Silicon is a promising contender in the race for low-bandgap absorbers for use in a solar driven monolithic water splitting cell (PEC). However, given its role as the low-bandgap material the silicon must be situated behind the corresponding high-bandgap material and as such, it will be exposed to (red) light from the dry back-side – not from the wet front side, where the electrochemistry takes place.^1,2 Depending on the configuration of the selective contacts (junctions) this may lead to compromises between high absorption and low recombination.^2,3 We discuss the tradeoffs and compare modeling results to measurements. Regardless of configuration, the wet surface of the silicon is prone to passivation or corrosion and must therefore be carefully protected in service in order to remain active. TiO₂ has been found as an effective protection layer for both photoanodes and photocathodes in acid electrolyte ⁴ and NiCoO_x for photoanodes in alkaline electrolyte. ³ This is discussed in the context of general considerations for photoelectrode protection and state-of-the-art performance. ⁵

References:
[1]: B. Seger et al., Energ. Environ. Sci., 7 (8), 2397-2413 (2014), DOI:10.1039/c4ee01335b
[2]: D. Bae et al., Energ. Environ. Sci., 8 (2), 650-660 (2015), DOI: 10.1039/c4ee03723e
[3]: D. Bae et al., ChemElectroChem, 3 (10), 1546-1552  (2016), DOI: 10.1002/celc.201500554
[4]: B. Mei et al., J. Phys. Chem. C., 119 (27), 15019-15027 (2015), DOI: 10.1021/acs.jpcc.5b04407
[5]: D. Bae et al., Chem. Soc. Rev. (2017), DOI:10.1039/C6CS00918B