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

Abstract

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.