Star-shaped bridge-linking polymer for robust buried interface in high-efficiency and stable perovskite solar cells

Metal halide perovskite solar cells (PSCs) are anticipated to play a pivotal role in the next generation of photovoltaic technologies, but their unsatisfactory stability hinders further commercial applications. Particularly, numerous interfacial defects and poor mechanical adhesion at the perovskite buried interface present a critical obstacle hindering power conversion efficiency (PCE) and long-term stability of perovskite solar cells. Here, different from conventional small-molecule or linear polymer interface modifiers, we introduce a star-shaped PMMA-<em>b</em>-PDMAEMA polymer (S-MD) as a multifunctional bridge-linking polymer for simultaneous defect passivation and mechanical reinforcement at the buried interface of inverted (p-i-n) PSCs. S-MD features a three-dimensional architecture with multiple extended conjugated arms, offering multiple Lewis base functional groups (e.g., C=O and R-N(CH₃)₂) with a high density of multidentate coordination sites. These groups can effectively coordinate with electron-deficient defects at the perovskite buried interface, enabling improved crystallization, reduced defect density, and enhanced interfacial adhesion. As a result, the interfacial fracture strength increases from 0.13 MPa to 1.66 MPa. The resultant device achieves a PCE of 26.35% (certified steady-state PCE of 25.96%). The flexible device retains over 90% of its initial efficiency after 3000 flexing cycles at a curvature radius of 6 mm (R = 6 mm). This work highlights a multidentate coordinating, star-shaped polymer interface strategy that offers a promising pathway toward highly efficient and stable inverted PSCs.