Both film thickness and surface morphology of the film changes the optical properties. As you know nanocrystals and organic materials are showing amorphous alignment in the thin film. Thats why they are not forming a real crystal structure. Every nanocrstal you gonna add to the thin film will change the optical way, so "n&k" values will be depended to the thichness.
You can find a series of experimental data and detail description in the 'Chemical Solution Deposition' by Gary Hodes. If thickness of CdS is already over 100-nm then the band gap change is not significant.