The Dual Effect of Metal Halide Passivation on Perovskite Solar Cell Performance and Stability

Wide band gap perovskite solar cells (PSCs) have shown notable improvements in performance in recent years; however, there are still significant open-circuit voltage (V_oc) losses to overcome, and stability poses a significant obstacle to commercialization. It is widely understood that non-radiative recombination between the perovskite and electron transport layer (ETL) is a primary source of V_oc losses, which can be largely mitigated through incorporating passivation materials at the interface.^[1] However, the usefulness of passivation materials also depends on their effect on device stability, which is an area that requires further investigation. In this work, we present a study of the impact of vacuum deposited metal halide passivation (namely MgF_x)^[2] on the performance and stability of wide band gap PSCs.<br/><br/>The primary focus of this work is understanding the dual effect of the MgF_x passivation through analysis of device performance and stability, as well as electronic properties, under ISOS-L-2 stress conditions.^[3] PSCs with a 1.77 eV bandgap were passivated with a range of thicknesses of MgF_x. With the optimum thickness of 0.9 nm of MgF_x, devices exhibit up to a 50 mV improvement in V_oc and 1.5% absolute improvement in power conversion efficiency compared to unpassivated devices. This performance enhancement is countered by a negative impact on stability, however, dominated by a decrease in short-circuit current (J_sc). Under full-spectrum simulated sunlight at 85 °C and open-circuit in ambient air, a 50% reduction in J_sc was observed in 15 hrs for the passivated devices, compared to 100 hrs in the unpassivated devices.<br/><br/>We utilize variable rate J-V scanning to determine the extent to which this device performance loss can be ascribed to effects from mobile ions.^[4] These measurements reveal that the accelerated performance degradation observed in devices passivated with MgF_x is largely due to enhanced impact of mobile ions, which impede current extraction, rather than significant material degradation. We performed analysis of charge collection quality to further investigate the mechanism of the J_sc degradation, as well as x-ray diffraction characterization to observe the extent of any material degradation at this interface. Additionally, we explore the alternative of CsF_x passivation to probe the impact of different metal ions introduced at the interface. This work reveals the significant role that small changes in architecture can play in device degradation under ISOS-L-S stress conditions and highlights the importance of careful selection of passivation materials to achieve both high performing and stable wide band gap PSCs.<br/><br/><b>References:</b><br/>[1] Caprioglio, P. et al. <i>Nat Commun</i> <b>2023</b>, <i>14</i>, 932.<br/>[2] Liu, J. et al. <i>Science </i><b>2022</b>, <i>377</i>, 302-6.<br/>[3] Khenkin, M.V. et al. <i>Nat Energy</i> <b>2020</b>, <i>5</i>, 35–49<br/>[4] Thiesbrummel, J. et al. <i>Adv. Energy Mater</i> <b>2021</b>, <i>11</i>, 2101447.