Abstract
Cu2ZnSnS4 (CZTS) has been recognized as a promising thin-film absorber material of chalcopyrite-related solar cells. A two-stage method for fabricating CZTS films using CZTS nanoparticles was developed. Nanocrystal inks fabricated by a ball-milling method was utilized to °C deposit CZTS precursors by spin-coating approach. The CZTS precursors were annealed in the sulfur atmosphere under different annealing temperatures ranging from 550 °C to 650 °C. Influences of annealing temperature on grain growth, composition, crystallinity, and photovoltaic properties of CZTS films were characterized. With the increase of annealing temperature, grain growth was enhanced, while the sulfur atomic ratio fist increased then decreased. The crystallinity of the films was significantly improved after the annealing, and the obvious peak of the secondary phase of ZnS, were observed from the X-ray diffraction results, when the annealing temperature increased to 625 °C. However, the secondary phase was not detected from the surface Raman spectrum. Through comparing the Raman spectrum of different areas of the CZTS film, secondary phases of ZnS and SnS were observed, indicating the decomposition of CZTS films, due to the high temperature. The highest conversion efficiency of 7.5% was obtained when the annealing temperature was 600 °C.
Highlights
The quaternary semiconductor Cu2ZnSnS4 (CZTS) using just earth-abundant materials and its counterpart Cu2ZnSn(S, Se)4 (CZTSSe), in which some of the sulfur atoms are replaced by selenium, are very promising thin film photovoltaic absorber layers of solar cell in the future, due to the ideal optical properties, nontoxicity, and low cost of the constituent elements [1,2,3]. 1.5 eV bandgap of the pure CZTS is nearly ideal for single-junction solar cell, and the bandgap can be tuned to less than 1.0 eV by substituting the sulfur atoms by selenium atoms [4]
We report our unique approach for fabricating CZTS films by a so-called ‘two-stage’ method from the CZTS nanocrystal inks, that is, CZTS nanoparticles are firstly deposited on the substrate by a spin-coating method to obtain CZTS precursor
The basic morphology for annealing temperature 550–600 ◦C (Figure 1b–d) are identical; when the temperature is raised to 625 ◦C the large grains over 1 μm begin to dominate, accompanied by a small number of small size grains (Figure 1e)
Summary
The quaternary semiconductor Cu2ZnSnS4 (CZTS) using just earth-abundant materials and its counterpart Cu2ZnSn(S, Se) (CZTSSe), in which some of the sulfur atoms are replaced by selenium, are very promising thin film photovoltaic absorber layers of solar cell in the future, due to the ideal optical properties, nontoxicity, and low cost of the constituent elements [1,2,3]. 1.5 eV bandgap of the pure CZTS is nearly ideal for single-junction solar cell, and the bandgap can be tuned to less than 1.0 eV by substituting the sulfur atoms by selenium atoms [4]. 1.5 eV bandgap of the pure CZTS is nearly ideal for single-junction solar cell, and the bandgap can be tuned to less than 1.0 eV by substituting the sulfur atoms by selenium atoms [4]. Due to this merit, CZTS-based devices are expected to have the same theoretical efficiency as Cu(In, Ga)Se2 (CIGS) solar cell [5]. Up to now efficiency of pure CZTS solar cell has surpassed 10%, while CZTSSe solar cell has achieved a record efficiency of 12.6% by a typical hydrazine-solution based synthesis method recently [6]. It has been reported that the following reaction could move forward or reverse, depending on the annealing temperature and sulfur vapor pressure [13,14]: Cu2ZnSnS4
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