Wafer-Scale Ultrathin, Single-Crystal Si and GaAs Photocathodes for Photoelectrochemical Hydrogen Production.

Crystalline Si and III-V compound semiconductors with appropriate band edge positions for the reduction of water have been widely utilized in photoelectrochemical (PEC) cells for the hydrogen evolution reaction (HER). However, the high cost of manufacturing those PEC cell photoabsorbers makes it difficult to achieve cost-effective hydrogen production. To overcome this issue, a new approach to fabricate a photoabsorber with low cost yet high performance for the HER is highly necessary. Here, we present a controlled fracture method, the so-called spalling process, to fabricate a cost-effective thin semiconductor applicable to the PEC HER. Using this method, a wafer-scale thin Si, whose thickness can be controlled from a few micrometers to sub-50 μm, was fabricated from a thick Si mother substrate without material loss. Pt nanoparticle-decorated 16 μm thick spalled Si with an np+ rear junction exhibited an HER onset potential of 332 mV (vs reversible hydrogen electrode (RHE)) and a photocurrent density of 20.1 mA cm-2 at 0 V (vs RHE), which are the best performances among previously reported planar-type thin Si-based photocathodes. Finally, we demonstrated that 20 μm thick GaAs could also be successfully fabricated by the spalling process, while exhibiting a PEC HER performance comparable to 350 μm thick bulk GaAs.