Question

how the absorption coefficient and thickness of the material is related in photovoltaics?

Answer 1

The solar spectrum hitting the earth corresponds to black body radiation (apart from some absorption lines due to gasses in our atmosphere and inherent to the sun). In principle, one wants to harvest an as large as possible fraction of this spectrum.

Solar cells can be optimized by increasing the absorption at a certain wavelength (which mostly depends on the bandgap of the semiconductor material used). On the other hand, they can also be optimized by joining several layers, which convert different wavelengths into an electrical voltage. I admit that multi-junction solar cells are more at a research stage at the moment, but they present an interesting approach.