Electrodeposition and characterization of p and n sulphide semiconductors.

Semiconductors of interest in the solar energy conversion field were prepared by electrodeposition. In particular, we used the E-ALD (Electrochemical Atomic Layer Deposition) [1] method to build a p-n junction. The obtained ultra thin films were characterised through SXRD (Surface X-Ray Diffraction), with the aim of performing a structural characterization of the grown films. E-ALD is a layer-by-layer electrodeposition based on the alternate underpotential deposition (UPD) of atomic layers of the elements constituting a compound. The starting point was the deposition of the p layer, (Cu-Zn-S), prepared alternating the UPD of the binary sulfides, CuS and ZnS. After the optimization of the deposition condition of the p film [2-4], the electrochemistry behavior of the Cd solution on the substrate covered by the p films has been studied, in order to optimisethe deposition of the n film (CdS). To determine the UPD of Cd, voltammetric studies of the Cd solution on the Ag electrode covered by the p films have been done. In these condition the window of electroinactivity of the new substrate is small, thus it is possible to observe the UPD and the bulk reduction peak of Cd, but it is impossible determine the amount of Cd deposited by linear stripping voltammetric studies, because the redissolution potentials of Cd and Cu-Zn are overlapped . In order to verify the effective deposition of Cd and the UPD nature of the deposition, some different samples have been prepared and then characterized by X-Ray Photoelectron Spectroscopy (XPS) and SEM microscopy and microanalysis. The samples have been growth layer-by-layer, or by EC-ALD, or checking the amount of Cd deposited integrating the charge deposited during time with an appropriate software, in a way to assure the deposition of a single monolayer. XPS results show the presence of cadmium as sulfides in all the samples and in comparable amount, thus verifying the UPD nature of the cathodic peak at less negative potential. In situ SXRD measurements performed at ESRF (Grenoble), allowed to investigate the growth mechanism of Cu-Zn-S thin films (the p layer). The growth of the film was monitored by following the evolution of the Bragg peaks. Analysis of these data are still in progress but the samples show crystallinity, proposing E-ALD as method to grow structurally ordered thin films.