Fabrication of wide-bandgap β-Cu(In,Ga) 3 Se 5 thin films and their application to solar cells

Abstract

Cu(In,Ga) 3 Se 5 is a candidate material for the top cell of Cu(In,Ga)Se 2 tandem cells. This phase is often found at the surface of the Cu(In,Ga)Se 2 film during Cu(In,Ga)Se 2 cell fabrication, and plays a positive role in Cu(In,Ga)Se 2 cell performance. However, the exact properties of the Cu(In,Ga) 3 Se 5 film have not been extensively studied yet. In this work, Cu(In,Ga) 3 Se 5 films were fabricated on Mocoated 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) 3 Se 5 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 Cu 0.3 (In 0.65 Ga 0.35 ) 3 Se 5 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)Se 2 solar cell, an external supply of Na with Na 2 S deposition further increased the cell efficiency of the Cu(In,Ga) 3 Se 5 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) 3 Se 5 film. The cell efficiency of CdS/Cu(In,Ga) 3 Se 5 was improved from 8.8 to 11.2% by incorporating Na with Na 2 S deposition on the CIGS film. The fill factor was significantly improved by the Na incorporation, due to a decrease of deep-level defects.