Influence of sintering temperature on screen printed Cu2ZnSnS4 (CZTS) films
Highlights
► The influences of sintering temperature on structure and properties of screen printed Cu2ZnSnS4 (CZTS) were investigated. ► It was found that the direct optical band gap increased with increasing the sintering temperature. ► The screen printed CZTS film after sintering at 450 °C had a high photosensitivity (Gi − Gd)/Gd of 14%. ► The hexagonal CuS phase aggregated after sintering at 500 °C and higher temperature.
Introduction
Thin film solar cells have an increasing stake in the photovoltaic market due to its much less material expense and low fabrication cost. Copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) solar cells are promising due to their high conversion efficiencies of about 20%. However, cadmium (Cd) in CdTe is toxic while CIGS is expensive. Therefore, alternatives are pursued, among which, the quaternary semiconductor Cu2ZnSnS4 (CZTS) has attracted remarkable attention [1], [2], [3], [4] due to the abundance and low toxicity of the materials involved. CZTS is a p-type semiconductor with a direct band gap of 1.5 eV and its solar cell has a theoretical conversion efficiency of about 30% [5] according to Shockley–Queisser limit.
CZTS films have been prepared by various methods, such as atomic beam sputtering [1], spray-deposition [6], and electron-beam deposition [7], [8]. In these methods, either expensive vacuum or polluting chemicals or H2S gas are used in the processes. Screen printing is a convenient and economic method to coating layers. In recent years, this method has been adopted for coating layers of solar cell materials, such as TiO2 [9], ZnS0.5Se0.5 [10], and CIGS [11]. A few leading thin film solar cell companies such as Nanosolar Pte Ltd. have industrially and massively manufactured CIGS thin film solar cells by utilizing an ‘industrial’ printing process to coat CIGS nanoparticle inks on flexible and light weight substrates for cost-efficient solar cell products. Although the screen printing method is successful in coating the above mentioned solar cell materials, only quite recently a few reports appeared to report screen printed CZTS films [12], [13], [14], [15]. In those reports, CZTS solar cell has been fabricated by printing CZTS powders, and printing is proved to be a cheap and effective method. However, processing conditions were far from well-studied, especially, the influence of sintering temperature on CZTS films is not clear. As the sintering after screen-printing is a necessary step to remove paste chemicals [16], it is important to study the sintering effect of screen printed CZTS films especially since CZTS is sensitive to sintering [17].
In this study, we report the fabrication of CZTS films by screen printing of a paste of our fabricated CZTS nanocrystallites. The influence of sintering temperature on structure and properties of CZTS films is investigated. Furthermore, we also studied electronic, optical and photosensitivity properties of the CZTS samples.
Section snippets
Experimental
CZTS was synthesized by solid state reaction of CuS, SnS, and ZnS nanocrystallites prepared by chemical bath deposition [17]. The CZTS precursor paste was prepared by having 1 g synthesized CZTS powder in acetone, followed by adding ethyl cellulose (viscosity 5–15 mPa s) ethanol solution, ethyl cellulose (30–60 mPa s) ethanol solution, and terpineol. This mixture in the container was stirred using a magnetic stirrer under ultrasonic wave. The prepared uniform mixture was put in a vacuum oven
Results and discussion
Fig. 1 shows TGA of CZTS powder under N2 ambient. CZTS powder had large weight loss at about 750 °C, and this is similar to that reported by Shin et al. [18]. Therefore, the sintering temperatures lower than 750 °C were adopted in this study. The weight loss of CZTS at temperatures between 100 and 700 °C was possibly due to S vaporization or SnS vaporization [19] from a CZTS compound.
Fig. 2 shows the XRD patterns of the CZTS initial powder and CZTS films screen-printed and sintered at temperatures
Conclusions
The influence of sintering temperatures on structure and properties of CZTS films was investigated. X-ray diffraction showed CZTS in samples after sintering 450 °C, but hexagonal CuS phase aggregated after sintering at 500 °C and higher temperatures. Optical band gap and resistivity were found varying with sintering temperatures. CZTS film sintering at 450 °C appeared better and exhibited an optical band gap of 1.54 eV and resistivity of 158 Ω cm. The photosensitivity (Gi − Gd)/Gd of the CZTS film
Acknowledgment
This work was supported by the Clean Energy Research Programme Grant Nos. NRF2008EWT-CERP002-041 and NUS R284-000-081-592 under Singapore EDB and DuPont Apollo.
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