In order to surpass the Shockley-Queisser efficiency limit multijunction solar cells have been designed and developed for many years now. However, apart from band-gap engineering there are still many issues to consider. For instance, fabricating monolithic multijunctions 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 semiconductor nanowires on the –mismatched, yet band gap compatible- ultrathin substrate. This design has many advantages: Not only the wires can overcome the lattice mismatching problem thanks to their intrinsic strain relaxation properties, they also create a natural anti-reflection coating. Besides, the periodic design of the wires adds unique optical features that are of great interest for PV applications. In particular, two aspects have been the focus of this design to improve solar energy conversion: Waveguiding properties in each wire and the grating properties of the periodic structure of such wires.
In this work, we study light-matter interactions in GaAs-based nanowire arrays on ultrathin silicon films with the dual goal to obtain large absorption in the array and to improve light trapping in the bottom thin film cell. There are two functions for the top cell here. The first one is not to lose absorption efficiency in the cell by using a thinner layer of absorbing material (which reduces the cost, size and weight of the solar cell). To do so, the geometry of wires (the radius and height) has been optimized so that the incoming light is coupled to the waveguiding modes of the wires. In other words, the intrinsic waveguiding property of high refractive index wires will make them absorb the light with much larger absorption cross section with respect to the geometrical cross section. The second function of the top cell is to help the bottom cell to absorb more efficiently as well. The bottom cell is an ultrathin silicon slab which can be viewed as a 2D waveguide in which the electromagnetic field is bounded in one direction and is forced to propagate in the other two. By optimizing the grating geometry (the pitch of the wires) for the wavelengths close to the bandgap of silicon –where the absorption coefficient is very low- the transmitted light’s momentum is manipulated to match the momentum of the waveguiding modes of the slab.
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.