Solar water splitting with a composite silicon/metal oxide semiconductor electrode

Abstract

We have studied solar water splitting with a composite semiconductor electrode, composed of an n-i-p junction amorphous silicon (a-Si, E g 1.7 eV) layer, an indium tin oxide (ITO) layer, and a tungsten trioxide (WO 3 , E g 2.8 eV) particulate layer. The n-i-p a-Si layer, which had more accurately a structure of n-type microcrystalline ( c) 3C-SiC:H (25 nm)/i-type a-Si:H (400 nm)/p-type a-SiC x :H (25 nm), was prepared on a TiO 2 -covered F-doped SnO 2 (FTO)/glass plate by a Hot-Wire CVD method. The ITO layer (100 nm thick) was deposited on the p-type a-Si by the DC magnetron sputtering method, and the WO 3 particulate layer was formed by a doctor-blade method, using a colloidal solution of commercial WO 3 powder of 10-30 nm in diameter. The composite electrode thus prepared was finally heat-treated at 300 C for 1 h. The anodic (water oxidation) photocurrent for the composite electrode in 0.1 M Na 2 SO 4 yielded an IPCE (incident photon to current efficiency) of about 6 % at 400 nm and was stable for more than 24 h. Besides, the onset potential lay a little (by about 0.05 V) more negative than the equilibrium hydrogen evolution potential, indicating a possibility of solar water splitting with no external bias. A preliminary result for the water photooxidation with an "nGaP/p-Si/Pt dot" electrode is also reported briefly.

Efficient Solar Water Splitting with a Composite “n-Si/p-CuI/n-i-p a-Si/n-p GaP/RuO2” Semiconductor Electrode

“…The main target in recent studies on photovoltaic solar energy conversion is to realize a practically applicable low-cost conversion system with a sufficiently high efficiency and long-term durability. Of a number of approaches thus far studied, an interesting approach is direct solar to chemical conversion, such as solar water splitting, by use of a semiconductor/electrolyte junction. − This approach has merits that (1) no current collection is necessary and a decrease in conversion efficiency by ohmic losses is minimized and (2) a storable fuel such as hydrogen is directly produced.…”

“…Direct solar to chemical conversion can be divided into two types: a photocatalyst type − and a photoelectrode type. − ,− The photocatalyst type has a strong merit in that the fabrication cost can be made extremely low but has a demerit in that it is quite difficult to obtain high conversion efficiencies owing to efficient carrier recombination and reverse reactions. In fact, the solar conversion efficiencies reported to date have remained very low values. ,− ,, …”

“…Fairly recently, stacked high-quality p-n junction solar cells prepared by the MOCVD technique were used as electrodes for high-efficiency solar water splitting, − but unfortunately, these electrodes were expensive for practical application. Very recently, several studies have been made on low-cost oriented composite semiconductor electrodes. ,− The conversion efficiencies for these electrodes are not high enough yet, but the electrodes of this type are of much interest because they have the potential ability to achieve both a high efficiency and low cost.…”

Advances in the application of nanotechnology in enabling a ‘hydrogen economy’

“…It is one of the few semiconductors that are chemically inert and photostable (capable of maintaining its activity over a period of time) in acidic pH. Nakato et al [55] fabricated a composite Si/ITO/WO 3 semiconductor electrode for photosplitting of water, where WO 3 was added to stabilize Si in aqueous solution. However, the anodic photocurrent density was found to be very low for efficient water splitting.…”