2, Department of Engineering Sciences, Uppsala University, Uppsala, , Sweden
The incorporation of Ag in Cu(In,Ga)Se2 (CIGSe) is understood to induce certain improvements in the opto-electronic properties of the absorber, which is reflected in enhanced conversion efficiencies . Ag atoms substitute for Cu forming a pentenary (Ag,Cu)(In,Ga)Se2 (ACIGSe) alloy with a wider band gap compared to CIGSe absorber. The Ag alloying also allows to reduce the deposition temperatures due to the lower melting point of Ag, which as a result decreases the density of lattice defects in the absorber. Synthesizing such alloy on high strain K-rich substrates along with KF post deposition treatment displays higher device performance compared to similarly processed CIGSe devices without Ag . The diffusion of alkali metals from the substrate in ACIGSe compared to CIGSe as well as the distribution of Ag in the absorbers are matters that can elucidate differences in the opto-electronic properties and therefore requires further attention.
Amongst various analytical techniques, atom probe tomography (APT) is an extremely promising tool to characterize the chemical distribution in three dimensions with sub-nm resolution . APT is used in the study here as a main tool to examine local compositions within ACIGSe and the distribution of alkali metals within absorbers grown on different substrates; soda lime glass (Na-rich) and high strain point glass (K-rich), respectively. Na and K were found to segregate to different lattice defects in the absorber structures. Correlative measurements using transmission Kikuchi diffraction and APT were therefore carried out to investigate the crystallographic and chemical nature of grain boundaries and twin boundaries detected within the ACIGSe structures. Although ACIGSe is a promising system for thin film solar cell technology, more research is certainly required to demonstrate if the beneficial effects observed for Ag additions are worth to pay for, considering its higher cost compared to Cu.
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