KCl induces a “slow-release effect” on PbI2 to strengthen the crystallization of perovskite and the reverse-bias stability of carbon-electrode-based solar cells

Abstract

The crystallization behavior of lead halide perovskites (PVSKs) is regulated by blending KCl in PbI₂ during the two-step growth method, and the effects of KCl blending on photovoltaic and reverse-bias stability are studied for the hole-conductor-free, carbon-electrode based perovskite solar cells (CPSCs). It is observed that KCl compresses the PbI₂ lattice and slows down the reaction rate between PbI₂ and organic salt, resulting in a “slow-release effect” on PbI₂. This effect helps improve PVSK crystallization and prolong the lifetime of photo-generated charge carriers, leading to an open-circuit voltage (V_OC) of 1.16 V and a V_OC deficit as low as 0.37 V (non-radiative V_OC deficit of ∼0.1 V) for the hole-conductor-free CPSCs. Furthermore, power conversion efficiency (PCE) increase from 17.36% to 19.66%. Reverse-bias stability is studied through the examination of electrical stress and thermal image techniques. Owing to the improved crystallization, KCl blending results in a higher breakdown voltage and prolonged electrical stability. A type of “reverse-bias luminescence” (RBL) is newly exhibited. An external quantum efficiency (EQE) of ∼5% is obtained at reverse bias of −4 V. The RBL behavior can be ascribed to the defect-assisted charge-tunneling process. This study not only provides a strategy to further improve the stability of hole-conductor-free CPSCs, but also expands the application to the light-emitting field.