Formation of sol-gel based Cu2ZnSnS4 thin films using ppm-level hydrogen sulfide

Abstract

This study reports the preparation of copper zinc tin sulfide (Cu2ZnSnS4 or CZTS) thin films using parts-per-million (ppm) level of hydrogen sulfide (H2S) gas as opposed to high percentage level of H2S (e.g., ≥5%) in many other studies. Sol-gel technique was adopted for the preparation of metal oxide films from precursor solutions with different ratios of Cu, Zn, and Sn metal ions. Sulfurization process was carried out in tube furnace using 100ppm H2S balanced by hydrogen at different temperatures. Generally, it was observed that the sulfur content of the thin films obtained showed strong dependence on the sulfurization temperature. Samples sulfurized at higher temperatures (e.g., ~450 and 550°C) in ppm-level H2S experienced significant sulfur loss as confirmed by EDS, which was accompanied by the formation of metal alloys such as Cu3Sn as identified by XRD. On the other hand, samples sulfurized at lower temperatures of ~350°C and even 125°C revealed the formation of CZTS phase as confirmed by XRD, Raman micro-spectroscopy, and sheet resistance measurement. Moreover, local EDS analysis indicates that CZTS films prepared at those low temperatures have near-stoichiometric composition and are sometimes accompanied by the formation of Cu2−xS phase(s). In addition, stoichiometric and Cu-rich precursor solutions tend to yield CZTS samples with better crystallinity and superior optical properties compared with the Cu-deficient solution. These results are discussed from thermodynamics perspective and the directions for future research for safer and more economical processing of CZTS solar cells are pointed out.