Na incorporation into Cu2ZnSnS4 thin film absorbers from RF-sputtered NaF precursors

• Published in: Solar energy Vol. 217; pp. 280 - 291
• Main Authors: Akcay, N., Baskose, U. Ceren, Ozcelik, S., Gremenok, V.F., Zaretskaya, E.P.
• Format: Journal Article
• Language: English
• Published: New York Elsevier Ltd 15.03.2021; Pergamon Press Inc
• Subjects: Absorbers; Alkali diffusion; Barrier layers; BSIT; Carrier density; Copper; Copper zinc tin sulfide; Cu2S secondary phase; CZTS; Diffusion barriers; Diffusion layers; Glass substrates; Molybdenum; Na incorporation; NaF; Phases; Photoluminescence; Photons; Precursors; Radio frequency; Raman spectra; Raman spectroscopy; Soda-lime glass; Sodium; Sodium diffusion; Sodium fluoride; Solar energy; Substrates; Sulfides; Thin films; Tin; Titanium nitride; Zinc; CZTS; Cu2S secondary phase; NaF; BSIT; Alkali diffusion; Na incorporation
• ISSN: 0038-092X; 1471-1257
• DOI: 10.1016/j.solener.2021.02.013

•Na incorporation into CZTS precursors by adding RF-sputtered NaF layers was studied.•Na additions from both NaF layers and SLG substrates were evaluated together.•The films with additional NaF layers showed better crystalline and compact structure.•SIMS analysis showed Na diffusion in two directions resulting from NaF layers and SLG.•The films had high carrier density and low resistivity due to incorporation of Na. In this work, we report the incorporation of sodium (Na) alkaline element into copper-zinc-tin-sulphide (Cu-Zn-Sn-S) precursor films by adding radio frequency (RF)-sputtered sodium fluoride (NaF) intermediate layers bringing a new approach to the bifacial sodium-incorporated treatment (BIST) and its effect on the properties of Cu2ZnSnS4 (CZTS) absorber films. Due to the absence of alkaline diffusion barrier layers on soda lime glass (SLG) substrates, Na additions from both NaF intermediate layers and SLG substrate were evaluated together. With additional NaF intermediate layers among the elemental metal stacks, the films exhibited better crystalline properties and a more compact structure. However, the amount of the Cu2-xS secondary phases formed on the surfaces increased by the additional NaF layers. Raman spectra confirmed the formation of the kesterite phase and showed the peaks corresponding to Cu3SnS4 and Cu2-xS secondary phases. Na diffusion originating from NaF layers deposited on molybdenum (Mo) films occurred in both directions of the CZTS film and the SLG substrate. The photoluminescence (PL) bands located at around 1.40 eV were attributed to the band to band (BB) transitions. The film with additional NaF intermediate layers exhibited higher carrier concentrations and lower resistivity than the other film due to the presence of the higher amount of Cu2-xS secondary phases on its surface.