Advanced Functional Antireflection Coatings for Broadband Multijunction Solar Cells

High efficiency III‒V semiconductor multijunction solar cells hold the record of the highest achieved conversion efficiency. Solar cells based on new materials enabling more than 4-junction architectures will most likely push the highest efficiency above 50% within the next decade. To be able to achieve this goal, every aspect of the solar cell structure needs to be designed and fabricated spot on, minimizing any possible optical and electrical losses. To this end, broadband antireflection coatings are instrumental for suppressing the amount of reflected light from the surface of the solar cell. This work contributes to the development of broadband antireflection coatings for primary use in connection with high efficiency multijunction solar cells. As the bandwidth of the utilized solar irradiation is getting increasingly wider, the antireflection coatings based on standard planar structures become harder to optimize, requiring fabrication of more complex films. On the other hand, there is a need to deploy simple and cost effective fabrication techniques to enable economical deployment of new photovoltaic technologies.

This work focuses on developing multilayer antireflection coatings that utilize a nanostructured top layer to surpass the limitations of the conventional planar structures. As a first strand of work, material properties and their relation to the fabrication processes are investigated for low refractive index MgF2 films deposited by electron beam evaporation and the high refractive index Ta2O5 films deposited by ion beam sputtering. The second major part is related to the investigation of a novel technique to fabricate nanostructures with antireflective properties employing a simple de-ionized water treatment. The process is applied to form randomly distributed nanostructures on thin planar amorphous Al2O3 layer. A key result introduced in this work is the novel integration of the alumina nanostructuring with an underlying multilayer antireflection coating, specifically aimed to be used in lattice-matched III‒V semiconductor multijunction solar cells. The performance of the nanostructured coating was assessed in practical III-V multijunction solar cells, revealing its suitability for practical applaiction. Finally, the stability and durability of the nanostructrure has been improved using a hydrophobicity treatment based on fluoropolymerization, and evaluated under atmospheric icing conditions.