Epitaxial layer structures for preparing high-efficiency tandem structures for water splitting

Invited Speaker, T. Hannappel , presentation 073
Publication date: 15th December 2014

Monolithically integrated, solar-driven water splitting systems, which are exposed to an electrolyte, need to be developed with regard to different challenges: Among these are light absorption, electronic and chemical passivation, catalysis, and the interface to the electrolyte, determining conversion (solar-to-hydrogen) efficiency and stability. Also the costs of ingredients have to be considered.
In the design of a water-splitting architecture, a single absorber structure could be used. Here, the energy gap must be large enough (Egap > 2.2 eV) for water splitting, including the thermodynamic minimum of the chemical potential difference together with the over-potentials of anode and cathode providing the driving force for the chemical interfacial reaction. Considering these points, the conversion efficiency that could be achieved for a single junction structure is limited to less than 15% without light concentration and under standard conditions (AM1.5 spectrum).
Significantly higher efficiencies could be achieved utilizing high-performance semiconductor tandem structures, which can potentially reach conversion efficiencies up to 25%. In photovoltaic as well as in water splitting devices, tandem structures based on III-V compounds have been used in record devices. Stability of light-driven water splitting structures can be addressed by using passivation layers out of transition metal oxides, which could be fabricated by atomic layer deposition. In this paper, it is shown that high-performance water-splitting device layer structures can be realized by preparing appropriate III-V-based tandem structures, e.g. via metal organic chemical vapor deposition.
Recently, we have demonstrated a solar-to-hydrogen efficiency of 14%, exceeding existing benchmarks in both efficiency and stability. In situ monitoring and control is an indispensable tool during critical surface-preparation processes with a high complexity. It provides the realization of layer structures and junctions, consisting of high-quality semiconductors for suitable devices with unprecedented performance. In a further achievement, appropriate III-V layers can be directly grown on silicon substrate. Such a material is GaPN(As)/Si(100), exhibiting a perfect band gap combination and also being cost effective. Furthermore, potential future device developments will be discussed, such as silicon/III-V nanowire arrangements.

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