Materials for light harvesting or photonic applications are at the heart of many recent technological developments and innovation, to address the main issues of the XXIst century, such as health care, clean water, sanitation, affordable and clean energy, environmental sensors, climate action, computing or communication. In this context, epitaxy is often considered as the ultimate technology for growing materials with dedicated physical properties, and especially optical ones. It however comes with the inevitable formation of several different crystal defects. In this contribution, the link between crystal defects and optoelectronic properties of semiconductors will be first presented. We will then review the different optical characterization tools commonly used with epitaxial materials, both for light emission or light harvesting. Prospects will be given on the development of single photons characterization tools, in the context of the recent development of quantum technologies.