Band gap engineering of atomic layer deposited Zn_xSn_1-xO buffer for efficient Cu(In,Ga)Se₂ solar cell

Ternary zinc tin oxide (ZTO) is one of the few environmental compatible buffer materials with the potential of replacing the n-CdS buffer in Cu(In,Ga)Se₂ (CIGS) solar cells and other photovoltaic systems once its properties are fully understood and optimized. In this work, ZTO films were grown by atomic layer deposition and were logically characterized with the aim of understanding the correlations between compositional changes and film properties. The ZnO:SnO₂ pulse ratio significantly affected the growth rate, crystal structure, morphology, and band gap of the ZTO films. By controlling the Sn/(Sn + Zn) atomic ratio, the optical band gap of the ZTO films was tuned between 3.05 and 3.36 eV. Integrating the ZTO films as buffer layers in CIGS solar cells, we observed that films with Sn concentrations of 9 to 16 at.% yielded photo-conversion efficiency close to 14%, which was very comparable to efficiency attained with the commonly used CdS buffer. Furthermore, using X-ray photoelectron spectroscopy analysis, we correlated the current-voltage behavior of the cells to the conduction band offset at the ZTO/ CIGS interface.