Understanding the Long-Term Stabilizing Effect of Alumina via Atomic Layer Deposition in Perovskite Solar Cells

Atomic layer deposition of aluminum oxide (ALD-Al₂O₃) layers has been extensively studied for stabilizing perovskite solar cells (PSCs) against environmental stressors, such as humidity and oxygen. In addition, the ALD-Al₂O₃ layer acts as a protective barrier, mitigating the pernicious halide ion migration from the perovskite toward the hole transport interface. ¹ However, its effectiveness in preventing the infiltration of ions and additives from the hole-transport layer into perovskites remains insufficiently understood. Herein, we demonstrate the deposition of a compact ultrathin (~0.75 nm) ALD-Al₂O₃layer that conformally follows the morphology of a triple-cation perovskite film over a large area. This promotes effective mechanical adhesion of the spiro-OMeTAD layer on top of the perovskite, improving the charge carrier collection between these two layers. Upon systematically investigating the layer-by-layer structure of the PSC stack, we discovered that ALD-Al₂O₃ also acts as a diffusion barrier layer for the degraded species from the adjacent transport layer into the perovskite. In addition to all protection capabilities, ALD-Al₂O₃ impedes the transition of crystalline perovskites to an undesired amorphous phase instead of a yellow delta phase.² Consequently, the dual functionality (i.e., enhanced mechanical adhesion and diffusion barrier) of the ALD-Al₂O₃ protection enhanced the device performance from 19.1% to 20.5%, retaining 75% of its initial power conversion efficiency compared to 10% for pristine devices after 180 days of shelf-aging, followed by 1000 min of maximum power point tracking under ambient conditions. Further, we have conducted 2000 hours of outdoor stability tests of the devices highlighting the PCE stability of &gt;90% with ALD-protected devices. Finally, this study deepens our understanding of the mechanism of ALD-Al₂O₃ as a two-way diffusion barrier, highlighting the multifaceted role of buffer layers in interfacial engineering for the long-term stability of PSCs.<br/><br/>References:<br/><b>1. Das, C.</b>, Kot, M., Hellmann, T., Wittich, C., Mankel, E., Zimmermann, I., Schmeisser, D., Khaja Nazeeruddin, M., and Jaegermann, W. (2020) Atomic Layer-Deposited Aluminum Oxide Hinders Iodide Migration and Stabilizes Perovskite Solar Cells. <i>Cell Rep Phys Sci</i>, <b>1</b> (7).<br/><br/><b>2. Kedia, M.</b>, Das, C., <b>Saliba, M.</b>, (2024) Understanding the long-term stabilizing effect of alumina via atomic layer deposition in perovskite solar cells. <i>(in review)</i>