White OLEDs for lighting applications

White OLEDs for lighting applications

Peter Loebl

, Volker van Elsbergen

, Herbert Boerner

, Claudia Goldmann

, Stefan Grabowski

,

Dietrich Bertram

a

_{Philips Technologie GmbH Forschungslaboratorien, Solid State Lighting, Weißhausstr. 2, D-52066}

Aachen, Germany

b

_{Philips Technologie GmbH, Business Center OLED Lighting, Philipsstrasse 8, D-52068 Aachen,}

Germany

ABSTRACT

Efficient white OLEDs are becoming attractive as large area light sources for illumination and in future also for general lighting. We discuss device concepts for white OLEDs and their potential to achieve high efficacy and good lumen- and color-maintenance at the same time. We focus on OLEDs using a combination of fluorescent blue and phosphorescent red and green emitters (hybrid OLEDs). Hybrid OLEDs have high efficacy and lifetime in the white to warm white color region (color points B and A on the black-body-curve). Near illuminant A efficacy values of 28-29 lm/W without optical out-coupling can be achieved with a hybrid OLED. The external quantum efficiency (EQE) is 14%. A typical color rendering index (CRI) is 84. Recent results for monochrome OLEDs and for hybrid OLED stacks are presented.

Keywords: OLED, white OLED, hybrid OLED, lighting, fluorescent, phosphorescent

1. INTRODUCTION

White OLEDs are attractive large area light sources and can be produced in almost any shape and size. These features make new lighting and design applications possible. First lighting products are on the market and stimulate designers and application engineers. Figure 1 shows LumibladeTM _{lighting tiles from Philips [1] and Figure 2 shows an interactive}

light installation from Philips unveiled recently at a trade fair (EUROLUCE 2009) [2].

While all-fluorescent white OLEDs using red, green, and blue fluorescent emitters show already remarkably high lifetimes of several 10.000 hours, their efficacy is still limited to values below 17 lm/W [3]. Nevertheless, the brightness of such OLEDs can be high enough for lighting applications, if several white OLEDs are stacked. Because of the limited efficacy of all-fluorescent devices we expect to see these devices mainly in attractive designed lighting products, sometimes in combination with LED spotlights.

To reach higher efficacy values and to expand into the general lighting market, more efficient OLED stacks are investigated intensively. These stacks use either phosphorescent red, green, and blue emitters or combine fluorescent blue emitters with red and green phosphorescent emitters (hybrid OLEDs). These OLED concepts, when combined with effective light out-coupling techniques, promise an efficacy exceeding 100 lm/W [4].

This article briefly summarizes the state-of-art in monochrome OLEDs, compares and validates available stack concepts for white OLEDs and finally focuses on hybrid white OLED stacks.

*[email protected]; phone +49-241-6003-597

Invited Paper

Organic Light Emitting Materials and Devices XIII, edited by Franky So, Chihaya Adachi, Proc. of SPIE Vol. 7415, 74151A · © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.828574

Fig. 1. Example of a white OLED lighting tile (LumibladeTM _{) from Philips [1].}

Fig. 2. Interactive OLED wall installation from Philips shown at the EUROLUCE fair in Milano / Italy 2009 [2] featuring 1.200 white OLED tiles (5 cm x 5 cm in size).

2. EXPERIMENTAL

All OLEDs described in this article were made by thermal evaporation of small molecule materials. The base pressure in the deposition chambers of the cluster tool was in the low 10-7 _{mbar range. Deposition was done onto ITO coated glass}

substrates (n=1.52) which were cleaned and subsequently treated in an O2 plasma for several minutes prior to deposition.

Deposition rates were in the range of 0.1 nm/s controlled by Inficon quartz balances. IV curves were measured using a Keithley 2400 programmable source meter from Keithley Instruments Inc., Cleveland. Luminance was determined in the

perpendicular direction using a luminance meter LMT L1009 from Lichtmesstechnik GmbH Berlin. Spectra were measured in the same manner using a SpectraScan PR705 spectrophotometer from Photo Research Inc., Chatsworth. In order to check whether the distribution of emitted light was Lambertian, devices were measured as well in a calibrated integrating sphere equipped with a CAS140B Compact Array fiber spectrometer from Instrument Systems Optische Messtechnik GmbH, Munich. The optical measurements in the integrating sphere were done according to the OLLA white paper excluding any side emission from the glass substrates [5]. For comparability reasons we communicate all our data at a brightness level of 1000 cd/m2 _{without additional optical out-coupling. Most white OLEDs were optimized}

for an emission color close to the black-body curve.

3. RESULTS

3.1 Monochrome OLEDs

OLEDs using efficient phosphorescent emitters should have100% internal quantum efficiency. For OLEDs deposited on glass substrates with a refractive index of 1.52, it is assumed that only 20% of the emitted light is in the escape cone, while 80% of the light remains in the glass substrate and in the organic layers. Therefore phosphorescent emitters show 20% external quantum efficiency (EQE) without additional improved optical light out-coupling (ILO) applied to the glass substrate. We do achieve such high EQE values for phosphorescent red emitters: For these, even values of 21-22% EQE have been determined (see Figure 3). Common phosphorescent green emitters like Ir(ppy)3 yield more than 17%

EQE in monochrome devices. Phosphorescent blue (FIrpic) is approximately at 16% EQE, and fluorescent blue yields more than 6% EQE, which is again slightly higher than the theoretically expected 5% (assuming 25% internal quantum efficiency for fluorescent emitters and again only 20% of the light to escape from the glass substrate).

Figure 3 shows the EQE values of the different emitters and their CIE1931 color coordinates. Please note that the data are measured at a brightness of 1000 cd/m2 _{which we see as a minimum requirement for lighting applications. Due to the}

roll-off caused by triplet-triplet quenching even higher EQE values are possible at lower brightness levels.

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Fig. 3. CIE1931 color coordinates [6] and external quantum efficiency in percent at 1000 cd/m2 _{of monochrome OLEDs}

from Philips without improved optical out-coupling. The yellow device contained phosphorescent red and green emitting layers.

Figure 3 shows that for many phosphorescent emitters we are close to 20% EQE, which is the maximum estimated without improved light out-coupling. In phosphorescent green and blue there is still slight improvement possible. Fluorescent blue is already better than 6% EQE which is exceeding the expected value of 5% for these emitters. Efficient white OLEDs can be realized by combining these red (R), green (G) and blue (B) emitters in different stacks: all-fluorescent, all-phosphorescent, and different hybrid stacks combining fluorescent and phosphorescent emitters can be realized. All the different white OLED stack concepts have certain advantages and issues which we will discuss in section 3.2. In this discussion we will focus on the direct combination of light emitting layers and exclude R, G, B stacked devices with transparent electrodes or charge generation layers between the monochrome OLEDs.

Before we discuss the different white concepts in detail, we look into the combination of red and green phosphorescent emitters into a phosphorescent yellow device which allows achieving high EQE values. An efficient yellow device is a necessary building block for all-phosphorescent as well as for hybrid white stacks. This device should have an EQE without ILO near 20%. Figure 4 shows that without ILO we achieve between 51 lm/W and 64 lm/W in yellow depending on the color point. EQE values between 18.5% and 19.2% have been demonstrated. With a suitable blue it is possible to cover the range between color points A-B easily as can be seen in the figure.

400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 Int e nsity [a. u .] Wavelength [nm] Fig. 4. Left: CIE1931 color coordinates of a yellow device combining a layer with phosphorescent red and a layer with

phosphorescent green emitter. EQE values close to 20% have been achieved. Right: Yellow spectra which can be realized in this way. Combining this yellow with a suitable blue allows covering color points between A and B.

3.2 White OLED concepts

In the following we discuss the predicted EQE values of the different concepts for white OLEDs. The Table 1 lists these concepts. In column 3 we indicate the EQE values which have been reporteded by different groups. All data are given without improved light out-coupling (ILO).The all-fluorescent concept promises 5% EQE and is the least efficient way to realize white OLEDs. EQE values claimed by several groups, however, reach up to 10% [3], [7]. Triplet-triplet annihilation might be the cause of this high value. The all-fluorescent concept has some advantages which might partially compensate the limited efficiency: Little dependence of the color on the driving voltage (color stability), and very good lumen- and color- maintenance (commonly described with the word ‘lifetime’).

Table. 1. Different concepts for white OLEDs; the red, green, and blue emitter types used are indicated in the left column (F = Fluorescent, P = Phosphorescent). The theoretically achievable EQE values are listed in column 2; the values reported by different groups are listed in column 3 (references in column 4). Color stability (dependence of color on driving voltage) is judged in column 5. Color and lumen maintenance is judged in columns 6 and 7. Column 8 describes the main issues of the different concepts. IL = interlayer, ++ very good, + good, 0 acceptable, - poor, ? unknown. Approach Theoretical EQE predicted without ILO Reported EQE without ILO References Color stability (voltage) Color maintenance Lumen maintenance Issues All-fluorescent F,F,F 5% 10% [3] [7] + + ++ Limited efficacy All-phos-phorescent P,P,P 20% 20% [8] [9] [10] [11] + + 0 Lifetime blue emitter Hybrid P,P,(IL),F

16% 14-15% This work 0 0 + Interlayer Triplet harvesting P, P, F 20% 15% [12] [13] [14] + + - Stable fluorescent blue emitter with small S-T splitting Extra-fluorescence P, F 20% 3% [15] ? ? ? Only small EQE values demonstrated

The all-phosphorescent approach is certainly most attractive [8-11], but relies on the stability of the blue emitter molecule. Here only very few light blue emitters are available, which still suffer from lifetime limitations. The hybrid approach combines the advantages of both worlds: Using a long living blue state-of-the art fluorescent emitter and combining it with red and green phosphorescent emitters. The theoretically achievable EQE value at a warm white color point (color point A: x=0.4476, y=0.4074) is on the order of 16%. In experiments, we achieve more than 14% near illuminant A. The approach is attractive because it can make use of many available fluorescent blue emitters. However, an interlayer between the green and the blue emitting layer is quite often needed to avoid quenching of green triplet excitons by the low lying triplet level of the blue layer. Finding good ambipolar interlayers with good electron and hole conduction is often a difficult task. Slight degradation of the interlayer may cause a color shift. Therefore, color maintenance of such stacks is often limited. Despite these facts hybrid OLED stacks are attractive because they promise quite efficient and reasonably stable devices in the warm white region.

A special kind of hybrid device is the so-called ‘triplet-harvesting’ approach [12], [13], [14], which combines red and green phosphorescent emitters with a blue singlet emitter which has a high triplet energy. If the triplet energy level of the blue layer is high enough no interlayer is needed. The triplet excitons which are created in the blue layer are used to pump the phosphorescent red and green emitters and are not lost. This makes the triplet-harvesting approach very attractive. It relies, however, on the availability of a stable and efficient blue emitter material with small singlet-triplet splitting (S-T splitting) which has a triplet energy high enough to pump both red and green phosphorescent emitters. Unfortunately the number of blue emitter materials for this approach fulfilling this requirement is rather limited. Color stability and color maintenance of this approach seems to be quite good; the achieved lumen maintenance is still quite low.

Device concepts using extra-fluorescence by mixing charge transfer states also have the potential to achieve 20% EQE. The concept is working nicely showing an improvement in EQE by a factor of 3 from 1% to 3% of a fluorescent red emitter when a suited phosphorescent emitter material is added [15], which is of course still too low for practical applications.

Figure 5 shows the color coordinates of devices realized using the different concepts shown in Table 1. We note that our hybrid devices are very close to the black-body line according to the ENERGY STAR® benchmark for OLEDs [16]. One device showing triplet harvesting is quite greenish [13], making it difficult to judge how efficiently triplet harvesting works and whether the concept is already better in performance than the standard hybrid white approach. The (double-stacked) triplet harvesting device from Kodak [14] is close to the black-body line and has approximately 8.5% EQE. The device from Massachusetts Institute of Technology (MIT) showing extra-fluorescence is probably quite reddish (we estimated color point from the reported spectrum) [15].

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ID E K O , a ll-flu o res cen t, E Q E 9 -10% , U D C , a ll-p h o sp h o res cen t, E Q E 20 % , K o n ica M in o lta, all-p h o sp h o re scen t, EQ E 20% P h ilips H yb rid , E Q E 1 4.4 % P h ilips , H y b rid , E Q E 1 0.6 % P h ilips , H y b rid , E Q E 9 .5 %

K o d ak , T rip let h arvestin g , E Q E 8.5 % T U D , T rip let h arve stin g , E Q E 15% M IT, Ex tra-fluo re scen c e, E Q E 3%

y

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Idemitsu K,

Fig. 5. CIE1931 color coordinates and external quantum efficiency (EQE) of different white concepts without optical out-coupling. Only the hybrid white devices from Philips [this work] are very close to the black–body curve (solid black line) according to the ENERGY STAR® benchmark for OLEDs [16] . The color point of the extrafluorescent device from MIT was estimated from the published spectrum [15]. References: Idemitsu Kosan [3,7], UDC [8,9], Konica Minolta [10], TUD [13], Kodak [14].

Since the triplet-harvesting and the extra-fluorescence approaches are still in an early state, the following three concepts are most promising at the moment: All-fluorescent, all-phosphorescent, and hybrid white. In the following, we will discuss the hybrid white concept in more detail.

3.3 Hybrid white devices

Combining red and green phosphorescent emitters with highly efficient fluorescent emitters requires the use of an ambipolar interlayer which conducts both electrons and holes. This is necessary because most commercial fluorescent stable blue emitters having high efficiency are optimized for large singlet triplet splitting and have therefore low triplet energies. The interlayer has a triplet level high enough to avoid transfer of green excitons to the blue emitter system. This is schematically depicted in Figure 6.

Hole injection layer Hole transport layer

Phosphorescent red Phosphorescent green Fluorescent blue

Hole blocking layer Electron injection layer

ITO Cathode

Interlayer: exciton blocker, ambipolar

Fig. 6. Schematic of a hybrid white OLED device. Left side: Energy levels. Note that the triplet energy of the interlayer is high enough to avoid loss of green triplet excitons to the fluorescent blue emitter system. Right side: Schematic cross section of a hybrid OLED with an ambipolar interlayer. The arrows indicate the hole (up) and electron (down) currents across the interlayer.

To obtain high device efficacy and high color rendering index (CRI) it is important to choose the right combination of emitters. The maximal achievable efficacy without improved optical out-coupling (ILO) can be calculated using the luminous equivalent of the spectra of the red, green and blue emitters, taking into account an EQE value for the phosphorescent emitters of 20% and of 5% for the blue emitter. When we choose Ir(MDQ)2(acac) as red emitter,

Ir(ppy)3 for the green emitter, and a typical fluorescent blue (x=0.134, y=0.198), the corresponding luminous equivalent

values are 226 lm/W, 498 lm/W, and 172 lm/W, respectively. Combining these emitters to get a warm white spectrum at illuminant A (x=0.4476, y=0.4074) we need approximately 51% red emission, 25% green emission and 24% blue emission. The luminous equivalent of such a device at color point A is 245 lm/W. Without improved optical out-coupling we would extract only 20% of the emitted light into air and get an efficacy of 49 lm/W (Figure 7, right). Of course this is an idealized case assuming that the voltage to drive an emitter is equivalent to the optical bandgap. While this is not realistic, it sets an upper limit for the efficacy which can be achieved in a hybrid white OLED device without ILO. 0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 142 172 294 226 484 lm/W y Ir(MDQ)₂(acac) red Ir(ppy)₃ blue Deep blue x 245 Deep blue blue Simulation Combination of emitters Amount of emission Luminous equivalent at illuminat A (without ILO) CRI [%] [lm/W] Ir(MDQ)2(acac) 51 49 86 Ir(ppy)3 25 blue 24 Red emitter 59 55 73 Ir(ppy)3 19 blue 22 Ir(MDQ)2(acac) 51 51 89 Ir(ppy)3 28 Deep blue 21

Fig. 7. Left: CIE1931 color coordinates and luminous equivalents (in lm/W) calculated from the spectra of different red, green and blue emitters which can be used in hybrid white devices. Combining Ir(MDQ)2(acac), Ir(ppy)3 and blue to a

hybrid white device at color point A yields a luminous equivalent of 245 lm/W (white inverted triangle). Right: calculated luminous efficacy (including 80% out-coupling loss) for three different R, G, B emitter combinations for a hybrid device without ILO at color point A. The amount of red, green and blue emission in % is also given. The combination of Ir(MDQ)2(acac), Ir(ppy)3 and fluorescent ‘blue’ or ‘deep blue’ show high CRI, whereas the use of

another red emitter reduces CRI.

We have calculated the achievable efficacy at color point A also using other emitter combinations. Figure 7 indicates that one could use a red / orange emitter with higher luminous equivalent (e.g. 294 lm/W instead of 226 lm/W for Ir(MDQ)2(acac)), leading to efficacy values close to 55 lm/W for the hybrid white device without ILO. The spectrum of

this red emitter, however, reduces the CRI to values below 80, while with Ir(MDQ) 2(acac) we achive typically values

well above 80. Using a ‘deep blue’ (x=0.136, y=0.145) can also improve device efficacy, since there is more room in the spectrum for efficient green emission. We find 51 lm/W for the combination Ir(MDQ)2(acac), Ir(ppy)3, and ‘deep blue’.

The improvement due to the ‘deep blue’, however, is relatively small. In conclusion, to get a high CRI value and acceptable efficacy the combination of Ir(MDQ)2(acac), Ir(ppy)3, with either fluorescent blue or ‘deep blue’ seems a

good choice.

In the previous section we saw that the combination of phosphorescent red and green emitter results in a yellow device which shows an efficacy (without ILO) in the order of 50-60 lm/W depending on color coordinates. To build a hybrid device the yellow device has to be combined with a blue fluorescent emitter. This requires an exciton blocking interlayer as explained above. Figure 8 shows the performance of such a hybrid white OLED device with and without interlayer. Omitting the interlayer leads indeed to a strong reduction of green emission and a loss in efficacy. With interlayer the device performance at 1000 cd/m2 _{and at a driving voltage of 3.5 V is: 32 cd/A, 29 lm/W, EQE 14%.}

0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 IL no IL y x 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 IL no IL Intensity [a .u.] Wavelength [nm] 0 2000 4000 6000 0 20 40 no IL IL Current efficie n cy [cd/A] Luminance [cd/m2]

Fig. 8. Hybrid white OLED device combining a phosphorescent yellow unit with a fluorescent blue layer. The yellow unit contains two emitters. The device performance with (IL) and without interlayer (no IL) between the yellow and the blue unit can be seen in the CIE diagram (top left) and in the emission spectrum (top right): Without interlayer the spectrum looses green emission (top right), and the current efficiency is reduced by almost a factor of 2 (bottom).

Improving hybrid devices further by careful concentration and layer thickness control allows making efficient devices with a range of color points covering the whole black body curve. Figure 9 shows the performance of a hybrid white stack which has been optimized such that it can cover the whole black body curve by controlling the properties of the green layer in the middle of the device. Depending on the color coordinates the EQE values for this stack vary between 9.6 % and 12.2 % at 1000 cd/m2 _{without ILO. The current efficiency and the efficacy vary between 22 cd/A to 32 cd/A}

and 18 lm/W to 28 lm/W, respectively, depending on the color point.

0.30 0.35 0.40 0.45 0.50 0.30 0.35 0.40 0.45 y cd/A lm/W EQE % 32 28 12.2 29 25 12.0 25 20 10.5 22 18 9.6

Fig.9. Left: Color coordinates [16] for a hybrid white stack which was optimized that the color coordinates fall within the McAdams ellipses between color point B and illuminant A. Right: The table shows the relevant performance data of the devices at 1000 cd/m2

without ILO.

Figure 10 shows an example how the color coordinates of the hybrid stack can be optimized by simply tuning the emission layers of the device. The circular dot (labeled by an arrow) lies just at the edge of a Mc Adams ellipse around illuminant A. The color of this OLED may be still regarded as ‘close to color point A’. The efficacy of the four devices in Figure 10 increases of course with increasing y coordinate, as can be seen in Figure 11. Therefore, the device marked by an arrow in Figure 10 (circular dot) would be a good compromise between color and efficacy.

Fig.10. Hybrid stack optimized by tuning the emission layer thickness to obtain high efficacy and at the same time a white color point which is still on the edge of the MacAdams ellipse around illuminant A (circular dot labeled by an arrow). Efficacy increases with increasing y coordinate.

Figure 11 shows the current efficiency and efficacy of the devices in Figure 10. To maximize the efficacy one can certainly go to the upper edge of the McAdams ellipse around color point A. Both current efficiency and efficacy are reduced when the emission color is closer to the black body curve. The device represented by circular dots (Figures 10 and 11) is certainly a good compromise with respect to color (according to the ENERGY STAR® color benchmark [16]) and efficacy. Please note that the devices are carefully balanced and show only small roll-off in current efficiency with increasing brightness. 0 2000 4000 6000 0 10 20 30 40 50 Curre nt ef ficiency [Cd/A] Luminance [cd/m2] 0 2000 4000 6000 0 10 20 30 40 50 Effic acy [lm /W ] Luminance [cd/m2]

Fig.11. Current efficiency and efficacy as function of brightness of hybrid stacks optimized near color point A. Green shifted samples have higher efficacy. The devices of the corresponding triangular and circular dots are still within the Mc Adam ellipse around illuminant A (see Figure 10) according to the ENERGY STAR® color benchmark [16].

The spectrum of the warm white OLED at the edge of the Mc Adams ellipse around illuminant A (marked in Figure 10 by an arrow) is close to a black-body radiator as can be seen in Figure 12. The color rendering index (CRI) of this OLED is with 84 remarkably high; the color temperature is 2815 K.

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Fig. 12. Spectrum of a warm white hybrid OLED close to color point A (cf. to the circular symbol in Fig. 10) showing a high color rendering index of 84.

To verify the efficacy data of this hybrid white OLED and to evaluate its potential with improved optical out-coupling, the device was measured also in an integrating sphere. The current efficiency and efficacy without ILO agree well with the values measured perpendicular.

Table 2 below shows the current efficiency, efficacy, and EQE of this OLED without and with improved optical out-coupling. Improved light out-coupling using a hemisphere glass lens as a macro-extractor to couple out all the light from the substrate improves the EQE by a factor of 1.9 and yields 27% EQE and an efficacy of 51 lm/W. Using a scattering foil improved the EQE for this OLED by a factor of 1.3 (18.3% EQE and 35 lm/W), which means that nearly 70 % of the light in the glass are coupled out into air.

Table.2. Current efficiency, efficacy, and EQE of the hybrid white OLED from Figure 11 measured at 1000 cd/m2 _{in an integrating}

sphere. The OLED substrate was a low index glass (n=1.52). Out-coupling was done by a scattering foil (ILO) or using a hemisphere lens as macro-extractor. During the measurements the emission from the substrate edge has been cut-off according to ref. [5].

Current

efficiency Efficacy EQE

cd/A lm/W %

OLED without

ILO 32 28 14.4

OLED with ILO 40 35 18.3

OLED with hemisphere lens

59 51 27.0

The stability of this hybrid device (lumen and color maintenance) was found in the order of 6.000 hours without ILO and 10.000 h with improved light out-coupling. The properties of interlayer of this hybrid white OLED are relevant for the lifetime of the device and also for the dependence of the color of the OLED on driving voltage (color stability). The effect of interlayer on color stability will be addressed in the next section.

3.4 Color tunable hybrid OLED.

Generally hybrid white OLEDs show some slight drift of color with brightness or with driving voltage. This limited color stability is undesired and can be minimized to some extent by a careful stack design. However, this effect can also be optimized to realize color tunable OLEDs. The working principle is shown in Figure 13.

Fig. 13. Schematic energy diagram of an OLED consisting of a yellow and a blue emitting layer. Thin arrows describe the electron transport, thick arrows the hole transport. Left: device without interlayer; emission in the blue layer mainy. Right: device with thick interlayer and large energy barrier for holes, and smaller barrier for electrons; emission in the yellow layer only. Middle: thin interlayer with transport over trap states or tunneling through barrier. Color tuning is possible.

The left side in Figure 13 depicts the situation without interlayer. When we have alignment of the transport levels (no large barriers for electrons or holes), the emission is on the cathode side (blue layer) since the mobility of holes is generally larger the mobility of electrons. When we use a thick interlayer, however, which creates large barrier for holes, recombination will be on the anode side (yellow layer), since the transport of holes is blocked by the energy barrier (right side of Figure 13). When we make the interlayer thinner (Figure 13 middle) the position of the recombination zone varies strongly with the applied voltage and therefore with brightness: The device becomes color tunable. Whether this color tuning effect is due to a field dependent tunneling through the thin interlayer barrier or the result of field depended transport across trap states in the interlayer is still unclear. Figure 14 shows the different situations in the CIE diagram: Without interlayer we find mainly blue emission; the color changing is not much with the driving voltage. For a thick interlayer the emission is yellow and does not depend on the driving voltage. For a thin interlayer, however, the color of the device is strongly dependent on the driving voltage. It is straightforward to construct an OLED which can e.g. be tuned in its color between blue, white, and yellow along the black body curve (Figure 15). These types of devices are attractive since color tuning can be realized without introducing transparent conductive electrodes into the stack. For these devices, color tuning is possible always along a straight line between two color points in the CIE color diagram. Devices which change color from blue towards yellow, or between any other emitter colors can be realized.

Figure 14: CIE diagrams of a hybrid stack with different interlayer thickness. Left: No interlayer; mainly blue emission. Right: Thick interlayer; yellow emission only. The middle shows the situation with a thin interlayer, where the device becomes color tunable: increasing the driving voltage from 3.3V to 4.5 V shifts the emission color from blue to yellow along the black body curve.

3.3 V

_{4.5 V}

With thin IL

No IL

No IL

With thin IL

Figure 15: Array of 16 hybrid test OLEDs combining red and green phosphorescent emitters with a blue fluorescent emitter. The upper 8 OLEDs do not have an interlayer (IL) whereas the lower 8 OLEDs have an IL between the blue and the green emission zone. At low driving voltage the emission is blue (left picture). For higher driving voltage (right picture) the emission of the devices without IL remains blue, whereas the devices with thin IL show a strong shift of the emission wavelength towards yellow.

4. CONCLUSION

Hybrid white OLED devices combining proven stable red and green phosphorescent emitters with state-of-the-art fluorescent blue emitters are attractive since they promise high efficacy and good lumen maintenance. Near illuminant A an external quantum efficiency (EQE) of 14% and an efficacy of 28 lm/W can be achieved without improved light out-coupling (ILO). The color rendering index (CRI) is 84. Lumen and color maintenance is in the order of 6.000 hours without ILO and 10.000 h with improved light out-coupling. Hybrid white OLEDs can be optimized to show only small dependence of their color on the driving voltage (good color stability). On the other hand devices can be realized which show a very strong dependence of their color on the driving voltage. By a careful adjustment of the exciton blocking interlayer this effect can be optimized and used for color tunable OLED devices.

5. ACKNOWLEGEMENTS

We acknowledge the help of our Philips colleagues Peter Janiel, Henning Ohland, Georg Gaertner, and Silvia Golsch. We thank Reinder Coehoorn for fruitful discussions. The work leading to some of these results has received funding from the European Community's Seventh Framework Program under grant agreement n° FP7–224122 (OLED100.eu) and from the European Community's Program No. FP7-213708 (AEVIOM.eu).

6. REFERENCES

[1] _{www.lumiblade.com.}

[2] _{Euroluce trade fair Milano / Italy (2009):}

http://www.lightnowblog.com/2009/04/philips-launches-oled-products-at-euroluce/.

[3] _{Kuma, K. H., Jinde, Y., Kawamura,M., Yamamoto, H., Arakane, T., Fukuoka K.,Hosokawa, C., ‘Highly Efficient}

White OLEDs using RGB Fluorescent Materials’, SID Int. Symp. Digest of Tech. Papers, 1504 (2007).

[4] _{http://www.usdc.org/technical/downloads/OLED_Techroadmap_nbtext.pdf}

[5] _{http://www.olla-project.org/: ‘OLLA White Paper on the Necessity of Luminous Efficacy Measurement}

Standardization of OLED Light Sources’, IST- 004607, (2008).

[6] _{http://en.wikipedia.org/wiki/File:PlanckianLocus.png}

[7] _{Nishimura, K., Kawamura, M., Jinde, Y., Yabunouchi, N., Yamamoto, H., Arakane, T., Iwakuma, T., Funahashi,}

M., Fukuoka, K., Hosokawa, C., ‘The Improvement of White OLED’s Performance ‘, SID Int. Symp. Digest of Tech. Papers, 1971 (2008).

[8] _{Universal Display Corporation: http://206.106.174.125/pholed.htm.}

[9] _{D’Andrade, B.W., Tsai, J.-Y., Lin, C., Weaver, S., Mackenzie; P.B., Brown, J.J., ‘Efficient White Phosphorescent}

Organic Light Emitting Devices’, SID Int. Symp. Digest of Tech. Papers, 1026 (2007).

[10] _{Nakayama, T., Hiyama, K., Furukawa, K., Ohtani ,H., ‘Development of Phosphorescent White OLED with}

Extremely High Power Efficiency and Long Lifetime’, SID Int. Symp. Digest of Tech. Papers, 1018 (2007).

[11] _{BASF-Osram press release November 25, 2008:http://www.basf.com/group/pressemitteilungen/P-08-512.}

[12] _{Sun, Y., Giebink, N.C., Kanno, H., Thompson, M.E., Forrest S.R., ‘Management of Singlet and Triplet Excitons for}

Efficient White Organic Light-Emitting Devices, Nature 440, 908 (2006).

[13] _{Schwartz, G., Reineke, S., Walzer, K., Leo, K., ‘Reduced Efficiency Roll-off in High-efficiency Hybrid White}

Organic Light Emitting Diodes’, Applied Physics Letters 92, 053311 (2008).

[14] _{Kondakova, M.E., Giesen, D.J., Deaton, J.C., Liang-Sheng Liao, L-S., Pawlik, T.D., Kondakov, D.Y., Miller,}

M.E., Royster, T.L., Comfort, D.L., ‘Highly Efficient Fluorescent / Phosphorescent OLED Devices using Triplet Harvesting’, SID Int. Symp. Digest of Tech. Papers 39, 219 (2008).

[15] _{Segal, M., Singh, M., Rivoire ,K., van Voorhis, T., Baldo, M.,’ 'Extrafluorescent Electroluminescence in Organic}

Light Emitting Devices', Nature Materials 6, 374(2007).

[16] _{ENERGY STAR® Program Requirements for Solid State Lighting Luminaires, Version 1.0, 22 (2007).}