The present invention increases the conversion efficiency of a photoelectric conversion element that uses cadmium zinc telluride or cadmium telluride (Cd(Zn)Te) compound semiconductor single crystals containing a group 1A element as an impurity. A heat-resistant pot is filled with raw material and a group 1A element, which is reacted with a portion of the raw material, and the container is heated, thereby melting the raw material into a melt and diffusing the dissociated group 1A element in the melt, producing single crystals from the melt. Compound semiconductor single crystals for photoelectric conversion elements having a hole concentration of 4×1015 cm−3 to 1×1018 cm−3 are produced in this manner. Using a substrate (2) that has been cut out from the compound semiconductor single crystals for photoelectric conversion elements enables the conversion efficiency of a photoelectric conversion element (10) to be increased.