Highly efficient inverted top-emitting organic light ... - OSA Publishing

Highly efficient inverted top-emitting organic light-emitting diodes using a lead monoxide electron injection layer Qiang Wang, Zhaoqi Deng, and Dongge Ma* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences, Changchun 130022, People’s Republic of China *[email protected]

Abstract: By introducing an effective electron injection layer (EIL) material, i.e., lead monoxide (PbO), combined with the optical design in device structure, a high efficiency inverted top-emitting organic lightemitting diode (ITOLED) with saturated and quite stable colors for different viewing angles is demonstrated. The green ITOLED based on 10-(2benzothiazolyl)-1, 1, 7, 7-tetramethyl-2, 3, 6, 7-tetrahydro-1H, 5H, 11H-[1] benzopyrano [6, 7, 8-ij] quinolizin-11-one exhibits a maximum current efficiency of 33.8 cd/A and a maximum power efficiency of 16.6 lm/W, accompanied by a nearly Lambertian distribution as well as hardly detectable color variation in the 140° forward viewing cone. A detailed analysis on the role mechanism of PbO in electron injection demonstrates that the insertion of the PbO EIL significantly reduces operational voltage, thus greatly improving the device efficiency. More importantly, the optically optimized device structure by setting the resonant wavelength at the peak wavelength of the intrinsic emission of the emitter and adding an effective outcoupling layer further enhances the device efficiency, at the same time, also reduces the color shift with viewing angles, leading to the simultaneous optimization in efficiency and angular emission characteristics in the fabricated ITOLEDs. ©2009 Optical Society of America OCIS codes: (220.0220) Optical design and fabrication; (230.3670) Light-emitting diodes.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9.

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Received 22 Jun 2009; revised 1 Sep 2009; accepted 5 Sep 2009; published 14 Sep 2009

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1. Introduction Organic light emitting diodes (OLEDs) have attracted much attention as flat panel displays in the future due to their merits such as its light weight, fast response time, low operation voltage, wide color gamut, wide viewing angle, high brightness, and low power consumption [1–3]. Active matrix OLEDs (AMOLEDs) driven by thin film transistors (TFTs) will be main pathway. In the design of AMOLEDs, a disadvantage of conventional bottom-emitting OLEDs (BOLEDs) for high resolution display applications is that the pixel aperture ratio (the ratio of the active emissive area to the subpixel area) is diminished by the accompanying backplane TFTs and electronic circuitry. In the case, higher current density is needed to compensate the lower aperture ratio, detrimental to the efficiency and lifetime of OLED operation. By contrast, top-emitting OLEDs (TOLEDs) are the best choice because the topemitting structure does not diminish the pixel aperture ratio on account of its geometrical merit, thus allowing a high pixel resolution [4]. Moreover, the stacking of either a color filter (CF) or a color change medium (CCM) can be more facile onto the top of the TOLEDs, which can allow more choices of substrates and control the color purity more easily by the microcavity effects in TOLEDs than in BOLEDs [5]. Up to now, the structure of most TOLEDs has been composed of a reflective bottom anode, organic layers, and a (semi)transparent top cathode for light out-coupling. Inverted top-emitting OLEDs (ITOLEDs), i.e., making OLEDs with a reflective cathode at the bottom and a (semi-)transparent anode on the top, would be more preferable in AMOLEDs due to the use of significantly superior n-type transistors that usually have higher carrier mobility and thus lower operation voltage in polycrystalline silicon (poly-Si) TFTs rather than p-type transistors that cannot be adequately produced in amorphous Si (a-Si) TFTs [6]. #113212 - $15.00 USD

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In ITOLEDs, it is necessary to introduce an electron injection layer (EIL) [7–10] to enhance electron injection if utilizing high function work metal such as aluminum (Al) as a highly reflective bottom cathode. Recently, we found that lead monoxide (PbO) resulted in an excellent electron injection and therefore significantly improved performance for ITOLEDs. Therefore, the exploring of new electron injection layer materials shows so important. Furthermore, ITOLEDs consisting of two reflective electrodes and organic films sandwiched in between whose dimensions are on the order of a single wavelength exhibit strong optical microcavity effects. As known, the microcavity effects affect not only the electroluminescence (EL) efficiency and luminance, but also the spectral purity and color. This means that it is essential to optically optimize the device structure which is directly relative to the organic layer thickness and the reflectivity of electrode [8,11]. In this paper, we introduce the PbO as the EIL in ITOLEDs, optimize the organic layer thickness, and add an outcoupling layer on the surface of emission end. As a result, an ITOLED with the simultaneous optimization in efficiency and angular emission characteristics is realized. We give detailed analysis with respect to structure design and improvement mechanism. 2. Optical design

Fig. 1. Schematic diagram of the planar inverted top-emitting device structure studied. On the basis of the optical design and calculation, the optimum structure is fixed as follows: Al/LiF or PbO/Alq3(25 nm)/BCP(5 nm)/Alq3: C545T(30 nm)/NPB(30 nm)/MoO3(8 nm)/Ag(18 nm)/Alq3(60 nm). The optical effect of the ultrathin LiF or PbO layer is ignored in our calculations because its thickness is much smaller than the wavelength of visible light. The thicknesses of LiF, PbO, and MoO3 were experimentally optimized to insure excellent electrical properties of the devices besides the optical optimization.

Optically, an ITOLED with the above-mentioned superior structure may be considered as a Fabry-Pérot cavity embedded with a source, as shown in Fig. 1. The optical properties of such a microcavity can be described by the reflectivities of the mirrors, its cavity length, and the position of the emitting dipoles. The irradiance that can be emitted from such a microcavity can be given by [12]:

  4π L1  T2 1 + R1 + 2 R1 cos  − φ1   λ    I ( λ ,θ ) = 2 φ +φ 2 L π  1 − R1 R2 + 4 R1 R2 sin 2  − 1 2 2  λ

(

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)

  

I0 ,

(1)



where I 0 represents the free-space EL irradiance of the radiating molecules, R1 and R2 are the inside reflectivities of the bottom and top contact, T2 is the transmittance through the top contact, φ1 and φ2 are the phase shifts on reflection from the mirrors, L1 and L represent the effective optical distance of the emitting dipoles from the highly reflecting mirror and the total optical thickness of the cavity, respectively. According to Eq. (1), if the values of R1 , R2 , and T2 are fixed, the maximum value of

I ( λ , θ ) can be achieved when: 4π L1 λ − φ1 = 2nπ

(n : integer)

(2)

and 2π L λ − (φ1 + φ2 ) 2 = mπ (m : integer). (3) An inspection of Eq. (2) reveals the importance of the spatial location of the emitting dipoles in determining the nature of the light outcoupling. By placing the active layer exactly at the first antinode of the standing wave within the microcavity, strong enhancement caused by efficient coupling between the emission dipoles and the optical mode can be achieved, and, likewise, if the emitting dipoles are located at a node, then the emission would be greatly suppressed. According to Eq. (3), the resonance wavelengths of the cavity can be tuned by changing the cavity length. Microcavity OLEDs previously reported usually adopt a metal mirror and a very thick distributed Bragg reflector (DBR) to produce the cavity, where the cavity lengths are significantly longer than emission peak wavelengths and bring multiple optical modes in the emission output [13–15]. Since our devices, consisting of two metal mirrors sandwiching organic films, have the total optical length shorter than one optical wavelength, we should set the resonant cavity at the lowest order of mode. It is noteworthy that minimization of the cavity length would maximize the integrated irradiance, a benefit caused by an overlapping of the various resonant optical modes and the active emission region [16]. For device design, the values of R1 , R2 , and T2 are calculated by the transfer matrix theory. A dielectric capping layer is applied on top of the semitransparent metal anode to tune the emission characteristics [17]. Since the reflective cathode is thick enough, the value of R1 can be regarded as a constant in the wavelength range of emission. Thus, on the basis of choosing the proper values of L1 and L , through varying the thicknesses of the anode and the capping layer, one can obtain an optimized configuration for ITOLEDs. The criteria for optimization are maximum of extracted power while having saturated colors and angular independent quantity. 3. Experimental details

We apply our analysis to the ITOLED configuration shown in Fig. 1. The ITOLEDs were fabricated by thermal evaporation in a high-vacuum system with a base pressure less than 5 × 10−4 Pa without breaking the vacuum. All devices were fabricated on glass substrate subjected to a routine cleaning procedure. A structured Al bottom cathode was first prepared on glass substrate through a shadow mask. For comparison, two kinds of EIL, PbO and LiF, were separately deposited onto the Al cathode. The thickness of PbO layer was changed for optimization. The thickness of LiF layer was fixed at 1 nm by experiment optimizing. The organic multilayer structures followed sequentially consisted of tris-(8-hydroxyquinoline) aluminum (Alq3, 25 nm) as the electron transport layer (ETL), 2, 9-dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP, 5 nm) as the hole blocking layer (HBL), Alq3 (30 nm) doped with 1 wt% of 10-(2-benzothiazolyl)-1, 1, 7, 7-tetramethyl-2, 3, 6, 7-tetrahydro-1H, 5H, 11H- [1] benzopyrano [6, 7, 8-ij] quinolizin-11-one (C545T) as the light emitting layer (EML) and 4, 4’-N, N’-bis [N-(1-naphthyl)-N-phenyl-amino] biphenyl (NPB, 30 nm) as the hole transport

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layer (HTL). After that, a 8 nm of molybdenum oxide (MoO3) was deposited as the hole injection layer (HIL). An 18 nm thin layer of Ag, which has relatively low optical absorption and the highest conductivity among all metals, was patterned through another shadow mask as the semi-transparent top anode. Finally, a high-refractive-index dielectric layer of Alq3 (n ~1.75) was deposited onto the Ag as an index-matching outcoupling layer to change the optical properties of the thin metal anode and therefore to improve the emission characteristics. The resulting device area is 16 mm2. The organic materials and metal oxide were evaporated at a rate of between 0.2 and 0.3 nm/s, and the metals were deposited at a rate of about 1 nm/s. The thickness of the deposition was monitored in situ by using an oscillating quartz thickness monitor and calibrated by Dektak 6M profiler. In our experiment, control devices with nearly the same configuration but without the ultrathin EIL were also fabricated. The current-voltage-luminance characteristics were recorded using a computer controlled sourcemeter (Keithley 2400) and multimeter (Keithley 2000) with a calibrated silicon photodiode. The EL spectra were measured by JY SPEX CCD3000 spectrometer. The interface electronic structure was performed by X-ray photoelectron spectroscopy (XPS) with Al Ka X-ray source (1486.6 eV). The absorption spectra were measured by Shimadzu UV3600. All the measurements were carried out in ambient atmosphere at room temperature. 4. Results and discussion 4.1 EL performance

Table 1 summarizes the luminance at 20 mA/cm2, maximum current efficiency, and maximum power efficiency of the C545T dopant based devices with different thicknesses of PbO and 1 nm thickness of LiF and the device without EIL. It can be seen that the optimal thickness of PbO to obtain high EL efficiency is 0.3-2.5 nm. When the electron injection layer is getting thicker (i.e., 3.0 nm or over), the electrons are difficult to be injected into the organic layer because PbO is an insulating material. The maximum current efficiency and power efficiency reach 33.8 cd/A and 16.6 lm/W due to the utilization of PbO EIL. By contrast, the maximum current efficiency and power efficiency reach 22.9 cd/A and 7.3 lm/W when using LiF as the EIL, and are reduced to 11.9 cd/A and 2.8 lm/W if no EIL to be used. It is noted that the power efficiency is enhanced more significantly when using PbO as the EIL, which is evidently related to the great reduction of operational voltage. This indicates that PbO should be more effective electron injection material, considering the same device structure designed in terms of the microcavity effects. Table 1. Comparison of the EL performance of the devices with different thicknesses of PbO and the device with 1 nm thickness of LiF

Thickness (nm) Luminance at 20 mA/cm2 (cd/m2) Maximum current efficiency (cd/A) Maximum power efficiency (lm/W)

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0.3

0.5

PbO 1.0 1.5

2.0

2.5

LiF 1.0

1837

5821

6670

6454

6444

5848

5513

4162

11.9

29.4

33.8

33.7

32.9

29.4

28.3

22.9

2.8

10.3

16.6

13.5

13.6

13.4

12.8

7.3

0



Fig. 2. Comparison of the current density-luminance-voltage (J-L-V) characteristics of the devices with PbO EIL (0.5 nm), LiF EIL (1 nm), and without EIL

The role of PbO as EIL can also be illuminated from the current density -luminancevoltage (J-L-V) characteristics of ITOLEDs with various EILs (see Fig. 2). As shown, the operational voltage is indeed significantly reduced by inserting a PbO EIL. For example, the operating voltages at a current density of 20 mA/cm2 are 8.1 V, 11.1 V, and 14.2 V, respectively, for the cases of PbO, LiF, and no EIL. Although the turn-on voltage (defined at 1 cd/m2) is approximately the same, which is about 4.1 V, for the case of PbO and LiF as the EIL, as we see, the utilization of PbO EIL shows higher luminance in the high voltage region than that of LiF EIL. For example, at a bias of 6.5 V, the LiF based device gives a brightness of 71 cd/m2, which is enhanced to 899 cd/m2 in the device with PbO EIL. This demonstrates that PbO EIL also leads to more significantly improvement in electron injection with the increase of the operational voltage, resulting in a better balance between the injected holes and electrons. Therefore, once integrated with the TFTs for application, ITOLEDs can still maintain a high efficiency at a desired brightness. For instance, from Fig. 3, we can see that at the maximum current efficiency is obtained at a quite high luminance of 19000 cd/m2 for the ITOLED with 0.5 nm PbO EIL and a trivial efficiency drop of only about 0.4 cd/A occurs when the brightness arrives at as high as 40000 cd/m2. The power efficiency decreases slightly from the maximal value 16.6 lm/W to 16.0 lm/W at a typical display luminance of 100 cd/m2 and rolls off to 14.7 lm/W at 1000 cd/m2. Such outstanding EL performance promises a good prospect of this type of ITOLEDs in practical application.

Fig. 3. Current efficiency and power efficiency as a function of luminance for the ITOLED with 0.5 nm PbO EIL.

4.2 Role mechanism of PbO

To evaluate the effect of the ultrathin PbO EIL on electron injection from the bottom cathode, the electron-only devices were fabricated employing the structure shown in the inset of Fig. 4 and their current density-voltage (J-V) characteristics were measured. Figure 4 shows the J-V characteristics of Al/PbO(0.5 nm)/Alq3(80 nm)/Al, Al/LiF(1 nm)/Alq3(80 nm)/Al, and Al/Alq3(80 nm)/Al devices. It is clearly seen that the Al/PbO/Alq3/Al device shows much #113212 - $15.00 USD

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larger injection current than Al/LiF/Alq3/Al and Al/Alq3/Al devices, indicating that PbO as the EIL is more effective than LiF in this form of device structure. We note that the effect of LiF as EIL is dependent on the sequence of electrode structure in devices. The Al/Alq3/LiF/Al device indeed exhibits higher electron injection capacity than Al/LiF/Alq3/Al device from LiF/Al side. This can be explained well by the role mechanism of LiF in electron injection because the evaporation of Al on LiF liberates Li which moves into the Alq3 and so acts as dopant for the underlying organic layer and the high temperature during the Al deposition can facilitate the chemical reaction [18,19]. This process does not exist in Al/LiF/Alq3/Al device where nonreactive LiF is deposited onto the cooled Al electrode.

Fig. 4. Current density-voltage (J-V) characteristics of electron-only devices with different EILs. The inset shows the structure of the electron-only devices.

Being different from LiF EIL, the enhancement of electron injection by PbO EIL is attributed to the interfacial dipole effect. As shown in Fig. 5(a), the formation of the interfacial dipole layer at the Al/Alq3 interface leads to a slight downward shift of the vacuum level [20,21]. This shift corresponds to the change of the work function of Al cathode. After inserting PbO between Al and Alq3, due to much more dipoles induced by the charge transfer between PbO and Alq3, a larger vacuum level shift is achieved, resulting in a largely reduced electron injection barrier and thus significantly improving electron injection (see Fig. 5(b)). To elucidate the fact, XPS spectra of Alq3 and Alq3:PbO films are studied. As shown in the inset of Fig. 6, the Al 2p binding energy shift to higher level in Alq3:PbO film with respect to Alq3 film. This indicates that there occurs electron transfer from Alq3 to PbO. The interaction between Alq3 and PbO can also be testified by the difference of the absorption spectra of Alq3 film and the Alq3: PbO film. Figure 6 displays that the absorption peaks of the Alq3 film around 260 and 400 nm are decreased by doping with PbO, implying that an interaction indeed exists between PbO and Alq3. As a result, more interfacial dipoles are formed at electrode interface due to charge transfer. The dipole layer induces the surface potential shift that contributes to improve the electron injection efficiency. Thus, when a forward bias is applied to the device with PbO layer, a considerable voltage can be dropped across the PbO layer through the formation of dipoles, resulting in the reduction of energy barrier for electron injection from Al to Alq3.

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Fig. 5. Energy level diagram (a) without and (b) with the PbO EIL.

Fig. 6. The absorption spectra of intrinsic Alq3 and Alq3:PbO films. Inset: XPS spectra of Alq3 film and Alq3:PbO film.

To demonstrate the effect of PbO on electron injection quantitatively, we analyze the J-V characteristics of the electron-only devices on the basis of the Richardson-Schottky (RS) thermionic emission model [22,23]. The RS model is based on the lowering of barrier by the image charge potential under an external electric field F = V d ( V is the applied voltage across the organic layer and d is the thickness of the organic layer) [24]. By the thermionic emission mechanism, the relationship between Schottky emission current density J and the electric field F can be expressed as [25]:

 φ − β RS F  J = A∗T 2 exp  − B  ,  k BT  

(4)

where A∗ = 4π qm∗ k B h3 ( = 120 A/cm2K2 for m∗ = m0 ) is the Richardson constant,

β RS = q3 4πεε 0 , φB is the zero-field injection barrier, k B is the Boltzmann constant, and T is the temperature. According to Eq. (4), a plot of log J vs

F should be liner with a Y-

axis intercept equal to log A T − φB log e k BT and a slope equal to β RS log e k BT . Figure 7 ∗

2

clearly shows the linear relationship of log J as a function of F for the electron-only device without EIL. The intercept of the line in Fig. 7 gives the barrier height of 1.096 eV for the electron injection from Al into Alq3 at zero electric field, being in very good agreement with the barrier (0.90 ± 0.06 eV) obtained from the Al/Alq3/Al electron-only device in which the top Al electrode is used as the electron injecting electrode [22]. For the electron-only device with PbO thin film, we fit the J-V data with RS thermionic emission model according to Eq. (4) by plotting log J vs F as shown in the inset of Fig. 7 and get the energy barrier

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of 0.686 eV, indicating that when thin PbO film is inserted between the Al cathode and the Alq3 layer, the energy barrier for electron injection is considerably lowered. Combining the work function of Al and the lowest unoccupied molecular orbital (LUMO) of Alq3, the energy level diagrams of the two devices are drawn in Fig. 5, where ∆φB refers to the decrement of the electron injection barrier by the PbO thin layer.

Fig. 7. Fitting of the J-V data of the Al/Alq3(80 nm)/Al electron-only device to the thermionic emission model plotted as current density vs (electric field)1/2 in log-linear scale. Inset: Fitting the J-V data of the Al/PbO(0.5 nm)/Alq3(80 nm)/Al electron-only device.

4.3 EL spectrum characteristics Figure 8(a) shows the EL spectra of ITOLED with PbO EIL at different viewing angles off the surface normal, compared to the normalized 0° EL spectrum of the conventional bottomemitting device. We note that the EL peak of the ITOLED along the normal direction is quite close to that of the BOLED, indicating that the microcavity structure of the ITOLED meets the resonant condition for the intrinsic emission of C545T. Due to the microcavity effects, the top-emitting device exhibits a narrower EL spectrum with a full width at half maximum (FWHM) of 52 nm in normal direction, showing a more saturated color compared with the FWHM of 64 nm for the BOLED. In addition, angular dependence of emission spectra and irradiance in TOLEDs/ITOLEDs is important because the viewing characteristics are sensitive to the dimension of the devices and related to the viewing angle problem. Surprisingly, the present ITOLED displays a small spectrum shift where the spectral peak shifts only by 6 nm from the normal direction to 70° off the normal direction. This can be attributed to the satisfaction of the resonant conditions and the role of the antireflection outcoupling layer. In Eq. (1), L can be expressed by: L = ∑ ni di cos(θi )

(5)

i

where ni and d i are the refractive index and thickness of the corresponding layer between the two mirrors. θi is the internal angle between the direction of light in the layer and the normal to the plane of the device, which relates to the viewing angle θ via Snell’s law. As expected, the resonance shifts to a lower wavelength with increasing observation angle. In order to suppress the angular dependence, an outcoupling capping layer acting as antireflection coating is deposited onto the top mirror. The blueshift of the outcoupled light can be almost eliminated through optimizing the thicknesses of the silver anode and capping layer, resulting from the different phase changes upon reflection at the top contact. At large angles, the phase shift upon reflection at the anode compensates the change of phase length between the two metal mirrors, which is the root for the angular independence of the main peak of extracted light [11]. Additionally, the rapid falloff of the intrinsic emission irradiance at shorter wavelengths for C545T suppresses the tendency of blueshift to a certain extent. Consequently, the ITOLED shows saturated and stable colors for different observation angles. It is #113212 - $15.00 USD

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noteworthy that the ITOLED also yields a nearly ideal Lambertian emission profile, rendering the device suitable for display applications, as shown in Fig. 8(b).

Fig. 8. (a) EL spectra of device with PbO EIL as a function of angle from the surface normal. EL spectrum (normalized) in normal direction of the BOLED with the same organic layer structure is also shown for comparison. (b) Angular distribution of radiation irradiance for the ITOLED and the Lambertian emission pattern (normalized to the 0° irradiance).

5. Conclusion

We have successfully demonstrated a highly efficient ITOLED with outstanding angular emission characteristics. We achieve our objective through combining the optical design of the microcavity structure and the balance of charge carriers resulting from improved electron injection. We have shown that the introduction of the PbO EIL significantly reduces operational voltage and greatly improves the EL efficiency of the ITOLEDs. The improvement in EL performance by the PbO EIL is attributed to the formation of the interfacial dipoles at electrode interface due to charge transfer between PbO and Alq3, leading to substantially improved electron injection. Furthermore, based on optimization of device structure via a comprehensive theoretical analysis, the ITOLED exhibits saturated colors from the intrinsic emission of C545T. The angular dependence of the emission spectra has almost been eliminated by an outcoupling layer with optimal thickness. The emission pattern of the device is nearly Lambertian. All these features may make the present ITOLED attractive for utilization in various practical applications. Acknowledgments

The authors thank the Foundation of Jilin Research Council (20080337), Foundation of Changchun Research Council (2007GH06), Science Fund for Creative Research Groups of NSFC (20621401), State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, and Ministry of Science and Technology of China (863 program No. 2006AA03A161 and 973 program Nos. 2009CB930603 and 2009CB623604) for financial support.

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