  Antireflection coatings are an interesting challenge for multijunction solar
cells due to their broadband spectrum absorption and the requirement of current
matching of each subcell. A new design for multijunction solar cell
antireflection coatings is presented in this work in which alternative high and
low index materials are used to minimize the reflection in a broadband
(300nm-1800nm). We compared the short circuit current density of high-low
refractive index stacks designs with optimum double-layer antireflection
coatings by considering two optical materials combinations (MgF2/ZnS and
Al2O3/TiO2) for the AM0 and AM1.5D spectra. The calculations demonstrate that
for lattice-matched triple-junction solar cells and inverted metamorphic
quadruple-junction solar cells, high-low refractive index stacks outperform the
optimum double-layer antireflection coatings. The new design philosophy
requires no new optical materials because only two materials are used and
exhibits excellent performance in broadband spectra. The angle performance of
these antireflection coatings is slightly better than classical double-layers
whereas the analysis for thickness sensitivity shows that small deviations from
deposition targets only slightly impact the performance of antireflection
coatings. Finally, some technical solutions for depositing these high-low
refractive index multilayers are discussed.
