In-situ volatilization of solid additive assists as-cast organic solar cells with over 20 % efficiency

Complex pre/post-processing will cause cost multiplication and severe technical challenges for the mass production of organic solar cells (OSCs). Herein, we report a solid additive, 1,4-dibromo-2,5-difluorobenzene (DBDF), with a low melting point and rational interaction force with the donor and acceptor. Film-formation kinetics and morphological investigation reveal that DBDF volatilizes completely in situ during spin-coating and induces preferable molecular packing and vertical phase separation, contributing to efficient charge generation and extraction in devices. In addition, the ordered molecular packing assists in the red-shifted absorption spectra of both donor and acceptor for harvesting more photons, which is also conducive to improving charge generation. Based on the classical PM6:Y6 matrix, the DBDF-based OSCs without additional engineering achieve a much higher power conversion efficiency (PCE) of 18.1 % compared with control devices (16.6 %). Moreover, the D18:N3:Y6–1O-based OSCs with DBDF incorporation deliver a top-ranked PCE of 20.2 % for as-cast devices. It is noteworthy that subsequent thermal annealing offers negligible support in performance enhancement because of the increased voltage loss. This work puts forward an ingenious strategy to simplify the device fabrication process to boost the development of low-cost commercial manufacture of OSCs.