In Situ Surface Sulfidation of CsPbI₃ for Inverted Perovskite Solar Cells

Inverted CsPbI₃ commonly exhibits a more p-type surface than bulk, which induces severe interfacial recombination and, thus, limits the device’s V_oc and efficiency in inverted perovskite solar cells (PSCs). Here, a gradual CsPbI₃/PbS heterojunction is constructed to inhibit such recombination through in situ chemical sulfidation with N,N′-diphenylthiourea (DPhTA). DPhTA can directly react with CsPbI₃ to form PbS and induce a p- to n-type transition at the CsPbI₃ surface, which leads to the energy level bending downward and establishing a gradual CsPbI₃/PbS heterojunction at the top of the surface region. PSCs with DPhTA exhibit a high V_oc of 1.20 V and reach over 20% efficiency (stabilized efficiency of 19.5%), which is among the highest efficiencies of inverted CsPbI₃ PSCs. In addition, the strong Pb-S bond and well-matched crystal lattice of PbS will protect and stabilize the CsPbI₃ layer beneath, thereby greatly improving the device’s stability. Resulting PSCs retain over 95% of the initial efficiency whether after maximum power point (MPP) tracking for 1200 h or N₂ storage for 300 days.