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High efficiency all phosphorescent white light-emitting diodes based on conjugated polymer host
Synthetic Metals 161 (2011) 2149–2153
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
High efficiency all phosphorescent white light-emitting diodes based on conjugated polymer host Qiaoli Niu a,b,∗ , Junbiao Peng a , Yong Zhang a , Yong Zhang b , Bo Liang a a
Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, Guangdong, China Institute of Optoelectronic Materials and Technology, South China Normal University, Key Laboratory of Electroluminescent Devices of Department of Education of Guangdong Province, Guangzhou 510631, Guangdong, China b
a r t i c l e
i n f o
Article history: Received 28 April 2011 Received in revised form 26 July 2011 Accepted 2 August 2011 Available online 27 August 2011 Keywords: Phosphorescent Back energy transfer White polymer light-emitting diode Conjugated polymer
a b s t r a c t High efficiency all phosphorescent white electroluminescence was realized by double doping of bluelight-emitting bis(2-(4,6-difluorophenyl)-pyridinato-N,C2 ) picolinate (FIrpic) and red iridium complex Ir(DMFPQ)2 pbm into conjugated polymer host poly(9,9-dioctylfluorene) (PFO). Effects of hole-transport layer (HTL) on the performances of white polymer light-emitting diodes (WPLEDs) were investigated. First of all, PVK as the HTL was essential, because the back energy transfer from FIrpic to PFO caused by the low-lying triplet energy level of PFO was suppressed by PVK. Furthermore, performances of the WPLEDs were enhanced by introducing CBP into PVK owing to improved balance of electron and hole current. The resulting all phosphorescent WPLEDs have a peak luminous efficiency of 15.5 cd/A and a peak power efficiency of 6 lm/W. Commisssion Internationale de L’Eclairage (CIE) coordinates of (0.41, 0.38) were realized at a current density of 18 mA/cm2 . The obtaining of the efficient all phosphorescent white electroluminescence with PFO as host polymer will broaden the approaches of white light generation and be a big promote for the application of phosphorescent WPLEDs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction White organic/polymer light-emitting diodes (WOLEDs/ WPLEDs) based on phosphorescent dyes have gained increasing research interest in both scientific and industrial communities due to their potential applications in full-color flat-panel displays, back-lighting sources for liquid-crystal displays, and especially in next generation solid-state lighting sources [1–5]. Compared with WOLEDs, WPLEDs are more promising for large size or flexible products due to their solvent processing manufacture technology [6,7]. Non-conjugated polymer PVK has been a commonly used polymer host for phosphorescent dyes [8,9]. Indeed, PVK is an excellent material as hole-transport layer (HTL) of polymer light-emitting diodes (PLEDs) [10,11]. As a host polymer, PVK has however an inherent defect in that its electron and hole mobility difference is too large. Electron-transport materials must be used together with PVK to enhance the electron current [12,13]. However, in fabricating functional layers on top of the emissive layer (EML), electron-transport materials may be selectively removed resulting in poor PLEDs efficiency [14–16]. Therefore, it ∗ Corresponding author at: Institute of Optoelectronic Materials and Technology, South China Normal University, Key Laboratory of Electroluminescent Devices of Department of Education of Guangdong Province, Guangzhou 510631, Guangdong, China. Tel.: +86 20 85212667x805. E-mail address: Nql [email protected] (Q. Niu). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.08.018
is necessary to find other host polymers for phosphorescent dyes. Poly(9,9-dioctylfluorene) (PFO) is a commonly used blue material, as well as a well-known host polymer for red phosphorescent dyes [17,18]. However, because of the low-lying triplet energy level of PFO, it was thought to be a bad choice as a host for green or blue phosphorescent dyes. But efficient green phosphorescent PLEDs were realized based on PFO host recently by inserting HTL PVK [19,20]. If efficient all phosphorescent white electroluminescence (EL) can be achieved with PFO as host polymer, it will broaden the approaches of white light generation and will be a big promote for the application of phosphorescent WPLEDs. In this letter, we report a high efficiency all phosphorescent white EL with conjugated polymer PFO as the matrix of two iridium complexes, blue-light-emitting bis(2-(4,6-difluorophenyl)pyridinato-N,C2 ) picolinate (FIrpic) and red dye Ir(DMFPQ)2 pbm. EL spectra of PLEDs were tuned by varying the weight ratio of FIrpic and Ir(DMFPQ)2 pbm in PFO, and white EL was obtained at 10:1. Moreover, effects of HTL on the performances of WPLEDs were investigated. First of all, PVK as the HTL was essential, because the back energy transfer from FIrpic to PFO caused by the low-lying triplet energy level of PFO was suppressed by PVK. Furthermore, performances of the WPLEDs were enhanced by introducing CBP into PVK. The resulting all phosphorescent WPLEDs have a peak luminous efficiency of 15.5 cd/A and a peak power efficiency of 6 lm/W. Commisssion Internationale de L’Eclairage (CIE) coordinates of (0.41, 0.38) were realized at a current density of
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Fig. 1. The molecular structures of FIrpic and Ir(DMFPQ)2 pbm.
18 mA/cm2 . This efficiency was comparable with that of the phosphorescent WPLED based PVK host, which was about 20 cd/A [9]. To the best of our knowledge, this is the first report of high efficiency all phosphorescent WPLEDs based on conjugated polymer host. The obtaining of the efficient all phosphorescent white EL with PFO as host polymer will broaden the approaches of white light generation and be a big promote for the application of phosphorescent WPLEDs.
Fig. 2. EL spectra of PFO, FIrpic and Ir(DMFPQ)2 pbm.
2. Materials and methods Chemical structures of FIrpic and Ir(DMFPQ)2 pbm [21] are shown in Fig. 1. Devices configurations consist of ITO/PEDOT:PSS (40 nm)/PVK (40 nm)/EML (70 nm)/CsF (1 nm)/Al (100 nm), where PEDOT (Baytron P4083, purchased from Bayer AG) and PVK (purchased from Aldrich) were served as hole injecting and transporting layer, respectively. The EML consists of two phosphorescent iridium complexes simultaneously dispersed in a PFO host matrix: bluelight-emitting FIrpic and red iridium complexes Ir(DMFPQ)2 pbm. To prepare the device, an ITO-coated glass substrate was thoroughly cleaned in an ultrasonic bath. Acetone, detergent, deionized water, and isopropanol were sequentially used. Subsequently, the substrate was baked in a vacuum oven at 80 ◦ C for 2 h. Before PEDOT coating, the substrate was treated with oxygen plasma for 4 min. 40 nm of PEDOT was then spin-coated onto the ITO surface, and the resulting film was baked in a vacuum oven at 90 ◦ C for 12 h to remove residual water. In the next step, PVK and the EML were spin-coated onto the substrate sequentially. The thickness of PVK and EML were determined by profilometry (Tencor AlfaStep 500). Sequential depositions of CsF (1 nm) and Al (100 nm) were carried out at a base pressure of 3 × 10−4 Pa by thermal evaporation. An active emission area of 0.15 cm2 was defined by a shadow mask. Apart from the spin-coating of PEDOT layer, all other processes were carried out in a drybox (Vacuum Atmospheres) under N2 atmosphere. The luminous efficiency (LE)–current density (J)–voltage (V) data was collected by using a Keithley 236 source measurement unit and a calibrated silicon photodiode. The emission spectra and CIE coordinates were measured using a PR705 SpectraScan Spectrophotometer (Photo Research). And the absorption spectra were measured by using a UV–Visible spectrophotometer (Agilent 8453). 3. Results and discussion Monochromatic, blue- and red-light-emitting PLEDs with devices structures of ITO/PEDOT:PSS (40 nm)/PVK (40 nm)/
Fig. 3. EL spectra of PLEDs, ITO/PEDOT:PSS/PVK/PFO:FIrpic (10 wt%):Ir(DMFPQ)2 pbm/CsF/Al, with ratio of FIrpic to Ir(DMFPQ)2 pbm increased from 20:1, 10:1 to 10:2.
PFO:FIrpic (10 wt%) or PFO: Ir(DMFPQ)2 pbm (2 wt%) (70 nm)/CsF (1 nm)/Al (100 nm) were characterized, respectively. Fig. 2 shows the EL spectra of PFO, FIrpic and Ir(DMFPQ)2 pbm. Their 1931 CIE coordinates are (0.17, 0.10), (0.20, 0.41) and (0.65, 0.34), respectively. In 1931 CIE chromaticity diagram, the connection of FIrpic and Ir(DMFPQ)2 pbm across the white region, suggesting that white light can be obtained by blending them with an appropriate ratio. First, doping concentration of FIrpic in PFO was considered. High efficiency blue emission from FIrpic was the precondition for efficient white light in this all phosphorescent PFO-phosphorescent dopant system. As we know, efficiencies of phosphorescent PLEDs were strongly depended on the doping concentration of phosphorescent dyes [17,18]. According to a previous report, 10 wt% was the optimum doping concentration of FIrpic in polymer matrix [22]. Hence, we fixed PFO (90 wt%):FIrpic (10 wt%) as the basis for the matrix, and tuned the emission color by changing the ratio of Ir(DMFPQ)2 pbm to PFO from 0.5, 1 to 2 wt%. Fig. 3 shows the EL spectra of the PLEDs, ITO/PEDOT:PSS/PVK/PFO:FIrpic (10 wt%):Ir(DMFPQ)2 pbm/CsF/Al, with ratio of FIrpic to Ir(DMFPQ)2 pbm increased from 20:1, 10:1 to 10:2. All of the spectra show two intense peaks, 471 and 610 nm, assigned to the emissions of FIrpic and Ir(DMFPQ)2 pbm, respectively. The EL spectra showed strong concentration dependence.
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Fig. 5. Devices characterization of the WPLEDs with PVK or PVK:30 wt% CBP as the HTL: (a) J–V-external quantum efficiency (EQE), (b) LE–J-power efficiency (PE) and (c) EL spectra. Fig. 4. EL spectra of the blue-light-emitting PLEDs with doping concentration of FIrpic in PFO varied from 0, 1, 8, 10, to 16 wt%: (a) without PVK layer and (b) with PVK layer; and (c) the schematic energy level diagram of PFO, FIrpic and PVK.
With increasing content of Ir(DMFPQ)2 pbm, the relative intensity of the red band increased rapidly, implying energy transfer from FIrpic to Ir(DMFPQ)2 pbm. White emission can be realized utilizing the incomplete energy transfer from FIrpic to Ir(DMFPQ)2 pbm. According to Fig. 3, with simultaneous doping of 10 wt% FIrpic and 1 wt% Ir(DMFPQ)2 pbm in PFO, the EL spectrum showed a white light with the 1931 CIE coordinates of (0.40, 0.38). To further improve the efficiency of the white light, effects of HTL on the performances of WPLEDs were investigated. Three kinds of PLEDs with different HTLs were studied, no HTL, PVK and PVK:30 wt% CBP. Fig. 4 shows the EL spectra of the blue-lightemitting PLEDs without (a) or with (b) PVK layer with doping
concentration of FIrpic in PFO varied from 0, 1, 8, 10, to 16 wt%; and the schematic energy level diagram of PFO, FIrpic and PVK (c). Obviously, EL spectra of the two series of PLEDs, with or without PVK layer, were very different. Without PVK layer, even at high FIrpic doing concentration, for example 16 wt%, the relative intensity of FIrpic was still much weaker than that of PFO. However, in the presence of PVK layer, PFO emission was almost quenched even when doped with only 1 wt% FIrpic. This phenomenon was observed before in the conjugated polymer-green phosphorescent dopant system [19,20,23]. According to previous reports, back energy transfer from phosphorescent dyes to conjugated polymer host was responsible for the phosphorescent quenching happened in the devices without PVK layer, and which could be suppressed by inserting PVK layer [19,20,23]. As shown in Fig. 4(c), back
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Table 1 Devices performance of the WPLEDs with different hole-transport layers. Hole transporting layer
PVK PVK:30 wt% CBP
EQE (%)
6.8 7.6
Luminous efficiency (cd/A)
13.7 15.5
Power efficiency (lm/W)
4.73 6
@Peak luminous efficiency
CIE
Voltage (V)
Current density (mA/cm2 )
Luminance (cd/m2 )
9.2 8
2.29 2.35
315 364
(0.40, 0.38) (0.41, 0.38)
Fig. 6. The electronic band structures of the materials.
Fig. 8. EL spectra of the white light under different bias.
Fig. 7. Current density–electric field characteristics of the hole-only devices with different HTLs: PVK and PVK:30 wt% CBP.
energy transfer from the triplet excitons on the FIrpic to the triplet states of PFO happened because of the lower triplet energy level of PFO (2.3 eV) [19] than that of FIrpic (2.65 eV) [24]. And it would be suppressed in the PVK/EML interface because of the higher triplet energy level of PVK (3.0 eV) [23]. Therefore, HTL PVK plays a vital role in conjugated polymer-phosphorescent dopant system. Without HTL PVK, efficient all phosphorescent WPLEDs based on conjugated polymer matrix could not be achieved. Furthermore, WPLED with PVK doped with 30 wt% CBP as the HTL was studied. Fig. 5 shows the J–V-external quantum efficiency (EQE), LE–J-power efficiency (PE) characteristics and EL spectra of the WPLEDs with PVK or PVK:30 wt% CBP as the HTL, respectively. After the doping of CBP in PVK, the EQE, LE and PE of WPLEDs increased from 6.8 to 7.6%, 13.7 to 15.5 cd/A and 4.73 to 6 lm/W, respectively, as summarized in Table 1. And the 1931 CIE coordinates were almost the same with only a little shift from (0.40, 0.38) to (0.41, 0.38). The current density decreased after the doping of CBP in PVK, which maybe because of the decreased hole injection from PEDOT. This can be deduced from the electronic band structures of the materials shown in Fig. 6. The highest occupied molecular orbit (HOMO) of CBP (6.0 eV) [25] was 0.8 eV lower than that of PEDOT (5.2 eV) [26], and the difference between that of PVK (5.8 eV) [26] and PEDOT was 0.6 eV, suggesting more difficult hole injection from
PEDOT to CBP than to PVK. Current density–electric field intensity characteristics of the hole-only devices, ITO/PEDOT/HTL/EML/Al, with PVK or PVK:30 wt% CBP as HTL were shown in Fig. 7. After the doping of CBP, the hole current reduced under the same electric field. PFO is a hole-dominated material with hole current larger than that of electron [27]. Moderate hole injection decrease was better for the balance of hole and electron current in the EML, and therefore the WPLEDs performances were enhanced. The voltage stability of the white light was measured, as shown in Fig. 8. With the bias increased from 9.4 to 13.6 V, the EL spectra were stable. And the 1931 CIE coordinates have only a little shift from (0.43, 0.38) to (0.40, 0.39). That was because the relative intensity of FIrpic increased a little with raising of bias. 4. Conclusion Efficient all phosphorescent WPLEDs based on conjugated polymer host were fabricated. HTL PVK was essential, because of which the back energy transfer from blue phosphorescent dye FIrpic to PFO was suppressed. By tuning the double doping ratio of FIrpic and Ir(DMFPQ)2 pbm in PFO to 10:1, white light was obtained, whose 1931 CIE coordinates were (0.40, 0.38). When further doping 30 wt% CBP into PVK, the EQE, LE and PE of WPLEDs increased from 6.8 to 7.6%, 13.7 to 15.5 cd/A and 4.73 to 6 lm/W, respectively. To the best of our knowledge, this is the first report of high efficiency all phosphorescent WPLEDs based on conjugated polymer host. The white light was stable when voltage increased from 9.4 to 13.6 V. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (Grant Nos. 10904042 and 61078046), the Key Project of Chinese Ministry of Education (Grant No.
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210157), the Natural Science Foundation of Guangdong Province, China (Grant Nos. 8251063101000007 and 10151063101000009), the Scientific and Technological Plan of Guangdong Province (Grant Nos. 2008B010200004 and 2010B010600030).
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Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
High efficiency all phosphorescent white light-emitting diodes based on conjugated polymer host Qiaoli Niu a,b,∗ , Junbiao Peng a , Yong Zhang a , Yong Zhang b , Bo Liang a a
Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, Guangdong, China Institute of Optoelectronic Materials and Technology, South China Normal University, Key Laboratory of Electroluminescent Devices of Department of Education of Guangdong Province, Guangzhou 510631, Guangdong, China b
a r t i c l e
i n f o
Article history: Received 28 April 2011 Received in revised form 26 July 2011 Accepted 2 August 2011 Available online 27 August 2011 Keywords: Phosphorescent Back energy transfer White polymer light-emitting diode Conjugated polymer
a b s t r a c t High efficiency all phosphorescent white electroluminescence was realized by double doping of bluelight-emitting bis(2-(4,6-difluorophenyl)-pyridinato-N,C2 ) picolinate (FIrpic) and red iridium complex Ir(DMFPQ)2 pbm into conjugated polymer host poly(9,9-dioctylfluorene) (PFO). Effects of hole-transport layer (HTL) on the performances of white polymer light-emitting diodes (WPLEDs) were investigated. First of all, PVK as the HTL was essential, because the back energy transfer from FIrpic to PFO caused by the low-lying triplet energy level of PFO was suppressed by PVK. Furthermore, performances of the WPLEDs were enhanced by introducing CBP into PVK owing to improved balance of electron and hole current. The resulting all phosphorescent WPLEDs have a peak luminous efficiency of 15.5 cd/A and a peak power efficiency of 6 lm/W. Commisssion Internationale de L’Eclairage (CIE) coordinates of (0.41, 0.38) were realized at a current density of 18 mA/cm2 . The obtaining of the efficient all phosphorescent white electroluminescence with PFO as host polymer will broaden the approaches of white light generation and be a big promote for the application of phosphorescent WPLEDs. © 2011 Elsevier B.V. All rights reserved.
1. Introduction White organic/polymer light-emitting diodes (WOLEDs/ WPLEDs) based on phosphorescent dyes have gained increasing research interest in both scientific and industrial communities due to their potential applications in full-color flat-panel displays, back-lighting sources for liquid-crystal displays, and especially in next generation solid-state lighting sources [1–5]. Compared with WOLEDs, WPLEDs are more promising for large size or flexible products due to their solvent processing manufacture technology [6,7]. Non-conjugated polymer PVK has been a commonly used polymer host for phosphorescent dyes [8,9]. Indeed, PVK is an excellent material as hole-transport layer (HTL) of polymer light-emitting diodes (PLEDs) [10,11]. As a host polymer, PVK has however an inherent defect in that its electron and hole mobility difference is too large. Electron-transport materials must be used together with PVK to enhance the electron current [12,13]. However, in fabricating functional layers on top of the emissive layer (EML), electron-transport materials may be selectively removed resulting in poor PLEDs efficiency [14–16]. Therefore, it ∗ Corresponding author at: Institute of Optoelectronic Materials and Technology, South China Normal University, Key Laboratory of Electroluminescent Devices of Department of Education of Guangdong Province, Guangzhou 510631, Guangdong, China. Tel.: +86 20 85212667x805. E-mail address: Nql [email protected] (Q. Niu). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.08.018
is necessary to find other host polymers for phosphorescent dyes. Poly(9,9-dioctylfluorene) (PFO) is a commonly used blue material, as well as a well-known host polymer for red phosphorescent dyes [17,18]. However, because of the low-lying triplet energy level of PFO, it was thought to be a bad choice as a host for green or blue phosphorescent dyes. But efficient green phosphorescent PLEDs were realized based on PFO host recently by inserting HTL PVK [19,20]. If efficient all phosphorescent white electroluminescence (EL) can be achieved with PFO as host polymer, it will broaden the approaches of white light generation and will be a big promote for the application of phosphorescent WPLEDs. In this letter, we report a high efficiency all phosphorescent white EL with conjugated polymer PFO as the matrix of two iridium complexes, blue-light-emitting bis(2-(4,6-difluorophenyl)pyridinato-N,C2 ) picolinate (FIrpic) and red dye Ir(DMFPQ)2 pbm. EL spectra of PLEDs were tuned by varying the weight ratio of FIrpic and Ir(DMFPQ)2 pbm in PFO, and white EL was obtained at 10:1. Moreover, effects of HTL on the performances of WPLEDs were investigated. First of all, PVK as the HTL was essential, because the back energy transfer from FIrpic to PFO caused by the low-lying triplet energy level of PFO was suppressed by PVK. Furthermore, performances of the WPLEDs were enhanced by introducing CBP into PVK. The resulting all phosphorescent WPLEDs have a peak luminous efficiency of 15.5 cd/A and a peak power efficiency of 6 lm/W. Commisssion Internationale de L’Eclairage (CIE) coordinates of (0.41, 0.38) were realized at a current density of
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Fig. 1. The molecular structures of FIrpic and Ir(DMFPQ)2 pbm.
18 mA/cm2 . This efficiency was comparable with that of the phosphorescent WPLED based PVK host, which was about 20 cd/A [9]. To the best of our knowledge, this is the first report of high efficiency all phosphorescent WPLEDs based on conjugated polymer host. The obtaining of the efficient all phosphorescent white EL with PFO as host polymer will broaden the approaches of white light generation and be a big promote for the application of phosphorescent WPLEDs.
Fig. 2. EL spectra of PFO, FIrpic and Ir(DMFPQ)2 pbm.
2. Materials and methods Chemical structures of FIrpic and Ir(DMFPQ)2 pbm [21] are shown in Fig. 1. Devices configurations consist of ITO/PEDOT:PSS (40 nm)/PVK (40 nm)/EML (70 nm)/CsF (1 nm)/Al (100 nm), where PEDOT (Baytron P4083, purchased from Bayer AG) and PVK (purchased from Aldrich) were served as hole injecting and transporting layer, respectively. The EML consists of two phosphorescent iridium complexes simultaneously dispersed in a PFO host matrix: bluelight-emitting FIrpic and red iridium complexes Ir(DMFPQ)2 pbm. To prepare the device, an ITO-coated glass substrate was thoroughly cleaned in an ultrasonic bath. Acetone, detergent, deionized water, and isopropanol were sequentially used. Subsequently, the substrate was baked in a vacuum oven at 80 ◦ C for 2 h. Before PEDOT coating, the substrate was treated with oxygen plasma for 4 min. 40 nm of PEDOT was then spin-coated onto the ITO surface, and the resulting film was baked in a vacuum oven at 90 ◦ C for 12 h to remove residual water. In the next step, PVK and the EML were spin-coated onto the substrate sequentially. The thickness of PVK and EML were determined by profilometry (Tencor AlfaStep 500). Sequential depositions of CsF (1 nm) and Al (100 nm) were carried out at a base pressure of 3 × 10−4 Pa by thermal evaporation. An active emission area of 0.15 cm2 was defined by a shadow mask. Apart from the spin-coating of PEDOT layer, all other processes were carried out in a drybox (Vacuum Atmospheres) under N2 atmosphere. The luminous efficiency (LE)–current density (J)–voltage (V) data was collected by using a Keithley 236 source measurement unit and a calibrated silicon photodiode. The emission spectra and CIE coordinates were measured using a PR705 SpectraScan Spectrophotometer (Photo Research). And the absorption spectra were measured by using a UV–Visible spectrophotometer (Agilent 8453). 3. Results and discussion Monochromatic, blue- and red-light-emitting PLEDs with devices structures of ITO/PEDOT:PSS (40 nm)/PVK (40 nm)/
Fig. 3. EL spectra of PLEDs, ITO/PEDOT:PSS/PVK/PFO:FIrpic (10 wt%):Ir(DMFPQ)2 pbm/CsF/Al, with ratio of FIrpic to Ir(DMFPQ)2 pbm increased from 20:1, 10:1 to 10:2.
PFO:FIrpic (10 wt%) or PFO: Ir(DMFPQ)2 pbm (2 wt%) (70 nm)/CsF (1 nm)/Al (100 nm) were characterized, respectively. Fig. 2 shows the EL spectra of PFO, FIrpic and Ir(DMFPQ)2 pbm. Their 1931 CIE coordinates are (0.17, 0.10), (0.20, 0.41) and (0.65, 0.34), respectively. In 1931 CIE chromaticity diagram, the connection of FIrpic and Ir(DMFPQ)2 pbm across the white region, suggesting that white light can be obtained by blending them with an appropriate ratio. First, doping concentration of FIrpic in PFO was considered. High efficiency blue emission from FIrpic was the precondition for efficient white light in this all phosphorescent PFO-phosphorescent dopant system. As we know, efficiencies of phosphorescent PLEDs were strongly depended on the doping concentration of phosphorescent dyes [17,18]. According to a previous report, 10 wt% was the optimum doping concentration of FIrpic in polymer matrix [22]. Hence, we fixed PFO (90 wt%):FIrpic (10 wt%) as the basis for the matrix, and tuned the emission color by changing the ratio of Ir(DMFPQ)2 pbm to PFO from 0.5, 1 to 2 wt%. Fig. 3 shows the EL spectra of the PLEDs, ITO/PEDOT:PSS/PVK/PFO:FIrpic (10 wt%):Ir(DMFPQ)2 pbm/CsF/Al, with ratio of FIrpic to Ir(DMFPQ)2 pbm increased from 20:1, 10:1 to 10:2. All of the spectra show two intense peaks, 471 and 610 nm, assigned to the emissions of FIrpic and Ir(DMFPQ)2 pbm, respectively. The EL spectra showed strong concentration dependence.
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Fig. 5. Devices characterization of the WPLEDs with PVK or PVK:30 wt% CBP as the HTL: (a) J–V-external quantum efficiency (EQE), (b) LE–J-power efficiency (PE) and (c) EL spectra. Fig. 4. EL spectra of the blue-light-emitting PLEDs with doping concentration of FIrpic in PFO varied from 0, 1, 8, 10, to 16 wt%: (a) without PVK layer and (b) with PVK layer; and (c) the schematic energy level diagram of PFO, FIrpic and PVK.
With increasing content of Ir(DMFPQ)2 pbm, the relative intensity of the red band increased rapidly, implying energy transfer from FIrpic to Ir(DMFPQ)2 pbm. White emission can be realized utilizing the incomplete energy transfer from FIrpic to Ir(DMFPQ)2 pbm. According to Fig. 3, with simultaneous doping of 10 wt% FIrpic and 1 wt% Ir(DMFPQ)2 pbm in PFO, the EL spectrum showed a white light with the 1931 CIE coordinates of (0.40, 0.38). To further improve the efficiency of the white light, effects of HTL on the performances of WPLEDs were investigated. Three kinds of PLEDs with different HTLs were studied, no HTL, PVK and PVK:30 wt% CBP. Fig. 4 shows the EL spectra of the blue-lightemitting PLEDs without (a) or with (b) PVK layer with doping
concentration of FIrpic in PFO varied from 0, 1, 8, 10, to 16 wt%; and the schematic energy level diagram of PFO, FIrpic and PVK (c). Obviously, EL spectra of the two series of PLEDs, with or without PVK layer, were very different. Without PVK layer, even at high FIrpic doing concentration, for example 16 wt%, the relative intensity of FIrpic was still much weaker than that of PFO. However, in the presence of PVK layer, PFO emission was almost quenched even when doped with only 1 wt% FIrpic. This phenomenon was observed before in the conjugated polymer-green phosphorescent dopant system [19,20,23]. According to previous reports, back energy transfer from phosphorescent dyes to conjugated polymer host was responsible for the phosphorescent quenching happened in the devices without PVK layer, and which could be suppressed by inserting PVK layer [19,20,23]. As shown in Fig. 4(c), back
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Table 1 Devices performance of the WPLEDs with different hole-transport layers. Hole transporting layer
PVK PVK:30 wt% CBP
EQE (%)
6.8 7.6
Luminous efficiency (cd/A)
13.7 15.5
Power efficiency (lm/W)
4.73 6
@Peak luminous efficiency
CIE
Voltage (V)
Current density (mA/cm2 )
Luminance (cd/m2 )
9.2 8
2.29 2.35
315 364
(0.40, 0.38) (0.41, 0.38)
Fig. 6. The electronic band structures of the materials.
Fig. 8. EL spectra of the white light under different bias.
Fig. 7. Current density–electric field characteristics of the hole-only devices with different HTLs: PVK and PVK:30 wt% CBP.
energy transfer from the triplet excitons on the FIrpic to the triplet states of PFO happened because of the lower triplet energy level of PFO (2.3 eV) [19] than that of FIrpic (2.65 eV) [24]. And it would be suppressed in the PVK/EML interface because of the higher triplet energy level of PVK (3.0 eV) [23]. Therefore, HTL PVK plays a vital role in conjugated polymer-phosphorescent dopant system. Without HTL PVK, efficient all phosphorescent WPLEDs based on conjugated polymer matrix could not be achieved. Furthermore, WPLED with PVK doped with 30 wt% CBP as the HTL was studied. Fig. 5 shows the J–V-external quantum efficiency (EQE), LE–J-power efficiency (PE) characteristics and EL spectra of the WPLEDs with PVK or PVK:30 wt% CBP as the HTL, respectively. After the doping of CBP in PVK, the EQE, LE and PE of WPLEDs increased from 6.8 to 7.6%, 13.7 to 15.5 cd/A and 4.73 to 6 lm/W, respectively, as summarized in Table 1. And the 1931 CIE coordinates were almost the same with only a little shift from (0.40, 0.38) to (0.41, 0.38). The current density decreased after the doping of CBP in PVK, which maybe because of the decreased hole injection from PEDOT. This can be deduced from the electronic band structures of the materials shown in Fig. 6. The highest occupied molecular orbit (HOMO) of CBP (6.0 eV) [25] was 0.8 eV lower than that of PEDOT (5.2 eV) [26], and the difference between that of PVK (5.8 eV) [26] and PEDOT was 0.6 eV, suggesting more difficult hole injection from
PEDOT to CBP than to PVK. Current density–electric field intensity characteristics of the hole-only devices, ITO/PEDOT/HTL/EML/Al, with PVK or PVK:30 wt% CBP as HTL were shown in Fig. 7. After the doping of CBP, the hole current reduced under the same electric field. PFO is a hole-dominated material with hole current larger than that of electron [27]. Moderate hole injection decrease was better for the balance of hole and electron current in the EML, and therefore the WPLEDs performances were enhanced. The voltage stability of the white light was measured, as shown in Fig. 8. With the bias increased from 9.4 to 13.6 V, the EL spectra were stable. And the 1931 CIE coordinates have only a little shift from (0.43, 0.38) to (0.40, 0.39). That was because the relative intensity of FIrpic increased a little with raising of bias. 4. Conclusion Efficient all phosphorescent WPLEDs based on conjugated polymer host were fabricated. HTL PVK was essential, because of which the back energy transfer from blue phosphorescent dye FIrpic to PFO was suppressed. By tuning the double doping ratio of FIrpic and Ir(DMFPQ)2 pbm in PFO to 10:1, white light was obtained, whose 1931 CIE coordinates were (0.40, 0.38). When further doping 30 wt% CBP into PVK, the EQE, LE and PE of WPLEDs increased from 6.8 to 7.6%, 13.7 to 15.5 cd/A and 4.73 to 6 lm/W, respectively. To the best of our knowledge, this is the first report of high efficiency all phosphorescent WPLEDs based on conjugated polymer host. The white light was stable when voltage increased from 9.4 to 13.6 V. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (Grant Nos. 10904042 and 61078046), the Key Project of Chinese Ministry of Education (Grant No.
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210157), the Natural Science Foundation of Guangdong Province, China (Grant Nos. 8251063101000007 and 10151063101000009), the Scientific and Technological Plan of Guangdong Province (Grant Nos. 2008B010200004 and 2010B010600030).
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