Metal electrodeposition onto (photo)electrochemically prepared nanostructured silicon surfaces: solar applications

Self-organised electrochemical processes such as oscillatory currents on silicon electrodes produce surface patterns that can be used for low temperature preparation of scaleable efficient photoactive structures, Nanoporous oxides, formed during such processes, provide the template for device fabrication. Electrodeposition of suited metals into the oxide nanopores produces Schottky type, i.e. rectifying semiconductor-metal, junctions. With n-Si/SiO2/Pt/I--I-3(-) photoelectrochemical solar cells, solar to electrical conversion efficiencies above 10% have been realised. With an analogous structure based on p type Si, light assisted H-2 evolution is found. In system synchrotron radiation photoelectron spectroscopy (SRPES), high resolution scanning electron miroscopy (HRSEM) and tapping mode atomic force microscopy (TM-AFM) are employed to characterise the surface chemistry and nanotopography upon Pt electrodeposition in model experiments on (hydrogen terminated) Si (111) (1 x 1): H and step bunched Si (111) surfaces. The successive occurrence of valence band induced Pt deposition at anodic polarisation of n-Si and of conduction band induced reduction when the semiconductor is in an accumulation condition is observed and discussed. The concurrent formation of silicon oxide in the valence band deposition reaction appears to be favourable for the device performance, suggesting the presence of a semiconductor-oxide-metal junction. Comparing the output power performance of the Pt system with that prepared with other metals where the deposition reaction potential does not overlap with the valence band shows superior performance of structures made with Pt nanoislands.