Photoelectrochemistry (PEC) is theoretically one of the most efficient methods to produce alternative fuels, although the efficiency, cost, and durability of lab-scale systems are currently not at the level required to make this technology economically feasible. The chalcopyrite material class provides exceptional candidates to meet the requirements for cheap, sustainable solar fuels production. As we have previously reported, co-evaporated 1.7 eV CuGaSe₂ offers very high-saturated photocurrent densities (>15 mA.cm^-­2) and high Faradaic efficiency (>85%). However, CuGaSe₂ photocathodes suffer from non-ideal surface energetics, requiring additional PV cells to drive the water splitting process. The hybrid photo-electrode (HPE) device, in which a PEC electrode is placed on top of a solar cell, is by far the most efficient approach for unbiased solar water splitting. Unfortunately, CuGaSe₂’s narrow bandgap prohibits this integration scheme.

In the present communication, we report on our efforts to synthesize 1.8-2.2 eV band-gap chalcopyrite materials for PEC water splitting. Specifically, we investigated the effect of sulfur on the optical and photoelectrochemical characteristics of the copper chalcopyrite material class. Using co-evaporated 1 μm-thick CuGaSe₂ as a baseline system, we demonstrate that the substitution of selenium with sulfur can be accomplished through a simple annealing step. As a result, a dramatic change in optical properties was observed, with a bandgap increase from 1.6 eV (CuGaSe₂) to 2.4 eV (CuGaS₂). Then, by simply adjusting the indium content during the initial growth process, the bandgap of sulfurized copper chalcopyrite was decreased from 2.4 eV to 2.0 eV. X-ray diffraction data indicated successful bulk sulfurization by the shift of the prominent (112), (220), and (312) reflections to higher angles. Saturated photocurrent densities of 5 mA/cm² were achieved with 2.0 eV red CuIn_0.3Ga_0.7S₂ photocathodes in 0.5M H₂SO₄ under AM1.5_G simulated illumination. From this proof of concept experiment, we have further developed new synthesis approaches and innovative characterization protocols to achieve PV-grade wide bandgap sulfide-based chalcopyrites capable of generating photocurrent densities over 10 mA.cm^-2.