For an effective conversion of solar energy to electric power or a chemical fuel a lot of different materials as well as device structures have been suggested but only very few provide technological competitive conversion efficiencies so far. Especially thin film devices originally considered to provide the most cost competitive solution have not fulfilled their promises. Limitations and loss processes can be deduced from a detailed consideration of the involved photovoltaic and electrochemical elementary steps. Optimized performance can only be reached when the photovoltaic and electrolytic boundary conditions of integrated systems are comparable to those of separated devices with no extra loss due to the coupling process.

Feasible solutions seem to be possible for thin film PV and water splitting generation, as we will show with a number of investigations performed recently combining thin film synthesis, electrical or electrochemical investigations with surface science studies. Especially photoemission results on the demands on the bulk and surface electronic structure provide clear boundary conditions on the material’s and surface properties: i) the semiconductors must provide a wide splitting of quasi Fermi levels which will be provided by multi-junctions of classical semiconductors but are not to be expected for semiconductors with localized electron states as e. g. oxides as e. g. hematite due to polaron formation. ii) The growth of the materials must provide compact, grain boundary poor absorber layers to avoid charge recombination and/or transport barriers to adjust the diffusion lenghts to the absorption lengths. Iii) The interfaces between the semiconductor absorber material and the contact layers also including the electrocatalyst must be prepared avoiding Fermi level pinning and non-adjusted electron transfer states by using proper aligned passivation layers. As a consequence a buried junction is needed. We will compare PV and/or PEC cells based on Si, GaInP, and SnS to illustrate our conceptual considerations.

In summary, we do believe that effective thin film PV and PEC devices need the input and coupled scientific approach from different disciplines and expertise ranging from solid state physics, surface science to electrochemistry. Promising advanced technological solutions seem possible based on improved materials science approaches in correlating material’s engineering, manufacturing and resulting properties to the involved physical boundary conditions of the elementary steps of energy conversion.