The effect of Cu concentration in the photovoltaic efficiency of CIGS solar cells prepared by co-evaporation technique

•ODC phase within CIGS layers was studied by Raman spectra.•Band gap of CIGS layers are affected by Cu content.•Effect of ODC phase on electrical property of CIGS layers is studied.

Structural and electrical properties of polycrystalline CIGS thin films have been studied by changing the Cu/(In + Ga) ratio in the films. CIGS thin films with various Cu/(In + Ga) ratios are grown over Mo-coated soda-lime glass substrates using a co-evaporation technique. The Raman spectra provide information about the existence of the order defect compound (ODC) at Cu-poor compositions (Cu/(In + Ga) ratio < 0.8). The order defect compound (ODC) decreases with an increase of Cu concentration and disappears when the Cu/(In + Ga) ratio exceeded 0.8. The energy gap of the CIGS thin films reduce from 1.23 to 1.18 eV with an increase of Cu content. An increased carrier concentration and a decreased carrier mobility and resistivity of CIGS films occur at specimen CIGS-5 due to disappearance of ODC. The open circuit voltage (Voc) also enhances with an increment of Cu/(In + Ga) ratios from 410 to 540 mV. Finally, the experimental results reveal an optimum Cu/(In + Ga) ratio of 0.85 where the solar cell shows the highest efficiency of 10.3% with Jsc, Voc and FF of 28.0 mA cm−2, 520 mV and 0.706, respectively.