Prof. Achim J. Lewerenz
The fundamentals of photovoltaic and photoelectrocatalytic modes of solar energy conversion are reviewed. Novel approaches such as excitonic solar cells, monolithically integrated semiconductor structures for photoelectrocatalysis and bio-inspired catalytic systems will be emphasized. Photonic excitation energy transfer (Foerster and Dexter processes) will be shortly discussed. The (photo)electrochemical fabrication of a nanoemitter device by a self-organized process that operates in the photovoltaic and photoelectrocatalytic mode of a photoelectrochemical cell is described. Solar-to-electricity conversion efficiencies reach presently 11.2% for the n-Si/SiO2-Pt/HI-I- I3-/C cell. The cell is produced by site selective deposition of Pt into nanopores of silicon oxide that are formed during current oscillations of Si in fluoride containing electrolytes. Photoelectrocatalytic half cells with p-Si/SiO2-Pt,Ir, PtIr/H+-H2/C show reduced solar-to-fuel efficiencies which is attributed to the insufficient contact potential difference between p-Si and the hydrogen redox couple. Surface analyses on the noble metal electrodeposition processes are performed using synchrotron radiation photoelectron spectroscopy, atomic force microscopy, transmission electron microscopy and scanning tunnelling spectroscopy on Pt nanoemitter islands. The data suggest that an MOS-type junction is formed between deposited Pt (Ir) nanoislands and the silicon substrate. Future routes to efficient and stable devices are outlined.