Dissertations / Theses on the topic 'Tin diselenide'

Thesis (Ph. D.)--University of Oregon, 2008.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 112-117). Also available online in Scholars' Bank; and in ProQuest, free to University of Oregon users.

xvi, 117 p. ; ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number.
High efficiency thin film solar cells have the potential for being a world energy solution because of their cost-effectiveness. Looking to the future of solar energy, there is the opportunity and challenge for thin film solar cells. The main theme of this research is to develop a detailed understanding of electronically active defect states and their role in limiting device performance in copper indium gallium diselenide (CIGS) solar cells. Metastability in the CIGS is a good tool to manipulate electronic defect density and thus identify its effect on the device performance. Especially, this approach keeps many device parameters constant, including the chemical composition, grain size, and interface layers. Understanding metastability is likely to lead to the improvement of CIGS solar cells. We observed systematic changes in CIGS device properties as a result of the metastable changes, such as increases in sub-bandgap defect densities and decreases in hole carrier mobilities. Metastable changes were characterized using high frequency admittance spectroscopy, drive-level capacitance profiling (DLCP), and current-voltage measurements. We found two distinctive capacitance steps in the high frequency admittance spectra that correspond to (1) the thermal activation of hole carriers into/out of acceptor defect and (2) a temperature-independent dielectric relaxation freeze-out process and an equivalent circuit analysis was employed to deduce the dielectric relaxation time. Finally, hole carrier mobility was deduced once hole carrier density was determined by DLCP method. We found that metastable defect creation in CIGS films can be made either by light-soaking or with forward bias current injection. The deep acceptor density and the hole carrier density were observed to increase in a 1:1 ratio, which seems to be consistent with the theoretical model of V Cu -V Se defect complex suggested by Lany and Zunger. Metastable defect creation kinetics follows a sub-linear power law in time and intensity. Numerical simulation using SCAPS-1D strongly supports a compensated donor- acceptor conversion model for the experimentally observed metastable changes in CIGS. This detailed numerical modeling yielded qualitative and quantitative agreement even for a specially fabricated bifacial CIGS solar cell. Finally, the influence of reduced hole carrier mobility and its role in limiting device performance was investigated.
Adviser: J. David Cohen

Thesis (Ph. D.)--University of Oregon, 2007.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 127-132). Also available online in Scholars' Bank; and in ProQuest, free to University of Oregon users.

Thesis (Ph. D.)--University of Oregon, 2002.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 143-148). Also available for download via the World Wide Web; free to University of Oregon users.

Thesis (Ph. D.)--University of Oregon, 2007.
Typescript. Includes vita and abstract. Includes bibliographical references (leaves 81-84). Also available online in Scholars' Bank; and in ProQuest, free to University of Oregon users.

Exploring the properties of layered materials as a function of thickness has largely been limited to semiconducting materials as thin layers of metallic materials tend to oxidize readily in atmosphere. This makes it challenging to further understand properties such as superconductivity and charge density waves as a function of layer thickness that are unique to metallic compounds. This dissertation discusses a set of materials that use the modulated elemental reactants technique to isolate 1 to 3 layers of VSe2 in a superlattice in order to understand the role of adjacent layers and VSe2 thickness on the charge density wave in VSe2. The modulated elemental reactants technique was performed on a custom built physical vapor deposition to prepare designed precursors that upon annealing will self assemble into the desired heterostructure. First, a series of (PbSe)1+δ(VSe2)n for n = 1 – 3 were synthesized to explore if the charge density wave enhancement in the isovalent (SnSe)1.15VSe2 was unique to this particular heterostructure. Electrical resistivity measurements show a large change in resistivity compared to room temperature resistivity for the n = 1 heterostructure. The overall change in resistivity was larger than what was observed in the analogous SnSe heterostructure. v A second study was conducted on (BiSe)1+δVSe2 to further understand the effect of charge transfer on the charge density wave of VSe2. It was reported that BiSe forms a distorted rocksalt layer with antiphase boundaries. The resulting electrical resistivity showed a severely dampened charge density wave when compared to both analogous SnSe and PbSe containing heterostructures but was similar to bulk. Finally, (SnSe2)1+δVSe2 was prepared to further isolate the VSe2 layers and explore interfacial effects on the charge density wave by switching from a distorted rocksalt structure to 1T-SnSe2. SnSe2 is semiconductor that is used to prevent adjacent VSe2 layers from coupling and thereby enhancing the quasi two-dimensionality of the VSe2 layer. Electrical characterization shows behavior similar to that of SnSe and PbSe containing heterostructures. However, structural characterization shows the presence of a SnSe impurity that is likely influencing the overall temperature dependent resistivity. This dissertation includes previously published and unpublished co-authored materials.

xv, 132 p. : ill. A print copy of this title is available from the UO Libraries, under the call number: SCIENCE TK7871.15.F5 H325 2007
The effects of sulfur alloying on the electronic properties of CuIn(SeS) 2 and CuInGa(SeS) 2 materials has been investigated using sophisticated junction capacitance techniques including drive-level capacitance profiling and transient photocapacitance and photocurrent spectroscopies. CISSe and CIGSSe materials are used as absorber layers in thin-film photovoltaic devices. By characterizing the electronic properties of these materials we hope to understand how these materials can be improved to make thin-film devices with better conversion efficiencies. Sulfur widens the bandgap of these materials by moving the valence band to lower energies and the conduction band to higher energies. This significantly affects the electronic structure of these devices by increasing the activation energies of dominant acceptor levels and lowering room temperature free hole carrier densities. Using optical spectroscopies we observe a large, broad defect that also changes its apparent energetic depth with sulfur alloying. The occupation of this defect was controlled both optically and thermally, and showed a striking temperature dependence. This temperature dependence was measured by recording the relative defect signal, the ratio of the TPC signal in the defect regime to the above bandgap regime, as a function of temperature. As the temperature of the measurement was decreased, steps in the relative defect signal were observed, indicating the turning off of the thermal pathway that emptied trapped charge from the defect. Remarkably, such steps were seen at the same temperature in CISSe and CIGSSe devices with similar sulfur content. In addition, no steps were seen in CMS devices. This points to a defect state specific to the incorporation of sulfur in the absorber material. We hope that a better understanding of the electronic structure of these materials will assist in the creation of improved wide-bandgap thin-film photovoltaic devices.
Adviser: J. David Cohen

碩士
國立交通大學
電子工程學系 電子研究所
102
In 2004, the renowned 2D material, graphene, was successfully produced by Andre Geim and Konstantin Novoselov, the laureates of 2010 Nobel Prize in Physic. This amazing achievement has proved that the 2-D structured monolayer is not only a scientific hypothesis but a truly existing material system. It also inspired the explosive research on graphene and other 2-D materials, for example, Transition-Metal- Dichalcogenides (TMD). From bulk to few layers, TMD has shown promising potentials in electronic and optical devices. When it comes to Integrated Circuits (IC), large-area and high-quality TMD thin films are required. Therefore, we have coworked with the group led by Dr. Lance Li, Institute of Atomic and Molecular Sciences, Academia Sinica. The WSe2 film is produced using a CVD method at Academia Sinica. Then we produce top-gate WSe2 Thin Film Transistor (TFT) in Nano Facility Center, National Chiao Tung University and National Nano Device Laboratories. We have already constructed the basic process flow of making top-gate WSe2 TFT with reasonable characteristics. In this thesis, I will show our experiment results, including statistics of device parameters. We focus particularly on the different gate oxide deposition methods and their impacts on the device performance. The 150℃ low-temperature ALD deposited HfO2 is the best in our devices, and the SiO2/high-k dielectric bilayer oxide also improve yield but limited device performance. Although there are still many problems to be solved before realizing high-performance devices, the current study serves as a good starting point for future research on WSe2 TFT.

碩士
國立臺灣大學
化學工程學研究所
98
CuInSe2 thin films were synthesized via selenizing the Cu-In alloys-containing precursors. The alloy nanoparticles with CuIn and Cu2In were obtained via the NaBH4-assisted chemical reduction method in ethylene glycol. The nanoparticles with a size around 40-60 nm were formed. On selenization, CuInSe2 phase started to be formed at as low as 250℃. The significant grain growth of the particles was observed in the thin films selenized at elevated temperatures because of the presence of Cu-Se compounds. The monophasic CuInSe2 thin film with large grains was formed at 550℃. The formation of CuInSe2 with a chalcopyrite structure was confirmed via Raman spectroscopy. However, densified thin films were not observed. In the second part of the study, the nanopowders consisting of Cu-In alloys and In were synthesized via the chemical reduction method using NaBH4 as the reductant and water as the solvent. On selenization, CuInSe2 was found to coexist with the intermediate compounds of In2O3, CuSe, and Cu2Se. The sizes of the grains in the thin films were significantly increased because of the existence of Cu-Se phases. When the selenization temperature was raised to 550℃, the densified CuInSe2 films with a grain size of about 2 um were formed. The Raman spectroscopy and the Rietveld refinement method confirmed the formation of pure chalcopyrite phase of CuInSe2. The band gap of the CuInSe2 thin film was measured to be 0.97 eV via the UV-visible-NIR spectroscopy. The formation of CuInSe2 throughout the thin film was verified via grazing incident X-ray diffraction. Using the alloy nanoparticles for the coating process was demonstrated to be a potential way for preparing the CuInSe2 thin films. The CuInSe2 thin-film solar cells exhibited a photovoltaic conversion efficiency of 1.47% under AM 1.5G illumination.

碩士
長庚大學
光電工程研究所
102
In the thesis we investigate the characteristics of Chalcopyrite CuInSe2 (CIS) absorber layers by using evaporated selenization method. This method is a two step process: (i) deposition of stacked metallic bilayers of indium and copper by evaporation, (ii) selenization of the precursor layers in a closed quartz box by the furnace. Chalcopyrite is well known to form a stable phase at significant deviations from stoichiometry ranging from copper poor ([Cu]/[In]<1) to copper rich ([Cu]/[In]>1). In today’s best CIS solar cells, the composition is in the range of [Cu]/[In]≒0.9. In copper excess usually have CuxSe it suggested that CuxSe formation at the surface may be a limiting step for the CuInSe2 formation. The study is observed that the copper over indium([Cu][In]) ratio from 0.88~1.79 with CIS thin film. The XRD spectra indicate that the peak position of (112) direction in the CIS absorber layer will shift to higher angle when the copper rich. In order to understand the influence of various process parameters the depth-resolved XPS of the absorber layer is required. Binding energy of Cu 2p3/2 are increase with CuxSe. And having secondary phase the element of Se are existence in Se 3d5/2. By Chemical bath deposition (CBD), cadmium sulfide (CdS) is also studied in this thesis. IV measurement of [Cu]/[In]≒0.9 cell has a small leakage current. The efficiency of Al/ ZnO:Al /i-ZnO/ CdS/ CuInSe2/ Mo/ SLG solar cell is 4.62%, Voc=0.34 V, Jsc=29.1 (mA/cm2), FF=0.49.

碩士
長庚大學
化工與材料工程學系
99
In this study, copper indium diselenide (CuInSe2) thin films were grown on indium-tin-oxide (ITO) coated glass and soda lime glass(SLG) substrates using chemical bath deposition.0.4M of Cu(NO3)2 and 0.4M of In(NO3)3 and Na2SeSO3(0.4M) were cationic and anionic precursors, respectively. The pH value of solution bath was set to 10 and reaction temperature was kept at 30℃ for precursor solution in this study . The CuInSe2 thin films were annealed in a evauate quartz tube at 550℃ with 6×10-3 torr . According to X-ray diffraction patterns (XRD) and energy dispersive analysis of X-ray (EDAX) analysis of the CuInSe2 samples, the impurity such as Cu2Se and In2O3 in samples were decrease with an increase in the [Cu/In] molar ratio in precursor solutions. The thicknesses and direct band gaps of the CuInSe2 samples, determined from the surface profile measurements and transmittance spectra, were in the ranges of 517~619 nm and 1.63~1.05 eV, respectively. All CuInSe2 samples were p-type conductivity. The carrier concentration and resistivity determined from the Hall effect measure were 3.06×1013~8.9×1012 cm-3 and 1.79×104 ~1.25×105Ω-cm, respectively.

碩士
長庚大學
電子工程學系
100
In this thesis, we prepare the CIGS absorber layer by two step methods, sputter and selenization. First, the CIG precursor films with different stack order were deposited by sputtering CuGa binary target and In target. Then, the precursor was selenized by the temperature profiles of three steps in Se vapor environments. In the stack layer experiment, we prepare three types of precursor structure, CuGa/In, CuGa/In/CuGa and CuGa/In/CuGa/In. We observe that CuGa/In/CuGa/In structure have denser, smoother and more uniform surface from SEM characterization, and the FWHM of chalcopyrite phase(112) is about 0.2 from XRD measurements. Raman spectroscopy shows the Cu2-xSe secondary phase at 260cm-1, which can be successfully eliminated by KCN etching. In the structure of CuGa/In/CuGa precursor, we change the thickness ratio of top and bottom CuGa layer. When the thickness ratio is 1:1.4, it can grow the smoother and denser surface. The CGI and GGI ratios are 0.83 and 0.19 respectively. Further, we discuss the reaction mechanism of selenization process. It is found that the Cu11In9 phase have a strong orientation in the first temperature step of 175℃. In the second temperature step of 350℃, we observe a strong diffraction peak of the CIS phase. In addition, the Cu3Ga phase completely disappears. It indicates that selenium has been completely diffused into the bottom CuGa layer to form CuGa and selenium compounds, like Cu2Se and Ga2Se3 etc.

碩士
長庚大學
光電工程研究所
101
In the thesis, the characteristics and growth mechanisms of copper-indium-diselenide (CIS) thin films is investigated. One-step electrodeposition was used to deposit CIS thin film on the Mo/SLG stack layer, and then the samples were treated by selenization. Cyclic voltammogram was used to study the growth mechanisms of CIS thin film. The film properties are characterized by SEM, XRD, XPS, SIMS and Raman, etc. For studying the growth mechanisms of CIS thin film, we measured reduction potential in unitary(Cu2+、In3+、Se4+), binary(Cu-Se、In-Se) and ternary(CuInSe2) electrolyte solutions, which contains the critic acid in each solution. It indicated that the copper is deposited first, and then induced H2SeO3 reduction to Se in the binary Cu and Se solutions. The binary phase of Cu3Se2 was formed. Moreover, the reduction potential of indium positively shifted due to the existing of Se element or Cu3Se2 phase. The indium selenide of In2Se3 was formed. The binary phase of Cu3Se2 will react with indium or indium selenide to form the phase of CIS thin film. Following the electrodeposition, selenization treatment was performed on the preparing samples to improve the film quality. The characterization of CIS thin film was studied by changing deposition potential, electrolyte solution and selenization profile. It was found that the main peak position of XRD transferred gradually from Se element to Cu-Se binary phase, and then to CIS phase with increasing deposition potential. There is a crossing point for Cu and In content in the range of -0.5V to -0.6V. In addition, the XRD peak ratio of Cu2Se/CIS(112) and In2Se3/CIS(112) decreases with increasing CuSO4 concentration. The half maximum of full width of XRD peak (112) also decreases with increasing CuSO4 concentration. However, there is an opposite behavior for In2(SO4)3 concentration. Base on the study, we can obtain a good quality of CIS thin film by the following parameters. The deposition potential is in the range of -0.5V to -0.6V. The electrolyte solution included 3mM CuSO4 , 25mM In2(SO4)3, 12mM SeO2 and 5mL sodium dodecyl sulfate at the pH value of 2.0. The electrodepositing time is 30min in room temperature. The ramp rate of temperature is 10°C/min and it stays the final temperature of 450 °C for 30 min during annealing or selenization.

碩士
國立臺灣大學
化學工程學研究所
99
In this study, the sol-gel method followed by the selenization process was developed to synthesize AgGaSe2 powders. The molar ratios of gallium ions to silver ions were controlled to prepare pure AgGaSe2 powders. Two kinds of selenization processes were employed in this study for investigating the formed phases of the obtained powders. As the sol-gel derived precursors were mixed with the Se powders, pure phase AgGaSe2 powders were successfully prepared. When the precursors without mixing Se powders were selenized using selenium vapor, AgGaSe2 coexisted with the impurity-Ag9GaSe6. The reaction mechanism was proposed as a three-step reaction. Based on the results, the selenium liquid in the selenization process promotes the reaction of Ag and Se when the precursors were mixed with selenium powders. In the second part of the study, pure phase of AgIn1-xGaxSe2 powders were successfully prepared by selenizing the mixture of sol-gel derived precursors and Se powders. Lattice parameters and grain sizes were decreased with increasing the gallium-ion contents of the prepared powders. The pastes containing the sol-gel derived precursors and Se powders were used to prepare the precursor films, followed by the selenization process. Single phase of AgIn1-xGaxSe2 films were prepared without using selenium vapor. As the molar ratios of gallium ions to IIIA ions were elevated, the band gap energy of the obtained films were nonlinearly increased.

碩士
國立臺灣大學
化學工程學研究所
98
AgInSe2 powders were successfully prepared via mixing the sol-gel derived precursors, followed by a selenization process. A figure depicted the relation between resultant compounds and different selenization temperatures were constructed according to the formed phases. The Raman spectrum and the Rietveld refinement confirmed that the prepared AgInSe2 belonged to the chalcopyrite structure. With increasing the selenization temperatures, the particle sizes of AgInSe2 powders as well as the crystallinity of AgInSe2 powders increased significantly. The formation mechanism of AgInSe2 during the selenization process was proposed as a two-step process. Ag2Se is formed in the first step and then induces the second-step reaction to produce AgInSe2. The sol-gel route with a selenization process is introduced as a new approach to fabricate the pure AgInSe2 powders for using in thin-film solar cells. In the second part of this study, single-phased AgInSe2 thin films were successfully prepared via depositing the sol-gel derived precursors on the substrates, followed by a selenization process. The pure-phased AgInSe2 thin films were obtained at the selenization temperature as low as 400℃ via adding the excess amount of In3+ ions. Adjusting the In3+/Ag+ molar ratios can effectively prevent the formation of impurities Ag2Se and AgIn5Se8 in thin films. A Raman spectrum indicated that the obtained films belonged to chalcopyrite structure. The optical absorption revealed that the obtained AgInSe2 had the band gap of 1.23 eV. According to the GIXD analysis of the prepared films, the formation mechanism of AgInSe2 thin films is proposed. At first, Se vapor reacted with Ag to produce Ag2Se. Then Se vapor, In2O3 and the formed Ag2Se react with each other to form AgInSe2. The sol-gel route with a selenization process provides a potential way to obtain AgInSe2 thin films with close-packed microstructures.

碩士
國立臺灣大學
化學工程學研究所
98
Copper gallium diselenide (CuGaSe2) powders were synthesized via the sol-gel method followed by a selenization process. The sol-gel process can effectively reduce the required synthesis temperature to 400oC due to enhanced reactivity and improved composition homogeneity. The amount of the impurity phase Cu2Se was decreased when sufficient Ga3+ was added during the reactions. CuGaSe2 powders were successfully prepared when the Ga3+/Cu2+ molar ratio was increased to 2. The formation of CuGaSe2 with a pure chalcopyrite structure was confirmed via Rietveld refinement results. With decreasing the Ga3+/Cu2+ molar ratios, the particle size of the prepared CuGaSe2 powders was significant enlarged because of the copper selenide phases act as the flux for the particle growth. The optical absorption spectra reveal that the band gap of obtained CuGaSe2 is 1.68 eV. The sol-gel method combine with the selenization process was demonstrated to provide a potential approach to fabricate CuGaSe2 materials. A simple process for preparing CuGaSe2 (CGS) absorber layers was developed in this study. The sol-gel derived CGS precursor pastes were coated via a doctor blade technique followed by the selenization process. On selenization at 450oC single-phased CGS thin-films were obtained. The Raman analysis confirmed that the obtained CuGaSe2 thin films belonged to chalcopyrite structure. The amounts of Cu2Se particles on the surface of the film were reduced when the Ga3+/Cu2+ molar ratio was increased. The uniform film was obtained at the Ga3+/Cu2+ molar ratio of 1.5. The GIXD analysis reveals that the CuGaSe2 phase distributes through the whole film. Because the existence of the highly conductive Cu2Se, the resistivity of obtained CuGaSe2 films were raised with increasing the molar ratio of Ga3+ to Ga2+. The formation mechanism of CuGaSe2 thin films is proposed. The copper selenide phases formed at first and then these phases lead the formation reaction of CuGaSe2.

碩士
國立交通大學
照明與能源光電研究所
101
As the increasing crisis of energy depletion and greenhouse effect, the development and research of alternative energy becomes more and more important around the whole world. Photovoltaics processes the advantages of abundant resources, noise-free…etc, so it has become a thriving industry. Copper-indium-diselenide(CIS) thin film solar cells show high conversion efficiency, high absorption properties, high radiation resistance and high stability, which make it be one of the most potential products beyond all thin film solar cells. In this study, we apply hydrogen to the selenization process, the CIS grain size increases with the increasing hydrogen content ratio. As the hydrogen content ratio reaches 20%, grain size is about 2um. In addition, hydrogen also enhances the diffusion behavior of metal Se vapors into the precursor film and formed MoSe2 between the back electrode and CIS thin film. So hydrogen is helpful to enhance the conversion efficiency of CIS thin film solar cells and improve the large area (30cm*40cm) uniformity. We observe the CIS thin film morphology by SEM cross-section inspection, and verify the reduction of CIS thin film defects via PL intensity analysis. MoSe2 formation thickness effect in CIS thin film solar cells is also discussed, MoSe2 form an ohmic contact between the Mo and CIS thin film and play an important role of reducing the series resistance and further enhance the fill factor. Finally, we have successfully produced high efficiency CIS thin film solar cells up to 9.47%, and exhibit Voc of 0.46 V, Jsc of 31.39mA/cm2 and fill factor of 65.6%. Besides, we also integrate the existing lithography technology and pattern CIS thin film within 60um*60um pixel size, which show the great potential to future thin film transistor applications.

碩士
長庚大學
光電工程研究所
100
In the thesis we investigate the characteristics of copper-indium-gallium-selenium (CIGS) absorber layers by using a low cost sol-gel method. We examine various precursors to find a good stoichiometric CIGS thin film. It is observed that the copper-to-indium- and-gallium ratios (CGI) of 1:1.3 in the sol-gel solution are the best ratio to obtain a CIGS thin film with the CGI ratio from 0.9~0.95. In addition, five different gallium-to-indium-and-gallium (GGI) ratios are also examined. The XRD spectra indicate that the CIGS thin films exhibit a chalcopyrite structure. The peak position of (112) direction in the CIGS absorber layer will shift to higher angle due to smaller bong length of gallium to selenium when the GGI ratio increases. The bandgap of CIGS absorber layer increases from 1.01eV to 1.65eV with the increasing GGI ratio from 0 to 1. The carrier concentration, mobility and resistivity of the CIGS films are about 1015-1016 cm-3, 5-28 cm2/vs and 102 -cm, respectively. It is within an acceptable range for making a high efficiency CIGS solar cell. The achieving solar efficiency of the fabricating CIGS solar cells is 0.804%. The open circuit voltage, short circuit current and the fill factor of the solar cells are 0.154V, 15.3mA/cm2 and 0.341, respectively.

碩士
長庚大學
光電工程研究所
100
The reduction temperature influence on the characteristics of CIGS thin films prepared by sol-gel method is investigated. Five different reduction temperatures of 250, 280, 350, 380 and 450 oC are studied in this thesis. The EDS results indicate that the copper content in the precursor increases with the increasing reduction temperature. The XRD spectra exhibit that the precursor shows an amorphous structure when the reduction temperature is lower than the temperature of 280 oC and begins to form a crystalline structure of In2O3 phase when the reduction temperature is higher than the temperature of 350oC. After the selenization, both of the grain size and copper content in CIGS thin films increase with the increasing reduction temperature. In addition, there exist many large grain sizes of Cu-Se compounds on the surface of CIGS thin films when the ratio of copper to indium and gallium is higher than the value of 0.9. The forming In2O3 phase in the precursor cannot be removed after selenization processing when the reduction temperature is higher than 350 oC. From Raman measurements, it was also observed that the In2O3 phase existed in the CIGS thin films. It is noted that the gel cannot be removed completely when the reduction temperature is lower than 250 oC. Thus, it is suggested that the reduction temperature should be in the range of 250 to 280 oC to obtain a good quality of CIGS thin films.

碩士
逢甲大學
材料科學所
95
The research uses alloy target for innovative process which can simplify the process control. It combines sputtering CIG thin film precursor layer and evaporation selenium layer, and deposit CIGS solar cell thin film absorber on Mo back contact layer. Sputtering CIG precursor layer in room and high temperature are Cu7In3 and Cu16In9 structures with gallium substituent. As temperature rises up, a relatively In-rich Cu11In9 phase will be in appearance. The analysis of SEM surface morphology figures that these In-rich and Cu-rich phases present obviously different grain size and distribution. The size of Cu-rich grains is 400 nm constructed matrix grain layer and In-rich grains is 1.5μm with bulge type. The differences refer to higher fluidness of In-rich phases. Precursor selenization adopt Mo/CIG/Se, CIG/Se/CIG and Se/CIG/Se three structures. In order to improve the selenium homogenization and prevent rapid selenium loss in high temperature, low temperature (100~150℃) diffusion process is proceeded first and ensure the diffusion uniformity by RBS, and then proceed the selenization process in high temperature. However, the studies indicate that selenium under supply is still the problems, so In-Se, Cu-Se bi-element compounds and CIGS chalcopyrite phase mixture are grown, and single phase CIGS can not be get. Otherwise, in-situ selenium vapor resupply in high temperature selenization not only cancels the mixed phases but grows the single phase CIGS chalcopyrite structure with smooth surface and large grain.

博士
國立臺灣大學
化學工程學研究所
99
To overcome the shortcomings of the vacuum process for preparing Cu(In,Ga)Se2 films, including the requirement for high-cost equipment, a low production rate and high complexity, alternative low-cost and simple methods must be developed. In this investigation, two non-vacuum processes which were the hydrothermal process and the modified spin coating method were utilized to prepare Cu(In,Ga)Se2 powders and films. The effects of preparation conditions on the efficiency of Cu(In,Ga)Se2 solar cells are thoroughly investigated. In first part of this thesis, CuInSe2 powders and films were successfully prepared via a hydrothermal method. Well-crystallized CuInSe2 particles were obtained after heating at 180oC for 1 h at low temperatures. When the hydrothermal temperature was increased, the crystallinity and particle size both increased. In the second part of the thesis, a modified spin coating method followed by a subsequent selenization process was developed to prepare Cu(In,Ga)Se2 films. Densified Cu(In,Ga)Se2 films with homogeneously distributed gallium ions were successfully prepared. The grain sizes of the prepared films increased with the increase of the molar ratio of copper ions to IIIA ions. As the selenization temperature increased, the crystallinity and the grain size of the prepared films both increased. The third part of this thesis the effects of the gallium-ion content and the band-gap grading on the microstructure and the electrical properties of Cu(In,Ga)Se2 films . The efficiency of the solar cells was increased as Cu(In,Ga)Se2 was prepared with double grading of gallium-ion contents. In the fourth part of this thesis, Cu(In,Ga)Se2 films with various contents of sodium ions were coated onto a sodium-free substrate. The efficiency of the solar cell was significantly increased when sodium ions were added. In this thesis, Cu(In,Ga)Se2 film were prepared using two non-vacuum methods which were the hydrothermal method and the modified spin coating method. CuInSe2 powders were prepared at low temperature in a short time. Densified Cu(In,Ga)Se2 films were fabricated via the modified spin coating method followed by selenization. Carefully controlling the preparation conditions increased the efficiency of solar cells.

Nanostructured materials play a vital role in almost all aspects of science and technology in the 21st century. The materials include nanoparticles, nanofilms, biological membranes etc. whose physicochemical properties are size-dependent. Thin films have wide range of applications in various branches of science. One of the efficient methods to form miniaturized structures for device applications is to fabricate nanostructured films on different substrates. Surfactant assembly on metallic and non-metallic surfaces based on self assembly and Langmuir-Blodgett technique offers a unique way to form thin films at molecular levels. The process of formation of unimolecular assemblies gives the flexibility of tuning the properties of underlying substrates for various applications including wetting characteristics, lubrication, passivation, mimicking biological phenomena etc. Towards this direction, self assembled monolayers (SAMs) of alkanethiols on gold and silver surfaces have been studied comprehensively for the past two decades. The reported literature on short chain length thiol-based monolayers is however, limited since the formation using conventional methods yield poor quality monolayers. The short chain length monolayers are useful in various applications like tribology, layer-by-layer assemblies, biosensors etc. Hence, it is essential to reproducibly form SAMs of various chain lengths and understand their properties. The present study is related to the formation of SAMs of alkanethiols and diselenides on gold and silver surfaces to form ordered and well-oriented monolayers. Monolayers of varying chain lengths (CH3(CH2)nSH where n = 3, 5, 7, 9, 11, 15) have been formed on gold and silver surfaces using different methods, (1) adsorption from neat thiols; (2) adsorption under electrochemical control and (3) adsorption from alcoholic solutions of the thiols. The characteristics features of the SAMs have been followed based on three different aspects, (i) structure and stability of the methylene groups (ii) interfacial characteristics involving the end group and the solvent and (iii) metal-head group interactions. The structure and stability of the monolayers have been followed based on vibrational spectroscopy and electrochemistry under different environment including thermal perturbations. The stability of the SAMs at different temperatures and subsequent changes associated with the orientation / packing has been monitored both in the dry state using reflection absorption infrared spectroscopy (RAIRS) and under electrochemical conditions using cyclic voltammetry and impedance analysis. Monolayers adsorbed from neat thiols show superior quality in terms of stability and structural arrangement. Short chain thiols with n = 3, 5, 7 show substantial stability when the adsorption is carried out from neat thiols. Figure 1 shows the RAIR spectra of hexanethiol SAM on gold adsorbed by three different procedures. Monolayers adsorbed under potential control behave very similar to the monolayers adsorbed from neat thiol as for as stability and structural orientation are concerned. Monolayers prepared using conventional methods of adsorption from alcoholic solutions are of inferior quality in terms of stability and arrangement especially for the short chain lengths. This is likely to be due to the fact that monolayers prepared using conventional methods may have intercalated solvent molecules within the monolayer assembly that degrade the integrity of the SAM leading to poor quality. The blocking characteristics of the monolayers for diffusing redox couple have been followed by determining the heterogeneous electron transfer rate constant using electrochemical techniques. The spectroscopic data and the electrochemical data follow similar trend indicating the superior quality of monolayer adsorbed from neat thiol in terms of stability as compared to conventionally prepared monolayers. Figure 1. RAIR spectra of hexanethiol-SAMs on Au(111) surface at 25C. The monolayers are formed by adsorption (A) from neat thiol (B) under potential control and (C) from alcoholic solution of the thiol. Wavenumber (cm-1) The interfacial characteristics of the monolayers (effect of end group functionality on the solvent properties) have been monitored on the basis of capacitance, contact angle and atomic force microscopy- measurements. Well-organized monolayers behave like good capacitors with relatively low values of double layer capacitance in presence of a liquid electrolyte as compared to the expected values based on known thickness and dielectric constant of the SAMs. This behavior can be explained by invoking the depletion of water density at the methyl terminated SAM-water interface where the solvent properties are different from that of bulk. Variation of one such property, dielectric constant, has been mapped using force measurement based on AFM. Dielectric constant of water changes from the bulk value of 78 to a low value as given in figure 2. This cross-over occurs within a span of 1-3 nm depending on the chain length of the thiol. Of the three procedures used, the ones based on the use of neat thiol and electrochemical adsorption result in well-oriented alkyl chains followed by highly oriented methyl terminal groups. This is responsible for the high hydrophobic nature of the interface and the subsequent observation of interfacial water properties. The SAMs prepared from ethanol fail to show the hydrophobic effects. Hydrophilic monolayers (NH2 terminated monolayers) fail to show depletion of water density at the interface indicating the importance of end group functionality in altering the interfacial characteristics of the monolayer. Figure 2. Spatial variation of dielectric permittivity of water at the hexanethiol SAM - water interface. The SAM is formed on gold (111) surface; (a) from ethanolic solution of the thiol (b) under electrochemical control (c) from neat thiol. The origin on the x-axis is the position of the methyl groups of SAM and the direction towards right side is in to the bulk water. The well-oriented SAMs have been used to follow the adsorption of a biopolymer. Zein protein is a prolamine of maize and is projected to be a biocompatible coating for food products and food containers. Hence, it is essential to prepare impermeable coatings of zein with different surface wetting properties. The adsorption of zein on highly ordered SAMs with hydrophobic or hydrophilic end group functionality has been studied and the orientation of the protein followed using spectroscopy, microscopy and electrochemistry. It is observed that zein shows higher affinity towards hydrophilic than hydrophobic surfaces with small foot print size on the Figure 3. Orientation of zein protein on hydrophilic and hydrophobic SAM as deciphered from the experimental data. hydrophilic surface resulting in large surface coverage. Figure 3 shows the schematics of zein deposits on hydrophilic and hydrophobic SAM surfaces determined based on spectroscopy, quartz crystal microbalance and electrochemical studies. The AFM shows cylindrical, rod-like and disc-like features of zein on hydrophilic surfaces that form the base units for the growth of cylindrical structures of zein. The published literature on the studies on SAMs on silver surfaces reveals that there is no consensus on the structure of the monolayers on silver. This may be due to the difficulty in getting pristine oxide-free surfaces in the case of silver and this is likely to affect the monolayer quality. Hence, it is decided to prepare SAMs of alkanethiols on silver and study their characteristics. Subtle differences between the monolayers adsorbed from neat thiol and from alcoholic solutions of thiols have been observed in terms of stability and permeability. Atomic force microscopic studies illustrate the presence of depletion of water at the SAM-aqueous interface. Diselenide-based monolayers have been formed on gold to understand the head group-substrate interactions on the monolayer properties. The disorder observed on short chain diselenide-based monolayers formed from alcoholic solutions can be eliminated by adsorption from neat compounds as described for the thiols. A preliminary account on the stability of SAMs under hydrodynamic conditions has been given based on rotating disc electrode voltammetry. It is observed that the SAMs get well-ordered when the electrode is rotated at a fast rate leading to the hypothesis that the monolayer assembly gets annealed as a function of the rotation rate. The thesis is planned as follows: Chapter 1 gives general introduction about organic thin films with particular emphasis on self-assembled monolayers on gold and silver, their characteristics in terms of stability, interfacial properties and adsorption behaviour. Chapter 2 deals with the experimental methodologies and schematics used for the preparation and characterization of the monolayers. Chapter 3 is on the contribution of alkyl spacer to the stability of the monolayers studied using spectroscopy and electrochemistry. Chapter 4 deals with the interfacial properties of the SAMs in presence of aqueous medium. In order to emphasize the importance of the terminal functional groups, adsorption of zein has been demonstrated on surfaces of controlled wettablity. Chapter 5 explains the formation and stability of monolayers of short and long chain alkyl diselenides on gold surfaces. Chapter 6 gives the structural and interfacial characteristics of alkanethiol monolayers on silver surfaces. The stability and subsequent changes of alkanethiol monolayers under hydrodynamic conditions has been discussed in the appendix section.(For fig pl refer pdf file.)

A new approach to the discovery of high absorbing semiconductors for solar cells was taken by working under a set of design principles and taking a systemic methodology. Three transition metal chalcogenides at varying states of development were evaluated within this framework. Iron pyrite (FeS���) is well known to demonstrate excellent absorption, but the coexistence with metallic iron sulfides was found to disrupt its semiconducting properties. Manganese diselenide (MnSe���), a material heavily researched for its magnetic properties, is proposed as a high absorbing alternative to iron pyrite that lacks destructive impurity phases. For the first time, a MnSe��� thin film was synthesized and the optical properties were characterized. Finally, CuTaS���, a known but never characterized material, is also proposed as a high absorbing semiconductor based on the design principles and experimental results.
Graduation date: 2013