Origin of Open-Circuit Voltage Losses from Energy-Level Misalignment between Metal Halide Perovskites and Hole Transport Layers

Metal halide perovskites (MHPs) show great potential in multijunction photovoltaics applications due to their tunable bandgaps through compositional mixing on the halide site. However, wide-bandgap MHPs (&gt;1.7 eV) typically suffer from greater open-circuit voltage (V_OC) losses compared to their narrow-bandgap counterparts (~1.6 eV) owing to energy-level misalignment with charge extraction layers. Herein, we investigate the origin of such losses, focusing on the energy level misalignment between the valence band maximum and the highest occupied molecular orbital (HOMO) for a commonly employed combination of perovskite with various halide compositions and hole transport layers.[1] Our research combines time-resolved photoluminescence spectroscopy and numerical modelling to reveal the origin of V_OC losses, which stem from the accumulation of holes in the HOMO of hole trasport layers, followed by subsequent non-radiative across-interfacial recombination via interfacial defects. By simulating an ideal choice of hole transport material to pair with a mixed-halide MHP whose 1.8-eV bandgap is optimized for tandem solar cells, we demonstrate a potential reduction in V_OC losses originating from energy level misalignment. Our findings underscore the pressing need for tailored charge-extraction materials with improved energy level alignment to enhance the efficiency of solar cells based on wide-bandgap mixed halide MHPs.<br/><br/>[1] Lee, JE et al. Energy & Environmental Science. (2023) manuscript under review.