The heterostructure n-CdO/a-C/p-Si is proposed for use as a solar cell device. The heterostructure consists of two semiconductor layers having different optical band gaps. An ultrathin layer of a-C with a narrow optical band gap is located between these layers. The photovoltaic effect in this device has been investigated. It is shown that the short-circuit current I-sc = 46 mA/cm(2) for heterostructure n-CdO/a-C/p-Si corresponds to the values obtained in the best solar cells based on crystalline silicon. It is also shown that the heterostructure n-CdO/p-Si (without a-C) has a short circuit current which is much weaker. (C) 2000 Elsevier Science B.V. All rights reserved.