Lihao Han

Efficient solar water splitting by enhanced charge separation in a bismuth vanadate-silicon tandem photoelectrode

Metal oxides are generally very stable in aqueous solutions and cheap, but their photochemical activity is usually limited by poor charge carrier separation. Here we show that this problem can be solved by introducing a gradient dopant concentration in the metal oxide film, thereby creating a distributed n(+)-n homojunction. This concept is demonstrated with a low-cost, spray-deposited and non-porous tungsten-doped bismuth vanadate photoanode in which carrier-separation efficiencies of up to 80% are achieved. By combining this state-of-the-art photoanode with an earth-abundant cobalt phosphate water-oxidation catalyst and a double- or single-junction amorphous Si solar cell in a tandem configuration, stable short-circuit water-splitting photocurrents of ~4 and 3 mA cm(-2), respectively, are achieved under 1 sun illumination. The 4 mA cm(-2) photocurrent corresponds to a solar-to-hydrogen efficiency of 4.9%, which is the highest efficiency yet reported for a stand-alone water-splitting device based on a metal oxide photoanode.

Efficient Water‐Splitting Device Based on a Bismuth Vanadate Photoanode and Thin‐Film Silicon Solar Cells

A hybrid photovoltaic/photoelectrochemical (PV/PEC) water-splitting device with a benchmark solar-to-hydrogen conversion efficiency of 5.2% under simulated air mass (AM) 1.5 illumination is reported. This cell consists of a gradient-doped tungsten-bismuth vanadate (W:BiVO4 ) photoanode and a thin-film silicon solar cell. The improvement with respect to an earlier cell that also used gradient-doped W:BiVO4 has been achieved by simultaneously introducing a textured substrate to enhance light trapping in the BiVO4 photoanode and further optimization of the W gradient doping profile in the photoanode. Various PV cells have been studied in combination with this BiVO4 photoanode, such as an amorphous silicon (a-Si:H) single junction, an a-Si:H/a-Si:H double junction, and an a-Si:H/nanocrystalline silicon (nc-Si:H) micromorph junction. The highest conversion efficiency, which is also the record efficiency for metal oxide based water-splitting devices, is reached for a tandem system consisting of the optimized W:BiVO4 photoanode and the micromorph (a-Si:H/nc-Si:H) cell. This record efficiency is attributed to the increased performance of the BiVO4 photoanode, which is the limiting factor in this hybrid PEC/PV device, as well as better spectral matching between BiVO4 and the nc-Si:H cell.

Extracting large photovoltages from a-SiC photocathodes with an amorphous TiO2 front surface field layer for solar hydrogen evolution

A thin film heterojunction photocathode is fabricated by depositing an n-type amorphous titanium dioxide (TiO2) onto a p-type/intrinsic hydrogenated amorphous silicon carbide (a-SiC). Using this configuration, the photovoltage of the photocathode increases from 0.5 V to 0.8 V under open circuit conditions, indicating the change in band-edge energetics from the semiconductor–liquid junction to the isolated solid p–i–n junction. The p–i–n structure produces an internal electric field that increases the operating photovoltage, and subsequently improves the drift mechanism of photogenerated charge carriers across the intrinsic layer. The enhancement of the photovoltage leads to a very positive photocurrent onset potential of +0.8 V vs. RHE and exhibits a photocurrent density of 8.3 mA cm−2 at 0 V vs. RHE with only a 100 nm absorber layer. The a-SiC photocathode with a front surface field amorphous TiO2 layer shows a high stability for 12 hours of operation under photocatalytic conditions. This high performance, very thin, and earth-abundant photocathode is very promising for integration with smaller band gap solar absorbers to form a multijunction system for highly efficient bias-free solar water splitting devices.

Gradient dopant profiling and spectral utilization of monolithic thin-film silicon photoelectrochemical tandem devices for solar water splitting

A cost-effective and earth-abundant photocathode based on hydrogenated amorphous silicon carbide (a-SiC:H) is demonstrated to split water into hydrogen and oxygen using solar energy. A monolithic a-SiC:H photoelectrochemical (PEC) cathode integrated with a hydrogenated amorphous silicon (a-SiC:H)/nano-crystalline silicon (nc-Si:H) double photovoltaic (PV) junction achieved a current density of −5.1 mA cm−2 at 0 V versus the reversible hydrogen electrode. The a-SiC:H photocathode used no hydrogen-evolution catalyst and the high current density was obtained using gradient boron doping. The growth of high quality nc-Si:H PV junctions in combination with optimized spectral utilization was achieved using glass substrates with integrated micro-textured photonic structures. The performance of the PEC/PV cathode was analyzed by simulations using Advanced Semiconductor Analysis (ASA) software.

Size control, quantum confinement, and oxidation kinetics of silicon nanocrystals synthesized at a high rate by expanding thermal plasma

The growth mechanism of silicon nanocrystals (Si NCs) synthesized at a high rate by means of expanding thermal plasma chemical vapor deposition technique are studied in this letter. A bimodal Gaussian size distribution is revealed from the high-resolution transmission electron microscopy images, and routes to reduce the unwanted large Si NCs are discussed. Photoluminescence and Raman spectroscopies are employed to study the size-dependent quantum confinement effect, from which the average diameters of the small Si NCs are determined. The surface oxidation kinetics of Si NCs are studied using Fourier transform infrared spectroscopy and the importance of post-deposition passivation treatments of hydrogenated crystalline silicon surfaces are demonstrated.

Reliable Performance Characterization of Mediated Photocatalytic Water-Splitting Half Reactions

Photocatalytic approaches using two sets of semiconductor particles and a pair of redox-shuttle mediators are considered as a safe and economic solution for solar water splitting. Here, accurate experimental characterization techniques for photocatalytic half reactions are reported, investigating the gas as well as the liquid products. The methods are exemplified utilizing photocatalytic titania particles in an iron-based aqueous electrolyte for effective oxygen evolution and mediator reduction reactions under illumination. Several product characterization methods, including an optical oxygen sensor, pressure sensor, gas chromatography, and UV/Vis spectroscopy are used and compared for accurate, high-resolution gas-products and mediator conversion measurements. Advantages of each technique are discussed. A high Faraday efficiency of 97.5±2 % is calculated and the reaction rate limits are investigated.

Optical modeling of an efficient water splitting device based on bismuth vanadate photoanode and micromorph silicon solar cells

We demonstrate a photoelectrochemical/photovoltaic (PEC/PV) device based on a combined configuration with a bismuth vanadate (BiVO4) photoanode in the front and a thin film solar cell as power source in the rear. An optical model of this PEC/PV configuration is built by our in-house developed Advance Semiconductor Analysis (ASA) software. Micromorph (a-Si:H/nc-Si:H) cells are investigated in the optical model in reference to the photoanode filtered solar spectrum. The optimum micromorph cell with 400 nm a-Si:H i-layer is confirmed to have the best performance when combined with BiVO4 photoanode. A solar-to-hydrogen efficiency of 5.2%, the highest conversion efficiency of metal oxide materials ever reported is achieved.