臺灣博碩士論文加值系統

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.

目錄
摘要 I
Abstract II
致謝 V
目錄 VI
圖目錄 VIII
第一章 引言 1
第二章 文獻回顧 4
2.1太陽能電池原理 4
2.1.1光傳導效應 (Photoconductive effect) 4
2.1.2光伏特效應 (Photovoltaic effect) 4
2.2太陽能材料簡介 5
2.2.1太陽能材料物理特性限制 5
2.3可調控能隙材料 7
2.4應力效應(Stress effect)對光學能隙的影響： 9
2.5 SnS 合成與調控能隙文獻： 10
2.6 CZTS、CZTSe合成與調控能隙文獻 14
2.7研究動機 17
第三章 實驗步驟與方法 19
3.1濕式化學法在氮氣中合成SnS及Sn0.8GexS 19
3.2濕式化學法在氮氣中合成Cu2Zn(Sn1-xGex)Se4(x=0、0.3、0.5、0.7、1.0) 20
3.3材料特性分析 21
3.3.1 X光繞射儀(X-ray Diffractometer) 21
3.2.2 掃瞄式電子顯微鏡(Scanning Electron Microscope, SEM) 22
3.2.3 穿透式電子顯微鏡(Transmission Electron Microscope, TEM) 23
3.2.4 X光能量散佈分析儀(Energy Dispersive X-ray Spectrometer, EDS) 25
3.2.5 紫外/可見光(UV-vis)光譜儀 26
3.2.6 化學分析電子光譜儀(Electron Spectroscopy for Chemical Analysis，ESCA) 27
3.2.7拉曼光譜儀(Raman Spectrometer) 28
第四章 結果與討論 30
4.1 SnS與Sn0.8GexS奈米晶 30
4.1.1濕式化學法在氮氣中合成SnS奈米晶 30
4.1.2濕式化學法在氮氣中合成Sn0.8GexS奈米晶 30
4.1.3 Sn0.8GexS奈米晶的微結構 32
4.1.4 Ge摻雜之Sn0.8GexS奈米晶光學能隙 33
4.1.5退火對Sn0.8Ge0.8S薄膜光學性質造成的影響 34
4.2 Cu2Zn(Sn1-xGex)Se4 (CZTGSe) 35
4.2.1合成Cu2Zn(Sn1-xGex)Se4的反應動力研究 35
4.2.2 Cu2Zn(Sn1-xGex)Se4 (x= 0、0.3、0.5、0.7、1.0)奈米晶的性質分析 38
4.2.3 Cu2Zn(Sn1-xGex)Se4 (x=0、0.3、0.5、0.7、1.0)奈米晶形貌、微結構與化學組成 39
4.2.4 Cu2Zn(Sn1-xGex)Se4 (x=0、0.3、0.5、0.7、1.0)的光學能隙 40
第五章 結論 41
5.1 濕式化學法在氮氣中合成Sn0.8GexS奈米晶 41
5.2 濕式化學法在氮氣中合成CZTGSe奈米晶 42
參考文獻 43
附錄 105
JCPDS Cards No. 00-039-0354 (Orthorhombic SnS) 105
JCPDS Cards No. 01-077-3356 (Cubic SnS) 106
JCPDS Cards No. 01-070-8930 (Cu2ZnSnSe4) 107
JCPDS Cards No. 01-070-7623 (Cu2ZnGeSe4) 109
JCPDS Cards No. 01-072-8034 (Cu2SnSe3) 110
JCPDS Cards No. 01-089-2878 (Cu2GeSe3) 112
JCPDS Cards No. 00-037-1463 (ZnSe) 113

圖目錄
圖 1 p-n junction示意圖 50
圖 2 p-n junction外接導線示意圖 51
圖 3 太陽光能量分布光譜 51
圖 4 實驗流程簡圖 52
圖 5 迴流系統裝置圖 53
圖 6 布拉格定律示意圖 53
圖 7 SnS試料在210℃反應12 h的XRD圖 54
圖 8 SnS orthorhombic層狀結構 54
圖 9 在230℃, 12 h合成SnS的XRD圖 55
圖 10 Sn0.8Ge0.4S試料在230℃, 12 h反應的XRD圖 55
圖 11 Sn0.8Ge0.4S在不同溫度下反應XRD圖 56
圖 12 Sn0.8Ge0.4S在不同溫度下反應的EDS分析結果 56
圖 13 Sn0.8GexS(x=0.4, 0.6, 0.8) 270℃, 12 h XRD圖 57
圖 14 SnS、Sn0.8GexS(x=0.4, 0.6, 0.8) 270℃, 12 h XRD疊圖(a)20~80°(b)(111)peak 58
圖 15 SnS、Sn0.8GexS SEM/EDS元素分析 59
圖 16 SnS、Sn0.8GexS EDS result 60
圖 17 SnS、Sn0.8GexS晶格常數對Ge/(Sn+Ge)作圖 60
圖 18 Sn0.8Ge0.6S奈米晶之ESCA光電子能譜圖 (a)全區段 (b)Sn (c)S (d)Ge 61
圖 19 SnS，230℃, 12 h之SEM影像 (a)倍率50K(b)倍率100K 62
圖 20 Sn0.8Ge0.4S，270℃, 12 h之SEM影像 (a)倍率50K(b)倍率100K 63
圖 21 Sn0.8Ge0.6S，270℃, 12 h之SEM影像(a)倍率50K(b)倍率100K 64
圖 22 Sn0.8Ge0.8S，270℃, 12 h之SEM影像(a)倍率50K(b)倍率100K 65
圖 23 230℃, 12 h合成SnS奈米晶之TEM影像、EDS、繞射圖 66
圖 24 Sn0.8Ge0.8S之TEM分析 68
圖 25 吸收係數光譜 (a)SnS (b)Sn0.8Ge0.4S (c) Sn0.8Ge0.6S (d)Sn0.8Ge0.8S……………69
圖 26 SnS, Sn0.8GexS(x=0.4, 0.6, 0.8)奈米晶之反射率圖譜 70
圖 27 SnS, Sn0.8GexS(x=0.4, 0.6, 0.8)，(F(R)hν)2對hν作圖 70
圖 28 SnS、Sn0.8GexS(x=0.4、0.6、0.8)能隙對Ge/(Sn+Ge)作圖 71
圖 29 未退火SnS 薄膜 SEM影像 71
圖 30 未退火Sn0.8Ge0.8S薄膜 SEM影像 (1)平面形貌 (2)厚度 72
圖 31 200℃退火20分鐘Sn0.8Ge0.8S薄膜SEM影像 72
圖 32 300℃退火20分鐘Sn0.8Ge0.8S薄膜SEM影像 73
圖 33 350℃退火20分鐘Sn0.8Ge0.8S薄膜SEM影像 73
圖 34 400℃退火20分鐘Sn0.8Ge0.8S薄膜SEM影像 74
圖 35 Sn0.8Ge0.8S薄膜(a)反射率圖譜 (b)(F(R)hν)2對hν作圖 75
圖 36 Sn0.8Ge0.8S薄膜在200, 300, 350, 400℃退火之SEM/EDS分析 75
圖 37 CZTSe, CZGSe, CTSe, CGSe, ZnSe JCPD cards疊圖 76
圖 38 CZTSe (270℃, 12 h) Raman圖譜 76
圖 39 CZTSe (270℃, 48 h) Raman圖譜 77
圖 40 CZTSe (270℃, 60 h) Raman圖譜 77
圖 41 CZGSe (270℃, 12 h) Raman圖譜 78
圖 42 CZGSe (270℃, 48 h) Raman圖譜 78
圖 43 CZGSe (270℃, 60 h) Raman圖譜 79
圖 44 CZGSe (270℃, 72 h) Raman圖譜 79
圖 45 Cu2Zn(Sn0.5Ge0.5)Se4 (270℃, 5 h) Raman圖譜 80
圖 46 Cu2Zn(Sn0.5Ge0.5)Se4 (270℃, 12 h) Raman圖譜 80
圖 47 Cu2Zn(Sn0.5Ge0.5)Se4 (270℃, 48 h) Raman圖譜 81
圖 48 Cu2Zn(Sn0.5Ge0.5)Se4 (270℃, 60 h) Raman圖譜 81
圖 49 Cu2Zn(Sn0.5Ge0.5)Se4 (270℃, 72 h) Raman圖譜 82
圖 50 Cu2Zn(Sn0.3Ge0.7)Se4 (270℃, 12 h) Raman圖譜 82
圖 51 Cu2Zn(Sn0.3Ge0.7)Se4 (270℃, 24 h) Raman圖譜 83
圖 52 Cu2Zn(Sn0.3Ge0.7)Se4 (270℃, 36 h) Raman圖譜 83
圖 53 Cu2Zn(Sn0.3Ge0.7)Se4 (270℃, 48 h) Raman圖譜 84
圖 54 Cu2Zn(Sn0.3Ge0.7)Se4 (270℃, 60 h) Raman圖譜 84
圖 55 Cu2Zn(Sn0.3Ge0.7)Se4 (270℃, 72 h) Raman圖譜 85
圖 56 270℃下不同反應時間CZTSe、CZGSe、CZTGSe相分析 86
圖 57 270℃, 60-72 h CZTGSe(x=0、0.3、0.5、0.7、1.0) Raman圖譜 87
圖 58 270℃, 60-72 h CZTGSe(x=0、0.3、0.5、0.7、1.0) XRD圖譜 87
圖 59 CZTSe tetragonal structure 88
圖 60 CZGSe tetragonal structure 88
圖 61 CZTGSe(x=0.7) ESCA光電子能譜圖 89
圖 62 CZTGSe(x=0、0.3、0.5、0.7、1.0)晶格常數與Ge/(Sn+Ge)作圖 90
圖 63 CZTSe，270℃, 60 h之SEM影像 (a)倍率50K(b)倍率100K 91
圖 64 CZTGSe(x=0.3) 270℃, 72 h之SEM影像 (a)倍率50K(b)倍率100K 92
圖 65 CZTGSe(x=0.5) 270℃, 72 h之SEM影像 (a)倍率50K(b)倍率100K 93
圖 66 CZTGSe(x=0.7) 270℃, 72 h之SEM影像 (a)倍率50K(b)倍率100K 94
圖 67 CZGSe 270℃, 72 h之SEM影像 (a)倍率50K(b)倍率100K 95
圖 68 CZTGSe(x=0、0.3、0.5、0.7、1.0) EDS分析結果 96
圖 69 CZTSe TEM影像、EDS、繞射圖 97
圖 70 CZTGSe(x=0.7)之TEM分析 99
圖 71 CZGSe TEM(a)影像、EDS、繞射圖 (b)HRTEM影像、line scan 101
圖 72 CZTGSe奈米晶之吸收係數光譜 102
圖 73 CZTGSe (x=0.0、0.3、0.5、0.7、1.0) 反射率圖譜 103
圖 74 CZTGSe (x=0.0、0.3、0.5、0.7、1.0) (F(R)hν)2對hν作圖 103
圖 75 理論計算/實驗能隙值與Ge濃度的關係 104

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