One promising approach to achieving stable and efficient photoelectrochemical (PEC) water splitting is the composite photoelectrode architecture known as a metal-insulating-semiconductor (MIS) photoelectrode. [1,2,3,4]  Within the MIS design, metal catalyst is deposited on top of a thin insulating layer ( < 2 nm thick) that separates the metal from the semiconducting absorber layer. The insulating layer is particularly important because it can protect the underlying semiconductor from corrosion and avoid fermi-level pinning while still enabling efficient charge transfer between the semiconductor and metal through quantum mechanical tunneling. Through this composite design, the MIS architecture is able to decouple efficiency and stability, and has been applied to c-Si and InP photoelectrodes with great success in recent years, [1,2,3,4] For example, InP MIS photocathodes for the hydrogen evolution reaction have achieved photoelectrode conversion efficiencies up to 14%,[2] and n-Si MIS photoanodes have been demonstrated with stable operation in the highly oxidative environment of the oxygen evolution reaction for > hrs.[1]. Despite these encouraging results, further improvement in MIS photoelectrode performance is needed if this technology is to become commercially viable. In particular, improved photovoltage and maximized photo-current density generated by MIS photoelectrodes is essential for achieving DOE solar-to-hydrogen conversion efficiency targets.

            In this work, electrodeposition has been explored as a potentially low-cost and scalable means of depositing ultra-low loadings (1 to 20 ng cm^-2) of Pt nanoparticles onto SiO₂-covered p-Si photoelectrodes. As-deposited MIS photoelectrodes are found to exhibit very poor durability, but we describe how this issue can be overcome through application of a thin secondary insulating layer to form an insulator-metal-insulator-semiconductor (IMIS) geometry. Surprisingly, it is found that the secondary insulating layer also results in a substantial improvement in the fill factor and short circuit current of IMIS devices compared to MIS photoelectrodes of identical loading. The combination of electrodeposition with the IMIS architecture thus offers an exciting opportunity for improving both the efficiency and durability of photoelectrochemical energy conversion. After presenting a side-by-side comparison of MIS and IMIS photoelectrode performance, we highlight reasons for the differences in their performance.     

References:

[1] Y.W. Chen, P. McIntyre, et al., Nat. Mater., 10 (2011)

[2] M.H. Lee, et al., Angew. Chem. Int. Ed., 51 (2012).

[3] J.C. Lee, A. Bard, J. Ekerdt, et al., Nature Nanotech., 10, 84-90 (2015)

[4] D.V. Esposito, I. Levin, T.P. Moffat, and A.A. Talin. Nature Materials, 12, 562-568 (2013).