The fabrication process of high performance quadruple junction thin film silicon solar cells is described and the application of the solar cells in an integrated photoelectrochemical water splitting device is demonstrated. It is shown that the performance of solar cells can be adjusted by varying the process parameters and the thickness of the absorber layers of the individual sub cells and by integrating microcrystalline silicon oxide as intermediate reflecting layers. Thereby current matching of the sub cells was improved and a high open-circuit voltage of 2.8 V was achieved. Furthermore, the solar cell stability against light-induced degradation was investigated. Efficiencies of 13.2% (initial) and 12.6% (after 1000 h of light-soaking) were achieved. Bias-free water splitting with a solar-to-hydrogen efficiency of 7.8% was demonstrated in an integrated photovoltaic–electrochemical device using the developed quadruple junction photocathode. Finally, it is shown that in the case of quadruple junction solar cells the light-induced degradation has a lower effect on the photovoltaic–electrochemical efficiency as on the pho-tovoltaic efficiency.