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Illumination & Displays

Polymers shine the light

Light-emitting polymer technology comes to the forefront.

From oemagazine June 2002
31 June 2002, SPIE Newsroom. DOI: 10.1117/2.5200206.0003

Organic light-emitting-diode (OLED) display technology has been grabbing headlines in recent years. Now one form of OLED displays—light-emitting-polymer (LEP) technology—is rapidly emerging as a serious candidate for next-generation flat-panel displays. LEP technology promises thin, lightweight emissive displays with low drive voltage, low power consumption, high contrast, wide viewing angle, and fast switching times. One of the main attractions is the compatibility of this technology with plastic substrates and with a number of printer-based fabrication techniques, which offer the possibility of roll-to-roll processing for cost-effective manufacturing.

materials

Electroluminescent polymers are long chain hydrocarbon-based conjugated molecules featuring alternating single and double carbon bonds, with molecular weights of several hundred thousand atomic units. Unlike small molecule OLEDs, which must be produced using vacuum deposition techniques, LEPs can be applied to substrates by spin coating or printing to form amorphous films.

The pz orbitals of the neighboring carbon atoms in the polymer molecule overlap to form delocalized π molecular orbitals. The lower energy bonding orbital is called the highest occupied molecular orbital (HOMO) level, and the higher-energy π* (or anti-bonding orbital) forms the lowest unoccupied molecular orbital (LUMO) level. These orbitals are analogous to the valence and conduction band edges in an inorganic semiconductor in that states below the HOMO are occupied and those above the LUMO are empty. The HOMO and LUMO are separated by an energy gap, normally in the optical energy range, which allows the material to generate visible emission.

Although the semiconducting properties of these materials were known, it was not until 1989 that researchers at Cambridge University (Cambridge, UK) discovered that these polymers emit light when an electric current is passed through them.1 This discovery of electroluminescence led to the launch of Cambridge Display Technology (CDT; Cambridge, UK), which develops and licenses LEP technology. Since the initial discovery, there have been continuous and rapid advances in device efficiency and stability that are now enabling LEP-based products to enter the marketplace and compete with existing display technologies.

The first polymer to exhibit electroluminescence was poly p-phenylene vinylene (PPV). Since then, many new classes of soluble conjugated polymers have been developed, and both materials and the processes for producing them have steadily improved. One of the most widely used is the polyfluorene family, including poly(9,9'-dioctylfluorene). By modifying the substituent groups in the polymer and/or the molecular weight, we can tune the electrical properties of band gap, electron affinity, and charge transport, and rheological properties such as viscosity and solubility, thus tailoring the material to the specific application and deposition method. We can thus access a wide range of colors simply by blending different polymers together or by changing the chemical composition. Although polymers can reproduce the full gamut of CRT colors, the blue and green materials currently showing the best efficiencies and durability are not fully saturated for NTSC and PAL applications. Thus, material development continues.

design and performance

Figure 1. In an idealized LEP device, an electron injected from the cathode recombines with a hole from the anode to produce light emission at a wavelength governed by the bandgap of the polymer material.

The basic LEP device structure is very simple; indeed this is one of the most attractive features of the technology. To build a device, we sandwich a polymer layer between two metallic contacts, at least one of which is transparent. Upon application of a voltage greater than the threshold bias of 2 to 3 V, a current flows. Electrons are injected from the cathode into the LUMO of the polymer layer and holes from the anode into the HOMO level (see figure 1), and the structure emits light.

The injected charges move from molecule to molecule through the disordered polymer film by hopping conduction. Due to the highly disordered nature of polymer films, the conductivities of electroluminescent polymers are orders of magnitude lower than their inorganic counterparts, but the thinness of the devices (50 to 150 nm) helps operating voltages remain low. The injected electrons and holes combine to form excitons, or electron-hole pairs, bound together by their mutual attraction. An exciton may recombine radiatively to produce light. The wavelength of the emission depends on the bandgap of the polymer, and the intensity of the light is proportional to the current. The ratio of light emitted to current flowing in the external circuit—the quantum efficiency—has improved from only 0.01% in the first devices to more than 7% today.


Figure 2. Cross section of the basic LEP structure shows the main components of the device.

Ideally, the electrode work functions should match the HOMO and LUMO levels in the polymer so that it is easy to inject a steady supply of electron and hole pairs into the polymer so they can generate light. For the anode, we typically use a thin layer of transparent conductor indium tin oxide (ITO), which has a work function around 4.8 eV, deposited on a glass or plastic substrate (see figure 2). One or more organic thin films 50 to 150-nm thick are deposited from solution using spin coating or a printing technique. The first layer is a non-emissive conducting polymer, poly-ethylenedioxythiophene (PEDOT) or polyaniline doped with poly-styrenesulphonic acid (PSS), which serves as a hole-injecting layer and has an even higher work function than indium tin oxide (5.2 eV). The second layer deposited is the emissive polymer. Finally, we evaporate a metallic cathode on top of the emissive layer. The cathode is a reflective metal layer with a low work function to match the LUMO level of the polymer—for example, calcium (work function 2.8 eV). Light is emitted from the polymer through the glass substrate. This emission is Lambertian, giving a very wide viewing angle for the display.

The difference in work function between the cathode and anode materials described above is only 2.4 eV, which is less than both the polymer bandgap and the energy of the emitted photon. Thus, there is an energy barrier preventing the electrons and holes entering the polymer, which impairs light emission, reducing efficiency. This potential energy barrier to charge injection is usually present at the cathode/polymer interface. One solution to this problem is to insert a thin layer of dielectric, such as lithium fluoride, at the cathode, which enhances electron injection and improves device efficiency.

Alternative structures are also possible, for example, using a transparent cathode or a black-layer contrast enhancement technology (Luxell Technologies; Mississauga, Canada) that incorporates nonreflecting cathodes to provide high display contrast without the requirement for expensive polarizer films that reduce light output.

The active device area must be hermetically sealed to prevent the ingress of water and oxygen that can degrade the polymer and the reactive metal cathode. Present-generation structures typically use a metal or glass can encapsulation, with a glue seal to the substrate. There is continuing development in the use of conformal encapsulation layers, which will be compatible with roll-to-roll manufacture while remaining lighter, thinner, and more economical (see oemagazine, December 2001, page 18).

performance

In test displays, we have obtained very good performance for red, green, and blue polymers (see table). In the case of green LEPs, for example, we have achieved efficiencies of better than 20 Lm/W, which is higher than a traditional tungsten filament light bulb. To achieve office-level luminescence, the devices require relatively low voltages (3 to 4 V), though it is important that this voltage remain stable over time, typically rising by less than 0.1 mV/hr over the lifetime of the device. Although the luminance decrease of LEP devices (defined as time to half initial luminance) for red and green materials is acceptable for many applications, blue polymer stability remains the focus of much material development work.

LEP efficiency depends on the probability of an injected charge forming an exciton, the probability that this exciton will decay radiatively, and the amount of generated light that exits the structure. One of the limiting factors is spin statistics; when an exciton is formed it can be either in a spin singlet state—which can decay radiatively at the required wavelength—or in a spin triplet state that has a vanishingly small probability of emitting light. Simple statistics indicate that there should be three times as many triplets formed as singlets, so the maximum possible efficiency achievable would be 25%.2 Recent experimental measurements have confirmed this limit for small-molecule-based fluorescent devices. For polymers, however, the experiments demonstrated that the ratio of singlets to triplets is better than 1:13. Thus, the achievable efficiency in LEPs is intrinsically higher, which allows devices constructed from simpler materials, formulations, and structures to achieve higher efficiencies and lower power consumption.

fabrication

Figure 3. Ink-jet printing enables a full-color display to be produced by depositing different color LEPs into pre-patterned wells with polyimide bank structures on top of a polysilicon TFT substrate.

A full-color display requires the patterning of individual red, green, and blue pixels. This is where the superior ease of processability of the polymer materials offers big advantages. Because they are deposited from solution, LEPs can be patterned using a wide variety of printing techniques. The most advanced technique is ink-jet printing (figure 3). Resolutions as high as 360 dpi have been demonstrated, and the approach is scalable to large-screen displays. Printing promises the potential of a much lower manufacturing cost than equivalent LCD or small-molecule OLED technologies. Commercial inkjet printing systems for LEP manufacturing are being developed by CDT subsidiary Litrex Corp. (Pleasanton, CA) and by Seiko-Epson (Suwa, Japan).

One of the visionary goals for LEP technology is the development of flexible-substrate displays, which opens the potential for products that are thin, lightweight, robust, and able to be molded and conformed to different shapes. The potential for lower manufacturing cost is another attraction of flexible-substrate displays. Achieving these cost reductions is predicated on realizing roll-to-roll production, which requires the further development of substrate, barrier, and encapsulant layers as well as the integration of roll-to-roll production equipment. The prospect of economical, high-quality, robust, emissive displays has captured the interest of a number of display manufacturers, including Dupont Displays (Research Triangle Park, NC), Osram Optoelectronics (Regensburg, Germany), and Delta Optoelectronics (Hsin-chu, Taiwan).

To display an image on an LEP screen, we must control the current passing through each individual pixel—and therefore its luminance. There are two approaches to this goal: passive matrix and active matrix. Passive matrix schemes pattern the anode and cathode into perpendicular rows and columns and apply a data signal to the columns while addressing the rows sequentially. As the number of rows in the display increases, each pixel must be pulsed to higher brightness by a factor of the number or row times the desired brightness, which can exceed 20,000 cd/m2. The current required to achieve these brightness levels limits this architecture to relatively small screen sizes. Philips Flat Display Systems (Sunnyvale, CA) and Dupont Displays have demonstrated full-color passive-matrix displays.

In active-matrix display architecture, a thin-film polysilicon transistor on the substrate addresses each pixel individually. Active matrix displays are not limited by current considerations. Seiko-Epson, Toshiba (Tokyo, Japan), and Samsung (Seoul, Korea) have now demonstrated full-color active-matrix displays. One exciting possibility is that polymer transistors, which can be manufactured by techniques similar to those used for LEP patterning, could be used to drive an LEP display. Such an approach would potentially lend itself to roll-to-roll processing on flexible substrates.

LEP technology combines simple solid-state construction, low drive voltage, and high-efficiency OLEDs with large-area patternability and the possibility of flexible substrates. This combination of characteristics provides a powerful base for building high-information-content displays ranging from microemissive displays to large-area screens. We expect that LEP technology will initially be used to extend the performance of current displays in mobile communications, computers, and consumer electronics but will quickly lead to new applications. The necessary manufacturing infrastructure and supply chain are under development for this emerging industry, including commercial material systems, production techniques, and capital equipment such as ink-jet printers. Building from the fundamental advantages of polymer chemistry, LEP display technology is now set to significantly change the products we use to view the world. oe

References

1. J H Burroughes et al., Nature 347 p. 539 (1990).

2. R H Friend et al., Nature 397 p. 121 (1999).

3. J Wilson et al., Nature 413 p. 828 (2001).


Mark Leadbeater

Mark Leadbeater is manager of device research at Cambridge Display Technology, Cambridge, UK.