Organic Light-Emitting Diodes (OLEDs) with Polarised Emission
In this chapter the use of reactive mesogens in polarized organic light emitting diodes (OLEDs) is described. The first part also serves as an introduction to the basic principles of OLEDs, which have attracted enormous interest since 1986 when Tang and v
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Organic Light-Emitting Diodes (OLEDs) with Polarised Emission E. Scheler and P. Strohriegl
In this chapter the use of reactive mesogens in polarized organic light emitting diodes (OLEDs) is described. The first part also serves as an introduction to the basic principles of OLEDs, which have attracted enormous interest since 1986 when Tang and van Slyke described the first thin layer OLED with high brightness and a low operating voltage.
7.1 Organic Light Emitting Diodes: Basic Principles The materials used in Tang’s OLED [1] and its basic structure are shown in Fig. 7.1. Two different organic materials have been used, the electron conductor and green emitter Alq3 and the hole conducting aromatic amine TAPC. Both materials are deposited on top of an indium tin oxide (ITO) glass by vacuum evaporation and covered with a magnesium/silver cathode. The OLED emits green light with a maximum brightness of more than 1,000 cd m2 and an efficiency of 1.5 lm W1 . Meanwhile OLEDs have reached a high level of brightness and exhibit high efficiencies. This goal has been reached by optimizing each single layer and by adding additional layers to form multi-layer OLEDs. State-of-the-art OLEDs possess up to seven functional layers, which are responsible for example, for hole injection, hole transport, light emission, electron transport and electron injection, respectively. Additional hole- and electron-blocking layers are also frequently used [2–7]. The development of phosphorescent iridium organometallic complexes was another major breakthrough [8]. Such iridium complexes are triplet emitters and enhance the theoretical quantum efficiency of OLEDs from 25% for fluorescent E. Scheler • P. Strohriegl () Lehrstuhl fuer Makromolekular Chemie I, Bayreuther Institut fuer Makromolekulforschung BIMF, University of Bayreuth, Bayreuth D-95440, Germany e-mail: [email protected] R.J. Bushby et al. (eds.), Liquid Crystalline Semiconductors, Springer Series in Materials Science 169, DOI 10.1007/978-90-481-2873-0 7, © Springer ScienceCBusiness Media Dordrecht 2013
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E. Scheler and P. Strohriegl CH3
CH3
Mg Ag Alq3
N N
O
O
N CH3
TAPC
Diamine ITO Glass
Al
N
H3C
O
N
Alq3
Fig. 7.1 Basic structure and materials used in the first efficient thin layer OLED [1]
2.6eV
2.7eV
N
3.2eV α-NPB 40 nm 4.7eV
ITO
N
Ir
3.7eV Ir(ppy)33 BCP in CBP 6 nm 20 nm
Alq3 20nm
N
MgAg
Irpy3
5.7eV N
6.0eV
6.3eV
N
6.7eV
CBP N N
N
N H3C
α-NPB
O
O
N
N
BCP
Al O
N
CH3
Alq3
Fig. 7.2 Energy level diagram and materials used in an efficient green phosphorescent OLED [8]
singlet emitters to 100%. The energy level diagram and the materials for a green phosphorescent OLED are shown in Fig. 7.2. Holes are injected from the ITO anode into the aromatic amine ’-NPB, a typical hole conductor. From the magnesium/silver cathode, electrons are injected into Alq3, which acts as an electron conductor in this setup. Carrier recombination takes place in the emission layer, which consists of 6% of the green phosphorescent emitter
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