Absorption and carrier generation
Absorbing photons with suitable energies can create mobile electrons and corresponding holes in a semiconductor. The band gap affects which wavelengths can contribute. Illumination supplies energy and electronic structure governs the response, so incident light is not converted identically across all wavelengths or material systems.
Separating and collecting charge
A junction or another suitable structure separates photogenerated carriers, while contacts connect them to an external circuit. Generation alone does not guarantee useful output because recombination and collection losses remove carriers. Material selection, contacts and internal architecture must therefore work together within a functioning cell.
Cells and material choices
Crystalline silicon is widely used, while thin-film cells employ other semiconductors such as cadmium telluride. Absorption, fabrication, stability and cost vary between materials. Multijunction structures combine band gaps to use more of the spectrum, and laboratory cells, modules and complete generating systems require different comparisons.
References: [1]
What conversion efficiency measures
Conversion efficiency compares electrical output with incident optical power and depends on illumination and device losses. Photovoltaic operation differs from a typical external photoemission experiment ejecting electrons from a surface. It organizes charge generation and transport within a material to provide useful work through an external circuit.