High-contrast organic light-emitting diodes on flexible substratesAlexey N. Krasnov Citation: Applied Physics Letters 80, 3853 (2002); doi: 10.1063/1.1480473 View online: http://dx.doi.org/10.1063/1.1480473 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/80/20?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Patterning organic light-emitting diodes by cathode transfer Appl. Phys. Lett. 81, 4165 (2002); 10.1063/1.1525878 High-performance polymer light-emitting diodes doped with a red phosphorescent iridium complex Appl. Phys. Lett. 80, 2308 (2002); 10.1063/1.1462862 Graded mixed-layer organic light-emitting devices Appl. Phys. Lett. 80, 725 (2002); 10.1063/1.1446992 Transparent organic light-emitting diodes using metal acethylacetonate complexes as an electron injective bufferlayer Appl. Phys. Lett. 78, 3343 (2001); 10.1063/1.1359485 High-efficiency organic polymer light-emitting heterostructure devices on flexible plastic substrates Appl. Phys. Lett. 76, 661 (2000); 10.1063/1.125854

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.114.34.22 On: Wed, 03 Dec 2014 08:55:10

High-contrast organic light-emitting diodes on flexible substratesAlexey N. Krasnova)

Luxell Technologies Incorporated, 2145 Meadowpine Boulevard, Mississauga, Ontario L5N 6R8, Canada

~Received 22 January 2002; accepted for publication 22 March 2002!

Present organic light-emitting diode~OLED! displays, including those on flexible substrates,demonstrate relatively poor contrast due to the reflective metal electrode at the rear of the stack. Inthis letter we report on the use of a contrast-enhancing optical interference filter to improve thecontrast of flexible OLED displays. Some other benefits of using the contrast-enhancing technologyare also discussed. ©2002 American Institute of Physics.@DOI: 10.1063/1.1480473#

Display technologies using organic light-emitting diodes~OLEDs! are capturing growing attention these days. Advan-tages such as full color capability, potentially low manufac-turing costs, small thickness, light weight, and wide viewingangle make this new type of emitting device extremely at-tractive for automotive stereo panels, cellular phone displays,personal digital assistants, etc. A typical OLED device can besmall molecule or polymer based and consists1 of two elec-trodes, a hole-transport layer, an electron transport layer, andan emissive layer. The front electrode is a transparent con-ducting material, such as a combination of indium–tin–oxide~ITO! and poly~ethylenedioxy! thiophene~PEDOT!, whichprovides good hole injecting properties. The rear counterelectrode must possess good electron injecting properties,thus have a low work function. All presently available metalsand alloys suitable for this application, including Al, AlLi,Mg, MgAg, etc., are highly reflective, which explains thepoor contrast of presently available OLEDs. A simple andeffective solution by which to reduce the reflectance of or-ganic displays using a contrast-enhancing stack~CES!,~trademarked Black Layer™! has been suggested recently byour group.2 A significant contrast increase of devices fabri-cated on rigid glass by Luxell as well as of devices made byother manufacturers~Cambridge Display Technology, Uni-versal Display Corporation, etc.! has been achieved. It wouldalso be very desirable to extend the application of CES torecently introduced flexible organic light-emitting devices~FOLEDs! on unbreakable and lightweight plasticsubstrates,3 thus bringing new potential for high-contrast rollup, avionics, automotive, advertising, or other types of dis-plays, which can be affixed to virtually any curved surface.

In this letter we discuss a successful application of acontrast-enhancing optical interference stack to high-contrastflexible OLED devices.

The device is fabricated on PEDOT flexible substrates~from AGFA, Belgium! with either small-molecule or poly-mer organic stacks, complete with a refractive electrode orCES. We discuss in detail our studies performed on OLEDdevices using tris~8-hydroxyquinoline!aluminum ~Alq! asthe emission material andN,N8-diphenyl-N,N8-(3-methylphenyl)-@1,18biphenyl#-4,48-diamine ~TPD! as thehole transport material in a PEDOT/TPD/Alq/LiF/CEF con-figuration. The principles of the CES operation are discussed

elsewhere.4 This is a destructive-optical-interferencemultilayer filter, which is incorporated between the opaqueelectrode and the organic stack. The CES, also playing therole of cathode, has a combination of an ultrathin (;20 Å)semitransparent metal layer~such as Al or Cr!, a phase-changing layer of transparent conducting oxide~TCO! ~usu-ally SiO based!, and a thick layer of a reflective metal~typi-cally Al! ~Fig. 1!. The contrast-enhancing stack is depositedon the organic layers in a single pumpdown and is separatedfrom the organic part by an ultrathin (;10 Å) LiF layer5 toavoid destructive reaction of metal with Alq and to enhanceelectron injection.6 Other cathode configurations, includingthose of metal and metal/TCO, were also prepared to com-pare the properties of OLED devices. Fabrication of the de-vices was completed by encapsulation in a dry argon box.

The interface between the organic stack and the CES hasbeen found to have a significant influence on device perfor-mance, particularly in determining the chemical stability ofthe device and the energy barrier for the injected electrons.Three main factors have been taken into account while opti-mizing the Black Layer for flexible displays:~i! optical in-terference properties;~ii ! mechanical and chemical stability;and ~iii ! injection properties. Calculation of the optical re-flectance from the device was performed in accordance withthe technique described in Ref. 7.

Figure 2 demonstrates the experimental reflectance spec-tra from Al/AlSiO/Al flexible and rigid OLED contrast-enhancing stacks. The effect of the Black Layer is cancella-tion of the reflection over a wide bandwidth to provide a darkbackground in spite of the use of a reflective electrode. Be-cause the components of the organic stack and plastic sub-strate have well-matching refractive indices, the CES helpsto reduce specular reflectance from the display to less than1%.

a!Electronic mail: akrasnov@luxell.com FIG. 1. Schematic cross section of the CES-assisted OLED.

APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 20 20 MAY 2002

38530003-6951/2002/80(20)/3853/3/$19.00 © 2002 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.114.34.22 On: Wed, 03 Dec 2014 08:55:10

Our preliminary experiments demonstrated a shift of theI –V curve toward lower voltages~Fig. 3! with the use of theCES compared to a conventional device, which can be ex-plained by alteration of the potential barrier. At the sametime, no operating voltage decrease was observed in the caseof the device with the thick-metal/TCO cathode, suggestingthe important role of the ultrathin metal film—a componentof the contrast-enhancing filter. We suggest that the ultrathinmetal layer results in both further lowering of the highestoccupied molecular orbital~HOMO! level of Alq under theapplied voltage and improving the electron injection throughthe enhanced ‘‘chemistry’’ of the modified cathode. Theformer is attributed to the fact that metals stop being goodconductors of electricity once they are below a certain thick-ness. They also become more transparent due to decreasedelectron–photon interaction, which is exactly the case whenthe ultrathin metal layer is used. In this situation, a voltagedrop and, therefore, a space charge layer exist across theultrathin metal layer to compensate for the difference invacuum levels between the Alq and TCO, which pushes theAlq HOMO level down. The latter is due to partial reactionof Al with LiF, 8 which results in the formation of a LiAlalloy, known for its excellent electron-injecting properties.Undergoing ultraviolet photoelectron spectroscopy~UPS!measurements9 are expected to contribute to further under-standing of the mechanism of electron injection in the CES-assisted devices.

The emission efficiency of OLEDs is known to generallyincrease with the shift of theI –V curve toward lower volt-ages. In the case of CES-assisted OLED devices, however,this trend is less pronounced than in the case of conventionaldevices. This can be explained by the fact that the recombi-nation zone in the devices having TPD as the hole transportlayer and Alq as the electron transport layer is situated nearthe TPD/Alq interface due to the low mobility of holes in theAlq layer.10 The lower the work function of the cathode, thebetter the confinement of the recombination zone within the

TPD/Alq interface. Higher emission efficiencies of the de-vice can be achieved, therefore, by tuning the thickness ofthe Alq layer, thus using a constructive interference effect ofthe emitted light. This, however, places additional restric-tions on optimization of the organic stack and makes its pa-rameters extremely sensitive to the Alq thickness variations.The use of the CES eliminates this effect, and provides anexplanation for the relative insensitivity of the emission ef-ficiency to the cathode work function.

Simulations by Chanceet al.11 predicted nonradiativeenergy transfer from the exited state of the organic material

FIG. 4. Scanning electron micrograph of the CES-OLED on a flexible sub-strate~a!. The effect of bending on a nonoptimized~b! and an optimizedCES stack~c!.

FIG. 2. Experimental reflectance spectra of conventional and CES-assistedOLEDs.

FIG. 3. Current–voltage dependence of a conventional~a! and a CES-assisted flexible OLED~b!.

3854 Appl. Phys. Lett., Vol. 80, No. 20, 20 May 2002 Alexey N. Krasnov

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.114.34.22 On: Wed, 03 Dec 2014 08:55:10

to a thick metal film, which efficiently quenches lumines-cence in the proximity of the metal. At the same time, it hasbeen demonstrated12 that very thin~20–30 Å! metal films donot cause the reduction in efficiency. These authors sug-gested a strong influence of interference effects on the activelayer efficiency. The first absorbing film of the Black Layeris about 20 Å thick and, according to the model suggested,does not cause nonradiative energy transfer from the deviceemitter and, therefore, provides better emitting conditions.

In multilayer devices intrinsic mechanical stress cancause films to either crack or peel off the underlying sub-strate layer. Luckily, most OLED thin films are rugged undermechanical bending due to flexible van der Waals forces,which bound them. Use of inorganic films for the contrast-enhancing filter, however, requires special attention to ensuretheir compatibility with the organic stack and between eachother. The optimized CES stack has been found to behavemechanically friendly when incorporated into the device~Fig. 4!. As a result, CEF flexible displays were found toexhibit similar electro-optical characteristics before and afterbending with the radius of; 2 mm.

OLEDs are known to be sensitive to exposure to mois-ture and oxygen traces, which results in fast cathode delami-nation and device active layer degradation~‘‘dark spots’’!.OLEDs on rigid substrates are packaged in an inert environ-ment using protective glass sealed with an epoxy. There aresome unsolved problems in finding a reliable packaging so-lution for flexible displays. The preliminary experiments sug-gest, however, that the use of the inorganic multilayer CESmay stabilize the interface and prevent diffusion of the ag-gressive elements into the organic film within the pixel area~Fig. 5!. Known for its excellent environmental stability~SiOfilms are perfect as anticorrosion coatings for aluminum sur-face mirrors! and ease of evaporation, silicon monoxide andits metal compositions are welcome components of thecontrast-enhancing stack. In the case of the CES-assisted de-vice, degradation starts primarily from the pixel edges. Fur-ther optimization of the Black Layer, including use of othermetals and compounds, is under way. The results will bepublished in separate reports.

In conclusion, we have described the application of adestructive contrast-enhancing optical interference stack toflexible organic light-emitting diode displays. We have dem-onstrated that the CES architecture contributes remarkably tothe reduced reflectance from the device. It also improvesbarrier properties of the OLED stack and, in combinationwith other packaging solutions, may prolong the lifetime ofthe device.

The author would like to acknowledge the contributions

of Dr. D. Johnson and Dr. P. Hofstra. Special thanks are dueto R. Wood for device preparation and discussions.

1C. W. Tang and S. A. Van Slyke, Appl. Phys. Lett.51, 913 ~1987!.2P. G. Hofstra and A. N. Krasnov, U.S. Patent IPN No. WO 01/08240~pending!.

3M. S. Weaveret al., Inf. Disp. 17, 26 ~2001!.4J. Dobrowolski, B. Sullivan, and R. Bajcar, U.S. Patent No. 5,049,780~filed 1991!.

5L. S. Hung, C. W. Tang, and M. G. Mason, Appl. Phys. Lett.70, 152~1997!.

6T. Wakimoto, S. K. Awami, K. Nagayama, Y. Yonemoto, R. Murayama, J.Funaki, H. Sato, H. Nakada, and K. Imai, International Symposium Inor-ganic and Organic Electroluminescence, 1994.

7J. A. Dobrowolski, B. T. Sullivan, and R. C. Bajcar, Appl. Opt.31, 5988~1992!.

8G. Greczynski, M. Fahlman, and W. R. Salaneck, J. Chem. Phys.113,2407 ~2000!.

9H. Ishii and K. Seki, IEEE Trans. Electron DevicesED-44, 1295~1997!.10R. G. Kepler, P. M. Beeson, S. J. Jacobs, R. A. Anderson, M. B. Sinclair,

V. S. Valencia, and P. A. Cahill, Appl. Phys. Lett.66, 3618~1995!.11R. R. Chance, A. Prock, and R. Silbey, Adv. Chem. Phys.37, 1 ~1978!.12H. Becker, S. E. Burns, and R. H. Friend, Phys. Rev. B56, 1893~1997!.

FIG. 5. Degradation of conventional~a! and CES-assisted~b! OLEDdevices.

3855Appl. Phys. Lett., Vol. 80, No. 20, 20 May 2002 Alexey N. Krasnov

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

128.114.34.22 On: Wed, 03 Dec 2014 08:55:10