In this study, p-type chalcopyrite CIGSS films were deposited on soda lime glass substrates by a novel co-sputtering technique using individual quaternary alloy, Cu(In,Ga)Se2 and In2S3 targets followed by post-annealing treatment. This technique is easily reproducible and features in both energy and materials saving which is very suitable for large area deposition in producing more efficient photovoltaic devices.
And in the conclusion, we still divided into three parts to list about the conclusion of each section.
Substrate Temperature:
At first, a co-sputtering CIGS precursor films deposited at various substrate temperatures were prepared. Experimental results indicated that amorphous CIGSS films is dominant as Tsub.=323 K. Subsequent melting of indium at Tsub.=423 K resulted in the formation of In4Se3 phase while the crystallinity of the film was improved. As Tsub.=473 K, the α-In2Se3 phase emerged. Eventually, β-In2Se3 appeared as Tsub.=523 K. All CIGSS films exhibit an In-rich structure as evidenced by EDS analysis. In addition, the atomic ratios of Ga/(In+Ga) and Cu/(In+Ga) are almost constant with values of 0.16 and 0.70, respectively regardless of the substrate temperature. The surface morphologies of the as-deposited CIGSS films resemble to the Zone I area in the Thornton structural zone model.
Annealing temperature:
For better performance, a CIGSS precursor films co-sputtered at Tsub.= 473 K are chose as samples to employ post-annealing treatment with various annealing temperature, and discuss with the influence of
annealing temperature on the structural, electrical, and optical properties of CIGSS films were extensively investigated and concluded that:
1. The as-deposited CIGSS precursor films were mainly comprised of In4Se3 phase with no sign of chalcopyrite structure. After two-step post annealing, the XRD patterns reveals that all the CIGSS films exhibited sharp crystallinity chalcopyrite peaks with a preferred orientation along the (112) plane. Furthermore, the position of the (112) peaks slightly shifted to lower diffraction angles as the annealing temperature increased.
2. EDS profile shows that the ratios of each constituent element were near stoichiometry after post-annealing. The SIMS depth profiles reveal an enhanced diffusivity at an annealing temperature of 763 K.
3. As the annealing temperature is higher than 823 K, which is close to the softening temperature of soda-lime glass substrates, the films peeled off due to the residual thermal stress. At this temperature, the crystallinity of the CIGSS films also began to deteriorate.
4. Polycrystalline CIGSS films with the best crystallinity were obtained at an annealing temperature of 763 K, with a carrier concentration and film resistivity of 4.86×1016 cm-3and 32 Ωcm, respectively. The optical band gap of the CIGSS absorber layer was estimated to be 1.18 eV.
Annealing time:
To improve the graded band structure, a CIGSS precursor films co-sputtered at Tsub.= 473 K and the annealing temperature was set at 763 K to employ post-annealing treatment with various annealing time.
1. The XRD patterns clearly show that all the resultant films exhibited a chalcopyrite structure with well-resolved preferred orientation in the (112) diffraction plane. Binary selenides phase onset as the annealing time extended to 25 minutes. Therefore, a shorter interval of 5 minutes will suffice for optimal chalcopyrite structure. SIMS depth profiles reveals two distinct region divided by an abrupt boundary.
First, an indium-rich region (I) exists underneath the film surface in which all of the compositions of indium, gallium and sulfur are at higher concentration level. In contrast, in the copper-rich region (II), both the concentrations of copper and selenium are higher than those in the region (I) while the concentrations of other constituents drop suddenly.
2. The optical transmittance in the infrared region (800~1800 nm) is improved significantly as the annealing time increased. All films have Eopt lying between 1.13~1.18 eV. As the annealing time is longer than 25 minutes, the Eopt dropped to ~1.13 eV. This effect was caused by the significant loss of sulfur and selenium after the CIGSS precursor films were annealed at 763 K for longer period.
In summary, the polycrystalline CIGSS films prepared by a novel co-sputtering process followed by substrate temperature of 473K, and with a post annealing at 763 K for 5 minutes shows good crystallinity with near-stoichiometry composition. The CIGSS film has optimal value of carrier concentrations, 4.86×1016cm-3 and resistivity of 45 Ωcm, respectively.
List of table captions
Table 1. Dependence of the composition determined by EDS for CIGSS films deposited at different substrate temperatures.
Table 2 EDS data of as-deposited and CIGSS films annealed at different temperatures.
Table 3.EDS analysis of CIGSS films annealed at 763K for different annealing time.
List of figure captions
Fig. 6 XRD diffraction patterns of CIGSS films deposited at different substrate temperatures.
Fig. 7 Surface morphologies of CIGSS thin films deposited at different substrate temperatures of (a) 323 K, (b) 423 K, (c) 473 K, and (d) 523 K.
Fig. 8 X-ray diffraction patterns of CIGSS films before and after annealing at different temperature.
Fig. 9 Dependence of the composition determined by DES for CIGSS films at different annealing temperatures.
Fig. 10 SIMS depth profile of an as-deposited CIGSS films (a) and one annealed at 763K (b).
Fig. 11 SEM morphologies of CIGSS films annealed at different temperatures: (a) as-deposited, (b) 733 K, (c) 763 K, (d) 793 K, (e) 823 K, and (f) 853 K.
Fig. 12 AFM micrograph of CIGSS films annealed at different temperatures: (a) as-deposited, (b)733 K, (c)763 K, (d)793 K, (e) 823 K, (f) 853 K.
Fig. 13 Transmittance plots of CIGSS films as a function of annealing temperature.
Fig. 14 Plot of (αhν)2 versus photon energy for CIGSS films as a function of annealing temperature.
Fig. 15 The resistivity, hall mobility, and carrier concentration of the CIGSS films that as a function of annealing temperature.
Fig. 16 X-ray diffraction patterns of CIGSS films annealed at 763 K for different annealing time.
Fig. 17 SIMS depth profile of CIGSS films deposited at 423 K (a) and annealed at 763 K for 5 minutes (b).
Fig. 18 SEM morphologies of CIGSS films annealed at 763 K for different times: (a) 5 minutes, (b) 15 minutes, (c) 25 minutes.
Fig. 19 Transmittance plots of CIGSS films annealed at 763 K for different annealing time.
Fig. 20 Plot of (αhν)2 versus photon energy for CIGSS films annealed at 763 K for different annealing time.
Fig. 21 Resistivity, Hall mobility, and carrier concentration of CIGSS films annealed at 763 K for different annealing time.
七、 計畫成果自評
“Growth characteristics and properties of ZnO:Ga thin films prepared by pulsed DC magnetron sputtering” Applied Surface Science 256 (2010) 3432-3437, (SCI).
2. W. T. Yen, Y. L. Chen, J. H. Ke, Y. C. Lin*, C. Y. Nieh and S. C. Liang, 2009, “The study of surface sulfurization to improve pentanary Cu(In,Ga)(Se,S)2 films by co-sputtering quaternary alloy and In2S3 targets” Thin Solid Films, (Submitted) (SCI).
3. W. T. Yen, Y. C. Lin*, P. C. Yao, Y. L. Chen, J. H. Ke, and S. T. Hang, 2009,“Influence of annealing temperature on properties of Cu(In,Ga)(Se,S)2 thin films prepared by co-sputtering from quaternary alloy and In2S3 targets” Vacuum, (Submitted) (SCI).
研討會論文
1. Wen-Tsai Yen(嚴文材), Jia-Hong Ke (柯佳宏), Yi-Cheng Lin (林義 成), Cuo-Yo Nieh (倪國裕), Shih-Chang Liang (梁仕昌), Hsiao-Min Wu ( 吳 曉 旻 ), “Study of molybdenum thin films prepared for Cu(In,Ga)Se2 solar cells back contact using DC magnetron sputtering”, Acceptance Letter of your abstract submitted to TACT 2009 Conference.
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