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Posted on September 21, 2012 by  & 

Latest progress on solution-processed OLEDs

IDTechEx recently attended the SID Organics meeting held at Imperial College London. Here we will provide a brief update on the activities of several major chemical companies that are actively pursuing OLED.
Dr Anna Hayer tells us that 100+ researchers are working on OLEDs at Merck. They have an active research programme on solution- processable small-molecule OLEDs. The main advantages of small molecules are that they have high-purity and high-performance, but they are also typically difficult to solubilize and print as high-quality film.
Merck has made significant progress towards solution-processing these devices. Their best devices are not however fully solution-processed. The ETL (electron transport layer) and the HBL (hole blocking layer) are evaporated. This enhances performance and, Dr Hayer argues, does not compromise the scalability that solution-processing affords. This is because the top two layers are put down during blank deposition process, meaning that patterning will not be required.
This hybrid manufacturing approach enables Merck to reach efficiency and lifetime levels of 10-15 cd/A and 50,000-75,000 hours for red emitters, respectively, and 75 cd/A and 200,000-225,000 hours for green ones, respectively. All measurements are at 1000 cd/m2. It is noted that without the evaporated ETL layers, the performance is constrained to <10cd/A and <50 cd/A for red and green emitters, respectively.
Merck uses spin-coating in the main. It however has joint programmes with customers on inkjet printing the OLEDs. Merck cannot publically share their result on this front.
Merck has also investigated the stability of their OLEDs. They now understand that the EML layer is the most sensitive layer to air. More critically, Merck understands that the EML layer also suffers from photo-oxidisation. This suggests that device processing should take place mostly under yellow light. It is also found that red emitters are more prone to photo-oxidisation.
Interestingly, degradation under light is irreversible, whereas that under dark is reversible, e.g., under N2 environment. Moreover, the EML layer is more sensitive to oxygen than water. This is partly because oxygen becomes ionised upon interaction with the EML layer, which in turn makes it more harmful to EML. This generates a self-destructive positive feedback loop.
Dr Olivier Gaudin discussed the latest progress on Solvay's programme on solution-processable small-molecule devices. Dr Gaudin thinks that solution-processing results in simple structures with lower cost, but they also tend to result in a lower performance level (compared to evaporation).
Solvay works with Plextronics in making their devices. The latter supplies the HIL (hole injection layer) and HTL (hole transport layers), while the former works mostly on the EML (emissive) and ETL (electron transport) layers.
Solvay has conducted a detailed study to understand the maximum performance level achievable using solution-processable layers. This study consists of a series of experiments in which one or multiple layers are solution-processed in an otherwise evaporated device. The idea is that the evaporated version gives the maximum performance level and therefore replacing a layer with a solution-processed one will give the best achievable result for that layer.
Solvay finds the switching the processing of HIL and HTL layers from evaporation to solution-processing (Plextronics materials) involves no penalty in performance. This is because the performance levels remain largely the same between the fully and partially evaporated devices. These devices offer 10,000 hours of lifetime (@3,000 nits) and 25-27.5 lm/W at 1,000 nits.
In contrast, Solvay finds that switching the processing of the EML and ETL layers from evaporation to solution-processing results in a degradation of lifetime and power efficiency by a factor of 4 and 1.3, respectively. Devices with solution-processed HIL, HTL and EML layers offer 4K hrs lifetime (@3,000 nits) 17-20 lm/w at 1,000 nits. Dr Gaudin argues that morphology and solvent choice are critical to forming high-quality printed EML and ETL layers.
Solvay is also participating in a joint-research programme to develop large-sized flexible solution-processed OLEDs. In this programme, the Holst Centre performs the printing and provides the thin-film flexible encapsulant, Plextronics provides the HILK and HTL layers, Solvay develops the EML and ETL layers, Dupont Teijin supplies the flexible PEN substrate and Phillips carriers out the measurements. It is noted that the transparent conductor consists of ITO layers supplemented with a printed silver grid network.
Cambridge Display Technology, Dr Martina Pintani
Cambridge Display Technology (CDT) (now bought by Sumitomo) uses polymeric OLEDs for solution-processing their devices. In house, they mostly perform spin-coating. They however also run a project on inkjet printing their OLEDS and Dr Martina Pintani claims that they have almost reached performance parity with spin-coated devices.
Dr Martina Pintani tells us that the electronic properties of polymeric OLEDs are determined by the polymer backbone, while side chains control the physical properties. CDT uses ITO as the transparent conductor and PDOT:PSS as the hole-injection layer. In their device, a hybrid manufacturing technique is employed whereby all the organic layers are solution-processed.
Interestingly, CDT introduces a 15-nm iL (interlayer) between the EML and HIL. This is to separate the emission area from the HIL where quenching can take place. This enhances the performance. Their best devices offer an efficiency of 31, 73, and 10 cd/A for red, blue and green emitters, respectively. The lifetimes are 200,000, 100,000 and 13,000 hours for red, blue and green emitters, respectively.
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Top image: ACS

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Research Director

Posted on: September 21, 2012

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