Developing and Producing Specialty Materials for Printed Electronics

The field of printed electronics continues to advance at an impressive rate with new applications such as printed organic light emitting diodes (OLEDs), organic photovoltaic cells (OPVs), printed batteries and even micro printed sensors and electronic paper.

All of these concepts are essential to the development of thinner, lightweight more powerful hand held devices, new display technology and even personal and wearable electronics.

Nanograde is preeminent in the development and production of specialty materials for printed electronic components. Based in the technology corridor adjacent to Lake Zurich, Nanograde has created a range of metal oxide and semiconducting nanoparticles and has formulated them into inks ideal for printing electronic components. In addition, by optimizing their inks to industrial printing/coating systems their materials yield high quality optical and electronic films cheaper than ever before.

Nanotechnology has barely scratched the surface of its potential and the range of applications is growing all the time, particularly in electronics. In order to help develop new nanoparticle-based products Nanograde has set up a custom formulation unit to produce high specification nanoparticles and formulations according to customer requirements.

Organic Light Emitting Diodes (OLED) Light Out Coupling Materials

OLED (Organic Light Emitting Diode) is a flat light emitting device produced by sandwiching a series of thin films of organic and inorganic material between two conductors. When an electrical current is applied, light is produced and hence OLEDs are used to make displays and lighting. The light generation mechanism in OLEDs is largely due to the recombination of excited electrons on electrically excited organic molecules; electrons lose energy by emitting light. This light is, however, generated from the thin organic emitting layer spontaneously in all the directions and propagates via various modes, that is, externally (out of the screen) and guided or total internal reflection. The amount of light lost to total internal reflection can be up to 80% meaning a very dim light or screen. There are however various ‘light out-coupling’ techniques that can enhance the external efficiency of OLEDs and some of the best ones are based on nanoparticle printing techniques.

OLED - Image Credit: How Stuff Works

Various nano-patterning techniques have been developed such as nano-contact printing and nano-imprinting to improve light extraction from OLEDs. One example uses a nano-patterned photonic crystal in a glass substrate to improve light-coupling efficiency. This gave an enhancement factor of 1.5 over viewing angle of ±40° and the periodic modulation helped to convert the guided waves (internally reflected) in the high refractive-index (RI) indium-tinoxide/organic layers into the more desirable externally directed waves.

Nanograde can provide high quality OLED out coupling coatings for high refractive index planarization and scattering and is able to offer a range of materials with high refractive index (e.g., 1.85) for this purpose. The refractive index of these materials can actually be adapted in increments to the maximum refractive index of 1.95 if required.

Low-Cost Anti-Reflection Coatings for LCD Displays

Reflection of light from glass surfaces is a significant problem and impedes the performance of solar panels, smart phone and tablet displays. Glass is commonly used in solar panels and displays and has a refractive index of 1.5 with reflection of about 4% of incident light at its surface.

Anti-reflection multi-layer coatings are not new but have to be processed using expensive vacuum techniques, which increases the costs of device displays, spectacles and solar panels. A less expensive method of applying multi-layer anti-reflection coatings is to use a printing method to create a nano-thin coating on a glass or polymer substrate. This modifies the refractive index of the glass surface and in this way reduces the reflection from about 8% to around 0.6% over a range of angle of incidence of up to 50°.

Image Credit: Oleksiy Mark/Shutterstock.com

Nanograde are experts in anti-reflective coating and can offer a range of high refractive index materials that can be printed in multiple layers to modify the refractive index of the surface to a maximum of 1.95 if required. The final cured film surface of the glass is robust and water repellent and not only provides anti-reflective properties but also physical protection.

Semiconducting Materials for Printed OLED and OPV

OLEDs and OPVs are set to revolutionise the display and energy harvesting markets, allowing for the first time materials to be produced in quantity at a low cost. Currently OLED devices are manufactured at very high cost. By using printing techniques to manufacture these devices the costs drop dramatically.

Modern OLEDs generally have a bilayer structure, comprising a conductive layer and an emissive layer. More recently OLEDs have used a graded heterojunction where the composition of hole and electron-transport materials is varied constantly within the emissive layer using a dopant emitter (nanoparticle based). This has the effect of increasing emissive efficiency and thus brighter, more efficient displays.

Nanograde has developed the first surfactant-free inorganic buffer layer inks that enables solution processing of inorganic WOx or ZnO or Al:ZnO layers in both OLED devices as hole or electron transport layer. An innovative feature of the inks are their compatibility with polymeric substrates and a curing temperature, which can be as low as 80°C.

Organic photovoltaics (OPVs) have a similar principle except in reverse, i.e., instead of emitting light they adsorb light energy and convert it into excited electrons using the photoelectric effect. These devices usually consist of an electron- or hole-blocking layer on top of an indium tin oxide (ITO) conductive glass followed by electron donor and an electron acceptor, a hole or electron blocking layer, and metal electrode on top. The hole or electron blocking layers are used to increase the overall conversion efficiency and provide a longer service life for the device.

To produce solution processed OPV or OLED devices Nanograde offers a portfolio of 20 different inks, which provide a choice of properties for the hole injection/electron blocking and electron injection/hole blocking materials. Solution processed OPV or OLED devices require different work functions and have a range of different production processes. The Nanograde range of inks is able to fulfil all the production requirements and they can achieve their full performance at annealing temperatures as low as 80°C.

Conclusion

The technology offered by nanoparticles alongside high precision printing techniques is set to provide lighter, thinner and less expensive electronics.

Nanograde has expertise in both nanotechnology and printing techniques and also a comprehensive product range of inks and antireflective coatings, which will be essential to the manufacture of the next generation of OLEDs and OPVs.

The range of new products that are possible using Nanograde technology could make a fundamental adjustment to the way we live and how we can save and use energy. Products of importance include electronic paper, thin flexible displays OLEDs (televisions and computer monitors), low power OLED lighting fixtures, wearable sensors and OLEDs, and finally very low cost photovoltaic cells allowing energy to be harvested directly from sunlight. The combination of all of these factors points to a low energy and greener future for everyone.

References

1. D. S. Mehta and K Saxena,  Light out-coupling strategies in organic light emitting devices, Proc. of ASID’06, 8-12 Oct, New Delhi, 2006.

2. H. J. Peng, Y. L. Ho, X. J. Yu, and H. S. Kwok, Enhanced coupling of light from organic light emitting diodes using nanoporous films, J. Appl. Phys., vol. 94, pp1649-1654, 2004.

3. Y. J. Lee, S. H. Kim, J. Huh, G. H. Kim, and Y. H. Lee, A high-extraction-efficiency nano-patterned organic light-emitting diode,  Appl. Phys. Lett., vol. 82, pp3779-3781, 2003.

4. J. van de Groep, P. Spinelli, and A. Polman, Single-step soft-imprinted large-area nano-patterned anti-reflection coating Nano Letters (2015) | DOI: 10.1021/acs.nanolett.5b01623

5. C. Sekine, Y. Tsubata, T. Yamada, M. Kitano and S. Doi, Recent progress of high performance polymer OLED and OPV materials for organic printed, Electronics, Sci. Technol. Adv. Mater. 15 034203, pp15, 2014 (http://iopscience.iop.org/1468-6996/15/3/034203)

6. N. Tessler, Y. Preezant, N. Rappaport, and Y. Roichman, Charge Transport in Disordered Organic Materials and Its Relevance to Thin-Film Devices: A Tutorial Review, Adv. Mater., 21, pp2741–2761, 2009.

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