Halogenated Hole-Transport Molecules with Enhanced Isotropic Coordination Capability Enable Improved Interface and Light Stability of Perovskite Solar Cells

Interfacial defects are one of the main origins of the hysteresis effect and limit the efficiency and light stability of perovskite solar cells (PSCs). Herein, the authors propose to grant the hole-transport materials' (HTMs) improved isotropic coordination and defect passivation through simple halogenation, enabling a robust perovskite/hole-transport layer interface while avoiding the use of an external passivation layer. First-principles simulations and experimental results show that the halogenated HTMs offer more isotropic coordination sites for Pb2+ ions than the halogen-free ones, thus providing the enhanced passivating ability of defects regardless of their molecular orientation at the surface of perovskite films. Consequently, the PSCs based on the chlorinated spiro[fluorene-9,9 '-xanthene]-based HTM show suppressed nonradiative recombination, delivering a remarkable open-circuit voltage (V-OC) enhancement (from 1.07 to 1.14 V) and a minimal hysteresis index of as low as 0.07%. The corresponding cells also show much improved light stability, retaining 81% of the initial efficiency after 1000 h of continuous illumination at the maximum power point. This work demonstrates that a solid isotropic coordination capability of HTMs with Pb2+ is critical to forming a robust interface and improving the PSCs' light stability.