Tandem solar cells fabricated from thin films provide promise of improved efficiency while keeping the processing costs low. CdSe as top cells are investigated in this work. CIGS has been a standardized process with lab efficiencies reaching 18% [53]. This dissertation focuses on the development of conductive window layer for the development of a high performance, high bandgap solar cell. ZnSe, Cu2-xSe, and ZnSexTe1-x are investigated as viable window layers of the top cell. ZnSe in undoped form forms a good junction with CdSe films, but the Voc from these devices could never exceed the 360mV mark, while the current densities approached 17.5mA/cm2 [61].To improve Voc's, the high contact energy at the ZnSe/Cu interface has to be overcome by replacing Cu with a metal having higher work function or doping the window layer to form a tunneling contact with Copper.Deposition of ZnSe from binary sources in presence of nitrogen plasma resulted in films with proper stoichiometry. However, doping could not be accomplished. ZnTe is easily dopable, and was the next alternative. ZnTe doping in presence of Nitrogen plasma resulted in Zn rich films. Hence doping of the ternary compound ZnSexTe1-x was considered. This work focuses on studying the effects of compositional variation on the conductivity of the ZnSexTe1-x films. ZnSexTe1-x films were doped using Nitrogen. Films were deposited by co-evaporation from ZnTe, ZnSe and Se sources. Te/Se ratio was varied by varying the ZnTe thickness and Se Thickness. Films with Zn/Group VI ratio close to 1 were measured for conductivity using IV measurements. Highest conductivity of 2* 10-8 ohm-cm was obtained at ZnSe, ZnTe, and Se thicknesses of 2000Ã?, 1500Ã?, and 500Ã? respectively. The actual carrier concentration could be concealed by the current limiting Cu contacts. All films with Zn/Group VI ratio close to 1 showed slight conductivity in the 10-10 ohm-cm range. Layered ZnSexTe1-x Films doped with Nitrogen had targeted Zn/Group VI ratio of 1, but with a higher Te content. The films were also slightly conductive, in the 10-10 ohm-cm range. The mechanism limiting the doping in all the films seems to be the same.

abstract: Satisfying the ever-increasing demand for electricity while maintaining sustainability and eco-friendliness has become a key challenge for humanity. Around 70% of energy is rejected as heat from different sectors. Thermoelectric energy harvesting has immense potential to convert this heat into electricity in an environmentally friendly manner. However, low efficiency and high manufacturing costs inhibit the widespread application of thermoelectric devices. In this work, an inexpensive solution processing technique and a nanostructuring approach are utilized to create thermoelectric materials. Specifically, the solution-state and solid-state structure of a lead selenide (PbSe) precursor is characterized by different spectroscopic techniques. This precursor has shown promise for preparing thermoelectric lead selenide telluride (PbSexTe1-x) thin films. The precursor was prepared by reacting lead and diphenyl diselenide in different solvents. The characterization reveals the formation of a solvated lead(II) phenylselenolate complex which deepens the understanding of the formation of these precursors. Further, using slightly different chemistry, a low-temperature tin(II) selenide (SnSe) precursor was synthesized and identified as tin(IV) methylselenolate. The low transformation temperature makes it compatible with colloidal PbSe nanocrystals. The colloidal PbSe nanocrystals were chemically treated with a SnSe precursor and subjected to mild annealing to form conductive nanocomposites. Finally, the room temperature thermoelectric characterization of solution-processed PbSexTe1-x thin films is presented. This is followed by a setup development for temperature-dependent measurements and preliminary temperature-dependent measurements on PbSexTe1-x thin films. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2020

Doping in bulk semiconductors (e.g., n- or p- type doping in silicon) allows for precise control of their properties and forms the basis for the development of electronic and photovoltaic devices. Recently, there have been reports on the successful synthesis of doped semiconductor nanocrystals (or quantum dots) for potential applications in solar cells and spintronics. For example, nanocrystals of ZnSe (with zinc-blende lattice structure) and CdSe and ZnO (with wurtzite lattice structure) have been doped successfully with transition-metal (TM) elements (Mn, Co, or Ni). Despite the recent progress, however, the underlying mechanisms of doping in colloidal nanocrystals are not well understood. This thesis reports a comprehensive theoretical analysis toward a fundamental kinetic and thermodynamic understanding of doping in ZnO, CdSe, and ZnSe quantum dots based on first-principles density-functional theory (DFT) calculations. The theoretical predictions of this thesis are consistent with experimental measurements and provide fundamental interpretations for the experimental observations. The mechanisms of doping of colloidal ZnO nanocrystals with the TM elements Mn, Co, and Ni is investigated. The dopant atoms are found to have high binding energies for adsorption onto the Zn-vacancy site of the (0001) basal surface and the O-vacancy site of the (0001) basal surface of ZnO nanocrystals; therefore, these surface vacancies provide viable sites for substitutional doping, which is consistent with experimental measurements. However, the doping efficiencies are affected by the strong tendencies of the TM dopants to segregate at the nanocrystal surface facets, as indicated by the corresponding computed dopant surface segregation energy profiles. Furthermore, using the Mn doping of CdSe as a case study, the effect of nanocrystal size on doping efficiency is explored. It is shown that Mn adsorption onto small clusters of CdSe is characterized by high binding energies, which, in conjunction with the Mn surface segregation characteristics on CdSe nanocrystals, explains experimental reports of high doping efficiency for small-size CdSe clusters. In addition, this thesis presents a systematic analysis of TM doping in ZnSe nanocrystals. The analysis focuses on the adsorption and surface segregation of Mn dopants on ZnSe nanocrystal surface facets, as well as dopant-induced nanocrystal morphological transitions, and leads to a fundamental understanding of the underlying mechanisms of dopant incorporation into growing nanocrystals. Both surface kinetics (dopant adsorption onto the nanocrystal surface facets) and thermodynamics (dopant surface segregation) are found to have a significant effect on the doping efficiencies in ZnSe nanocrystals. The analysis also elucidates the important role in determining the doping efficiency of ZnSe nanocrystals played by the chemical potentials of the growth precursor species, which determine the surface structure and morphology of the nanocrystals.

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Leung Yee Pan = 利用有機金屬化學氣相沉積方法生長一維硒化鋅鈉米結構 / 梁懿斌. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2004. / Includes bibliographical references (leaves 80-82). / Text in English; abstracts in English and Chinese. / Leung Yee Pan = li yong you ji jin shu hua xue qi xiang chen ji fang fa sheng chang yi wei xi hua xin na mi jie gou / Liang Yibin. / Acknowledgements --- p.ii / Abstract --- p.iii / Chapter Chapter 1 - --- Introduction --- p.1 / Chapter 1.1 --- Background --- p.1 / Chapter 1.2 --- Motivation --- p.3 / Chapter 1.2.1 --- ZnSe --- p.3 / Chapter 1.2.2 --- MOCVD --- p.3 / Chapter 1.3 --- Our Work --- p.4 / Chapter Chapter 2 - --- Experiment --- p.5 / Chapter 2.1 --- MOCVD System --- p.5 / Chapter 2.2 --- Metalorganic Sources --- p.5 / Chapter 2.3 --- Substrates --- p.7 / Chapter 2.4 --- Substrate Preparations --- p.7 / Chapter 2.5 --- Preheating (Applied Only when Using GaAs Substrates) --- p.7 / Chapter 2.6 --- Growth of Epi-layer (Applied Only when Using GaAs Substrates) --- p.8 / Chapter 2.7 --- Growth of ZnSe Nanowires on Si(100) and ZnSe/GaAs(100) --- p.8 / Chapter 2.8 --- The Samples --- p.9 / Chapter Chapter 3 - --- Characterization --- p.10 / Chapter 3.1 --- Surface Morphologies --- p.10 / Chapter 3.1.1 --- Scanning Electron Microscopy --- p.10 / Chapter 3.1.2 --- Atomic Force Microscopy --- p.12 / Chapter 3.2 --- Structural Properties - X-Ray Diffraction --- p.13 / Chapter 3.3 --- Optical Properties - Photoluminescence --- p.15 / Chapter 3.4 --- Other Techniques --- p.16 / Chapter Chapter 4 - --- Results --- p.17 / Chapter 4.1 --- ZnSe Nanowires Grown on Si(100) --- p.17 / Chapter 4.1.1 --- Effect of Growth Temperature --- p.17 / Chapter 4.2 --- Growth of ZnSe Nanowires on GaAs( 100) - The First Trial --- p.20 / Chapter 4.3 --- Optimizing the ZnSe Epi-layer --- p.21 / Chapter 4.3.1 --- Surface of GaAs(100) --- p.21 / Chapter 4.3.2 --- ZnSe Epi-layer Grown at Different Reactor Pressures --- p.22 / Chapter 4.4 --- Importance of Au --- p.26 / Chapter 4.5 --- Growth of ZnSe Nanowires on GaAs(lOO) - A Systematic Study --- p.28 / Chapter 4.5.1 --- Growth Rates --- p.28 / Chapter 4.5.2 --- Overall Morphologies --- p.32 / Chapter 4.5.3 --- Classifying the Morphologies --- p.37 / Chapter 4.5.4 --- Abundances of Different Morphologies of Different Samples --- p.40 / Chapter 4.5.5 --- Growth Direction --- p.45 / Chapter 4.5.6 --- Structure of the Nanowires --- p.50 / Chapter 4.5.7 --- Optical Properties of the Nanowires --- p.54 / Chapter Chapter 5 - --- Discussions --- p.57 / Chapter 5.1 --- Overview of the MOCVD Process --- p.57 / Chapter 5.1.1 --- Effects of Growth Temperature on Growth Rate of MOCVD --- p.58 / Chapter 5.1.2 --- Effects of Reactor Pressure on Growth Rate of MOCVD --- p.59 / Chapter 5.2 --- Effect of Reactor Pressure on the Growth Rate of the Nanowires --- p.60 / Chapter 5.3 --- Growth Mechanisms of the Nanowires --- p.64 / Chapter 5.3.1 --- VLS Mechanism --- p.64 / Chapter 5.3.2 --- Spiral Growth Mechanism --- p.66 / Chapter 5.3.3 --- Reentrant Corner Mechanism --- p.67 / Chapter 5.3.4 --- Roles of Au Particles and ZnSe Epi-layer --- p.68 / Chapter 5.3.5 --- Growth Mechanisms of Different Types of Nanowires --- p.69 / Chapter 5.3.6 --- Effect of Growth Temperature --- p.71 / Chapter 5.4 --- Quality of the Nanowires --- p.72 / Chapter 5.5 --- "Remarks of the AFM Experiments and the ""Transferred"" Samples" --- p.72 / Chapter Chapter 6 - --- Conclusions --- p.75 / Appendices --- p.77 / Chapter I - --- "Estimation of the mass, other than the nanowires, contributed to the sample" --- p.77 / Chapter II - --- Calculation of the growth angle with respect to the surface normal --- p.78 / References --- p.80

Pei, Congjian = 銅銦硒/硫和銅鋅錫硒/硫一維納米結構的合成與表徵 / 裴聰健. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 76-78). / Abstracts in English and Chinese. / Pei, Congjian = Tong yin xi/liu he tong xin xi xi/liu yi wei na mi jie gou de he cheng yu biao zheng / Pei Congjian. / Abstract --- p.i / 論文摘要 --- p.ii / Acknowledge: --- p.iii / Contents: --- p.iv / List of Figures: --- p.vii / Chapter Chapter 1 --- Introduction --- p.1 / Chapter Chapter 2 --- Background --- p.3 / Chapter 2.1. --- Overview --- p.3 / Chapter 2.2 --- Methodology --- p.3 / Chapter 2.2.1 --- General growth strategies for synthesizing nanowires (NWs) --- p.3 / Chapter 2.2.2 --- Synthesis NWs from vapor phase (VLS mechanism) --- p.4 / Chapter 2.2.3 --- Synthesis NWs from solution phase --- p.7 / Chapter 2.2.4 --- Synthesis NWs assist by template --- p.8 / Chapter 2.3 --- Instrumentation --- p.10 / Chapter 2.3.1 --- XRD (X-Ray Diffraction) --- p.10 / Chapter 2.3.2 --- SEM (Scanning Electron Microscopy) --- p.11 / Chapter 2.3.3 --- TEM (Transmission Electron Microscopy) --- p.13 / Chapter Chapter 3 --- Synthesis and characterization of In2Se3 nanowires --- p.17 / Chapter 3.1 --- Overview: --- p.17 / Chapter 3.2 --- Experimental Section: --- p.17 / Chapter 3.3 --- Results and Discussions: --- p.19 / Chapter 3.3.1 --- Results of high temperature (~800oC) synthesized sample --- p.20 / Chapter 3.3.2 --- Results of the low temperature (~600oC) synthesized sample --- p.26 / Chapter 3.3.3 --- Results of thermal evaporate CuInSe2 source: --- p.30 / Chapter 3.4 --- Conclusion: --- p.31 / Chapter Chapter 4 --- Synthesis and Characterization of ID CuInSe2 and CuInS2 structures via template assist method --- p.33 / Chapter 4.1 --- Overview: --- p.33 / Chapter 4.2 --- Experimental: --- p.33 / Chapter 4.2.1 --- Fabrication of CuInSe2 nanowire arrays --- p.34 / Chapter 4.2.1 --- Fabrication of CuInS2 nanowire arrays --- p.35 / Chapter 4.3 --- CuInSe2 nanowire arrays: --- p.36 / Chapter 4.4 --- CuInS2 nanotube &nanowire array: --- p.42 / Chapter 4.5 --- Discussion of the formation mechanisms: --- p.45 / Chapter 4.6 --- Conclusion: --- p.50 / Chapter Chapter 5 --- Synthesis of ordered single-crystalline nanowires arrays of Cu2ZnSnS4 and Cu2ZnSnSe4 --- p.51 / Chapter 5.1 --- Overview: --- p.51 / Chapter 5.2 --- Experimental: --- p.52 / Chapter 5.3 --- Results and discussion: --- p.54 / Chapter 5.4 --- Discussion of the formation mechanisms: --- p.68 / Chapter 5.5 --- Conclusion: --- p.72 / Chapter Chapter 6 --- Summary --- p.73 / Reference: --- p.76

Ho, Yee Man Martina = 硒化鎘三維納米結構之製作及其感應脫氧核糖核酸之應用潛能 / 何綺雯. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references. / Abstract also in Chinese. / Ho, Yee Man Martina = Xi hua ge san wei na mi jie gou zhi zhi zuo ji qi gan ying tuo yang he tang he suan zhi ying yong qian neng / He Qiwen. / Chapter Chapter 1 --- Introduction / Chapter 1 --- Photovoltaic properties of CdSe --- p.1 / Chapter 1.1 --- Quantum size effect --- p.1 / Chapter 1.2 --- Synthesis of CdSe nanostructures --- p.3 / Chapter 1.3 --- Electrochemical sensing of CdSe nanostructures --- p.4 / Chapter 1.3.1 --- Surface passivation and functionalization of CdSe nanostructures --- p.5 / Chapter 1.4 --- Electronic properties of nanocrystalline semiconductor electrode --- p.6 / Chapter 1.4.1 --- Band alignment --- p.6 / Chapter 1.4.2 --- Interfacial charge transfer process --- p.9 / Chapter 1.4.3 --- Surface traps and adsorbed molecules --- p.10 / Chapter 1.4.4 --- DNA molecules as a capping group --- p.11 / Chapter 1.5 --- Literatures review in DNA sensing --- p.12 / Chapter 1.6 --- Present study --- p.14 / Chapter 1.6.1 --- Objective --- p.14 / Chapter 1.6.2 --- General methodology --- p.15 / Chapter Chapter 2 --- Experimental / Chapter 2.1 --- Introduction into the instrumentation of this project --- p.21 / Chapter 2.2 --- CHI Electrochemical workstation --- p.22 / Chapter 2.2.1 --- Linear sweep voltammetry --- p.24 / Chapter 2.2.2 --- Cyclic voltammetry --- p.24 / Chapter 2.2.3 --- Multiple potential step --- p.25 / Chapter 2.3 --- CEM Microwave-assisted chemical synthesizer --- p.27 / Chapter 3.1 --- Morphological examination by scanning electron microscopy --- p.28 / Chapter 3.2 --- Elemental analysis by energy dispersive x-ray spectroscopy --- p.30 / Chapter 3.3 --- Crystal structure analysis by x-ray diffraction --- p.31 / Chapter 3.4 --- Surface compositional analysis by x-ray photoelectron spectroscopy --- p.32 / Chapter 3.5 --- Transmission electron microscopy --- p.34 / Chapter Chapter 3 --- Synthesis of 3D nanostructured CdSe multipod electrodes / Chapter 3.1 --- Introduction into the synthesis of CdSe MP electrode --- p.35 / Chapter 3.2 --- Recipe for the synthesis of CdSe NPs --- p.36 / Chapter 3.3 --- The synthesis of CdSe MPs --- p.37 / Chapter 3.3.1 --- Tuning the experimental parameters: Reaction temperature --- p.37 / Chapter 3.3.2 --- Tuning the experimental parameters: Reaction hold time --- p.46 / Chapter 3.3.3 --- Tuning in experimental parameters: Precursor molar ratio --- p.50 / Chapter 3.4 --- The fabrication of MP CdSe on a conductive substrate --- p.54 / Chapter 3.4.1 --- The electrodeposition of CdSe thin films on ITO/glass substrates --- p.55 / Chapter 3.4.2 --- The growth of CdSe MPs on CdSe/ ITO/glass --- p.57 / Chapter 3.5 --- The characterization of MP CdSe electrode --- p.57 / Chapter Chapter 4 --- Electrical and opto-electric characteristics of CdSe MP electrodes and their applications as platforms for the DNA recognition / Chapter 4.1 --- Introduction to the property characterization of CdSe MP electrodes --- p.62 / Chapter 4.2 --- DNA surface attachment --- p.64 / Chapter 4.2.1 --- Mechanism of DNA surface anchoring --- p.65 / Chapter 4.3 --- I-V characterization in PBS --- p.69 / Chapter 4.3.1 --- Experimental procedures of the I-V tests in PBS --- p.70 / Chapter 4.3.2 --- Results and discussions of I-V tests in PBS --- p.72 / Chapter 4.3.2.1 --- Exercising as-prepared CdSe MP electrode --- p.74 / Chapter 4.3.2.2 --- I-V characteristics of CdSe MP electrodes before and after ssDNA attachment --- p.75 / Chapter 4.3.2.3 --- I-V characteristics of CdSe MP electrodes before and after the dsDNA attachment --- p.76 / Chapter 4.3.2.4 --- "Photo-response of bare CdSe MP, ssDNA/CdSe MP and dsDNA/CdSe electrodes" --- p.77 / Chapter 4.4 --- "Photovoltaic I-V measurement in I3""/I"" redox electrolyte" --- p.79 / Chapter 4.4.1 --- Experimental procedures --- p.79 / Chapter 4.4.2 --- Results and discussions --- p.80 / Chapter 4.5 --- Possible application implied by the results --- p.88 / Chapter 4.5.1 --- DNA base pair mismatch identification --- p.91 / Chapter 4.5.2 --- Field-assisted DNA hybridization acceleration process --- p.92 / Chapter Chapter 5 --- Conclusions / Chapter 5.1 --- Conclusions --- p.95

Thesis (Ph.D.)--University of Delaware, 2007. / Principal faculty advisor: Babatunde Ogunnaike, Dept. of Chemical Engineering, and Robert W. Birkmire, Dept. of Materials Science & Engineering. Includes bibliographical references.

M.W.P. Strandberg, T. Wentink, Jr. [and] A.G. Hill. / "June 21, 1948." / Bibliography: p. [14] / Army Signal Corps No. W-36-039-sc-32037.

This thesis investigates the biodistribution and toxicological effects of amphiphilic polymer coated CdSe/ZnS quantum dots (QDs) in two aquatic species, Daphnia magna (daphnia) and Danio rerio (zebrafish). The use of QDs in the life sciences has become common practice over the past decade. In addition QDs are being incorporated in commercially available light emitting diodes and photovoltaic solar cells. As the widespread commercial use of QDs increases, environmental release is inevitable, and water will contain the highest environmental concentrations based on life cycle assessments. Despite increased attention to the aquatic toxicology of nanomaterials in recent years, little information exists on the biological fate of QDs in aquatic organisms. Quantitative data on the uptake and excretion of QDs from daphnia and zebrafish were collected using fluorescence imaging paired with metal analysis. First, daphnia were examined after aqueous and dietary exposure to amphiphilic polymer coated CdSe/ZnS QDs. Surface coating influenced QD acute toxicity and high particle aggregation correlated with daphnia mortality. QDs were readily ingested by daphnia and accumulated in the intestines. High body burdens of 150-200 μg/g were found in the daphnia, with intestinal QD concentrations significantly elevated above the exposure media concentration. The slow elimination observed in daphnia suggested that trophic transfer of QDs to higher organisms may occur. Using daphnia and zebrafish as a model food chain revealed that QDs can transfer to zebrafish through dietary exposure with body burdens of 8-9.5 μg/g found. However, no biomagnification between daphnia and zebrafish was observed and the biomagnification factor (BMF = 0.04) was significantly less than one. This work demonstrates that aqueous and dietary exposures to QDs can result in high total body concentrations in aquatic organisms with little to no gross toxicity. The low acute toxicity observed for some surface coated QDs encourages further design optimization to improve the biocompatibility and reduce the environmental impact of QDs.