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Right-click to download an interview with Lei Ying from South China University of Technology talking about a new approach for depositing protective laminates on OLEDs to extend their lifetimes. (14.19 MB MP3)

“No one has used MgO for OLED encapsulation before,” says Lei Ying, a SCUT researcher, after listing the various inorganic materials that have been considered. Yet as the SCUT researchers tell, MgO has a number of advantages: a low refractive index giving high transmittance, a wide bandgap, high dielectric constant, high chemical stability and a lack of UV irradiation treatment requirements. It is also cheap, commercially available, and with the new protocol reported in Nanotechnology, it is readily deposited on an Al2O3-coated device.

Engineers have traditionally used Al2O3 as a protective coating as it is quite successful in blocking the transmission of damaging water vapour and other gases. However, OLEDs’ extreme susceptibility to water vapour and gas damage has spurred researchers on to investigate ways of developing improved protective laminates by combining with other inorganic semiconductors. In fact, MgO’s string of advantages has already encouraged attempts to add it to enhance protective Al2O3 layers, but without success.

“Previously people tried to make MgO layers using Mg(Cp)2,” explains Ying. “But the processing temperature needed is as high as 300 °C – for thin-film encapsulation we need very low temperatures like 80 °C, so this approach cannot be used for thin-film encapsulation.”

Led by Junbiao Peng, professor in the School of Materials Science and Engineering at SCUT, the researchers developed a protocol to successfully deposit MgO layer-by-layer at temperatures of just 80 °C. The OLED was then protected without any signs of damage for up to 600 hours in an extreme environment of 60 °C and 100% humidity, extending the lifetime of the device by around an order of magnitude. What is more, they identified the critical thickness below and above which the MgO layer is not effective.

Not too thick, not too thin

The researchers deposited the MgO layer-by-layer using atomic layer deposition, with the precursor Mg(CpMe)2 along with water and tetramethylammonium (TMA). The ligands of Mg(CpMe)2 evaporate at around 70 °C as opposed to the 150–200 °C needed for Mg(Cp)2, so the MgO can be deposited at much lower temperatures.

While Al2O3 protects the OLED by blocking the transmission of water vapour, MgO enhances this protection by absorbing water vapour so that it does not reach the device. This means there is a minimum thickness of 1 nm below which the MgO layer will not be uniform, and there will not be enough of it to effectively absorb water vapour.

In addition, the MgO layer must be amorphous as water molecules can penetrate between the domains of crystalline MgO films. The researchers found that above 1 nm the layer forms in the crystalline phase; 1 nm is the critical MgO layer thickness for effectively enhancing the protective behaviour of the coating.

Next steps

The SCUT researchers suggest that the protective laminate technique may also be useful for organic photovoltaics, organic light-emitting field-effect transistors and other printed circuits. Because OLEDs have a particularly low tolerance to water vapour and gas exposure, the success of the laminate with these devices suggests it will more than suffice for the others.

The next steps for the team, however, will be to extend the work with OLEDs. “The most impressive thing for flexible OLEDs is the large area,” says Ying. “However, if you can make this laminate for a small area it does not necessarily mean you can make it work for a large-area device. So what we’re going to do is see if we can transfer the technology to large-area devices.”

Full details are reported in Nanotechnology, in an article that features in Nanotechnology Select.

For more on the latest developments in organic LEDs visit the Nanotechnology focus collection.