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Hybrid Approach to OLEDs

For OLED displays to be competitive with established display technologies like LCD or PDP, they need to be comparable in price to these technologies, at least in the mid-term future. This will require that OLED production be optimized such that it is simple and scalable to enable competitive pricing of the finished product. This is even more important for emerging lighting applications using OLEDs, which directly compete with traditional (and extremely cheap) incandescent or fluorescent light sources.
OLED manufacturing is typically done by either ultra high vacuum vapor deposition or by solution processing. While high vacuum vapor deposition is done with so called small molecule organic materials, solution-processing methods can employ either small molecule or polymeric organic materials. Both approaches have their specific advantages and challenges, which will be discussed in the following paragraphs.
A hybrid OLED combines solution processing and vapor deposition steps of organic materials. Although at first glance, this seems to make OLED production more complicated, there are a number of advantages using this method.
Exhibit 1 shows a typical layer sequence of an OLED using Novaled's PIN structure. The layer sequence starts at the bottom with an inorganic anode. The organic layer stack consists of a doped hole transport layer (HTL), blocking layers around the central light emitting layer and is finished by a doped electron transport layer (ETL). The cathode is typically again made from an inorganic material. Redox doping of organic transport layers with molecular dopants offers a number of advantages like low driving voltages, wide choice of materials for cathode and anode, and good power efficiency [1]. Novaled's redox doping transport materials have long been used successfully for OLEDs in which all layers were deposited using vapor thermal evaporation (VTE).
Exhibit 1
Typical OLED Structure*

* Schematic layer sequence of an OLED. Most OLEDs today are fully vapor deposited. Hybrid OLEDs can contain either one or more solution processed layer (shown in red), while the rest of the layer stack is deposited in vacuum. The solution-processed part is shaded to indicate that it depends on the concrete manufacturing process, how many layers are actually processed in solution.
Introducing Solution Process Steps
The simplest approach to start with hybrid OLEDs is to introduce a single solution processed layer, which in the case sketched in Exhibit 1 is the HTL. Using a solution processed HTL has the distinctive advantage that the substrate is better planarized than it would be with a vapor deposited HTL. Films formed from solution generally have better surface smoothing properties than vacuum deposited layers. Additionally, it is possible to coat very thick layers in one step from solution by changing solution formulation. In a vacuum thermal evaporation process, the evaporation temperature determines the deposition speed and hence the time required to deposit a layer of a certain thickness. The deposition speed can only be increased in a limited range, otherwise the evaporation temperature will become too high and the organic material will be damaged. Therefore, the deposition of thick layers in vacuum is inherently time-consuming and a solution-processing step is preferable if a thick organic layer is required.
Thick organic layers are for example useful on rough substrates, when the inorganic anode material is not sufficiently smooth to enable good quality OLEDs to be built upon. In display applications, OLEDs are typically deposited on top of a backplane driving circuit, which forms a textured surface. Also in this case, the smoothing properties of a solution-processed layer can be advantageous. The high conductivity of the Novaled p-doped layer enables layer thicknesses of several hundred nanometers without significant impact on driving voltages and OLED performance. Furthermore, the use of a solution-processed layer prevents pin-holes, which can cause leakage currents and accelerate device degradation. In this respect, introducing a solution-processed layer can also increase device stability.
For hybrid OLEDs with solution processed HTL, small-molecule doped transport materials from Novaled can be used. Materials are stored as solids and dissolved in organic solvents before use. Since organic solvents are employed, the solution is water-free and non-acidic. This can be beneficial for applications where the high boiling point and chemical properties of water pose a problem.
Complete hybrid OLEDs can be fabricated by spin-coating of the HTL, transfer of the substrates into vacuum and subsequent deposition of the following layers by VTE. In a series of experiments using this single layer hybrid approach it was found that the device yield can be improved compared to a reference sample, which was completely vapor deposited. Since the finished film has the same optical properties as a vapor deposited layer, there is no need for extensive re-optimization if a vapor deposited HTL is replaced by a solution processed HTL in an OLED layer stack.
For the example discussed in the previous section, the solution-processed layer has only hole transporting properties. But in general, it is possible to process several layers of an OLED from solution, which poses stringent requirements on the used materials. If a layer stack is made by solution processing, it is imperative that a finished layer is not attacked and dissolved by subsequent coating steps. This can be achieved by using orthogonal solvents for each layer or by making the deposited film insoluble by e.g. a subsequent cross-linking step. Since cross-linking steps require high temperatures and may lead to unwanted diffusion of material, a fully solution processed OLED with good performance is very difficult to obtain.
Combining Manufacturing Methods
Here, a hybrid OLED which is mainly solution processed but employs one or several vapor deposited layers may be the most advantageous combination of both manufacturing methods in one OLED. A possible production process for such a device is sketched in Exhibit 2. The OLED stack up to the light-emitting layers is made from solution processable materials. This enables the use of printing methods like ink-jet for the formation of RGB-subpixels; these methods are available for large substrate sizes and deliver high resolution. In this way, the scaling issues, which are encountered with very high-resolution shadow masks for vapor deposition are avoided.
Exhibit 2
Hybrid OLED Production Process*

* Production process for a hybrid OLED with multiple solution processed layers. (a) Solution processing steps are used to build the layer stack up to the emitting layers. (b) Vacuum processing steps add ETL and metal layers to finish the hybrid OLED structure (c).
On top of the solution processed light emitting layers, organic electron transport layers are deposited in vacuum together with the final metal cathode. By contacting several sub-pixels with one cathode, the resolution and alignment requirements are considerably relaxed. The use of the doped organic electron transport layer enables a far wider choice of materials for the metal contact, even transparent contacts are possible. Additionally, it is possible to maximize the light output from the OLED layer stack by tuning the doped ETL thickness such that the distance between emissive layer and cathode contact forms an optimal cavity inside the OLED. Since the cathode needs to be deposited in vacuum anyway, the added process complexity is small.
In conclusion, we have presented the advantages of combining solution processing and vapor deposition techniques in the production of OLEDs. These hybrid OLEDs promise to fulfill market demands for high performance and price-competitive OLED devices.

by Dr. Ulrich Denker, researcher/physics group at Novaled AG
References:
[1] K. Walzer, B. Maennig, M. Pfeiffer, and K. Leo, Chem. Rev. 107, 1233 (2007)

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