To see the other types of publications on this topic, follow the link: EMITTING DIODES.
Dissertations / Theses on the topic 'EMITTING DIODES'
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 50 dissertations / theses for your research on the topic 'EMITTING DIODES.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
1
Stevenson, Stuart G. "Dendrimer light-emitting diodes." Thesis, St Andrews, 2008. http://hdl.handle.net/10023/581.
Full text
2
Choi, Wai Kit. "Organic light-emitting diodes." HKBU Institutional Repository, 1999. http://repository.hkbu.edu.hk/etd_ra/190.
Full text
3
Rosenow, Thomas. "White Organic Light Emitting Diodes." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2011. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-67342.
Full textAbstract:
Die vorliegende Arbeit beschäftigt sich mit drei Ansätzen der hocheffizienten Erzeugung von weißem Licht mit organischen Leuchtdioden (OLEDs) auf der Basis kleiner Moleküle.
Ein Ansatz kombiniert die Emission eines fluoreszenten und zweier phosphoreszenter Emitter in einer einzelnen Emissionsschicht. Da das Triplettniveau des verwendeten Blauemitters niedriger ist als die Triplettniveaus der phosphoreszenten Emitter, werden die Konzentrationen der Emitter so gewählt, dass ein Exzitonenübertrag zwischen ihnen unterbunden wird. Die strahlungslose Rekombination von Tripletts auf dem fluoreszenten Blauemitter begrenzt die Effizienz dieses Ansatzes, jedoch besticht die resultierende weiße OLED durch eine bemerkenswerte Farbstabilität.
Der zweite Ansatz basiert auf dem “Triplet Harvesting” Konzept. Ansonsten ungenutzte Triplett Exzitonen werden von einem fluoreszenten Blauemitter auf phosphoreszente Emitter übertragen, wodurch interne Quanteneffizienzen bis zu 100 % möglich sind. Der zur Verfügung stehende Blauemitter 4P-NPD erlaubt aufgrund seines niedrigen Triplettniveaus nicht den Triplett übertrag auf einen grünen Emitter. Daher wird das “Triplet Harvesting” auf zwei unterschiedliche phosphoreszente Emitter, anhand des gelben Emitters Ir(dhfpy)2acac und des roten Emitters Ir(MDQ)2acac untersucht. Es wird gezeigt, dass beide phosphoreszente Emitter indirekt durch Exzitonendiffusion angeregt werden und nicht durch direkte Rekombination von Ladungsträgern auf den Emittermolekülen. Eine genaue Justage der Anregungsverteilung zwischen den phosphoreszenten Emittern ist durch Schichtdickenvariation in der Größenordnung üblicher Schichtdicken möglich. Spätere Produktionsanlagen brauchen daher keinen speziellen Genauigkeitsanforderungen gerecht zu werden.
Der dritte und zugleich erfolgreichste Ansatz beruht auf einer Weiterentwicklung des zweiten Ansatzes. Er besteht zunächst darin den Tripletttransfer auf den Übertrag von einem fluoreszenten blauen auf einen phosphoreszenten roten Emitter zu beschränken. Die sich ergebende spektrale Lücke wird durch direktes Prozessieren einer unabhängigen voll phosphoreszenten OLED auf diese erste OLED gefüllt. Verbunden sind beide OLEDs durch eine ladungsträgererzeugende Schicht, in welcher durch das angelegte Feld Elektron/Loch-Paare getrennt werden. Dieser Aufbau entspricht elektrisch der Reihenschaltung zweier OLEDs, welche im Rahmen dieser Arbeit individuell untersucht und optimiert werden. Dabei ergibt sich, dass die Kombination von zwei verschiedenen phosphoreszenten Emittern in einer gemeinsamen Matrix die Ladungsträgerbalance in der Emissionszone sowie die Quanteneffizienz der vollphosphoreszenten OLED stark verbessert. Als Ergebnis steht eine hocheffiziente weiße OLED, welche durch die ausgewogene Emission von vier verschiedenen Emittern farbstabiles Licht mit warm weißen Farbkoordinaten (x, y) = (0.462, 0.429) und ausgezeichneten Farbwiedergabeeigenschaften (CRI = 80.1) erzeugt. Dabei sind die mit diesem Ansatz erreichten Lichtausbeuten (hv = 90.5 lm/W) mit denen von voll phosphoreszenten OLEDs vergleichbar.
4
Thomschke, Michael. "Inverted Organic Light Emitting Diodes." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2013. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-106255.
Full textAbstract:
This study focuses on the investigation of the key parameters that determine the optical and electrical characteristics of inverted top-emitting organic light emitting diodes (OLED). A co-deposition of small molecules in vacuum is used to establish electrically doped films that are applied in n-i-p layered devices. The knowledge about the functionality of each layer and parameter is important to develop efficient strategies to reach outstanding device performances.
In the first part, the thin film optics of top-emitting OLEDs are investigated, focusing on light extraction via cavity tuning, external outcoupling layers (capping layer), and the application of microlens films. Optical simulations are performed to determine the layer configuration with the maximum light extraction efficiency for monochrome phosphorescent devices. The peak efficiency is found at 35%, while varying the thickness of the charge transport layers, the
semitransparent anode, and the capping layer simultaneously. Measurements of the spatial light distribution validate, that the capping layer influences the spectral width and the resonance wavelength of the extracted cavity mode, especially for TM polarization. Further, laminated microlens films are applied to benefit from strong microcavity effects in stacked OLEDs by spatial mixing of external and to some extend internal light modes. These findings are used to demonstrate white top-emitting OLEDs on opaque substrates showing power conversion efficiencies up to 30 lm/W and a color rendering index of 93, respectively.
In the second part, the charge carrier management of n-i-p layered diodes is investigated as it strongly deviates from that of the p-i-n layered counterparts. The influence of the bottom cathode material and the electron transport layer is found to be negligible in terms of driving voltage, which means that the assumption of an ohmic bottom contact is valid. The hole transport and the charge carrier injection at the anode is much more sensitive to the evaporation sequence, especially when using hole transport materials with a glass transition
temperature below 100°C. As a consequence, thermal annealing of fabricated inverted OLEDs is found to drastically improve the device electronics, resulting in lower driving voltages and an increased internal efficiency. The annealing effect on charge transport comes from a reduced charge accumulation due to an altered film morphology of the transport layers, which is proven for electrons and for holes independently. The thermal treatment can further lead to a device degradation. Finally, the thickness and the material of the blocking layers which usually control the charge confinement inside the OLED are found to influence the recombination much more effectively in inverted OLEDs compared to non-inverted ones.
5
Gray, Jonathan William. "Resonant cavity light emitting diodes." Thesis, Imperial College London, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.399518.
Full text
6
Hemingway, Leon Robert. "Dendrimers for light emitting diodes." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.325840.
Full text
7
Liu, Yee-Chen. "Polymer blend light-emitting diodes." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610709.
Full text
8
Chen, Chih-Lei. "Processing light-emitting dendrimers for organic light-emitting diodes." Thesis, University of Oxford, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.489420.
Full textAbstract:
Light-emitting dendrimers with iridium(III) complex cores have given rise to some of the simplest and most efficient organic light-emitting diodes. However, whilst monochrome devices can be prepared there is currently no method for the patterning of the dendrimer films to give rise to pixelated colour displays. The main aim of this project was to develop methodology for the patterning of dendrimer films. In particular, dendrimers are designed that have an oxetane surface group that can be crosslinked to form patterns by a photo-generated acid.
9
Najafabadi, Ehsan. "Stacked inverted top-emitting white organic light-emitting diodes." Diss., Georgia Institute of Technology, 2014. http://hdl.handle.net/1853/52990.
Full textAbstract:
The majority of research on Organic Light-Emitting Diodes (OLEDs) has focused on a top-cathode, conventional bottom-emitting architecture. Yet bottom-cathode, inverted top-emitting OLEDs offer some advantages from an applications point of view. In this thesis, the development of high performance green electroluminescent inverted top-emitting diodes is first presented. The challenges in producing an inverted structure are discussed and the advantages of high efficiency inverted top-emitting OLEDs are provided. Next, the transition to a stacked architecture with separate orange and blue emitting layers is demonstrated, allowing for white emission. The pros and cons of the existing device structure is described, with an eye to future developments and proposed follow-up research.
10
Fang, Fang. "Investigation of green light emitting diodes." Thesis, University of Cambridge, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.610094.
Full text
11
Wang, Yan. "Highly flexible top-emitting phosphorescent organic light emitting diodes (OLEDs)." Thesis, University of British Columbia, 2014. http://hdl.handle.net/2429/50921.
Full textAbstract:
Organic Light Emitting Diodes (OLEDs) have become attractive for flat panel display industry, with applications ranging from mobile phone screens to TVs. They have several advantages over inorganic LEDs such as high contrast ratio, wide viewing angle, faster response time, scalable large area processing and most importantly mechanical flexibility. OLEDs on flexible substrates can endure certain level of mechanical deformation such as bending, rolling or folding without disruption of the performance. The current demonstrated flexibility for OLEDs is up to a few centimeters or millimeters bending radius, depending on the materials, substrates and device structures. More flexible OLEDs with bending radius of curvature on the order of microns will be needed for applications in wearable, roll-up, or foldable displays and bezel-free screens and flexible signage systems.
This thesis presents the design, fabrication and characterization of highly flexible and foldable top-emitting OLEDs made on 50 micron thick polyimide (PI) plastic substrates, which can achieve approximately 200 microns bending radius of curvature (folding) without visible damage or impact on emission brightness and uniformity. To the best of our knowledge these are the most flexible phosphorescent OLEDs and first foldable OLEDs ever reported. We believe such flexibility is the benefit of the mechanical stability and low film thickness of the PI substrate. The surface roughness of PI had been the major limitation of its application as OLED substrates, and in this thesis a special side-angle evaporation method is proposed to improve the step coverage of deposited thin films of materials on PI without the requirement of buffer layers. The same method is also proved to be applicable for fabricating OLEDs on much rougher substrates such as Scotch tapes, and fiberglass and transparency sheets. The OLEDs fabricated on above substrates are also presented and characterized.Applied Science, Faculty of
Electrical and Computer Engineering, Department of
Graduate
12
Galata, Sotiria. "Sulphur doped silicon light emitting diodes." Thesis, University of Surrey, 2005. http://epubs.surrey.ac.uk/842933/.
Full textAbstract:
In this thesis light emission from sulphur related impurity in silicon has been reported. Although, sulphur related luminescence from silicon has been stated since the 1980's, no room temperature luminescence has been achieved and no compatible devices that can be integrated to the silicon technology have been invented. Photoluminescence and electroluminescence experiments were made on a set of samples implanted with only with sulphur at doses ranging from 1011-1014 S cm-2 at 30 keV, annealed at 1000 °C or 1100 °C for 10 s and on another set of samples implanted with sulphur as above and further implanted with boron at 1015 B cm-2 at 30 keV, further annealed at 950 °C for 1 min. The experiments revealed two major emissions at 1129.5 nm (1.0997 eV) which is due to the Si TO phonon assisted transition and at 1363 nm (0.9097 eV) which is due to sulphur related impurities. Variable temperature experiments were done at both PL and EL experiments. From the EL variable temperature measurements, it was observed that the two main lines were shifting towards longer wavelengths with the increase of temperature. Sulphur emission was present at room temperature with low intensity compared to the silicon emission which was more dominant at room temperature. Of great interest was the effect of power on silicon and sulphur emission. It has revealed a sublinear and a superlinear behaviour for the sulphur and silicon integrated intensity respectively with the increase of the injection condition, which can be attributed to the saturation of sulphur related levels responsible for the 1.33 nm emission at the high excitation levels. A model of the diffusion of sulphur concentration after the annealing treatments was presented, introducing the two cases of perfect reflection and perfect loss from the samples surfaces. Finally a model explaining our PL and EL power dependence experiments was provided which showed that there are two major radiative routes via the silicon and the sulphur that take place, which are competing at each other along with a non-radiative route coming from the sulphur related level. Our model describes the trends in our experimental data well. Finally, the energy related to the sulphur peak quenching was calculated to be 32.2 +/-1.4 meV.
13
Siddiqui, Saiful Anam. "Erbium doped silicon light emitting diodes." Thesis, University of Surrey, 2003. http://epubs.surrey.ac.uk/843408/.
Full textAbstract:
Erbium, a rare earth element, has been shown to exhibit characteristic luminescence at 1.54mum due to its internal 4f transition from the first excited state (4pi3/2) to the ground state (4pi5/2). As this emission wavelength falls inside the maximum transmission window of silicon based optical fibers, erbium doped silicon might lead to the opportunity of silicon based optoelectronics. The introduction of erbium in silicon allows excitation through electron-hole recombination and subsequent radiative emission from the rare earth centers. The works reported here describe the structural, electrical and optical properties of crystalline silicon codoped with erbium and boron by ion implantation technique. Four sets of samples, co-implanted with erbium and boron at different Er dose, implantation energy and at different conditions, were prepared. Post-implantation annealing has been performed to recover the implantation damage to an acceptable value and to activate the dopant atoms optically and electrically. PL and EL measurements have been performed in the temperature range between 80K to room temperature. The sample with the lowest erbium concentration and energy gives the best PL and EL results. The observed emission peaks in both PL and EL measurements were at around 1.129mum, ~1.303mum, 1.50mum and 1.597mum at 80K. At higher temperatures, a broader peak at around 1.50mum with long tail towards the both end of wavelength has been observed. The peak at 1.129mum corresponding to the Si band edge emission, the reason for the peaks at around l.303mum has not been identified while the remaining two peaks correspond the Er3+ emission. Virtually no temperature quenching of Er luminescence is observed in some samples rather room temperature intensity is higher than that at 80K. The improvement of the temperature quenching effect on Er luminescence at room temperature has been attained in our results, which is significant improvement in comparison to the result found in the literature. The structural properties were studied by TEM in both cross-sectional and plan view configurations. TEM analyses showed dislocation loops and other defects of random size and distribution from the surface to 600nm below the surface. Er precipitates defects were also seen in the sample doped with Er comparatively at higher dose (1x1015Er/cm2) and energy (1.0 MeV). No detectable room temperature PL and EL signals were observed from the sample implanted at higher doses and energies.
14
Webster, Graham R. "Advanced polymers for light emitting diodes." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.393551.
Full text
15
Liedtke, Alicia. "Liquid crystals for light emitting diodes." Thesis, University of Hull, 2009. http://hydra.hull.ac.uk/resources/hull:2429.
Full textAbstract:
In this work a series of new semiconducting liquid crystals (LCs), which are applicable for organic light emitting diodes (OLEDs), were investigated. Semiempirical calculations were carried out on monomers and anti-cofacial dimers built from our molecules, representing molecules in solution and thin film respectively. Compared to the monomer a doubling of the oscillator strength in the dimer was found for longitudinal offsets larger than 20 A. Smaller shifts showed a forbidden absorption transition from ground to the lowest excited state. Assuming that the absorption transition is equivalent to the emissive transition, this might explain the reduced optical quantum efficiency observed for all of our materials in the solid state. OLEDs made from blends of three different blue/green emitters with a red component showed white light emission with voltage independent CIE coordinates close to the ideal white. With polarised microscopy nematic phases frozen in a glassy state at room temperature were observed for all blends. Thus the blends were homogeneous and no phase separation occurred. This is important for homogeneous white emission and the alignment of the LCs due to a rubbed alignment layer below. Polarised white electroluminescence with an average polarisation ratio of 8:1 was shown from an OLED made with a blend deposited onto an alignment layer. Polarised background light for LC displays is desirable as this minimises the losses at the polarisers in the display and thus increases its brightness or lowers the power consumption. The low efficiency of the red emitter however limited the OLED performance. Surface relief gratings (SRGs) with periods of a few hundred nm and a maximum depth of 66 nm and periods in the nm-range with a depth of 140 nm were spontaneously induced on our films. They were formed through molecular mass transport from the dark to bright regions during crosslinking by irradiation with a sinusoidal light pattern created by a phase mask. The anisotropic properties of LCs are shown to enhance transport. SRGs were formed at room temperature and an elevated sample temperature of 65deg. They are suitable feedback structures for optically pumped organic lasers and can also be employed to enhance the outcoupling of OLEDs.
16
Farrow, T. "Quantum dot single-photon emitting diodes." Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598951.
Full textAbstract:
Significant advances have been made in the past five years in realizing photon sources based on various kinds of emitters in the 500-1000 nm spectral region. However, reliable sources emitting at wavelengths compatible with standard telecommunication optical fibre are still lacking. Self-assembled quantum dots offer a way to generate wavelength-tunable photons. They offer also relatively high radiative efficiencies, short spontaneous emission lifetimes (~1 ns) and, importantly, can easily be incorporated into compact semiconductor devices fabricated with established technologies. In this thesis, single photon sources based on InAs/GaAs quantum dots were studied and developed for applications at the technologically important wavelength around 1.3 μm compatible with standard telecommunication optical fibre. A single photon source suitable for use in a quantum key distribution system was demonstrated by optically pumping an InAs/GaAs quantum dot incorporated inside a micropillar optical cavity (chapter 4). Single photon emission driven by short electrical pulses was then realised for the first time at λ ~ 1.3 μm using an InAs/GaAs quantum dot embedded in a planar optical microcavity (chapter 5). Electrically driven single photon emission was also achieved for quantum dot devices based on the micropillar geometry, demonstrating the viability of such compact cavities with small optical mode volumes at both λ ~ 900 nm and 1.3 μm (chapters 6 and 7). The realization of these devices surmounted the difficulty of contacting the small cross-sectional diameters (~ 2 – 2.5 μm) of the micropillars. Positioning quantum dots accurately inside optical microcavities can improve the efficiency of single photon sources. Chapter 3 presents results of a study of semiconductor wafers with intentionally positioned quantum dots, demonstrating the viability of a technique for placing quantum dots reproducibly.
17
Visweswaran, Bhadri. "Encapsulation of organic light emitting diodes." Thesis, Princeton University, 2014. http://pqdtopen.proquest.com/#viewpdf?dispub=3665325.
Full textAbstract:
Organic Light Emitting Diodes (OLEDs) are extremely attractive candidates for flexible display and lighting panels due to their high contrast ratio, light weight and flexible nature. However, the materials in an OLED get oxidized by extremely small quantities of atmospheric moisture and oxygen. To obtain a flexible OLED device, a flexible thin-film barrier encapsulation with low permeability for water is necessary.
Water permeates through a thin-film barrier by 4 modes: microcracks, contaminant particles, along interfaces, and through the bulk of the material. We have developed a flexible barrier film made by Plasma Enhanced Chemical Vapor Deposition (PECVD) that is devoid of any microcracks. In this work we have systematically reduced the permeation from the other three modes to come up with a barrier film design for an operating lifetime of over 10 years.
To provide quantitative feedback during barrier material development, techniques for measuring low diffusion coefficient and solubility of water in a barrier material have been developed. The mechanism of water diffusion in the barrier has been identified. From the measurements, we have created a model for predicting the operating lifetime from accelerated tests when the lifetime is limited by bulk diffusion.
To prevent the particle induced water permeation, we have encapsulated artificial particles and have studied their cross section. A three layer thin-film that can coat a particle at thicknesses smaller than the particle diameter is identified. It is demonstrated to protect a bottom emission OLED device that was contaminated with standard sized glass beads.
The photoresist and the organic layers below the barrier film causes sideways permeation that can reduce the lifetime set by permeation through the bulk of the barrier. To prevent the sideways permeation, an impermeable inorganic grid made of the same barrier material is designed. The reduction in sideways permeation due to the impermeable inorganic grid is demonstrated in an encapsulated OLED.
In this work, we have dealt with three permeation mechanisms and shown solution to each of them. These steps give us reliable flexible encapsulation that has a lifetime of greater than 10 years.
18
Weaver, Michael Stuart. "Electroluminescence from organic light emitting diodes." Thesis, University of Sheffield, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.265610.
Full text
19
Gray, Dodd (Dodd J. ). "Thermal pumping of light-emitting diodes." Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/76963.
Full textAbstract:
Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 129-135).
The work presented here is a study of thermally enhanced injection in light-emitting diodes (LEDs). This effect, which we refer to as "thermal pumping", results from Peltier energy exchange from the lattice to charge carriers when current is injected into an LED. For an applied voltage V such that qV < (hw), where q is the electron charge and (hw) is the average emitted photon energy, thermal pumping can greatly enhance the wall plug efficiency of an LED. Thermal pumping can even give rise to LED wall plug efficiency greater than one, which corresponds to electroluminescent cooling of the diode lattice. Thermal pumping and electroluminescent cooling will be studied through numerical modeling and experiment. Our results include the first ever experimental demonstration of electroluminescent cooling in an LED. Finally we use the intuition gained from the study of thermal pumping to design an LED for maximized optical power output with 100% wall plug efficiency.
by Dodd Gray.
M.Eng.
20
Lupton, John Mark. "Nanoengineering of organic light-emitting diodes." Thesis, Durham University, 2000. http://etheses.dur.ac.uk/1597/.
Full text
21
Fehrman, Stephen A. "Passivation of polymer light-emitting diodes." Click here to view, 2009. http://digitalcommons.calpoly.edu/eesp/19/.
Full textAbstract:
Thesis (B.S.)--California Polytechnic State University, 2009.Project advisor: David Braun. Title from PDF title page; viewed on Jan. 28, 2010. Includes bibliographical references. Also available on microfiche.
22
譚祐怡 and Yau-yee Tam. "Dual use of visible light-emitting diodes." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2000. http://hub.hku.hk/bib/B31223436.
Full text
23
Tam, Yau-yee. "Dual use of visible light-emitting diodes /." Hong Kong : University of Hong Kong, 2000. http://sunzi.lib.hku.hk/hkuto/record.jsp?B21949001.
Full text
24
Zou, Zhengzhong. "GaAs-based quantum dot vertical-cavity surface-emitting lasers and microactivity light emitting diodes /." Full text (PDF) from UMI/Dissertation Abstracts International, 2002. http://wwwlib.umi.com/cr/utexas/fullcit?p3089497.
Full text
25
Huang, Qiang. "High efficiency top-emitting organic light-emitting diodes: design and fabrication." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2007. http://nbn-resolving.de/urn:nbn:de:swb:14-1193677683674-43826.
Full textAbstract:
This thesis focuses mainly on the techniques to achieve high-performance top-emitting OLEDs, regarding device efficiency and lifetime for both non-inverted and inverted structures. It is thus organized as follows: In Chapter 2, the basic physics of organic semiconductor materials are reviewed, including the electronic properties of organic semiconductor materials, molecular excitations and their electronic transitions etc., which are believed to be critical for understanding of the work. Then, the general device physics of OLEDs are reviewed in detail, which includes almost every important electrical and optical process involved in the device. Finally, techniques and methods used to improve the device performance are summarized, which includes electrical doping of charge carrier transport layers. In Chapter 3, all organic materials, experimental techniques, and characterization methods used in this study are briefly described. In the following Chapter 4, techniques that are used for device optimization of non-inverted top-emitting OLEDs are discussed. Also, the mechanism of light outcoupling enhancement by a capping layer is discussed there. In the last part of Chapter 4, the influence of the optical device structure on the intrinsic quantum yield of the emitters is studied. Chapter 5 is focused on inverted top-emitting OLEDs, which are believed to be better applicable with current mainstream n-type amorphous silicon thin film transistor (TFT) technology. In this Chapter, the organic/metal and metal/organic interfaces are investigated in detail and their influence on device performance is discussed. In Chapter 6, the degradation of top-emitting OLEDs is studied, with a focus on the influence of electrode material and electrode thickness on the lifetime of top-emitting devices.
26
Li, Zonglin, and 李宗林. "Reliability study of InGaN/GaN light-emitting diode." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2009. http://hub.hku.hk/bib/B43224155.
Full text
27
Li, Zonglin. "Reliability study of InGaN/GaN light-emitting diode." Click to view the E-thesis via HKUTO, 2009. http://sunzi.lib.hku.hk/hkuto/record/B43224155.
Full text
28
Lu, Li Ping. "Highly efficient hybrid polymer light-emitting diodes." Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.607808.
Full text
29
Yu, Xiaoming. "Organic light emitting diodes (OLEDs) for lighting /." View abstract or full-text, 2009. http://library.ust.hk/cgi/db/thesis.pl?ECED%202009%20YU.
Full text
30
Potfajova, Jaroslava. "Silicon based microcavity enhanced light emitting diodes." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2010. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-25451.
Full textAbstract:
Realising Si-based electrically driven light emitters in a process technology compatible with mainstream microelectronics CMOS technology is key requirement for the implementation of low-cost Si-based optoelectronics and thus one of the big challenges of semiconductor technology. This work has focused on the development of microcavity enhanced silicon LEDs (MCLEDs), including their design, fabrication, and experimental as well as theoretical analysis. As a light emitting layer the abrupt pn-junction of a Si diode was used, which was fabricated by ion implantation of boron into n-type silicon. Such forward biased pn-junctions exhibit room-temperature EL at a wavelength of 1138 nm with a reasonably high power efficiency of 0.1%. Two MCLEDs emitting light at the resonant wavelength about 1150 nm were demonstrated: a) 1-lambda MCLED with the
resonator formed by 90 nm thin metallic CoSi2 mirror at the bottom and semitransparent distributed Bragg reflector (DBR) on the top; b) 5.5-lambda MCLED with the resonator formed by high reflecting DBR at the bottom and semitransparent top DBR. Using the appoach of the 5.5-lambda MCLED with two DBRs the extraction efficiency is enhanced by about 65% compared to the silicon bulk pn-junction diode.
31
Potfajova, J. "Silicon based microcavity enhanced light emitting diodes." Forschungszentrum Dresden-Rossendorf, 2010. http://nbn-resolving.de/urn:nbn:de:bsz:d120-qucosa-27756.
Full textAbstract:
Realising Si-based electrically driven light emitters in a process technology compatible with mainstream microelectronics CMOS technology is key requirement for the implementation of low-cost Si-based optoelectronics and thus one of the big challenges of semiconductor technology. This work has focused on the development of microcavity enhanced silicon LEDs (MCLEDs), including their design, fabrication, and experimental as well as theoretical analysis. As a light emitting layer the abrupt pn-junction of a Si-diode was used, which was fabricated by ion implantation of boron into n-type silicon. Such forward biased pn-junctions exhibit room-temperature EL at a wavelength of 1138 nm with a reasonably high power efficiency of 0.1% [1]. Two MCLEDs emitting light at the resonant wavelength about 1150 nm were demonstrated: a) 1 MCLED with the resonator formed by 90 nm thin metallic CoSi2 mirror at the bottom and semitranparent distributed Bragg reflector (DBR) on the top; b) 5:5 MCLED with the resonator formed by high reflecting DBR at the bottom and semitransparent top DBR. Using the appoach of the 5:5 MCLED with two DBRs the extraction efficiency is enhanced by about 65% compared to the silicon bulk pn-junction diode.
32
Habtemichael, Yishak Tekleab. "Packaging designs for ultraviolet light emitting diodes." Thesis, Georgia Institute of Technology, 2012. http://hdl.handle.net/1853/45764.
Full textAbstract:
Aluminum Gallium Nitride (AlGaN) / Gallium Nitride (GaN) based deep ultraviolet (DUV) light emitting didoes (LEDs) with emission wavelengths between 200-280 nm enable key emerging technologies such as water/air purification and sterilization, covert communications and portable bio-agent detection/identification systems for homeland security, and surface and medical device sterilization. These devices produce a large amount of undesired heat due to low quantum efficiencies in converting electrical input to optical output. These low efficiencies are attributed to difficulties in the growth&doping of AlₓGa₁₋ₓN materials and UV absorbing substrates leading to excessive joule heating, which leads to device degradation and a spectral shift in the emission wavelength. With this regard, effective thermal management in these devices depends on the removal of this heat and reduction of the junction temperature. This is achieved by decreasing the package thermal resistance from junction-to-air with cost-effective solutions. The use of heat sinks, thermal interface materials, and high conductivity heat spreaders is instrumental in the reduction of the overall junction-to-air thermal resistance.
This thesis work focuses on thermal modeling of flip-chip packaged deep UV LEDs to gain a better understanding of the heat propagation through these devices as well as the package parameters that have the biggest contributions to reducing the overall thermal resistance. A parametric study focusing on components of a lead frame package is presented to ascertain the thermal impacts of various package layers including contact metallizations, thermal spreading sub-mounts, and thermal interface materials. In addition the use of alternative thermal interface materials such as phase change materials and liquid metals is investigated experimentally.
33
Mitchell, William John. "Polymer-electrode interactions in light emitting diodes." Thesis, University of Oxford, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.289406.
Full text
34
Mirza, Benjamin Imran. "Mid-infrared INSb/A1InSb light-emitting diodes." Thesis, University of Bristol, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.503928.
Full textAbstract:
Electrical and optical properties of bulk and quantum-well (QW) InSb/AlˣIn₁-ˣSb midinfrared light-emitting diodes (LEDs) have been investigated as a function of temperature and injection current. Measured current-voltage characteristics are rectifying across all temperatures and aluminium compositions for each of the devices. For the QWLEDs, experimentally measured emission spectra are compared to the theoretical equivalent, enabling individual optical transitions in the QW to be attributed to observable features.
35
Evans, N. R. "Phosphorescent conjugated polymers for light-emitting diodes." Thesis, University of Cambridge, 2004. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598900.
Full textAbstract:
This dissertation describes work done to improve the efficiency of conjugated-polymer organic light-emitting diodes (OLEDs) by harvesting both singlet and triplet excitons as electroluminescence. The electroluminescence quantum efficiencies of OLEDs based on the fluorescent conjugated polymer poly(9,9-dioctyl-9H-fluorene-2,7-diyl) 1.08 have been improved through the design, synthesis and application of new phosphorescent conjugated polymers. A study of simple blends of phosphorescent dopants with the polymer 1.08 showed that the red-phosphorescent dopant 2.15 is better suited for observing phosphorescent emission in a blend with the polymer 1.08 than are green- or orange-phosphorescent dopants. (Fig 8453A) The problem of phase separation in the simple blends was solved by tethering the red-phosphorescent dopants to the polyfluorene chains, to give the new phosphorescent conjugated copolymer 3.30. The maximum electroluminescence efficiencies in OLEDs of the copolymer 3.30, are greater than those of the corresponding fluorescent conjugated polymer, and typically an order of magnitude greater than those of the corresponding simple blends. (Fig. 8453B) Dexter triplet-energy transfer reduces the photoluminescence efficiency of copolymers in which the red-phosphorescent dopants are bonded directly to the polyfluorene chains - a nonconjugated manner but without the separation of a tether. In addition, the polymerisation of monomers with small singlet-triplet energy gaps has been proposed as a route for the synthesis of conjugated polymers with small singlet-triplet energy gaps, and some preliminary results are presented.
36
Ho, P. K. H. "Novel architectures for polymer light-emitting diodes." Thesis, University of Cambridge, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604103.
Full textAbstract:
This dissertation work aims to develop novel optoelectronic polymer device architectures through appropriate control of the interfacial and bulk structures of the semiconducting polymer. The two approaches developed here are the hole-injection interlayer engineering via the layer-by-layer polyelectrolyte assembly, and refractive-index engineering via the use of polymer-inorganic nanocomposites. The poly-electrolyte assembly route enables vertical interfacial structuring of the electronic properties to be achieved at the anode contact for improved hole injection and electron confinement. The development of the nanocomposites provides solution-processable photonic building blocks that are semiconducting and that could provide optical feedback in polymer devices. Both of these developments have made possible new high-performance device architectures. For example, polymer light-emitting diodes (LEDs) with efficiencies of 20 cd/A in the green spectral region (estimated internal quantum yield, 15-20% photon/electron), and polymer microcavity LEDs with emission full-width-at-half-maximum of 25 nm and less have been demonstrated. This dissertation also addresses a few of the scientific issues encountered in the course of the work. The relationship between external and internal electroluminescence quantum yields of these LEDs is analysed, and the role played by optical properties of the metal cathode-reflector delineated. The effects of embedding SiO2 nanoparticles on the optical, electronic and morphological properties of the poly(p-phenylenevinylene) matrix, together with the extent of increased localisation of its p-electrons, are also studied.
37
Hui, Kwun-nam, and 許冠南. "Light-emitting diodes incorporating microdisks and microspheres." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2008. http://hub.hku.hk/bib/B41633969.
Full text
38
Ng, Man-ching Alan, and 吳文政. "Light emitting diodes based on ZnO nanorods." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2010. http://hub.hku.hk/bib/B43572005.
Full text
39
Schober, Matthias. "Charge Transport in Organic Light-Emitting Diodes." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-100071.
Full textAbstract:
This thesis is about the development and validation of a numerical model for the simulation of the current-voltage characteristics of organic thin-film devices. The focus is on the analysis of a white organic light-emitting diode (OLED) with fluorescent blue and phosphorescent red and green emitters. The simulation model describes the charge transport as a one-dimensional drift-diffusion current and is developed on the basis of the Scharfetter-Gummel method. It incorporates modern theories for the charge transport in disordered organic materials, which are considered by means of special functions for the diffusion coefficient and the charge-carrier mobility. The algorithm is designed such that it can switch between different models for mobility and calculates both transient and steady-state solutions. In the analysis of the OLED, electron and hole transport are investigated separately in series of single-carrier devices. These test devices incorporate parts of the layers in the OLED between symmetrically arranged injection layers that are electrically doped. Thereby, the OLED layer sequence is reconstructed step by step. The analysis of the test devices allows to obtain the numerous parameters which are required for the simulation of the complete OLED and reveals many interesting features of the OLED.
For instance, it is shown how the accumulation of charge carriers in front of an interface barrier increases the mobility and the transfer rate across the interface. Furthermore, it is demonstrated how to identify charge-trapping states. This leads to the detection of deep trap states in the emission zone of the OLED -- an interesting aspect, since these states can function as recombination centers and may cause non-radiative losses. Moreover, various other effects such as interface dipoles and a slight freeze-out of active electric dopants in the injection layers are observed. In the simulations of the numerous test devices, the parameters are consistently applied. Thereby, the agreement between simulation and experiment is excellent, which demonstrates the correctness and applicability of the developed model. Finally, the complete OLED is successfully simulated on the basis of the parameters that have been obtained in the analysis of the single-carrier devices. The simulation of the OLED illustrates the transport levels of electrons and holes, and proofs that the OLED efficiency is low because of non-radiative recombination in the interlayer between the phosphorescent and fluorescent emission zones. In this context, many interesting issues are discussed, e.g. the applicability of the Langevin model in combination with the mobility models for the description of recombination and the relevance of interactions between free charge carriers and excitons.
40
Smith, Lucy Hannah. "Optical optimisation of organic light-emitting diodes." Thesis, University of Exeter, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.425499.
Full text
41
Lai, May Ling. "Perovskite light-emitting diodes with tunable emission." Thesis, University of Cambridge, 2018. https://www.repository.cam.ac.uk/handle/1810/283638.
Full textAbstract:
Solid-state lightings are becoming the popular choice for lightings due to its higher efficiency, improved colour rendering index and the flexibility of various size and shape. Halide perovskite has tunable colour emission, low disorder and is solution processable making it one of a popular choice as emitters. This thesis demonstrates the versatility of using halide perovskite material in light-emitting diodes. We demonstrate the first working perovskite light-emitting diode at room temperature by introducing thin layer of perovskite emitter which is crucial to confine the inherent free carriers in the material. We show that the 3D lead-halide bulk perovskite is bandgap tunable with emission in the green and red visible spectrum. Light-emitting diodes in the visible spectrum are common however near-infrared emission is a rarity. Lead is a heavy metal which is known for its toxicity. We tackled the issue of toxicity by replacing with tin and demonstrate tunable emission in the near-infrared region. Bulk perovskites have large binding energy which makes it difficult to confine the charges and form radiative recombination which is crucial for emission and efficiency of the device. We move into lower dimensionality perovskites by utilising all-inorganic perovskite nanoplatelets and show emission in the blue region.
42
Santhanam, Parthiban. "Thermo-electrically pumped semiconductor light emitting diodes." Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/87935.
Full textAbstract:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 217-227).
Thermo-electric heat exchange in semiconductor light emitting diodes (LEDs) allows these devices to emit optical power in excess of the electrical power used to drive them, with the remaining power drawn from ambient heat. In the language of semiclassical electron transport, the electrons and holes within the device absorb lattice phonons as they diffuse from their respective contacts into the LED's active region. There they undergo bimolecular radiative recombination and release energy in the form of photons. In essence the LED is acting as a thermodynamic heat pump operating between the cold reservoir of the lattice and the hot reservoir of the outgoing photon field. In this thesis we report the first known experimental evidence of an LED behaving as a heat pump. Heat pumping behavior is observed in mid-infrared LEDs at sub-thermal forward bias voltages, where electrical-to-optical power conversion at arbitrarily high efficiency is possible in the limit of low optical output power. In this regime, the basic thermal physics of an LED differs from that seen at conventional higher voltage operating points. We construct a theoretical model for entropy transport in an LED heat pump and examine its consequences both theoretically and experimentally. We use these results to propose a new design for an LED capable of very high efficiency power conversion at power densities closer to the limit imposed by the Second Law of Thermodynamics. We then explore the potential application of these thermo-photonic heat pumps as extremely efficient sources for low-power communication and high-temperature absorption spectroscopy.
by Parthiban Santhanam.
Ph. D.
43
Rosser, Saul 1979. "Underwater photographic lighting using Light Emitting Diodes." Thesis, Massachusetts Institute of Technology, 2003. http://hdl.handle.net/1721.1/91794.
Full text
44
Sharma, Nikhil. "Characterisation of InGaN/GaN light emitting diodes." Thesis, University of Cambridge, 2001. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.621315.
Full text
45
Potfajova, J. "Silicon based microcavity enhanced light emitting diodes." Forschungszentrum Dresden-Rossendorf, 2009. https://hzdr.qucosa.de/id/qucosa%3A21604.
Full textAbstract:
Realising Si-based electrically driven light emitters in a process technology compatible with mainstream microelectronics CMOS technology is key requirement for the implementation of low-cost Si-based optoelectronics and thus one of the big challenges of semiconductor technology. This work has focused on the development of microcavity enhanced silicon LEDs (MCLEDs), including their design, fabrication, and experimental as well as theoretical analysis. As a light emitting layer the abrupt pn-junction of a Si-diode was used, which was fabricated by ion implantation of boron into n-type silicon. Such forward biased pn-junctions exhibit room-temperature EL at a wavelength of 1138 nm with a reasonably high power efficiency of 0.1% [1]. Two MCLEDs emitting light at the resonant wavelength about 1150 nm were demonstrated: a) 1 MCLED with the resonator formed by 90 nm thin metallic CoSi2 mirror at the bottom and semitranparent distributed Bragg reflector (DBR) on the top; b) 5:5 MCLED with the resonator formed by high reflecting DBR at the bottom and semitransparent top DBR. Using the appoach of the 5:5 MCLED with two DBRs the extraction efficiency is enhanced by about 65% compared to the silicon bulk pn-junction diode.:List of Abbreviations and Symbols
1 Introduction and motivation
2 Theory
2.1 Electronic band structure of semiconductors
2.2 Light emitting diodes (LED)
2.2.1 History of LED
2.2.2 Mechanisms of light emission
2.2.3 Electrical properties of LED
2.2.4 LED e ciency
2.3 Si based light emitters
2.4 Microcavity enhanced light emitting pn-diode
2.4.1 Bragg reflectors
2.4.2 Fabry-Perot resonators
2.4.3 Optical mode density and emission enhancement in coplanar Fabry-Perot resonator
2.4.4 Design and optical properties of a Si microcavity LED
3 Preparation and characterisation methods
3.1 Preparation techniques
3.1.1 Thermal oxidation of silicon
3.1.2 Photolithography
3.1.3 Wet chemical cleaning and etching
3.1.4 Ion implantation
3.1.5 Plasma Enhanced Chemical Vapour Deposition (PECVD) of silicon nitride
3.1.6 Magnetron sputter deposition
3.2 Characterization techniques
3.2.1 Variable Angle Spectroscopic Ellipsometry (VASE)
3.2.2 Fourier Transform Infrared Spectroscopy (FTIR)
3.2.3 Microscopy
3.2.4 Electroluminescence and photoluminescence measurements
4 Experiments, results and discussion
4.1 Used substrates
4.1.1 Silicon substrates
4.1.2 Silicon-On-Insulator (SOI) substrates
4.2 Fabrication and characterization of distributed Bragg reflectors
4.2.1 Deposition and characterization of SiO2
4.2.2 Deposition of Si
4.2.3 Distributed Bragg Reflectors (DBR)
4.2.4 Conclusions
4.3 Design of Si pn-junction LED
4.4 Resonant microcavity LED with CoSi2 bottom mirror
4.4.1 Device preparation
4.4.2 Electrical Si diode characteristics
4.4.3 EL spectra
4.4.4 Conclusions
4.5 Si based microcavity LED with two DBRs
4.5.1 Test device
4.5.2 Device fabrication
4.5.3 LED on SOI versus MCLED
4.5.4 Conclusions
5 Summary and outlook
5.1 Summary
5.2 Outlook
A Appendix
A.1 The parametrization of optical constants
A.1.1 Kramers-Kronig relations
A.1.2 Forouhi-Bloomer dispersion formula
A.1.3 Tauc-Lorentz dispersion formula
A.1.4 Sellmeier dispersion formula
A.2 Wafer holder
List of publications
Acknowledgements
Declaration / Versicherung
46
Fernandes, Ricardo Liz de Castilho. "Green emitting diodes for solid state lighting." Master's thesis, Universidade de Aveiro, 2016. http://hdl.handle.net/10773/17763.
Full textAbstract:
Mestrado em Engenharia FísicaNos anos recentes a iluminação de estado sólido impulsionou alternativas de iluminação efí cientes e ecológicas. Os desafi os correntes envolvem o desenvolvimento de materiais emissores de luz que convertem radiação de uma determinada energia para radiação de energia mais baixa, na gama do visível. Esta tese estuda um complexo novo, Tb(NaI)3(H2O)2 onde NaI é o ácido nalidíxico, que emite na região do verde e é estável sob iluminação no ultravioleta. Este foi incorporado em materiais híbridos orgânico-inorgânico tripodais com dois pesos moleculares médios (3000 e 5000 g.mol-1, denominados t- U(3000) e t-U(5000) respetivamente) que permitem o processamento de monólitos e fi lmes com forma e espessura controlada. Estes híbridos também aumentam o rendimento quântico absoluto de emissão de 0.11 medidos para o Tb(NaI)3(H2O)2 isolado para ~0.82 após incorporação no t-U(5000). Foi também demonstrado o potencial de usar estes materiais híbridos como emissores na região verde para uso em iluminação de estado sólido através do revestimento do díodo emissor na região ultravioleta (365 nm). Este LED apresenta uma efi cácia de 1.3 lm.W1.
In the last few years, solid state light-emitting diodes (LEDs) have been driving the lighting industry towards energy e cient and environmental friendly lighting. Current challenges encompass e cient and low-cost downconverting photoluminescent phosphors with emission in the visible region. This thesis will cover a novel UV-photostable green emitting complex, Tb(NaI)3(H2O)2 where NaI is nalidixic acid, was incorporated into organic-inorganic tripodal hybrid materials with two average molecular weights (3000 and 5000 g.mol{1, termed as t- U(5000) and t-U(3000), respectively) which enable the easy shaping of monoliths and lms with controlled thickness. Moreover, the hybrid hosts boost the Tb3+ green absolute emission quantum yield from 0.11 measured for the isolated Tb(NaI)3(H2O)2 complex to 0.82 after incorporation into t-U(5000). The potential use of the hybrid materials as UV-down converting green-emitting phosphors for solid state lighting was demonstrated by means of coating a near-UV LED (365 nm). This LED shows an e cacy of 1.3 lm.W1.
47
Hui, Kwun-nam. "Light-emitting diodes incorporating microdisks and microspheres." Click to view the E-thesis via HKUTO, 2008. http://sunzi.lib.hku.hk/hkuto/record/B41633969.
Full text
48
Ng, Man-ching Alan. "Light emitting diodes based on ZnO nanorods." Click to view the E-thesis via HKUTO, 2010. http://sunzi.lib.hku.hk/hkuto/record/B43572005.
Full text
49
Shaheen, Sean E. "Device physics of organic light-emitting diodes." Diss., The University of Arizona, 1999. http://hdl.handle.net/10150/289012.
Full textAbstract:
This work investigated several aspects of OLED device physics. The mechanisms responsible for the efficiency enhancement typically seen when a dye molecule is doped into the emission layer were examined. By comparing the spectra and efficiencies of single-layer devices for varying dopant concentrations, it was found that both charge transfer and energy transfer from the host molecule to the dye dopant are important processes. The measured efficiencies for photoluminescence and electroluminescence were found to be consistent with a simple model developed to explain the functional dependence on the dopant concentration. Work was also done on the enhancement of electron injection from an aluminum cathode using a thin layer of LiF. A double-layer device with the blue emitter DPVBi showed a factor of 50 enhancement in quantum efficiency upon insertion of a LiF layer. This technique has important practical application since it allows for the use of an environmentally-stable aluminum cathode while retaining high device efficiency. The effect of the ionization potential of the hole transport layer on the efficiency of a double-layer device was also investigated. TPD side-group polymers were used as the hole transport layer. The device efficiency was shown to increase as the ionization potential of the hole transport layer was pushed further from the work-function of ITO. This trend was attributed to an improved balance between the injection rates of holes and electrons. A Monte Carlo simulation of a single-layer device was developed which demonstrated the importance of balanced injection to obtain high efficiency. Drawing upon these results, an optimized OLED was fabricated which exhibited a luminous efficiency of 20 lm/W for green emission. This is one of the highest OLED efficiencies reported as of the date of this writing.
50
CHEN, FANG. "Halide perovskite nanocrystal-based light emitting diodes." Doctoral thesis, Università degli studi di Genova, 2021. http://hdl.handle.net/11567/1041035.
Full textAbstract:
Lead halide perovskite (LHP)-based optoelectronic device has been a hot research topic in the last years, owing to the versatile optoelectronic properties of this class of materials, such as large light absorption coefficient, long-range charge carrier mobility, high defect tolerance, direct bandgap, and to the facile synthesis process. Since their synthesis by Kovalenko group in 2015, LHP nanocrystals (NCs) gained increasing attention also for application in light-emitting diodes (LEDs), owing to the possibility to easily tune their emission wavelength across the whole range of visible light and their high color purity. A detailed introduction on the chemistry and physics of LHP NCs, some basic knowledge about LEDs and the development of LEDs based on the LHP NCs are provided in Chapter 1.
In Chapter 2, the chemicals, the basic experiments and the characterization techniques adopted in the thesis are introduced.
In Chapter 3, I identified four issues that need to be solved before the colloidal LHP NCs could be efficiently applied into the fabrication of high efficient and stable LEDs. Given that long-chain and insulating ligands are used during the NC synthesis to ensure their good solubility and stability in solution, conductivity of LHP NCs films is too low to transport carriers efficiently. Secondly, post-treatments on the LHP NCs, often aimed at improving the aforementioned transport properties, are easy to introduce new defects and compromise the optical properties of the LHP NCs. Thirdly, since LHP NCs are ionic crystals and sensitive to humid air as well as to post-treatments, their long-term stability is still a big issue. Finally, it is difficult to obtain smooth and compact LHP NCs films, which are necessary for low current leakage and high-efficient devices. I addressed the above four issues step-by-step and obtained highly stable LHP NCs films. A combination of benzoyl bromide, ammonium thiocyanate and ethyl acetate was used to treat the pristine LHP NC solution. The method demonstrated to be able to improve the conductivity of the LHP NCs films and well retain the optical properties and morphology even after ten months.
In Chapter 4, I employed the obtained NCs as emissive layer in green LEDs based on the proposed treatment method in Chapter 3. The champion device showed high efficiency of 1.2% at 518 nm with a maximum brightness near 3000 cd/m2 and high stability during operation with a half-lifetime of 27 min at a constant bias of 5 V as well as during storage (23 days in air). Furthermore, I conducted a mechanism study on the efficiency roll-off of the NCs-based LEDs using conductive atomic force microscopy (c-AFM). Morphology and current distribution of the NCs films under increasing bias were collected and a new insight about the efficiency roll-off was proposed.
In Chapter 5, I further focused on the improvement of the efficiency of blue LEDs based on LHP NCs, which is still lower than that of green and red ones. In this context, I studied the effect of addition of various metal halides during the synthesis of LHP NCs on their optical properties. I found that the post-synthesis addition of CuCl2 leads to the formation of NCs with sky-blue emission and high stability in air. I applied the obtained NCs in the fabrication of sky-blue LEDs. The champion device, based on NCs with further optimized ligands, produces the up-to-date highest external quantum efficiency (EQE) of 5.02% and the highest luminance of 130 cd/m2 at the maximum EQE.
In summary, this thesis firstly provides a promising route and proposes a possible mechanism to achieve high stable LHP NCs film. Secondly, efficient green LHP LEDs were obtained. Thirdly, a possible mechanism of the efficiency roll-off in LHP NC LED was proposed and may give guides for the design of NC LEDs with suppressed efficiency roll-off in the future. Last but not least, high efficient and stable sky-blue LED was fabricated based on CuCl2-treated LHP NCs, paving a promising way towards the fabrication of highly efficient LHP NC-based blue LEDs.