Spray deposition of Cu2ZnSnS4 thin films
Introduction
Cu2ZnSnS4 (CZTS) is a quaternary semiconductor that has emerged as a potential absorber substitute for CuInGaSe2 in photovoltaic devices. This material has excellent optical properties (α ∼ 105 cm−1) and a direct band gap value that matches well the solar spectrum (1.4 ⩽ Egap ⩽ 1.5 eV). Furthermore, it does not contain toxic elements such as selenium or expensive and scarce ones as indium and gallium. Due to their excellent properties, CZTS films are being intensively studied and are promising for developing low-cost, highly efficient and environmentaly friendly solar cells [1], [2].
Numerous techniques are being currently investigated to prepare CZTS films. Among them, non-vacuum techniques are crucial to achieve cost reduction in the production of thin film solar cells. The highest efficiency for a CZTS solar cell has been reported as 11.1% by Barkhouse et al. using a low-cost method based on the preparation of CZTS nanoparticles [3].
Spray pyrolysis is a versatile technique that has been used before for the deposition of thin films of many types of compounds [4]. Moreover, it is cost effective, it does not require high vacuum, it can be used for irregular geometries and it can be used even on top of flexible substrates.
Spray pyrolysis of CZTS films was first reported by Nakayama and Ito [5]. As-deposited films resulted in sulfur-deficient compositions, so that stoichiometric films could only be obtained after a high temperature treatment in H2S. Since then, several studies have been reported focused in the preparation of CZTS films by spray pyrolysis [6], [7], [8], [9], many of them from alcohol-based precursor solutions. Here, we aim at preparing high quality CZTS from aqueous solutions, where the stoichiometry can be optimized by a sulfurization treatment with sulfur vapor that avoids the use of toxic H2S.
Section snippets
Deposition of Cu2ZnSnS4 films
Cu2ZnSnS4 (CZTS) films were deposited by spray pyrolysis. Bare glass and glass coated with fluorine-doped tin oxide (FTO, from Pilkington TEC Glass, TEC 8, ∼8 Ohms/sq) were used as substrates (2 × 2 × 0.2 cm3). Prior to the deposition, both substrates were cleaned successively in detergent and isopropylic alcohol solution in an ultrasonic bath. Later, the substrates were placed in a hot plate during 1 h to reach the temperature of the spray process (Ts). CZTS thin films were prepared at 325, 375, and
Structural characterization
Fig. 1 displays GXRD patterns of CZTS films sprayed on glass substrates at different temperatures. For every temperature shown in Fig. 1a, diffraction planes match the standard XRD pattern of the Cu2ZnSnS4 phase (PDF file n° 26-0575). Unfortunately, in our experimental set-up, unfiltered Cu Kβ radiation produces spurious peaks. However these can be easily identified. The major diffraction peaks appear at 2θ = 28.5°, 47.4° and 56.2° which can be attributed to the (1 1 2), (2 2 0), and (3 1 2)
Conclusions
Cu2ZnSnS4 thin films have been successfully prepared by spray pyrolysis from an aqueous precursor. The effect of substrate temperature and sulfurization on the properties of the deposit was investigated. Structural characterization carried out using XRD and Raman spectroscopy indicated that CZTS is the dominant component; it is homogeneously distributed even on as-deposited samples. Raman mapping showed evidence of Cu2−xS in some spots, particularly when using the lowest temperature.
Deposited
Acknowledgement
Financial support from Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Agencia Nacional de Promoción Científica y Tecnológica (FONCYT Project Code PICT 0110/11) and Universidad Nacional de Mar del Plata (UNMdP) from Argentina is highly acknowledged. The authors wish to thank to Dr. Thembela Hillie, from CSIR South Africa, for his assistance with FIB-SEM images; Dr. Mariela Desimone and Mr. Sebastián Rodriguez for their support in the XRD measurement.
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