129. EFFICIENT PHOTOELECTROCHEMICAL SOLAR CELLS WITH 3D METAL-OXIDE/SI BRANCHED NANOWIRE HETEROSTRUCTURES

Department: Electrical & Computer Engineering
Faculty Advisor(s): Deli Wang | Sungho Jin

Primary Student
Name: Alireza Kargar
Email: akargar@ucsd.edu
Phone: 858-822-4629
Grad Year: 2015

Student Collaborators
Soheil Seena Partokia, spartokia@gmail.com | Chulmin Choi, cnchoi@eng.ucsd.edu | Ke Sun, k3sun@ucsd.edu | Yi Jing, yijing@ucsd.edu

Abstract
Photoelectrochemical (PEC) solar cells, which convert solar energy to hydrogen through water splitting, have been considered as one of promising approaches for energy demand in the near future. They make possible to store solar energy in H2 molecules. Advanced nanostructures can pave the PEC water splitting towards a viable technology in the near future. Branched nanowire (NW) heterostructure is a promising advanced nanostructure, which offers enhanced surface area, improved light absorption, and increased gas evolution (compared to the most studied advanced nanostructure for solar water splitting; core/shell NW array). Herein, we show the fabrication of highly ordered 3D branched NW heterostructures consisting of Si NW cores and metal oxide branches (ZnO and α-Fe2O3) using solution growth method. The enhanced photocathodic behaviors of 3D ZnO/Si branched NW heterostructures have been demonstrated. We have optimized their PEC performances based on different sizes of NWs. The optimized heterostructure provides a high H2 production observed with the naked eyes. Besides the photocathodic behaviors, these ZnO/Si branched NW arrays can also provide photoanodic behaviors for O2 production through changing the level of doping in p-Si NW cores. The challenge for these ZnO/Si branched NWs is that ZnO is not that stable in electrolyte to protect the Si NWs. Hematite (α-Fe2O3) NW, with a bandgap of ~2.2 eV, is a promising stable photoelectrode material, which we have grown on Si NW cores to replace ZnO NW branches. Using different dopants such as Sn and Ti for α-Fe2O3 NW branches, the PEC performance of branched NWs is studied to have the optimum PEC performance. Furthermore, the PEC performances of 3D α-Fe2O3/Si branched NWs are optimized based on various lengths of Si and α-Fe2O3 NWs. These simple-fabricated and cost-effective branched NW photoelectrodes open up promising approaches for high-efficiency, low-cost and scalable photoelectrochemical solar cells for H2 production.

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