A general survey of the solar energy and its conversion to other types of energy, has been made from the view point of applied physics. Physical processes of the energy conversion from the sun beam are classified, and material sciences for the photo-voltaic, photo-thermal and photo-chemical power generation systems are also discussed. Device Physics for the low cost, high efficiency energy conversion elements are reviewed from the recent investigations in the fields. It has been demonstrated that there are some possibilities remained to improve the solar cell performances with reducing the cost by combining several new principles reviewed, apart from the progressing practical approaches such as scale, up of production process with the automatization and standardizations etc.