Photocathodes Based On N And P Type Si Substrates For Solar Water Splitting

Photoelectrochemical（PEC） water splitting using semiconductor is a promising technology for converting solar energy to produce hydrogen, an important industrial reagent and potential future fuel. Silicon is a promising semiconductor for application as a photocathode in a PEC water splitting system due to its nearly ideal band structure, excellent charge carrier transport properties, and relatively low cost. However, poor stability, surface light reflection and low surface kinetics remain key challenges for silicon used as photocathodes. To create durable, efficient and economical silicon photocathodes, additional strategies for overcoming these challenges must be developed. The main contents of the present research are as follows:（1） We explored a highly stable and efficient multi-crystalline（mc） p type silicon photocathode. A pyramid-like surface nanostructure on mc-Si wafer was fulfilled through a two-step metal-catalyzed chemical etching process, and then a n+p junction photocathode protected by a thin Al₂O₃ layer was constructed. The photocathode exhibits a high stability of continuous photoelectrochemical H2 production for above 100 h after a thin layer of Al₂O₃ is coated on its surface, and its energy conversion efficiency can be up to 6.8% after Pt loading, due to the lowered surface light reflection, increased surface area and minority carrier life time on the electrode surface. The corresponding results have been published on Applied Physics Letters 2015, 106, 013902.（2） 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. In this thesis, we constructed an efficient and stable photocathode based on singlecrystal 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 m W/cm2 simulated solar illumination and Pt catalyzing. The corresponding results have been published on Applied Physics Letters 2015, 106, 213901.