Department of Physics, Chemistry and Biology, Linköping University, Linköping, S-581 83, Sweden.
Small. 2014 Nov 12;10(21):4403-8. doi: 10.1002/smll.201401342. Epub 2014 Jul 9.
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.
半导体纳米线（NWs）由于在光伏方面的巨大潜力，最近引起了越来越多的关注。基于 GaNP NWs 的新型材料系统被认为非常适合高效多结和中间能带太阳能电池的应用。这项工作表明，尽管 GaN(x)P(1-x) 合金的能带隙能量位于可见光光谱范围内（即对于目前可实现的 x < 3%，位于 540-650nm 范围内），但在 Si 衬底上生长的同轴 GaNP NWs 也可以利用能量上转换来收集红外光。可以通过 GaNP 的反斯托克斯近带边光致发光（PL）来监测这种能量上转换，即使从单个 NW 也可以看到。基于对反斯托克斯 PL 对激发功率和波长的依赖性的测量，确定这种效应的主要过程是由于位于价带上方约 1.28eV 的深能级的两步双光子吸收（TS-TPA）。参与 TS-TPA 过程的缺陷的形成被认为是由氮掺入促进的。所揭示的缺陷介导的 TS-TPA 过程可以提高 GaNP NWs 中太阳能的收集效率，有利于这种新型材料系统在第三代光伏器件中的应用。
