… nanocone structures have been considered as one of the highly promising candidates for high-efficiency thin-film photovoltaics. However, few studies have explored nanocone-based PEC [photoelectrochemical] devices, particularly with porous photoactive layers on nanocone structures. In PEC cells, the photoactive layers deposited on the nanocone conductive substrates may not only enhance the light absorption of the photoactive material but may also maintain efficient charge separation and provide a large contact surface area at the electrode/electrolyte interface to promote the surface water oxidation process.

We report here a facile strategy for the deposition of an approximately 700-nm-thick nanoporous Mo-doped BiVO₄ (Mo:BiVO₄) layer on an engineered cone-shaped nanostructure and demonstrate that the unique photoanode achieves a remarkable water-splitting photocurrent at low applied voltage with the best-reported STH conversion efficiency to date. Our study presents the first successful case for realizing a thick nanoporous photoabsorption layer with highly efficient charge separation through the engineered cone-shaped nanostructure and solves the urgent issue concerning the incompatibility of light absorption capability with carrier transport length. The strategy of depositing photoactive materials on the engineered light-trapping architectures offers a new photoelectrode architecture for high-performance PEC water-splitting cells.

—Qiu et al.

Schematic illustration of the fabrication process of the conductive nanocone substrate and electron microscope images of Mo:BiVO4 on the nanocone substrate. (A) First, the glass/Si substrate was deposited with one close-packed monolayer of SiO2 to produce a mask for etching and then the shrinking stage was used to adjust the diameter and spacing of the SiO2 nanoparticles through a selective and isotropic RIE process. Second, a Si nanocone array structure was generated via Cl2-based RIE of the Si substrate. The subsequent third process is to oxidize Si nanocone arrays at high temperature in air. The fourth step is to prepare conductive nanocone substrates by coating one layer of Pt and another functional layer of SnO2. The final step is to deposit the nanoporous BiVO4 photoactive layer through a sol-gel process. (B) Scanning electron microscope (SEM) images (60° tilting) of the final SiOx/Pt/SnO2 nanocone arrays. (C) Cross-sectional SEM images of Mo:BiVO4 on the SiOx/Pt/SnO2 nanocone substrate. Some exposed nanocones were also marked in the figure. Scale bars, 500 nm. Qiu et al. Click to enlarge.

Bismuth vanadate is an inexpensive compound that absorbs sunlight and generates modest amounts of electricity—but is a poor conductor of electricity. To carry a current, a solar cell made of bismuth vanadate must be sliced very thin, 200 nanometers or less, making it virtually transparent. As a result, visible light that could be used to generate electricity simply passes through the cell.

To capture the sunlight before it escapes, Cui’s team used the silicon nanocones, each about 600 nanometers tall.

Nanocone structures have shown a promising light-trapping capability over a broad range of wavelengths. Each cone is optimally shaped to capture sunlight that would otherwise pass through the thin solar cell.

—Prof. Cui

To suppress photocorrosion under illumination, the researchers deposited an active OER catalyst of Fe(Ni)OOH on the Mo:BiVO₄-absorbed layer using a facile two-step electrochemical deposition technique.

The photoanode delivered a “remarkable” photocurrent density of 5.82 ± 0.36 mA cm⁻² at 1.23 V versus RHE. In tandem with a single perovskite solar cell, the photoanode produced a photocurrent of 5.01 mA cm⁻², corresponding to the theoretical STH efficiency of 6.2%.

The tandem solar cell continued generating hydrogen for more than 10 hours, an indication of good stability, said Cui.

Resources

• Yongcai Qiu, Wei Liu, Wei Chen2, Wei Chen, Guangmin Zhou, Po-Chun Hsu, Rufan Zhang, Zheng Liang, Shoushan Fan, Yuegang Zhang, and Yi Cui (2016) “Efficient solar-driven water splitting by nanocone BiVO4-perovskite tandem cells” Science Advances Vol. 2, no. 6, e1501764 doi: 10.1126/sciadv.1501764