Back-contact charge selectivity enables efficient Cs₂AgBi₂I₉ indoor photovoltaics under ambient office lighting

Abstract

Lead-free perovskite-inspired materials are promising absorbers for indoor photovoltaics (IPVs), yet their performance is often limited by interfacial recombination, leakage currents, and non-ideal charge-selective contacts. Here, we demonstrate that the efficiency of Cs2AgBi2I9 IPVs is governed by hole-transport-layer (HTL) selectivity rather than by intrinsic hole-injection kinetics alone. Mesoscopic n–i–p devices employing doped Spiro-OMeTAD, P3HT, PCPDTBT, and PPDT2FBT as HTLs were systematically compared under AM 1.5G and white light-emitting diode (WLED) illumination. The PPDT2FBT-based device achieves a power conversion efficiency of 8.50% (8.30% stabilized) under 6500 K, 1000 lux WLED illumination, among the highest reported for wide-bandgap Bi-based IPVs under comparable conditions. By correlating photovoltaic performance with transient photocurrent/photovoltage, electrochemical impedance spectroscopy, dark current–voltage analysis, and interfacial electronic-coupling calculations, we show that faster hole-transfer kinetics alone do not determine device efficiency. Instead, the dominant factor under low-light operation is back-contact selectivity, which governs electron leakage, interfacial recombination, and voltage loss. PPDT2FBT provides the most selective contact, simultaneously enabling efficient charge extraction, suppressed leakage currents, and enhanced recombination resistance, leading to improved open-circuit voltage and fill factor. Finally, proof-of-concept measurements under mixed office illumination, with simultaneous spectral irradiance monitoring at the device plane, confirm stable photovoltaic operation under realistic indoor lighting conditions. These results identify HTL selectivity as a key design parameter for minimizing voltage losses in wide-bandgap perovskite-inspired photovoltaics and establish a general strategy for advancing low-toxicity IPVs toward practical indoor energy harvesting.