Abstract
The flat-band potential (UFB) of HF-etched single-crystal n-Si(111) and (100) electrodes in concentrated hydrogen halide (HX) solutions shifts toward the negative in the order of HF, HCI, HBr, and HI, i.e., in the order of decreasing electronegativity of halogen atoms. The results can be explained by taking account of formation of Si-X (X=halogen) termination bonds through substitution reaction at Si-OH bonds on the HF-etched Si surfaces. The formation of Si-X bonds leads to changes in surface bond dipole and hence in surface potential. This finding makes it possible to obtain an n-Si chip having different surface band energies at the front and rear surfaces. Such an n-Si chip with surface-band asymmetry can be used to achieve efficient solar-to-chemical energy conversion. It is shown that a high conversion efficiency of 3.4 % was obtained by use of only one n-Si chip through decomposition of HI into H2 and I2 (or I3-) by simulated solar AM 1.5 G (100 mW/cm2) illumination.
