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Illumination & Displays

Toward efficient, flexible flat-panel displays

Redesigning the electrodes may help to unlock the full potential of organic light-emitting diodes on low-cost flexible plastic substrates.
13 December 2011, SPIE Newsroom. DOI: 10.1117/2.1201112.004075

Organic light-emitting diodes (OLEDs) have attracted considerable attention, driven by the promise of low-cost flexible displays and light sources.1 Flexible devices are made possible by the fact that the organic materials used in OLEDs consist of discrete molecular units that do not require perfect single-crystal films, perfectly aligned with their substrate, to operate in real devices. However, much of the recent progress in high-efficiency OLEDs has focused exclusively on rigid glass substrates. So much so, that many of the recent device optimizations are incompatible with flexible plastic substrates.

For example, the recent focus for OLED development has been on using high-refractive-index (n≥1.8) substrates in place of standard glass (n=1.5)2–4 to enhance the outcoupling (that is, exit) of light trapped within the device. Unfortunately, enhancements in optical outcoupling that rely on high-refractive-index substrates are not practical on flexible plastic substrates because most plastics have a low refractive index (n≤1.6) that is comparable to standard glass. One challenge is thus to extract the trapped light without using a high-refractive-index material as the substrate in order to maintain compatibility with common engineering plastics. Another challenge is achieving a sufficiently conductive transparent electrode for large-area devices that is compatible with flexible plastic. To achieve high-conductivity indium tin oxide (ITO), the most commonly used transparent electrode, a high annealing temperature (>300°C) is required. A high conductivity is essential to achieve high power efficiency in OLEDs and to ensure uniform emission from large-area OLEDs (i.e., to reduce the electric field drop across the large-area electrode). However, most plastics cannot withstand such high annealing temperatures.

Figure 1. Schematic of an organic light-emitting diode (OLED) device structure with a flexible plastic substrate. Ta2O5: Tantalum pentoxide.

To overcome these challenges, we replaced the standard transparent ITO electrode with a thin semi-transparent gold (Au) film. We also inserted a 50–100nm-thick layer of high-refractive-index tantalum pentoxide (Ta2O5, n∼2.1) between the substrate and semi-transparent Au electrode (see Figure 1). By changing the thickness of the Au and Ta2O5 layers, we were able to extract light trapped in different optical modes in the device, resulting in significant efficiency enhancements. We also further modified the Au surface with a thin layer of high-work-function molybdenum trioxide (MoO3) to enable the use of a hole-transport material with a very deep highest-occupied molecular orbital (HOMO)5.

We first performed comprehensive optical simulations to optimize the thickness of the Au and Ta2O5 layers for a device with bis(2-phenylpyridine)(acetylacetonate)iridium(III), also known as Ir(ppy)2(acac), a highly efficient green phosphorescent emitter. We then fabricated flexible OLED devices based on the simulation results. Figure 2 summarizes the measured external quantum efficiency (EQE) and power efficiency of the optimized device (i.e., 18nm Au and 70nm Ta2O) fabricated on a flexible polycarbonate substrate in comparison to a reference device fabricated on ITO-coated glass.6

Figure 2. External quantum efficiency (EQE), above, and power efficiency (PE), below, for the novel device optimized for bis(2-phenylpyridine)(acetylacetonate) iridium(III) as a function of luminance and compared to a reference device fabricated on glass coated with indium tin oxide (ITO). Au: Gold.

Figure 3. Photograph of a flexible OLED (50×50mm) at high luminance of 5000cd/m2.

We found that the Ta2O5/Au/MoO3 device on flexible plastic reached an unprecedented high EQE of ∼40%. In comparison, the EQE of the ITO-based device is less than 30%. Figure 3 shows a photograph of a large-area (50×50mm) working device on flexible plastic at high luminance of 5000cd/m2. Although the Ta2O5/Au/MoO3 electrode greatly enhances the outcoupling of light, a significant amount is still trapped in the plastic substrate. To demonstrate the upper limit of the efficiency achievable on flexible plastic, we coupled a lens-based macro-extractor to the plastic substrate to out-couple light trapped in the substrate modes. Using this approach, we found that a maximum EQE and power efficiency of 63% and 290lm/W, respectively, could be achieved. More remarkably, the EQE dropped by less than 5% from low luminance (<100cd/m2) to high luminance (>10,000cd/m2), remaining at an extremely high value of 60% with a power efficiency of 126lm/W at 10,000cd/m2.

In summary, we have demonstrated a novel OLED design concept to unlock the full potential of OLEDs on flexible plastic. The EQE of this device reached 40% without outcoupling enhancement and could be further increased to 63% using a macro-extractor. We are now applying our unique outcoupling strategy to enable low-cost mass-production of flexible OLEDs using roll-to-roll processing for next-generation flat-panel displays and solid-state lighting.

Zhibin Wang, Michael Helander, Zhenghong Lu
Organic Optoelectronics Research Group
Department of Materials Science and Engineering
University of Toronto
Toronto, Canada

Zhibin Wang is a PhD candidate and senior researcher whose research is focused on the device physics of organic optoelectronics, such as charge injection and transport and optical design.

Michael G. Helander is a PhD candidate and Vanier Canada graduate scholar. He is also a senior researcher whose research is focused on interface engineering for simplified and high-4efficiency organic optoelectronics.

Zhenghong Lu is a professor and Tier-1 Canada Research Chair in Organic Optoelectronics. He leads the Organic Optoelectronics Research Group, overseeing a dynamic team of about a dozen researchers working on organic optoelectronics device design and engineering.

1. S. R. Forrest, The path to ubiquitous and low-cost organic electronic appliances on plastic, Nature 428, pp. 911, 2004.
2. A. Mikami, T. Koyanagi, High efficiency 200-lm/W green light emitting organic devices prepared on high-index of refraction substrate, SID Symp. Dig. 40, pp. 907, 2009. Paper 60.4L
3. S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer, B. Lussem, K. Leo, White organic light-emitting diodes with fluorescent tube efficiency, Nature 459, pp. 234, 2009.
4. S. Mladenovski, K. Neyts, D. Pavicic, A. Werner, C. Rothe, Exceptionally efficient organic light emitting devices using high refractive index substrates, Opt. Express 17, pp. 7562, 2009.
5. M. G. Helander, Z. B. Wang, J. Qiu, M. T. Greiner, D. P. Puzzo, Z. W. Liu, Z. H. Lu, Chlorinated indium tin oxide electrodes with high work function for organic device compatibility, Science 332, pp. 944, 2011.
6. Z. B. Wang, M. G. Helander, J. Qiu, D. P. Puzzo, M. T. Greiner, Z. M. Hudson, S. Wang, Z. W. Liu, Z. H. Lu, Unlocking the full potential of organic light-emitting diodes on flexible plastic, Nat. Photon. 5, pp. 753, 2011. doi:10.1038/nphoton.2011.259