Device Physics of Dilute-Donor Narrow Band Gap Organic Solar Cells with Highly Transparent Active Layers

Organic Photovoltaics (OPVs) offer sustainable, solution-processable, and cost-effective energy harvesting solutions. While opaque devices underwent enormous progress in the past decade, it remains a challenge to meet the high transparency requirements for integrated semitransparent OPVs for windows, skylights, or greenhouses, and only a few examples with average visible transmittance (AVT) over 60% exist. Moreover, a systematic understanding of the photoelectronic processes in visibly transparent devices is still absent. In this work, we investigate the generation-recombination dynamics in three semitransparent blend systems with varied donor concentrations to understand their performance limitations. [1]<br/>Specifically, we study charge generation, multi-mechanism recombination, and extraction in three OPV systems that are based on an ultra-narrow non-fullerene acceptor COTIC-4F and a polymer donor that absorbs in the visible range PCE10. The systematic reduction of the donor content from 40% to 30% and 20% leads to highly transparent active layers with blend AVTs of 64%, 70%, and 77%, respectively. Opaque devices in the optimized highly-reproducible device configuration comprising these transparent active layers lead to PCEs of 7.0%, 6.5%, and 4.1%. The investigation of these structures yields quantitative insights into changes in the charge generation profile, non-geminate recombination losses via the bimolecular, bulk and interface trap-assisted mechanisms, and extraction dynamics upon dilution of the donor. Lastly, we give an outlook for employing the highly transparent active layers in semitransparent OPV devices.<br/><br/>[1] N. Schopp, G. Akhtanova, P. Panoy, A. Arbuz, S. Chae, A. Yi, J. Kim, V. Promarak, T.-Q. Nguyen and <u> V.V. Brus</u>, Unraveling Device Physics of Dilute-Donor Narrow Band Gap Organic Solar Cells with Highly Transparent Active Layers, <b><i>Advanced Materials</i></b> (2022) in press. DOI: 10.1002/adma.202203796