Thermally Co-Evaporated Wide-Bandgap Perovskite Photovoltaics via BaI ₂ Vapor Surface Treatment

Thermally evaporated wide-bandgap perovskite subcells offer distinct advantages for perovskite-silicon tandem photovoltaics, particularly regarding conformal deposition on textured silicon and industrial compatibility. However, these devices suffer from severe open-circuit voltage ( V oc ) losses, limiting their efficiency improvement and potential in tandem applications. This study elucidates that Pb–I diatomic vacancy and iodine single-atomic vacancy on the Cs-rich but Pb-poor surface of thermally evaporated Cs 0.94 DMA 0.06 PbI 3 films are the primary origin of V oc loss, although these defects are benign for surface phase stability. Therefore, a non-destructive surface passivation strategy through BaI 2 vapor is proposed without altering the lattice structure that induces phase transformation. This approach successfully suppresses surface defect-induced non-radiative recombination, achieving a significant enhancement in V oc (1.166 V) and a device efficiency of 18.15%. This achievement sets a new efficiency record for thermally evaporated wide-bandgap perovskite solar cells with a bandgap over 1.7 eV, paving the way for tandem photovoltaic applications.