Practical limitations to selenium annealing of compound co-sputtered Cu₂ZnSnS₄ as a route to achieving sulfur-selenium graded solar cell absorbers

The suitability of selenium annealing as a technique to introduce energy band gap gradients via sulfur-selenium substitution in Cu₂ZnSnS₄ (CZTS) films is evaluated. Compound co-sputtered CZTS precursors are annealed in selenium atmosphere at 425°C, either as-deposited or after a short time sulfur pre-anneal. The films are investigated by Raman spectroscopy and X-ray diffractometry, and the spatial distribution of elemental species measured by secondary ion mass spectrometry and energy dispersive X-ray spectroscopy. Sulfur-selenium gradients are not achieved for the as-deposited precursor. Sulfur-selenium gradients are achieved in the early stages of annealing for pre-anneal samples, where Cu₂ZnSn(S,Se)₄ (CZTSSe) formation is found to be correlated spatially with sodium distribution. These gradients are lost as the annealing progresses. Selenisation occurs by CZTSSe grain growth, rather than by direct substitution of selenium for sulfur. The spatial correlation of high sodium concentration with CZTSSe formation suggests that liquid-phase sodium selenide facilitates selenium incorporation during recrystallisation, limiting the practicality of anion-grading of CZTSSe during the annealing step as a means of establishing a graded band gap. - Highlights: • Selenium annealing of compound co-sputtered CZTS precursors is investigated. • Sulfur-selenium gradients in sulfur pre-annealed precursors form near the back contact. • Selenisation is correlated with sodium distribution. • Sodium assisted recrystallisation limits practicality of Se annealing for S-Se gradient formation.