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Electrochemical preparation and structural characterization of CuGaSe2 thin films

CuGaSe2 thin films have been synthesized by one-step electrodeposition on titanium and SnO2/glass substrates from aqueous solutions that contain thiocyanate ion as a complexing agent. According to composition, and morphology analysis at deposition potential of -0.55V vs SCE (Standard Calomel Electrode) for 20 minutes from aqueous solutions of 4.6mM CuCl, 25.0mM GaCl3 y 9.2mM SeO2 in 2.0M de KSCN at pH 2.5 was found the optimum condition for obtain the CuGaSe2 thin films with good morphology and good stoichiometric. The samples were annealed at temperature of 520°C in inert atmosphere for 30 minutes to improve their crystallinity. The obtained CuGaSe2 thin film has a direct band gap (Eg) of 1.65eV. In addition, the cyclic voltammetry (CV) studies were performed in order to elucidate the electrodic processes that occur for the formation of semiconductor CuGaSe2. Therefore, the electrochemical behaviour for each ion (Cu+, Ga3+ and Se4+), for binary (Cu-Se, Ga-Se, and Cu- Ga) and for ternary (Cu-Ga-Se) systems are described. This material has potential application in solar cells.


Introduction
Thin solar cells based on Cu(In,Ga)Se 2 (CIGS) absorber layers have demonstrated 21.7% conversion efficiencies in laboratory devices [1]. Moreover, CuGaSe 2 (E g =1.65eV and α=1,2x10 4 cm -1 ) is one of the most-promising wide-gap chalcopyrite absorber and a strong candidate material high open-circuit voltage in thin films solar cells [2]. The current record for thin films solar cell devices with this ternary compounds prepared by physical vapour deposition (PVD), have reached 9.7% efficiencies [3]. This technique produces high-quality materials, but involves a process that is difficult to increased to largescale. Electrodeposition is an alternative method to obtain good quality and large-scale thin films. However, to obtain high quality layers, parameters such as pH, reactant concentrations, reduction potential, etc., are critical and need optimization, since these parameters influence the deposits stoichiometry and the semiconductor electrical properties [4]. Previously, several research groups have reported the preparation of CuGaSe 2 using electrochemical techniques [5][6][7][8]. In this paper, we present the detailed procedure to obtain CuGaSe 2 thin films grown by one-step electrodeposition on Ti and SnO 2 /glass from aqueous solutions containing thiocyanate ions.

Experimental procedure
The CV studies and the electrodeposition of CuGaSe 2 thin films, were performed in a three-electrode cell, connected to a BAS Epsilon potentiostat. Pt foil was the reference electrode, SCE (Saturated Calomel Electrode) was the reference electrode, and Ti foil or SnO 2 /glass was the working electrode.

Electrochemical studies
The electrochemical behaviour of the individuals constituents, binary and ternary systems were systematic studied in order to elucidate the electrodic processes that occur for the formation of semiconductor CuGaSe 2 . Figure 1 shows the CV reduction potentials for the three ions. The thiocyanate ion effect is to shift the Cu + reduction potential in the negative direction to about -0,78V, where metallic gallium is also deposited. Moreover, a reduction peak at -0.68V probably corresponds to Se 4+ reduction to Se 0 . The Cu-Ga-Se system voltammogram ( Figure 2) presents a multiplicity of cathodic peaks that by comparing with the electrochemical behaviour of the binary systems each process was analysed. At cathodic potentials between -0.35 to -0.75V a peak is observed which is attributed to the formation of binary phases Cu 2-X Se [9,10]. At more negatives reduction potentials, beyond -0.75V, two reduction peaks were observed which are assigned respectively to Cu and Ga metallic, respectively. Furthermore, in the Ga-Se system, a cathodic peak was observed at -0.52V corresponding to the formation of Ga 2 Se 3 , which could to gallium incorporation in the film, increasing its concentration (Table 1). These studies suggest that the formation of CuGaSe 2 could occur through the reaction [8]:

Compositional and morphology studies
The atomic composition and morphology of the Cu-Ga-Se films were analysed by EDS and SEM, respectively. Table 1 summarizes the influence of deposition potential on the film compositions and morphology. In all cases, a solution containing 5.0mM CuCl, 25.0mM GaCl 3 and 5.5mM SeO 2 was   used. In the range between -0.35 and -0.45V, the amount gallium is too low, this is because the reduction reaction of Ga 3+ to Ga 0 cannot take place, the gallium insertion into the film proceed by formation of gallium oxide or hydroxide [7]. In addition, in this range a Cu 2-x Se phase is also deposited, giving an induced deposition mechanism [10]. The micrograph image (Figure 4) showed dispersed deposits attributed to copper selenide phases. At -0.55V cathodic potentials, it is observed that gallium atomic concentration increases due to gallium, incorporation in the film by metal selenides formation (Figure 2). At this potential, Cu 2-X Se formation is observed (Figure 2). In the range between -0.55 and -0.75V it can be seen that the Ga/Cu ratio remains almost constant. This demonstrates that the stoichiometry of the films is independent of the applied deposition potential in this range. Additionally, the synthesized films have a stoichiometric ratio very close to the ternary compound CuGaSe 2 ( Figure 3, Table 1) but with Cu 2-X Se excess. Moreover, the micrograph image ( Figure 4) showed a homogeneous granular morphology. The compositional data obtained in the Table 1 have been expressed into deviation from molecularity (Δm) and deviation from stoichiometry (ΔS) in Figure 3 and take into account the role of the deviation from the ideal composition. In Figure 3 such a schematic ternary phase diagram for CuGaSe 2 is shown. To describe the Cu x Ga y Se z composition of a point within the ternary field relative to the composition of the chalcopyrite, Groenick and Janse [11] have introduced two independent parameters, i.e. the deviation in molecularity Δm and the deviation in valence stoichiometry ΔS: where x, y, and z are the atomic concentrations of the individual atoms. As indicated in the Figure  3, ΔS describes compositional deviations from the pseudo binary Cu 2 Se-Ga 2 Se 3 line, where ΔS<0   corresponds to anion poor and ΔS>0 to anion rich material. Δm divides the triangle into Cu-rich and Cu-poor compositions. In our case, it can be seen that the films synthesized at potentials between -0.35 to -0.85V have ΔS>0 and Δm>0, indicating that these films contain an excess of selenium and copper, respectively. This means that these potentials would form the ternary semiconductor with an excess of the Cu 2-X Se phase. Based on the morphologic analysis a -0.55V potential was selected as the optimum for the CuGaSe 2 deposition, since it requires less energy to produce uniform and homogeneous films. However, it is necessary to do further studies changing the concentration of the ions, trying to reduce the binary Cu 2-x Se phase formation. In Table 2, is shown the atomic percent of the deposits, changing the concentration of the precursor ions. It is shown that with the 4.6:25.0:9.2 ion concentrations give a stoichiometric relation close to the expected ternary.

Optical absorption spectroscopy analyses
Using the transmittance (T) values and film thickness (5.6 microns) the absorption coefficient (α) it was obtained. The band gap energy was estimated by extrapolating the linear portion of ( ℎ ) # vs (ℎ ) plot to the intercept on the x-axis. Figure 8 reveals that the obtained CuGaSe 2 thin film has a direct band gap (E g ) of 1.65eV.

Conclusions
In this paper, it was found that we could obtain by electrochemical method good thin films of CuGaSe 2 over Ti and SnO 2 /glass substrate with applying at potential of -0.55V during 20 minutes of deposition. The electrolytic medium was 4.6mM of CuCl, 25.0mM of GaCl 3 y 9.2mM of SeO 2 in 2.0M of KSCN at pH 2.5. Annealing at 520 0 C in inert atmosphere for 30 minutes improved the CuGaSe 2 thin films crystallinity. The crystal structure of thin films was chalcopyrite type with space group 42 , with the direct energy gap of 1.65eV and high optical absorption coefficient. This material has potential application in solar cells.