and current.