This paper is to study for efficiency improvement of solar cells by utilizing impurity traps introduced in the band gap of semiconductor, that is, impurity photovoltaic (IPV) effect. It is revealed theoretically that there is a certain energy range where impurity-traps act as stepping stones in two-step excitation of electrons from the valence band to the conduction band under suppression of carrier recombination through such traps. Indium is selected as one of proper impurities that satisfy this condition in crystalline silicon, and theoretical prediction is experimentally verified. A good agreement between theory and experiment is obtained concerned with photoconductive properties. It is concluded that the IPV effect is useful to improve the cell efficiency.