yes, there is indeed a correlation. To be more precise, the spectral absorption coefficient is related to the complex index of refraction. The real part of the latter is the index of refraction that you measure in your material. If this real part is large, so is the absorption coefficient. Hence, I don't think that there is a material with high index of refraction but low absorption.

The relationship between the refractive index and absorption is indeed described through the complex refractive index, where the real part defines the refractive index itself, and the imaginary part accounts for absorption. However, this correlation is not universal. In certain spectral ranges, it is possible to identify materials with relatively high refractive indices and low absorption.

For elastomers, developing a material with a refractive index between glass (1.5) and silicon (3.9) that also maintains high transparency is a challenging but achievable task. For example, specially modified polymers, doped elastomers, or organic-inorganic hybrids can exhibit the desired properties within specific wavelength ranges. In such materials, the refractive index can be effectively tuned by incorporating nanoparticles or introducing chemical functional groups, without significantly increasing absorption.

For the encapsulation of silicon solar cells, materials based on silicones, fluoropolymers, or other transparent elastomers with adjustable optical properties may be good candidates. Additionally, the use of anti-reflective coatings or graded-index structures could significantly improve optical coupling and reduce reflection losses.

All the best!

Nikita Dubitskiy : Thanks for this good explanation, I was not aware of that - again, learned something new.

Thanks for reasonable answers! @Nikita Dubitskiy and @Johannes Gruenwald,

EVA and polyolifins are typically used in silicon solar cells for encapsulation. However, it is good to explore matheatial calculation steps to how to select a mixing/material. It will reduce experimations to add materials with quantitative calculations of 'optical constant vs diferent materials or doping'.