Electrosynthesis of Cu-Se Films on Copper Electrodes in Alkaline Media: A Voltammetric, Electrochemical Quartz Crystal Microbalance and I/t Transient Study Ricardo Co ´ rdova,* Cristina Lo ´ pez, Marco Orellana, Paula Grez, Ricardo Schrebler, and Rodrigo del Rı ´o Instituto de Quı ´mica, Pontificia UniVersidad Cato ´ lica de Valparaı ´so, Casilla 4059, Valparaı ´so, Chile ReceiVed: June 10, 2004; In Final Form: December 15, 2004 The electroformation of Cu-Se phases, obtained by selenizing a thin film of copper deposited on the quartz/ gold electrode system, was studied with an electrochemical quartz crystal microbalance (EQCM) and by cyclic voltammetry (CV) in an alkaline solution (0.05 M Na 2 B 4 O 7 ) containing selenide ion. Potentiodynamic parameters showed that the formation of the initial Cu-Se phases (Cu 2-x Se/Cu 3 Se 2 ) is ruled by an irreversible diffusion controlled mechanism, where a first electron transfer is the rate-determining step. A CV study was also performed with a bulk copper electrode in 1 M NaOH solution containing selenide ion. The deconvolution of the anodic and cathodic I/E profiles corresponding to the electroformation and electroreduction of the Cu-Se film formed allowed us to establish that, depending on the anodic potential limit of the potentiodynamic scan, the Cu-Se phases formed were either a mixture of Cu 2-x Se/Cu 3 Se 2 or Cu 2-x Se/Cu 3 Se 2 /CuSe. An EQCM study showed that, during the initial stage of Cu-Se phase electroformation, water molecules were released from the electrode. In advanced stages of the process, when the electrode was completely covered by Cu-Se compounds, selenide anions were adsorbed on the formed phase. When the anodic potential limit was extended to -0.2 V, copper oxide compounds were formed. The analysis of the cathodic charge related to Cu-Se phase electroreduction and Energy Dispersive X-ray Spectroscopy (EDXS) analysis confirmed that when the anodic limit was -0.8 V, a mixture of different Cu-Se phases was formed. A I/t transient study performed with a bulk copper electrode in alkaline solution containing selenide established that the nucleation and growth mechanism (NGM) of the Cu-Se phases takes place through an initial bidimensional-instantaneous nucleation (IN2D), followed by four bidimensional-progressive nucleations (PN2D). These results and atomic force microscopy (AFM) experiences supported that the growth of the Cu-Se films occurs through a layer-by- layer mechanism. 1. Introduction Metal chalcogenide compounds have a semiconductor nature, and they are of considerable technical interest in the field of electronics and electrooptical devices. Intensive research has been performed in the past to study the production and characterization of these compounds under the form of thin films. The electrodeposition of multinary compounds of practical use offers a reduced cost in comparison with the chemical bath deposition method and the chemical vapor deposition method. This is the case, for instance, of copper indium diselenide compound (CuInSe 2 , CIS) which has received considerable attention in recent years in photovoltaic devices. CIS thin films are grown by co-deposition of Cu, In, and Se simultaneously. 1-5 Both CIS and its homologue Cu (In,Ga)Se 2 (CIGS) are highly promising absorber materials to obtain low cost polycrystalline thin film solar cells. CIS has an almost ideal band gap of ∼1 eV and a high absorption coefficient at photon energies above the band gap. The study of the copper/selenide system has been considered in the intermediate stages of CIS electrosynthesis as well as in the application of Cu 2 Se as a window layer for solar cells. 6-7 Pejova et al. 8 studied the deposition of different phases of Cu-Se compounds using a chemical method. This method was based on the decomposition of selenosulfate in an alkaline medium containing a Cu(II) salt and a suitable complexing agent. The different phases obtained (Cu 2 Se and Cu 3 Se 2 ) were characterized by X-ray diffraction (XRD) and other optical measurements. Marlot et al. 9 used a rotating electrochemical quartz crystal microbalance (REQCM) to study the electrodeposition of Cu- Se compounds from aqueous solutions of CuSO 4 and H 2 SeO 3 . The composition of the films obtained was determined in situ by means of the simultaneous measurements of the quantity of charge and mass gains. Besides, the compositions of the films deposited were dependent on hydrodynamic conditions of the solution and the electrolyte composition used. The electroformations of Cu-Se phases by the deposition of copper onto selenium covered gold electrodes were studied by different electrochemical techniques. 10 Through the analysis of the I/t transients, the potential edge where the initial stages of Cu-Se compounds start was established. 3 A general reaction scheme that considers the formation of CuSe and Cu 2-x Se compounds was proposed by taking into account the charges involved in the anodic redissolution of the Cu-Se deposits previously electrosynthesized. Kemell et al. 11 studied the one-step electrodeposition of Cu 2-x Se thin films from a thiocyanate bath using combined cyclic voltammetry and microgravimetry studies. The deposition of the Cu 2-x Se thin film took place on a previously formed Se layer on the working electrode. The mechanism of the deposition * Corresponding author. Fax: +56-32-273422. E-mail: rcordova@ucv.cl. 3212 J. Phys. Chem. B 2005, 109, 3212-3221 10.1021/jp047485o CCC: $30.25 © 2005 American Chemical Society Published on Web 01/27/2005