Chapter 2 OLED - Organic Light Emitting Device
2.2 Luminescence of organic materials
Organic materials refer to a base of the IV-group carbon and with additional elements such as hydrogen (H), nitrogen (N), oxygen (O), phosphorus (P) and sulfur (S). Since polymers and organic materials are not referred to conducting materials because of the large band gap between conduction and valence bands, a very high electrical field has to be applied. When applying an electrical field, charge carriers including hole and electron are injected from the electrodes into the organic layer respectively and result in geometrical defects on the symmetric organic structure and exhibit a lower band gap according to
Fig. 2-2. The charged carriers move along the structure and the attraction
between the carriers results in a recombination of electron and hole to form an exciton which has a possibility to emit light as photons. The exciton is either in singlet or in triplet-state according to the Pauli’s principle [7]. The exciton will form two new energy bands inside the band gap. The energy level of the exciton is below the conduction band and the released energy is emitted as photons. Upon the relaxation of exciton, heat and photons will be emitted with an energy set according to the gap between the energy bands. The states of the exciton have certain influence of the emission of light and quantum efficiency since the singlet states release its energy as an emission of photons. However the triplet-state is regarded as the heat forming state and can not transfer to light. In some special cases, the triplet-state generates light as well.The backbone of the organic materials is strongly localized bonds between the carbon atoms. The conductivity is enabled through bonds that are orthogonal to the backbone. The length of the conjugation, i.e. the conjugation length, set certain characteristics of the molecule. The conjugation length defines the length where the
electron is free to move within. In general, small molecules tend to have short conjugation length, while longer conjugated molecules, polymers, may have a longer.
Longer conjugation length results in a smaller band gap. It is therefore easier to produce red light with conjugated polymers compared to small molecules, and consequently small molecules can more easily generate blue light.
cathode anode
+
-
-+ +
hole-transporting layer
electron-transporting layer
HTL ETL
HOMO
LUMO
Fig. 2-2. Schematic energy band diagram of OLED and electron-hole recombination in forward bias condition.
2.3 The composition of OLED 2.3.1 The mono-layer device
The simplest OLED, a mono-layer device, is constructed of a thin layer of organic material sandwiched between two electrodes. As shown in Fig. 2-3(a), the function of the anode is to supply positively charged holes and a frequently used material, due to the requirement of transparency, is the Indium Tin Oxide (ITO). The cathode electrode supplies electrons to the organic layers. The charged carriers of electrons and holes are injected into the thin emitting layer material where they form an exciton and generate photons afterwards. The fact of the singlet and triplet states of the excitons has a certain influence and limitation on the device quantum efficiency.
Because of the disordered structure in organic materials, the behaviors of the charged carriers in organic materials are not similar to that in metal. This fact impedes a recombination and decreases its probability [8]. Therefore, there is always one dominant charge carrier that moves through the material without recombining with the opposite charge carrier. Another problem related to mono-layer devices is that the charge carriers tend to remain at one of the electrodes and creating space charges that prevent further hole and electron injections. Furthermore, if charges form an exciton near a metal, quenching mechanism can destroy the exciton and reduce the light generation efficiency.
+ + + + + - - - -
-glass anode cathode
emitting layer
+
glass anode cathode
emitting layer + + + +
-ETL
(a) (b)
Fig. 2-3. Configurations of OLED for (a) fundamental single-layer structure and (b) double-layer heterostructure structure.
2.3.2 The double-layer device
A minimization of the energy barrier between the electrodes and the organic material is another approach to enhance the injection of the carriers. The mono-layer device is then upgraded to a double-layer device. Instead of one organic layer, two layers, an emission and an electron-injection layer, are placed between the electrodes, as shown in Fig. 2-3(b). By choosing the materials according to their properties of mobility, band gap, the charged carriers can be injected and transported in an easier
manner into the emission zone where an emission of light takes place [9].
The band diagram of the materials can be matched better to the specific electrodes through the double-layer construction and result in improved equilibrium of the currents of holes and electrons. The electrical field can consequently be decreased, which increases the efficiency and lifetime of the device. The difference in energy levels between the two organic layers creates a potential barrier at the interface.
The barrier confines the holes and electrons and contributes to an increased recombination probability [10].
2.3.3 The multi-layer device
The functionality of the double-layer device and its components can be further improved with regard to several layers and minimal potential barriers. The greatest advantage of a multi-layer structure, compared to a double-layer, is the possibility to separate transport regions from the emitting region. This multi-layer structure accomplishes a better performance and an increased range of colors of the emitted light. By tuning the voltage over the electrodes, a proper placement of the emitting zone can be achieved, which may improve the efficiency of the device. The structure in Fig. 2-4 is an example of a multi-layer bottom-emission OLED, which has its light emitted through the transparent anode.
The substrate on which the OLED is fabricated consists of a material with a rigid or a flexible structure, e.g. glass or plastics. Compared to flexible substrates, rigid materials are of advantage for their ability to prevent the device from moisture and air.
An outer exposure of moisture and air without any protection significantly degrades the lifetime and performance of the device.
To enhance the injections from the anode, according to Fig. 2-4, a hole injection
layer (HIL) is introduced for control and an enhancement of the injection of the holes into the hole transport layer (HTL). An effective HTL of p-type material can effectively enhance the transportation of holes to the emitting zone. The excitons are expected to be formed in the emitting layer (EL) and eventually emit light. Since equilibrium of mobilities between the carriers is difficult to achieve, a blocking layer can be used to confine the charge carriers to perform a maximum recombination.
glass anode
Emitting Layer (EL)
Hole Transporting Layer (HTL) Hole Injection Layer (HIL) Electron Injection Layer (EIL) Electron Transporting Layer (ETL) cathode
Fig. 2-4. Multi-layer OLED structure with additional functioning HIL, HTL, EIL and ETL.
The electron transport layer (ETL) is of n-type doped material which can enhance the mobility of the charge carriers and the transportation of electrons to the emitting layer. The layer also functions as hole blocking and is suitable to control specific charged carries. The ETL can, similar to the HTL, be used as an emitting layer. Similar to the holes, an organic electron injection layer (EIL) is used to assist the electrons to cross the barrier between the cathode and the ETL. Due to the enhanced transportation of electrons, a low electrical field is adequate for the multi-layer device and leads to a more power efficient OLED. It is furthermore possible to use the same material for the two electrodes, yet at a reduced efficiency.
2.3.4 Modifications
A bottom-emission OLED has a transparent substrate such like the glass. A silicon backplane can be used as the substrate as well and with a different combination of the electrodes and the organic layers to form a top-emission device can be accomplished. In comparison with conventional bottom-emission OLED, the top-emission OLED presents more advantageous for some micro-displays appllications[11][12]. When using a transparent cathode in the bottom-emission OLED, the device has become a transparent OLED (TOLED). The transparency of the device then is nearly as transparent as the chosen substrate. For advanced multi-stacked OLED devices, e.g. stacked OLED (SOLED), the transparent device is required.
It is also possible to apply the OLED on a flexible material to make a bendable or foldable display, enabling a completely new era of display types, e.g. displays directly deposited on the windshield or the helmet visor. Although more research has to be done in order to make the efficient non-glass encapsulation against humidity and oxidation, flexible OLED also brings a more rugged structure due to no need for the glass substrate and their bendable nature. Since standard OLED uses a cathode of a metallic material, approximately 75% of the incident light is reflected back and causes a poor contrast when ambient light passes through the display [13]. The use of a TOLED in combination with an absorber or an optical interference structure improves the contrast, e.g. Luxell’s black layer results in a 180° phase shift to cancel out the ambient radiation [14].
2.4 Fabrication of OLED devices
The difference between the small molecules and conjugated polymers is mainly
the fabrication and patterning process. All processes need a pure deposition of the organic material on a cleaned substrate. As moisture and UV-light have certain effects on the degradation mechanism of the device during manufacturing [ 15 ][ 16 ]
, the processes for either small molecules or conjugated polymers should be elaborated for long device lifetime.
2.4.1 Patterning small molecules
Small molecule layers are deposited through a vacuum evaporation process, as shown in Fig. 2-5. The surfaces roughness of the organic layers deposited by evaporation process sequentially are important for the lifetime of the device. A disadvantage of vacuum evaporation process is the different for large-size substrate.
turbo pump
cold trap
rough pump
holder
shutter
source boat
vacuum chamber substrate
power supply
Fig. 2-5. Schematic diagram of vacuum evaporation system for OLED fabrication.
2.4.2 Patterning polymers
Conjugated polymers can be applied to a surface through either dip-coating or spin-coating. Dip-coating is a method where the substrate is slowly dipped into the polymer solution, which results in a cover of polymers at both sides. The spin-coating
method is performed by dripping the polymer solution onto a rotated plate where the polymer solution is spread out to form a uniform thin film, as revealed in Fig. 2-6(a).
The thickness of the layers is dependent on the composition of the polymer and the concentration of the polymer solution. However, spin-coating results in non-smooth and non-parallel surfaces, which would cause degradation of devices because of the different distances between the electrodes.
(a) (b)
Fig. 2-6. Fabrication approaches for solvent based PLED. (a) spin-coating process and (b) ink-jet printing process.
An advantage of the spin-coating method compared to the vacuum deposition is the simplicity. However, it is also a less material efficient utilized process, and it should be taken into account in fabricating large size display, the spin-coating method can result in much higher costs. A drawback of spin-coating is the requirement of the removal of water and oxygen before the metal deposition [ 17 ]. However, the conventional spin-coating process is difficult to integrate the multiple color materials on the pixels individually with high resolution to realize a full-color display.
Ink jet printing is another promising candidate for the patterning of pixels to achieve the full-color display with polymers. This allows direct pattering of polymer
layers with high resolution and large-size substrate to yield full-color displays without the use of expensive vacuum deposition systems [18][19][20]
, as shown in
Fig. 2-6(b).
The ink-jet printing technology requires optimization of the solvents used to dissolve the polymers to formulate the inks. Selection of a suitable solvent is important since it affects the film morphology, which can influence the device performance in terms of device efficiency and stability. In addition, the surface energy of ITO electrode and the material of bank which defines the pixel region are as well the key parameters that control the spreading of polymer solvent and the resulted film uniformity.
2.5 Full-color approaches in manufacturing
In a similar manner for small molecules and polymers, fabrication technologies to achieve higher color purity have been proceed. The synthesizing and doping of organic materials have modified their chain structure and color characteristics. For emission of several colors, a host material can be doped with dye and new luminescent properties are then accomplished [21]. In order to achieve the full-color displays, several approaches will be introduced in following sections.
2.5.1 Patterning RGB
The patterning RGB is a straightforward approach in much the same way as CRT to achieve colors without an additional color-filter. As shown in Fig. 2-7(a), each pixel is divided into three parts called subpixels. By the combination of various gray levels of three primary colored subpixels, an arbitrary color can be generated. Since each OLED must have a different organic thin film as its light emitting layer, a drawback of this approach is in high-resolution because the arrangement of the subpixels becomes three times tighter to achieve a similar resolution compared to other approaches. In
other words, for an achievement of high-resolution, the patterning process is considered to be a difficulty for a small pixel pitch. A patterning technique called shadow masking was proposed. Besides, to enhance the color gamut, it is proposed to combine the emitting subpixel with an additional color filter.
substrate
R G B
substrate
R G B
white OLED OLED
CF
(a) (b)
Fig. 2-7. Schematic diagrams for achieving full-color displays based on OLEDs. (a) patterning RGB approach with separate subpixels. (b) Filtering white light with color-filter.
2.5.2 White light OLED combined with color-filter (CF)
Filtering white light with color-filter is commonly used in AM-LCD to achieve full color. The main advantage with this method is no need to pattern the bottom substrate with different types of OLEDs and therefore only one white light OLED is used over the whole surface. The different colors are then obtained through a band-pass filtering, as depicted in Fig. 2-7(b). The principal drawback of this approach is that much of the white OLED output must be absorbed by the filter to obtain the required primary colors. For example, up to 66% of the optical power from the white OLED is filtered in order to obtain three originals in ideal case, with the result that the OLED must be driven up to three times brighter than the required RGB pixels brightness. Since the rate of OLED degradation is a function of drive current,
this technique substantially results in a high power consumption, which can shorten the lifetime of OLED [22]. Besides, degradation is further enhanced as the filtered light generates heat in the substrate. In practical, the eMagin OLED micro-display light throughput reduction is 88 %, because of the color-filters [23].
2.5.3 Stacked OLED
Another approach to achieve full-color is through the stacked OLED (SOLED) device. Due to the fact that a complicated multi-layer structure is used in SOLED, as shown in Fig. 2-8(a), the approach is suitable for small molecules but not appropriate for OLED based on conjugated polymers [24]. Instead of patterning RGB, three separately addressed devices are placed on top of each other. The bottom and the middle device are OLEDs with both transparent cathode and anode, allowing down emitting light from the top layer. A patterning RGB approach with a large pixel pitch requires a certain viewing distance for an accurate color representation, which is not the fact for SOLED. SOLED is therefore suitable for helmet-mounted and head-up displays for a short eye relief and high-resolution [25].
The ability to tune specific colors is an additional advantage of the SOLED compared to the patterning RGB approach. Another advantage can be observed in the fabrication process, since the pixel pitch is minimized with maximum fill factor, consequently, resulting in a factor of 3 higher in resolution. The entire SOLED can be as thin as 500 nm, but a lack of proper efficiency makes the method unattractive at present [25].
bottom OLED middle OLED top OLED
substrate G→R
blue OLED
B→G B→G
color conversion layer
(a) (b)
Fig. 2-8. Schematic diagrams for achieving full-color displays based on OLEDs. (a) OLED of stacked RGB materials and (b) Down conversion of blue OLED.
2.5.4 Down conversion of blue light
Emission of blue light from a layer can be filtered and produce red, green and blue. The method, shown in Fig. 2-8(b), converts blue to green and further green to red or directly converts blue to respective colors in terms of pre-patterned films of fluorescent material which efficiently absorbs blue light and re-emits the energy as either green or red light, depending on the compound used. Luminescent organic systems can have a conversion quantum efficiency approaching 100%, although the power efficiency is reduced since the energy of the emitted photon is of less than that of the absorbed photons. If necessary, color-filter may be adopted to sufficiently narrow the spectrum to achieve saturated color [26] [27].
2.6 Power efficiency in an OLED
The power efficiency is defined as the ratio between the power the display consumes and the amount of light emitted. For OLEDs, the power efficiency is of great concern because improved efficiency can lead to a longer device lifetime. The power efficiency for an OLED device is the same as the external quantum efficiency.
The external efficiency not only includes the internal efficiency but also takes outer factors as light emission from the side of the device and internal refraction into consideration. Internal quantum efficiency is defined as the amount of emitted photons in comparison to the amount of charges injected into the emitting layer. The internal quantum efficiency for OLEDs depends on the electron-hole recombination in the emitting layer, where an increased amount of recommendations leads to a better efficiency. It is therefore important to choose a material with good recombination property.
Even if all electrons and holes form an excited state, the quantum efficiency can not be theoretically higher than approximately 50%, depending on a fact that only the singlet excited states can contribute to the light emission. In conjugated polymers there are approximately 50% singlet and 50% triplet excited states [28]. Instead of emitting light, the triplet state emits heat that contributes to the degradation of the device. For small molecules there is about 25% singlet state so that the small molecule materials therefore inherently possess lower power efficiency compared to conjugated polymers [29][30][31]
.
According to the limitation of the quantum efficiency, due to the triplet excited state, an approach of improvement can be through doping with phosphorescent materials [28][ 32 ]. The contribution of a phosphorescent dye, such like platinum, accomplishes a mixture of the singlet- and triplet excited states, which generates a higher speed of the emission of light, known as phosphorescence [33]. Phosphorescent dopants enable small molecule OLEDs to have internal quantum efficiencies approaching 100%, as compared to an approximated 25% maximum for conventional fluorescent devices [34]. The increase in OLED efficiency directly translates into a reduction of display power consumption.
The conjugated polymers are considered to yield higher quantum efficiency due to the 1:1 singlet-triplet state ratio compared to small molecules with a singlet-triplet ratio of 1:3 [28][29]. It is considered that the conjugated polymers do not have any particular need for doping [35]. However, the internal quantum efficiency of a real device is much lower than the theoretical value but doping of the materials does enhance their performance. The reduction of the internal efficiency is mainly due to the absorption of the emitted light due to “Stokes shift” [36].
Table 2-1 shows a
comparison of luminous efficiencies for red, green and blue materials with their respective CIE coordinates, for the three main types of organic light emitting devices:phosphorescent device system [37], fluorescent small molecule materials [38] and spin coated polymer light emitting materials [39].
Table 2-1. Comparison of luminance efficiencies and CIE coordinates of phosphorescent OLEDs, fluorescent OLEDs, and polymer OLEDs (PLED)
[37][38][39].
Redcd/A, CIE
Green cd/A, CIE
Blue cd/A, CIE phosphorescent OLED 11, (0.65, 0.35) 24, (0.30, 0.63) 11, (0.16, 0.32)
fluorescent OLED 3, (0.63, 0.37) 7, (0.31, 0.63) 3, (0.15, 0.17) PLED 2, (0.60, 0.31) 13, (0.39, 0.59) 3, (0.15, 0.17)
2.7 The degradation process for OLEDs
The mechanisms affecting the degradation process are strongly linked to the physical properties of the materials used and result in different degradation properties between inorganic and organic LEDs [21]. The degradation mechanisms for OLEDs are
The mechanisms affecting the degradation process are strongly linked to the physical properties of the materials used and result in different degradation properties between inorganic and organic LEDs [21]. The degradation mechanisms for OLEDs are