英文延伸摘要 SUMMARY In this study, phase formation and optical properties of (Sb1-xSnx)2Se3 (x = 0.0, 0.1, 0.2, 0.3) and Cu2FeSnSe4 (CFTSe) nanocrystals synthesized by various solvothermal processes were explored respectively. On synthesis at 190˚C in teflon-lined stainless steel autoclave, pure Sb2Se3 nanorods formed for 12 h, while pure (Sb1-xSnx)2Se3 nanorods formed for a longer time, 24-36 h, indicating that the Sn dopant increased the activation energy of Sb2Se3. The substitution solubility of Sn in Sb2Se3 was about 6 at%. The Sb2Se3 and (Sb1-xSnx)2Se3 nanorods were 50-150 nm in width and several μm in length. The growth direction of Sb2Se3 and (Sb1-xSnx)2Se3 nanorods was [001]. The bandgaps of (Sb1-xSnx)2Se3 (x = 0.0, 0.1, 0.2, 0.3) were 1.17, 1.21, 1.25, and 1.27 eV, respectively, which increased with the Sn concentration (x). The Sn-doped Sb2Se3 with tunable bandgap may be a promising candidate for photovoltaic applications. Pure and stoichiometric CFTSe nanocrystals were readily acquired for the Cu2Fe1.2Sn0.8Se4 samples using Fe(CH3COO)2 as the Fe source subjected to synthesis in oleylamine at 270˚C for 96 h. Two sharp Raman peaks at 170 and 186 cm-1 for CFTSe nanocrystals were identified. The higher reactivity of the Fe(CH3COO)2 precursor relative to the FeCl2 one enhanced the growth of pure CFTSe nanocrystals, while more Fe concentration in the Cu2FexSn0.8Se4 (x ≧ 1.3) samples led to the formation of CuFeSe2. The direct bandgap of CFTSe nanocrystals is about 1.16 eV. Key word：(Sb1-xSnx)2Se3、Cu2FeSnSe4 nanocrystals、one-pot system、wet chemical synthesis、 autoclave INTRODUCTION Recently, thin-films solar cells like copper indium gallium selenide (CIGS), and cadmium telluride (CdTe) have made impress improvements in device efficiency. However, due to the scarcity of Ga and In, and toxicity of Cd, both of CIGS and CdTe solar cells can’t popularize. Therefore, erth-abundant, non-toxic and low-cost materials should be explored for high-efficiency solar cells. V-VI compounds such as Sb2Se3, and Sb2S3, and the Cu2-II-IV-VI4 quaternary chalcopyrite compounds such as Cu2ZnSnS4 (CZTS), and Cu2ZnSnSe4 (CZTSe) containing non-toxic, abundant elements, and excellent opitical properties have drawn great attention as potential candidates for CIGS for use as the solar cell absorber layer. In order to opitimize the device conversion efficiency, the band gap of an ideal PV absorber layer should be around 1.3 eV for the single-junction cell and 1.0-1.9 eV for the two-junction cell. These requirements can be achieved by tunning the bandgaps of PV materials via adjusting the chemical compositions. Because few studies about the tunable bandgaps of Sb2Se3 and CFTSe were reported. In the present study, the tunable bandgap of Sn-doped Sb2Se3 nanocrystals and Cu2FeSnSe4 alloys were synthesised by one-pot system. MATERIALS AND METHODS For Sn-doped Sb2Se3 nanocrystals, a mixture of SnCl2∙2H2O, SbCl3 was dissolved in deionized water with magnetic stirring, and Se powders was dissolved in hydrazine. The resultant solution was transferred into a Teflon-lined stainless steel autoclave and maintained at 180-190℃ for 24-48 h. For Cu2FeSnSe4 alloys, a mixture of CuCl, Fe(CH3COO)2, SnCl4∙5H2O and Se powders was dissolved in OLA with magnetic stirring, and then heated at 270˚C in N2. The products was centrifuged and then washed with hexane and ethanol to remove the dissoluble by-product, and finally dried at about 50℃. The microstructure of samples was observed using scanning electron microscopy (SEM), and transmission electron microscopy (TEM).The chemical compositions of samples were measured with energy dispersive spectroscopy (EDS). The phases in the samples were analyzed using a X-ray diffractometer (XRD) and Raman spectra. The optical properties of samples were characterized using diffuse reflectance UV-vis spectroscopy in the range of 400-2000 nm. The valence states of the chemical elements in the samples were measured using X-ray photoelectron spectroscopy (XPS). RESULTS AND DISCUSSION Sb2Se3 and (Sb1-xSnx)2Se3 (x = 0.1, 0.2, 0.3) powders with single orthorhombic Sb2Se3 phase (JCPD 00-072-1184) were synthesized with autoclave at 190°C for 24-36 h. The XRD peaks of (Sb1-xSnx)2Se3 samples shift to lower diffraction angles as compared with that of the Sb2Se3 sample. From XPS measurement Sn and Sb in the (Sb1-xSnx)2Se3 samples are in the oxidation state of 2+ and 3+ respectively. The substitution of Sn for Sb in (Sb1-xSnx)2Se3 reduces its lattice constant because the ion radius of Sb3+, 0.076 nm, is smaller than that of Sn2+, 0.093 nm. The chemical compositions of (Sb1-xSnx)2Se3 samples show that the limit substitution soliblity of Sn in Sb2Se3 lattice is about 6 at%. The optical bandgap of Sb2Se3 and (Sb1-xSnx)2Se3 (x = 0.0, 0.1, 0.2, 0.3) nanocrystals are in the range of 1.17-1.27 eV, which are obtained from their reflectance spectra by performing the Kubelka-Munk transformation, showing that the optical bandgaps increase with the Sn concentrations. The Cu2FeSnSe4(CFTSe) samples were synthesized at 270˚C for 84-96 h to obtain pure CFTSe phase which was characterized by the sharp Raman peaks. The Raman spectrum of the sample with Fe(CH3COO)2 precursor synthesized at 270℃ for 96 h shows two sharp peak at 170 and 186 cm-1, both of which shift to lower frequencies as compared with the corresponding ones, 173 and 196 cm-1, of CZTSe. The variation in frequencies of two peaks is due to the change in the force constant of bond.From EDS/TEM analyses, the average chemical compositions of the CFTSe sample were Cu：Fe：Sn：Se = 25.1：13.0：12.4：49.5. CFTSe samples comprised nanoparticles, about 20-50nm in size, and nanosheets, about 50-100 nm in thickness and several hundred nm in size. When the sample with more concentration of Fe(CH3COO)2, Cu2FexSn0.8Se4(x ≧ 1.3), were synthesized at 270℃ for 72-120 h.The CuFeSe2(44-1305) along with the CFTSe phase was formed. The optical bandgap, 1.16 eV, of pure CFTSe was obtained from its reflectance spectra by performing the Kubelka-Munk transformation, which is close to the reported values. CONCLUSION 1. Sb2Se3 and (Sb1-xSnx)2Se3 (x = 0.1, 0.2, 0.3) powders with single orthorhombic Sb2Se3 phase (JCPD 00-072-1184) were synthesized with autoclave at 190°C for 24-36 h respectively. 2. The maximum substitution solubilities of Sn in Sb2Se3 were about 6 at%. 3. The bandgaps of (Sb1-xSnx)2Se3 (x = 0.0, 0.1, 0.2, 0.3) were 1.17, 1.21, 1.25, and 1.27 eV, respectively, which increased with the Sn concentration (x). 4. pure and stoichiometric CFTSe nanocrystals can be obtained when the Cu2Fe1.2Sn0.8Se4 samle using Fe(CH3COO)2 as Fe source is subjected to synthesis at 270℃ for 96 h in OLA 5. The Raman spectrum of the sample shows two sharp peak at 170 and 186 cm-1. 6. The direc bandgap of CFTSe nanocrystals is about 1.16 eV.
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