Physical Characterization of Thin Films of Cu_xZn_yS_z for Photovoltaic Applications

Abstract

Kesterite, Cu2ZnSn(S,Se)4, are considered promising materials for energy conversion devices, encompassing reduced production costs and low environmental risks. The Electrochemical Atomic Layer Epitaxy (ECALE) method was used to obtain compound semiconductors, in the form of thin films, whose composition belongs to the compositional field of kesterite. Namely, CuxSy and CuxZnyS thin films were considered in this study. Films were characterised through Scanning Electron Microscopy, for film morphology, and X-ray Absorption Spectroscopy, to determine the structure of the film, the metal valence states, the nature of the coordinating ligand. In particular, films are found to always exhibit a Cu2-xS type structure, where Zn is likely tetrahedrally coordinated. The role of the two transition metal cations in modulating the overall properties of the thin layer results fundamental and it opens interesting perspectives in the chemical tuning of the photovoltaic properties.

Thermochemistry of the E-ALD process for the growth of CuxZnyS on Ag(111): Interpretation of experimental data

“…Our approach reproduced the trends found in literature 16,18,19 , giving a clear (although qualitative) insight in the growth process and allowing to discriminate the possible causes for the Zn-deficiency and the thread-like structure in the Cu x Zn y S films.…”

“…The element specific predominance charts have been presented in order to understand the overall stability of solvated and solid species in the Cu-Zn-S-O-H system, the system conventionally adopted to synthesize phases of the Cu-Zn-S system over Ag(111) through E-ALD [16][17][18] . The Pourbaix diagrams allowed to identify the four main metal-bearing species involved in the E-ALD process: chalcocite, Cu, zincite and sphalerite.…”

“…In this context, the E-ALD steps are implicitly assumed as independent one to the others. Still, in some cases, different authors found compositional, structural or morphological features conflicting with these assumptions, in particular when growing metal chalcogenides ultra-thin films of practical thickness: a) Unexpected film thickness: 30 E-ALD cycles of CdS are reported to have half the expected thickness 8 b) Unexpected surface morphology: micrometric scale thread-like structure for the growth of ternary sulfide of copper and zinc (Cu x Zn y S) 14,[16][17][18] . c) Unexpected chemical composition: the E-ALD growth of copper sulfide results in a Cu:S ratio of 2:1 (Cu 2 S) instead of 1:1 (CuS) 19,20 .…”

“…c) Unexpected chemical composition: the E-ALD growth of copper sulfide results in a Cu:S ratio of 2:1 (Cu 2 S) instead of 1:1 (CuS) 19,20 . Similarly, for the growth of Cu x Zn y S, the expected Cu:Zn ratio was 1:1 while the experimental ratio is 6:1 14,[16][17][18] . A Zn-deficiency is also found for the growth of Cd x Zn y S 21 .…”

Operando SXRD of E-ALD deposited sulphides ultra-thin films: Crystallite strain and size

“…Moreover, the roughness changes with the stoichiometry, reaching a minimum for x = 0.28 [15]. Conversely, for the Cu x Zn y S film the morphology is found to be dramatically different with respect to the Cu 2 S, presenting a thread-like structure of nanowires reaching the micrometer scale [16,17], whereas for Cu 2 S a roughness of 24 Å is reported elsewhere [7]. X-ray absorption spectroscopy experiments pointed to the possible influence of the Zn deposition step in the formation of the nanowires net in the Cu x Zn y S E-ALD scheme [16].…”