Understanding the varying mechanisms between the conformal interlayer and overlayer in the silicon/hematite dual-absorber photoanode for solar water splitting†
Dual-absorber photoelectrodes have been proved to have great potential in the photoelectrochemical (PEC) water splitting application due to their broadband absorption and suitable energy-band position, while the surface/interface issues are still not clearly resolved and understood. Here, during the preparation of a silicon/hematite dual-absorber photoanode achieved via synthesizing a Sn-doped hematite film on the silicon nanowire (SiNW) substrate, we separately introduced the conformal overlayer and interlayer of an Al2O3 thin film by atomic layer deposition. With the thickness-optimized interlayer (overlayer) of the Al2O3 thin film, the photocurrent density at 1.23VRHE can be enhanced from 0.85 mA cm−2 to 1.51 mA cm−2 (1.25 mA cm−2), and the on-set potential has a cathodic shift of ∼0.32 V. Although both the overlayer and interlayer modification can substantially improve the PEC performance, the underlying mechanisms are obviously different. The overlayer can only reduce the carrier recombination on the top surface and in the bulk of the hematite film; in contrast, the interlayer not only passivates the SiNW surface and bottom surface of the hematite film, but also the top surface of the photoanode due to Al3+ thermal diffusion from the bottom to the top surface of the hematite film and the resultant Al2O3 formation. This work deepens our understanding for the roles of the surface and interface engineering in the achievement of high-performance PEC systems based on dual or more absorbers.