p‐CuGaSe2/n‐CdS heterojunctions have been prepared by depositing CdS films onp‐type CuGaSe2single crystals whose initial resistivity was 10−2&OHgr; cm and changed to 1 &OHgr; cm after the CdS film deposition. The CdS films, which were grown by a multisources method, exhibit a room‐temperature resistivity of 0.1 &OHgr; cm. The absolute quantum efficiency of these devices as photovoltaic detectors reaches the value of 80% at a wavelength of 5800 A˚. As solar cells, these heterojunctions at 25 °C display a solar power conversion efficiency of 5% when they are exposed to the solar light whose intensity is 71 mW/cm2. When the heterojunctions are directly polarized, they emit light in a broad band which is centered at ∼7700 A˚. An external electroluminescent emission efficiency of about 0.05% has been measured at liquid‐nitrogen temperature.