New Advances in Stabilizing High-Efficiency Semiconductors for Use in Solar Fuels Applications

The photoelectrochemical conversion of sunlight into gaseous hydrogen or liquid hydrocarbon fuels can be an efficient method for renewable energy production, but significant materials challenges hinder the fabrication of integrated devices that are both efficient and stable. In particular, many moderate band gap semiconductors (E_g = 1-2 eV) photocorrode rapidly (within seconds or minutes) during electrochemical operation in aqueous solution. This process is exacerbated in water oxidation photoanodes that operate at high potentials in acidic or alkaline environments. The stable and reliable use of moderate band gap semiconductors is integral to JCAP’s mission of creating efficient solar-to-fuels devices. As such, recent work at JCAP has focused on developing thin films that protect a number of high-efficiency semiconductors, including Si, CdTe, and GaP. Such protection layers need to satisfy a number of requirements: they must resist corrosion in alkaline or acidic environments, they must be transparent, and they must allow for the transport of photogenerated carriers from the semiconductor to the electrolyte interface. Research at JCAP has led to the development of thin film protection layers based on TiO₂ and NiO_x that fulfill these requirements and have enabled stable operation of Si-based photoanodes for >1000 h during water oxidation in corrosive alkaline electrolytes (Fig. 1). The NiO_x and TiO₂ layers are prepared via atomic layer deposition (ALD) or reactive sputtering over large areas, and the effects of composition, crystal structure, morphology, and electronic properties on photoelectrochemical performance have been studied in detail. Heterojunctions between TiO₂ or NiO_x protection layers and n-type photoactive semiconductors (n-Si, n-CdTe, n-GaP) are rectifying and yield photovoltages in the range of 0.3-0.6 V. Strategies to increase photovoltage via tailoring the interfacial chemistry/structure, as well as by utilizing buried homojunctions, have been successfully pursued. Furthermore, band alignments at various interfaces (semiconductor/protective layer and protective layer/electrolyte) have been investigated and must be controlled for optimal performance. Overall, this presentation will address both the fundamental scientific issues related to the development of these protective layers, as well as the application of these materials towards stabilizing planar and structured electrodes for solar fuels applications.

Figure 1. Long-term photocurrent stability test of Si-NiO_x photoanodes for water oxidation.