Wide Bandgap Copper Chalcopyrite Materials for Photoelectrochemical Hydrogen Production

Photoelectrochemistry (PEC) is 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, typically identified by its most popular alloy Cu(In,Ga)Se₂, provides exceptionally good candidates to meet the requirements identified for cheap, sustainable solar fuels production. As we recently reported[1], co-evaporated 1.6 eV CuGaSe₂ offers very high-saturated photocurrent densities (-16 mA.cm^-­2 in 0.5M H₂SO₄ under AM1.5G illumination), long durability (up to 400 hours), and 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 driverin a monolithic hybrid photoelectrode device. Nevertheless, a solar-to-hydrogen efficiency of 3.7% was achieved using a co-planar integration scheme. In order to improve the water-splitting efficiency further, novel chalcopyrite alloys with bandgap 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[2]. 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). X-ray diffraction data 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 CuIn_0.3Ga_0.7S₂ photocathodes in 0.5M H₂SO₄ under AM1.5_G simulated illumination.

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[1] N. Gaillard, D. Prasher, J. Kaneshiro, S. Mallory, and M. Chong, MRS Spring Meeting, Z2.07 (2013).

[2] M. Bär, W. Bohne, J. Rohrich, E. Strub et al., Appl. Phys. Lett. 96, 3857 (2004).

See more of: Invited Presentations on Solar Water Splitting II
See more of: A8: Solar Fuels and Photocatalysts 4
See more of: Batteries and Energy Storage

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