The effect of Zn1‐xMgxO buffer layer deposition temperature on Cu(In,Ga)Se2 solar cells: A study of the buffer/absorber interface

The effect of atomic layer deposition temperature of Zn1‐xMgxO buffer layers for Cu(In,Ga)Se2 (CIGS) based solar cell devices is evaluated. The Zn1‐xMgxO films are grown using diethyl zinc, bis‐cyclopentadienyl magnesium and water as precursors in a temperature range of 105 to 180°C. High efficiency devices are produced in the region from 105 up to 135°C. At a Zn1‐xMgxO deposition temperature of 120°C, a maximum cell efficiency of 15·5% is reached by using a Zn1‐xMgxO layer with an x‐value of 0·2 and a thickness of 140 nm. A significant drop in cell efficiency due to large losses in open circuit voltage and fill factor is observed for devices grown at temperatures above 150°C. No differences in chemical composition, structure and morphology of the samples are observed, except for the samples prepared at 105 and 120°C that show elemental selenium present at the buffer/absorber interface. The selenium at the interface does not lead to major degradation of the solar cell device efficiency. Instead, a decrease in Zn1‐xMgxO resistivity by more than one order of magnitude at growth temperatures above 150°C may explain the degradation in solar cell performance. From energy filtered transmission electron microscopy, the width of the CIGS/Zn1‐xMgxO chemical interface is found to be thinner than 10 nm without any areas of depletion for Cu, Se, Zn and O.