Two-Dimensional BiOSe with High Mobility for High-Performance Polymer Solar Cells.

Carrier mobility is a critical factor for power conversion efficiency (PCE) of polymer solar cells (PSCs), and the low charge carrier mobility still limits the performance improvement of PSCs. Adding high-mobility material into the active layer is one of the better ways to enhance the PCE of PSCs. Two-dimensional (2D) BiOSe can be an ideal additive material for improving the carrier mobility of PSCs because of its ultrahigh mobility and high thermal stability. In this work, the BiOSe few-layer 2D nanoflakes are fabricated by combining lithium intercalation with shear force-assisted liquid phase exfoliation and applied as an additive to promote charge transport in PSCs for the first time. The 2D BiOSe nanoflakes, when introduced into the active layer, not only provide a new interface between a donor and an acceptor and efficient charge transfer pathways but also induce crystallization of the photosensitive layer and form continuous interpenetrating networks, which promotes the exciton separation and charge transfer in the photosensitive layer. As a result, the PCE of a device based on PBDB-T/ITIC is increased from 10.09% (0 wt %) to 12.22% (2 wt %). Meanwhile, the PCE of a device based on PM6/Y6 is also increased from 14.59% for a binary device to 16.28% for an optimized ternary device (2 wt %). Moreover, the optimized ternary device shows excellent air stability by suppressing the mixing of the two phases. This work provides a good method to enhance the PCE of PSCs and also shows that the BiOSe material has a good prospect in photovoltaic devices.