Lowering Cost for OLED Lighting Manufacturing

Heike Landgraf, Uwe Hoffmann, Manuel Campo, Applied Materials GmbH & Co. KG, Siemensstraße 100, D-63755 Alzenau, Germany

Heinrich Becker, Hubert Spreitzer, Joachim Kaiser Merck KGaA, Frankfurter Strasse 250, D-64293 Darmstadt, Germany

Thomas Reichenbacher, Stephan Fabig, Johannes Ostermann, Reinhard Caspary, Hans-Hermann Johannes, Wolfgang Kowalsky

Technische Universität Braunschweig, Institut für Hochfrequenztechnik, Schleinitzstrasse 22, 38106 Braunschweig, Germany

Sebastian Franke FhG-IPMS, Institutsteil COMEDD, Maria-Reiche-Str. 2, 01109 Dresden, Germany

Abstract Manufacturing of Organic Light Emitting Diodes (OLED) for lighting is still some distance from the industry recognized cost tar-get of 100 € /m². Within the government funded Light In-Line (LILi) project, a team of material supplier, machine manufacturers and basic researchers have investigated potential ways to reduce OLED manufacturing costs. The concept developed within the LILi-project shows that a competitive cost structure can now be reached.

1. Objective and Background Fabricated on sheets of glass, OLED lighting tiles have been shown to emit bright white light, more uniformly and more energy efficiently than fluorescent light fixtures, making them well-suited for ceiling lights in homes and offices. Whilst a number of OLED display and lighting products have been developed in recent years, one of the remaining key challenges is the reduction of the large area manufacturing cost. This challenge must be addressed before the technology can be widely adopted especially for general lighting applications. Consequently Applied Materials, Merck KGaA and the University of Braunschweig (TU-BS) have collaborated together in a three-year project, known as Light InLine or LILi. The work is funded by Germany’s Federal Ministry of Education and Research (BMBF) and the objective is to develop processes designed to lower the cost of manufacturing OLED for general illumination applications.

The project has focused on the development of both reliable and stable processes rather than the demonstration of world record device performance because the currently achieved performance data already either match or are close to matching existing requirements stated in DOEs Energy Star specification [3], [6].

A baseline device stack was employed showing good lifetime perfor-mance combined with an efficiency high enough to demonstrate large area bright emission. Typical performance levels are of the order 17 lm/W at warm-white (0.45/0.44) color coordinates.

The first objective was to identify the most cost intensive part of the production process. This involved developing an understanding of the principle physical problems within the manufacturing process and providing solutions to overcome these technological hurdles. Proof of concept was provided by testing all organic materials and processes under production-like conditions before transfer to manufacturing large area prototype OLED devices.

The processes and technological challenges investigated included but were not limited to:

Design of the white OLED layer stack Innovative ways of providing the organic material supply Investigations on organic material evaporation behavior Analyses of the coated film quality using HPLC, NMR [2] Long term stability of all OLED materials Test of a new evaporation source design to reduce thermal load

on the organic material itself [5] Material mixture using linear source technology Automatic rate control for organic and metal evaporation New optical methods for organic rate measurement Uptime enhancement of the shadow mask Continuous production demonstration over several weeks Influence of base pressure on OLED performance Effect of doping gradients in organic devices Evaluation of different substrate pretreatments including UV-

Ozone, ion bombardment or molecular radicals treatment [4]

The results presented in this paper will focus on some topics.

2. Results The display industry has consistently demonstrated reduction in manufacturing cost through the increase in throughput. This is typically achieved by increasing the substrate size, reducing the takt time and reducing the tool downtime. As such, one principal objective of the project was to increase the uptime of the organic evaporation source by increasing the amount of material that fits within the existing crucible design (the size of the evaporation crucible is itself limited). This was achieved by compressing the organics to provide an average increase in density of 30 % as shown in Figure 1.

Figure 1: Increasing the 500 ml crucible material capacity (left powder, right compressed pellets)

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As OLED performance (especially for white OLED devices) very strongly depends on the organic layer thickness and doping concentration, a stable deposition rate is an absolute prerequisite for the increase in production yield of the machine. The principle challenge in this instance is found in terms of accurate measurement of the deposition rate itself. As such efforts were focused on the implementation of improvements of the inline metrology equipment including quartz crystal oscillators located at each evaporation source or optical thickness measurement directly on the substrate. Fundamental research on novel metrology solutions was also performed.

Of vital importance to the implementation of these new measure-ment techniques is to provide the same level of reproducibility and accuracy demonstrated by current quartz crystal oscillators coupled with a minimum uptime of 4 weeks without the need of maintenance. Optical absorption techniques proved the most viable metrology method available with the results from measure-ments made on a prototype test chamber shown in Figure 2.

Figure 2: Comparison of rates measured via quartz oscillator and absorption coefficients measured via optical spectroscopy.

This measurement methodology was more closely scrutinized on an industrial setting suitable for a Gen7 linear evaporation source. A series of experiments were performed using organic materials utilized in white OLED devices in order to validate the overall performance of this inline measurement method.

A further advantage provided by these optical absorption mea-surements is the simultaneous detection of different organic species. This feature is unique among the range of potential inline metrology tools available for OLED processing and can be used reproducibly either for co-deposition of organic materials or during the doping process. This also provides an innovative path-way towards reduction of both complexity and, more importantly, capital equipment cost (hence cost of ownership).

The quartz crystal technology has also been improved, in terms of the thermal management of the sensor head and the algorithms required to analyze the data from the quartz crystal itself. This has permitted the continuous operation of a Gen4 organic evaporation source with automatic rate control for periods of more than 600 hours with a rate stability better than ± 3.5 % (Figure 3).

A white light emitting OLED reference layer stack was employed as shown in Figure 4 to investigate economic manufacturability of OLED devices under production conditions. This stack structure was proposed by Merck in collaboration with IPMS and utilizes

Merck’s own high performance OLED materials. These materials were then evaporated using Gen4 linear evaporation sources [5].

Figure 3: More than 650 hours automatic rate control for linear organic evaporation source

Figure 4: Layer stack

The evaporation test procedure for each material included measurement of the dynamic rate as a function of temperature, validation of the stability and accuracy of the automatic rate control, layer thickness uniformity at production thicknesses and deposition rates, measurement of both the powder and film density, determination of the material utilization efficiency and chemical analysis of the coated material and residual material in the crucible with HPLC and NMR respectively.

Figure 5 shows the dynamic rate for the hole transport material (HTM). For production with 90 s takt time and 40 nm HTM thickness, a dynamic deposition rate of 27 nm m/min is required.

Figure 5: Rate curves for various crucible loads of HTM

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Figure 6: Thickness distribution on Gen 4 substrate

Further material savings can be made by improving the coating uniformity. With the optimized Generation 4 evaporation source design film thickness uniformities better than ± 3 % were obtained as shown in Figure 6.

A series of tests were also performed to demonstrate the stability of the organic materials over a 2 week period. Figure 7 shows chemical analysis results of a blue host material using HPLC indicating material purity levels > 99.99 %, confirming no detectable material decomposition.

Figure 7: HPLC Chromatogram of blue host material before (above) and after (below) long term test evaporation

Further tests were performed to evaluate the stability of all the organic materials used within the stack under production conditions. The HPLC results obtained from Merck’s measure-ments indicated that each material remained stable for periods in excess of 4 weeks even at the elevated temperatures required for evaporation.

Further experiments were performed utilizing the LILi layer stack and performed on an inline Gen2 production tool at IMPS in Dresden. This tool was designed for mass production of large area organic devices on glass substrates and was developed by Applied Materials (formerly Applied Films) in 2002 [1]. This tool consists of 11 high vacuum chambers with base pressure < 10-6 mbar and two glove boxes on both ends for substrate load/unload operations. Within the context of the LILi project, this tool was equipped with a total of 3 metal and 12 linear organic evaporators for OLED device manufacture. An additional pretreatment zone was also installed to evaluate the effect of different plasma pretreatment processes on device yield and performance.

Figure 8: The hole-injection current into a single carrier device in respect to the ITO substrate pre-treatment

Each of the pretreatment methods used was shown to improve the injection of holes into the first organic layer. However, the highest current measured within the hole-only device was obtained by utilizing a UV-Ozone clean in atmosphere for each ITO coated glass substrate prior to organic deposition as shown in Figure 8. These tests also indicated that the nature of the pretreatment method had to be tailored specifically to the type of OLED stack to be processed.

A cost of ownership (CoO) model was used to optimize the machine layout in order to minimize the white light OLED device manufacturing cost. This model provided a means of offsetting the influence of the number of deposition sources within the tool by improvements in the uptime based upon the hardware and process improvements described previously. The tool specification, as set out within the context of the LILi project, is shown in Table 1 below for reference.

Table 1: Requirements of white OLED production tool according to LILi project results

Parameter Requirements

Substrate size 0,98 x 0,76 m² = 0,74 m²

Machine uptime ≥ 4 weeks

Takt time < 90 s

Coated area per year 254.000 m²

Organic material utilization > 60 %

Pretreatment Linear ion source

Vacuum requirements Vacuum loading for substrates

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Figure 9: Gen4 production coater based on LIli results

The optimized tool design included each of the pretreatment and deposition sources necessary for the organic layer and metallic contact deposition steps, in addition to mask and carrier exchange, and loading and unloading of the substrates (Figure 9). This tool design therefore provided a significant cost of ownership reduction for mass production of OLED tile based lighting devices thereby achieving the primary objective of the LILi project. Further economy of scale cost reductions can be achieved particularly in terms of material cost reductions by ramping up the manufacturing volume.

3. Summary The paper summarizes the results of a 3 year project on improving the manufacturability of white OLED devices for lighting applications. Significant improvements in manufacturing tool uptime have been demonstrated permitting continuous manu-facturing for periods greater than 4 weeks. This paper also pro-vides the first reported successful integration of optical absorption based inline metrology for deposition rate control in order to overcome the current limitations of standard inline, real time organic material deposition rate measurements.

4. Acknowledgements This work was funded by the Ministry of Education and Research (Bundesministerium für Bildung und Forschung, BMBF, project number 13N10611).

5. References [1] C. May, Y. Tomita, M. Toerker, M. Eritt, F. Loeffler, J.

Amelung, K. Leo, “In-line deposition of organic light-emitting devices for large area applications”, Thin Solid Films 516, 4609–4612 (2008)

[2] H. Becker, I. Bach, M. Holbach, J. Schwaiger, H. Spreitzer, “Purity of OLED-Materials and the Implication on Device-Performance”, SID Symposium Digest 41, 39–42, (2010)

[3] P. A. Levermore, A. B. Dyatkin, Z. M. Elshenawy, H. Pang, R. C. Kwong, R. Ma, M. S. Weaver, J. J. Brown, “Phosphorescent OLEDs: Enabling Solid State Lighting with Lower Temperature and Longer Lifetime”, SID Symposium Digest 42, 1060–1063, (2011)

[4] S. Franke, C. May, H. Landgraf, M. Campo, U. Hoffmann, “Physical and chemical plasma pre-treatment of indium tin oxide and influence on organic light emitting diodes”, Plastic Electronics Conference (2010)

[5] U. Hoffmann, H. Landgraf, M. Campo, J. Bruch, S. Keller, M. Koenig, “New Concept for Large Area White OLED Production for Lighting”, SID Symposium Digest 41, 688–691, (2010)

[6] www.lumiblade.com , www.orbeos.com, www.novaled.com

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