Nicolas Gaillard1
University of Hawaii1
Photoelectrochemical (PEC) water splitting has the potential to become an efficient method for renewable hydrogen production. However, the efficiency, projected cost, and durability of lab-scale systems are not yet at the level required to make this technology economically feasible. Amongst all materials studied to date, the chalcopyrite class is arguably one of the most promising classes for PEC water splitting, as it has already demonstrated low-cost and high photoconversion capabilities as a photovoltaic material. In the context of PEC, our group has shown that co-evaporated 1.65 eV bandgap (E<sub>G</sub>) CuGaSe<sub>2</sub> is capable of evolving hydrogen with Faradaic efficiency greater than 85% and generate photocurrent densities over 15 mA/cm2, as measured in a 3-electrode configuration in 0.5M H<sub>2</sub>SO<sub>4</sub> under simulated AM1.5G illumination. Unfortunately, CuGaSe<sub>2</sub>’s narrow E<sub>G</sub> limits 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-E<sub>G</sub> (1.8-2.0 eV) chalcopyrites, compatible with the HPE integration scheme, and capable of generating saturated photocurrent densities greater than 10 mA/cm<sup>2</sup>. We present specifically results on E<sub>G</sub> tunable Cu(In,Ga)(S,Se)<sub>2</sub>, Cu(In,Ga)S<sub>2</sub>, CuGa(S,Se)<sub>2</sub> and CuGa<sub>3</sub>Se<sub>5</sub>. We also discuss some of the strategies developed to improve their surface energetics for the hydrogen evolution reaction, including the use of bandgap tunable MgZnO n-type buffer layers. Finally, we introduce the concept of semi-monolithic tandems, a new integration scheme where wide- and narrow-bandgap chalcopyrites are first integrated on separate substrates, then exfoliated, and finally bonded to create fully functional tandem device.