By improving cost and manufacturing efficiencies of OLED displays, hole-injection-layer technology could be one factor in enabling the commercialization of flexible OLED-based displays.

by Brian Woodworth, Mathew Mathai, Ritesh Tipnis, and Eric Boughner

ELECTRONICS continue to transition from being tools for doing business and daily living to the point where they fit unobtrusively in our natural environment and intuitively deliver new functionality. Printed electronics (PE) will exponentially impact this trend, resulting in new ways of managing information, delivering power, and communicating (see Fig. 1). The continued development of PE is critical to this trend as it provides cost efficiencies and product design options as well as integration options that are not possible with products fabricated with current electronics manufacturing techniques.

New low-cost applications in power, circuitry, and lighting are expected to fuel the growth of the PE industry, whose value is expected to grow from the current $1 billion to $300 billion in the next 20 years.¹ The near future of PE holds the possibility of printed lighting that allows organic light-emitting-diode (OLED) displays to deliver high-definition quality viewing for a fraction of the powerconsumed by a liquid-crystal-display (LCD) screen. Other applications on the horizon include printed solar modules that are more cost efficient than their silicon-based counterparts and printed Radio Frequency Identification (RFID) devices that enable $0.01 tags for item-level tagging. All of these applications achieve maximum utility when designed and implemented on flexible substrates.

Fig. 1: Printed electronics will create a ubiquitous electronic environment.

Applications in the FPD Industry

Displays based on organic conductor (OC) materials are set to capture a significant portion of the FPD market if critical parameters such as device efficiency, lifetime, and operation voltage are optimized. Also, in order for this application to compete on a cost basis, the ability to use print, or solution-processed, manufacturing techniques is of paramount importance. Flexible displays utilizing OLED technology are a leading candidate to satisfy these requirements.

OLED devices can take a variety of forms, each actively undergoing advanced research and/or commercialization. Some OLED displays incorporate small organic molecules (SMOLEDs) as the emissive layer. These require a considerable investment into the manufacturing set-up due to the cost associated with a series of vacuum-deposition processes through very thin shadow masks for each required color. Also, this process is less efficient in material usage and energy consumption, making it cost ineffective, especially for large-area displays. This method also represents challenges in moving to larger substrate sizes.

This has led to wide-scale interest in LEDs comprised of electroluminescent polymers that are typically solution-processed (similar to inks), commonly referred to as polymer light-emitting diodes (PLEDs). Not only is solution processing an economic technology, it also results in a variety of application techniques such as ink-jet printing and contact printing, both of which are leading candidates used for the deposition of these materials on flexible substrates. Also, while SMOLEDs migration to larger, more economical substrate sizes is restricted due to inherent limitations in its current manufacturing approach, these print-ing technologies allow PLEDs to be suitable for both large and small substrate sizes, with sig-nificant potential benefits in larger panel sizes. As such, the potential of PLED technology to be a leading frontplane technology for flexible active-matrix displays is quite high.

Typical bilayered PLED device architecture comprises four layers of materials on a glass or plastic substrate (Fig. 2). Sandwiched between a low-work-function metallic cathode and a transparent anode – usually indium tin oxide (ITO) – is a solution-processed thin layer of light-emitting polymer (LEP) and a transparent hole-injection layer (HIL).

Importance of the Hole-Injection Layer

The importance of the HIL in increasing efficiency, extending lifetime, and lowering threshold voltage cannot be overlooked if PLED devices are to become widely commercialized in active-matrix displays. The purpose of the HIL in the function of a PLED is to 

• Allow for the efficient injection and transport of holes from the anode to the LEP layer by bridging the energy gap between these two layers.²

• Planarization of the anode surface to prevent shorts.

• Block electrons from flowing out of the LEP layer and into the anode without recombination.³

Thus, an effective HIL enables holes and electrons to be more evenly concentrated in the emitting layer, maximizing their recombination at low-power input levels that leads to enhanced device performance in all of the categories listed above. The dense film structure and morphology of a well-applied film provides a uniform conducting path for hole migration to the LEP/HIL interface, which could lead to decreased turn-on voltages and to superior luminescent performance.⁴

Several technologies have been employed as HIL materials. The most commonly used is based on poly(3,4-ethylenedioxythiophene) (PEDOT). Alternative approaches range from the use of Nafion-based systems that act as effective electron-blocking materials to the use of conducting polymers such as thieno[3,4-b] thiophene, polyaniline, and polypyrrole. Other materials that are widely used are non-protic compounds based on a polymeric benzidine/borane/hypervalent iodine ternary system as well as siloxane/carbazole-based HIL materials. All of these are solution processable, thus making them commercially viable in the printed-electronics industry. However, certain vapor-deposited systems based on benzidine/F4TCNQ continue to be employed by small-molecule (SM) display manufacturers.

Fig. 3: General structure of regio-regular polythiophene (RRPT) polymer.

Commercially available HIL technologies, while providing some positive impact on PLED device performance and lifetime, exhibit significant drawbacks that limit the use of these materials and have ultimately impeded the widespread commercialization of PLEDs.⁵These limitations include

• The high acidity of conventional HIL causes the ITO to degrade and leach indium into the HIL layer. This instability of the anode–HIL interface affects device performance and shortens its lifetime. Also, this results in difficulties and increased wear and tear on equipment when printing these materials.

• The hygroscopic nature of conventional HIL materials leads to absorption of moisture, which is extremely detrimental for display-manufacturing processes.

• Conventional HIL materials are quite intractable in their design and engineering. It is extremely difficult, if not impossible, for display manufacturers to tailor their work functions to align with a wide range of LEPs, thus restricting the desired device performance.

• The current commercially available technology is limited to aqueous-based systems, which limits the display manufacturers choice of printing techniques.

In order for PLEDs to find consistent applications such as serving as the frontplanes of choice for active-matrix televisions greater than 30 in. on the diagonal, the properties of existing HIL materials have to be considerably upgraded from what is currently available.

Improved Performance and Manufact-urability with Plexcore^® HIL

Plextronics has developed Plexcore^® HIL, a key technology that will enable the successful commercialization of PLED-based displays. The highly tunable nature of Plexcore helps overcome the limitations of the current technology by enabling the utilization of a greatly expanded platform of polymer design. This is particularly important because the variety of device architectures and material choices, such as LEP, available to display manufacturers results in the need for flexibility in the formulation of an HIL film that is compatible with other materials in the device stack.

Plexcore HIL is a multi-component system that includes a regio-regular polythiophene (RRPT) polymer (see Fig. 3), an integral dopant package, a multi-component solvent system (including both aqueous and organic solvents), a matrix polymer, and other additives that influence either (or both) application characteristics of the HIL or film properties. The specific components and formulation vary with the end-use application and emitter system used in a specific OLED device.

Fig. 5: Plextronics's active-matrix program.

This technology is a platform system that allows scientists to tailor the fundamental properties of materials used in HIL product. In the past year, more than 850 unique ink formulations were screened with up to six commercial emitters in approximately 2750 devices. An integrated, iterative development approach, as depicted in Fig. 4, is utilized to tune Plexcore HIL to the desired device-performance requirements. Trials with display manufacturers indicate that this technology will help achieve the broad commercialization of high-performance low-cost AMPLED full-color displays with improved lifetime and efficiency.

Impact of HIL on Important Commercial Requierments

Device performance data obtained to date illustrates how Plextronics is utilizing this systems-based technical insight to solve key issues for broad commercialization. As can be seen in Fig. 5, Plextronics's Active Matrix Program has benefited enormously by successful implementation of improved HILs along with corresponding improvements in emitter technology, optimization of device architectures, and increased capacity for iterative development. This high throughput in developmental activities will ultimately enable the lifetime performance required for OLED commercialization by the display industry.

Conclusion

Printed electronics opens the possibility to a new world of flexible-display functionalities. OLED displays, specifically PLED displays manufactured via printing methods, are leading this charge. Continued improvements in HIL technology as described above will improve the efficiency and operating lifetime of these displays while reducing their manufacturing cost. These developments will help AMPLED displays capture an increasing share of the FPD marketplace.

References

¹Nanomarkets, "Organic Electronics: A Market & Technology Assessment" (Sept 2006).
²X. M. D. Gong, D. Moses, A. J. Heeger, S. Liu, and K. Jen, Appl. Phys. Lett. 83, 183 (2003).
³A. Kraft, A. C. Grimsdale, and A. B. Holmes, Angew. Chem. Int. Ed. 37, 402 (1998).
⁴S. Chen and C. Wang, Appl. Phys. Lett. 85, 765 (2005).
⁵T. Lee, Y. Chung, O. Kwon, and J. Park, Adv. Funct. Mater.17, 390 (2007). •