Formation of an interfacial MoSe2 layer in CVD grown CuGaSe2 based thin film solar cells

Thin polycrystalline films of CuGaSe2 (CGSe) have been grown on Mo coated glass substrates by halogen supported chemical vapor deposition (CVD) with two different binary source materials, CuSe and Ga2Se3. Solar cells based on these absorber films prepared in a sequential two-stage process show efficiencies exceeding 6%. High resolution transmission electron microscopy investigation of the complete solar cell structure reveals a 170-nm thick MoSe2 interfacial layer at the CGSe/Mo back contact. The crystallites of the MoSe2 layered structure are found to be mainly oriented perpendicular to the Mo surface. The main focus of this investigation was to study the influence of the CVD process on the growth of MoSe2 and the role the interfacial layer may have in the performance of the solar cell. For a detailed analysis we studied the growth and morphology of the interfacial layer dependent on the [Cu]/[Ga]-ratio in the gas phase during the CGSe deposition process and the Na content of the glass substrate. It was found that Na influences the growth of the MoSe2 layer. By means of temperature dependent IV (IVT)-measurements the electrical properties of the CGSe/MoSe2/Mo heterostructure were investigated. In the heterostructure under investigation the MoSe2 interfacial layer mediates an ohmic contact to the CGSe film. (C) 2003 Elsevier Science B.V. All rights reserved.