R4N+ and Cl− stabilized α-formamidinium lead triiodide and efficient bar-coated mini-modules

The higher thermodynamic stability of formamidinium lead triiodide (FAPbI3) in the yellow non-perovskite (δ-phase) than the black perovskite (α-phase) at room temperature causes spontaneous α-phase to δ-phase transition. Stabilization of α-FAPbI3 by alloying the perovskite composition is limited by band gap broadening and halide segregation. Furthermore, commercial PSCs require coating methods suitable for large-area modules. Herein, we report α-phase stabilization of FAPbI3 without band gap broadening using R4N+ cations and Cl− anions. Subsequently, high-efficiency perovskite solar mini-modules (PSMs) were fabricated using a bar-coating process with simultaneous defect passivation and hole-transport promotion which exhibited a maximum power conversion efficiency (PCE) of 21.23% (certified 20.33%, 36.4-cm2 area). The PCE in the 1-cm2 area fabricated by bar-coating was 23.24% (certified 22.79%, the highest in those fabricated by scalable bar-coating method). Furthermore, the encapsulated PSM retained 93% of its initial PCE, even after 870 h under continuous one-sun illumination.