Effects of Cu(In,Ga)3Se5 defect phase layer in Cu(In,Ga)Se2 thin film solar cells

Abstract

Cu(In,Ga)3Se5 (135-CIGS) layers with various thicknesses were deposited on the surface of Cu(In,Ga)Se2 (112-CIGS) photon absorber in the fabrication of CIGS thin film solar cells. This significantly affects the shift of the optical band gap energy from 1.15 eV (112-CIGS) to 1.19 eV, with only 10 nm thick of 135-CIGS capping layer, leading to the increase in open-circuit voltage (Voc) of the solar cells. The optical transmission spectra show no sign of single 135-CIGS layer. The maximum Voc of 670 mV was obtained from 5 to 10 nm thick 135-CIGS capping layer on 112-CIGS compared to 646 mV of only 112-CIGS. The power conversion efficiencies of the devices covered with 135-CIGS with thickness of less than 80 nm are slightly lower than that of the uncovered 112-CIGS solar cells due to lower generated photocurrents. The solar cell parameters become dramatically deteriorate with thicker 135-CIGS capping layer. The XRD also shows the shift of the diffraction peak toward larger 2θ without peak broadening or splitting when the thickness of 135-CIGS is increased. The external quantum efficiency (EQE) measurements indicate the shift of absorption threshold towards shorter wavelength when the thickness of 135-CIGS is increased, which is consistent with the optical transmission measurements.