Effect of Cu(In,Ga)₃Se₅ ultra-thin layer on optical properties and photovoltaic efficiency of Cu(In,Ga)Se₂ thin film solar cells

Cu(In,Ga)3Se5 (135-CIGS) layers with various thicknesses were deposited on the surface of ~1.8 micron thick 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 the open-circuit voltage (Voc) of the solar cells. The optical transmission spectra show no sign of separated 135-CIGS layer. The maximum Voc of 670 mV is obtained from 5-10 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 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 diffraction peak toward larger 2-theta 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 that is consistent with the optical transmission measurements. The photoluminescence (PL) spectra of 135-CIGS/112-CIGS heterostructure with various thicknesses of 135-CIGS layer are identified as donor-to-acceptor pairs (DAPs) and free (conduction band) -to-bound (acceptor) transitions and show the temperature and excitation power dependence on the PL spectra. On the contrary, when the thin 112-CIGS layer is deposited on top of 135-CIGS (112-CIGS/135-CIGS), the PL spectra show more pronounced and resolved peaks which are surprisingly independent of temperature and excitation power because of the interference effect. The thin 135-CIGS (1-200 nm) capping layer on the 1.8 micron 112-CIGS films show the nature of p-type, while the thin 112-CIGS (5-300 nm) capping layer on the 1.8 micron 135-CIGS films exhibit the nature of n-type.