Abstract: A detailed introduction to the concepts, current status and research targets for the next generation of solar cells is presented. Their structure and design characteristics are analyzed from the viewpoint of the full absorption of sunlight energy and low conversion loss. Wide band-gap solar cells, quantum dot hot-carrier solar cells, and multi-exciton generation quantum dot solar cells based on nanotechnology and tandem structures which can utilize high energy photons are described. To decrease the loss of low energy photons, narrow band-gap photovoltaic materials and intermediate band photovoltaic devices may be employed. Various modes of light conversion, solar cells with up-conversion and down-conversion, and the limits to their efficiency are discussed.