銅銦鎵硒(CIGS)薄膜太陽能電池因具有高轉換效率、在太陽光譜區間有高吸光係數且和矽晶太陽能電池相比成本較低等特性，被認為是下一代光伏元件中有潛力的競爭者。而在薄膜表面製備微結構具有減少反射率、增加光學吸收進而改善元件效率的潛力，且有機會能將元件應用在軟性基板上，減少材料的使用量。本論文基於目前銅銦鎵硒薄膜太陽能電池的發展，著眼於發展製備銅銦鎵硒薄膜表面結構的新技術，此技術有機會能解決超薄銅銦鎵硒薄膜太陽能電池吸光不足的問題，且相對於現有技術較為簡便，成本較低。
在論文的第一部分中，使用了具有微米或奈米結構的凝膠模具，浸泡於含溴蝕刻液後，對銅銦鎵硒薄膜做壓印。藉由模具輔助式化學蝕刻法可以成功製備出不同的結構，也分析了表面化學性質與結構，同時微結構的形成機制與表面富硒的情形也在本論文中被討論。
而論文的第二部分則集中探討微米結構銅銦鎵硒薄膜太陽能電池的元件光學性質與電性。在此選取了具有不同週期與結構的銅銦鎵硒微米圓頂陣列來製備元件以探討元件表現和表面結構與膜厚的關係。
在本論文中展示了具有微結構的銅銦鎵硒薄膜太陽能電池，其捕捉光的能力比平坦化減薄後的銅銦鎵硒薄膜太陽能電池要來得好。因此，即使厚度降低，其電池效率依然可以被維持，說明微米結構具有應用於銅銦鎵硒超薄吸收層太陽能電池上之潛力。

This dissertation, based on the current development of Cu(In, Ga)Se2 (CIGS) thin film solar cells, is focused on developing a new technology for CIGS surface texturing, which could solve the critical issue of insufficient light absorption of ultra-thin CIGS devices.
In first part of this dissertation, micro- and nanopatterned agarose stamps soaked in bromine-methanol etchant were used for imprinting of CIGS thin films. Various structures were successfully fabricated by this mold-assisted chemical etching process. The surface chemical and structural properties were analyzed, and the mechanisms of microstructures formation and the selenium enrichment are discussed as well.
In the second part, we focus on the characterization of optical and electrical properties of the devices based on microstructured CIGS thin films. The CIGS microdome arrays with different periods and diameters were chosen to investigate the effects of surface morphologies and film thickness on the performance of devices.
Finally, we have shown that the light trapping abilities of microstructured CIGS devices are better than those of the flattened CIGS devices, and thus the efficiency could be maintained even the thickness is reduced.

Table of contents
Chapter 1 Introduction 2
Chapter 2 Literature Reviews 7
2.1 CIGS thin film solar cells 7
2.2 Thinning of CIGS 10
2.3 Micro- and nanostructures fabrication 13
2.4 Motivation 14
Chapter 3 Experimental Techniques 16
3.1 Fabrication of hierarchical Cu(In, Ga)Se2 micro- and nano-patterns by mold-assisted chemical etching process 16
3.1.1 Thinning and flattening of CIGS thin films 17
3.1.2 Micro- and nanoprinting of CIGS thin films 18
3.1.3 Cross-printing of CIGS thin films 19
3.2 Characterization and analysis 20
3.3 Devices fabrication 21
3.4 Instruments list 22
3.4.1 SEM 22
3.4.2 AFM 23
3.4.3 XPS 24
3.4.4 Raman Spectroscopy 24
3.4.5 UV-Vis spectrophotometer 25
3.4.6 J-V measurement system 26
Chapter 4 Results and Discussion 28
4.1 Thinning and flattening of CIGS thin films 28
4.2 Micro- and nanoprinting of CIGS thin films 30
4.3 Surface chemical and structural characterization 39
4.4 Devices fabrication and characterization 46
Chapter 5 Conclusions 53
References 55
 
List of figures
Figure 1.1 Efficiency and cost projections for first- (I), second- (II), and third-generation (III) PV technologies (wafer-based, thin films, and advanced thin films, respectively). 3
Figure 1.2 Solar cell efficiencies of various cell technologies as tracked by NREL. 5
Figure 2.1 Cross-sectional view of general CIGS solar cells. 7
Figure 2.2 The fill factor FF, efficiency η, open circuit voltage Voc and short-circuit current densities (determined from QE measurements) plotted against the CIGS film thickness. 11
Figure 2.3 SEM images of non-etched CIGSe (~2500 nm) and etched CIGSe with different etching time with and without CdS/ZnO/ZnO:Al windows layer. 12
Figure 3.1 Process flow of thinning and flattening of CIGS thin films by flat agarose stamps. 17
Figure 3.2 Process flow of micro- and nanostructure fabrication via mold-assisted chemical etching process. 18
Figure 3.3 Process flow of cross printing with different arrangement angles. 19
Figure 3.4 Photographs of (A) FESEM Hitachi S-4000, (B) JEOL JSM-6500F, and (C) Hitachi SU-8010 for analyzing micro- and nanostructures. 22
Figure 3.5 A photograph of the Dimension Icon® atomic force microscope for analyzing surface morphologies. 23
Figure 3.6 Horiba Jobin Yvon HR 800 Confocal Raman Spectrometer for analyzing CIGS microstructures. 24
Figure 3.7 Hitachi U-4000 UV/Vis spectrophotometer for measuring the reflectance spectrum. 25
Figure 3.8 Keitheley 4200 semiconductor analyzer for measuring the performances of CIGS solar cells. 26
Figure 4.1 AFM 3D images of pristine (A) and after etching (B) 1 min. (C) 2 min. (D) 3 min. (E) 4 min. (F) 5 min. CIGS thin films for checking the surface roughness. 29
Figure 4.2 SEM cross-section images of pristine (A) and after etching (B) 1 min. (C) 2 min. (D) 3 min. (E) 4 min. (F) 5 min. CIGS thin films for calculating film thickness. 29
Figure 4.3 Diagram of root mean square (RMS) roughness and film thickness versus etching time. 29
Figure 4.4 (A, B) SEM images of micro-patterned Si and agarose stamps. Insects show the corresponding cross-section images. (C, D) Bright field (BF) and dark field (DF) OM images of CIGS MDAs and MHAs. Insects show the corresponding photo images. 30
Figure 4.5 (A, D) SEM images of CIGS MHAs and MDAs. (B, E) AFM 3D images of CIGS MHAs and MDAs. (C, F) Schematic diagrams of the mold-assisted chemical etching process with and without additives and proposed formation mechanisms of MHAs and MDAs. 31
Figure 4.6 Diagrams of (A) measured film thickness versus etching time of CIGS thin films etched by flat agarose stamps, and measured film thickness and pattern height difference of (B) MDAs and (C) MHAs versus etching time. The dashed lines show the parabola fit through the average values, and inset figures show the geometrical parameters for calculation. 33
Figure 4.7 Simplified configuration of the (A) smoothed CIGS with a flat agarose stamp, (B) CIGS MDAs, and (C) CIGS MHAs for etched volume calculation. (D) The diagram of calculated etched volume versus etching time. 33
Figure 4.8 SEM images of (A) Star-shaped MHAs. (B) Flower-shaped MHAs. (C) 45∘tilt MDAs fabricated on CIGS thin films. Insect shows the corresponding cross-section image. (D) Gratings with 833.3 nm period and 416 nm line width. (E) Gratings with 606 nm period and 303 nm line width. (F) Gratings with 416.6 nm period and 208 nm line width. 34
Figure 4.9 (A, B, C) SEM images of cross printing microarrays at 45˚, 60˚, and 90˚intersection angles. (D, E, F) AFM 3D images of cross printing microarrays at 45˚, 60˚, and 90˚ intersection angles. 35
Figure 4.10 (A-D) SEM images of MHAs on Cu-poor, Cu-rich, Co-evaporation, Electrodeposition CIGS thin films respectively. (E-H) SEM images of MDAs on Cu-poor, Cu-rich, Co-evaporation, Electrodeposition CIGS thin films respectively. 36
Figure 4.11 (A-C, G, H) SEM images of MHAs after etching 1hr with etching solution concentration of 0.05M, 0.075M, 0.1M, 0.15M, and 0.2M. and (D-F, J, K) corresponding SEM cross-section images. (I, L) SEM images of MHAs after etching 1hr with etching solution concentration of 0.25M, and 0.4M. 38
Figure 4.12 (A) Diagram of film thickness of CIGS thin films versus etching solution concentration for MHAs fabrication. (B) Diagram of pattern height difference of MHAs versus etching solution concentration. 38
Figure 4.13 XPS analysis of Cu 2p3/2 (A), In 3d5/2 (B), Se 3d5/2 (C) in CIGS films before and after 6 minutes 30 seconds etch. 39
Figure 4.14 XPS results of (A) Cu 2p3/2, (B) In 3d5/2, (C) Ga 2p3/2, (D) Se 3d5/2 in CIGS films with 10, 20, 30, 40 minutes etching process without additives. 41
Figure 4.15 (A-D) The OM images of different regions (point A-E) on the CIGS MDAs. (E, F) The SEM images of CIGS MDAs at different regions. (G) The Raman spectra of CIGS MDAs at different regions. 43
Figure 4.16 SEM images of (A) pristine CIGS MDAs and evolution of surface morphologies of CIGS MDAs after (B) 5 min and (C) 10 min KCN wash. 45
Figure 4.17 The Raman spectra of the pristine CIGS thin film and CIGS MDAs before and after KCN wash and CBD CdS. 45
Figure 4.18 SEM images of the silicon microstamps with periods and diameters of (A) 5 μm/2.2 μm, (B) 9 μm/3.6μm, (C) 13 μm/5.5 μm, and (D) 15 μm/7.8 μm, and (E-F) the resulting CIGS MDAs with corresponding surface morphologies. 46
Figure 4.19 Reflectance measurements at 10° of devices of (A) CIGS thin films smoothed for 0, 20 and 40 minutes, CIGS MDAs with periods and diameters of (B) 5 μm/2.2 μm, (C) 9 μm/3.6μm, (D) 13 μm/5.5 μm, and (E) 15 μm/7.8 μm with different etching time. 48
Figure 4.20 Reflectance measurements by integrated sphere of devices of (A) CIGS thin films smoothed for 0, 20 and 40 minutes, CIGS MDAs with periods and diameters of (B) 5 μm/2.2 μm, (C) 9 μm/3.6μm, (D) 13 μm/5.5 μm, and (E) 15 μm/7.8 μm with different etching time. 48
Figure 4.21 Devices parameters (shunt resistance (Rsh), series resistance (Rs), open circuit voltage (Voc), short circuit current density (Jsc), saturation current density (J0), fill factor (FF), and efficiency (η), ) form J-V characterization for devices of (A) CIGS thin films smoothed for 0, 20 and 40 minutes, CIGS MDAs with periods and diameters of (B) 5 μm/2.2 μm, (C) 9 μm/3.6μm, (D) 13 μm/5.5 μm, and (E) 15 μm/7.8 μm with different etching time. 50
 
List of tables
Table 4.1 Geometrical parameters for calculation 33
Table 4.2 XPS analysis of surface composition of Copper/Indium/Gallium/Selenium before and after 6 minutes 30 seconds etch. 40

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