Dr Paulina Morawska, research associate at Heriot-Watt University, discusses how ultrafast lasers can be used to address the encapsulation challenges of OLEDs
Anyone with an interest in displays cannot have failed to notice the interest in organic LEDS (OLEDs) in recent years, given their excellent colour performance, high viewing angles, brightness, flexibility and efficiency. OLEDs have been used in a wide range of consumer products, but their high sensitivity to oxygen and humidity means they require encapsulation to prevent lifetime issues.
This is not easy to achieve for an appropriate cost, requiring highly impervious materials and hermetic bonding or sealing via a reproducible process. Another requirement is that any heat generated during the bonding process must be very localised to avoid damaging or affecting the functionality of the organic or electronic parts inside the sealed region.
A number of techniques have been developed in recent years to address the highly demanding encapsulation requirements of OLEDs, but each has its own issues. For example, a thin polymer film can be deposited on top of the device – which, while having the advantage of high flexibility, does not achieve perfect hermeticity; hence there are lifetime issues. Alternatively, an impervious oxide coating (typically Al2O3) could be deposited on top of the device using atomic layer deposition. However, this is not only a somewhat complex process, but the thin Al2O3 layer is also vulnerable to damage. Lastly, encapsulation between two sheets of thin or ultra-thin (flexible) glass is an option, however while glass has excellent hermetic properties, a polymer is required for the edge sealing – which leads to hermeticity being compromised.
Sketch of the OLED encapsulation process with an ultrashort pulsed laser. Two ultra-thin flexible glasses are directly welded together, with no interlayer required.
Myself, Professor Duncan Hand and others at Heriot-Watt University (HWU) have therefore been working with partners including the Centre for Process Innovation (CPI) within the Innovate UK UltraWeld project to develop an improved technique for encapsulating OLEDs with ultra-thin flexible glass, in this case using an ultrashort-pulsed (picosecond) laser to weld the two glass sheets together in order to provide a true hermetic seal. The lower sheet forms the OLED device substrate while the upper provides optically transmissive encapsulation. They are brought into close contact and the ultrashortpulsed laser is tightly focused at the interface. The very high intensity at the focus (1.3 x 107W/cm2) gives rise to non-linear absorption, and the generation of a small plasma region surrounded by a melt volume no more than 50μm across and 100μm high. The focused laser beam is translated across the material, and the melt volume re-solidifies, resulting in the generation of a continuous weld seam. As the weld seam is very narrow, closely-spaced parallel laser scans are used in order to generate a number of parallel weld seams, and hence build up the total width of the weld.
In their most recent advances, HWU and CPI have shown that it is possible to bond two ultrathin (100μm thickness) flexible glasses together in the presence of the deposited device electrodes. In addition, they have demonstrated that it is possible to bridge small gaps between the glasses of up to 0.5μm, simplifying the process setup. Also, they report that electrical conductivity can be maintained, provided that the weld is structured appropriately. Hermeticity tests are still ongoing.
Microscope image of the weld seam cross section obtained for two ultrathin flexible glasses successfully bonded with an ultrashort-pulsed laser.
This glass welding technique has the potential to be transferred to other planar applications that could benefit from thin flex glass encapsulation, providing a combination of formability and excellent physical protection. Of particular relevance are various types of solar cells, for example organic photovoltaics or perovskite structures. This work builds on previous developments by myself, Hand and others at HWU on the ultrashort-pulsed laser welding of highly dissimilar materials, for example of glass to metals.
The UltraWeld project is led by Oxford Lasers and seeks to further develop and exploit this technology, via the development of a commercial prototype machine. Dimitris Karnakis of Oxford Lasers said: 'We have had interest from many companies in this novel laser process and in our machine development, for a wide range of applications. It clearly provides great commercial opportunity not just for OLEDs but also in many other areas.’