This work comprises different approaches for the efficiency enhancement of white organic light-emitting diodes (OLEDs). In particular, diffusion and transfer processes of excited singlet and triplet states are investigated. Generation of white light is realized by using the so-called triplet harvesting method where the otherwise nonradiatively decaying triplets of a blue fluorescent emitter are transferred to a highly efficient phosphorescent emitter and result in additional emission at lower energies. Triplet harvesting significantly increases the internal quantum efficiency in OLEDs. First, the well-known blue emitter 4P-NPD is investigated as model case. Using time-resolved spectroscopy, triplet harvesting by a yellow and red phosphorescent emitter, respectively is directly proven. However, triplet harvesting by a green emitter is not possible due to the low triplet energy of 4P-NPD. Using quantum chemical calculations, two new emitter molecules, 8M-4P-NPD and 8M-4P-FPD, are synthesized with the aim to rise the triplet energy. Their properties and their ability to facilitate triplet harvesting by a green emitter are studied. For the first time, a white triplet harvesting OLED is demonstrated where triplet harvesting occurs directly from a blue emitter to a green and a red emitter. Furthermore, an additional singlet transfer is observed in the triplet harvesting OLEDs under investigation. Using the phosphorescent emitter as singlet sensor, this effect allows the determination of the singlet diffusion length in 4P-NPD. By varying the distance between singlet generation zone and singlet sensor, a singlet diffusion length of 4.6 nm is found. One further approach to increase the efficiency is the optimization of a tandem OLED which comprises two single OLED units stacked on top of each other. At a luminance of 1,000 cd/m², the white tandem OLED shows an external quantum efficiency of 25%, a luminous efficacy of 33 lm/W, a color rendering index (CRI) of 62, and Commission Internationale de l’Eclairage (CIE) color coordinates of (0.53/0.43). These efficiencies are comparable to state-of-the-art efficiencies of white OLEDs. Finally, the highly efficient white tandem structure is applied on an alternative electrode consisting of flattened silver nanowires. In comparison to the conventional OLED with indium-tin oxide (ITO) electrode, this OLED shows similarly high efficiencies as well as a superior color stability in terms of viewing angles. The color stability can be assigned to the light scattering properties of the nanowires. The OLED with silver nanowire electrode shows efficiencies of 24% and 30 lm/W at 1,000 cd/m² with a CRI of 69 and CIE coordinates of (0.49/0.47). / In dieser Arbeit werden verschiedene Ansätze zur Effizienzsteigerung in weißen organischen lichtemittierenden Dioden (OLEDs) erforscht. Hierfür werden im Besonderen Diffusions- und Transferprozesse von angeregten Singulett- und Triplettzuständen untersucht. Zur Erzeugung von weißem Licht wird die sogenannte “triplet harvesting” Methode verwendet, bei der die sonst nicht zur Emission beitragenden Triplettzustände eines fluoreszenten blauen Emitters auf einen hocheffizienten phosphoreszenten Emitter übertragen werden. Dieser liefert dann zusätzliche Emission im niederenergetischen Spektralbereich. Durch triplet harvesting kann die interne Quantenausbeute in OLEDs beträchtlich gesteigert werden. Zunächst wird der bekannte blaue Emitter 4P-NPD als Modellbeispiel untersucht. Mittels zeitlich aufgelöster Spektroskopie kann triplet harvesting auf einen gelben bzw. roten Emitter direkt nachgewiesen werden. Allerdings ist auf Grund der niedrigen Triplettenergie triplet harvesting auf einen grünen Emitter nicht möglich. In Anbetracht dieser Tatsache werden unter Zuhilfenahme quantenchemischer Betrachtungen zwei neue Emittermoleküle, 8M-4P-NPD und 8M-4P-FPD, synthetisiert und auf ihre Eigenschaften und ihre Eignung für triplet harvesting untersucht. Dabei wird zum ersten Mal eine weiße OLED realisiert, in der triplet harvesting von einem blauen Emitter direkt auf einen grünen und einen roten Emitter erfolgt. Des Weiteren wird bei den untersuchten triplet harvesting OLEDs ein zusätzlicher Singulettübertrag auf den phosphoreszenten Emitter beobachtet. Dieser Effekt wird zur Bestimmung der Singulettdiffusionslänge in 4P-NPD genutzt. Der phosphoreszente Emitter dient dabei als Singulettsensor. Über eine Variation des Abstands zwischen Singulettgenerationszone und Sensor wird eine Singulettdiffusionslänge von 4,6 nm bestimmt. Ein weiterer Ansatz zur Effizienzsteigerung besteht in der Optimierung einer aus zwei OLEDs zusammengesetzten Tandem OLED. Bei einer Leuchtdichte von 1000 cd/m² erzielt diese weiße Tandem OLED eine externe Quanteneffizienz von 25% und eine Leistungseffizienz von 33 lm/W mit einem Farbwiedergabeindex (CRI) von 62 und Commission Internationale de l’Eclairage (CIE) Farbkoordinaten von (0,53/0,43). Diese Effizienzen sind vergleichbar mit dem aktuellen Forschungsstand weißer OLEDs. Schließlich wird diese hocheffiziente weiße Tandemstruktur auf eine alternative Elektrode bestehend aus flachgedrückten Silbernanodrähten aufgebracht. Im Vergleich zur konventionellen OLED mit Indiumzinnoxid (ITO) Elektrode erreicht diese ähnlich hohe Effizienzen sowie eine verbesserte Farbstabilität bezüglich des Betrachtungswinkels, was auf die Streueigenschaften der Nanodrähte zurückgeführt werden kann. Bei einer Leuchtdichte von 1000 cd/m² zeigt die OLED mit Silbernanodrahtelektrode Effizienzen von 24% und 30 lm/W bei einem CRI von 69 und CIE Koordinaten von (0,49/0,47).

The high performance organic light-emitting diodes (OLEDs) have been studied. First, we have fabricated a WOLED with AlF3 and m-MTDATA as a hybrid buffer layer. Results indicate that the turn-on voltage can be reduced to 3.1V, and the luminous efficiency can be improved to 14.7 cd/A when a hybrid buffer layer was used. Since the turn-on voltage decreases and the efficiency increases, the power consumption as well as lifespan are then improved. Moreover, the luminous efficiency of the hybrid buffer layer devices also immunes to drive voltage variations. Second, we studied the properties of transportation in OLEDs. The study presented the device of a WOLED with a combination of a graded hole transport layer (GH) structure and a gradually doped emissive layer (GE) structure as a double graded (DG) structure. The DG structure: ITO/MTDATA(15 nm/NPB(15 nm)/NPB:25% BAlq (15 nm)/NPB : 50% BAlq (15 nm)/BAlq:0.5% Rubrene (10 nm)/ BAlq : 1% Rubrene (10nm) /BAlq:1.5%Rubrene (10 nm) / Alq3 (20 nm)/ LiF (0.5 nm)/Al (200 nm) is beneficial for improving both electrical and optical performances. The luminous efficiency of the DG device is 11.8cd/A, which is larger than that of 7.9cd/A with the HJ device. This improvement is attributed to the discrete interface between hole transport layer and emissive layer can be eliminated, surplus holes can be suppressed, electron-hole pairs can obtain optimal transportation and recombination in the emissive layer, and quenching effects can be significantly suppressed. Moreover, the spectra were almost not changed with an increasing drive current. As the efficiency was improved, it is expected that the power consumption can be reduced as well. Third, high efficiency and brightness p-i-n OLEDs with a CsI-doped Alq3 layer as a n-ETL has been studied. The p-i-n WOLED with a 15 % CsI-doped Alq3 layer exhibits a luminous efficiency of 5.75 cd/A at a driving current of 20mA/cm2 as well as a maximum power efficiency of 4.67lm/W. This improved performance is attributed to the increased electron carriers of the n-ETL and the balance of electrons and holes in the recombination zone. The X-ray photoelectron spectroscopy (XPS) have shown that doping of CsI caused chemical reaction, attributing to the increase of carriers. Finally, we focus on the improvement of contrast ration (CR) of OLEDs. We successfully fabricated a conductive organic-metal light-absorbing layer with a high CR and low reflectance for use as a black cathode in an OLED. The black cathode that was fabricated using vacuum deposition has the advantages of low cost and simple fabrication. Moreover, the J-V characteristic of the black cathode device is almost identical to that of a conventional device. Additionally, the reflectance can be reduced from 66.2% to 11.3% and a small reflectance variation around 3.3% over the visible spectrum is appealed. At an ambient illumination of 250 lx, the CR can be increased from 4.2 to 10.8 at a brightness of 250 cd/m2.

In last decade, white light emitting diodes(LEDs) have become used widely from traditional indicator to general illumination. The increase of its power is the key improving issue. The current light efficiency of white LED about 30%. In other words ,more than 70% of the input electrical energy will be generated in the form of heat. So, how to get rid of the heat damage in high power LED is a severe problem. The finite element analysis is employed to simulate high power white LEDs temperature distribution and thermal stress distributions caused by the dissipated heat. The effects of package parameters, i.e. die attach, solder material, solder thickness, and chip substrate, on the temperature and thermal stress distributions on high power LED packages are simulated and studied in this thesis. A comparison between the 40mil single chip package and the chip on board(CoB) package has also been executed in this study. Simulated results indicate that the highest power of a single 40mil chip package is 7watt. The thermal stress distribution , i.e. the peak value of local thermal stress is over its yield strength, is occurred as the power up to 7watt. Numerical results also reveal that the appropriate fin design can improve the heat dissipation significantly in high power LED package.

Pure m-plane p-GaN/InGaN/n-GaN on the m-sapphire grown by plasma assisted molecular beam epitaxy (PAMBE) had been achieved. V/III ratio of the first layer m-plane GaN is 20 and growth temperature is 665 ¢XC. ¢½/¢» ratio and the growth temperature are the most important factors in the growth sequence. M-InGaN film with better crystal quality was grown successfully by tuning these two factors. We have obtained a narrow window for epitaxial growth of m-plane InGaN/GaN on m-sapphire at 450 ¢XC. The striated surface is along (1120) a-axis direction of m-InGaN epilayer. As the growth temperature is increased further to 550 ¢XC, there is no InGaN signal from x-ray diffraction (XRD). We study the effect of growth condition on the structural properties and morphology of these films using high-resolution x-ray diffractometer (XRD) and scanning electron microscopy (SEM)

In this thesis, we design electrode shape of light emitter diode (LED) to help the current diffusing uniformly. The purpose of the uniform current is to avoid the waste heat from the devices and enhance the efficiency of active region. The LED samples adopted in this study are GaN base materials grown on sapphire. The P-N electrodes must be processed on the same side since the poor conductivity of sapphire. The same side P-N electrode will results in current crowding phenomena. We design special electrode shapes to make the current diffuse uniformly and reduce the current crowding phenomena. First, we use COMSOL simulation software to simulate the current spreading between the electrodes. We adopt the same parameters from the reference papers to confirm the reliabilities of the simulation. Then we simulate several electrode shapes with highly uniform current spreading. Second, we use the simulation results to fabricate electrode on chips. The first set is LED without transparent conductive layer. This set is to confirm whether the fabrication processes is feasible and adjust the simulation parameters at the same time. The second set is LED with transparent conductive layer. The experimental emission intensity has deviation from the simulation results. We deduce the emission intensities from smaller LED chip size will have great influence on illumination surface. The third set is electrodes fabricated on large size LED chip. The electrode patterns successfully enhance the uniformity of current spreading, and enhance the output light intensity of 21%. The current density distribution trend from simulation is matched with the illumination intensities.

This research plans the power capacities of the obstacle lighting on the power transmission towers, which are located in the areas where the utility cannot reach. The obstacle lighting is formed by light emitting diodes (LEDs), which are powered mainly by solar cells and subordinately by rechargeable batteries. The solar cells charge the batteries during the sunny daytime with plenty sunlight. When the sunlight is insufficient and the obstacle lamp is turned on, batteries and solar cells supply the obstacle lighting simultaneously. The power capacities of the solar cells and batteries are designated to keep the obstacle lighting system uninterruptible either under the drastic weather variation or a long period of insufficient sunlight. Under the specified operation rules of the obstacle lighting, a more economical and precise method is proposed for planning the power capacities of solar cells and batteries based on the weather data from Central Weather Bureau in recent 6 years following. The power planning method is implemented in the areas of Kaohsiung, Chiayi and Ali mountain to demonstrate the feasibility and the accuracy in reality.

In this thesis, we use self-assembled nano metal particles as a dry etching mask to from nanopillars. The nanopillars integrated with traditional light-emitting diode (LED) p-type GaN surface is designed to increase the light extraction efficiency. The initial fabrication process adopted in this study is using 100nm SiO2 as thermal aggregation layer. The poor thermal conductivity of SiO2 material will help to accumulate heat on the surface. Then, 10nm Ni thin film is deposited on the SiO2, and rapid annealed at 900oC (working pressure of 1~3¡Ñ10-6 Torr). The Ni nanospheres are prepared to integrate with LED chip processes. We use the etching times (pillar heights) as experimental parameters to study the degree of light extraction efficiency. Traditional right angle branch electrode samples of as grown, 20, 30, 40 sec etching time are analyzed by LI curve measurement. Under 20mA injection current, samples with 20, 30, 40 sec etching times have better light extraction than as grown, an increase of 6.54%, 3.27%, 1.63%, respectively. The experimental results reveal that self-assembled nano metal particles as a dry etching mask on the p-type GaN LED surface can increase the light extraction efficiency.

White light generation through phosphors excitation by UV-LED has become an important subject in WLED. In our study, compounds of SrSiO3¡GDy3+ white-emitting phosphors are synthesized via hydrothermal method. In this study, the effects of annealing temperature, annealing time, contents of Dy3+ on the relative emission intensity of the phosphors are judicially investigated. Meanwhile, WLED produces a plenty of heat energy to cause phosphors high temperature when WLED is excited by electrical energy. The emission spectra of the phosphors are investigated excited by Hg arc lamp (£f=365nm) under high temperature environment. Another, two distinct emission bands from SrSiO3¡GDy3+ phosphors are found to be around 480nm and 572nm when the phosphor is excited by Hg arc lamp(£f=365nm). The combination of these two emissions forms a white light like color to naked eyes, showing that SrSiO3¡GDy3+ presents to be a promising phosphor for applications in fabricating white-light-emitting diodes through UV pumping.

ABSTRACT Optoelectronics of polymer light emitting diode (LED) depends significantly on polymer molecular structure and charge conjugation. This study focused on the optoelectronics of freestanding films and LEDs of a colinear, fully conjugated heterocyclic aromatic rigid-rod polymer (PBT) and its mixtures with a partially conjugated coil-like polymer (Pbi). A deuterated PBTd4 was also mixed with a fully conjugated coil-like polymer (ABPBI) for UV-Vis absorption spectrum, photoluminescence (PL), diodic current-voltage response, and electroluminescence (EL). Rigid-rod PBT was only soluble in strong protic acid. PBT films were processed using methanesulfonic acid. PBT free-standing films showed maximum absorptions at 468 nm and 640 nm; PBTd4 having all hydrogen atoms on the phenylene moiety substituted by deuterium retaining same electron orbitals thus showed same absorption and PL spectra. It was likewise for the PBTd4 and ABPBI mixtures at ABPBI concentrations of 1 % and 10 %. For mixtures of PBT and Pbi, the absorption spectra indicated super- position of individual optical absorption response and no energy transfer. However, PL spectra showed a blue shift with increasing Pbi content. This was attribed for PBT rod-like configuration, or PBT aggregation perturbed by mixing with Pbi. Monolayer LED of Al/PBT/ITO and Al/Pbi/ITO yielded a threshold voltage of 4 V. When PBT/Pbi mixtures of 75/25, 50/50, 25/75, were used as the light emitting layer, the threshold voltage altered to 10 V, 7 V and 17 V, respectively. This threshold voltage deviation from 4 V is due mainly to difference in layer thickness, or phase separation affecting the tunneling effect. To enhance LED stability, an Ag layer was evaporated onto the Al electron injection electrode. For Ag/Al/PBT/ITO devices and mixed PBT/Pbi (75/25,50/50,25/75) devices, the maximum EL wavelength exhibited no systematic change at 753 nm, 714 nm, 727 nm, and 697 nm, respectively, due to using different bias voltage.

Polymers containing bulky tetraphenylthiophene (TP) moieties were prepared by different coupling reactions. Firstly, 2,5-bis(4-bromophenyl)-3,4-diphenylthiophene (TP-Br) was coupled together by either NiCl2/PPh3 or n-BuLi to form polymers with TP as the repeat unit. The resulting polymers (PTP-NiCl2 and PTP-BuLi) are easily soluble in organic solvents and are photoluminescent (PL) materials (