**Abstract**

Solar energy has always been an obvious choice for solving the energy issues for the humans for centuries. The two most popular choices, out of many, to harness this infinite source of energy are: solar cells and photoelectrochemical cells. Although both these techniques are quite attractive, they have inherent limitations for tapping all of the incident photons. Maximizing the absorption of incident photons to produce maximum possible electrical output is always the main impetus for the researchers working to streamline these two techniques and making them compatible with existing sources of electrical energy. It has been well established that the light trapping in the solar cells and photoelectrochemical cells can play a vital role in improving their performance. To design light harvesting structures for both these applications, periodic nanophotonic structures have demonstrated stupendous results and shown that they have the real potential to enhance their performance. The chapter, in this regard, presents and reviews the current and historical aspects of the light harvesting structures for these two interesting applications and also discusses about the future of the research to further the performance of these large-area solar-to-electrical conversion transducers.

**Keywords:** periodic nanophotonic structures, solar cells, photoelectrochemical cells, photonic band gap, diffraction grating
