title: Energy upconversion in GaP/GaNP core/shell nanowires for enhanced near-infrared light harvesting.
abstract: Semiconductor nanowires (NWs) have recently gained increasing interest due to their great potential for photovoltaics. A novel material system based on GaNP NWs is considered to be highly suitable for applications in efficient multi-junction and intermediate band solar cells. This work shows that though the bandgap energies of GaN(x)P(1-x) alloys lie within the visible spectral range (i.e., within 540-650 nm for the currently achievable x < 3%), coaxial GaNP NWs grown on Si substrates can also harvest infrared light utilizing energy upconversion. This energy upconversion can be monitored via anti-Stokes near-band-edge photoluminescence (PL) from GaNP, visible even from a single NW. The dominant process responsible for this effect is identified as being due to two-step two-photon absorption (TS-TPA) via a deep level lying at about 1.28 eV above the valence band, based on the measured dependences of the anti-Stokes PL on excitation power and wavelength. The formation of the defect participating in the TS-TPA process is concluded to be promoted by nitrogen incorporation. The revealed defect-mediated TS-TPA process can boost efficiency of harvesting solar energy in GaNP NWs, beneficial for applications of this novel material system in third-generation photovoltaic devices.
authors.affiliation: Department of Physics, Chemistry and Biology, Linköping University, Linköping, S-581 83, Sweden.
authors.forename: Alexander
authors.initials: A
authors.lastname: Dobrovolsky
authors.forename: Supanee
authors.initials: S
authors.lastname: Sukrittanon
authors.forename: Yanjin
authors.initials: Y
authors.lastname: Kuang
authors.forename: Charles W
authors.initials: CW
authors.lastname: Tu
authors.forename: Weimin M
authors.initials: WM
authors.lastname: Chen
authors.forename: Irina A
authors.initials: IA
authors.lastname: Buyanova
keywords: nanowires; optical properties; photoluminescence; solar cells; upconversion