In this study, we improved the hydrogen production efficiency by combining a photosystem I/platinum nanoparticle composite with an artificial light harvesting dye, Lumogen Red.

Photon upconversion via triplet–triplet annihilation (TTA-UC) is a process that converts two lower-energy photons to a higher-energy photon, which is expected to increase the maximum solar cell efficiency beyond the Shockley–Queisser limit. To incorporate TTA-UC into a dye-sensitized solar cell (DSSC), we used a co-adsorption approach, in which both a TTA-UC donor with an alkyl carboxylic acid chain and a TTA-UC acceptor dye were adsorbed onto mesoporous TiO 2 . Incident photon-to-current conversion efficiency spectra and excitation intensity dependence indicated that a photocurrent was generated under irradiation at wavelengths above 490 nm by the TTA-UC mechanism. The power-conversion efficiency of the DSSC was increased to 0.72%, and the photocurrent contributed by TTA-UC was 0.036 mA cm –2 under 1 sun irradiation.

Visible-light-responsive photocatalysts used in the highly efficient hydrogen production exhibit several disadvantages such as photocorrosion and fast recombination. Because of the potential important applications of such catalysts, it is crucial that a simple, effective solution is developed. In this respect, in this study, we combined SiC (β modification) and TiO 2 with CdS to overcome the challenges of photocorrosion and fast recombination of CdS. Notably, we found that when irradiated with visible light, CdS was excited, and the excited electrons moved to the conduction band of TiO 2 , thereby increasing the efficiency of charge separation. In addition, by moving the holes generated on CdS to the valence band of SiC, in the opposite direction of TiO 2 , photocorrosion and fast recombination were prevented. As a result, in the sulfide solution, the CdS/SiC composite catalyst exhibited 4.3 times higher hydrogen generation ability than pure CdS. Moreover, this effect was enhanced with the addition of TiO 2 , giving 10.8 times higher hydrogen generation ability for the CdS/SiC/TiO 2 catalyst. Notably, the most efficient catalyst, which was obtained by depositing Pt as a cocatalyst, exhibited 1.09 mmol g –1 h –1 hydrogen generation ability and an apparent quantum yield of 24.8%. Because water reduction proceeded on the TiO 2 surface and oxidative sulfide decomposition proceeded on the SiC surface, the exposure of CdS to the solution was unnecessary, and X-ray photoelectron spectroscopy confirmed that photocorrosion was successfully suppressed. Thus, we believe that the effective composite photocatalyst construction method presented herein can also be applied to other visible-light-responsive powder photocatalysts having the same disadvantages as CdS, thereby improving the efficiency of such catalysts.

Photosynthetic pigment–protein-based biophotovoltaic devices are attracting interest as environmentally friendly energy sources. Photosystem I (PSI), a photosynthetic pigment–protein, is a proven biophotovoltaic material because of its abundance and high charge separation quantum efficiency. However, the photocurrent of these biophotovoltaic devices is not high because of their low spectral response. We have integrated an artificial light-harvesting antenna into a PSI-based biophotovoltaic device to expand the spectral response. To fabricate the device, a perylene di-imide derivative (PTCDI) was introduced onto a TiO2 surface as an artificial antenna. In the photovoltaic cells formed by the PTCDI/PSI-assembled TiO2 electrode, the magnitude of the incident photon-to-current conversion efficiency spectrum was significantly enhanced in the range 450–750 nm, and the photocurrent increased to 0.47 mA/cm2. The result indicates that the photons absorbed by PTCDI transfer to PSI via Förster resonance energy transfer.

Photosystem I-biophotovoltaics based on p-type dye-sensitized solar cells and a tandem cell was demonstrated.