Monolayer and opaque allows an opportunity to catch the energy while a form of transparency and a multilayer basis would be logically much more efficient as the energy that is not captured in the first layer go to the second and subsequent layers.
Multijunction based solar cells are already layered to direct sunlight through several receptors, each one designed for a specific frequency. A single junction solar cell has a theoretical limit of less than 40% while with multilayered solar cells this efficiency could be simply multiplied by two.
The problem right now for those cells are the production price as no one was capable of designing a manufacturing line with a reasonable price output. Although this is the tree to shake, thus everyone should be thinking in how to make it possible.