p-n Junction Silicon Electrode Coated with Noble Metal for Efficient Solar Photoelectrolysis of Hydrogen Iodide

Abstract

The photooxidation of iodide ions on n-type silicon electrodes highly doped with boron from the surface in hydrogen iodide or hydrogen iodide/iodine solutions was investigated. Two doping methods were employed: thermal diffusion and ion implantation. The electrode was remarkably stabilized by coating with a 2 to 3 nm-thick platinum layer, followed by heat-treatment at about 400 °C for ca. 10 min. The photocurrent was stable for more than 500 h at a density of the order of 10 mA cm−2. The boron-doped and Pt-coated n-Si electrode also generated a photovoltage much higher than that for a Pt-coated n-Si electrode, showing an advantage of forming the p-n junction. Photoelectrochemical cells made of a boron-doped-Pt-coated n-Si electrode, a platinized Pt counter electrode, and a commercial cation exchange membrane were found to photoelectrolyze hydrogen iodide without externally applied voltage into hydrogen and iodine with a solar-to-chemical energy conversion efficiency (φchems) under simulated AM 1 solar radiation of 8.2%, probably the highest of the φchems values observed for the cells working without external voltage.

Metal oxide coated p–n junction silicon electrodes for photoelectrochemical solar energy conversion

“…showed an open circuit photovoltage of 0.58 V, a short-circuit photocurrent of 26.6 mA crn-,, a fill factor of 0.6 1, and an energy conversion efficiency of 9.3% under AM1 radiation. This is nearly the same as the best value obtained by us with a platinum-covered p'n-Si electrode (27).…”

“…I, b and d ) means that there is a high resistance at the interface between the Ti02 film and the solution. The electrode coated with TiO, and RuOz (or Ru) gave a j-U curve nearly as good as that for a Pt coated ptn-Si electrode (27). This implies that the pt-Si layer and the TiOz film are in ohmic contact.…”

“…Recently, we reported that the decomposition of hydrogen iodide into hydrogen and iodine was achieved using a silicon single crystal electrode with a p+-n junction inside (abbreviated as p-'n-Si) coated with platinum or its silicide, achieving an approximately 8% conversion efficiency and 4500 h stability (26)(27)(28). A similar efficiency has been obtained for hydrogen iodide decomposition using Pt-coated n ' p-Si electrodes (29).…”

Hydrogen Generation from Irradiated Semiconductor-Liquid Interfaces

“…567 These electrodes, when connected to a RuO 2 counterelectrode (where OER occurred) caused sustained water splitting without external bias with solar-to-chemical conversion efficiencies of 1.98% and 2.93%. 567 Studies on the use of Si for splitting solvents other than water (e.g., HBr, HI 131,203,567 ), will be discussed in a subsequent section below.…”

“…In another study, heavy doping with acceptor impurities introduced p + regions on a n-Si wafer and these p-n junctions in turn (modified with either Pt or Pd islands) were used to drive the photoelectrolysis of HI to H 2 and I 2 . 203 An interesting hybrid configuration involves a AgCl photoanode operated in conjunction with a Pt cathode. 571 The latter, however, was biased with an amorphous Si solar cell; the AgCl photoanode and the a-Si:H solar cell were simultaneously illuminated.…”