The molecular solar thermal (MOST) energy storage systems based on photo responsive phase change materials are one of the effective ways to store and convert solar energy in recent years. Photo responsive phase change materials can undergo photo induced reversible solid–liquid transition during photoisomerization at room temperature or low temperature. In the process of solar energy storage and conversion based on photo responsive phase change materials, the synergistic storage of photoisomerization enthalpy and phase transition enthalpy occur. When stored energy is released in the form of thermal energy, this provides a new model for the development and utilization of solar energy. This chapter revolves around the topic of “photo responsive phase change materials for energy storage”, introduces azo-based MOST materials with photo induced reversible solid–liquid transformation, the typical photo responsive phase change behavior characteristics, discusses their transformation and energy storage mechanisms, design principles, and introduces the potential applications of the azo-based MOST photo responsive phase change materials in solar energy storage.