Determination of optimum Cu/(Ga+In) ratio at different fabrication temperatures for Cu(In,Ga)(Se,Te)2 thin film solar cells

In this study, tellurium-dopped Cu(In,Ga)(Se,Te)2 completed solar cells with different Cu/(Ga+In) ratios were fabricated at different substrate temperatures and investigated to understand the effect of Cu/(Ga+In) ratio on the solar parameters and the structure of the solar cells. It was found that the Cu/(Ga+In) ratio affected the short circuit current and fill factor more than the open circuit voltage by changing the microstructure. Therefore, the increase in the current collection and short circuit current density had a greater effect on the efficiency value. Increasing the Cu/(Ga+In) ratio from 0.88 to 0.90 decreased the average grain size from 0.47 µm to 0.38 µm at 480℃ substrate temperature. However, further increasing the Cu/(Ga+In) ratio from 0.90 to 0.92 made the structure more compact having an average grain size of 0.41 µm with less thin film quality. The samples with highest Cu/(Ga+In) ratios having the most compact structures in their temperature groups showed very low current collection through all wavelengths when compared to other samples. The short circuit current density results were found to be in consistency with the external quantum efficiency graphs of the completed solar cell samples. Optimum Cu/(Ga+In) values for the samples produced at 480℃ and 620℃ substrate temperatures were found to be 0.88 and 0.90, respectively. The sample with 0.90 CGI ratio which produced at 620℃ performed the best efficiency with 14.23 %.