Single elementary target-sputtered Cu2ZnSnSe4 thin film solar cells
Graphical abstract
Introduction
Quaternary p-type semiconducting Cu2ZnSnSe4 (CZTSe) compounds have been studied recently as alternative absorber materials for high efficiency thin film solar cells due to the abundance of non-toxic constituents and high absorption coefficient (>104 cm−1) [1], [2]. A variety of processing techniques, such as thermal evaporation [2], solution-based process [3], [4], pulsed-laser deposition [5], electrodeposition [6] and sputtering [7], have been investigated to prepare high quality CZTSe thin films. Sputtering is one of the most viable deposition techniques for producing a large-scale panel of thin film solar cells with demonstrable productivity and easy adjustment. So far, all reported sputtering methods for CZTSe absorber are primarily based on the utilization of multi-targets of the elements of Cu, Zn and Sn and the subsequent selenization of the deposited precursors [7], [8], [9]. Control of the thickness of each deposited layer by sequential deposition of Cu, Zn and Sn on Mo-coated substrates has been critical in producing a conversion efficiency of 3.2–4.8% [7], [8]. However, there has not been any report on a simple single-target approach that can avoid the complex sequential deposition process. A simple sputtering method is preferred for a large-scale production and cost-effectiveness.
Here we present an effective sputtering method for producing high quality absorber films by adjusting the relative content of Cu, Zn, Sn and Se in a single-target without the extra control of deposition sequence. A cell efficiency of ~4.16% obtained for a Cu/Zn-rich target demonstrates the great potential of the elementary single target approach. The effects of substantial variations in target compositions are primarily discussed in conjunction with desirable grain structures and phase distributions for better photovoltaic characteristics.
Section snippets
Experimental
Single targets of four selected compositions, Cu2ZnSnSe4 (designated as 2Cu 1Zn), Cu2Zn1.5SnSe4 (2Cu 1.5Zn), Cu2.5Zn1.5SnSe4 (2.5Cu 1.5Zn) and Cu3Zn1.5SnSe4 (3Cu 1.5Zn), were prepared from elementary metal sources. Cu (99.85% Kanto Chem. Co., Inc.), Zn (>99%, Aldrich), Sn (>99%, Aldrich) and Se (>99%, Dae Jung) were mixed by simple ball milling in ethanol for 6 h. The mixture was dried and uniaxially pressed into a 2-in. disk pellet. The pellet was fired at 300 °C for 30 min in N2 atmosphere. Thin
Results and discussion
Fig. 1 shows XRD patterns of the absorber thin films prepared on Mo-coated glass substrates using different sputtering targets of 2Cu 1Zn, 2Cu 1.5Zn, 2.5Cu 1.5Zn and 3Cu 1.5Zn. The XRD patterns exhibit several distinct CZTSe peaks [JCPDS file: 52-0868] including (1 1 2), (2 2 0)/(2 0 4) and (3 1 2)/(1 1 6) planes at approximately 27.1°, 45° and 53.5°, respectively. As shown in insets of the XRD pattern, the clear peak splitting of the (2 2 0)/(2 0 4) and (3 1 2)/(1 1 6) planes indicates the stannite structure (space
Conclusions
We have demonstrated the practical application of one-stage sputtering of Cu2ZnSnSe4 using a single elementary target with a conversion efficiency of 4.16%. A critical step for the successful demonstration is based on the adoption of excessive Zn and Cu contents, which are assumed to decrease significantly during the active deposition. The best target composition Cu2.5Zn1.5SnSe4 resulted in enlarged grains with promising ratios of Zn/Sn ~1.06 and Cu/(Zn+Sn) ~0.81. Excess Zn seems to bring
Acknowledgements
This work was financially supported by a grant (no. 2011-0020285) from the National Research Foundation of Korea funded by the Korean government.
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Present address: School of Physics & Materials Science, Thapar University, Patiala 147004, India.