Bandgap Optimization of Copper Indium Gallium Disulfide Chalcopyrites for Solar-Assisted Water Splitting
Authors: Chong Marina , Prasher Dixit , Gaillard Nicolas 
Published: February 2020

Photoelectrochemistry (PEC) is an attractive method for producing hydrogen as an alternative fuel to oil. Limitations arising from device efficiencies, cost, and durability demonstrated in laboratories prevent the implementation of this technology on a larger scale. The chalcopyrite material class, exemplified by its most popular alloy Cu(In,Ga)Se2, encompasses some of the most promising candidates to meet the criteria for cheap, sustainable solar fuel production. As we recently reported[i], co-evaporated 1.6 eV CuGaSe2 offers very high-saturated photocurrent densities (20 mA.cm-

2 in pH 0 under AM1.5G illumination), long durability (up to 400 hours), and relatively high Faradaic efficiency (>85% for non-catalyzed systems). Although CuGaSe2 has the highest bandgap of the copper chalcopyrite class, its optical characteristics are still too close to that of amorphous silicon (a-Si), a low-cost material our research team has identified as an ideal photovoltaic driver in a monolithic hybrid photoelectrode device. Nevertheless, a solar-to-hydrogen efficiency of 3.7% was achieved using a co-planar integration scheme, where CuGaSe2 was connected in series with three a-Si solar cells. In order to increase the water-splitting efficiency, novel chalcopyrite alloys with bandgaps greater than 1.6 eV must be developed.
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 baseline system, we demonstrate that the substitution of selenium with sulfur is 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), in good agreement with theoretical predictions[ii]. Then, by simply adjusting the indium content in the film during the initial growth process, the bandgap of sulfurized copper chalcopyrite was decreased from 2.4 eV [GGI=Ga/(Ga+In)=1] to 2.2 eV (GGI»0.8) and finally to 2.0 eV (GGI»0.7), as presented in Fig. 1. X-ray diffraction data (Fig. 2) indicated successful bulk sulfurization by the shift of the prominent (112), (220), and (312) reflections to higher angles. Preliminary PEC analyses revealed an anodic shift of the flatband potential with increasing bandgap, when compared to CuGaSe2. This suggests that the bandgap modification in sulfurized films primarily stems from a downward shift of the valence band, an ideal situation for p-type PEC systems. Saturated photocurrent densities greater than -5 mA/cm2 were achieved with 2.0 eV red CuIn0.3Ga0.7S2 photocathodes in 0.5M H2SO4 under AM1.5G simulated illumination.

[i] N. Gaillard, D. Prasher, J. Kaneshiro, S. Mallory, and M. Chong, MRS Spring Meeting, Z2.07 (2013).
[ii] M. Bär, W. Bohne, J. Rohrich, E. Strub et al., Appl. Phys. Lett. 96, 3857 (2004).