Performance improvement in Cd-free Cu(In,Ga)Se-2 solar cells by modifying the electronic structure of the ZnMgO buffer layer

ZnMgO thin film is a potential buffer layer in Cu(In,Ga)Se-2 (CIGS) solar cells. ZnMgO film can be uniformly deposited, and the composition of Mg can be precisely controlled by the atomic layer deposition process. The conduction band offset at the ZnMgO/CIGS interface can be reduced by increasing the Mg content in the ZnMgO buffer. The ZnMgO buffer layer with 20% Mg content has shown the best cell efficiency in the AZO/i-ZnO/ZnMgO/CIGS structure. In this study, with the removal of i-ZnO from the structure, the cell efficiency was improved from 13.9 to 15.2% due to a decrease in the series resistance and an increase in the shunt resistance. Then, the 40 nm thick Zn0.8Mg0.2O buffer layer was modified to a Zn0.9Mg0.1O (20 nm)/Zn0.8Mg0.2O (20 nm) bilayer, in which the thickness of the Zn0.8Mg0.2O layer was reduced while the total thickness was maintained to minimize plasma damage from the subsequent AZO deposition by sputtering. With the bilayer buffer, the efficiency of the CIGS solar cell was further increased from 15.2 to 16.4%. In particular, the open circuit voltage and fill factor were significantly improved. Fewer deep level states in the Zn0.9Mg0.1O/Zn0.8Mg0.2O bilayer compared to a single Zn0.8Mg0.2O layer may be responsible for the improved performance.