Abstract
AbstractSodium and oxygen are prevalent impurities in kesterite solar cells. Both elements are known to strongly impact performance of the kesterite devices and can be connected to efficiency improvements seen after heat treatments. The sodium distribution in the kesterite absorber is commonly reported, whereas the oxygen distribution has received less attention. Here, a direct relationship between sodium and oxygen in kesterite absorbers is established using secondary ion mass spectrometry and explained by defect analyses within the density functional theory. The calculations reveal a binding energy of 0.76 eV between the substitutional defects NaCu and OS in the nearest neighbor configuration, indicating an abundance of NaO complexes in kesterite absorbers at relevant temperatures. Oxygen incorporation is studied by introducing isotopic 18O at different stages of the Cu2ZnSnS4/Mo/soda‐lime glass baseline processing. It is observed that oxygen from the Mo back contact and contaminations during the sulfurization are primary contributors to the oxygen distribution. Indeed, unintentional oxygen incorporation leads to immobilization of sodium. This results in a strong correlation between sodium and oxygen, in excellent agreement with the theoretical calculations. Consequently, oxygen availability should be monitored to optimize postdeposition heat treatments to control impurities in kesterite absorbers and ultimately, the solar cell efficiency.
Highlights
Introduction grain boundaries withNa acting as a catalyst.[16,17,18] further studies confirmed that Na and O can coexist on the surfaceEarth-abundant kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is a prom- and GBs.[19,20] for both CIGS and CZTSSe, postdepoising solar cell absorber with a record 12.6% power conversion sition heat treatments were shown to yield improved device performance.[21]
The Na and O profiles are correlated, which is a trend previously observed for CZTSSe and CIGS.[28,29,30]
To understand the correlation between Na and O in kesterite absorbers at the atomic level, first-principles density functional theory (DFT) was employed to study the point defects formed by these impurities in CZTS
Summary
The binding energies were calculated for the complexes formed by OS and interstitial Xi defect (X = Ag, Li, Na, K, Rb, and Cs), as shown in Figure S3 (Supporting Information), further evincing that the interaction between the alkali elements and oxygen is driven by the ionic bonding. These results corroborate the conclusion that Na and O concentrations in CZTS are correlated, as seen in the SIMS depth profiles, and that the origin of this correlation is Na O defect complexes. Different to Na, O in kesterite absorbers has several additional potential sources: outdiffusion from the Mo back contact, from the precursors as well as from the air during and after processing
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