Proceedings of nanoGe Fall Meeting 2018 (NFM18)
DOI: https://doi.org/10.29363/nanoge.nfm.2018.148
Publication date: 6th July 2018
As an effective way to surpass the Shockley-Queisser efficiency limit multijunction solar cells have been designed and developed for many years now. Silicon, in particular, is very appealing as the low bandgap material in a tandem design since we already have a mature understanding of its optical and electronic properties and its fabrication technology is widely available. For the high bandgap, III-V materials are shown to be promising in both light management and carrier management. However, fabricating monolithic III-V on Si multijunction is still challenged by various limiting factors such as lattice mismatching, high fabrication cost, and the size/weight of the tandem designs which makes them unsuitable for many applications. One way to tackle these issue is epitaxial growth of vertically standing III-V nanowires on Si ultrathin substrate. This design has many advantages: Not only the wires can overcome the mismatching problem thanks to their intrinsic strain relaxation properties, they also create a natural anti-reflection coating.
In this work, we study light-matter interactions in GaAs-based nanowire arrays on ultrathin silicon film with the dual goal of obtaining large absorption in the array and improving light trapping in the bottom thin film cell. By performing FDTD simulations we show that the coupling of the incident light to the HE11 waveguiding mode of the wires not only boosts the absorption in the wires themselves, but also efficiently transfers the non-absorbed light to Si bottom cell. Moreover, the grating properties of the array is capable of changing the momentum of the transmitted light. In this way, light is trapped in the Si thin film. As a result, we induce higher absorption for the wavelengths close to the bandgap of silicon where the absorption coefficient is very low.
To conclude, by optimizing the geometry of both each wire and the grating we are able to firstly couple the light into waveguiding modes of each wire and later couple the transmitted/scattered light into waveguiding modes of the ultrathin silicon layer underneath. By combining these 1D and 2D waveguiding properties a high efficiency ultrathin and flexible tandem cell is designed.
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