Suppression of the redox reaction between the IGZO surface and the reducing agent TMA using fluorine oxidizing agent treatment.

We propose that the post-deposition oxidation of the IGZO surface is essential for improving the interface quality, with Al2O3 prepared by atomic layer deposition (ALD) employing a common metal precursor trimethylaluminum (TMA). Here, the ALD-Al2O3 process was conducted using H2O as an oxidant at a substrate temperature of 150 °C after IGZO deposition. The depth-resolved X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) data reveal the defect-rich and poor interface of the standard Al2O3/IGZO stack due to the redox reaction between the IGZO surface and TMA. The anion character of the IGZO was modified by introducing fluorine, which is known as a stability enhancer for oxide semiconductors. We highlight that the presence of the fluorine also improves the interface quality with ALD-Al2O3. As a consequence of the fluorine incorporation prior to the ALD-Al2O3 process, the chemical reduction reaction of the IGZO surface was effectively alleviated, resulting in a defect-passivated and sharp interface owing to the strong oxidizing nature of the fluorine.

Passivation of Deep-Level Defects by Cesium Fluoride Post-Deposition Treatment for Improved Device Performance of Cu(In,Ga)Se2 Solar Cells.

Heavy-alkali post-deposition treatments (PDTs) utilizing Cs or Rb has become an indispensable step in producing high-performance Cu(In,Ga)Se2 (CIGS) solar cells. However, full understanding of the mechanism behind the improvements of device performance by heavy-alkali treatments, particularly in terms of potential modification of defect characteristics, has not been reached yet. Here, we present an extensive study on the effects of CsF-PDT on material properties of CIGS absorbers and the performance of the final solar devices. Incorporation of an optimized concentration of Cs into CIGS resulted in a significant improvement of the device efficiency from 15.9 to 18.4% mainly due to an increase in the open-circuit voltage by 50 mV. Strong segregation of Cs at the front and rear interfaces as well as along grain boundaries of CIGS was observed via high-resolution chemical analysis such as atomic probe tomography. The study of defect chemistry using photoluminescence and capacitance-based measurements revealed that both deep-level donor-like defects such as VSe and InCu and deep-level acceptor-like defects such as VIn or CuIn are passivated by CsF-PDT, which contribute to an increased hole concentration. Additionally, it was found that CsF-PDT induces a slight change in the energetics of VCu, the most dominant point defect that is responsible for the p-type conductivity of CIGS.