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 2 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.

Optimizing GaNP Coaxial Nanowires for Efficient Light Emission by Controlling Formation of Surface and Interfacial Defects

“…Recently we have shown that these functionalities can be further enhanced in the case of nanowires. For example, the light harvesting efficiency of GaNP NW arrays can be improved via energy upconversion caused by two-step two photon absorption [15].…”

Raman spectroscopy of GaP/GaNP core/shell nanowires

“…8 A promising material system attractive for applications in solar cells and light-emitting diodes is GaN x P 1-x / GaN y P 1-y core/shell NWs. [9][10][11][12] Incorporation of several percent of nitrogen in gallium phosphide reduces its lattice constant, thereby minimizing lattice mismatch with Si. 13 Also, a strong interaction between the N-related localized states and the extended states of GaP modifies the band structure of the forming alloy leading to an increased oscillator strength of the band-to-band optical transitions [14][15][16] and a splitting of the conduction band states into two subbands.…”

Dual-wavelength excited photoluminescence spectroscopy of deep-level hole traps in Ga(In)NP

“…There are in principle several possible physical mechanisms that could lead to this PL enhancement. The first possibility is that P2 with a below-bandgap photon energy could generate free electrons and holes via defect-mediated upconversion or two-photon nonlinear processes, 35,36 leading to enhanced PL emissions. This possibility is not plausible here because photo-excitation with only the NIR light P2 did not result in any sizable PL emissions.…”