Fluorination-Assisted Interfacial Dipole for CsPbI Perovskite Solar Cells with Over 22% Efficiency.

Inorganic CsPbI perovskite attracts widespread attention in photovoltaic applications due to its superior thermal stability and optoelectronic properties. However, CsPbI perovskite solar cells (PSCs) still suffer from severe energy losses due to interface nonradiative recombination and undesirable charge carrier transfer, predominantly limiting their photovoltaic performance. Herein, an interfacial dipole engineering is introduced for CsPbI PSCs, in which azetidinium chloride (Az) and its fluorinated derivative 3,3-difluoroazetidinium chloride (DFAz) are employed to manipulate the interface properties of PSCs and thus diminish energy losses. Systematically theoretical calculations and experimental studies reveal that the fluorination-assisted ammonium molecule could form a stronger interaction with perovskites and thereby arrange the dipole alignment on the superficial layer of perovskites, which could simultaneously ameliorate the passivation effect and energy level alignment of the perovskite and hole transport layers, thereby suppressing interface recombination. Meanwhile, the coordinated bonding between the ammonium and hole transport layer facilitates charge transfer at the heterojunction interface by offering additional carrier transport channels. Consequently, the CsPbI PSCs deliver a high efficiency of up to 22.05%. This work provides important design principles of interface engineering for high-performance solar cells to minimize energy losses.