Process optimization of Al-doped zinc oxide films as a window layer for Cu(In,Ga)Se2 thin film solar cells

Al-doped zinc oxide (AZO) thin film was deposited by an in-line pulsed-DC magnetron sputtering system for Cu(In, Ga)Se2 (CIGS) solar cell and optimum process conditions were analyzed. Of several process parameters, oxygen ratio to argon gas in the sputtering ambient was intensively investigated. As increasing the oxygen concentration from 0% to 2%, carrier concentration decreased from 4.8 × 1020 to 9.7 × 1017 cm−3 and Hall mobility reduced from 6.1 to 1.1 cm2/Vs. Resultantly, the resistivity of the AZO film increased significantly from 2.2 × 10−3 up to 5.9 × 10−1 Ω cm as increasing the oxygen concentration. This was due to the effect of the scattering mechanism depending on the role of the oxygen in the AZO film. The average transmittance in the range of visible light was lowest as about 74% in the film deposited without oxygen. By the way, the optical band gap was obtained with the most large value of 3.6 eV in the film deposited without oxygen. For examining the feasibility as a window layer, the optimal AZO film deposited without oxygen was applied for making the CIGS solar cell. In this CIGS solar cell, the absorber layer was formed by using the rapid thermal annealing method of CuGa/In metal precursors. The cell was satisfactorily operated with the efficiency of 7.69% even though the cell was not optimized for the buffer layer and the composition of the absorber, and had no metal grids and anti-reflection layer.