Strain uniformity and defect passivation via 2D organic halide salts to improve the air stability of perovskite solar cells

Abstract

Minimizing lattice strain and interfacial non-radiative recombination losses is crucial for enhancing the photovoltaic performance and long-term stability of perovskite solar cells (PSCs). However, this requires a deeper understanding of the mechanisms by which interfaces and grain boundaries can be modified. Herein, we applied and compared various ammonium halide salts (I-, Br-, and Cl-) to the surface of the 3D CsMAFA perovskite to induce 2D capping layer formation, which promotes surface and grain boundary recrystallization, modifies the bulk and surface structure, and passivates interfacial defects. We theoretically and experimentally reveal how ammonium halides diffuse and modulate the perovskite crystal lattice (defect suppression at both surfaces and grain boundaries), and their interaction strength with undercoordinated lead (Pb2+) ions. Diffusion analysis showed that chloride salts penetrated more effectively into the perovskite bulk than iodide and bromide salts. Consequently, the strong electronic properties of chloride species facilitate robust hydrogen bonding with FA/MA and effectively passivate the point defects. This process improved surface morphology, crystallinity, and crystal orientation while suppressing PbI2 formation and reducing trap density, thereby enhancing charge carrier dynamics. Remarkably, chloride species facilitate the alleviation of lattice micro-strain and suppression of lattice distortion, yielding a power conversion efficiency (PCE) of 23% and excellent operational stability, with the device retaining 91% of its initial efficiency. In contrast, the control device exhibits a PCE degradation of 71% at 15-25% relative humidity. These findings provide critical insights into halide-dependent surface engineering strategies for defect passivation, lattice strain relaxation, and enhanced environmental stability, offering a promising pathway toward highly efficient and durable PSCs.