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The present invention relates to an organic semiconducting layer and to an organic electronic device comprising such layer.
Organic semiconductors can be used to fabricate simple electronic components e.g. resistors, diodes, field effect transistors, and also optoelectronic components like organic light emitting devices (e.g. OLED organic light emitting diodes), and many others. The industrial and economical significance of the organic semiconductors and their devices is reflected in the increased number of devices using organic semiconducting active layers and the increasing industry focus on the subject.
A simple OLED is demonstrated in
Such organic semiconducting layers mainly comprise conjugated organic compounds, which can be small molecules, for instance monomers, or oligomers, polymers, copolymers, copolymers of conjugated and non-conjugated blocks, completely or partially cross-linked layers, aggregate structures, or brush like structures. A device made with different types of compounds, in different layers or mixed together, for example with polymer and small molecule layers, is also called a polymer - small molecule hybrid device.
Organic electronic semiconductors can be used in organic electronic devices, and in organic-inorganic hybrid devices.
Despite the large electronic gap, usually up to 3 eV, formed between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecule, it is normally still low enough such that both positive and negative charge carriers can be injected by special electrodes. It is especially difficult to provide a suitable cathode for organic semiconducting layers, because the LUMO energy is very high, in a region in which the cathode is usually highly reactive to oxidation by oxygen or moisture. There are several solutions for electron injection in electron transport layers (ETL), typically a low work-function metal is used in the cathode, such as Ca, or Mg. Alternatively, electrically doped electron transport layers are used, the large density of charge carriers provided by the dopants forces moves the Fermi level of the ETL close to the LUMO, facilitating the electron injection due to the Fermi level alignment with the cathode. Another solution is the use of electron injection layers (EIL), such as a thin insulating layer, which decouples the Fermi level of the cathode from the Fermi level of the ETL, creating a thin barrier which can be tunneled.
However all these approaches have disadvantages, the low work-function materials used for the cathode are very unstable and the OLEDs tend to have a short lifetime. Furthermore, these materials are difficult to be handled during production. Also, the electrical doping requires very reactive materials such as Li or Cs, not only are these materials difficult to handle during production, but they also are required in high concentration, which decreases the required transparency of the ETL. Molecular dopants do not require high concentration, however there are only a very few known molecules with a very low ionization potential which is required for electrical dopants. As with the inorganic dopants, those molecular dopants are also highly reactive towards moisture and oxygen requiring special handling procedures. ElLs have the disadvantage that the additional barrier increases the operating voltage of the device. By using ElLs it is also required that the adjacent ETL has a very high mobility, otherwise an additional voltage loss over the undoped ETL further increases the operating voltage of the device.
The problem to be solved is how to efficiently inject electrons into an ETL, without compromising the ETL's transparency, transport properties, and operation stability (lifetime).
It is an object of the present invention to provide an organic semiconducting layer, especially an electron transport layer, which overcomes the drawbacks of the prior art, especially to provide a layer without compromising the ETLs transparency, transport properties, and operation stability (lifetime).
It is a further object to provide a layer sequence and an OLED comprising such an organic semiconducting layer.
This object is achieved by the organic semiconducting layer according to claim 1, and the layer sequence and OLED according to claims 11 and 13, respectively. Preferred embodiments are disclosed in the sub claims.
In the formulas given in the claims, i.e. formula (1), (1a), (1b) and (2), any structural elements can be substituted by suitable substituents, whenever such a substitution is possible at all. For example if somebody select for R3 in formula (1) C1-C20-alkenyl, this alkenyl compound can be, of course, substituted by one or more suitable radicals.
Phenanthroline and derivatives thereof are frequently used electron transport materials (ETM) for ETLs. These materials have a reasonable LUMO and electron mobility and behave rather well with injection layers. Compounds like BPhen and BCP are also used in conjunction with dopants such as Cs. When doped with Cesium, BPhen allows to make OLEDs with minimal operating voltage. Despite the relatively high electron mobility, however, the operating voltage of OLEDs using undoped BPhen is considerably increased compared to the doped reference device. Example of such ETMs are given in
It was now surprisingly found that compounds according to Formula 1 do extraordinarily improve the performance of an OLED. More surprisingly is that very similar compounds such as 2,4,7,9-tetraphenyl-1,10-phenanthroline, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, or 4,7-diphenyl-1,10-phenanthroline have very poor performance as further described below.
The inventive organic semiconducting layer comprises an electron transport material according to Formula (1) and an electron injecting material (EIM) according to Formula (2). In one embodiment the organic semiconducting layer comprises the ETM and the EIM mixed in a single layer. Preferentially, the concentration of the EIM in the ETL is between 27-75 weight%. It should be noted that the EIM according to the formula (2) is not a dopant like Li or Cs, or an electrical dopant mentioned above. Those EIM materials do not have electron donating character and are thereby air stable and easy to handle compared to electrical dopants.
In an additional embodiment the inventive layer comprises two sublayers, wherein the first sublayer comprises the ETM and the second sublayer comprises the EIM. Preferentially, the ETM is the main component in the first sublayer and the EIM is the main component of the second sublayer.
Alternatively the first sublayer contains ETM, and the EIM contains EIM only. In this embodiment, it is preferred that the first sublayer thickness is less or equal to 50 nm, more preferably less or equal to 30 nm. It is also preferred that the second sublayer thickness is between ≥0.5 and ≤3 nm. Thicker ETL and/or EIL layers could increase the operating voltage by more than 0.5 V. An advantage of this layer is that it does not require co-evaporation for its deposition (as required for electrical doping).
Optionally, the first sublayer comprises the ETM and also comprises the EIM in one single mixed layer, preferentially in a smaller portion, more preferentially less than 30mol%. In this embodiment, the first sublayer can be thicker, and it is preferred that the first sublayer thickness is between ≥5 nm and ≤70 nm.
It is preferable that the inventive semiconducting layer contains only two materials, namely the ETM and EIM.
As a preferred electron transport material 2,9-di(biphenyl-4-yl)-4,7-diphenyl-1,10-phenanthroline can be used.
Further preferred electron transport material can be taken from the following examples:
Further preferred the electron injecting materials can be selected from:
also preferred are 2,3-diphenyl-5-hydroxyquinoxalinolato lithium, cesium quinolate, potassium quinolate, rubidium quinolate. Additional information of such materials can be found in
A further development of the invention is a stack of layers comprising the inventive semiconducting layer as described above wherein a metal cathode is provided in direct contact with the electron injecting material.
It is preferred that the cathode comprises a metal selected from: Mg, Ca, Ba, Al. Those materials provide best performance as cathode.
It is further preferred that the cathode comprises Al with a purity higher or equal to 99.95%. This embodiment enables the use of a very simple cathode whereas otherwise, Ca/Al, Mg/Al or alloys with a few percent of low work function materials in Al or Ag are necessary to achieve a good injection in non-doped ETLs.
In a preferred embodiment of the invention an OLED is provided comprising: an anode, an emitter layer, and a stack of layers as described above.
Preferably the OLED structure is a non-inverted structure, comprising: an anode provided over a substrate, an emitter layer provided over the anode, the inventive semiconducting layer provided over the emitter layer, and the cathode provided over the inventive semiconducting layer.
With this combination comprising the electron transport material (ETM) and electron injecting material (EIM) it is possible to achieve exceptionally good results. OLEDs according to the invention have better performance to state of the art OLEDs using injection layers. For instance, contrary to the expectations of Pu (see
Additional advantages and features of the present invention can be taken from the attached drawings which are not to be taken to restrict the scope of protection which is only to be defined by the appending claims.
Here follows a summary of the performance and lifetime data comparing an ETM according to the invention with typical state-of-the-art ETMs.
Generic OLED stack: glass / ITO (90nm) / p-dopant:NPD (30nm, 3 wt%) / NPD (10nm) /blue doped Emitter layer (20nm) / Electron transport material (30nm) / LiQ (2 nm) /Aluminum (100 nm). NPD is N,N'-Bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine.
An overview of the performance of the different Electron transport materials in the above generic stack with LIQ injection layer is given in table 1 above. All OLEDs emit blue light with color coordinates of (0.14; 0.20). As seen from the last column of the table, the Electron transport materials M2, M1 and M4 lead to approximately the same external quantum efficiency, whereas M6, M3 and M5 show reduced quantum efficiency. Thus, already independent of the driving voltage the last three materials are much less suited for OLED application as compared to the three materials from the first group. The driving voltage on the other hand side is much lower for M1 as compared to either M2 or M4. In consequence the external power efficiency is strongly reduced for M2 and M4 as compared to M1. In conclusion, M1 with LiQ injection layer, delivers the outstanding combination of low voltage and high internal quantum efficiency.
For further visualization,
Generic OLED stack: glass / ITO (90nm) / p-dopant:NPD (30nm, 3 wt%) / NPD (10nm) /blue doped Emitter layer (20nm) / M1:LiQ (36nm, 10, 20, or 40wt%) / LIQ (0 or 1 nm) /Aluminum (100 nm) (NDP=N,N'-Bis(naphthalene-1-yl)-N,N"-bis(phenyl)-benzidine)
An overview of the performance of several OLEDs according to the generic stack given above are shown in Table 2. The OLEDs emit deep blue light around 470 nm.
The features disclosed in the foregoing description, in the claims and in the drawings may, both separately and in any combination thereof, be material for realizing the invention in diverse forms thereof.
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