CIGS based devices for solar hydrogen production spanning from PEC-cells to PV-electrolyzers: A comparison of efficiency, stability and device topology

Sustainable solar hydrogen production could be accomplished along several, somewhat different routes. In this paper, the focus is directed towards photoelectrochemical cells and PV-electrolyzers. In particular, the differences and similarities between these two seemingly different approaches are investigated. In a previous article, a theoretical analysis of absorption, charge carrier separation, charge carrier transfer, and catalysis demonstrated that the difference between PEC-cells and PV-electrolyzers for solar hydrogen production is so small that they can be perceived as one unifying concept, termed PDC, or photodriven catalytic hydrogen production. In this paper this perspective is explored further by experimentally constructing a series of devices that stepwise and gradually are spanning from classical PEC-cells to traditional PV-electrolyzers. In each step, the difference and similarities are measured and discussed in terms of the underlying physical processes, the efficiency, the stability, and the device geometry. All the constructed devices were based on CIGS, CuIn(x)Gai(1-x)Se(2), which was demonstrated to be a highly interesting material for the purpose of solar hydrogen production. Devices were constructed for both the hydrogen evolution half-reaction, as well as for the full reaction. Among the most notably of these devices is a stable monolithic device based on CIGS cells interconnected in series, which reached 10% solar-to-hydrogen efficiency for unbiased water splitting. Experimentally, it is found that small changes in the device geometry can transform one device concepts into another with only minute changes in the underlying physics. The series of constructed devices thus bridge and merge the notions of photoelectrochemical cells and PV-electrolyzers. It is further demonstrated that small topological differences in the device architecture can have a profound impact on both the stability and the efficiency of the devices. (C) 2014 Elsevier B.V. All rights reserved.