Electrodeposition and Characterization of Cu(In, Al)Se₂ for Applications in Thin Film Tandem Solar Cells

Omar Meglali, Nadhir Attaf, Assia Bouraiou, Mohamed Salah Aida

Sciences and Technologies Faculty, Ziane Achour University, Djelfa, Algeria.
Sciences Faculty, Mohamed Boudiaf University, M’sila, Algeria.
Thin Films and Interfaces Laboratory, Department of Physics, Mentouri University, Constantine, Algeria.

DOI: 10.4236/msa.2013.411089                

Abstract

Cu(In, Al)Se₂ thin films were prepared by electrodeposition from the aqueous solution consisting of CuCl₂, InCl₃, AlCl₃ and SeO₂ onto ITO coated glass substrates. The as-deposited films were annealed under vacuum for 30 min at temperature ranging between 200°C and 400°C. The structural, composition, morphology, optical band gap and electrical resistivity of elaborated thin films were studied, respectively using x-ray diffraction, energy dispersive analysis of x-ray, scanning electron microscopy, UV spectrophotometer and four-point probe method. The lattice constant and structural parameters viz. crystallite size, dislocation density and strain of the films were also calculated. After vacuum annealing, x-ray diffraction results revealed that all films were polycrystalline in nature and exhibit chalcopyrite structure with (112) as preferred orientation. The film annealed at 350°C showed the coexistence of CIASe and InSe phases. The average crystallite size increases linearly with annealing temperature, reaching a maximum value for 350°C. The films show a direct allowed band gap which increases from 1.59 to 1.78 eV with annealing temperature. We have also found that the electrical resistivity of films is controlled by the carrier concentration rather than by their mobility.

Keywords

Electrodeposition; Cu(In, Al)Se₂; Thin Film

1. Introduction

One of the main challenges in photovoltaic research is the development of lower cost and higher conversion efficiency devices. This primary objective can be achieved by using tandem solar cell structures. For the thin film tandem cell, the top cell requires a wide band gap absorber layer with optical band gap in the range 1.7 - 1.9 eV whereas the bottom cell requires an absorber layer with a band gap around 1.1 eV [1]. Fabrication of thin film tandem cell gets relatively simplified if the desired band gap semiconductor absorber layers for the top and bottom cells can be obtained from a single ternary or quaternary alloy system by varying the alloy composition suitably [1].

Cu(In, Al)Se₂, abbreviated as CIASe, is considered as an quaternary interesting absorber material for the wide band gap cell in the thin film tandem structures, because it requires smaller relative alloy concentration than gallium (Cu(In, Ga)Se₂) or sulphur (CuIn(S, Se)₂) alloys to achieve comparable band gap. Furthermore, the aluminum is a much cheaper and abundant material [2]. The optical band gap of Cu(In_1−xAl_x)Se₂ semiconductor can be controlled from 1 eV (for x = 0) to 2.7 eV (for x = 1) by the partial replacement of indium by aluminum [3,4]. The conversion efficiency has already exceeded 16.9% using CIASe material as absorber layer in solar cell [5]. These progresses open up new perspectives.

The CIASe thin films have been prepared by various methods such as chemical vapor transport [6], evaporation [1,7], selenization of evaporated precursors [8] chemical bath deposition [1], pulsed laser deposition [9], electrodeposition [10], etc. Amongst them, electrodeposition is an appealing technique that offers low-cost equipment, high deposition speed and possibility of large-area polycrystalline films deposition. In the present work, Cu(In, Al)Se₂ films have been deposited on the indium tin oxide (ITO) coated glass substrate by one step electrodeposition process using two electrodes system. After vacuum annealing of the as-deposited samples, they have been characterized by using XRD, SEM, optical absorption and electrical resistivity. The relationship between the properties of the Cu(In, Al)Se₂ films and the annealing temperature is also studied.

2. Experimental

In the present work, electrochemical experiments were carried out in a simple two-electrode cell configuration with a indium tin oxide (ITO) coated glass substrate as a working electrode (area 5 mm × 15 mm), and a palatine sheet as the counter electrode. The electrode position bath containing 10 mM CuCl₂, 20 mM InCl₃, 20 mM AlCl₃ and 20 mM SeO₂ dissolved in de-ionized water. The pH of the solution was adjusted to 2.6. Deposition was carried out at a room temperature, without stirring the solution. The as-deposited films were annealed under vacuum for 30 min at temperature ranging between 200˚C and 400˚C.

The crystalline structure of the resulting films was analyzed by means of x-ray diffractometer using CuK_α radiation of wavelength λ = 1.5418 Å. Since no standard files are available for CIASe material, standard file of CuInSe₂ was used to identify Cu(In, Al)Se₂ elaborated films. The crystallite size C_s, was estimated from the full width at half maximum (FWHM) of the diffraction peak by using Scherr’s formula [11].

The surface morphology of the films was investigated using a scanning electron microscope (SEM). The optical transmittance of the films was recorded using UV-VisNIR spectrophotometer (Shimadzu UV-3101) at room temperature in the wavelength range of 350 - 1500 nm in steps of 2 nm. An identical ITO coated glass substrate was used as reference for recording the optical absorption spectra.

The thicknesses of the films were measured by means of a DEKTAK 3030 surface profile system. Electrical resistivity of the films was determined using four point probe method.

3. Results and Discussion

The x-ray diffraction pattern of the as-deposited film, before annealing, is shown in Figure 1. The XRD spectra appear the peaks correspond to the most intense peaks of ITO phase [12]. It is noted that before annealing we have observed only the XRD patterns related to the ITO phase. This indicates that the as deposited films are amorphous in nature or are composed with micro crystallite that cannot be detected.

The EDAX pattern for the as deposited films is given in Figure 2. The spectrum shows the typical emission lines of copper (CuL), indium (InL), aluminum (AlK) and selenium (SeL) elements in the investigated energy range. On the other hand, the appearance of silicon (SiK), oxygen (OK) and tin (SnL) peaks comes mainly from the

Figure 1. x-ray diffraction pattern of the as deposited thin film (before annealing).

Figure 2. EDAX pattern of the as deposited film.

ITO coated glass substrate.

The x-ray diffraction patterns of the elaborated thin films, after annealing at different temperature are shown in Figures 3(a)-(d). All specimens are composed of polycrystalline CIASe and ITO phases, and no distinct peaks particular to CuInSe₂ and CuAlSe₂ are detected. Thereafter, the annealing results in changing CIASe films from amorphous to polycrystalline structure.

The XRD spectra appear the peaks located at 2θ ≈ 26.78˚ and 45.19˚. The present XRD pattern is most satisfactorily indexed on the basis of the chalcopyrite structure of CuInSe₂ phase. The last peaks are the three most intense peaks for CIASe phase in its chalcopyrite structure [13], they correspond respectively to the (112) and (204)/(220) planes. The presences of these most intense reflections confirm the chalcopyrite structure of CIASe material.

Figure 4 shows the XRD patterns of the CIASe films near (112) diffraction peak. As can be seen from this

Conflicts of Interest

The authors declare no conflicts of interest.

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