A probe into underlying factors affecting utrafast charge transfer at Donor/IDIC interface of all-small-molecule nonfullerene organic solar cells

Abstract To gain a deep understanding on structure-property relationship of all-small-molecule nonfullerene organic solar cells, donor molecules SM1 and SM2 with and without cyano group substituents, respectively, were selected, for which SM1 -based device has obvious higher power conversion efficiency (10.11%) than SM2 -based device (5.32%) in the same condition. Here a systemic study combining quantum chemistry with molecular dynamics was conducted to explore what improve the performance of SM1 -based organic solar cells from the theoretical perspective. The results indicate that the cyano substituents in SM1 not only could induce a redshift absorption spectrum through decreasing the energy gap but also lead to better intramolecular charge transfer, which is beneficial to charge transfer at donor/acceptor interface. More interestingly, the SM1 /acceptor interface presents an additional charge transfer state (CT state) below the first optically Frenkel exciton state (FE state) except that its FE state has stronger absorption compared with SM2 /acceptor interface, which plays an important role in improving its charge transfer rate ( k inter-CT ) through increasing electronic coupling of charge transfer ( V DA-CT ). Thus it is speculated that molecular design considering additional CT states below FE state is a promising strategy in organic solar cells like SM1 -based device with good performance.