29-01-2013, 02:32 PM
Organic Light-Emitting Diodes (OLEDs) and Optically-Detected Magnetic Resonance (ODMR) studies on organic materials
Abstract
Organic semiconductors have evolved rapidly over the last decades and currently
are considered as the next-generation technology for many applications, such as
organic light-emitting diodes (OLEDs) in flat-panel displays (FPDs) and solid state
lighting (SSL), and organic solar cells (OSCs) in clean renewable energy. This
dissertation focuses mainly on OLEDs.
Although the commercialization of the OLED technology in FPDs is growing and
appears to be just around the corner for SSL, there are still several key issues that
need to be addressed: (1) the cost of OLEDs is very high, largely due to the costly
current manufacturing process; (2) the efficiency of OLEDs needs to be improved.
This is vital to the success of OLEDs in the FPD and SSL industries; (3) the lifetime
of OLEDs, especially blue OLEDs, is the biggest technical challenge. All these issues
raise the demand for new organic materials, new device structures, and continued
lower-cost fabrication methods.
Brief history of OLED technology
A. Bernanose and co-workers at the Université de Nancy, France, first discovered
electroluminescence (EL) in organic materials in the early 1950s by applying highvoltage
alternating current (AC) to crystalline thin films of acridine orange and
quinacridone (Fig. 1-1), but the EL was only a short burst of light at that time. They
proposed a mechanism of either direct excitation of the dye molecules or excitation of
electrons [1-4].
In the 1960s, Martin Pope and his group at New York University made seminal
discoveries, including the ohmic, injecting electrode contacts to organic crystals,
direct current (DC) EL, under vacuum, from a single crystal of pure anthracene as
well as tetracene-doped anthracene, (Fig. 1-1) [5-8]. Also in the 1960s, W. Helfrich
and W. G. Schneider produced double injection recombination EL for the first time, in
an anthracene single crystal using hole and electron injecting electrodes whose work
functions satisfied the requirements specified by Pope’s group [9]. In parallel, in the
1970s, the EL from polymer films was first observed by Roger Partridge at the
National Physical Laboratory in the United Kingdom, and the first polymer LEDs
(PLEDs), consisting of a film of poly(N-vinylcarbazole) (PVK) up to 2.2 μm thick
located between two charge injecting electrodes, was reported. The results of the
project were patented in 1975 and published in 1983 [10-13]. However, at that time,
the conductivity σ of such materials was so low that the devices required very high
driving voltages V (> 100 V), which limited light output and did not attract industry
interest.
Organic semiconductor materials: conjugated aromatic hydrocarbons
Organic materials comprise ~90% of the two million known materials. However,
among them, only a small fraction are conductive. These are typically conjugated
aromatic molecules, i.e., they consist of alternating single and double bonds [19].
The electronic configuration of the carbon atom’s ground state is 1s22s22p2 (Fig. 1-
4). In the tetrahedral methane or diamond bonding configuration, the four valence
electrons in the n = 2 shell occupy sp3 orbitals that result from the hybridization of a
2s and three 2p orbitals. However, it is possible that the 2s orbital will hybridize with
only two of the three available 2p orbitals to form three sp2 orbitals with one p-orbital
(pz) remaining. The 3 sp2 orbitals are all coplanar and oriented at 120° from each
other; the bonds formed by these 3 sp2 orbitals are called σ-bonds
Device structure of OLEDs
The device structure of the early OLEDs was very simple, consisting of only a
single organic layer between the anode and the cathode. One example was the first
PLEDs demonstrated by Burroughes et al., which involved a single layer of PPV. The
quantum efficiency of the PLEDs was only ~0.05%, partially due to lack of a
heterostructure [15].
As mentioned, the first small molecule bilayer heterojunction OLEDs contained
two organic layers, the TPD hole transport material and the Alq3, emitting and
electron transport material [14]. By inserting the separated hole transport layer, the
quantum efficiency of the SMOLEDs was drastically improved, approximately ~100
fold, to ~ 1%, compared to the predated thermally deposited anthracene
electroluminescent devices [14,32]. Then in the first heterojunction PLEDs, reported
in 1992, a polymeric heterostructure was developed using an electron transport layer
of the molecular material 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(PBD) dispersed in insulating poly(methyl methacrylate) (PMMA). This improved the
EL quantum efficiency to 0.8% [33]. With these studies, organic materials first
showed their potential as an efficient emissive technology applicable to all aspects of
the display and lighting industry. An intense examination by scientists and engineers
followed.