N-type silicon photocathodes with Al-doped rear p+ emitter and Al2O3-coated front surface for efficient and stable H2 production

Abstract

Currently, p-type silicon has been studied as a photocathode in a photoelectrochemical cell for water splitting where an n+ thin layer is usually fabricated on electrode surface in order to increase band bending at the n+p interface relative to the aqueous solution/p-Si interface. However, this leads to high Auger recombination on the reaction interface. We report herein an efficient and stable photocathode based on single-crystal n-type Si with a rear np+ junction, different from the conventional one on p-type Si with a front n+p junction. Using a thin Al2O3 surface protecting layer, it shows no loss in photoelectrochemical performance after 138 h of continuous operation, and the energy conversion efficiency can be nearly doubled to 8.68%, compared with 4.51% for the corresponding normal n+p electrode under 100 mW/cm2 simulated solar illumination and Pt catalyzing. Our np+ Si photocathodes improve the H2 production by providing: (1) high on-set potential due to the rear junction; (2) high carrier life time on the electrode surface due to the low doping level of n-type Si; and (3) excellent passivating effect of Al2O3 on the surface of n-type Si.