Development of Wide Bandgap Copper Chalcopyrite Thin Film 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[i], co-evaporated 1.7 eV CuGaSe2 offers very high-saturated photocurrent densities (15 mA.cm-shy;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 driver in 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 1.7 eV must be developed.
In the present communication, we report on our effort to synthesize wide bandgap (1.8 eVG<2.2 eV) band-gap chalcopyrite materials for un-biased PEC water splitting using PEC/PV hybrid devices. Specifically, we investigate the impact of sulfur on the optical and photoelectrochemical characteristics of the copper chalcopyrite material class. Using co-evaporated 1 mu;m-thick CuGaSe2 as baseline system, we demonstrate that selenium can be substituted by sulfur using a simple annealing step. With this protocol, a dramatic change in optical properties was observed, with a bandgap increase from 1.7 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, red 2.0 eV CuIn0.3Ga0.7S2 was obtained. Mott-Schottky analysis indicated a 200 mV anodic shift of the flatband potential with increasing bandgap (300 meV), suggesting 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. Linear sweep voltammetry performed in 0.5M H2SO4 under AM1.5G simulated illumination revealed the excellent photo-conversion properties of 2.0 eV CuInGaS2, with photocurrent densities of 3.5 and 6.0 mA/cm2 at 0 VRHE and -0.4 VRHE, respectively, with negligible dark current from flatband potential (+0.5 VRHE) to photocurrent saturation (-0.5 VRHE). Preliminary results obtained on PV/CuInGaS2 hybrid structures will be also presented.
[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).