Transferable III-V thin films, combined with light trapping structures, present several interests for photovoltaics: cost, material usage and weight reduction, flexible devices… To obtain such films, remote epitaxy consists in growing above a graphene covered III-V substrate, providing detachable mono-crystals. We report the fabrication of large-area graphene/GaAs substrates by a metal-assisted dry transfer with a high yield (<95%), reduced damage to the lattice, negligible doping, and stress relaxation. After the optimization of chemical etching steps, XPS reveals a residue-free surface with low oxidation levels compared to conventional transfers. Nucleation studies using MBE resulted in the formation of microcrystals, with partial alignment with the underlying GaAs(001).
KEYWORDS: Solar cells, Nanowires, Silicon, Doping, Photovoltaics, Multijunction solar cells, Group III-V semiconductors, Beryllium, Electron beams, Microscopy
Today, the record in photovoltaic (PV) conversion efficiency is detained by multi-junction solar cells based on III-V semiconductors. However, the wide adoption of these devices is hindered by their high production cost, especially the expensive III-V substrates. As an alternative, a hybrid solar cell was proposed by LaPierre et al.1 The cell geometry, which combines a 2D Si bottom-cell with a nanowire (NW) top-cell in a tandem device, presents a theoretical efficiency record of 34% when the top-cell band gap lies around 1.7 eV[1],[2].
In this work, we report the elaboration, nanoscale characterization and device fabrication of solar cells based on axial junction GaAsP NWs. Organized GaAsP NWs were grown on patterned SiO2/Si(111) substrates by MBE. Junction was axially created during the growth by incorporating different doping impurities (Be for p- and Si for n-doping). In-situ surface passivation using a radial GaP shell was applied to reduce non-radiative recombinations on surface states[3]. Local I-V characteristics and electron beam induced current (EBIC) microscopy under different biases were used to probe the electrical properties and the generation patterns of individual NWs. The doping concentrations and the minority carrier diffusion lengths were extracted from the EBIC generation profiles. Macroscopic devices based on NW arrays were fabricated by dielectric encapsulation and ITO contacting. Top view EBIC analyses were applied to probe the device homogeneity.
References
[1] R.R. LaPierre et al., J. Appl. Phys. 110 (2011), 014310.
[2] S. Bu et al., Appl. Phys. Lett. 102 (2013), 031106.
[3] C. Himwas et al., Nanotechnology. 28 (2017), 495707.
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