Clothing CsPbI₃ perovskite in a robust phase-pure armor to boost the thermal stability

Phase instability seriously hinders the application of inorganic CsPbI₃ perovskite solar cells (PSCs), especially under elevated temperature. Herein, we developed a novel strategy, namely intermediate treatment (IT), to clothe the CsPbI₃ perovskite in a robust phase-pure armor to boost its thermal stability. During the IT process, instead of the post-treatment (PT) of the CsPbI₃ perovskite, the quinuclidin-1-ium iodide (QCDI) solution was employed to treat the intermediate phase DMAPbI₃. At the following annealing stage, a thin but robust 1D QCDPbI₃ outer layer was formed via ion exchange between the QCD⁺ and DMA⁺ ions. Such outer layer stabilized the CsPbI₃ perovskite phase well by serving as a protecting layer and reducing the grain size. As a result, the IT-CsPbI₃ perovskite maintained black phase after heating at 85 °C for 120 h, while the pristine CsPbI₃ film deteriorated seriously after 24 h. The IT-CsPbI₃ PSCs based on a carbon electrode (C-PSCs) retained over 85% of the initial efficiency after 200 h of heating at 85 °C in an N₂ glovebox and maintained almost unchanged efficiency in dry air for 150 days. In addition, the IT strategy improved the film morphology well and passivated the defect states effectively. Consequently, the IT-CsPbI₃ C-PSC achieved a record efficiency (16.64%) among inorganic C-PSCs.