Rozprawy doktorskie: „Photovoltaic Devices - Semiconductor Nanocrystals”

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Kinder, Erich W. "Fabrication of All-Inorganic Optoelectronic Devices Using Matrix Encapsulation of Nanocrystal Arrays". Bowling Green State University / OhioLINK, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1339719904.

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López, Vidrier Julià. "Silicon Nanocrystal Superlattices for Light-Emitting and Photovoltaic Devices". Doctoral thesis, Universitat de Barcelona, 2015. http://hdl.handle.net/10803/334396.

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During the last decades, silicon nanocrystals have focused great attention due to the size-dependent physical properties they present, attributed to the quantum confinement effect. This, added to the bulk silicon compatibility with the well-established microelectronics technology and the low mining and manipulation costs this material presents, makes silicon a potential candidate for the growing photonics and optoelectronics fields. In particular, the tunnability of the electronic properties of silicon nanocrystals can be reached by controlling the nanocrystal size. This has been recently achieved by means of the superlattice approach, consisting of the alternated deposition of ultra-thin (2-4 nm) stoichiometric and silicon-rich layers of a given silicon-rich material. After a high-temperature annealing treatment, the silicon excess precipitates and crystallizes in the final form of nanocrystals, whose properties strongly depend on the fabrication process. Consequently, an ordered arrange of size-controlled nanocrystals (the superlattice) is obtained. In this Thesis Project, the structural, optical, electrical and electro-optical properties of silicon nanocrystal superlattices have been studied, using two different silicon-based materials as host matrices: silicon oxide and silicon carbide. The fabrication of these material systems has been carried out at different European institutions, specialists in the controlled deposition of nm¬thick films. Aiming at the nanocrystal superlattices characterization, different experimental techniques have been employed, which yield structural (transmission and scanning electron microscopies, X-ray diffraction), optical (optical absorption, photoluminescence and Raman scattering spectroscopies) and electrical / electro-optical (current versus voltage analysis in dark and under illumination, and electroluminescence, electro-optical response and light-beam induced photocurrent spectroscopies) information. From the material's point of view, the optimum structural properties that allow an almost perfect nanocrystal arrangement, size control and crystalline degree have been determined, always aiming at an optimum light emission and/or light absorption. Within this frame, fundamental studies have been performed to assess the crystalline degree of the nanostructures (confirming an atomic-thin transition layer between the crystalline nanocrystal core and the surrounding matrix), and to carefully inspect the controversial origin of luminescence within the nanocrystals when embedded in a silicon oxide matrix; as well, the structural conditions under which size-confinement of nanocrystals is reached when embedded in silicon carbide are reported. Once the best structural and optical properties from silicon nanocrystal superlattices were found, these material systems have been employed as active layers for light emitting and light converter (i.e. photovoltaic) devices. In oxide-based systems, the mechanisms that govern charge transport through the superlattices have been studied, and impact ionization has been hypothesized as the main electroluminescence excitation mechanism according to the experimental observations. In addition, the structural conditions (sublayer thicknesses, silicon-rich layer stoichiometry) that yield a maximum electroluminescence efficiency have been determined. Regarding silicon nanocrystals embedded in silicon carbide, a correlation has been established between the charge photogeneration and extraction when acting as an absorber material, which allowed assessing the structural conditions that maximize charge transport while minimizing the non-desirable recombination. Finally, via spectral response measurements, quantum confinement of excitons within silicon nanocrystals has been reported in silicon carbide matrix for the first time. In conclusion, the study on silicon nanocrystal superlattices developed within the present Thesis Project reveals the potential of silicon oxide as host matrix for silicon nanostructures to be used as light-emitting devices; instead, silicon carbide has proved a more suitable host material for photovoltaic applications, which sheds light to the future application of silicon nanocrystals as the top cell of an all-Si tandem cell.
Els nanocristalls de silici han esdevingut objecte d'estudi durant l'últim quart de segle, degut a què presenten, a causa de l'efecte de confinament quàntic, unes propietats físiques dependents de la seva mida. A més, la compatibilitat del silici massiu amb la ben establerta tecnologia microelectrònica juga en favor de la seva utilització i el seu desenvolupament per a futures aplicacions en el camp de la fotònica i l'optoelectrónica. El control del creixement de nanocristalls de silici es pot dur a terme mitjançant el dipòsit de superxarxes d'entre 2 i 4 nm de gruix, on capes de material estequiomètric basat en silici s'alternen amb altres de material ric en silici. Un posterior procés de recuit a alta temperatura permet la precipitació de l'excés de silici i la seva cristal.lització, tot originant una xarxa ordenada de nanocristalls de silici de mida controlada. En aquesta Tesi, s'han estudiat les propietats estructurals, òptiques, elèctriques i electro-òptiques de superxarxes de nanocristalls de silici embeguts en dues matrius diferents: òxid de silici i carbur de silici. Amb tal objectiu, s'han emprat tot un seguit de tècniques experimentals, que comprenen la caracterització estructural (microscòpia electrònica de transmissió i d'escombrat, difracció de raigs X), òptica (espectroscòpies d'absorció òptica, de fotoluminescència i dispersió Raman) i elèctrica / electro-òptica (caracterització intensitat-voltatge en foscor o sota il.luminació, electroluminescència, resposta electro-òptica), entre d'altres. Des del punt de vista del material, s'han estudiat les propietats estructurals òptimes per tal d'obtenir un perfecte ordenament en la xarxa de nanocristalls, una major qualitat cristal.lina i unes propietats d'emissió òptimes. L'optimització del material s'ha dut a terme en vistes a la seva utilització com a capa activa dins de dispositius emissors de llum i fotovoltaics, l'eficiència dels quals ha estat monitoritzada segons els diferents paràmetres estructurals (gruix de les capes nanomètriques involucrades, estequiometria, temperatura de recuit). Finalment, els nanocristalls de silici embeguts en òxid de silici han demostrat un major rendiment com a emissors de llum, mentre que una matriu de carbur de silici beneficia les propietats d'absorció i extracció (fotovoltaiques) del sistema.

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Cattley, Christopher Andrew. "Quaternary nanocrystal solar cells". Thesis, University of Oxford, 2016. http://ora.ox.ac.uk/objects/uuid:977e0f75-e597-4c7a-8f72-6a26031f8f0b.

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This thesis studies quaternary chalcogenide nanocrystals and their photovoltaic applications. A temperature-dependent phase change between two distinct crystallographic phases of stoichiometric Cu₂ZnSnS₄ is investigated through the development of a one pot synthesis method. Characterisation of the Cu₂ZnSnS₄ nanocrystals was performed using absorption spectroscopy, transmission electron microscopy (TEM) and powder X-ray diffraction (XRD). An investigation was conducted into the effects of using hexamethyldisilathiane (a volatile sulphur precursor) in the nucleation of small (<7nm), mono-dispersed and solution-stable quaternary Cu₂ZnSnS₄ nanocrystals. A strategy to synthesize high quality thermodynamically stable kesterite Cu₂ZnSnS₄ nanocrystals is established, which subsequently enabled the systematic study of Cu₂ZnSnS₄ nanocrystal formation mechanisms, using optical characterization, XRD, TEM and Raman spectroscopy. Further studies employed scanning transmission electron microscopy (STEM) energy dispersive x-ray (EDX) mapping to examine the elemental spatial distributions of Cu₂ZnSnS₄ nanocrystals, in order to analyse their compositional uniformity. In addition, the stability of nanocrystals synthesised using alternative ligands is investigated using Fourier transform infrared spectroscopy, without solution based ligand substitution protocol is used to replace aliphatic reaction ligands with short, aromatic pyridine ligands in order to further improve Cu₂ZnSnS₄ colloid stability. A layer-by-layer spin coating method is developed to fabricate a semiconductor heterojunction, using CdS as an n-type window, which is utilised to investigate the photovoltaic properties of Cu₂ZnSnS₄ nanocrystals. Finally, three novel passivation techniques are investigated, in order to optimise the optoelectronic properties of the solar cells to the point where a power conversion efficiency (PCE) of 1.00±0.04% is achieved. Although seemingly modest when compared to the performance of leading devices (PCE>12%) this represents one of the highest obtained for a Cu₂ZnSnS₄ nanocrystal solar cell, fabricated completely under ambient conditions at low temperatures.

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Nemitz, Ian R. "Synthesis of Nanoscale Semiconductor Heterostructures for Photovoltaic Applications". Bowling Green State University / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1277087935.

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Chang, Jin. "Controlled synthesis of inorganic semiconductor nanocrystals and their applications". Thesis, Queensland University of Technology, 2013. https://eprints.qut.edu.au/63960/1/Jin_Chang_Thesis.pdf.

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This thesis is a comprehensive study of the synthesis of nanomaterials. It explores the synthetic methods on the control of the size, shape and phase of semiconductor nanocrystals. A number of important conclusions, including the mechanism behind crystal growth and the structure-relationship, have been drawn through the experimental and theoretical investigation. The synthesized nanocrystals have been tested for applications in gas sensing, photocatalysis and solar cells, which exhibit considerable commercialization potential.

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JANA, SOURAV KANTI. "Light harvesting methods in photovoltaic devices with superficial treatments". Doctoral thesis, Università degli Studi di Milano-Bicocca, 2012. http://hdl.handle.net/10281/28621.

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Photovoltaics is fast emerging as an attractive renewable energy technology due to concerns of global warming, pollution and scarcity of fossil fuels supplies. However to compete in the global energy market, solar cells need to be cheaper and more energy efficient. Silicon is the favourite semiconductor used in solar photovoltaic cells because of its abandoned in nature, well established technology and non toxicity. But due to its indirect band gap, silicon is poor absorber of light and theoretical limiting efficiency of single junction wafer based silicon solar cells is ~31% which is called Shockley Queisser limit. But up to now the maximum achievable efficiency in laboratory for single crystal single junction silicon solar cells is ~ 24.7%. So far the cost of the wafer based silicon photovoltaics is high. Also thin film cells play an important role in low cost photvoltaics, but efficiency of the cost reduced cells is lower compared to wafer based cells. So light trapping into photovoltaic cells is a great issue inorder to increase the carrier generation inside the active layer of both bulk as well as thin film cells with out disturbing their fabrication technology. There many light harvesting methods; among them Surface Plasmon method using metal nanoparticles and spectrum downshifting method using nanocrystals are discussed here. Metal nanoparticles support surface plamson when light is incident on them, which cause the scatter light into the underlying substrate. This process is realized on standard silicon solar cells. The feasible light scattering related enhancement was examined using spectral response and I-V measurements. Relative increases of the total delivered power under simulated solar irradiation were observed for cells both with and without antireflection coating using both silver and gold nanoparticles. The relative enhancement of External Quantum Efficiency derived from the spectral response measurements was observed for both the silicon cells. The better results obtained from both spectral response and I-V measurements were ascribed in the case of cells without antireflection coating. The results from I-V measurements under Air Mass 1.5 irradiation on the cells (without antireflection coating) correspond to a clear increase of the short circuit current due to both silver (relative increase of 7.5%) and gold (relative increase of 6.1%) nanoparticles. Also there is a relative enhancement (1.5%) of short circuit current was ascribed in the cells (with antireflection coating). Further realization of this method on copper indium gallium selenide based thin film solar cells attributed the enhancement of external quantum efficiency in the red wavelength region where these cells have already a poor spectral response. Spectral downshifting method by the nanocrystals was investigated on the silicon based solar cells. Down shifting of photons on the silicon solar cells is realized by the absorption and emission property of the manganese doped zinc sulfide nanocrystals. The variation of band gap and photoluminescence intensity of different nanocrystals due to different doping concentration was observed. Relative enhancement of External Quantum Efficiency has been attributed in UV region (where silicon solar cells have poor spectral response) due to lower concentration of nanocrystals. A strong concentration quenching effect which causes decrease of external quantum efficiency in both UV and visible region has been observed.

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Martínez, Montblanch Luis. "N-type bismuth sulfide coloidal nanocrystals and their application to solution-processed photovoltaic devices". Doctoral thesis, Universitat Politècnica de Catalunya, 2014. http://hdl.handle.net/10803/284207.

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Durante las últimas décadas, la energía solar fotovoltaica se ha convertido en una tecnología de creciente importancia para satisfacer las necesidades energéticas actuales sin sacrificar las futuras generaciones. Las células solares tradicionales basadas en silicio llevan asociados altos costes, tanto en materia prima como en su fabricación. Sin embargo, las tecnologías alternativas como las células solares orgánicas ofrecen prometedoras ventajas de bajo coste y fabricación, a expensas de inestabilidad química. Los nanocristales inorgánicos coloidales han atraído una creciente atención, debido a su combinación única de estabilidad química, aprovechamiento pancromático de la energía solar y procesado en disolución. No obstante, los semiconductores nanocristalinos más habituales generan dudas en cuanto a su aplicabilidad, debido a la presencia de metales pesados tóxicos (como el plomo y el cadmio). Además, muchos de estos materiales son tipo p y se usan junto con semiconductores de tipo n de "bandgap" ancho, que no contribuyen a la generación de fotocorriente. El campo de semiconductores nanocristalinos no tóxicos con niveles energéticos apropiados, alta absorción óptica y "bandgap" adecuado para el aprovechamiento de la energía solar aún está por explorar. El objetivo de esta tesis es investigar el potencial de los nanocristales de sulfuro de bismuto para ser empleados como nanomateriales no tóxicos tipo n para un aprovechamiento eficiente de la energía solar. En el Capítulo 2 se presenta un estudio detallado de las propiedades físico-químicas y electro-ópticas de los nanocristales de sulfuro de bismuto. Éstos son semiconductores tipo n y tienen unos niveles energéticos y "bandgap" apropiados para un aprovechamiento eficiente de la energía solar. Por tanto, los nanocristales de sulfuro de bismuto presentan el potencial para ser empleados como aceptores de electrones en células solares basadas en heterouniones con los mejores materiales investigados en la tercera generación fotovoltaica. Los nanocristales de sulfuro de bismuto se emplean en el Capítulo 3 como aceptores de electrones en células solares híbridas. Los materiales usados típicamente como aceptores de electrones así como los polímeros semiconductores no aprovechan la radiación infrarroja. Los nanocristales de sulfuro de bismuto pueden ser usados como materiales aceptores en células solares híbridas y así extender el rango de sensibilidad de las células solares basadas en P3HT a longitudes de onda en el rango del infrarrojo cercano. En el Capítulo 4 se investiga la nanomorfología y el rendimiento fotovoltaico de las células solares híbridas basadas en nanocristales de sulfuro de bismuto y polímeros semiconductores funcionalizados con tioles. Esta nueva clase de polímeros funcionalizados se enlaza a la superficie de los nanocristales de sulfuro de bismuto previniendo su aglomeración, así como presentan niveles de potencial de ionización más profundos y contribuyen a una mejor interacción electrónica entre el nanocompuesto orgánico-inorgánico. En el Capítulo 5, los nanocristales de sulfuro de bismuto se emplean conjuntamente con puntos cuánticos de sulfuro de plomo en dispositivos fotovoltaicos procesados en disolución basados en unión p-n totalmente inorgánicos. Este sistema abre la posibilidad de fabricar heterouniones tipo "bulk", una arquitectura menos limitada por el tiempo de vida de los portadores. De este modo, se puede explorar un rango más amplio de materiales inorgánicos nanocristalinos para dispositvos fotovoltaicos de tercera generación.
Photovoltaics has become a technology of increasing importance during the last decades as a platform to satisfy the energy needs of today without compromising future generations. Traditional silicon-based solar cells suffer from high material and fabrication costs. Alternative technologies such as organic photovoltaics offer promising low-cost material and processing advantages, however at the cost of chemical instability. Inorganic colloidal nanocrystals have attracted significant attention, due to the unique combination of chemical robustness, panchromatic solar harnessing and low-cost solution processability. However, the state-of-the-art nanocrystalline semiconductors raise some concerns regarding their suitability for industrial applications due to the presence of highly toxic heavy metals (such as lead or cadmium). Moreover, most of these materials are p-type, and are usually employed together with large bandgap n-type semiconductors that do not contribute to photocurrent generation. The field on non-toxic, electron-acceptor nanocrystalline semiconductors with appropriate energy levels, high optical absorption and bandgap suited to optimal solar harnessing still remains unexplored. The aim of this thesis is to investigate the potential of bismuth sulfide nanocrystals to be employed as environmental-friendly n-type nanomaterials for efficient solar harnessing. Chapter 2 presents an in-depth physicochemical and optoelectronic characterization of bismuth sulfide colloidal nanocrystals. Bismuth sulfide nanocrystals are n-type semiconductors and have the appropriate bandgap and energy levels for efficient solar harnessing. Therefore, bismuth sulfide nanocrystals have the potential to be employed as the electron accepting material in heterojunction-based solar cells with most high-performing materials investigated for third-generation photovoltaics. Bismuth sulfide nanocrystals are employed in Chapter 3 as electron accepting materials in hybrid organic-inorganic solar cells. Typical electron accepting materials and semiconducting polymers used in organic photovoltaics do not harness infrared radiation, thus limiting their solar harnessing potential. Bismuth sulfide nanocrystals can be used as electron accepting materials in hybrid organic-inorganic solar cells and extend the sensitivity range of P3HT-based solar cells into near-infrared wavelengths. Chapter 4 investigates the nanomorphology and photovoltaic performance of hybrid solar cells based on bismuth sulfide nanocrystals and thiol-functionalized semiconducting polymers. This novel class of functionalized polymers binds to the surface of bismuth sulfide nanocrystals, thus preventing nanocrystal agglomeration, shows deeper ionization potential levels and exhibits improved electronic interaction within the organic-inorganic nanocomposite. In Chapter 5, bismuth sulfide nanocrystals are employed together with lead sulfide quantum dots in p-n junction-based all-inorganic solution-processed photovoltaic devices. This system opens the possibility of fabricating all-inorganic solution-processed bulk heterojunctions, a device architecture where requirements on carrier lifetime are eased. This way, a broader range of inorganic nanocrystalline materials can be explored in the quest for novel non-toxic third-generation photovoltaics

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Holder, Jenna Ka Ling. "Quantum structures in photovoltaic devices". Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:d23c2660-bdba-4a4f-9d43-9860b9aabdb8.

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A study of three novel solar cells is presented, all of which incorporate a low-dimensional quantum confined component in a bid to enhance device performance. Firstly, intermediate band solar cells (IBSCs) based on InAs quantum dots (QDs) in a GaAs p-i-n structure are studied. The aim is to isolate the InAs QDs from the GaAs conduction band by surrounding them with wider band gap aluminium arsenide. An increase in open circuit voltage (V_OC) and decrease in short circuit current (J_sc) is observed, causing no overall change in power conversion efficiency. Dark current - voltage measurements show that the increase in V_OC is due to reduced recombination. Electroreflectance and external quantum efficiency measurements attribute the decrease in J_sc primarily to a reduction in InGaAs states between the InAs QD and GaAs which act as an extraction pathway for charges in the control device. A colloidal quantum dot (CQD) bulk heterojunction (BHJ) solar cell composed of a blend of PbS CQDs and ZnO nanoparticles is examined next. The aim of the BHJ is to increase charge separation by increasing the heterojunction interface. Different concentration ratios of each phase are tested and show no change in J_sc, due primarily to poor overall charge transport in the blend. V_OC increases for a 30 wt% ZnO blend, and this is attributed largely to a reduction in shunt resistance in the BHJ devices. Finally, graphene is compared to indium tin oxide (ITO) as an alternative transparent electrode in squaraine/ C₇₀ solar cells. Due to graphene’s high transparency, graphene devices have enhanced J_sc, however, its poor sheet resistance increases the series resistance through the device, leading to a poorer fill factor. V_OC is raised by using MoO₃ as a hole blocking layer. Absorption in the squaraine layer is found to be more conducive to current extraction than in the C₇₀ layer. This is due to better matching of exciton diffusion length and layer thickness in the squaraine and to the minority carrier blocking layer adjacent to the squaraine being more effective than the one adjacent to the C₇₀.

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Cheng, Cheng. "Semiconductor colloidal quantum dots for photovoltaic applications". Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:07baccd0-2098-4306-8a9a-49160ec6a15a.

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This thesis studies lead suphide (PbS) colloidal quantum dots and their photovoltaic applications. Different sizes of PbS QDs were synthesised and characterised using absorption spectroscopy and transmission electron microscopes. PbS QD Schottky junction devices were fabricated with AM1.5 power conversion efficiency up to 1.8 %. The Schottky junction geometry limits the device performance. A semiconductor heterojunction using ZnO as an electron acceptor was built and the device efficiency increased to 3%. By studying the light absorption and charge extraction profile of the bilayer device, the absorber layer has a charge extraction dead zone which is beyond the reach of the built-in electric field. Therefore, strategies to create a QD bulk heterojunction were considered to address this issue by distributing the junction interface throughout the absorber layer. However, the charge separation mechanism of the QD heterojunction is not clearly understood: whether it operates as an excitonic or a depleted p-n junction, as the junction operating mechanism determines the scale of phase separation in the bulk morphology. This study shows a transitional behaviour of the PbS/ZnO heterojunction from excitonic to depletion by increasing the doping density of ZnO. To utilise the excitonic mechanism, a PbS/ZnO nanocrystal bulk heterojunction was created by blending the two nanocrystals in solution such that a large interface between the two materials could facilitate fast exciton dissociation. However, the devices show poor performance due to a coarse morphology and formation of germinate pairs. To create a bulk heterojunction where a built-in electric field could assist the charge separation, a TiO₂ porous structure with the pore size matching with the depletion width was fabricated and successfully in-filled by PbS QDs. The porous device produces 5.7% power conversion efficiency, among one of the highest in literature. The enhancement comes from increased light absorption and suppression of charge recombination.

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Machui, Florian [Verfasser], i Christoph [Akademischer Betreuer] Brabec. "Formulation of Semiconductor Solutions for Organic Photovoltaic Devices / Florian Machui. Gutachter: Christoph Brabec". Erlangen : Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), 2014. http://d-nb.info/1065005687/34.

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Beattie, Meghan. "Semiconductor Materials and Devices for High Efficiency Broadband and Monochromatic Photovoltaic Energy Conversion". Thesis, Université d'Ottawa / University of Ottawa, 2021. http://hdl.handle.net/10393/42475.

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This thesis addresses barriers to the widespread adoption of high-efficiency photovoltaic devices through the use of innovative semiconductor materials and device design. The feasibility of various strategies is explored through experimental characterization and modeling of semiconductor materials and devices. High-efficiency photovoltaic devices are made from epitaxially grown III-V semiconductor materials. Epitaxial devices are highly sensitive to lattice mismatch between the epi-layers and the substrate, requiring sophisticated substrate engineering or growth strategies to access materials outside of the lattice-matched regime. One promising strategy involves the electrochemical porosification of germanium on a lattice-mismatched silicon substrate to create a compliant interface for high-quality epitaxial growth of Ge, GaAs, and other equivalent-bandgap III-V semiconductors on silicon. This results in a threading dislocation density of ~10^4 cm^-2, a reduction of 4 to 6 orders of magnitude compared to direct epitaxy of germanium on silicon. This technology could enable the development of highly efficient III-V multi-junction photovoltaic devices on cost-effective silicon substrates that benefit from well-established commercial supply chains. In the first part, I present characterization of the electrical properties of porous germanium. Experimental measurements revealed conductivities ranging from 0.6 to 33 (x10^-3) Ohm^-1 cm^-1, depending on the morphology. The relationship between the electrical properties and the morphology is described using an electrostatic model that can be generalized to other porous semiconductors including silicon. For a compliant interface designed to integrate a standard triple-junction solar cell onto a silicon substrate, the porous Ge/Si layers are predicted to introduce < 0.01 Ohm cm^2 of series resistance to the device, which is sufficiently low for concentrated photovoltaic applications. Optoelectronic device modelling of the triple-junction solar cell on silicon demonstrates that III-V triple-junction solar cells fabricated on silicon using this compliant Ge/Si porous interface could achieve 93% of the efficiency of a comparable defect-free device. The remainder of this thesis is concerned with the design and characterization of photovoltaic devices optimized for monochromatic illumination, known as photonic power converters. Most commercially available photonic power converters are based on GaAs and are suitable for short-range photonic power transmission through optical fiber (< 1 km). Extended reach power-over-fiber systems require the use of photonic power converters that are compatible with longer-wavelength light, which travels further in optical fiber. One candidate material for this application is the semiconductor quaternary alloy InAlGaAs lattice-matched to InP for photonic power converter operation in the telecommunications O-band, near 1310 nm. I describe the design and characterization of multi-junction InAlGaAs/InP photonic power converters grown by molecular beam epitaxy, including the analysis of material properties and characterization of single- and dual-junction devices under 1319-nm laser illumination. Optically thick devices are found to be diffusion-limited and device simulations suggest that non-radiative recombination is significant. The performance of InAlGaAs tunnel diodes, which act as interconnections for the absorbing junctions within a multi-junction device, is demonstrated to be highly dependent on the growth temperature, with peak tunneling current densities exceeding 1200 A/cm^2 in the best measured devices. In addition to molecular beam epitaxy-grown InAlGaAs/InP devices, I also characterize single-junction O-band photonic power converters grown by metal-organic vapour phase epitaxy with two alternative absorber materials. A lattice-matched InGaAsP/InP device is compared to a more cost-effective lattice-mismatched GaInAs device grown on GaAs using a metamorphic buffer layer. Both devices are measured under 1319-nm laser illumination with a variety of beam sizes and peak efficiencies of 52.9% and 48.8% were measured for the InGaAsP/InP and the metamorphic-GaInAs/GaAs devices respectively. At illumination powers exceeding 100 mW, the performance begins to degrade with increasingly non-uniform illumination, indicating that illumination profiles should be as uniform as possible to maximize device performance.

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Hu, Bing. "FABRICATION AND STUDY OF MOLECULAR DEVICES AND PHOTOVOLTAIC DEVICES BY METAL/DIELECTRIC/METAL STRUCTURES". UKnowledge, 2011. http://uknowledge.uky.edu/gradschool_diss/222.

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A new class of electrodes with nanometer-scale contact spacing can be produced at the edge of patterned metal/insulator/metal this film structures. A key challenge is to produce insulator layers with low leakage current and have pristine metal contacts for controlled molecular contacts. Atomic layer deposition of high quality Al2O3 thin films onto Au electrodes was enabled by surface modification with a self-assembled monolayer of -OH groups that react with a monolayer of trimethylaluminum gas source. Ar ion milling was then used to expose the edge of the Au/dielectric/Au structure for molecular electrode contacts. The junctions are characterized by atomic force microscope and tunnel current properties. The Au/self-assembled monolayer/Al2O3/Au tunnel junction, with a very thin oxide insulator layer (15.4 Å), is stable and has a small tunneling current density of about 0.20 ~ 0.75 A/cm2 at 0.5 V. Organometalic cluster molecules were attached to bridge the electrodes. Through tunnel current modeling, low temperature and photo current measurements, molecular current was found to be consistent with direct tunneling through the organic tethers to available states at the metal center. This novel electrode was also used to study the efficiency of organic conducting thin films where the photovoltaic efficiency can be improved when the electrode separation distance is below the exciton diffusion length. Copper (II) phthalocyanine (CuPc) was thermally evaporated between the nano-gap electrodes formed by Au/Al2O3/Au tunnel junctions. A large photocurrent enhancement over 50 times that of bulk CuPc film was observed when the electrode gap distance approached 10 nm. CuPc diffusion length is seen to be 10 nm consistent with literature reports. All devices show diode I-V properties due to a large Schottky barrier contact resistance between the small top Au electrode and the CuPc film. To add another dimension of nm-scale patterning, nanowires can be used as line-of-sight shadowmasks provided that nanowire location and diameter can be controlled. Lateral ZnO nanowires were selectively grown from the edge of a Si/Al2O3/Si multi-layer structure for potential integration into devices utilizing Si processing technology. Microstructural studies demonstrate a 2-step growth process in which the tip region, with a diameter ~ 10 nm, rapidly grew from the Al2O3 surface. Later a base growth with a diameter ~ 22 nm overgrew the existing narrow ZnO nanowire halting further tip growth. Kinetics studies showed surface diffusion on the alumina seed surface determined ZnO nanowire growth rate.

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Ghosh, Aheli. "Heteroepitaxial Germanium-on-Silicon Thin-Films for Electronic and Photovoltaic Applications". Thesis, Virginia Tech, 2017. http://hdl.handle.net/10919/78037.

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Developing high efficiency solar cells for lower manufacturing costs has been a key objective for photovoltaic researchers to drive down the levelized cost of energy for solar power. In this pursuit, III-V compound semiconductor based solar cells have steadily shown performance improvement at approximately 1% (absolute) increase per year, with a recent record efficiency of 46% under concentrator and 32% under AM0. However, the expensive cost has made it challenging for III-V solar cells to compete with the mainstream Silicon (Si) technology. Novel approaches to lower down the cost per watt for III-V solar cells will position them to be among the key contenders in the renewable energy sector. Integration of such high-efficiency III-V multijunction solar cells on significantly cheaper and large area Si substrate has the potential to address the future LCOE roadmaps by unifying the high-efficiency merits of III-V materials with low-cost and abundance of Si. However, the 4% lattice mismatch, thermal mismatch, polar on non-polar epitaxy makes the direct growth of GaAs on Si challenging, rendering the metamorphic cell sensitive to dislocations. The focus of this dissertation is to investigate heterogeneously integrated 1J GaAs solar cells on Si substrate using germanium (Ge) as an intermediate buffer layer that will address mitigation of defects and dislocations between GaAs active cell structure and Ge “virtual” substrate on Si. The all-epitaxial molecular beam epitaxy (MBE)-grown thin (<1 μm) hybrid GaAs/Ge “virtual” buffer approach provided 1J GaAs cell efficiency of ~10% on Si, as compared with cell structures with thick 3 μm GaAs buffers. Solar cell results were further corroborated with material analysis to provide a clear path for the reduction of performance limiting dislocations. The thin “Ge-on-Si” virtual buffer was then investigated comprehensively to understand the impact of the heterostructure on device performance. The growth, structural, morphological, and electrical transport properties of epitaxial thin-film Ge, grown by solid source MBE on Si using a two-step growth process, were investigated. High-resolution x-ray diffraction analysis demonstrated ~0.10% tensile strained Ge epilayer, owing to the thermal expansion coefficient mismatch between Ge and Si, and negligible epilayer lattice tilt due to misfit dislocations at the Ge/Si heterointerface. Micro-Raman spectroscopic analysis further corroborated the strain-state of the Ge thin-film on Si. Cross-sectional transmission electron microscopy revealed the formation of a 90° Lomer dislocation network at the Ge/Si heterointerface, suggesting the rapid and complete relaxation of the Ge epilayer during growth. Atomic force micrographs exhibited smooth surface morphologies with surface roughness < 2 nm. Hall mobility measurements, performed within a temperature range of 77 K to 315 K, and the modelling thereof indicated that ionized impurity scattering limited carrier mobility in the thin Ge epilayer. Additionally, capacitance- and conductance-voltage measurements were performed after fabricating the metal-oxide-semiconductor capacitors (MOS-Cs) in order to determine the effect of epilayer dislocation density on interfacial defect states (Dit), bulk trap density, and the energy distribution of Dit as a function of temperature for electronic device applications. Deep level transient spectroscopy was used to identify the location (within the Ge bandgap) of electrically active trap levels; however, no significant trap levels were detected. Finally, the extracted Dit values were benchmarked against previously reported Dit data for Ge MOS devices, as a function of threading dislocation density within the Ge layer. The results obtained in this work were found to be comparable with other Ge MOS devices integrated on Si via alternative buffer schemes. The understanding gained from this comprehensive study of Ge-on-Si will help optimize the 1J GaAs on Si via thin Ge buffer approach, to enable a future of high efficiency low cost solar cells for terrestrial applications.
Master of Science
The global energy landscape is projected to change remarkably in the coming decades with dwindling carbon based resource reserves and escalating energy demands, necessitating large-scale adoption of cleaner alternatives, such as solar energy. However, for widespread commercial and domestic adoption of photovoltaics, the cost of solar generated electricity must become competitive with non-renewable resources such as oil or coal. Thus, achieving high efficiency solar cells and driving down cell costs are key research objectives of the photovoltaic (PV) community in order to become more self-sufficient in the energy sector. In this pursuit, III-V compound semiconductor-based solar cells have steadily outperformed all other PV technologies, but cost-prohibitive for terrestrial deployment. Si is the undisputed standard in the PV industry; thus, to make a significant step forward in the pursuit of high efficiency solar cells, a promising approach will be to integrate the superior properties of compound semiconductors with the mature technology of Si. This research systematically investigates the integration of high efficiency III-V cells with low cost, abundant Si substrates via a germanium (Ge) layer to unify the performance merits of III-V cells with the cost benefits and superior mechanical and thermal properties of Si. Concurrently, Ge has also emerged as a strong candidate to boost transistor performance at low operating voltages, primarily owing to its superior carrier mobility and ease of integration into mainstream Si process flow. This research further delves into the structural and electrical properties of the Ge on Si structure. Overall, this research demonstrates the feasibility of the use of Ge directly integrated on Si for high efficiency solar cells and low-power electronic devices.

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Garduno, Nolasco Edson. "Nano-scale approaches for the development and optimization of state-of-the-art semiconductor photovoltaic devices". Thesis, University of Manchester, 2014. https://www.research.manchester.ac.uk/portal/en/theses/nanoscale-approaches-for-the-development-and-optimization-of-stateoftheart-semiconductor-photovoltaic-devices(927e70db-03ff-43e0-8b27-5472bc4a293f).html.

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This project is concerned with both the study of different Multiple Quantum Wells (MQWs) structures using the In0.53Ga0.47As/In0.52Al0.48As material system lattice matched to InP and a systematic investigation of the properties of InAs QD systems within GaAs with the aim of achieving enhancements of solar cell performance. The key challenge is the growth of QDs solar cell structures which exhibit sufficient absorption (enhanced infrared absorption) to increase short circuit current density (Jsc) but which can still maintains a high open circuit voltage (Voc). The research consists of epitaxial growth using state-of–the-art MBE, optical absorption, photoluminescence and high resolution x-ray diffraction measurements as well as device fabrication and characterization of novel solar cell structures. Optimization was performed on these novel cells to further improve their efficiency by inserting stacks of QD into different regions of the device. The effect of localized doping of such structures was used in an attempt to maintain and enhance the open-circuit voltage which in turn increases the device efficiency. The fabricated devices were characterized using measurements of the dark/light current-voltage (I-V) characteristics and spectral response (50-480 K). Solar cell external quantum efficiencies under standard air mass (AM) 1.5 spectrum were determined and the suitability of these new cells under solar concentration were assessed. Full physical simulations are performed using SILVACO semiconductors modelling software to generate models of multi-junction solar cells that were crucial in informing iterations to growth and fabrication and help to reconcile theory with experiment. One of the key findings, of this thesis, is the fact that Intermediate band photovoltaic devices using material based on InAs/GaAs vertically stacked quantum dot arrays, can be used in applications according to specific configuration criteria such as high temperature operation conditions. The intermediate band cell, including an inter-dot doped configuration, has been found to be a potential candidate as the inter dot doping profile reduces the efficiency degradation below the GaAs values including an enhancement in the open circuit voltage. It has been proved that these devices not only have a good performance at high temperatures but also by changing the vertical stacking QD layer periodicity can enhance the short circuit current density while keeping a large open circuit voltage. It was confirmed in practical device operation that thermal energy is required to enable the intermediate band in InAs/GaAs QD materials. The impact of this works can help in the future improvements of the intermediate band solar cells based on InAs on GaAs QD. The best overall efficiency of 11.6 % obtained in this work is an excellent value for so simple devices configuration. The Si3N4, tested for the first time on InAs/GaAs QD materials, reduces the reflectance on the device surface to a value of 2% and the operational wavelength can be tuned by controlling the layer thickness. A 100 nm Si3N4 antireflective coating proved to be an excellent coating from 700 to 1000 nm. In terms of short circuit current density a 37% enhancement was achieved.

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15

ROSINA, IRENE. "Exploiting Cation Exchange Reactions in Doped Colloidal NIR Semiconductor Nanocrystals: from synthesis to applications". Doctoral thesis, Università degli studi di Genova, 2020. http://hdl.handle.net/11567/1019427.

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Colloidal quantum dots (CQDs) have tunable optical properties through manipulation of their size, shape, and surface chemistry. Among pholuminescent QDs, near-infrared (NIR) emitting ones are of particular interest since they can be used in several applications, from the labeling in living tissues, to the integration in commercial optoelectronic devices, like photovoltaics for solar energy conversion or photodetectors from visible to the near-infrared and mid-infrared. In addition, the exciting promise of CQDs is that is associated with easy and low-cost device fabrication process. In fact, solution-based techniques like spin-coating, dip coating and ink-jet printing are typically used for solution CQDs readily to be used in large-area processing techniques. Thus, to obtain an ink solution of nanocrystals (NCs) ready to be used in device fabrication process, in this thesis, cation exchange (CE) reactions have been used as a convenient tool to finely transform NCs directly in solution or deposited as thin films. These reactions allow to substitute a fraction or all “host” metal cations of pre-synthesized NCs with new “guest” cations while preserving both NCs’ size, shape and, typically, crystal structure. Depending on the miscibility of the reactant and product materials, and on the kinetics of the CE reaction, different types of nanostructures can be accessed ranging from alloy NCs, doped systems, dimers, core@shell (or core@graded-shell) heterostructures even with elaborated architectures (i.e., quantum wells, multiple-cores@shell). Unlike ion substitution in solids, cation exchange at nanoscale results in fast reaction rate and an easy modulation of the thermodynamics through selective ion coordination in solution. This study provides an overview of the CE on semiconductor NCs, in particular on II-VI, I-III-VI2 and III-VI compounds. We first explore the exchange between cadmium chalcogenides and mercury ions to produce Cd1-xHgxTe CQDs which can be potentially employed in NIR photodetectors and photovoltaic devices. Our developed synthesis is a result of a wide systematic investigation process, in which we varied specific physical parameters, such as the reaction temperature, the feed molar ratio of the precursor and the solvent. More specifically, these aspects were studied to have control on the size, shape, surface composition and crystalline phase after mild conditions of annealing into stable connected crystals. This peculiarity could be exploited to boost the photogenerated charges diffusion in polycrystalline photoconducting films fabricated by means of an ink of NCs solution. Additionally, another aspect studied was the surface passivation of Cd1-xHgxTe colloidal NCs, in order to understand how to optimize the charge transfer efficiency among the nanocrystals. The carrier transport in QD devices differs fundamentally from band transport in bulk semiconductors. In nanocrystal film it is of fundamental relevance to improve the mobility of the photogenerated charges. Noteworthy, the granularity of the system and the consequent coupling between adjacent dots can produce additional physical parameters, as charge recombination. The carrier diffusion length can be limited by trapping sites1. To overcome these limitations, post-synthetic strategies that couple the high quality NCs solutions with ideal properties (band gap, absorption, monodispersivity) and high-quality films (quantum dot packing, passivation, and absorptive/conductive properties) are necessary. Indeed, to improve the inter-NCs conductivity in a NC film, ligand exchange and stripping procedures are widely used, with the aim of replacing insulating surfactants with more conductive species. These procedures have some drawbacks, for example metal cations can desorb from the surface of the NCs during the stripping. On the contrary, here we will show how our nano heterostructures (NHCs) enable to avoid the post-process ligand stripping and to perform the final annealing step in milder conditions. Above these considerations, CE can be exploited to address NCs solution through surface uniformity from the nano- to the macroscopic scale. This is the first step toward electronic coupling between the separate building blocks of nanocrystals. Apart from III-V QDs, we shifted our research activity on valid alternative material which do not contain toxic heavy metals such as Cd, Pb, As or Hg, and that offer a high flexibility for tuning band gap in the NIR window. In chapter 5, the results about the study of a III-V system are reported. Thus, we studied InP system, which is probably the only one that could provide a compatible emission color range similar to that of Cd-based QDs but without intrinsic toxicity. Nevertheless, the synthesis of III-V NCs, due to their covalent-bond character, is limited by long reaction times or an uncontrollably fast nucleation that may lead to the formation of amorphous or bulk compounds. The role of our work is to explore the reported InP synthesis and to further improve the luminescent properties of these systems Here we study the effect of different parameter (molar concentration in reaction mixture, the use of different phosphorous precursors) to enhance the control over the particle size and size distribution. After that, we studied different Sulphur source precursors to obtain InP@ZnS core@shell NCs with high quantum Yield (QY). In the last chapter, we describe also I-III-VI2 system as CuInS2 for photoluminescence modulation. In this Chapter Copper Indium Sulfide nanocrystals are prepared using a single-step heating up method relying on the low thermal stability of ter- dodecanethiol used as stabilizing agent, solvent, and sulfur precursors. The obtained particles exhibit an emission varying from 710 to 940 nm. This range depends on the extent of the heating time (pre-heating) before the threshold temperature of 230°C for the growth process of ternary semiconductor NCs such as of CIS nanocrystals. Afterwards we report on the procedure for the growth of a ZnS shell, which enables a blueshift of the PL emission wavelength with respect to those of their parent CIS, due to the widening of the band gap for the entrance of zinc ions into the CIS structures.

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16

Dickerson, Jeramy Ray. "Heterostructure polarization charge engineering for improved and novel III-V semiconductor devices". Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/51793.

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Innovative electronic device concepts that use polarization charges to provide improved performance were validated. The strength of the electric fields created by polarization charges (PCs) was suggested to act as an additional design parameter in the creation of devices using III-nitride and other highly polar materials. Results indicated that polarization induced electric fields can replace conventional doping schemes to create the charge separation region of solar cells and would allow for a decoupling of device performance from doping requirements. Additionally, a model for calculating current through polarization induced tunnel diodes was proposed. The model was found to agree well with experimental current values. Several polarization induced tunnel junction (PTJ) designs were analyzed. A novel double-barrier PTJ was conceived that would allow for the creation of a multi-junction solar cell using strained InGaN absorption layers. Future research would include the fabrication of these devices and the inclusion of thermal effects in the model for calculating current through PTJs.

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Roland, Paul Joseph. "Charge Carrier Processes in Photovoltaic Materials and Devices: Lead Sulfide Quantum Dots and Cadmium Telluride". University of Toledo / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1449857685.

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Maldei, Michael. "A study of the suitability of amorphous, hydrogenated carbon (a-C:H) for photovoltaic devices". Ohio : Ohio University, 1997. http://www.ohiolink.edu/etd/view.cgi?ohiou1175022667.

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Akeyo, Oluwaseun M. "ANALYSIS AND SIMULATION OF PHOTOVOLTAIC SYSTEMS INCORPORATING BATTERY ENERGY STORAGE". UKnowledge, 2017. http://uknowledge.uky.edu/ece_etds/107.

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Solar energy is an abundant renewable source, which is expected to play an increasing role in the grid's future infrastructure for distributed generation. The research described in the thesis focuses on the analysis of integrating multi-megawatt photovoltaics (PV) systems with battery energy storage into the existing grid and on the theory supporting the electrical operation of components and systems. The PV system is divided into several sections, each having its own DC-DC converter for maximum power point tracking and a two-level grid connected inverter with different control strategies. The functions of the battery are explored by connecting it to the system in order to prevent possible voltage fluctuations and as a buffer storage in order to eliminate the power mismatch between PV array generation and load demand. Computer models of the system are developed and implemented using the PSCADTM/EMTDCTM software.

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20

Docampo, Pablo. "Electronic properties of mesostructured metal oxides in dye-sensitized solar cells". Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:e97e90f9-47fe-4259-a462-c97f0bf81469.

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Solid-state dye-sensitized solar cells (ssDSCs) offer the possibility of high power conversion efficiencies (PCEs) of over 20%. However, after more than a decade of research, devices still barely reach over 7% PCEs. In this thesis, limitations to device performance are studied in detail, and solutions for future advancement are put forward. In the first part of the thesis, factors limiting charge generation are explored by studying the crystallization environment of mesoporous TiO2 self-assembled through block copolymers. It was found that the density and distribution of sub band gap states are a function of the synthesis conditions and critically affect the performance characteristics of the self-assembled titania used in ssDSCs. As a result, the self-assembled mesoporous oxide system presented in this thesis outperforms for the first time the conventional nanoparticle based electrodes fabricated and tested under the same conditions, with demonstrated PCEs of over 5%. In chapters 6, 7, and 8, the factors limiting the diffusion length and hence, the thickness of the fabricated devices, are carefully examined. Previous literature points towards insufficient pore-filling of the hole transporting material (HTM) as the main limiting factor. In chapter 6, a pore-filling study is shown where a new technique to evaluate the pore-filling fraction of the HTM in the conventional mesoporous metal oxide electrode is also presented and conclude that sufficient pore-filling of thick films can easily be achieved. Another usual strategy to extend the electron lifetime in the devices and thus, the charge diffusion length, involving thin film coatings of insulating metal oxides is examined in chapter 7, with satisfactory results for SnO2-based ssDSCs. The diffusion length can also be extended if the factors limiting the diffusion of charges through the device are identified and removed, as presented in chapter 8. Finally, a study on the stability of the ssDSC is presented in chapter 9. The developments achieved enable long term stability to be effectively targeted, and represent a key milestone towards commercial realization of ssDSCs.

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Hafiz, Shopan d. "Optical investigations of InGaN heterostructures and GeSn nanocrystals for photonic and phononic applications: light emitting diodes and phonon cavities". VCU Scholars Compass, 2016. http://scholarscompass.vcu.edu/etd/4199.

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InGaN heterostructures are at the core of blue light emitting diodes (LEDs) which are the basic building blocks for energy efficient and environment friendly modern white light generating sources. Through quantum confinement and electronic band structure tuning on the opposite end of the spectrum, Ge1−xSnx alloys have recently attracted significant interest due to its potential role as a silicon compatible infra-red (IR) optical material for photodetectors and LEDs owing to transition to direct bandgap with increasing Sn. This thesis is dedicated to establishing an understanding of the optical processes and carrier dynamics in InGaN heterostructures for achieving more efficient visible light emitters and terahertz generating nanocavities and in colloidal Ge1−xSnx quantum dots (QDs) for developing efficient silicon compatible optoelectronics. To alleviate the electron overflow, which through strong experimental evidence is revealed to be the dominating mechanism responsible for efficiency degradation at high injection in InGaN based blue LEDs, different strategies involving electron injectors and optimized active regions have been developed. Effectiveness of optimum electron injector (EI) layers in reducing electron overflow and increasing quantum efficiency of InGaN based LEDs was demonstrated by photoluminescence (PL) and electroluminescence spectroscopy along with numerical simulations. Increasing the two-layer EI thickness in double heterostructure LEDs substantially reduced the electron overflow and increased external quantum efficiency (EQE) by three fold. By incorporating δ p-doped InGaN barriers in multiple quantum well (MQW) LEDs, 20% enhancement in EQE was achieved due to improved hole injection without degrading the layer quality. Carrier diffusion length, an important physical parameter that directly affects the performance of optoelectronic devices, was measured in epitaxial GaN using PL spectroscopy. The obtained diffusion lengths at room temperature in p- and n-type GaN were 93±7 nm and 432±30 nm, respectively. Moreover, near field scanning optical microscopy was employed to investigate the spatial variations of extended defects and their effects on the optical quality of semipolar and InGaN heterostructures, which are promoted for higher efficiency light emitters owing to reduced internal polarization fields. The near-field PL from the c+ wings in heterostructures was found to be relatively strong and uniform across the sample but the emission from the c- wings was substantially weaker due to the presence of high density of threading dislocations and basal plane stacking faults. In case of heterostructures, striated regions had weaker PL intensities compared to other regions and the meeting fronts of different facets were characterized by higher Indium content due to the varying internal field. Apart from being the part and parcel of blue LEDs, InGaN heterostructures can be utilized in generation of coherent lattice vibrations at terahertz frequencies. In analogy to LASERs based on photon cavities where light intensity is amplified, acoustic nanocavity devices can be realized for sustaining terahertz phonon oscillations which could potentially be used in acoustic imaging at the nanoscale and ultrafast acousto-optic modulation. Using In0.03Ga0.97N/InxGa1-xN MQWs with varying x, coherent phonon oscillations at frequencies of 0.69-0.80 THz were generated, where changing the MQW period (11.5 nm -10 nm) provided frequency tuning. The magnitude of phonon oscillations was found to increase with indium content in quantum wells, as demonstrated by time resolved differential transmission spectroscopy. Design of an acoustic nanocavity structure was proposed based on the abovementioned experimental findings and also supported by full cavity simulations. Optical gap engineering and carrier dynamics in colloidal Ge1−xSnx QDs were investigated in order to explore their potential in optoelectronics. By changing the Sn content from 5% to 23% in 2 nm-QDs, band-gap tunability from 1.88 eV to 1.61 eV, respectively, was demonstrated at 15 K, consistent with theoretical calculations. At 15 K, time resolved PL spectroscopy revealed slow decay (3 − 27 μs) of luminescence, due to recombination of spin-forbidden dark excitons and effect of surface states. Increase in temperature to 295 K led to three orders of magnitude faster decay (9 − 28 ns) owing to the effects of thermal activation of bright excitons and carrier detrapping from surface states. These findings on the effect of Sn incorporation on optical properties and carrier relaxation and recombination processes are important for future design of efficient Ge1−xSnx QDs based optoelectronic devices. This thesis work represents a comprehensive optical study of InGaN heterostructures and colloidal Ge1−xSnx QDs which would pave the way for more efficient InGaN based LEDs, realization of terahertz generating nanocavities, and efficient Ge1−xSnx based silicon compatible optoelectronic devices.

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Vijh, Aarohi. "Triple Junction Amorphous Silicon based Flexible Photovoltaic Submodules on Polyimide Substrates". Connect to full text in OhioLINK ETD Center, 2005. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1122656006.

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Kovacik, Peter. "Vacuum deposition of organic molecules for photovoltaic applications". Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:98461a90-5ae3-4ae3-9245-0f825adafa72.

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Organic photovoltaics have attracted considerable research and commercial interest due to their lightness, mechanical flexibility and low production costs. There are two main approaches for the fabrication of organic solar cells – solution and vacuum processing. The former relies on morphology control in polymer-fullerene blends resulting from natural phase separation in these systems. The latter takes advantage of solvent-free processing allowing highly complex multi-junction architectures similar to inorganic solar cells. This work aims to combine the benefits of both by depositing conjugated polymers using vacuum thermal evaporation. By employing this unconventional approach it aims to enhance the efficiency of organic photovoltaics through increased complexity of the thin-film architecture while improving the nanoscale morphology control of the individual active layers. The thesis explores the vacuum thermal deposition of polythiophenes, mainly poly(3-hexylthiophene) (P3HT) and side-group free poly(thiophene) (PTh). A variety of chemical techniques, such as NMR, FT-IR, GPC, DSC and TGA, are used to examine the effect of heating on chemical structure of the polymers. Optimal processing parameters are identified and related to the resulting thin-film morphology and charge transport properties. Efficient photovoltaic devices based on polythiophene donors and fullerene acceptors are fabricated. Materials science techniques AFM, XRD, SEM, TEM and MicroXAM are used to characterize topography and morphology of the thin films, and UV-Vis, EQE, I-V and C-V measurements relate these to the optical and electronic properties. The results of the study show that polymer side groups have a strong influence on molecular packing and charge extraction in vacuum-deposited polymer thin films. Unlike P3HT, evaporated PTh forms highly crystalline films. This leads to enhanced charge transport properties with hole mobility two orders of magnitude higher than that in P3HT. The effect of molecular order is demonstrated on polymer/fullerene planar heterojunction solar cells. PTh-based devices have significantly better current and recombination characteristics, resulting in improved overall power conversion efficiency (PCE) by 70% as compared to P3HT. This confirms that the chemical structure of the molecule is a crucial parameter in deposition of large organic semiconductors. It is also the first-ever example of vacuum-deposited polymer photovoltaic cell. Next, vacuum co-deposited PTh:C60 bulk heterojunctions with different donor-acceptor compositions are fabricated, and the effect of post-production thermal annealing on their photovoltaic performance and morphology is studied. Co-deposition of blended mixtures leads to 60% higher photocurrents than in thickness-optimized PTh/C60 planar heterojunction counterparts. Furthermore, by annealing the devices post-situ the PCE is improved by as much as 80%, achieving performance comparable to previously reported polythiophene and oligothiophene equivalents processed in solution and vacuum, respectively. The enhanced photo-response is a result of favourable morphological development of PTh upon annealing. In contrast to standard vacuum-processed molecular blends, annealing-induced phase separation in PTh:C60 does not lead to the formation of coarse morphology but rather to an incremental improvement of the already established interpenetrated nanoscale network. The morphological response of the evaporated PTh within the blend is further verified to positively differ from that of its small-molecule counterpart sexithiophene. This illustrates the morphological advantage of polymer-fullerene combination over all other vacuum-processable material systems. In conclusion, this processing approach outlines the conceptual path towards the most beneficial combination of solution/polymer- and vacuum-based photovoltaics. It opens up a fabrication method with considerable potential to enhance the efficiency of large-scale organic solar cells production.

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Nadimpally, Bhavananda R. "Copper Indium Diselenide Nanowire Arrays in Alumina Membranes Deposited on Molybdenum and Other Back Contact Substrates". UKnowledge, 2013. http://uknowledge.uky.edu/ece_etds/28.

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Heterojunctions of CuInSe2 (CIS) nanowires with cadmium sulfide (CdS) were fabricated demonstrating for the first time, vertically aligned nanowires of CIS in the conventional Mo/CIS/CdS stack. These devices were studied for their material and electrical characteristics to provide a better understanding of the transport phenomena governing the operation of heterojunctions involving CIS nanowires. Removal of several key bottlenecks was crucial in achieving this. For example, it was found that to fabricate alumina membranes on molybdenum substrates, a thin interlayer of tungsten had to be inserted. A qualitative model was proposed to explain the difficulty in fabricating anodized aluminum oxide (AAO) membranes directly on Mo. Experimental results were used to corroborate this model. Subsequently, a general procedure to use any material that can be deposited using sputtering or evaporation as a back contact for nanowires grown using AAO templates was developed. Experimental work to demonstrate this by transferring thin AAO templates onto flexible Polyimide (PI) substrates was performed. This pattern transfer approach opens doors for a wide variety of applications on almost any substrate. Any material that can be deposited by physical means can then be used as a back contact. Electron-beam induced deposition using a liquid precursor (LP-EBID) was used to selectively grow preconceived patterns of compound semiconductor (CdS) nanoparticles. Stoichiometric CdS nanoparticle patterns were grown successfully using this method. They were structurally and optically characterized indicating high purity deposits. This approach is promising because it marries the precision of e-beam lithography with the versatility of solution based deposition methods.

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Teran-Escobar, Gerardo, David M. Tanenbaum, Eszter Voroshazi, Martin Hermenau, Kion Norrman, Matthew T. Lloyd, Yulia Galagan i in. "On the stability of a variety of organic photovoltaic devices by IPCE and in situ IPCE analyses – the ISOS-3 inter-laboratory collaboration". Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-139279.

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This work is part of the inter-laboratory collaboration to study the stability of seven distinct sets of state-of-the-art organic photovoltaic (OPV) devices prepared by leading research laboratories. All devices have been shipped to and degraded at RISØ-DTU up to 1830 hours in accordance with established ISOS-3 protocols under defined illumination conditions. In this work, we apply the Incident Photon-to-Electron Conversion Efficiency (IPCE) and the in situ IPCE techniques to determine the relation between solar cell performance and solar cell stability. Different ageing conditions were considered: accelerated full sun simulation, low level indoor fluorescent lighting and dark storage. The devices were also monitored under conditions of ambient and inert (N2) atmospheres, which allows for the identification of the solar cell materials more susceptible to degradation by ambient air (oxygen and moisture). The different OPVs configurations permitted the study of the intrinsic stability of the devices depending on: two different ITO-replacement alternatives, two different hole extraction layers (PEDOT:PSS and MoO3), and two different P3HT-based polymers. The response of un-encapsulated devices to ambient atmosphere offered insight into the importance of moisture in solar cell performance. Our results demonstrate that the IPCE and the in situ IPCE techniques are valuable analytical methods to understand device degradation and solar cell lifetime
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich

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26

Teran-Escobar, Gerardo, David M. Tanenbaum, Eszter Voroshazi, Martin Hermenau, Kion Norrman, Matthew T. Lloyd, Yulia Galagan i in. "On the stability of a variety of organic photovoltaic devices by IPCE and in situ IPCE analyses – the ISOS-3 inter-laboratory collaboration". Royal Society of Chemistry, 2012. https://tud.qucosa.de/id/qucosa%3A27818.

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This work is part of the inter-laboratory collaboration to study the stability of seven distinct sets of state-of-the-art organic photovoltaic (OPV) devices prepared by leading research laboratories. All devices have been shipped to and degraded at RISØ-DTU up to 1830 hours in accordance with established ISOS-3 protocols under defined illumination conditions. In this work, we apply the Incident Photon-to-Electron Conversion Efficiency (IPCE) and the in situ IPCE techniques to determine the relation between solar cell performance and solar cell stability. Different ageing conditions were considered: accelerated full sun simulation, low level indoor fluorescent lighting and dark storage. The devices were also monitored under conditions of ambient and inert (N2) atmospheres, which allows for the identification of the solar cell materials more susceptible to degradation by ambient air (oxygen and moisture). The different OPVs configurations permitted the study of the intrinsic stability of the devices depending on: two different ITO-replacement alternatives, two different hole extraction layers (PEDOT:PSS and MoO3), and two different P3HT-based polymers. The response of un-encapsulated devices to ambient atmosphere offered insight into the importance of moisture in solar cell performance. Our results demonstrate that the IPCE and the in situ IPCE techniques are valuable analytical methods to understand device degradation and solar cell lifetime.
Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG-geförderten) Allianz- bzw. Nationallizenz frei zugänglich.

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Chmielewski, Daniel Joseph. "III-V Metamorphic Materials and Devices for Multijunction Solar Cells Grown via MBE and MOCVD". The Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1534707692114982.

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Smith, Thomas. "Studies of p-type semiconductor photoelectrodes for tandem solar cells". Thesis, Loughborough University, 2014. https://dspace.lboro.ac.uk/2134/14522.

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Photoelectrodes and photovoltaic devices have been prepared via multiple thin film deposition methods. Aerosol assisted chemical vapour deposition (AACVD), electrodeposition (ED), chemical bath deposition (CBD) and doctor blade technique (DB) have been used to deposit binary and ternary metal oxide films on FTO glass substrates. The prepared thin films were characterised by a combination of SEM (Scanning Electron Microscopy), powder X-ray diffraction, mechanical strength tests and photochemical measurements. Nickel oxide (NiO) thin films prepared by AACVD were determined to have good mechanical strength . with a photocurrent of 7.6 μA cm-2 at 0 V and an onset potential of about 0.10 V. This contrasted with the dark current density of 0.3 μA cm-2 at 0 V. These NiO samples have very high porosity with crystalline columns evidenced by SEM. In comparison with the AACVD methodology, NiO films prepared using a combination of ED and DB show good mechanical strength but a higher photocurrent of 24 μA cm-2 at 0 V and an onset potential of about 0.10 V with a significantly greater dark current density of 7 μA cm-2 at 0 V. The characteristic features shown in the SEM are smaller pores compared to the AACVD method. Copper (II) oxide (CuO) and copper (I) oxide (Cu2O) films were fabricated by AACVD by varying the annealing temperature between 100-325°C in air using a fixed annealing time of 30 min. It was shown by photocurrent density (J-V) measurements that CuO produced at 325 °C was most stable and provided the highest photocurrent of 173 μA cm-2 at 0 V with an onset potential of about 0.23 V. The alignment of zinc oxide (ZnO) nano-rods and nano-tubes fabricated by CBD have been shown to be strongly affected by the seed layer on the FTO substrate. SEM images showed that AACVD provided the best seed layer for aligning the growth of the nano-rods perpendicular to the surface. Nano-rods were successfully altered into nano-tubes using a potassium chloride bath etching method. NiO prepared by both AACVD and the combined ED/DB method were sensitized to absorb more of the solar spectrum using AACVD to deposit CuO over the NiO. A large increase in the photocurrent was observed for the p-type photoelectrode. These p-type photoelectrode showed a photocurrent density of approximately 100 μA cm-2 at 0 V and an onset potential of 0.3 V. This photocathode was then used as a base to produce a solid state p-type solar cell. For the construction of the solid state solar cells several n-type semiconductors were used, these were ZnO, WO3 and BiVO4. WO3 and BiVO4 were successfully produced with BiVO4 proving to be the optimum choice. This cell was then studied more in depth and optimised by controlling the thickness of each layer and annealing temperatures. The best solid state solar cell produced had a Jsc of 0.541 μA cm-2 (541 nA) and a Voc of 0.14 V, TX146 made up of NiO 20 min, CuFe2O4 50 min, CuO 10 min, BiVO4 27 min, using AACVD and then annealed for 30 min at 600°C.

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29

Saliba, Michael. "Plasmonic nanostructures and film crystallization in perovskite solar cells". Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:fdb36a9e-ddf5-4d27-a8dc-23fffe32a2c5.

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The aim of this thesis is to develop a deeper understanding and the technology in the nascent field of solid-state organic-inorganic perovskite solar cells. In recent years, perovskite materials have emerged as a low-cost, thin-film technology with efficiencies exceeding 16% challenging the quasi-paradigm that high efficiency photovoltaics must come at high costs. This thesis investigates perovskite solar cells in more detail with a focus on incorporating plasmonic nanostructures and perovskite film formation. Chapter 1 motivates the present work further followed by Chapter 2 which offers a brief background for solar cell fabrication and characterisation, perovskites in general, perovskite solar cells in specific, and plasmonics. Chapter 3 presents the field of plasmonics including simulation methods for various core-shell nanostructures such as gold-silica and silver-titania nanoparticles. The following Chapters 4 and 5 analyze plasmonic core-shell metal-dielectric nanoparticles embedded in perovskite solar cells. It is shown that using gold@silica or silver@titania NPs results in enhanced photocurrent and thus increased efficiency. After photoluminescence studies, this effect was attributed to an unexpected phenomenon in solar cells in which a lowered exciton binding energy generates a higher fraction of free charge. Embedding thermally unstable silver NPs required a low-temperature fabrication method which would not melt the Ag NPs. This work offers a new general direction for temperature sensitive elements. In Chapters 6 and 7, perovskite film formation is studied. Chapter 6 shows the existence of a previously unknown crystalline precursor state and an improved surface coverage by introducing a ramped annealing procedure. Based on this, Chapter 7 investigates different perovskite annealing protocols. The main finding was that an additional 130°C flash annealing step changed the film crystallinity dramatically and yielded a higher orientation of the perovskite crystals. The according solar cells showed an increased photocurrent attributed to a decrease in charge carrier recombination at the grain boundaries. Chapter 8 presents on-going work showing noteworthy first results for silica scaffolds, and layered, 2D perovskite structures for application in solar cells.

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30

Stolle, Carl Jackson. "Low cost processing of CuInSe2 nanocrystals for photovoltaic devices". Thesis, 2015. http://hdl.handle.net/2152/30473.

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Semiconductor nanocrystal-based photovoltaics are an interesting new technology with the potential to achieve high efficiencies at low cost. CuInSe2 nanocrystals have been synthesized in solution using arrested precipitation and dispersed in solvent to form a “solar ink”. The inks have been deposited under ambient conditions to fabricate photovoltaic devices with efficiency up to 3%. Despite the low cost spray coating deposition technique, device efficiencies remain too low for commercialization. Higher efficiencies up to 7% have been achieved using a high temperature selenization process, but this process is too expensive. New nanocrystal film treatment processes are necessary which can improve the device efficiency at low cost. To this end, CuInSe2 nanocrystals were synthesized using a diphenyl phosphine:Se precursor which allows for precise control over the nanocrystal size. The size is controlled by changing the temperature of the reaction. The smallest size nanocrystals demonstrated extremely high device open circuit voltage. Ligand exchange procedures were used to replace the insulating oleylamine capping ligand used during synthesis with more conductive halide ions or inorganic chalcogenidometallate cluster (ChaM) ligands. These ligands led to improved charge transport in the nanocrystal films. A high-intensity pulsed light processing technique known as photonic curing was used which allows for high temperature sintering of nanocrystal films on temperature-sensitive substrates. High energy pulses cause the nanocrystals to sinter into large grains, primarily through melting and resolidification. The choice of metal back contact has a dramatic effect on the final film morphology, with Au and MoSe2 back contacts providing much better adhesion with the CuInSe2 than Mo back contacts. Nanocrystal sintering without melting can be achieved by replacing the oleylamine ligands with ChaM ligands prior to photonic curing. Low energy photonic curing pulses vaporize the oleylamine ligands without inducing sintering or grain growth. This greatly improved nanocrystal coupling and interparticle charge transport. Multiexcitons were successfully extracted from these nanocrystal films and external quantum efficiencies over 100% were observed. Transient absorption spectroscopy was used to study the multiexciton generation process in CuInSe2 nanocrystal films and colloidal suspensions. The multiexciton generation efficiency, threshold, and Auger lifetimes for CuInSe2 compare well with other nanocrystal materials.
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31

Abulikemu, Mutalifu. "Synthesis and Characterization of Colloidal Metal and Photovoltaic Semiconductor Nanocrystals". Diss., 2014. http://hdl.handle.net/10754/335794.

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Metal and semiconducting nanocrystals have received a great deal of attention from fundamental scientists and application-oriented researchers due to their physical and chemical properties, which differ from those of bulk materials. Nanocrystals are essential building blocks in the development of nanostructured devices for energy conversion. Colloidal metals and metal chalcogenides have been developed for use as nanocrystal inks to produce efficient solar cells with lower costs. All high-performing photovoltaic nanocrystals contain toxic elements, such as Pb, or scarce elements, such as In; thus, the production of solution-processable nanocrystals from earth-abundant materials using environmentally benign synthesis and processing methods has become a major challenge for the inorganic semiconductor-based solar field. This dissertation, divided into two parts, addresses several aspects of these emerging challenges. The first portion of the thesis describes the synthesis and characterization of nanocrystals of antimony sulfide, which is composed of non-scarce and non-toxic elements, and examines their performance in photovoltaic devices. The effect of various synthetic parameters on the final morphology is explored. The structural, optical and morphological properties of the nanocrystals were investigated, and Sb2S3 nanocrystal-based solid-state semiconductor-sensitized solar cells were fabricated using different deposition processes. We achieved promising power conversion efficiencies of 1.48%. The second part of the thesis demonstrates a novel method for the in situ synthesis and patterning of nanocrystals via reactive inkjet printing. The use of low-cost manufacturing approaches for the synthesis of nanocrystals is critical for many applications, including photonics and electronics. In this work, a simple, low-cost method for the synthesis of nanocrystals with minimum size variation and waste using reactive inkjet printing is introduced. As a proof of concept, the method was used for the in situ synthesis of gold nanoparticles as a model system. Relatively monodisperse gold nanoparticles were produced. The size and shape of gold nanoparticles can be controlled by the gold precursor and surfactant concentration in the ‘ink.’ This approach can be extended to the synthesis of other nanocrystals and is thus a truly impactful process for the low-cost synthesis of materials and devices incorporating nanocrystals.

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32

Yang, Jia-Ting, i 楊家婷. "Preparation and Characterization of Anisotropic Nanocrystals and its Application in Photovoltaic Devices". Thesis, 2012. http://ndltd.ncl.edu.tw/handle/dw7756.

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碩士
國立虎尾科技大學
材料科學與綠色能源工程研究所
100
In this study, the blends of organic polymer (P3HT) with inorganic nanocrystals (NCs) as a acceptor/donor active layer of hybrid photovoltaic device are studied. The CdSe NCs are synthesized by colloidal route using the LA/HDA and HPA/TOPO as ligands. The optical properties and crystal structure of CdSe is elucidated by UV-vis, FL, TEM and XRD analysis. The result shows that the CdSe-LA/HDA is spherical (QDs) and the particle size ranges from 4 to 4.5±0.4 nm with increasing the reaction time from 3 to 30 min. The emission and absorption wavelengths of CdSe-LA/HDA are between 606 to 617 nm and 600 to 607 nm, respectively. On the other hand, the CdSe-HPA/TOPO NCs is anisotropic and the morphology changes from nanorods (NRs) to tetrapods (TPs) with increasing the reaction time from 1 to 30 min, and the absorption wavelengths ranges from 550 to 600 nm. In order to increase the separation rate of carriers, the heterostructure of CdSe@PbSe-TPs NCs is prepared. The average diameter and length of the arms increase from 3.3±0.5 to 8±1.9 nm and 20.8±2.5 to 27.4±6.3 nm and the absorption range extends more than 1000 nm. The performance of the organic/inorganic (O/I) hybrid solar cell is measured by sun simulator under AM 1.5 (1000 W/m2) and the photovoltaic I-V curve is collected. The result reveals that the emission property of P3HT is quenched, and the surface roughness increases after annealing in the vacuum. When the ratio of P3HT and CdSe@PbSe-TPs is 1:0.6, shows the best conversion efficiency. Heterostructure and anisotropic structure of CdSe@PbSe-TPs NCs resulting in the separation rate of excitons increases under AM1.5. The open-circuit voltage (Voc), short-circuit current (Isc), fill factor (FF) and conversion efficiency of O/I hybrid solar cell are 0.76 V, 36.0 pA, 0.36 and 0.98×10-6 %, respectively.

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33

Huang, Hong-Heng, i 黃洪恆. "Preparation and Characterization of CdTe Nanocrystals and the Application in Photovoltaic Devices". Thesis, 2014. http://ndltd.ncl.edu.tw/handle/6v9byw.

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碩士
國立虎尾科技大學
材料科學與綠色能源工程研究所
102
In this study, the blends of organic polymer (P3HT:PCBM) with inorganic nanocrystals (NCs) as a acceptor/donor active layer of hybrid photovoltaic (PV) device are studied. The CdTe NCs are synthesized by colloidal route using the LA/HPA, LA/TOPO and LA/HPA+TOPO (called LA/Mix) as ligands. On the other hand, CdexcessTe and CdTeexcess NCs also are prepared by mixing ligands without using complex agents. The optical properties and crystal structures of CdTe NCs are elucidated by UV-vis, FL, TEM and XRD. The result shows that all NCs are nanorods (NRs) and the average diameter and length ranges from 4.8 to 7.5 nm and 7.2 to 11.8 nm with increasing the reaction time from 3 to 30 min. The absorption ranges of all NCs are between 500 to 776 nm. On the other hand, the major morphology of NCs is spherical (QDs) and only small portion are nanorods (NRs) by using HDA. The absorption of ranged is in the NCs from 500 to 720 nm, and the average diameter and length are 6.5±0.7 and 9.27±0.6 nm, respectively. LA-CdTe-H (QDs/NRs, absorption wavelength is 668 nm), LA-CdTe-T30 (NRs, absorption wavelength is 776 nm) and LA-CdTe-Mix (NRs, absorption wavelength is 690 nm) are usd acceptor materials in the organic/inorganic hybrid photovoltaic devices (O/I PV). The performance of the O/I PVs are measured by sun simulator under AM 1.5 (100 mW/cm2) and the photovoltaic I-V curves are collected. The result reveals that the short-circuit current density (Isc), open-circuit voltage (Voc), fill factor (FF) and conversion efficiency are 9.5 mA/cm2, 0.61 V, 0.59 and 3.44 % for adding 10 wt% of LA-CdTe-T30 into P3HT:PCBM. This sample has the best conversion efficiency in this study.

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34

Lokteva, Irina [Verfasser]. "Synthesis and surface characterization of semiconductor nanocrystals for photovoltaic application / von Irina Lokteva". 2010. http://d-nb.info/1007552646/34.

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35

Ntholeng, Nthabiseng. "Synthesis and characterization of Cu-based telluride semiconductor materials for application in photovoltaic cells". Thesis, 2017. http://hdl.handle.net/10539/23532.

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Submitted to the Faculty of Science, School of Chemistry at University of the Witwatersrand, in partial fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 05 June 2017.
The colloidal method has extensively been used to synthesize ternary and quaternary copper sulfides and selenides. Although tellurides form part of the chalcogenides, little has been reported on them particularly the synthesis of these nanostructures. Achieving high-quality nanocrystals through colloidal synthesis requires thorough monitoring of parameters such as time, solvent, precursor as they affect nucleation and growth of the nanocrystals. Herein, we report on the colloidal synthesis of ternary CuInTe2 and quaternary CuIn1-xGaxTe2 nanostructured semiconductor materials. A typical synthesis of CuInTe2 entailed varying reaction temperature. At temperatures below 250 °C, no formation of CuInTe2 was seen. At 250 °C formation of CuInTe2 could be observed with the formation of binary impurities. A change in the sequence in which precursors were added at 250 °C yielded pure CuInTe2. Applying different surfactants aided in achieving differently structured morphologies of CuInTe2 nanocrystals. Morphology varied from rods, cubes, nanosheets etc. Different morphologies resulted in different optical properties with the high optical band gap of 1.22 eV measured for 1D rods. Different precursors were employed in the synthesis of quaternary CuIn1-xGaxTe2. Precursor 2 (entailed the use of Cu (acac)2, In (acac)3 and Ga(acac)3) yielded pure CuIn1-xGaxTe2 phase with no formation of impurities. Variation in reaction time influenced the optical properties of the quaternary CuIn1-xGaxTe2 with high band gap obtained at low reaction time (30 min). A change in Ga and In concentration resulted in reduced lattice parameters a and c with lowest values obtained with the highest Ga concentration. However, achieving the intended concentration proved challenging due to the loss of the material during synthesis. Increasing the Ga concentration resulted in a high optical band gap. Conducting the reaction with Hexadecylamine (HDA) resulted in a relatively high optical band though the formation of impurities was evident. The obtained band gap can be attributed to small sized particles as evident from TEM results. Heterojunction ZnO/CIT and ZnO/CIGT solar cell devices were fabricated through a simple solution approach. The performance of ZnO/CIGT device was superior to that of ZnO/CIT in which efficiency increased from 0.26-0.78%. In the ZnO/CIT device, high Voc of 880 mV was recorded while 573.66 mV was measured for ZnO/CIGT device. Chemical and thermal treatments were performed on the ZnO/CIGT devices. The efficiency increased from 0.78 1.25% when the device was chemically treated with a short-chain EDT ligand. A high conversion efficiency of 2.14% was recorded for devices annealed at 300 °C. High annealing temperatures resulted in poor device performance with the lowest efficiency of 0.089% obtained at annealing temperatures of 500 °C attributed to the leaching out of In and Ga into the ZnO layer.
LG2017

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36

"The chemical deposition of semiconductor thin-films for photovoltaic devices". Tulane University, 1999.

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Initially, possible precursors to metal sulfide films formed by metal-organic chemical vapor deposition (MOCVD), the standard commercial technique for manufacturing photovoltaic semiconductors, were synthesized. Triple-junction GaInP 2/GaAs/Ge solar cells, prepared by this method, were studied to understand how chemical properties and material defects can effect the performance of photovoltaic devices. Finally, novel methods for the low-temperature, solution growth of CdS, CdSe, and CuInSe2 photovoltaic materials were targeted which will reduce manufacturing costs and increase the economic feasibility of solar energy conversion A series of dialkyldithiocarbamate copper, gallium and indium compounds were studied as possible metal sulfide MOCVD precursors. Metal powders were oxidized by dialkylthiurams in 3- or 4-methylpyridine using standard techniques for handling air and moisture-sensitive compounds. Metal chlorides reacted directly with the sodium dialkyldithiocarbamate salts. In these complexes, the metal was found in a roughly octahedral orientation, surrounded by dithiocarbamate ligands and/or solvent molecules Triple-junction GaInP2/GaAs/Ge cells were composed of thin-films of GaInP2 and GaAs grown monolithically on top of a germanium substrate. Each layer of semiconductor material had a different bandgap and absorbed a different portion of the solar spectrum, thus improving the overall efficiency of the cell. Work focused on dark current-voltage behavior which is known to limit solar cell open-circuit voltage, fill factor, and conversion efficiency. Cells were studied using microscopic and spectroscopic techniques to correlate the effect of physical defects in the materials with poor performance of the devices as evaluated through current vs. voltage measurements Films of US and CdSe were readily prepared in solution through an 'ion-by-ion' deposition of Cd2+ and S2- (or Se 2-) generated from the slow hydrolysis of thiourea (or dimethylthiourea). The bath chemistry was carefully controlled by the adjustment of pH to slow hydrolysis and with chelating agents to sequester the cadmium ions. Triethanolamine and ethylenediamine were both effective chelators with the latter producing thicker, clearer films. Finally, US films were grown over electrodeposited CuInSe2 to form working photovoltaic devices In summary, contributions were made which (a) advance current methods for manufacturing photovoltaic semiconductors and (b) offer an alternative route to producing new forms of thin-film solar cell devices
acase@tulane.edu

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37

Lee, Jong Jin Kwong Dim-Lee. "A study on the nanocrystal floating-gate nonvolatile memory". 2005. http://repositories.lib.utexas.edu/bitstream/handle/2152/1975/leej77040.pdf.

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38

Lee, Jong Jin. "A study on the nanocrystal floating-gate nonvolatile memory". Thesis, 2005. http://hdl.handle.net/2152/1975.

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39

Liu, Yueran 1975. "Novel flash memory with nanocrystal floating gate". Thesis, 2006. http://hdl.handle.net/2152/2819.

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40

Akhavan, Vahid Atar. "Photovoltaic devices based on Cu(In1-xGax)Se2 nanocrystal inks". Thesis, 2011. http://hdl.handle.net/2152/ETD-UT-2011-08-4285.

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Thin film copper indium gallium selenide (CIGS) solar cells have exhibited single junction power conversion efficiencies above 20% and have been commercialized. The large scale production of CIGS solar cells, however, is hampered by the relatively high cost and poor stoichiometric control of coevaporating tertiary and quaternary semiconductors in high vacuum. To reduce the overall cost of production, CIGS nanocrystals with predetermined stoichiometry and crystal phase were synthesized in solution. Colloidal nanocrystals of CIGS provide a novel route for production of electronic devices. Colloidal nanocrystals combine the well understood device physics of inorganic crystalline semiconductors with the solution processability of amorphous organic semiconductors. This approach reduces the overall cost of CIGS manufacturing and can be used to fabricate solar cells on flexible and light-weight plastic substrates. As deposited CIGS nanocrystal solar cells were fabricated by ambient spray-deposition. Devices with efficiencies of 3.1% under AM1.5 illumination were fabricated. Examining the external and internal quantum efficiency spectrums of the devices reveal that in nanocrystal devices only the space charge region is actively contributing to the extracted photocurrent. The device efficiency of the as-deposited nanocrystal films is presently limited by the small crystalline grains (≈ 15 nm) in the absorber layer and the relatively large interparticle spacing due to the organic capping ligands on the nanocrystal surfaces. Small grains and large interparticle spacing limits high density extraction of electrons and holes from the nanocrystal film. A Mott-Schottky estimation of the space charge region reveals that only 50 nm depth of the nanocrystalline absorber is effectively contributing to the photogenerated current. One strategy to improve charge collection involves increased space charge region for extraction by vertical stacking of diodes. A much longer absorption path for the photons exists in the space charge region with the stacked devices, increasing the probability that the incident radiation is absorbed and then extracted. This method enables an increase in the collected short circuit current. The overall device efficiency, however, suffers with the increased series resistance and shunt conductance of the device. Growth of nanocrystal grains was deemed necessary to achieve power conversion efficiencies comparable to vapor deposited CIGS films. Simple thermal treatment of the nanocrystal layers did not contribute to the growth of the crystalline grain size. At the same time, because of the loss of selenium and increased trap density in the absorber layer, there was a measurable decrease in device efficiency with thermal processing. For increased grain size, the thermal treatment of the absorber layer took place in presence of compensating amounts of selenium vapor. The process of selenization, as it is called, took place at 500°C in a graphite box and led to an increase of the grain size from 15 nm to several microns in diameter. Devices with the increased grain size yielded efficiencies up to 5.1% under AM1.5 radiation. Mott-Schottky analysis of the selenized films revealed a reduction in doping density and a comparable increase in the space-charge region depth with the increased grain size. The increased collection combined with the much higher carrier mobility in the larger grains led to achieved Jsc values greater than 20 mA/cm2. Light beam induced current microscopy (LBIC) maps of the devices with selenized absorber layers revealed significant heterogeneity in photogenerated current. Distribution of current hotspots in the film corresponded with highly selenized regions of the absorber films. In an effort to improve the overall device efficiency, improvements in the selenization process are necessary. It was determined that the selenization procedure is dependent on the selenization temperature and processing environment. Meanwhile, the reactor geometry and nanocrystal inks composition played important roles in determining selenized film morphology and the resulting device efficiency. Further work is necessary to optimize all the parameters to improve device efficiency even further.
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41

Chien, Chen-Yu, i 簡振宇. "Preparation of CdSe Nanocrystals with Different Morphologies and the Application in Organic Photovoltaic Devices". Thesis, 2013. http://ndltd.ncl.edu.tw/handle/36329215288265930674.

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碩士
國立中央大學
材料科學與工程研究所
101
In this study, CdSe quantum dots (QDs) with different morphologies have been synthesized and applied as the donor in the active layer in the OPV devices. CdSe nanocrystals (NCs) are synthesized by using trioctylphosphine oxide (TOPO)/ hexylphosphonic acid (HPA), hexadecylamine (HDA)/HPA, and oleic acid (OA)/ octadecene (ODE) as surfactants. Besides, CdSe tetrapods with zinc-blend seeds and wurtzite arms are prepared by seed growth method. After that, CdSe NCs are mixed with P3HT:PCBM and used as the active layer of the OPV devices. The morphologies, structures, surface chemical states, chemical compositions, optical properties, and solar cell efficiencies are detected by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma – atomic emission spectrometer (ICP-AES), UV-visible absorption spectroscopy (UV-vis)/fluorescence (FL), and high fidelity solar simulator/IV measurement (IV), respectively. CdSe-T NCs prepared by TOPO/HPA have tetrapod morphology and wurtzite structure when reacting after 10 mins. T60 sample has the diameter and length about 4.6 and 16.7 nm, respectively. The morphology of CdSe-H samples prepared by HDA/HPA is branches, and the largest diameter and length is about 4.2 and 26.4 nm, respectively. Compared with CdSe-T samples, the bonding of HDA to the NC surface is stronger and the growth rate of NCs is lower. In terms of the CdSe-O prepared by OA/ODE, their morphology is tetrapod with the shortest length of arm among all samples. Seed growth synthesis can produce a large amount of CdSe tetrapods with length about 14.0 nm and zinc-blend core/wurtzite arm structure. The Cd is electron supplier and Se is acceptor for the prepared CdSe tetrapods. The addition of T60, H60, and CdSe-SG samples can promote JSC from 9.6 to 10.3, 10.8, and 10.9 mA/cm2, and efficiency from 3.80 to 4.04, 4.17, and 4.30 %, respectively due to the enhancement in the light absorption ability and high balanced charge carrier mobility. When the concentrations of CdSe-SG increases from 0 to 25 and 80 mg, JSC changes from 9.6 to 10.9 and 9.4 mA/cm2, and efficiency changes from 3.80 to 4.30 and 3.19 %, respectively, suggesting that appropriate CdSe content in the active layer is essential for the transport of electrons and light absorption in the OPV devices.

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42

Sarkar, Joy 1977. "Non-volatile memory devices beyond process-scaled planar Flash technology". Thesis, 2007. http://hdl.handle.net/2152/3666.

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Mainstream non-volatile memory technology dominated by the planar Flash transistor with continuous floating-gate has been historically improved in density and performance primarily by means of process scaling, but is currently faced with significant hindrances to its future scaling due to fundamental constraints of electrostatics and reliability. This dissertation is based on exploring two pathways for circumventing scaling limitations of the state-of-the-art Flash memory technology. The first part of the dissertation is based on demonstrating a vertical Flash memory transistor with nanocrystal floating-gate, while the second part is based on developing fundamental understanding of the operation of Phase Change Memory. A vertical Flash transistor can allow the theoretical minimum cell area and a nanocrystal floating-gate on the sidewalls is shown to allow a thinner gate-stack further conducive to scaling while still providing good reliability. Subsequently, the application of a technique of protein-mediated assembly of preformed nanocrystals to the sidewalls of the vertical Flash transistor is also demonstrated and characterized. This technique of ordering pre-formed nanocrystals is beneficial towards achieving reproducible nanocrystal size uniformity and ordering especially in a highly scaled vertical Flash cell, rendering it more amenable to scaling and manufacturability. In both forms, the vertical Flash memory cell is shown to have good electrical characteristics and reliability for the viability of this cell design and implementation. In the remaining part of this dissertation, studies are undertaken towards developing fundamental understanding of the operational characteristics of Phase Change Memory (PCM) technology that is expected to replace floating-gate Flash technology based on its potential for scaling. First, a phenomenon of improving figures of merit of the PCM cell with operational cycles is electrically characterized. Based on the electrical characterization and published material characterization data, a physical model of an evolving "active region" of the cell is proposed to explain the improvement of the cell parameters with operational cycles. Then, basic understanding is developed on early and erratic retention failure in a statistically significant number of cells in a large array and, electrical characterization and physical modeling is used to explain the mechanism behind the early retention failure.

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Alattar, Yousef. "A Study of SAM Modified ZnO in Hybrid Bilayer ZnO/P3HT Photovoltaic Devices". 2013. http://hdl.handle.net/10222/35440.

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Hybrid organic/inorganic solar cells such as ZnO/P3HT offer promise in increasing efficiency of organic-based devices. However there are many unresolved issues such as poor short-circuit current and open-circuit voltage that are hampering their widespread, commercial use. It is thought that surface trap states on ZnO are providing an open avenue for carrier recombination thus creating devices with poor current transport characteristics. Using self assembled monolayers (SAMs) may provide some key answers and solutions to this problem by passivating trap states. In the course of this work, benzoic acid, 4-aminobenzoic acid, 4-methoxybenzoic acid, phenylphosphonic acid, and 4-methoxyphenylphosphonic acid SAMs were studied in large part due to their commercial availability. It was found that the phenylphosphonic acids had a clear impact on decreasing dark current; therefore strongly suggesting that exciton recombination has been inhibited to some degree. These molecules also caused a decrease in efficiency by an order of magnitude as compared to a plain ZnO/P3HT bilayer cell (standard). There were pronounced negative effects on the other device parameters such as open circuit voltage and short circuit current. In the case of 4-methoxybenzoic acid and benzoic acid the effects are not so clear in that parts of the dark J-V curve indicate a decrease in dark current while other regions show an increase. Interestingly for the negative effect on efficiency and other device parameters was not as pronounced as the phenylphosphonic acids. In both cases it is hypothesized that because of their wide band gaps and poor energy level matching, they ultimately impact device performance negatively. In the future, use of simulations to determine optimal SAM molecular structures that can be synthesized in the lab or purchased commercially is suggested.

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Tang, Shan 1975. "Protein-mediated nanocrystal assembly for floating gate flash memory fabrication". 2008. http://hdl.handle.net/2152/18156.

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As semiconductor device scaling is reaching the 45 nm node, the need for novel device concept, architecture and new materials has never been so pressing as today. Flash memories, the driving force of semiconductor memory market in recent years, also face the same or maybe more severe challenges to meet the demands for high-density, low-cost, low-power, high-speed, better endurance and longer retention time. As traditional continuous floating gate flash struggles to balance the trade-off between high speed and retention requirement, nanocrystal (NC) floating gate flash has attracted more and more interest recently due to its advantages over traditional flash memories in many areas such as better device scaling, lower power consumption and improved charge retention. However, there are still two major challenges remaining for embedded NC synthesis: the deposition method and the size and distribution control. Nowadays using bio-nano techniques such as DNA, virus or protein for NC synthesis and assembly has become a hot topic and feasible for actual electronic device fabrication. In this dissertation a new method for NC deposition wherein a colloidal suspension of commercially-available NCs was organized using a self-assembled chaperonin array. The chaperonin array was applied as a scaffold to mediate NCs into an assembly with uniform spatial distribution on Si wafers. By using this method, we demonstrated that colloidal PbSe and Co NCs in suspension can self-assemble into ordered arrays with a high density of up to 10¹²cm⁻². MOSCAP and MOSFET memory devices were successfully fabricated with the chaperonin protein mediated NCs, showing promising memory functions such as a large charge storage capacity, long retention time and good endurance. The charge storage capacity with respect to material work function, NC size and density was explored. In addition to NC engineering, the tunnel barrier was engineered by replacing traditional SiO₂ by high-k material HfO₂, giving a higher write/erase speed with a reduced effective oxide thickness (EOT). Suggestions for future research in this direction are presented in the last part of this work.
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McMurtry, Brandon Makana. "Synthesis and Formation Mechanism of Metal Phosphide and Chalcogenide Nanocrystals". Thesis, 2021. https://doi.org/10.7916/d8-nfgk-at97.

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Semiconductor nanocrystals, or quantum dots, have attracted significant interest for use in solid state lighting, biological imaging, photovoltaics, catalysis, and displays such as televisions or tablets. Quantum dots excel in these applications because of their narrow emission profiles, high absorptivity at high energies, and optoelectronic properties that can be easily tuned using colloidal chemistry. The last point in particular has driven the development of new synthetic methods for producing a range of semiconducting materials on the nanoscale. Academically, interest in the synthesis of quantum dots has also extended to the mechanism of their formation and its implications for the growth of nanoscale crystals more generally. This thesis addresses facets of both points above, first by developing several novel syntheses for indium and gallium phosphide nanocrystals, and second by leveraging the synthetic control it allows to study the mechanisms of homogeneous crystal growth. Chapter 1 provides a brief overview of the colloidal syntheses, optoelectronic properties, and formation mechanisms of quantum dots. Emphasis is placed on the development of new chemical syntheses for nanoscale materials and how the size, size distribution, and morphology can be carefully controlled by thoughtful reaction design. The progression of quantum dot synthesis is presented and specific innovations to the precursor and surfactant design are highlighted. Next, a brief discussion about nanocrystal surface chemistry and its impact on the photophysical properties of the inorganic core is described along with its proposed influence on the kinetics of nanocrystal growth. Finally, classical theories of homogeneous crystal growth are presented and used to explain the origin of the exceptionally narrow size distributions accessible in a wide range of materials. Chapter 2 introduces two novel synthetic pathways to InP nanocrystals. The first describes a small library of substituted aminophosphines that can control the precursor conversion reactivity by over an order of magnitude. Leveraging the collection of aminophosphines, it is demonstrated that at sufficiently high temperatures, the rate of precursor conversion can be used to vary the final nanocrystal size—disputing previous findings for InP nanocrystals. We show that the reactivity of the phosphine is governed by a pre-equilibrium between the precursor and an intermediate (P(NHR)3) that goes on to form InP. Variations to the initial aminophosphine substitution pattern change the position of the pre-equilibrium, thereby allowing the rate of [InP]i deposition to be controlled. The second synthetic method leverages metal phosphonate salts as a surfactant to synthesize large samples of InP. We find that the nanocrystals grow via a ripening mechanism and display excellent crystallinity as determined by powder X-ray diffraction and pair distribution function analysis. Finally, we demonstrate that the final nanocrystals are bound by both phosphonates and phosphines through the use of 31P nuclear magnetic resonance spectroscopy. Chapter 3 expands on the syntheses of InP in the previous chapter by developing methods to form GaP, InxGa1-xP, and InP-based core-shell structures. At the onset, two distinct syntheses of GaP are introduced, one similar to the metal phosphonate route used to form InP, and one that used a mixture of amines to stabilize GaP colloidally. The phosphonate method results in small GaP with somewhat indistinct scattering patterns, while the amine method results in large GaP whose morphology can be varied depending on the solvent selected. Leveraging the newly developed InP and GaP syntheses we demonstrate that InxGa1-xP alloys could be directly synthesized from mixtures of In3+ and Ga3+ salts. We also show that InxGa1-xP can be accessed indirectly via cation exchange of Zn3P2 or Cd3P2, however attempts at synthesizing alloys via cation exchange with phosphonate bound GaP were found to be largely unsuccessful. Finally, the chapter contains initial attempts at synthesizing GaP/InP core-shells with the intention of producing GaP/InP/GaP spherical quantum well architectures. Preliminary data show that InP can be deposited using several different methods, though it remains unclear whether the optical properties will be suitable for integration in solid state lighting applications. Chapter 4 examines the crystal growth processes that precede the formation of monodisperse ensembles of InP, PbS, and PbSe nanocrystals. Surprisingly, we find that nucleation persists for a substantial portion of the total reaction time—a stark departure from the canonical “burst” of nucleation proposed originally by Victor LaMer. We go on to measure the nucleation period for a variety of different reaction conditions and find that the fraction of reaction time nucleation extends over is sensitive to both the material and reaction temperature. This is consistent with a mechanism where faster kinetics of monomer attachment reduce the duration of crystal nucleation—a conclusion that can be surmised by nucleation mass balance models that show a clear material and temperature dependence on the rate of nanocrystal growth. We also interrogate the claim that solute molecules accumulate prior to the formation of mature nanostructures. In situ X-ray experiments clearly corroborate the appearance of solute-like species at early reaction times that build up prior to the appearance of crystals with extended structure. Finally, we propose a novel size-focusing mechanism predicated on a size dependent growth rate. Using population mass balance modeling we show that the measurements of size and size distribution are qualitatively consistent with a growth rate inversely proportional to nanocrystal size.

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Heng, C. L., Wee Kiong Choi, Wai Kin Chim, L. W. Teo, Vincent Ho, W. W. Tjiu i Dimitri A. Antoniadis. "Charge Storage Effect in a Trilayer Structure Comprising Germanium Nanocrystals". 2002. http://hdl.handle.net/1721.1/3969.

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A metal-insulator-semiconductor (MIS) device with a trilayer insulator structure consisting of sputtered SiO₂ (~50nm)/evaporated pure germanium (Ge) layer (2.4nm)/rapid thermal oxide (~5nm) was fabricated on a p-type Si substrate. The MIS device was rapid thermal annealed at 1000°C. Capacitance-voltage (C-V) measurements showed that, after rapid thermal annealing at 1000°C for 300s in Ar, the trilayer device exhibited charge storage property. The charge storage effect was not observed in a device with a bilayer structure without the Ge middle layer. With increasing rapid thermal annealing time from 0 to 400s, the width of the C-V hysteresis of the trilayer device increased significantly from 1.5V to ~11V, indicating that the charge storage capability was enhanced with increasing annealing time. High-resolution transmission electron microscopy results confirmed that with increasing annealing time, the 2.4nm amorphous middle Ge layer crystallized gradually. More Ge nanocrystals were formed and the crystallinity of the Ge layer improved as the annealing time was increased. When the measurement temperature was increased from –50°C to 150°C, the width of the hysteresis of the MIS device reduced from ~10V to ~6V. This means that the charge storage capability of the trilayer structure decreases with increasing measurement temperature. This is due to the fact that the leakage current in the trilayer structure increases with increasing measurement temperature.
Singapore-MIT Alliance (SMA)

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Sreeshma, D. "Investigations on deep-level defects in HgTe nanocrystals-based photovoltaic devices using a novel instrumentation for Deep Level Transient Spectroscopy". Thesis, 2023. https://etd.iisc.ac.in/handle/2005/6161.

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Colloidally produced nanocrystals (NCs) arranged in thin films hold promise for next-generation semiconductors. These NCs offer tunability in semiconductor properties due to their size, shape, composition, and surface characteristics. However, the performance of NC-based optoelectronic devices still lags behind theoretical predictions. This is primarily attributed to electronic deep-level trap states, which act as recombination centres and limit effective mobility. The large surface area, hybrid nature, and disordered structure of NCs contribute to the abundance of trap states. To improve device performance, it is crucial to identify these defects and understand their impact on electrical characteristics. This work employs Deep Level Transient Spectroscopy (DLTS) to identify deep-level defects in NCs and NC-based photovoltaic devices. DLTS allows for determining defect level energy, concentration, capture cross-section, and differentiation between minority and majority carrier traps. This technique is highly sensitive, capable of detecting low defect concentrations, and resolves signals from various traps. The conventional DLTS system suffers from drawbacks, including the need for multiple temperature cycles, which can lead to poor device contact and thin film adhesion. Additionally, maintaining a consistent temperature environment for each measurement is challenging, resulting in low-quality data. To address these issues, we develop a microcontroller-based DLTS system. This system utilizes a capacitance meter and electronic circuits controlled by an Arduino-Due microcontroller. We have used Arduino-Due to generate the filling pulse, monitor the capacitance, temperature, data acquisition, timing control and signal processing. By conducting measurements within a single temperature scan, our system saves time, improves accuracy, and reduces experimental failures. We validate the innovative instrumentation using a gold-doped silicon p-n junction sample. Furthermore, we apply this microcontroller-based DLTS system to study deep-level defects in Mercury Telluride (HgTe) nanocrystal-based photovoltaic devices. We fabricate photovoltaic devices based on HgTe NCs/TiO2 and employ capacitance-voltage (C-V) and DLTS techniques to investigate and collect quantitative data on deep-level trap states. DLTS confirms the presence of interface trap states, while frequency-dependent capacitance measurements support the influence of charge storage in these nanocrystal-based heterostructures, offering insights for advanced device development. Using DLTS, we measure trap energy, capture cross-section, and concentration. These traps in the photovoltaic devices can act as recombination centres and effectively interact with valence and conduction bands. Poor device responsiveness is observed in the ITO/TiO2/HgTe/Au configuration due to inefficient photo charge extraction. To enhance device performance, we optimize hole and electron extractions by introducing a Molybdenum Oxide (MoO3) hole extraction layer. We investigate the effect of this contact layer on trap level formation in the FTO/TiO2/HgTe/MoO3/Au photovoltaic device using low-temperature I-V, C-V, C-F, and microcontroller-based DLTS measurements. The obtained trap energy levels are comparable to those of the ITO/TiO2/HgTe/Au device, indicating the presence of trap levels at the TiO2/HgTe interface and no significant impact of the MoO3 contact layer on trap formation. Our microcontroller-based DLTS system proves to be an efficient tool for determining defect levels in heterojunctions based on nanocrystals. Surface states at the HgTe nanocrystals and oxygen vacancies in TiO2 are identified as the main contributors to trap levels, primarily located at the TiO2/HgTe interface. To further confirm the origin of trap states, we fabricate an ITO/HgTe/Al Schottky junction and measure the defect level energy using low-temperature I-V and C-F measurements. The obtained energy values support trap levels resulting from surface reconstruction at the TiO2/HgTe heterojunction interface. Passivating these trap states is crucial for improving device effectiveness.

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Koo, Bonil. "CdTe/CdSe/CdTe heterostructure nanorods and I-III-VI₂ nanocrystals: synthesis and characterization". 2009. http://hdl.handle.net/2152/7851.

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Semiconductor nanocrystals are interesting candidates as new light-absorbing materials for photovoltaic (PV) devices. They can be dispersed in solvents and cheaply deposited at low-temperature on various substrates. Also, the nanocrystals have unique optical properties depending on their size due to the quantum size effect and moreover it is easy to uniformly control their stoichiometry. CdTe/CdSe/CdTe heterostructure nanorods and I-III-VI₂ nanocrystals were selected to synthesize and investigate in order to utilize the benefits of colloidal nanocrystals described above. Colloidal nanorods with linear CdTe/CdSe/CdTe heterojunctions were synthesized by sequential reactant injection. After CdTe deposition at the ends of initially formed CdSe nanorods, continued heating in solution leads to Se-Te interdiffusion across the heterojunctions and coalescence to decreased aspect ratio. The Se-Te interdiffusion rates were measured by mapping the composition profile using nanobeam energy dispersive X-ray spectroscopy (EDS). The rate of nanorod coalescence was also measured and compared to model predictions using a continuum viscous flow model. The synthetic method of monodisperse chalcopyrite (tetragonal) CuInSe₂ nanocrystals was also developed. The nanocrystals have trigonal pyramidal shape with one polar and three non-polar surface facets. When drop-cast onto carbon substrates, the nanocrystals self-assemble into close-packed monolayers with triangular (honeycomb) lattice structure. Moreover, the effect of excess Cu precursor (CuCl) was studied for the formation of monodisperse trigonal pyramidal CuInSe₂ nanocrystals. The formation mechanism of monodisperse trigonal pyramidal CuInSe₂ nanocrystals was suggested with regard to excess amount of CuCl precursor, based on the nucleationgrowth model of colloidal nanocrystal formation. A new wurtzite phase of CuInS₂, CuInSe₂, and Cu(InxGa1-x)Se₂ (CIGS) was observed in nanocrystals synthesized by heating metal precursors and Se-(or S-)urea in alkylamine. X-ray diffraction (XRD) showed the predominant phase to be wurtzite (hexagonal) instead of chalcopyrite (tetragonal). High resolution transmission electron microscopy (TEM), however, revealed polytypism in the nanocrystals, with the wurtzite phase interfaced with significant chalcopyrite domains.
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Watt, Tony L. "Abberation-corrected atomic number contrast scanning transmission electrion [sic] microscopy of nanocrystals and nanomaterial-based systems for use in next-generation photovoltaic devices". Diss., 2008. http://etd.library.vanderbilt.edu/ETD-db/available/etd-07222008-122245/.

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"Properties of CuIn(Se,S)2 thin films prepared by a developed two-step growth process". Thesis, 2009. http://hdl.handle.net/10210/2556.

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