Abstract
We demonstrate for the first time the operation of GaInNAs and GaAs n-i-p-i doping solar cells with ion-implanted selective contacts. Multiple layers of alternate doping are grown by molecular beam epitaxy to form the n-i-p-i structure. After growth, vertical selective contacts are fabricated by Mg and Si ion implantation, followed by rapid thermal annealing treatment and fabrication into circular mesa cells. As means of characterisation, spectral response and illuminated current–voltage (I-V) were measured on the samples. The spectral response suggests that all horizontal layers are able to contribute to the photocurrent. Performance of the devices is discussed with interest in the n-i-p-i structure as a possible design for the GaInP/GaAs/GaInNAs tandem solar cell.
Highlights
The highest efficiency solar cells are GaAsbased tandem solar cells (GaAs, GaInP and Ge) achieving efficiencies greater than 32% (AM 1.5 G at one sun spectrum) [1]
We theoretically modelled dilute nitride n-i-p-i solar cells both alone and when incorporated as the third junction in a GaInP/GaAs/GaInNAs tandem solar cell [10]
Ion implantation of the selective contact offers the possibility of fabricating contacts on the front and back of the device without shorting it as the implants can start deep in the device
Summary
The highest efficiency solar cells are GaAsbased tandem solar cells (GaAs, GaInP and Ge) achieving efficiencies greater than 32% (AM 1.5 G at one sun spectrum) [1]. Multi-junction solar cells can achieve higher efficiency than single-junction cells as each junction is tuned to absorb a certain band of the solar spectrum. This reduces thermal relaxation and belowbandgap photon losses. The optimal bandgap for the bottom cell is 1 eV, but germanium with a bandgap of 0.67 eV is commonly used, limiting the efficiency of the tandem cell. Replacement of the Ge junction by dilute nitrides (Ga0.91In0.09N0.03As0.97 for example) with 1-eV bandgap could increase the theoretical limit for external efficiency from 42% to 52% at 1.5 AMD at 500 suns concentration [2] while remaining lattice-matched to GaAs
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