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Published July 2014 | Supplemental Material + Published
Journal Article Open

Operation of lightly doped Si microwires under high-level injection conditions


The operation of lightly doped Si microwire arrays under high-level injection conditions was investigated by measurement of the current-potential behavior and carrier-collection efficiency of the wires in contact with non-aqueous electrolytes, and through complementary device physics simulations. The current-potential behavior of the lightly doped Si wire array photoelectrodes was dictated by both the radial contact and the carrier-selective back contact. For example, the Si microwire arrays exhibited n-type behavior when grown on a n^(+)-doped substrate and placed in contact with the 1,1′-dimethylferrocene+/0–CH_(3)OH redox system. The microwire arrays exhibited p-type behavior when grown on a p^(+)-doped substrate and measured in contact with a redox system with a sufficiently negative Nernstian potential. The wire array photoelectrodes exhibited internal quantum yields of ~0.8, deviating from unity for these radial devices. Device physics simulations of lightly doped n-Si wires in radial contact with the 1,1′-dimethylferrocene^(+/0)–CH_(3)OH redox system showed that the carrier-collection efficiency should be a strong function of the wire diameter and the carrier lifetime within the wire. Small diameter (d < 200 nm) wires exhibited low quantum yields for carrier collection, due to the strong inversion of the wires throughout the wire volume. In contrast, larger diameter wires (d > 400 nm) exhibited higher carrier collection efficiencies that were strongly dependent on the carrier lifetime in the wire, and wires with carrier lifetimes exceeding 5 μs were predicted to have near-unity quantum yields. The simulations and experimental measurements collectively indicated that the Si microwires possessed carrier lifetimes greater than 1 μs, and showed that radial structures with micron dimensions and high material quality can result in excellent device performance with lightly doped, structured semiconductors.

Additional Information

© 2014 Royal Society of Chemistry. Received 18th January 2014; Accepted 18th March 2014. We acknowledge the Department of Energy Office of Basic Energy Sciences grant DOE DE-FG02-03ER15483, and BP for financial support. NCS acknowledges the NSF for an American Competitiveness in Chemistry postdoctoral fellowship (CHE-1042006). Critical support and infrastructure for this work were provided by the Kavli Nanoscience Institute and the Molecular Materials Research Center at Caltech. The angle-resolved optical characterization work was supported by the US Department of Energy 'Light-Material Interactions in Energy Conversion' Energy Frontier Research Center Award (grant DESC0001293).

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