Stanford solar tandem cell shows promise for efficient solar-driven water-splitting to produce hydrogen
Researchers at Stanford University, with colleagues in China, have developed a tandem solar cell consisting of an approximately 700-nm-thick nanoporous Mo-doped bismuth vanadate (BiVO4) (Mo:BiVO4) layer on an engineered Si nanocone substrate. The nanocone/Mo:BiVO4 assembly is in turn combined with a solar cell made of perovskite.
When placed in water, the device immediately began splitting water at a solar-to-hydrogen conversion efficiency of 6.2%—matching the theoretical maximum rate for a bismuth vanadate cell. Although the efficiency demonstrated was only 6.2%, the tandem device has room for significant improvement in the future, said Stanford Professor Yi Cui, a principal investigator at the Stanford Institute for Materials and Energy Sciences and senior author of an open access paper describing the work published in Scientific Advances.
… 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.
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.
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