Efficient production of hydrogen from solar energy is anticipated to be an important component in a future sustainable post-carbon energy system. Here we demonstrate that series interconnected absorbers in a PV-electrolysis configuration based on the compound semiconductor CIGS, CuIn x Ga 1Àx Se 2 , are a highly interesting concept for solar water splitting applications. The band gap energy of CIGS can be adjusted to a value close to optimum for efficient absorption of the solar spectrum, but is too low to drive overall water splitting. Therefore we connect three cells in series, into a monolithic device, which provides sufficient driving force for the full reaction. Integrated with a catalyst this forms a stable PV/photo-electrochemical device, which when immersed in water reaches over 10% solar-to-hydrogen efficiency for unassisted water splitting. The results show that series interconnected device concepts, which enable use of a substantial part of the solar spectrum, provide a simple route towards highly efficient water splitting and could be used also for other solar absorbers with similar electro-optical properties. We discuss how the efficiency could be increased for this particular device, as well as the general applicability of the concepts used in this work. We also briefly discuss advantages and disadvantages of photo-electrochemical cells in relation to PV-electrolysis with respect to our results. Broader contextSolar energy is one of the most attractive renewable energy sources as it is abundant, wide spread and basically free. The price for capturing this energy is now starting to get compatible with other energy sources which raises the concern of the intermittent nature of solar irradiation. The production of solar generated hydrogen is both a possible alternative for dealing with this intermittency, as well as for providing a potent fuel and an important feedstock for the industry. Much research has been done in the development of materials usable as photo-electrochemical cells for this purpose. An inherent problem in the design of these devices is the mismatch between the solar spectrum and the thermodynamic and kinetic requirements for the solar water splitting reaction. The standard proposed solution to this problem is to construct tandem devices. Here we instead explore the idea of connecting efficient photo-absorbers in series to obtain the required photo-potential and discuss the similarities between PEC and PV-electrolysis cells. To demonstrate the potential of this approach we manufacture a monolithic device based on series interconnected CIGS cells that reach 10% solar to hydrogen efficiency.

A theoretical analysis of different device concepts for solar hydrogen production, demonstrating the close similarities between photoelectrochemical cells and PV-electrolyzers.

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Reducing absorber layer thickness below 500 nm in regular Cu(In,Ga)Se2 (CIGS) solar cells decreases cell efficiency considerably, as both short-circuit current and open-circuit voltage are reduced because of incomplete absorption and high Mo/CIGS rear interface recombination. In this work, an innovative rear cell design is developed to avoid both effects: a highly reflective rear surface passivation layer with nano-sized local point contact openings is employed to enhance rear internal reflection and decrease the rear surface recombination velocity significantly, as compared with a standard Mo/CIGS rear interface. The formation of nano-sphere shaped precipitates in chemical bath deposition of CdS is used to generate nano-sized point contact openings. Evaporation of MgF2 coated with a thin atomic layer deposited Al2O3 layer, or direct current magnetron sputtering of Al2O3 are used as rear surface passivation layers. Rear internal reflection is enhanced substantially by the increased thickness of the passivation layer, and also the rear surface recombination velocity is reduced at the Al2O3/CIGS rear interface. (MgF2/)Al2O3 rear surface passivated ultra-thin CIGS solar cells are fabricated, showing an increase in short circuit current and open circuit voltage compared to unpassivated reference cells with equivalent CIGS thickness. Accordingly, average solar cell efficiencies of 13.5% are realized for 385 nm thick CIGS absorber layers, compared with 9.1% efficiency for the corresponding unpassivated reference cells.