Current Photovoltaic Research ISSN:2288-3274(Print) 2508-125X(Online)

Cu(In,Ga)₃Se₅ is a candidate material for the top cell of Cu(In,Ga)Se₂ tandem cells. This phase is often found at the surface of the Cu(In,Ga)Se₂ film during Cu(In,Ga)Se₂ cell fabrication, and plays a positive role in Cu(In,Ga)Se₂ cell performance. However, the exact properties of the Cu(In,Ga)₃Se₅ film have not been extensively studied yet. In this work, Cu(In,Ga)₃Se₅ films were fabricated on Mo‐ coated soda‐lime glass substrates by a three‐stage co‐evaporation process. The Cu content in the film was controlled by varying the deposition time of each stage. X‐ray diffraction and Raman spectroscopy analyses showed that, even though the stoichiometric Cu/(In+Ga) ratio is 0.25, Cu(In,Ga)₃Se₅ is easily formed in a wide range of Cu content as long as the Cu/(In+Ga) ratio is held below 0.5. The optical band gap of Cu0.3(In_0.65Ga_0.35)₃Se₅ composition was found to be 1.35eV. As the Cu/(In+Ga) ratio was decreased further below 0.5, the grain size became smaller and the band gap increased. Unlike the Cu(In,Ga)Se₂ solar cell, an external supply of Na with Na₂S deposition further increased the cell efficiency of the Cu(In,Ga)₃Se₅ solar cell, indicating that more Na is necessary, in addition to the Na supply from the soda lime glass, to suppress deep level defects in the Cu(In,Ga)₃Se₅ film. The cell efficiency of CdS/Cu(In,Ga)₃Se5 was improved from 8.8 to 11.2% by incorporating Na with Na₂S deposition on the CIGS film. The fill factor was significantly improved by the Na incorporation, due to a decrease of deep‐level defects.