Organic white light-emitting diodes based on luminescence down-conversion [Elektronische Ressource] = Erzeugung von weißem Licht durch die Konversion der Lumineszenz von organischen Leuchtdioden / vorgelegt von Benjamin Claus Krummacher

Kb_1983

Informations

• Publié par: friedrich-alexander-universitat_erlangen-nurnberg
• Publié le: 01 janvier 2008
• Nombre de lectures: 6
• Langue: English
• Poids de l'ouvrage: 3 Mo

Extrait

Organic White Light-Emitting
Diodes based on Luminescence
Down-Conversion


Deutsche Übersetzung des Titels:

Erzeugung von weißem Licht durch die Konversion der
Lumineszenz von organischen Leuchtdioden




Der Technischen Fakultät der
Universität Erlangen-Nürnberg
zur Erlangung des Grades

D O K T O R – I N G E N I E U R

vorgelegt von

Benjamin Claus Krummacher

























Als Dissertation genehmigt von
der Technischen Fakultät der
Universität Erlangen-Nürnberg


Tag der Einreichung: 26.11.2007
Tag der Promotion: 02.06.2008
Dekan: Professor Dr. J. Huber
Berichterstatter: Professor Dr. A. Winnacker
Professor Dr. R. Weißmann









To Fritz Arthur Uhlmann (*1906-†1992)
Content


1 1. Introduction
1 1.1. Motivation
3 1.2. Content of this Work

5 2. Theory and Fundamentals
5 2.1. Structure and Fundamentals of OLED Devices
5 2.1.A Organic Materials for Light-Emitting Devices
6 2.1.B Physical Processes in an OLED
13 2.1.C Device Structure and Fabrication
15 2.2. Theoretical Description of OLED Half-Cavities
15 2.2.A Light Outcoupling from an OLED Device
17 2.2.B The Half-Space Model
19 2.3. Physiological Sensation of Light
19 2.3.A Human Vision
20 2.3.B Photometry
22 2.3.C Colorimetry
25 2.4. Generation of White Light by Down-Conversion
25 2.4.A The Down-Conversion Concept and Luminescence Converting Materials
28 2.4.B Previous Work on Down-Conversion OLEDs
30 2.4.C Down-Conversion Model by Duggal et. al.
32 2.5. Scattering and Absorption by Small Articles
32 2.5.A Interaction between Light and Matter
35 2.5.B Description of Scattering and Absorption according to MIE-Theory

40 3. The Blue Light Source
40 3.1. State of the Art of Blue OLEDs
46 3.2. Highly Efficient Solution Processed Blue Organic Electrophosphorescent Diodes
46 3.2.A Device Structure
48 3.2.B Influence of Charge Balance on Resultant Device Efficiency
51 3.2.C Influence of Optical Half-Micro Cavity Effects on Resultant Device Efficiency
55 3.3.Conclusion

56 4. Light Extraction Enhancement due to Substrate Surface Modification
57 4.1. Approaches for Light Extraction Enhancement
58 4.2 General Method to Evaluate Substrate Surface Modification Techniques for Light
Extraction Enhancement
58 4.2.A Experiment
61 4.2.B Results and Discussion
69 4.3. Conclusion

71 5. Down-Conversion OLEDs
71 5.1. Optical Analysis of Down-Conversion OLEDs
72 5.1.A Ray-Tracing Model of a Down-Conversion OLED
78 5.1.B Determination of Model Inputs, Sample Fabrication
85 5.1.C Experimental Confirmation of Model, Interpretation


97 5.2. Influences on Extraction Efficiency and Angular Color Homogeneity
97 5.2.A Influence of OLED-Reflectance on Extraction Efficiency
99 5.2.B Role of the Phosphor Particle Size Distribution
105 5.2.C Reduction of the Dependence of Emission Color on Viewing Angle using Half-
Cavity Effect
113 5.3. Outlook: Realization of the Down-Conversion Approach in OLED Lighting
Applications
118 5.4. Conclusion

121 6. Summary and Conclusion

126 Appendix A The Kubelka-Munk Function
129 B Annotations to Chapter 3
132 C Annotations to Chapter 4
135 D The Henyey-Greenstein Scattering Function
136 E Logarithmic Plots of Scattering Functions
137 F Optical Data of Materials used within this Work
139 G Abbreviations

145 References

155 Einleitung (German)
155 Motivation
158 Inhalt dieser Arbeit

160 Zusammenfassung (German)

165 Inhaltsverzeichnis (German)


1. Introduction
1.1. Motivation
Clearly, lighting has played a major role in human life since a piece of burning wood
was invented 500,000 years ago. Torches, later candles and oil lamps, separated lighting
from heating. Gas lighting (1772), electric lighting (1876) and fluorescent lamps (1938) were
milestones in lighting technology.
Contemplating the total primary energy consumption, today lighting accounts for
about 20 % of all the electricity produced [Misr06], which brings out the relevance of lighting
in daily life. Furthermore, this number underlines the importance of developing highly
efficient light sources, considering increasing environmental problems due to the growing
global energy consumption. Since the invention of the inorganic red light emitting diode
(LED) in 1962 [Holo62], solid state lighting has been developed to a technology which allows
replacing incandescent and fluorescent lamps by more efficient and more durable devices. It
is estimated that by 2025 solid state lighting could reduce the global amount of electricity
used for lighting by 50%; no other electricity consumer has such a large energy-savings
potential [DOE01]. Now a new competitor for inorganic LEDs is coming onto the market that
is based on organic semiconductors.
Initial point of the development of organic light emitting diodes (OLEDs) was
research work published by C.W. Tang and S.A. Vanslyke in 1987 [Tang87].
Electroluminescence from thin layers of organic molecules processed by evaporation was
reported in this publication. The results demonstrated the capacity of OLEDs for the first time.
Three years later Burroughes et al. showed that light-emitting devices can also be fabricated
based on polymers [Burr90]. Today numerous academic and industrial research teams are
focusing on both technologies, i.e. solution processable polymer OLEDs and small molecule
OLEDs fabricated by an evaporation process. The first commercial OLED product was
available in 1997, when Pioneer brought the first display based on small molecules onto the
market. The first commercial application of a polymer OLED was the display of an electric
shaver by Phillips in 2002 [Phil03]. 2 1. INTRODUCTION
Now OLED technology is on the verge of creating commercial applications in the
lighting sector. The remarkable advantages of OLEDs will drive innovative products and
open new fields of application: They are thin, flat and lightweight. The thickness of the diode
itself comprising the electrodes and the organic layers sandwiched in between is below 1 μm.
However, the thickness of the device is basically determined by the substrate and the
encapsulation; at the state of the art the thickness of the resulting device can be reduced below
1 mm. Furthermore, the technology offers the production of large area lighting panels in a
cheap and simple process.

12
Single white stack Vertical RGB
stack
34
Horizontal RGB Blue OLED and
stack phosphor layer


Fig. 1-1. Schemes of the four general approaches to generate white light based on organic
light-emitting devices.


White light-emitting OLEDs can be generated by four approaches, schematically
shown in Fig. 1-1: (1) A single white emitting stack, where the white emission is achieved by
using a combination of different emissive components providing red, green, and blue light
from a single emitting layer [Slyk00]. This device architecture offers easy processing but it is
not easy to tune the color without affecting device performance. (2) A vertical red-green-blue
(RGB) stack where the output spectrum of such a device is determined by the three light- 1. INTRODUCTION 3
emitting components [Shen01]. This device architecture leads to color homogeneity over the
active area but relies on complex processing methods. (3) A horizontal RGB stack where the
output spectrum of a horizontal stack can be changed while operating the device when
addressing the patterns separately. Current methods to manufacture a device in this way rely
on expensive printing techniques. For all the above mentioned methods, color stability is
difficult to be achieved due to different lifetime aging rates of the emitters involved.
Method (4) is using a single blue emitting OLED in combination with a down-conversion
layer. Here a luminescence converting material (phosphor) coated on the underlying OLED
absorbs a part of the photons emitted by the light source and emits them at a different
wavelength. The non-absorbed fraction of the photons emitted by the light source and the
photons emitted by the phosphor constitute the output spectrum of the coated device. This
approach can be implemented by easy fabrication techniques and can provide better color
stability as the aging rate is determined by only one emitter. The efficiency of such a device is
limited by the efficiency of the blue OLED. White light-emitting devices based on an
inorganic blue LED and on down-conversion by phosphor were first published by Schlotter et
al. [Schl97] and are widely used in existing products. Duggal et al. were the first to
implement the idea to th