Chalcogen based materials like Cu2ZnSnS4 (CZTS) have drawn extensive attention for applications such as photovoltaics and water splitting. However, inability to monitor the sulfur partial pressure (PS2) during non-equilibrium annealing process at high temperature complicates the synthesis of CZTS with controlled optoelectronic properties. Here we demonstrated that PS2 can be monitored by investigating the Sn-S phase transformation. We showed that PS2 drops considerably over the annealing time, causing gradual alterations in CZTS: i) a change in defect type; ii) evolution of ZnS and SnxSy phases. With additional ordering treatment, we observed that the low room temperature photoluminescence energy usually seen in CZTS can result from insufficient PS2 during annealing. It is proven that remarkable Voc beyond 700 mV for solar cells with non-optimal CdS buffer can be repeatedly achieved when CZTS is prepared under sufficiently high PS2. An ordering treatment before CdS deposition can further improve Voc to 783 mV.
ASSOCIATED CONTENTSupporting Information. Additional tables including specific parameters of Raman results as well as figures of Raman spectra, SEM images and RT-PL spectra This material is available free of charge via the Internet at http://pubs.acs.org.
Sodium 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.
Dielectric functions of Cu2ZnSn(SxSe1-x)4 thin film absorbers with varied x were determined by spectroscopic ellipsometry and ab initio calculations. From the combination of experimental and theoretical studies, the fundamental interband transition energy E0 (∼1–1.5 eV) and the next following transition energy E1 (∼2–3 eV) were identified and found to blue-shift with increasing sulfur anion content, while keeping the energy separation E1−E0 almost constant, ∼1.4 eV from experiments, and 1 eV from theory. In addition, the average dielectric responses were found to decrease with sulfur anion content from both theoretical and experimental results. The Tauc optical bandgap value Eg determined on samples prepared on Mo and soda lime glass substrate showed a positive linear relationship between x and bandgap Eg. The bandgap bowing factor determined from the theoretical data is 0.09 eV.
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