Sang Youn Chae

Effect of the Si/TiO2/BiVO4 Heterojunction on the Onset Potential of Photocurrents for Solar Water Oxidation

BiVO4 has been formed into heterojunctions with other metal oxide semiconductors to increase the efficiency for solar water oxidation. Here, we suggest that heterojunction photoanodes of Si and BiVO4 can also increase the efficiency of charge separation and reduce the onset potential of the photocurrent by utilizing the high conduction band edge potential of Si in a dual-absorber system. We found that a thin TiO2 interlayer is required in this structure to realize the suggested photocurrent density enhancement and shifts in onset potential. Si/TiO2/BiVO4 photoanodes showed 1.0 mA/cm(2) at 1.23 V versus the reversible hydrogen electrode (RHE) with 0.11 V (vs RHE) of onset potential, which were a 3.3-fold photocurrent density enhancement and a negative shift in onset potential of 300 mV compared to the performance of FTO/BiVO4 photoanodes.

Insight into Charge Separation in WO3/BiVO4 Heterojunction for Solar Water Splitting

Recently, the WO3/BiVO4 heterojunction has shown promising photoelectrochemical (PEC) water splitting activity based on its charge transfer and light absorption capability, and notable enhancement of the photocurrent has been achieved via morphological modification of WO3. We developed a graft copolymer-assisted protocol for the synthesis of WO3 mesoporous thin films on a transparent conducting electrode, wherein the particle size, particle shape, and thickness of the WO3 layer were controlled by tuning the interactions in the polymer/sol-gel hybrid. The PEC performance of the WO3 mesoporous photoanodes with various morphologies and the individual heterojunctions with BiVO4 (WO3/BiVO4) were characterized by measuring the photocurrents in the absence/presence of hole scavengers using light absorption spectroscopy and intensity-modulated photocurrent spectroscopy. The morphology of the WO3 photoanode directly influenced the charge separation efficiency within the WO3 layer and concomitant charge collection efficiency in the WO3/BiVO4 heterojunction, showing the smaller sized nanosphere WO3 layer showed higher values than did the plate-like or rod-like one. Notably, we observed that photocurrent density of WO3/BiVO4 was not dependent on the thickness of WO3 film or its charge collection time, implying slow charge flow from BiVO4 to WO3 can be a crucial issue in determining the photocurrent, rather than the charge separation within the nanosphere WO3 layer.

Role of HA additive in quantum dot solar cell with Co[(bpy)3]2+/3+-based electrolyte

A strategy to improve the power conversion efficiency (η) in a quantum dot solar cell (QDSC) is demonstrated with a model system of TiO2/CdS QDSCs. When the electrolyte is changed from polysulfide to a [Co(bpy)3]2+/3+ complex, a higher Voc and η are observed because of its low redox potential. To resolve the slow diffusion nature of [Co(bpy)3]2+/3+ complexes within dense TiO2 nanoparticle/CdS film, a TiO2 nanorod (NR) array anode is applied, which increases η by more than a factor of 3. In addition, introduction of hexanoic acid (HA) in TiO2 NR/CdS film is found to improve η (Jsc as well as Voc) by alleviating the back recombination loss between CoIII and the TiO2 surface. Electrochemical impedance spectroscopy indicates that the charge transfer resistance on the photoanode decreases by suppressing the interfacial charge recombination after HA treatment, although adding HA in the electrolyte impedes diffusion resistance.

Highly stable tandem solar cell monolithically integrating dye-sensitized and CIGS solar cells

A highly stable monolithic tandem solar cell was developed by combining the heterogeneous photovoltaic technologies of dye-sensitized solar cell (DSSC) and solution-processed CuInxGa1-xSeyS1-y (CIGS) thin film solar cells. The durability of the tandem cell was dramatically enhanced by replacing the redox couple from to [Co(bpy)3]2+ /[Co(bpy)3]3+), accompanied by a well-matched counter electrode (PEDOT:PSS) and sensitizer (Y123). A 1000 h durability test of the DSSC/CIGS tandem solar cell in ambient conditions resulted in only a 5% decrease in solar cell efficiency. Based on electrochemical impedance spectroscopy and photoelectrochemical cell measurement, the enhanced stability of the tandem cell is attributed to minimal corrosion by the cobalt-based polypyridine complex redox couple.

Synthesis of Bi2WO6 photoanode on transparent conducting oxide substrate with low onset potential for solar water splitting

To enable water splitting in photoelectrochemical cells, the conduction-band (CB) and valence-band edges must straddle the hydrogen-reduction and water-oxidation potentials; however, the CB edge potential of many photoanodic semiconductors is insufficient. Here, we demonstrate the nanostructured Bi2WO6 for photoanodic application, which has a high, flat-band potential of 0.15 V vs. RHE. Single-phase, orthorhombic Bi2WO6 nano-structures were successfully grown on an FTO substrate through a two-step hydrothermal synthesis with subsequent thermal treatment at 600 °C. The synthesized Bi2WO6 photoanode shows a lower onset potential and improved photocurrents which were enhanced further by a factor of three by coating the surface with Co-Pi. The low onset potential of the Bi2WO6/Co-Pi photoanode results in a higher operating photocurrent in a p/n photodiode cell combined with a p-Si/n-Si/Pt photocathode.

Design of a Monolithic Photoelectrochemical Tandem Cell for Solar Water Splitting with a Dye-sensitized Solar Cell and WO 3 /BiVO 4 Photoanode

Photoelectrochemical cell (PEC) is one of the attractive ways to produce clean and renewable energy. However, solar to hydrogen production via PEC system generally requires high external bias, because of material`s innate electronic band potential relative to hydrogen reduction potential and/or charge separation issue. For spontaneous photo-water splitting, here, we design dye-sensitized solar cell (DSSC) and their monolithic tandem cell incorporated with a photoanode. has high conduction band edge potential and suitable band gap (2.4eV) to absorb visible light. To achieve efficient photoanode system, electron and hole mobility should be improved, and we demonstrate a tandem cell in which film is connected to cobalt complex based DSSC.