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Stress-induced property tunability of bulk and low-dimensional functional materials explored by density functional calculations

2012-08-07T07:44:46Z

Title: Stress-induced property tunability of bulk and low-dimensional functional materials explored by density functional calculations Authors: Zhang, Chao ( 張超) Abstract: Stress plays an important role in materials science, and applying stress is a useful and effective method of modulating the properties of materials. In this thesis, we investigate the effects of stress on the properties of materials using first-principles calculations based on the density functional theory. The research covers a broad range of materials systems, from three-dimensional bulk materials to low-dimensional nanostructured materials. In three-dimensional systems, hydrostatic stress fields can be considered as pressure, which can be routinely obtained from man-made high-pressure devices. At high pressures, most materials undergo phase transitions and exhibit novel properties, such as metallization and superconductivity. We study the high-pressure behaviors of GeH4 and alkaline earth hydrides. GeH4 undergoes a structural transformation from its low-pressure P21/c phase to a high-pressure Cmmm phase at about 15 GPa where insulator-metal transition occurs, followed by two other metallic phases with P21/m and C2/c symmetries up to 200 GPa. Our results indicate that the metallization of GeH4 can be realized through a band overlap within the material itself. Perturbative linear response calculations for Cmmm GeH4 at 20 GPa predict a strong electron-phonon interaction, and the resulting superconducting critical temperature is about 40 K. We investigate systematically the structural and electronic properties of alkaline earth hydrides under pressure, and emphasize the pressure-induced metallization of alkaline earth hydrides. While BeH2 and MgH2 have different semi-metallic phases, CaH2, SrH2, and BaH2 share the same metallic phase (P6/mmm). The metallization pressure shows an attractive decrease with the increment of alkaline earth metal radius and band gap of alkaline earth hydrides at ambient pressure. BaH2 has the lowest metallization pressure; hence its phase transition mechanism and vibrational properties under pressure are studied. Our results are consistent with current experimental data, and the obtained trend has significant implications for designing and engineering metallic hydrides for energy applications. For low-dimensional nanostructured systems, we focused on the effects of stress on Si sheets and Ge nanowires. For a (100) Si sheet, asymmetrical strain causes a direct-to-indirect band gap transition, whereas symmetrical strain keeps its direct band gap characteristics unchanged. Under asymmetrical strain along the <100> direction, the direct band gap of the (110) Si sheet exhibits unique characteristics, with the direct band gap varying linearly with the change in strain. Similar band gap variation is observed for (110) Si sheets applied with symmetrical and asymmetrical <110> strains. The various strain dependences are related to the modifications of the local density of states. Our results could be used to guide the strain engineering of the electronic properties of low-dimensional silicon materials. We investigate strain effects on the electronic properties of germanium nanowires (GeNWs) along the <112> direction. The <112> GeNWs possess direct band gaps when the cross-section aspect ratio of (111) to (110) facets is larger than 1. The strain does not change the direct band gap characteristics; however, compressive (tensile) strain tends to increase (decrease) the band gap. The variation in band gaps originates from the different strain dependences of valence bands and conduction bands. Our results suggest that both strain and size can be used to tune the band structures of GeNWs, which may help in designing future nanoelectronic devices. Notes: CityU Call Number: TA405 .Z45 2011; xiv, 125 leaves : ill. (some col.) 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2011.; Includes bibliographical references.

Theoretical design and characterization of sensitizers for dye-sensitized solar cells

2012-08-07T07:44:44Z

Title: Theoretical design and characterization of sensitizers for dye-sensitized solar cells Authors: Lu, Xiaoqing ( 魯效慶) Abstract: The growing worldwide demands for energy along with increasing concerns over global warming have stimulated the interest in seeking for renewable energy sources. Dye-sensitized solar cells (DSSCs) have gained widespread attention for their potential as low-cost solar energy conversion devices. One of key issues for the improvement of DSSCs is the design of light-absorbent dyes with cheaper, safer, more efficient, and more sustainable materials. In this thesis, we present theoretical investigations on various sensitizers using density functional theory (DFT) and time-dependent DFT (TD-DFT). The research covered a broad range of materials systems, including Ru(II)-, Cu(I)-, and Fe(II)-based metal-centered sensitizers and metal-free organic sensitizers. Ru(II)-based sensitizers containing functionalized bithiophene (btp) ligands (CYC-B1 and CYC-B11) have been investigated in both the gas phase and dimethylformamide (DMF) solution. The results characterized the role of the functionalized btp ligands with respect to the absorption properties of the dye. Frontier orbital analysis has shown that the three highest HOMOs are composed of nonbonding combinations of the Ru t2g orbitals with the p orbital and lone pairs of the SCN ligands, while the six lowest LUMOs are the π* combinations of the 4,4'-dicarboxy-2,2'-bipyridine (dcbpy) and/or btp-functionalized bipyridine (bpy) ligands calculated in the gas phase. The inclusion of solvent has resulted in great changes of the energies and compositions of the molecular orbitals of these complexes. It was found that the spectra are assigned to the intraligand π → π* transitions of the dcbpy ligand in the ultraviolet range, whereas spectra in the visible range show multitransition characteristic of metal-to-ligand charge transfer (MLCT), interligand π → π*, and intraligand π → π*. The functionalized btp ligands have provided additional contributions of intraligand π → π* in the visible range due to enhancement in the electron-donating capability and electron-accepting capability by the extension of the π-conjugation. The molecular geometries, electronic structures, and optical absorption spectra of a series of polypyridyl Cu(I)-based complexes have been investigated in both the gas phase and methyl cyanide (MeCN) solution. The frontier orbital analysis showed that the five HOMOs have predominantly Cu 3d character, resulting from a set of distorted degenerate orbitals, while the four LUMOs were composed of the antibonding combination of the C and N 2p orbitals of bipyridines. The absorption spectra of Cu(I)-based complexes displayed multitransition characters of Cu → bipyridine MLCT and ligand-to-ligand charge transfer (LLCT) transitions in the range of 350 - 700 nm. The solvent effect resulted in sharply upshifts of molecular orbital energies of Cu(I)-based complexes. Structural optimizations by enhancing the π-conjugation and introducing the heteroaromatic groups on ancillary ligands lead to the upshifts of molecular orbital energies, increase in oscillator strength, and red shift of the absorption spectra. When compared with Ru(II) sensitizers, polypridyl Cu(I)-based complexes showed similar optical properties and improving trend of DSSC performance along with the optimizations of structures. The results of this work have highlighted the point that polypyridyl Cu(I)-based complexes could provide promising sensitizers for efficient next-generation DSSCs. Fe(II)-based complexes incorporating polypyridyl ancillary ligands of [FeL2(SCN)2], [FeL3]2+, [FeL'(SCN)3]-, [FeL'2]2+ and [FeL''(SCN)2] (L = 2,2'-bipyridyl-4,4'-dicarboxylic acid, L' = 2,2',2''-terpyridyl-4,4',4''-tricarboxylic acid, L'' = 4,4'''-di-methyl-2,2':6',2'':6'',2'''-quaterpyridyl-4',4''-bis-carboxylic acid) have been investigated. The molecular geometries, electronic structures, and optical absorption spectra were predicted in both the gas phase and MeCN solution. The protonated/deprotonation effect was slight, while the isomerization effect and the solvent effect had great influence on the spectra characteristics of Fe(II)-based complexes. The spectra showed LLCT characteristic at shorter wavelength region, whereas the spectra display multitransition characters of Fe → polypyridine MLCT and LLCT transitions at the longer wavelength of the visible region. Fine tuning of electronic properties by proper choice of functionalized chromophores produced a series of bipyridyl Fe(II)-based complexes with general formula [FeL2(NCS)2] (L = 4,4'-substituted-2,2'-bipyridine), which leads to alterations of the spectral response ranges with different oscillator strengthes. The introduction of conjugated hydrocarbon at the 4,4'-substituted positions can enhance spectral response range and increase molar extinction coefficient yielding. The extension of the π-conjugation system of the bipyridine and conjugated electron-rich heteroaromatics in ancillary ligands can add donor groups and adjust the molecular orbital energy levels significantly. When compared with Ru(II) sensitizers, Fe(II)-based complexes showed similar characteristics and improving trend of optical absorption spectra along with the introduction of different polypridyl ancillary ligands. Metal-free organic sensitizers incorporating triphenylamine (tpa)-based donors and binary π-conjugated bridge have been investigated in the gas phase, DMF and chloroform solutions. The geometrical and electronic structures of the tpa-based sensitizers were used to unravel the relation between structural optimizations and optical properties. Based on the reference sensitizer of C219, novel organic sensitizers were designed by, (1) the extension of the donors by addition of a tpa subunit; (2) the alteration of the bridge groups by electron-rich furan and selenophene containing binary π-conjugated groups. By Substituting Na+ for H+ on the cyanoacrylic acid, it was confirmed the fact that the protonation/deprotonation effects had great effect on the spectra. The solvent effect leads to minor changes in the ground state geometries but great alterations in the excitation energies. The absorption spectra of tpa-based complexes exhibited intra-ligand LLCT characteristic with better response at the near-infrared region. Notes: CityU Call Number: TK2960 .L84 2011; xvi, 161 leaves : ill. (some col.) 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2011.; Includes bibliographical references (leaves 143-159)

Studies of surfaces and interfaces in organic electronic devices

2012-08-07T07:44:41Z

Title: Studies of surfaces and interfaces in organic electronic devices Authors: Liu, Zengtao ( 劉增濤) Abstract: There has been much progress in organic electronic devices in the past two decades due to their potential applications in optical and electronic fields. The performance of these devices, such as organic photovoltaic devices (OPVs), organic thin film transistors (OTFTs), and organic light-emitting diodes (OLEDs), depends critically on the properties of the electrode/organic and organic/organic interfaces. In this work, X-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) have been used to investigate the electronic and chemical structures at interfaces of different organic electronic devices. For OPV devices, we mainly focus on the donor/acceptor interfaces and discussed the relationship between the interfaces and the performance of OPV devices, especially the open circuit voltage. We found that the substrate, interface middle layer, and doping can all result in changes of the electronic structures at the donor/acceptor interface, which determines the performance of OPV devices. For OTFTs, we studied pentacene/molybdenum oxide (MoO3) and copper phthalocyanine (CuPc)/MoO3 interfaces and discuss how the electronic structures of these interfaces determine performance of the corresponding OTFTs. Via analysis of the electronic structures of these interfaces, we found that the space charge region formed in organic layer leads to high conductivity and normally-on operation mode in the OTFTs. We also studied the electronic structure of the n-n isotype copper hexadecafluorophthalocyanine (F16CuPc)/phthalocyanatotin (IV) dichloride (SnCl2Pc) heterojunction by UPS and XPS. Due to charge transfer across the heterojunction, energy level bendings were observed at the two organic layers. Formation of space charge regions strongly determine performance of the transistor consisting of the heterojunction. For OLEDs, we mainly studied the interface between carbon nanotube (CNT) film as a novel anode material and N, N'-diphenyl-N, N'-bis(1-naphthyl)-(1, 1'-biphenyl)-4, 4'-diamine (NPB). Due to the lower work function of the CNT film comparing to that of indium tin oxide (ITO), the height of the hole injection barrier is high and leads to severe performance degradation. Reducing the height in a controlled way is of scientific and technological importance for replacing ITO with the CNT film as anode material. We found that a simple and widely-used UV-ozone treatment can dramatically enhance the work function of CNT film. The height of hole injection barrier at the interface between NPB and treated CNT film is remarkably lowered, which will lead to improving the performance of devices. Then, we systematically studied the effects of O2 and H2O exposures on the surfaces and interfaces of organic materials in a control OLED by using UPS to understand the mechanisms of device degradation. For different surfaces and interfaces, the effects of O2 and H2O exposure were studied and discussed. Notes: CityU Call Number: TK7870 .L58 2011; xix, 144 leaves : ill. 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2011.; Includes bibliographical references.

Development and study of organic/inorganic hybrid solar cells

2012-08-07T07:44:39Z

Title: Development and study of organic/inorganic hybrid solar cells Authors: Liu, Chaoping ( 劉超平) Abstract: Both the increasing demand for energy and the environmental crisis lead to development of clean and renewable energy sources. Among a variety of new energy sources exploited in the past decades, solar energy is believed to be the ultimate solution to satisfy the energy demand and the environmental challenge. Solar energy can be converted to electricity using photovoltaic effect induced in electronic devices, known as solar cells. Since the first practical photovoltaic cell invented in 1954, significant progress has been made in the development of solar cells, but the cost for electricity produced by solar cells is still 2 to 3-times higher than that obtained from the conventional fuel resources. Therefore major breakthrough in technology of solar cells is still needed to meet the fundamental cost requirement. As a result several new technologies, devices and materials have been introduced and used to develop efficient and cost effective solar cells. Among them, organic/inorganic hybrid solar cells (HSCs) are devices that may meet the demands of high power conversion efficiency (PCE), low cost and environmental compatibility. The organic/inorganic hybrid system combines the merits of both organic and inorganic components showing great potential to fabricate low cost and highly efficient solar cells. In consistence with recent development, this work focuses on development and study of hybrid organic/inorganic devices. In particular, attention is given to investigations of organic/inorganic HSCs with the architecture combining conjugated p-type polymer, poly (3-hexylthiophene) (P3HT), and inorganic ZnO/Sb2S3 to form heterojunctions. Herein, the ZnO/Sb2S3/P3HT heterojunction solar cell is studied systematically via device design, modeling, and optimization. Both the planar and the bulk ZnO/Sb2S3/P3HT heterojunctions are used to construct the solar cells. The effects of thickness, annealing, and recombination for individual layer are investigated using the planar solar cells, while the effects of light trapping and the enlarged interfacial area on device performance are investigated with assistance of bulk heterojunction architectures. Firstly, numerical simulation employing the computer code Analysis of Microelectronic and Photovoltaic Structures (AMPS) is performed to explore fundamental mechanism and principle of planar ZnO/Sb2S3/P3HT heterojunction solar cells. The device performance dependent on some parameters, such as the thickness and carrier mobility of Sb2S3 (or P3HT) films, is studied by AMPS modeling. It is demonstrated that the performance significantly depends on the layer (Sb2S3 or P3HT) thickness, mobility, and the possible recombination presented; while the performance is hardly affected by the electron mobility (0.01~100 cm2/Vs) and thickness variations (60~400 nm) of ZnO film, which is mainly due to the large electron diffusion length in ZnO. The simulated results provide us a qualitative understanding of the performance solar cells based on ZnO/Sb2S3/P3HT heterojunctions, which in turn guides us to fabricate the solar cells with higher performances. With the insights based on numerical simulations, the HSCs with planar ZnO/Sb2S3/P3HT heterojunctions are fabricated and studied. The study shows that the device performance is significantly affected by the thermal annealing and the thickness of individual Sb2S3 or P3HT layers. The electronic structure of the Sb2S3 film is also investigated by ultraviolet photoelectron spectroscopy (UPS), which enables us to study energy alignment in the designed hybrid heterojunction solar cells. X-ray photoelectron spectroscopic (XPS) study further demonstrates that the surface of Sb2S3 layer is partially oxidized, and oxide layer is about 0.5 nm thick. This thin layer oxide film (Sb2O3) is in fact a passivation layer between the Sb2S3 and P3HT, which in turn reduces the carrier recombination and improves the device performance. The obtained analytical data qualitatively agree with the prediction based on the numerical simulation. In addition, ZnO nanowire arrays have been incorporated into the ZnO/Sb2S3/P3HT heterojunction solar cells (bulk heterojunction). Optical measurements demonstrate that the absorbance of Sb2S3 is increased by ~5% in the wavelength ranging from 450 to 650 nm due to the light trapping effect induced by scattering process in ZnO nanowire arrays. The solar cells with ZnO nanowire arrays show PCE of 2.9%, which is higher by 20% than that of the control device assembled without the ZnO nanowire arrays. AMPS modeling further evidences that the improved performance mainly arises from both the increased absorbance and the reduced bulk recombination in Sb2S3 layer. Compared to the inorganic materials, organic materials exhibit much lower carrier mobility, which significantly limits the performance of organic/inorganic HSCs. Improving the carrier transport properties of organic materials is thus of great importance for high performance of HSCs. The electrical properties of P3HT blended with square planar nickel complexes are also study here. Novel square planar nickel complexes with molecular alignment has been synthesized and introduced into the P3HT matrix. The study indicates that a variation in the cations-anions interaction of the prepared complex affects both the molecular packing and physical properties. An enhanced carrier transport is observed in the blends due to the additional charge carriers from the electronic states of nickel complexes. In the context of observed phenomena, a physical model is proposed to explain the enhancement of the charge transport property. The blends of P3HT and nickel complex with enhanced carrier transport may find their applications in polymer based HSCs. Notes: CityU Call Number: TK2960 .L58 2011; xv, 135 leaves : ill. (some col.) 30 cm.; Thesis (Ph.D.)--City University of Hong Kong, 2011.; Includes bibliographical references (leaves 116-133)