Impact of bromide incorporation on strain modulation in 2D Ruddlesden-Popper perovskite solar cells

Two-dimensional Ruddlesden-Popper perovskites (2D-RPPs) have emerged as promising candidates for efficient solar cells. However, compositional complexity and their multiphase nature make them particularly susceptible to strain, which can have detrimental effects on their device performance and stability. Here, we focus on cyclohexane methylamine (CMA)-based 2D-RPPs and modulate the strain by substituting iodide with bromide. These mixed-halide 2D-RPPs show excellent optical properties, with mixability and tunable band gap. As the substitution ratio increases, the 2D-RPP framework undergoes a sudden rearrangement in crystal lattice, effectively releasing the strain in lattices. Benefiting from the strain relaxation, the 2D-RPPs exhibit evident improvement in crystallinity, which significantly suppresses recombination in the device and enhances carrier transport across it. Consequently, we achieve an increase of 1.15% in efficiency with the strain-released devices containing (CMA)₂MA₈Pb₉I_26.4Br_1.6. This device shows significantly improved stability, retaining 93% of the initial efficiency after exposure to 55%–85% relative humidity (RH) for 120 days.