Stable perovskite solar cells with exfoliated graphite as an ion diffusion-blocking layer

Ion diffusion and metal diffusion in metal halide perovskites, charge-transporting layers, and electrodes are detrimental to the performance and stability of perovskite-based photovoltaic devices. As a result, there is intense research interest in developing novel defect and ion diffusion mitigation strategies. We present a simple, low-cost, scalable, and highly effective method that uses spray-coated exfoliated graphite interlayers to block ion and metal diffusion and humidity ingress within the perovskite, the hole transport material, and metal electrodes. The influence of inserting the exfoliated graphite films on the structure, surface morphology, and optoelectronic properties was examined through various methods, including X-ray diffraction, time-of-flight secondary ion mass spectrometry, scanning electron microscopy, atomic force microscopy, current-voltage (J-V) characteristics, transient photocurrent, and transient photovoltage. Our comprehensive investigation found that exfoliated graphite films reduced the I⁻ and Li⁺ diffusion among the layers, leading to defect mitigation, reducing non-radiative recombination, and enhancing the device stability. Consequently, the best-performing device demonstrated a power conversion efficiency of 25% and a fill factor exceeding 80%. Additionally, these devices were subjected to different lifetime tests, which significantly enhanced the operational stability.