Chalcogenides such as CdSe and CdTe thin-films are well known semiconductors, used in manufacturing of last generation photovoltaic components by means of vacuum techniques. Toxicity and shortage of the involved elements along with the EROEI (Energy Return Over Energy Investment) needed to assemble the devices, must be considered in its full life cycle assessment. Hence scientific community is focusing attention on new compounds based on economic and low-environmental impact elements such as Cu, Sn, Fe and Zn. In particular, quaternary semiconducting materials based on the kesterite (Cu2ZnSnS4) mineral structure are the most promising candidates to overtake the current generation of light-absorbing materials for thin-film solar cells. Electrodeposition is known as a low-cost semiconductor growth technique for applications in electronic devices. Surface limited electrodeposition of atomic layers, can be performed exploiting by means of Electrochemical Atomic Layer Deposition (E-ALD) technique to obtain sulphides ultra-thin films. In-situ SXRD measurements were performed at ESRF (Grenoble) and focused to the investigation of the growth mechanism of Cu2S ultra-thin films on the (1 1 1) crystal plane of a silver single crystal, commonly used as a working electrode. The growth of the film was monitored by following the evolution of the Bragg peaks after each E-ALD step. Results point to the occurrence of a self-standing film with a definite crystal structure after 15 E-ALD cycles. After the Bragg reflections are observed for the first time, only minor changes of the structural arrangement are registered.
Breadth and profile analysis of the Bragg peaks lead to a qualitative interpretation of the growth mechanism, in the normal and in-plane directions, with respect to the Ag surface. Namely, the contribution of crystallite’s strain and size were identified in the width of the Bragg reflections.
The interpretation of the experimental reciprocal lattice, coupled to the SEM investigation, suggests that the samples show a pseudo single-crystal diffraction pattern. This can be described by a hexagonal unit cell. The crystal structure of this electro-deposited Cu2S could be related to that of chalcocite, in particular considering the layering of triangular Cu sites and octahedral Cu sites. The influence of the applied electric potential on the stability of the electro-deposited crystal structure has been monitored by means of SXRD measurements performed during the switch off of the potential. A structural change was, in fact, registered, and correlated to the occurrence of the stable phases under conventional laboratory conditions.