The photoelectrochemical (PEC) conversion of solar energy to chemical fuels is an attractive and feasible route to a sustainable energy infrastructure. After the first demonstration by Fujishima and Honda in 1972,[1,2] there has been significant progress with notable conversion efficiency records established using III-V semiconductor and earth abundant silicon photocatalysts.[3,4,5,6] However, the current level of efficiency and durability are not yet sufficient for widespread commercial use of this technology. One particular challenge has been exploiting the proximity of charge carrier generation to the catalytically active sites, which is one advantage offered by a PEC device over photovoltaic-driven electrolysis. [7]

In this talk, we will review key aspects in realizing high efficiency and durability in conventional III-V PEC photocatalysts and focus on the interfacial properties that are integral to reliable PEC devices. Using ab-initio molecular dynamics simulations, we first show that the hydrogen bond networks at the water-InP interface exchange themselves more freely than those at water-GaP interface, which leads to more facile diffusion of hydrogen at the interface. [8] Interestingly, it was reported that Pt catalysts can activate hydrogen evolution on the surrounding SiO₂ surface of Si-based Metal-Insulator-Semiconductor solar-to-hydrogen conversion devices, with the activation suggested to take place due to hydrogen diffusion (H-spillover) at the electrolyte-insulator interface.[9] We relate these results to the III-P PEC systems and discuss a possible general device design strategy that takes advantage of this phenomena.

As a next step, we have investigated the electronic structure of water-semiconductor interfaces using many-body perturbation theory (GW approximation) optimized for large systems, so as to develop more comprehensive understanding of interfacial properties (structural, electronic, and chemical) relevant for the PEC device performance.[10] We discuss how this information can be used to engineer surface treatments for fine tuning the band alignment for improved PEC performance. [11]

In order to bridge the knowledge obtained from atomistic ab-initio simulations and the behavior of a macroscopic PEC device, we have also been developing highly collaborative surface validation capabilities based upon the state-of-art X-ray spectroscopy.[12] We demonstrate that X-ray spectroscopy, when combined with computational spectroscopy, can provide a detailed atomistic picture of the state of chemical species in semiconductors or on semiconductor surfaces. Such information is crucial in understanding the correlation between microscopic properties of device component materials and the device performance.

Lastly, we will discuss an effective design strategy of PEC device component materials, where we leverage a tightly coupled collaboration of theory, synthesis, and characterization. The case study has been performed based on a development of thin film Cu(In,Ga)(S,Se)₂ photo absorbers and the suitable buffer.[13]

[1] A. Fujishima and K. Honda, Nature 238, 37 (1972).

[2] J. Ager, M. Shaner, K. Walczak, I. Sharp, S. Ardo, Energy & Env. Sci. 8, 2811 (2015).

[3] O. Khaselev and J. Turner, Science 280, 425 (1998).

[4] R. Rocheleau, E. Miller, and A. Misra, Energy & Fuels 12, 3 (1998).

[5] R. Fan et al. App. Phys. Lett. 106, 213901 (2015).

[6] M. May et al. Nat. Comm. 6, 8286 (2015).

[7] A. Nakamura et al. App. Phys. Exp. 8, 107 (2015).

[8] B. Wood, E. Schwegler, W.-Ih Choi, T. Ogitsu, J. Am. Chem. Soc. 135, 15774 (2013).

[9] D. Esposito, I. Levin, T. Moffat, A. Talin, Nat. Mat. 12, 562 (2013).

[10] T. Pham et al. in preparation.

[11] B. MacLeod et al. ACS App. Mat. & Int. 7, 11346 (2015).

[12] W.-Ih Choi et al. in preparation.

[13] J. Varley et al. in preparation.