An efficient dual-emissive-layer white organic light emitting-diode: Insight into device working mechanism and origin of color-shift

Organic Electronics 19 (2015) 157–162

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Organic Electronics journal homepage: www.elsevier.com/locate/orgel

An efficient dual-emissive-layer white organic light emittingdiode: Insight into device working mechanism and origin of color-shift Qi Wang a,b, Dongge Ma a,⇑, Junqiao Ding a, Lixiang Wang a, Qiquan Qiao b, Huiping Jia c, Bruce E. Gnade c, Jason Hoshikawa-Halbert d a State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, PR China b Department of Electrical Engineering and Computer Sciences, College of Engineering, South Dakota State University, Brookings, South Dakota 57007, USA c Department of Materials Science and Engineering, Erik Jonsson School of Engineering and Computer Science, University of Texas at Dallas, Richardson, TX 75083, USA d Department of Synthetic Chemistry and Biological Chemistry, Kyoto University Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

a r t i c l e

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Article history: Received 12 September 2014 Received in revised form 20 December 2014 Accepted 20 January 2015 Available online 28 January 2015 Keywords: White organic light-emitting diode Phosphorescence Color-shift Charge transport

a b s t r a c t By using a single host for both blue and orange phosphorescent dopants, a simple and efficient white organic light emitting-diode is reported. The dual-emissive-layer white device achieves a peak external quantum efficiency of 16.9 ± 0.9% and power efficiency of 44.1 ± 2.3 lm/W without out-coupling enhancement. Analysis of the device working mechanism determines that the blue dopant molecules can form a bridge to facilitate electron transport into the adjacent orange emitting-layer. The orange emission originates from both the direct electron trapping by the orange dopant and incomplete blue–orange energy transfer mechanisms. The origin of the voltage-dependent color shift of the device is quantitatively determined according to the working mechanism. Possible solution to reducing the color-shift is also provided based on the calculation and analytical results. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction White organic light-emitting diode (WOLED) has attracted much attention in OLED research because it provides a great alternative for the current solid-state lighting source, e.g., inorganic LEDs [1]. Given the limited spectral bandwidth of an organic emitter generally used, multiple emitting species are often incorporated into one WOLED to realize a balanced white light. To achieve high efficiency in WOLEDs, using phosphorescent emitters is one effective method because phosphorescent dopants can convert both the singlet and triplet excitons into photons [2]. Recent

⇑ Corresponding author. Tel.: +86 431 85262357; fax: +86 431 85262873. E-mail address: [email protected] (D. Ma). http://dx.doi.org/10.1016/j.orgel.2015.01.027 1566-1199/Ó 2015 Elsevier B.V. All rights reserved.

conceptual advancement leads to many approaches to realize high-performance phosphorescent WOLEDs [3–15]. Among them, the single-host structure, i.e., using a single host for all of the phosphorescent dopants, has attracted more attention due to the following advantages [9–15]. First, it facilitates charge and exciton transport between different emission regions [9]. Second, it eliminates heterojunctions and therefore the interfacing energy barriers, thus favoring reduction of the operational voltage [12–15]. Finally, it offers a facile management of charge and exciton transport for a balanced white light and high efficiency in WOLEDs [9,10,12,15]. Today’s state-of-the-art single-host WOLEDs often focus on primary colors combination, i.e., blue, green and red, because they can provide a wide coverage of the visible wavelength [9–15]. However, the structure of such

Q. Wang et al. / Organic Electronics 19 (2015) 157–162

To illustrate the concept, a blue phosphorescent dye of iridium(III)[bis(4,6-difuorophenyl)-pyridinato-N,C20 ] picolinate (FIrpic) and an orange dye of bis(2-(9,9-diethyl-9Hfluoren-2-yl)-1-phenyl-1H-benzoimidazol-N,C3) iridium(acetylacetonate) [(fbi)2Ir(acac)] are selected [16,17]. A hole-transporting material 1,3-bis(9-carbazolyl)benzene (mCP) with a high triplet energy of 2.9 eV is introduced as the host for both the blue and orange dyes. To ensure blue emission, the FIrpic:mCP layer is placed near the main exciton formation zone in the device, as shown below [9,12,15]. The orange layer composed of (fbi)2Ir(acac):mCP is placed adjacent to the blue layer. The optimized structure of the WOLED is ITO/PEDOT:PSS/NPB (85 nm)/TCTA (5 nm)/3 wt.% (fbi)2Ir(acac): mCP (3.25 nm)/8.5 wt.% FIrpic:mCP (3.85 nm)/TPBi (50 nm)/LiF/Al. Here, NPB represents N,N0 -diphenyl-N,N0 bis(1-naphthylphenyl)-1,10 -biphenyl-4,40 -diamine, TCTA 4,40 ,400 -tri(N-carbazolyl)triphenylamine, TPBi 2,20 ,200 -(1,3, 5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole). The device was fabricated on a pre-cleaned ITO substrate with a sheet resistance of 10 X per square. All of the layers except PEDOT:PSS were grown by thermal evaporation in a high vacuum (2  104 Pa) system. The PEDOT:PSS layer was spin coated onto ITO and then heated under 120 °C for 2 h. The current–voltage–luminance characteristics were measured by using a Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a calibrated silicon photodiode. The EL spectra were measured by a calibrated PR650 spectrophotometer. The active dimension of the device is 4  4 mm2. All of the measurements were carried out in ambient atmosphere at room temperature. 3. Results and discussion Fig. 1 shows the electroluminescence (EL) performance of the WOLED. As shown in Fig. 1a, the WOLED achieves a forward viewing EQE and PE of 16.9 ± 0.9% and 44.1 ± 2.3 lm/W, respectively. The turn-on voltage of the

External quantum efficiency (%)

Power efficiency (lm/W)

10 LiF/Al TPBi (50 nm)

10

mCP:8.5 wt% FIrpic (3.85 nm) mCP:3 wt% (fbi)2Ir(acac) (3.25 nm) TCTA (5 nm) NPB (85 nm) PEDOT:PSS

ITO/Glass

1

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-3

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-1

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(b)

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Brightness (cd/m )

2. Materials and methods

100

(a)

Current density (mA/cm )

Current density (mA/cm )

devices is relatively complicated compared to WOLEDs with complementary colors, i.e., blue and orange/yellow [11]. Although complementary-color WOLEDs provide a simpler alternative, they have received less attention in single-host WOLEDs [10,11]. Furthermore, studies of the working mechanism for such WOLEDs, e.g., emission nature of each dopant, charge and exciton transporting behavior and their influence on device performance, are rare in previous literatures [9]. To resolve both issues, here we report a simple and efficient single-host WOLED with a dual-emissive-layer for both the blue and orange dopants. The white device achieves a forward viewing external quantum efficiency (EQE) of 16.9 ± 0.9% and power efficiency (PE) of 44.1 ± 2.3 lm/W. It is found that both the direct electron trapping by the orange dopant and incomplete blue–orange energy transfer mechanisms contribute to orange emission. The origin of the voltage-dependent color shift is quantitatively described according to the working mechanism.

12

Voltage (V)

1.4

CIE Voltage 4 V (0.43, 0.45) 5 V (0.40, 0.44) 6 V (0.37, 0.43) 7 V (0.35, 0.42) 8 V (0.34, 0.41)

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0.8 0.6 0.4 0.2 0.0

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-0.2 -0.4 350

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Wavelength (nm) Fig. 1. (a) External quantum efficiency and power efficiency of the optimized WOLED. Inset: structure of the white device. (b) Voltage– current density-brightness characteristics of the device. (c) Electroluminescent (EL) spectra of the WOLED from 4 to 8 V. The Commission Internationale de L’Eclairage coordinates (CIE) of the device are shown on the top-right.

device is 3.2 V, as shown in Fig. 1b. The voltages at the brightness of 500 and 1000 cd/m2 are 4.41 and 4.85 V, respectively. At a brightness of 500 cd/m2, the EQE and PE of the device still reach 13.7 ± 0.6% and 27.6 ± 1.1 lm/ W, respectively. Fig. 1c shows the EL spectra of the device at different voltages. The Commission Internationale de L’Eclairage coordinates (CIE) of the device at voltages from 4 to 8 V are shown in Fig. 1c. The color-rendering index (CRI) of the device is from 62 to 68, and the correlated color temperature (CCT) of the device is from 3468 to 5372,

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1 For interpretation of color in Figs. 1, 3 and 4, the reader is referred to the web version of this article.

60

20 WOLED-C1 WOLED-C2

15

50 40

10

30 20

5

10

(a)

0

10

-3

10

Power efficiency (lm/W)

External quantum efficiency (%)

respectively, at voltages from 4 to 8 V. As shown, both the blue1 emission from FIrpic and orange emission from (fbi)2 Ir(acac) are observed simultaneously, white emission being generated. However, the relative intensity of the blue emission is increased with increasing voltage, leading to a blue shift in the EL spectrum. To determine the origin of the voltage-dependent color shift, the working mechanism for the WOLED is described below [18]. Fig. 2 shows the energy levels and working principle of the device [1]. Due to the hole-transporting characteristic of mCP and electron-transporting characteristic of TPBi [19], the main exciton formation zone in the WOLED is at the mCP/TPBi interface. As reported previously, the mCP– FIrpic energy transfer is an efficient process [10]. Therefore, at the mCP/TPBi interface the excitons formed on mCP molecules are energy transferred to FIrpic due to the difference of the triplet-energy (T1) level between mCP and FIrpic (Fig. 2), ensuring blue emission (process 1 in Fig. 2). The triplet excitons on FIrpic molecules possess long diffusion length [1]. Given the thin (3.85 nm) FIrpic:mCP layer, the (fbi)2Ir(acac) molecules located adjacent to the FIrpic layer (Fig. 1a inset) can harvest the triplet energy from the excited FIrpic molecules (process 2 in Fig. 2) for orange emission due to the difference of the T1 level between FIrpic and (fbi)2Ir(acac). Furthermore, it has been demonstrated that FIrpic molecules can facilitate electron transport in the hole-dominating mCP layer [9,20,21]. The (fbi)2Ir(acac) molecules can also directly trap electrons transferred by the FIrpic molecules and holes from mCP (process 3 in Fig. 2) to form excitons, leading to orange emission. Moreover, due to the thin mCP–FIrpic layer (3.85 nm), a fraction of electrons could traverse this layer over the mCP

0 -2

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Fig. 2. Energy level diagram and proposed operating mechanism for the white OLED. Lines correspond to HOMO (solid) and LUMO (dashed) energies; filled boxes refer to the triplet energies. The excitons formed on the mCP molecules at the mCP/TPBi interface are energy transferred to the FIrpic molecules, ensuring blue emission (1). Given the thin (3.85 nm) FIrpic:mCP layer, (fbi)2Ir(acac) can either harvest the triplet energy from the excited FIrpic molecules (2) or directly trap electrons transferred by the FIrpic/mCP molecules and holes from mCP in the mCP-(fbi)2Ir(acac) layer (3) to form excitons, leading to orange emission. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

molecules and then transport to the (fbi)2Ir(acac) layer (included in process 3), contributing to orange emission. To further understand the emission mechanism of (fbi)2Ir(acac) (process 2 and 3 in Fig. 2) in the WOLED, we fabricated two control WOLEDs by increasing the concentration of either (fbi)2Ir(acac), i.e., from 3 wt.% to 8 wt.% (WOLED-C1), or FIrpic, i.e., from 8.5 wt.% to 15 wt.% (WOLED-C2), in mCP. Fig. 3a shows the EQE and PE of both control devices. They both are less efficient than the optimized WOLED (WOLED-O, Fig. 1), e.g., the peak EQE/PE of WOLED-C1 and C2 are 15.4 ± 0.3%/38.9 ± 0.9 lm/W and 15.7 ± 0.4%/38.8 ± 1.0 lm/W, respectively. The reduced efficiency originates from the concentration quenching effect, which causes strong triplet quenching at high doping concentration of either FIrpic or (fbi)2Ir(acac) [22]. Fig. 3b shows the EL spectra of both control devices. Compared to WOLED-O, WOLED-C1 shows a decreased blue emission. This is reasonable because more excitons or charge-carriers are harvested or trapped by (fbi)2Ir(acac) molecules when the (fbi)2Ir(acac) concentration is increased. However, when the FIrpic concentration is increased in WOLEDC2, the FIrpic emission is still decreased, which was

4V 5V 6V 7V 8V

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0.5 WOLED-C2

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400

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Wavelength (nm) Fig. 3. (a) External quantum efficiency and power efficiency of the optimized WOLED-C1 and C2. (b) EL spectra of both the control devices from 4 V to 8 V. The device structures of WOLED-C1 and C2 are ITO/PEDOT:PSS/NPB (85 nm)/TCTA (5 nm)/8 wt.% (fbi)2Ir(acac):mCP (3.25 nm)/8.5 wt.% FIrpic:mCP (3.85 nm)/TPBi (50 nm)/LiF/Al and ITO/PEDOT:PSS/NPB (85 nm)/TCTA (5 nm)/3 wt.% (fbi)2Ir(acac):mCP (3.25 nm)/15 wt.% FIrpic:mCP (3.85 nm)/TPBi (50 nm)/LiF/Al, respectively.

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that are directly converted into photons by FIrpic emission and energy transferred to (fbi)2Ir(acac) before the radiative decay, respectively. In Eq. (1), the first item corresponds to the process 1 in Fig. 2, the second item corresponds to the process 2 and the third item corresponds to the process 3. To calculate vFIrpic-emission in WOLED-O and WOLED-C1, the pure FIrpic device with the structure of ITO/PEDOT:PSS/ NPB (85 nm)/TCTA (5 nm)/mCP (3.25 nm)/8.5 wt.% FIrpic:mCP (3.85 nm)/TPBi (50 nm)/LiF/Al was fabricated. By fitting the spectrum of the white device to the EL spectra of FIrpic and (fbi)2Ir(acac), and considering photon energy in these power spectra, the ratio of the photons emitted from FIrpic and (fbi)2Ir(acac) in WOLEDs can be obtained. Given the EQE characteristics of the WOLEDs and pure FIrpic device (at the same current density), vFIrpic-emission can be calculated. Note that one assumption for the calculation is that adding 3 wt.% (fbi)2Ir(acac) in the mCP layer adjacent to the blue layer will not influence the intrinsic device efficiency of FIrpic [23]. Fig. 4a shows the calculated vFIrpic-emission of both WOLEDO and WOLED-C1 as a function of voltage. vFIrpic-emission increases with increasing voltage in both devices. This calculation result shows that the fraction of excitons consumed by the blue emission (process 1 in Fig. 2) is increased with increasing voltage, causing the blue shift in WOLED-O and WOLED-C1. This is consistent with the experimental results, as shown in Fig. 1c and Fig. 3b. Furthermore, as shown in Fig. 4a, at the same voltage a decrease in vFIrpic-emission is observed in WOLED-C1 relative to WOLED-O. This result quantitatively explains why the blue emission is decreased with increasing (fbi)2Ir(acac) concentration in WOLED-C1 (Fig. 3b), as compared to that of WOLED-O (Fig. 1c). The above calculations demonstrate that with increasing voltage more excitons are consumed by the FIrpic molecules

vFIrpicenergy transfer increases from only 1% to 24% when gðfbiÞ2 IrðacacÞ2 is increased from 17% to 20%. This calculation indicates that the contribution of the FIrpic–(fbi)2Ir(acac) energy-transfer mechanism for the orange emission becomes greater with increasing (fbi)2Ir(acac) efficiency, which is a reasonable result. The above calculations show that more excitons (vFIrpic-emission ) are consumed by FIrpic with increasing voltage (Fig. 4a), which quantitatively explain the blue shift in the optimized WOLED. This result indicates that more excitons are formed on mCP molecules at the mCP/TPBi interface with increasing voltage, thus enhancing the mCP– FIrpic energy-transfer process (process 1 in Fig. 2). Possible reason for this result is the following. Due to the significant difference of the highest occupied molecular orbital (HOMO) energy levels between (fbi)2Ir(acac) (5.1 eV) and mCP (5.9 eV), as shown in Fig. 2, the (fbi)2Ir(acac) molecules form deep traps for holes in the mCP–(fbi)2 Ir(acac) layer. Under low voltages, a fraction of the injected holes need to fill these traps formed on (fbi)2Ir(acac) molecule sites in the mCP–(fbi)2Ir(acac) layer, and then traverse the mCP–FIrpic layer. In this case, the direct charge (%)

where gFIrpic is the EQE of the pure FIrpic device (see below), gðfbiÞ2 IrðacacÞ2 the EQE of the pure (fbi)2Ir(acac) device, vFIrpic-emission and vFIrpicenergy transfer the fractions of excitons

of the pure FIrpic device and WOLED-O are 11.8 ± 0.6% and 14.8 ± 0.5%, respectively, at 4 V) using Eq. (1). With increasing gðfbiÞ2 IrðacacÞ2 , vFIrpicenergy transfer is increased, e.g.,

FIrpic-emission

ð1Þ

ged from 17% to 20%, Fig. 4b shows the calculated

vFIrpicenergy transfer as a function of gðfbiÞ2 IrðacacÞ2 at 4 V (the EQE

χ

  þ gðfbiÞ2 IrðacacÞ2 1  vFIrpic-emission  vFIrpicenergy transfer

located near the main exciton formation zone in WOLEDs; and its real efficiency cannot be reflected just by removing FIrpic in WOLED-O (Fig. 1a inset). Despite this issue, we can still estimate vFIrpicenergy transfer . We have demonstrated that the EQE of the optimized (fbi)2Ir(acac) device is 20% [24]. Here, the peak gFIrpic and EQE of WOLED-O are 13.3% and 17.9%, respectively. Assuming gðfbiÞ2 IrðacacÞ2 can be chan-

90 80 70 60 50 40 30 20 10 0

WOLED-O WOLED-C1

(a) 4

5

6

7

8

Voltage (V) (%)

gWOLED ¼ gFIrpic vFIrpic-emission þ gFIrpic vFIrpicenergy transfer

for blue emission (Fig. 4a), causing the voltage-dependent color shift. In fact, the excited FIrpic molecules may also transfer energy to (fbi)2Ir(acac) (process 2 in Fig. 2). However, vFIrpicenergy transfer cannot be calculated due to the lack of gðfbiÞ2 IrðacacÞ2 . This is because the (fbi)2Ir(acac) layer is not

FIrpic-energy transfer

unexpected. We explain this result as the following. As discussed above, the FIrpic molecules can facilitate electron transport in mCP. When the FIrpic concentration is increased, the electron hopping distance between neighboring FIrpic molecules is decreased. Therefore, more electrons will transport into the (fbi)2Ir(acac) layer and then form excitons, leading to an enhanced orange emission. From this, we conclude that the FIrpic molecules can form a bridge to facilitate electron transport into the (fbi)2 Ir(acac) layer; both direct electron trapping (assisted by either the FIrpic or mCP molecules) by the (fbi)2Ir(acac) molecules and incomplete FIrpic-(fbi)2Ir(acac) energytransfer mechanisms contribute to the orange emission in the WOLED. According to the device working mechanism, the EQE of the WOLEDs can be described as the following equation [9,23]

χ

160

25 20 15 10 5 0

WOLED-O at 4 V

(b) 17

18

19

20

η (fbi)2Ir(acac) (%) Fig. 4. (a) The calculated vFIrpic-emission of both WOLED-O and WOLED-C1 as a function of voltage. (b) The calculated vFIrpicenergy transfer as a function of gðfbiÞ2 IrðacacÞ2 at 4 V.

Q. Wang et al. / Organic Electronics 19 (2015) 157–162

trapping mechanism (process 3 in Fig. 2) by (fbi)2Ir(acac) molecules should be the main route for orange emission at low voltages. This analytical result can be supported by the calculation results. As shown in Fig. 4a, vFIrpic-emission is calculated to be 36% at 4 V in WOLED-O. The average value of vFIrpicenergy transfer is calculated to be about 14% at 4 V by taking the values of gðfbiÞ2 IrðacacÞ2 from 17% to 20%, as shown in Fig. 4b. Therefore, the fraction of excitons formed by directly trapping charges on (fbi)2Ir(acac) molecules is up to 50% at 4 V in WOLED-O, showing that this emission mechanism mainly accounts for orange emission at low voltages. When the voltage is increased, more holes will traverse the mCP layer since the hole traps in the mCP–(fbi)2Ir(acac) layer are filled. Therefore, more excitons will be formed at the mCP/TPBi interface with increasing voltage, thus enhancing the mCP–FIrpic energy-transfer process (process 1 in Fig. 2) and increasing the blue emission. This is consistent with the calculation result (Fig. 4a). We should note that although the origin of the color-shift is determined, it is an inherent characteristic of the device and cannot be eliminated at present stage. However, the above analysis provides a clue to reduce this issue. If using an orange dye with a HOMO energy level comparable to that of mCP, forming shallow hole traps in the mCP-(fbi)2Ir(acac) layer, the color-shift of the device should be reduced. 4. Conclusions In summary, a dual-emissive-layer white OLED is reported by using a single host for both blue and orange phosphorescent dyes. The white device achieves a peak external quantum efficiency of 16.9 ± 0.9% and power efficiency of 44.1 ± 2.3 lm/W. Analysis of the device working mechanism determines that the blue dopant molecules can form a bridge to facilitate electron transport into the adjacent orange emitting layer. Both the direct electron trapping and incomplete blue–orange energy transfer mechanisms contribute to the orange emission. According to this working mechanism, it is calculated that the fraction of excitons consumed by the blue dopant is increased with increasing voltage, leading to the blue shift in the EL spectra of the device. Possible solution to reducing the color-shift is also provided, according to the calculation and analytical results. Introducing an orange dye with a HOMO energy level comparable to that of mCP should reduce the color-shift of the device. Making such orange dyes and incorporating them into the device will be the next step of this study. Acknowledgements The authors acknowledge the National Natural Science Foundation of China (51333007, 91433201), Ministry of Science and Technology of China (973 program No. 2013CB834805), the Foundation of Jilin Research Council (2012ZDGG001, 20130206003GX), Chinese Academy of Sciences (KGZD-EW-303-3), CAS Instrument Project (YZ201103), and United States’ NSF CAREER (ECCS0950731) for the support of this research.

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