Direct Imaging of Cl- and Cu-Induced Short-Circuit Efficiency Changes in CdTe Solar Cells

To achieve high-efficiency polycrystalline CdTe-based thin-film solar cells, the CdTe absorbers must go through a post-deposition CdCl₂ heat treatment followed by a Cu diffusion step. To better understand the roles of each treatment with regard to improving grains, grain boundaries, and interfaces, CdTe solar cells with and without Cu diffusion and CdCl₂ heat treatments are investigated using cross-sectional electron beam induced current, electron backscatter diffraction, and scanning transmission electron microscope techniques. The evolution of the cross-sectional carrier collection profile due to these treatments that cause an increase in short-circuit current and higher open-circuit voltage are identified. Additionally, an increased carrier collection in grain boundaries after either/both of these treatments is revealed. The increased current at the grain boundaries is shown to be due to the presence of a space charge region with an intrinsic carrier collection profile width of ≈350 nm. Scanning transmission electron microscope electron-energy loss spectroscopy shows a decreased Te and increased Cl concentration in grain boundaries after treatment, which causes the inversion. Each treatment improves the overall carrier collection efficiency of the cell separately, and, therefore, the benefits realized by each treatment are shown to be independent of each other.