Stable AgBiS2 near-infrared absorbers for thin film tandem photovoltaics
AgBiS2 has rapidly risen in efficiency as single-junction photovoltaics (PVs), but its potential for tandem PVs is unexplored. Herein, we perform optical modeling based on the experimental absorption spectrum of AgBiS2, revealing that, as the NIR sub-cell, efficiencies exceeding 47.9% are achievable in triple-junction tandems, superseding tandems using Pb/Sn perovskite NIR cells (35.9%). However, fulfilling this potential requires overcoming the limited transport properties of AgBiS2 nanocrystal films, which constrains film thicknesses to only 30-50 nm, severely restricting NIR response. A solution-phase ligand exchange process is herein developed, allowing one-step deposition of passivated AgBiS2 nanocrystals that maintain efficient charge-carrier extraction despite an order of magnitude increase in film thickness. These films enable 24.0%-efficient four-terminal tandems with visible-light-absorbing lead-halide perovskite sub-cells, and monolithic tandems successfully demonstrating voltage addition. AgBiS2 PVs have high stability, retaining >90% of the original efficiency after 3 months of storage in dry air without encapsulation. This work establishes the significant potential of AgBiS2 as a stable and scalable NIR absorber for tandem PVs.