Stabilizing Buried Interface by Introducing a Multifunctional Poly(hexamethylenebiguanide) Hydrochloride for Efficient Perovskite Solar Cells

Defects at the buried interface compromise the stability of perovskite films, limiting the practical application of n–i–p perovskite solar cells (PSCs) in flexible and extreme‐environment scenarios. Therefore, optimizing the buried interface is a crucial strategy for enhancing the performance of PSCs. Here, Poly(hexamethylenebiguanide) hydrochloride (PHMB) was introduced into the SnO 2 /perovskite interface to optimize the interface contact. PHMB interacts strongly with SnO 2 and perovskite through biguanide groups and chloride ions (Cl − ), thereby improving the morphology of the SnO 2 film, passivating oxygen vacancies, suppressing defects at the bottom surface of perovskite, and providing better growth conditions for the deposition of perovskite. Moreover, the more significant improvement at the perovskite bottom surface than at the top surface further confirms the effectiveness of PHMB interfacial modification. Ultimately, the PHMB‐modified PSCs achieved an excellent power conversion efficiency (PCE) of 25.06%. The unencapsulated rigid PSCs can still maintain 92.42% of the initial efficiency after 120 low‐temperature cycles between 290 and 170 K. In addition, the PHMB‐modified flexible PSCs exhibit excellent bending stability. This study demonstrates that the introduction of a polymer interface passivation layer is a reliable approach for the application of PSCs in low‐temperature and flexible environments.