Solution-processed sub-1.1 eV chalcogenide absorbers for near-infrared energy conversion: materials chemistry, device physics, and emerging opportunities

The efficient capture of near-infrared (NIR) photons is essential for exceeding the Shockley–Queisser limit in single-junction photovoltaic devices and for advancing the performance of tandem and infrared technologies. This review focuses on solution-processed chalcogenides with bandgaps below 1.1 eV, emphasizing Ag–Bi and Cu–Sn-based materials as earth-abundant and low-toxicity alternatives to conventional III–V and II–VI semiconductor systems. We provide a critical evaluation of their chemical tunability, defect tolerance, and structural complexity, alongside recent progress in precursor chemistry, film fabrication, and device integration. Persistent challenges such as phase instability, band-tail states, and limited crystallinity are identified, while recent advancements in ink formulation, surface passivation, band alignment, and interface engineering are highlighted. Comparative benchmarking reveals the current limitations affecting cell performance and long-term stability in comparison to established NIR absorbers. Building upon these findings, we propose practical strategies for scalable processing, including single-step chemical syntheses, disorder control, and chalcogen-rich annealing treatments, as well as device-level optimizations involving buffer and back-contact selection, and tandem device compatibility. Collectively, solution-processed Ag–Bi and Cu–Sn chalcogenides represent a promising pathway toward stable, cost-effective NIR energy conversion technologies, contingent upon systematic mitigation of defect- and interface-related losses.