(Invited) Wide Bandgap Copper Chalcopyrite Candidates for Renewable Hydrogen Generation

Photoelectrochemical (PEC) water splitting has the potential to become an efficient method to produce renewable hydrogen. However, the requirements in terms of efficiency, projected cost, and durability of lab-scale systems required to make this technology economically feasible have still not been met. Amongst all materials studied to date, the chalcopyrite class is arguably one of the best classes for PEC water splitting, as it has already demonstrated low cost and high photoconversion capabilities as a solar material. As we have previously reported, co-evaporated 1.7 eV bandgap (EG) CuGaSe2 generates very high-saturated photocurrent densities (>15 mA.cm-­2) and high Faradaic efficiency (>85%). Unfortunately, CuGaSe2’s narrow EG prohibits its integration as top absorber into a dual junction stacked PEC device (also known as hybrid photoelectrode, HPE). In the present communication, we report on our latest efforts to synthesize wide EG (1.8-2.0 eV) chalcopyrites, compatible with the HPE integration scheme, and capable of generating a saturated photocurrent density greater than 10 mA/cm2. We present specifically results on EG tunable Cu(In,Ga)(S,Se)2, Cu(In,Ga)S2, CuGa(S,Se)2 as well as 1.85 eV CuGa3Se5. We discuss some of the strategies developed to improve their surface energetics for the hydrogen evolution reaction, including the use of In2S3 n-type buffer layers. Finally, we present solid-state techniques to identify possible pitfalls in some of these material systems and discuss potential paths for improvement.