TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Device: In the radiation conversion device, the photo-absorber unit comprises an optically active semiconductor structure having predetermined material composition and thickness being selected to operate as an anti-reflective structure for the certain wavelength range corresponding to maximal absorption of incident electromagnetic radiation by the semiconductor structure. The at least partially reflective structure is a single- or multi-layer structure, where the at least partially reflective structure is configured as a wavelength-selective reflector. The photo-absorber unit comprises the optically active semiconductor structure and an electrode structure which is substantially transparent for the certain wavelength range, the transparent electrode interfacing the at least partially reflective structure on one side and the optically active semiconductor structure at the opposite side. The photo-absorber unit has a thickness substantially not exceeding lambda bar one quarter n, where lambda bar is a weighted average wavelength of the certain wavelength range and n is an effective refractive index of the optically active semiconductor structure. The photo-absorber unit has a thickness smaller than a recombination length for photo-generated charge carriers in the optically active semiconductor structure. The at least partially reflective layer is a dielectric or dichroic mirror structure. The at least partially reflective structure comprises a substrate having the at least partially reflective coating comprising one of the following material compositions:silver-gold and silver-platinum alloys. The optically active semiconductor structure comprises an alpha -iron oxide (Fe2O3) layer. The at least partially reflective structure comprises a substrate having the at least partially reflective coating comprising one of the following material compositions: silver-gold composition with 5-15% gold; and silver-platinum alloys with 10-22% platinum. The device is configured as a photoelectrochemical device. Also, the device is configured for photoelectrolysis of water. The device comprises at least two radiation conversion cells configured to face one another by their radiation absorbing layers with a certain angle to allow incident electromagnetic radiation reflected from one of the cells to propagate towards and be absorbed by the other cell. The at least two radiation conversion cells are arranged in a V shape configuration, the certain angle ranging between 30 and 90 degrees . The device comprises photovoltaic cell located below the at least partially reflective structure, the at least partially reflective structure being configured to reflect light component of the certain wavelength range while transmitting light components with a different wavelength range corresponding the absorption spectrum of the photovoltaic cell; and partially transparent photovoltaic cell located on top of the photo-absorber unit, the partially transparent photovoltaic cell is configured to transmit light components of the certain wavelength range while absorbing a different wavelength range.