Abstract:
The photovoltaic absorber Cu2ZnSn(SSe1– x)4 (CZTSSe) has attracted interest in recent years due to the earth-abundance of its elements and the realization of high performance (13.8% power conversion efficiency, PCE). To raise the performance of CZTSSe-based solar cells, much effort has been applied to improving the quality of absorbers, band alignments/passivation at the p-n junction, and front/back interfaces/contacts.
This study focuses on the cation substitution of kesterite-based solar cells to suppress the cation disorder in the absorber layer and hence device performance. Due to the similarity between the covalent radii of Cu & Zn, the most prevalent Cu/Zn antisite defects will limit the Voc. Understanding the nature of and controlling the cation disorder in kesterite-based absorber materials remains a crucial challenge for improving their photovoltaic (PV) performances. Herein, the combination of neutron diffraction and synchrotron-based X-ray absorption techniques was implemented to investigate the relationships among cation disorder, defect concentration, overall long-range crystallographic order, and local atomic-scale structure for (AgxCu1−x)2ZnSnSe4 (ACZTSe) material. The joint Rietveld refinement technique was used to directly reveal the cation substitution's effect and quantify the defects' concentration in Ag-alloyed stoichiometric and non-stoichiometric Cu2ZnSnSe4 (CZTSe). The results showed that 10%-Ag-alloyed nonstoichiometric ACZTSe had the lowest concentration of detrimental antisite Cu Zn defects (∼8 × 1019 defects per cm−3), which was two times lower than pristine and five times lower than the stoichiometric compositions. Moreover, Ag incorporation maintained the concentrations of beneficial Cu vacancies (VCu) and antisite ZnCu defects to >2 × 1020 per cm−3. X-ray absorption measurements were performed to verify the disorder degree through the bond length changes and coordination number. Therefore, incorporating Ag into the CZTSe lattice could control the distribution of antisite defects in short- and long-range site disorders. This study paves the way to systematically understand and further improve the properties of kesterite-based materials for different energy applications.