Improved Eco-Friendly Photovoltaics Based on Stabilized AgBiS2 Nanocrystal Inks

AgBiS2 nanocrystals (NCs) have emerged as attractive absorbers in eco-friendly photovoltaics because of their nontoxic components and high absorption coefficient. Native long-chain ligands of AgBiS2 NCs should be replaced with short-chain ligands for their photovoltaics; however, conventional approaches have been performed using solid-state ligand exchange (SSLE), resulting in inhomogeneous NC aggregation, broad bandtail, large trap density, and resultantly low open-circuit voltage (V-OC) in devices. Herein, we first report that long-chain ligands of AgBiS2 NCs are replaced with halometallate-based short ligands via solution-phase ligand exchange (SPLE). AgI and BiI, are used as halometallate sources, and we find that colloidally stable, highly concentrated AgBiS2 NC inks in polar solvents are prepared via SPLE using AgI-based halometallates, enabling one-step-deposition suitable for roll-to-roll process. This leads to higher degree of ligand exchange, sharper bandtail, lower trap density, and resultantly higher V-OC in devices compared to conventional SSLE. We also first demonstrate that the photovoltaic performance can be improved by introducing ethanedithiol-exchanged AgBiS2 NCs on SPLE-prepared AgBiS2 NC solids because of favorable band alignment and extended depletion width. Thus, this enables improving device performance up to a power conversion efficiency of 4.08% with the highest V-OC of 0.55 V among the AgBiS2 NC photovoltaics reported so far.