Improvement in photovoltage and stability of porous n-Si electrodes coated with platinum by regulation of the thickness of nanoporous layers

Porous n-Si electrodes, prepared by photoetching in HF under appropriate conditions, have macroporous layers at the surface, consisting of micrometer-sized pores and Si pillars. The wall and top of the Si pillars are further covered with 0.2-0.5-mu m-thick nanoporous layers having nanometer-sized pores. The nanoporous layer can be thinned by immersion in HF. The solar cell characteristics (open-circuit photovoltage V-OC, fill factor, and stability) for the porous n-Si electrodes with Pt coating in 8.6 M HBr/0.05 M Br-2 were improved by thinning the nanoporous layer to an appropriate thickness, although the electrodes with no nanoporous layers gave only poor characteristics. The maximum solar energy conversion efficiency of 14% (V-OC 0.575 V, j(SC) 34.7 mA . cm(-2), and fill factor 0.701) was obtained, which is one of the highest for n-Si photoelectrochemical solar cells. A mechanism for the generation of high V-OC's as well as high fill factors in porous Si-based photoelectrochemical solar cells is discussed including a possibility of a low resistivity of the nanoporous layer for hole transport.