Photoelectrochemical water splitting by tandem type and heterojunction amorphous silicon electrodes

The tandem type hydrogenated amorphous silicon (a-Si) electrode having an n–i–p–n–i–p structure and a similar tandem a-Si electrode having n–i–p–n–i–p–n layers deposited on p-type crystalline Si showed cathodic photocurrents accompanied by hydrogen evolution starting at potentials 1.67 and 2.08 V, respectively, more positive than the thermodynamic hydrogen evolution potential in a sulfuric acid solution. These electrodes, when connected to an RuO 2 counterelectrode, caused sustained water splitting without external bias, giving solar-to-chemical conversion efficiencies of 1.98% and 2.93%, respectively, under simulated AM 1, 100 mW cm −2 solar radiation. These efficiencies are critically compared with the efficiency of another type of solar photoelectrolysis of water, namely, the electrolysis of water by the electrical output from solid-state solar cells.