Abstract
We investigate the photovoltaic performance of solar cells based on n-AlxIn1−xN (x = 0–0.56) on p-Si (100) hetero-junctions deposited by radio frequency sputtering. The AlxIn1−xN layers own an optical bandgap absorption edge tuneable from 1.73 eV to 2.56 eV within the Al content range. This increase of Al content results in more resistive layers (≈10−4–1 Ω·cm) while the residual carrier concentration drops from ~1021 to ~1019 cm−3. As a result, the top n-contact resistance varies from ≈10−1 to 1 MΩ for InN to Al0.56In0.44N-based devices, respectively. Best results are obtained for devices with 28% Al that exhibit a broad external quantum efficiency covering the full solar spectrum with a maximum of 80% at 750 nm, an open-circuit voltage of 0.39 V, a short-circuit current density of 17.1 mA/cm2 and a conversion efficiency of 2.12% under air mass 1.5 global (AM1.5G) illumination (1 sun), rendering them promising for novel low-cost III-nitride on Si photovoltaic devices. For Al contents above 28%, the electrical performance of the structures lessens due to the high top-contact resistivity.
Highlights
Nowadays, the main goal of the photovoltaic industry is to develop novel technologies that manage to simultaneously improve device efficiency and reduce fabrication costs
The growth of Alx In1−x N alloys has been reported by different techniques, but mainly by metal-organic chemical vapour deposition [5,6,7], molecular beam epitaxy [8,9,10], and sputtering [11,12,13]
To get rid of this double diode characteristic of the junction, we studied the effect of increasing the substrate temperature so that the mobility of the adatoms at the growing surface would be high enough to reduce the density of the non-desired recombination centres [25]
Summary
The main goal of the photovoltaic industry is to develop novel technologies that manage to simultaneously improve device efficiency and reduce fabrication costs. New low-cost materials are needed to complement the already established Si technology. III-nitride semiconductors and alloys are very promising for application in solar cells because of their tunable wide direct bandgap energy from the near-infrared (0.7 eV, InN) to the ultraviolet (6.2 eV, AlN) and their particular material properties, such as thermal and chemical stability, make them quite promising to be used in space applications [3]. The difficulties of growing high-quality and single-phase Alx In1−x N relies on the big differences between its binary constituents, InN and AlN (bonding energies, lattice mismatch, and growth temperature) being responsible for the immiscibility gap, and for the phase separation and composition inhomogeneities commonly present in the alloy [4]. The growth of Alx In1−x N alloys has been reported by different techniques, but mainly by metal-organic chemical vapour deposition [5,6,7], molecular beam epitaxy [8,9,10], and sputtering [11,12,13]
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