Multifunctional Buried Molecule-Bridge for High-Performance Inverted Perovskite Solar Cells

Carbazole-based self-assembled monolayers (SAMs) as an effective hole transportation layer have tremendously advanced the power conversion efficiency (PCE) of inverted perovskite solar cells (PSCs). However, the inhomogeneous distribution of SAMs on substrate and non-intimate interface contact can bring about significant interfacial energy loss at SAM/perovskite heterojunction. Herein, a small molecule 4-Bromobenzylphosphnic acid (4Br-BPA) is constructed as a molecule bridge connecting [4-(3,6-dimethyl-9H-carbazol-9-yl) butyl] phosphonic acid (Me-4PACz) and perovskite, exhibiting multifunctionality on improving the interfacial characteristics. First, the small-size 4Br-BPA molecules can partly fill some voids on NiO_x/Me-4PACz anchored with NiO_x via phosphonic acid group, meanwhile ameliorating the NiO_x surface state. Second, the 4Br-BPA post-deposited onto Me-4PACz interacting with Me-4PACz via π-π stacking has suppressed charge accumulation at interface, aligned the energy level of NiO_x/Me-4PACz consequently promoting the hole transportation. Third, the interplay between 4Br-BPA and perovskite enables effective passivation of interfacial traps, and the substrate NiO_x/Me-4PACz/4Br-BPA with improved wettability has facilitated the perovskite film growth with enhanced crystallization and released residual stress. Consequently, all these benefits have been transformed to an impressive PCE of 26.59% (certified 26.12%). The device based on 4Br-BPA also demonstrates much improved operational stability, maintaining ≈90% of initial efficiency under 1400 h continuous one-sun illumination.