What is the minimum coating thickness (but not zero) that will minimize the reflection at the wavelength of 705 nm where solar cells are most efficient?

d = 235 nm.

Explanation:

Let the layer be of glass. Refractive index of glass μ = 1.5. Let the required thickness be d.

For minimum reflection, that is destructive interference in thin films, the condition is

2μd = λ (for first order)

d = 705 / 2 x 1.5 = 235 nm [ λ = 705 nm ]