Solution processing is one potential method for enhancing the commercialization of OLED displays. In this article, DuPont discusses its recently announced solution-processing technology.

by William Feehery, David Flattery, Norman Herron, Charlie Lang, and Marie O'Regan

ACTIVE-MATRIX organic light-emitting-diode (AMOLED) displays are an attractive alternative to active-matrix liquid-crystal displays (AMLCDs) due to their excellent visual performance, thin form factor, and the potential for significantly reduced manufacturing cost. While LCD-manufacturing and supply-chain costs have been driven down relentlessly over the past 5 years due to innovation, volume growth, and glass scale, there is still a significant opportunity for AMOLEDs to be produced at even lower cost because AMLCDs remain a complex assembly of optical and electronic components. However, the low-cost potential of AMOLEDs has not yet been realized in commercial production due to the high costs of vapor-deposition manufacturing methods currently in use.

One of the most attractive attributes of an AMOLED display is the ability to build an emissive device directly on top of a thin-film-transistor (TFT) backplane. By doing so, the need for a color filter and a backlight unit (along with its associated light-management films) can be eliminated. These components can represent 20–45% of the overall cost of the display, depending on display size (Fig. 1).

In order to capitalize on this inherent edge, OLED makers must not cede these potential cost savings during the manufacturing process. Current experience shows there are four key challenges to exploiting this advantage:

Fig. 1: Potential manufacturing cost reduction for OLEDs.

William Feehery is Global Business Director at DuPont OLEDs; telephone 805/562-5307, e-mail: William.F.Feehery@usa.dupont.com. David Flattery is Operations Business Leader, Norman Herron is a Research Fellow, Charlie Land is a Senior Research Associate, and Marie O'Regan is Technology Director, all with DuPont OLEDs.

Yield: Overall, the yields from LCD manufacturing generally range from 85% to more than 95%, due to the maturity of the manufac-turing process. In contrast, most of the methods proposed for OLED production appear to be much less robust. Evaporation through fine metal masks suffers from problems with maskalignment, distortion, damage, and cleanliness. Ink-jet-printing technology, a potentially attractive alternative, is challenged by the need to deliver millions of drops of material over a sheet withinexacting tolerances for volume and placement.

Scale: For AMOLEDs to compete with LCDs, they must be manufactured on similar-sized motherglass. As a general rule, as LCD lines increase in generation, the fixed cost may increase by 30% while output doubles. To date, evaporation using fine metal masks has not been commercially demonstrated above a glass size of 730 x 460 mm, which is a significant disadvantage compared to LCDs.

Material Waste: OLEDs rely upon highly engineered and highly purified organic materials that are manufactured in relatively small volumes; hence, they are likely to remain quite expensive. Evaporation is an inefficient process for coating, with perhaps 3–10% of the material loaded into the evaporator deposited onto the sheet. Although using linear sources for evaporation can improve the material utilization, OLEDs ideally should be manufactured by a process in which the material is deposited only onto the substrate.

Low-Cost Encapsulation: The encapsulation process, which must protect the delicate OLED structure to a much higher degree than is required for the less-sensitive LC material, cannot add excessive cost. The encapsulation must be performed with low-cost materials, at high yield, without significantly increasing the display footprint. The most common configuration of machined cavities in glass lids and discrete pressure-sensitive getter tape adds significant cost to the AMOLED.

Several approaches have been proposed or utilized as alternatives to evaporation with fine masks. One method is to utilize white OLED materials with a red, green, blue (RGB) color filter so that fine shadow masks are not required. This approach may avoid some of the yield issues associated with evaporation, but the required color filter both adds significant cost and reduces the intensity of light emitted from the display. Others have explored transferring emissive materials from donor sheets to form a pixel pattern by laser-induced thermal imaging (LITI), radiation-induced sublimation transfer (RIST), or laser-induced patternwise sublimation (LIPS). Challenges still remain to achieve high-yield low-defect transfer of the materials from the donor sheet to the substrate.

Fig. 3: DuPont Displays's solution process flow for OLED fabrication.

Solution-Processing Technologies

Solution-processing technology, where the OLED materials are precisely delivered to only the required areas without waste, is an attractive, scalable approach to gaining a manufacturing-cost advantage over AMLCDs. Detailed cost modeling of solution-processed AMOLEDs predicts that manufacturing costs could be about 20% less than that for LCDs for small (2.2-in.) sizes. This savings should grow as the process is scaled to larger lines and larger displays, due to the much larger fraction of overall cost represented by the driver electronics for small displays and by the backlight unit (BLU) for larger LCDs, as shown in Fig. 2, which was generated from full models of display-manufacturing costs. Here, cost is measured on a per module basis, as referenced to the cost of an LCD module. The data are presented as the cost of a 2.2-in. OLED as a percentage of the cost of a 2.2-in. LCD module. For small OLED displays (~2 in.), roughly half the cost is the panel (versus the drive electronics). This fraction increases for larger displays. Our models indicate that evaporated OLEDs are disadvantaged versus LCDs in terms of panel cost. As the fraction of the display cost represented by the panel goes up, the dis-advantage of evaporated OLEDs increases versus LCDs. For solution OLEDs, the displays are advantaged versus LCDs in terms of panel cost. As the fraction of the display cost represented by the panel goes up, the advantage of solution-processed OLEDs increases versus LCDs. This relationship between cost advantage and size is an additional incentive to quickly scale the process and deliver display performance suitable for larger displays, including televisions.

Ink-jet printing has been widely explored as a candidate technique for solution processing – it is a potentially attractive technology because the higher-cost OLED materials are precisely transferred to the substrate in only the required areas and changes to display and substrate layouts can be digitally downloaded to the printer. Experience with a variety of ink-jet tools appears to indicate that while many, if not all, of the physical-quality issues may eventually be satisfactorily addressed, ink-jet printing is not yet sufficiently robust for low-cost manufacturing of OLED displays.

We have developed a scalable low-cost-solution OLED-manufacturing process where the number of printing steps has been minimized and existing flat-panel-display equipment has been utilized. The solution-based process outlined in Fig. 3 allows production of high-resolution active-matrix displays using soluable small-molecule emissive materials.

Fig. 8: Optical profilometer trace of emissive ink printed on a flat panel. The ink was printed in three distinct passes and each pass was allowed to dry before the neighboring line was printed.

Fig. 9: Materials Performance.

Figure 4 shows a detailed view of the typical layers in a solution-processed small-molecule OLED display. The hole-injection layer (HIL) and primer layer can be formulated so that the same thicknesses are used for all three colored emitters. This allows blanket coating of the hole-injection layer (HIL) and primer by any number of scalable non-contact coating techniques. Slot-die coating is generally preferred for preparing thin, very uniform layers, and this technique has been scaled up to (at least) Gen 8 substrates. We have found that slot-die coating can deposit very thin layers over relatively tall display topography – conductive bus lines, photoresist channels, cathode separators, etc. – with excellent uniformity and with tact times acceptable for manufacturing processes. Figure 5 below shows a DuPont^™ HIL coated over and into pixel wells via slot-die coating. The optical profilometer scan in Fig. 6 demonstrates the excellent uniformity in layer thickness.

Generally, the HIL is coated as an aqueous suspension and the primer from an organic solvent. If suitable coating fluids are available, these two layers can conveniently be coated in a single pass using a multilayer die.

Blanket coatings must later be removed from conductive traces and regions where the adhesive will bond the glass lid to the display. The materials are removed by a plasma cleaning process using the cathode layers as a mask.

Patterning fine pixels from solution requires some method to contain the inks on the plate while drying. Often, this has been accomplished with pixel wells and/or surface-tension variations (e.g., CF₄ plasma treatment), though typically it is difficult to obtain uniform films near the edges of the pixels with these methods. We have developed technology to modulate the surface energy on the primer layer in precise patterns, allowing excellent wetting and uniform film formation in the light-emitting region of the pixel, while simultaneously providing high repellency to ink outside the pixel region, thereby preventing color mixing (Fig. 7).

Proper positioning of the wetting and non-wetting regions, along with control of the ink-drying profiles result in emissive layers with very uniform thickness profiles in the active pixel regions (Fig. 8).

The ink-containment patterns are created during the OLED-fabrication process, therefore no physical containment structures such as wells need be installed on the active-matrix backplane, thus simplifying backplane manufacture.

Solution-Printed OLED Fabrication

In order to fabricate an OLED device using solution-printing techniques, the chosen materials must have some key properties:

• The solution-deposited layers must form flat, amorphous dry films.
• Deposited films must not be disrupted during processing of subsequent solution-processed layers.

The performance data shown in Fig. 9 is obtained using DuPont^™ HIL as the hole-injecting layer. DuPont^™ HIL has shown exceptional performance in all types of OLED devices with several key advantages over incumbent materials.

• It delivers a high work function, leading to excellent hole-injection and to improved OLED efficiency and lifetime.

• The material forms excellent transparent (95–97% across the visible spectrum, for a 100-nm-thick film) conductive films (RMS roughness, ~1 nm) with a range of conductivities available (from 10^-6 up to 10^-3 S/cm, useful, e.g., in addressing pixel crosstalk issues) and its water-based formulation allows for processing of subsequent layers from organic solvents without disruption.

• The material may be formulated for slot-die coating, spin-coating, and other printing methods.

It has been speculated that small-molecule-based systems could not be solvent-processed to give high-quality multilayer devices due to poor film-forming properties when cast from solution. However, with careful molecular design of the materials, the intrinsic high performance of a vapor-deposited device can be maintained in a solution-processed device. An example of the success of this approach is the current DuPont red emissive-layer system. Starting with a rigorous evaluation of the materials' performance in vapor-deposited test devices, a novel red emitter was developed – DuPont Red1 (DR1). This material was engineered to have OLED properties that closely match with a proprietary host matrix such that resulting vapor-deposited devices exhibit superior performance (Table 1). This system was then re-engineered to introduce the materials' characteristics, which make the emissive layer compatible with a solution-processing deposition process. In particular, solubility is enhanced by using solubilizing groups on both the host and emitter components. These groups further help to make compatible the discrete molecular components, resulting in good amorphous film-forming characteristics from solution. The result is performance approaching the high vapor-deposited result, but now in the desired solution-deposited device format (Table 1).

This same approach has also been adopted for the green and blue pixel emissive materials used in current demonstration devices from DuPont Displays shown in May at the Society for Information Display's Display Week 2007. The net effect has been the achievement of high performance coupled with good lifetimes in an all-solution processed set of RGB materials, as illustrated in Fig. 9. These results represent combinations of materials' design improvements, deposition technique, and device architecture advances. Overall, the current material set now shows impressive device performance, as demonstrated by the 4.3-in. WQVGA displays (Fig. 10).

Summary

The presumed manufacturing cost advantage of AMOLED displays over AMLCDs has not yet been realized in a commercial setting, due to the high cost of manufacturing using current vapor-deposition methods. To allow this cost advantage to be realized, DuPont Displays has developed a full set of materials that work in combination with a cost-effective solution-based fabrication process to deliver high-quality device performance for AMOLED displays. Further improvements to the materials and process are being pursued in order to achieve lifetimes that are suitable for large-format displays such as televisions. •