If you are getting enough light penetration that it would reflect off the material with enough intensity to make a difference, you are already getting 100% coverage. The underside of the leaves dont need light.
Also, I would think a impermeable membrane over your soil would cause it to dry out slowly as the water cant evaporate. Not to mention cutting off oxygen and providing a breeding ground for fungus and other unwanted residents.
Keep it simple. Worry about the things that matter. Good soil, adequate lighting, proper air circulation etc...