Patent Application
20070087220
The present invention is related to an electroluminescent device. More specifically, this invention relates to devices comprising an organic emission layer.
The basic mechanism of light emission of an electroluminescent device, such as an Organic Light-Emitting Diode (OLED), is the radiative recombination of an excited energy state into an energetically lower state. The excited energy state is originally formed by the combination of a positive and a negative charge carrier and potentially an energy transfer can occur from the originally excited energy state to another excited energy state, e.g., through exciton diffusion, Foerster transfer, Dexter transfer or the like. The combination of positive and negative charge carriers forms two types of excitations, namely, short-lived singlets (S) and long-lived triplets (T). Besides the desired radiative recombination of these excitations there exist competing non-radiative processes.
There exists a variety of transition processes an excited energy state can undergo, as described by Kao and Hwang, Electrical Transport in Solids, Pergamon Press, p. 470ff. In particular, the fusion of two excited energy states, e.g., S1+S1, T1+T1, S1+T1, leads to higher excited energy states, e.g., S1*, T1*, T2, T2*, etc. Molecules in such excited energy states are increasingly unstable and tend to decompose or initialize chemical reactions. With increasing density of excited energy states those fusion events become more and more probable. Therefore, the fusion of excited energy states can be a mechanism of significant degradation.
In U.S. Pat. No. 4,769,292 is described an electroluminescent device having a luminescent zone of less than one μm in thickness made of an organic host material capable of sustaining hole-electron recombination and a fluorescent material capable of emitting light in response to energy released by hole-electron recombination. A drawback of this bulk-emitting device is the low efficiency because only the emission of singlet excitons is used. Long-lived triplet excitons that are three times more often formed than singlet excitons are not utilized or deactivated. This may hence lead to a degradation of the device.
In known OLED systems conventional doping of organic layers is performed to improve the efficiency and color purity of organic light emitting devices. In these doped OLED systems the energy levels of the dopants lie within the energy bandgap of the organic host material. This allows effective exciton energy transfer from the host material to the dopant. Originally, fluorescent dyes were used as dopants which mainly utilize singlet excitons (S1). Since the triplet excitons are however not deactivated, an accelerated device degeneration can occur. More recently, luminescent or phosphorescent dyes are employed that utilize both singlet (S1) and triplet (T1) excitons. Though having a higher starting efficiency, the efficiency decrease over time of such triplet-exploiting devices is still substantial. Additionally, devices with these dyes suffer from a decreasing efficiency with increasing operation current due to triplet-triplet annihilation.
It is an object of the present invention to provide an organic electroluminescent device with a reduced degradation rate and an increased efficiency.
According to a first aspect of the invention the lifetime of organic and inorganic electronic and opto-electronic devices, e.g., OLEDs is increased. The lifetime and stability of organic and inorganic devices can be improved by addition of a material with an energy bandgap that is larger than the energy bandgap of a host material of an emitting layer, also referred to as active zone. Additionally, an increased efficiency of devices in particular of devices using phosphorescent dyes occurs.
The addition of a material, referred to as stabilizer, with an energy bandgap that is larger than the energy bandgap of the host material leads to an improvement in lifetime and stability without or with only minor negative effect on the emission and transport characteristics of the emitting layer. Stabilization arises from the fact, that the stabilizer deactivates high-energy excitations which are generated by excited energy state interactions in the active host material during operation. Therefore, degradation mechanisms such as photochemistry by excitations are reduced, resulting in a higher long-term stability of, for example, organic materials as host material. In addition, the additive stabilizer recycles a part of the energy of the deactivated excitations transferring the excitation energy back to the host material that can be a dye molecule. Hence, an increased efficiency is achieved.
The concept is not restricted to small-molecule host materials. It is more generally applicable, e.g. to polymers, organic/inorganic hybrid structures as well as host materials comprising polymers with a small-molecule additive.
In accordance with the present invention, there is provided an electroluminescent device that in sequence comprises an anode, a hole injecting and transporting layer, an emission layer comprising an emitting material, an electron transporting and injecting layer, and a cathode. The emission layer further comprises a stabilizing material capable of accepting energy of excited energy states of the emitting material. The stabilizing material has an energy bandgap that is larger than the energy bandgap of the emitting material. It also preferably has a reduction potential, also referred to as electron affinity, that is equal or less negative than the reduction potential of the emitting material.
In other words, the emission layer is enhanced with a material having a larger energy bandgap. This is achieved by the stabilizing material as additive.
In electroluminescent devices light emission is generated in a luminescent zone comprising a host material sustaining electron- and hole injection and a luminescent guest material capable of emitting light in response to hole-electron recombination. The introduction of the stabilizing material as an additional guest material leads to a reduction of the degradation rate. This stabilizing material as additional guest material, also referred to as stabilizer, is here selected to have a larger energy bandgap than the energy bandgap of the emitting material or host material. This is in contrary to conventional OLEDs which use luminescent guest materials with an energy bandgap that is smaller than the energy bandgap of the emitting material or host material. The larger bandgap of the stabilizing material provides a favored site for the excitation states of the emitting material. The excited energy states which are potentially causing degradation are hence faster depopulated and can cause less chemical degradation reactions. The excited energy state which was transferred to the stabilizing material can be further transferred back to the emitting material which equals a recycling of part of the energy. Alternatively, the excited energy state of the stabilizing material can undergo itself a recombination process. In another case, the stabilizing material itself can degrade with a certain probability which would correspond to a consumption of the stabilizing capability with time.
In order to achieve even better results the stabilizer can be adapted to the optical and electrical properties of the guest/host material within the emitting layer, e.g. by matching the energy levels of the stabilizer to the energy levels of the most probably occurring excited states of the guest/host material.
The emitting material can comprise an organic host material which can be selected from a wide range of materials. Further, the emitting material can comprise a luminescent material that allows the generation of a light emission. The stabilizing material can comprise a material from the class including carbazole, stilbene, fluorene, phenanthrene, and oligo-phenyls, which allows a selection from various suitable materials. A basic selection criterion can be that the molecule forms a solid at room temperature and its singlet and triplet energy states are higher than those of the emitting material.
In a preferred embodiment the stabilizing material can comprise a carbazole biphenyl or any of its derivatives such as 4,4′-N,N′-dicarbazole-biphenyl (CBP).
Such stabilizing material shows the advantage that besides a sufficiently high singlet and triplet energy state the glass transition temperature is relatively high, thereby reducing the negative effect of reducing the overall glass transition temperature of the device by the addition of the stabilizer material. The stabilizing material can also comprise a p-terphenyl or p-quarterphenyl or any of its derivatives, with the advantage of a sufficiently high singlet and triplet energy state combined with a sufficient chemical stability. The same is true for triphenylene. When the stabilizing material is provided in a concentration of 1-10% within the emission layer, then the advantage occurs that the device in a preferred manner exhibits a compromise between its improvement on efficiency and material degradation on one hand and stability and reliability on the other hand. The same applies to the stabilizing material in a concentration of 10−3 to 20 mole percent based on the moles of the emitting material.
It is particularly advantageous when the stabilizing material is chosen such as to provide sites for accepting energy of excited energy states of the emitting material, because then more reliable devices can be provided.
Preferred embodiments of the invention are described in detail below, by way of example only, with reference to the following schematic drawings.
The drawings are provided for illustrative purpose only and do not necessarily represent practical examples of the present invention to scale.
As indicated in the figure with the arrows, the fusion of two molecules that are in the T1 energy state can lead to one molecule in one of the energy states S1*, T1*, or T2 while the other molecule is in the ground energy state S0.
Organic molecules can have one of an excited singlet or an excited triplet energy state. In organic LEDs the presence of excited triplet energy states is undesired because the excited triplet energy states have the characteristic of being more stable than the excited singlet energy states while their relaxation does not contribute to light emission. Excited triplet energy states hence take away from the light emission efficiency of the OLED. Due to their longevity, the percentage of excited triplet energy states in the OLED material increases over time and hence continuously reduces the OLED efficiency. An alternative to an excited triplet energy state relaxing into a lower energy state can be the chemical alteration into a different material that does not emit light, which also exacerbates the OLED efficiency.
The host molecules that are in one of the energy states S1*, T1*, T2 that can result from triplet-triplet annihilation, as indicated in
The introduction of the stabilizer 8 as additional guest material leads to a reduction of the degradation rate. This stabilizer 8 is chosen to have an energy bandgap that is larger than the energy bandgap of the host material, i.e. of the emitting material 7. The larger energy bandgap of the additional guest material 8 provides to the emitting material 7 its first excited singlet energy state S1 or its first excited triplet energy state T1 as a favored site for receiving energy from the excited energy states: S1*, T1*, T2, T2*, etc., of the emitting material 7. The excited energy states resulting from the triplet-triplet annihilation of the emitting material 7 which are potentially causing degradation are hence faster depopulated and can thus cause less chemical degradation reactions. The excited energy state S1 or T1 which was created by the energy transfer at the stabilizer 8 can be further converted by transferring energy back to the emitting material 7, e.g. to its first excited singlet energy state S1, which transfer equals a recycling of part of the energy. Alternatively, the newly created excited energy state of the stabilizer 8 can undergo itself a recombination process. In another case, the stabilizer 8 itself can undergo degradation with a certain probability which would equal a consumption of the stabilizing capability with time.
To choose for the stabilizer 8 a material capable of providing one or more favored sites for higher excited energy states involves relating the properties of the stabilizing material to the emitting material 7. Relevant relationships are the energy bandgap and the reduction potential.
Thereby the energy transfer from the first excited singlet energy state S1 of the host material 7 is aggravated, such that the stabilizer 8 does not take away from the desired efficiency of luminescent relaxation. Preferably also the distance between the first excited triplet energy state T1 and the ground energy state S0 of the stabilizer 8 is larger than the distance between the first excited singlet energy state S1 of the host material 7 and its ground energy state S0.
Preferably, the stabilizer 8 should have an absorption band that is wide enough to accept a variety of higher excited energy states of the emitting material 7. Preferred stabilizing materials are carbazoles (CBP), oligo-phenylenes (quarterphenyl) or p-quarterphenyl of the formula (p-4P), stilbenes, or materials from the class of carbazole, stilbene, and oligo-phenyls.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05405494.5 | Aug 2005 | EP | regional |
