Preparation of β-Cu (In,Ga)3Se5 thin films for wide band gap absorber for top cell in CIGS tandem structure

Polycrystalline Cux(In,Ga)ySez films were deposited on Mo coated soda-lime glass substrate by three-stage co-evaporation process. Cu content x can be controlled by deposition times of each stage. The presence of β-Cu(In,Ga)3Se5 phase in films was confirmed by X-ray Diffraction and Auger Electron Spectroscopy when the x decreased below 0.5. The grain size became smaller as the x decreased. The absorption edge moved to shorter wavelength and the optical transmittance of long wavelength noticeably increased in β-Cu(In,Ga)3Se5 system comparing the conventional Cu(In,Ga)Se2. Its optical band gap was 1.49eV. The CdS/Cu(In0.3Ga0.7)3Se5 solar cell showed the efficiency of 8.09% with an active area of 0.44cm². High transmittance and band gap are desirable to be a light absorber for top cell, but further effort is necessary to improve cell efficiency for the top cell application in CIGS tandem solar cells.