IDTechEx technology analyst Dr Harry Zervos attended the meeting of the Society for Information Displays in London, where the latest research and development efforts were discussed and presented. Speakers included among others Dr Dago de Leeuw, Prof Donal Bradley of Imperial College London that hosted the event and speakers from the University of Manchester, University of Florida as well as Merck and Plastic Logic.
Some of the presentations are described below:
In his plenary address, Dr Dago de Leeuw from Philips- TU Eindhoven spoke about "Polymeric non-volatile memories"
Philips introduced the first ever 13.56 MHz first RFID system based on organic transponders (IEEE, J Sol. St. Circuits, 2007) and Poly-IC were the first to produce such RFID tags in a roll-2-roll process. The next step would be the ability to include programmable memory to store data, with requirements such as:
Program cycle endurance
Fast switch times
- Resistive switching
Switching observed on electron only PLED devices with aluminum electrodes, but with very low yield. Yet, yield improved with UV/O3 treatment. That's unexpected as the addition of a 50 nm alumina layer- an insulator (!)- increased yield. The forming of the memory was attributed to "soft breakdown" of Al2O3. The type of polymer used was found to be irrelevant, as was the type of electrode and type of oxide irrelevant. In the process of the soft breakdown in the oxide, the polymer acts as nothing more than a current-limiting series resistance.
Dr DE Leeuw finished his talk with the message to remember: "Let's store data by oxidizing the electrode before processing!"
Prof Jenny Nelson of Imperial College London spoke on the "Modeling of charge transport in disordered organic semiconductors".
Taking into account the fact that chemical structure, side chain length and processing/ macroscopic parameters (e.g. temperature, electric field) influence mobility, simulations showed that ordered/crystalline-regions increased with temperature. As the surface roughness affects simulated mobility, better behavior was observed when processing at higher temperatures.
The strong influence of molecular packing was also highlighted but was found difficult to compute and verify.
In his talk on the charge balance in organic light emitting devices, Professor Franky So of the University of Florida pointed out that charge balance is important to achieve high efficiency devices, whether small molecule or polymer. The bottleneck here is electron transport, the high triplet energy and high electron mobility needed while avoiding triplet exciton quenching to enhance efficiency.
Prof Donal Bradley gave an overview on Molecular Electronic Materials and Devices.
He focused on the following points:
- Electrode materials for cathodes: low work function suited to efficient injection. Encapsulation-permeability an issue for plastic substrates.
- Some of the oxides used as anodes can also be used as electron injection layers. Nanostructured TiO2 studied to this effect, due to its use in DSSC solar cells. In order to change its performance a layer of MoO3 was inserted which unfortunately had a detrimental effect to the device. Replacing titania with zirconia achieved a better performing device
- Vapor Phase Polymerized PEDOT: VPP PEDOT: Good work function for hole injection, makes up for ITO's shortcoming in that respect. Efforts are being made to optimize VPP PEDOT for OLEDs.
Added PEDOT:PSS leads to enhanced hole injection and much improved efficiency. There are hopes to be able to use VPP PEDOT instead of ITO, when using an additional PEDOT PSS layer.
A discrete component demonstrator of OLEDs and OPDs has also been fabricated.
According to Dr Madec of the University of Manchester, who are looking into the possibility of inkjet printing of organic TFTs, small molecules are best for TFTs due to better packing.
The group studied different formulations of TIPS pentacene; a soluble derivative of pentacene. The research proposes formulations of inks and use of different types of binders in order to achieve rheological properties that would be making the ink compatible with Inkjet printing and look into the links between morphology and mobility.
There was also a presentation from Dr Xiaojun Guo from Plastic Logic on the day of the opening ceremony of the company's production facility in Dresden.
PL has received over US$200 million in global investment and have now demonstrated an e-paper flexible display, 10-14'' in size, 25-50g in weight with the overall product weighing less than a pound , thin at a ¼ inch, robust, flexible and conformal with low power consumption.
The company uses solution processable polymer semiconductors and dielectrics, compatible with low cost PET substrates with a scalable manufacturing route based on direct writing and laser patterning technologies.
Active matrix driving scans each line quickly, approximately 20ms per frame. Passive matrix driving would take several minutes to update the display so each pixel uses a TFT for driving and a capacitor for data storage.
- TFT with low on resistance, low source/gate line resistance
- TFT with high off resistance,
- high yield and uniform performance
- stable performance for constant product use
The stability of the device was tested through electrical stressing. No change in device performance was recorded after 250 hours of electrical stress, (.8s ON, .8 s OFF, performance measured every 5.5 hours).