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Institute of Energy and Climate Research
IEK-5 Photovoltaics

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Water Splitting

The generation of fuels from renewable energies is considered a task of great importance for a sustainable energy supply in the future. Decomposition of water by a photo-electrochemical process is a possibility to harvest solar energy in the form of hydrogen. For this purpose a device can be used which, when immersed in an aqueous electrolyte and illuminated by sunlight, will be able to split water into hydrogen and oxygen. To generate hydrogen from water via electrolysis a minimum voltage of 1.23 V is required. In real systems due to so called overpotential losses, voltages around 1.5 – 2.0 V are needed, mainly depending on the type of catalyst used.
In a combination of a photovoltaic (PV) cell and an electrochemical (EC) cell, the solar cell illuminated by sunlight serves as a power source with an appropriate output voltage to drive the electrochemical reaction of splitting H2O into molecular H2 and O2. To supply a sufficient voltage at a reasonable solar-to-hydrogen efficiency, the solar cell has to comprise multiple junctions. In this case, the voltages of the sub cells, connected in series, add up to the required total voltage. Each sub cell has different optical and electrical properties (e.g. different band gaps), and the light passes one sub cell after another. An obvious advantage of this approach is an improved utilization of the solar spectrum and reduced thermalization losses compared to single junction devices.
We have developed vertically integrated multi-junction solar cells based on thin film silicon alloys which provide a wide range of appropriate output voltages for the water splitting process. In combination with an electrolyzer using noble metal catalysts such a device can produce hydrogen with a solar-to-hydrogen efficiency close to 10% [1].
However, the intimate contact of the light absorbing semiconductor material with the electrolyte creates challenges in the chemical stability of the device. The latter problem together with a lateral upscaling of the device has meanwhile also been successfully addressed [2].
An alternative approach uses the in-house know how on module integration by laser ablation processes to fabricate an PV-EC module with side-by-side arranged electrodes and promising potential for large area upscaling [3]

[1] F. Urbain et al., Energy & Env. Science 9 (2016) 145 - 154 [DOI:10.1039/C5EE02393A]
[2] J.-P. Becker, et al., J. Mat. Chemistry A 5 (2017) 4818 - 4826 [DOI: 10.1039/C6TA10688A]
[3] B. Turan et al., Nature Comm. 7, 12681 (2016) [DOI: 10.1038/ncomms12681]