本論文探討在210-270℃以油胺為溶劑合成SnS, Sn0.8GexS (x = 0.4, 0.6, 0.8)與Cu2Zn(Sn1-xGex)Se4 (x = 0.0, 0.3, 0.5, 0.7, 1.0)合金的相生成與可調控能隙之研究。SnS, Sn0.8Ge0.4S試料分別在230與240℃反應可得到orthorhombic SnS純相，顯示Ge的摻入提高了orthorhombic SnS相的活化能。Ge在SnS中的固溶度約6 at%。藉由提升Ge的濃度至6.2 at%， Ge-doped SnS奈米晶之能隙可由1.25調控至1.35 eV。將Sn0.8Ge0.8S薄膜在氮氣中以200℃退火，其能隙值無變化，而當退火溫度為300和350℃，能隙值由1.35 eV減小至1.32與1.28 eV，此現象可由退火造成Ge在試片中的揮發來解釋。
以溶熱法合成Cu2Zn(Sn1-xGex)Se4 (x = 0.3, 0.5, 0.7)合金過程中有Cu2SnSe3、Cu2GeSe3 (CGSe)、Cu2ZnSnSe4、ZnSe與包含CGSe的ZnSe等中間相生成，其中沒有觀察到Cu2ZnGeSe4。推論Cu2Zn(Sn1-xGex)Se4合金可能的反應機制為：(1) Cu2SnSe3 + ZnSe  Cu2ZnSnSe4, (2) (1-x)Cu2ZnSnSe4 + xCu2GeSe3 + xZnSe  Cu2Zn(Sn1-xGex)Se4。隨著Ge濃度由x = 0增加至1.0，Cu2Zn(Sn1-xGex)Se4的晶格常數呈線性減小，其能隙值由0.98呈線性增加至1.45 eV。本研究顯示可調控能隙之Ge-doped SnS與Cu2Zn(Sn1-xGex)Se合金有應用於光伏元件的潛力。

Solution-phase synthesis and tunable bandgaps of Sn1-xGexS and Cu2Zn(Sn1-xGex)Se4 nanocrystals

Hsuan-Tai Hsu
Wen-Tai Lin
Department of Materials Science and Engineering, National Cheng Kung University

SUMMARY

In this study, phase formation and tunable bandgaps of SnS, Sn0.8GexS (x = 0.4, 0.6, and 0.8), and Cu2Zn(Sn1-xGex)Se4 (x = 0.0, 0.3, 0.5, 0.7, and 1.0) alloys synthesized at 210-270˚C in oleylamine were explored. For SnS and Sn0.8Ge0.4S samples, pure orthorhombic SnS phase formed at 230 and 240˚C, respectively, indicating that introduction of Ge raised the activation energy of orthorhombic SnS phase. The substitution solubility of Ge in SnS was about 6 at%. The bandgap of Ge-doped SnS nanocrystals can be tuned from 1.25 to 1.35 eV by increasing the Ge concentration to 6.2 at%. On annealing in N2, the bandgap of 200˚C-annealed Sn0.8Ge0.8S films remained unchanged, while that of 300- and 350˚C-annealed Sn0.8Ge0.8S films decreased from 1.35 to 1.32 and 1.28 eV, respectively. This result can be explained in terms of evaporation of Ge during annealing.
On solvothermal synthesis of Cu2Zn(Sn1-xGex)Se4 (x = 0.3, 0.5, and 0.7) alloys the intermediate phases such as Cu2SnSe3, Cu2GeSe3(CGSe), Cu2ZnSnSe4, ZnSe, and inclusion of ZnSe to CGSe formed, while no Cu2ZnGeSe4 was observed. One can suggest the growth mechanisms for Cu2Zn(Sn1-xGex)Se4 alloys: (1) Cu2SnSe3 + ZnSe  Cu2ZnSnSe4, (2) (1-x)Cu2ZnSnSe4 + xCu2GeSe3 + xZnSe  Cu2Zn(Sn1-xGex)Se4. With increasing the Ge concentration (x) from 0.0 to 1.0, the lattice constant of Cu2Zn(Sn1-xGex)Se4 samples decreased linearly, while their bandgap increased linearly from 0.98 to 1.45 eV. The Ge-doped SnS and Cu2Zn(Sn1-xGex)Se4 alloys with tunable bandgap may be promising candidates for photovoltaic applications.

Key words: Ge-doped SnS, Cu2Zn(Sn1-xGex)Se4, wet chemical synthesis, nanocrystals, tunable bandgap

INTRODUCTION

Currently, the IV-VI compounds such as SnS, and SnSe, and the Cu2-II-IV-VI4 compounds such as Cu2ZnSnS4 (CZTS), and Cu2ZnSnSe4 (CZTSe) containing non-toxic and abundant elements have drawn great attention as alternatives to Cu(In,Ga)Se2 for use as the solar cell absorber layer. To maximize the 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.
Recently, few studies about the tunable bandgaps of SnS and CZTSe were reported. In the present study, the tunable bandgap of Ge-doped SnS nanocrystals and Cu2Zn(Sn1-xGex)Se4 alloys were explored.

MATERIALS AND METHODS

For Ge-doped SnS nanocrystals, a mixture of SnCl2∙2H2O, GeI4, CH4N2S and was dissolved in oleylamine (OLA) with magnetic stirring, and then heated at 230, 270˚C in N2 for 12 h. For Cu2Zn(Sn1-xGex)Se4 alloys, a mixture of CuCl, Zn(CH3COO)2∙2H2O, SnCl4∙5H2O, GeI4 and Se powders was dissolved in OLA with magnetic stirring, and then heated at 270˚C in N2 for 60-72 h. The product 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 SEM and TEM. The chemical compositions of samples were measured with SEM/EDS and TEM/EDS. The phases in the samples were analyzed using XRD and Raman spectrometer. The optical properties of samples were characterized using diffuse reflectance UV-vis spectrometer. The valence states of the chemical elements in the samples were measured using XPS.

RESULTS AND DISCUSSION

SnS and Sn0.8GexS (x = 0.4, 0.6, 0.8) powders with single orthorhombic SnS phase (JCPD 00-039-0354) were synthesized in OLA at 230 and 270°C for 12 h, respectively. The XRD peaks of Sn0.8GexS powders shift to higher diffraction angles as compared with that of the SnS powder. From XPS measurement Sn and Ge in the Sn0.8GexS samples are in the oxidation state of 2+. The substitution of Ge for Sn in SnS reduces its lattice constant because the ion radius of Ge2+, 0.073 nm, is smaller than that of Sn2+, 0.093 nm. Only a trace amount of the Sn0.8Ge1.0S powder could be synthesized, revealing that there may have a limit in the substitution solubility of Ge in the SnS lattice. In the present study, the substitution solubility of Ge in the SnS lattice is about 6 at%. The direct bandgaps of SnS, Sn0.8GexS (x = 0.4, 0.6, 0.8) nanocrystals are in the range of 1.25-1.35 eV, which are obtained from their reflectance spectra by performing the Kubelka-Munk transformation, showing that the direct bandgaps increase with the Ge concentrations.
The Cu2Zn(Sn1-xGex)Se4(x=0.3, 0.5, and 0.7) samples were synthesized at 270˚C for 72 h to obtain pure Cu2Zn(Sn1-xGex)Se4 phase which was characterized by the sharp Raman peaks. The Raman spectra of CZTSe, Cu2Zn(Sn1-xGex)Se4(x = 0.3, 0.5, and 0.7), and CZGSe samples showing that two main peaks of Cu2Zn(Sn1-xGex)Se4(x = 0.3, 0.5, and 0.7) locate in the range of 174-175 and 199-202 cm-1, respectively. With increasing the Ge concentration, the two main peaks progressively approach to that, 176 and 204 cm-1, of CZGSe respectively. The direct bandgaps of Cu2Zn(Sn1-xGex)Se4(x = 0.0, 0.3, 0.5, 0.7, and 1.0) samples are 0.98, 1.08, 1.20, 1.30, and 1.45 eV, which are obtained from their reflectance spectra by performing the Kubelka-Munk transformation, showing that the direct bandgaps increase with the Ge concentrations linearly.

CONCLUSION

SnS and Sn0.8GexS (x = 0.4, 0.6, 0.8) powders with single orthorhombic SnS phase (JCPD 00-039-0354) were synthesized in OLA at 230 and 270°C for 12 h, respectively. The maximum substitution solubilities of Ge in SnS were about 6 at%. The bandgaps of Ge-doped SnS nanocrystals could be tuned in the range of 1.25-1.35 eV. For the Sn0.8Ge0.8S film subjected to annealing in N2 the bandgaps of 200°C-annealed ones remained nearly unchanged, while those of 300- and 350°C-annealed ones decreased with the annealing temperature because of the evaporation of Ge and S. In the present study, the Ge-doped SnS nanocrystals with tunable bandgap may be the potential candidates as the photovoltaic materials.
On synthesis of CZTSe and CZGSe samples at 270℃, we find out that the growth rate of CZGSe is relatively lower than that of CZTSe. On synthesis of Cu2Zn(Sn1-xGex)Se4(x=0.3, 0.5, 0.7) samples at 270˚C, CTSe, ZnSe, and CGSe intermediate phases initially formed and then transformed to CZTGSe finally. Pure and composition-uniform Cu2Zn(Sn1-xGex)Se4 samples can be synthesized at 270˚C for 72 h. The bandgaps of Cu2Zn(Sn1-xGex)Se4 samples increased linearly from 0.98 to 1.45 eV with the Ge concentration (x) increasing from 0.0 to 1.0. The bandgap-tunable Cu2Zn(Sn1-xGex)Se4 alloy may be a promising candidate for photovoltaic applications.

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