Photocurrent-detected 2D electronic spectroscopy reveals ultrafast hole transfer in operating PM6/Y6 organic solar cells

The performance of nonfullerene-acceptor-(NFA)-based organic solar cells is rapidly approaching the efficiency of inorganic cells. The chemical versatility of NFAs extends the light-harvesting range to the infrared, while preserving a considerably high open-circuit- voltage, crucial to achieve power-conversion efficiencies >17%. Such low voltage losses in the charge separation process have been attributed to a low-driving-force and efficient exciton dissociation. Here, we address the nature of the subpicosecond dynamics of electron/hole transfer in PM6/Y6 solar cells. While previous reports focused on active layers only, we developed a photocurrent-detected two-dimensional spectroscopy to follow the charge transfer in fully operating devices. Our measurements reveal an efficient hole-transfer from the Y6- acceptor to the PM6-donor on the subpicosecond time scale. On the contrary, at the same time scale, no electron-transfer is seen from the donor to the acceptor. These findings, putting ultrafast spectroscopy in action on operating optoelectronic devices, provide insight for further enhancing NFA solar cell performance.This research received funding from the Clean Planet Program supported by Fundació Joan Ribas Araquistain (FJRA) and the Ignite program (Q-SPET) supported by the Barcelona Institute of Science and Technology. N.F.v.H. acknowledges the financial support by the European Commission (ERC Advanced Grant 670949-LightNet and Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agree- ment No 713729), the Ministry of Science & Innovations (“Severo Ochoa” program for Centers of Excellence in R&D CEX2019-000910-S and Plan Nacional PGC2018-096875−B- I00), the Catalan AGAUR (2017SGR1369), Fundació Privada Cellex, Fundació Privada Mir-Puig and the Generalitat de Catalunya through the CERCA program. J.M. and F.B. acknowledge the financial support by the European Commis- sion (grant 951843), Spanish Ministry MINECO and FEDER (grant MAT2017-89522-R), the Severo Ochoa program (Grant SEV-2015-0522) and “Agencia Estatal de Investiga- ción” (Grant PRE2018-084881). E.C. acknowledges the financial support of the H2020 FET Project COPAC (766563).Peer ReviewedPostprint (published version