Al bilayer cathode

Synthetic Metals 161 (2011) 1982–1986

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Efficient three-color white light-emitting diodes from a single polymer with PFN/Al bilayer cathode Jie Luo a,b , Xianzhen Li a , Junwu Chen a,∗ , Fei Huang a , Yong Cao a,∗ a Institute of Polymer Optoelectronic Materials & Devices, State Key Laboratory of Luminescent Materials & Devices, South China University of Technology, Guangzhou 510640, PR China b School of Physics & Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, PR China

a r t i c l e

i n f o

Article history: Received 14 June 2011 Received in revised form 26 June 2011 Accepted 6 July 2011 Available online 28 July 2011 Keywords: Three primary color white electroluminescence Non-doped white light-emitting diodes Alcohol-soluble conjugated polymer Bilayer cathode

a b s t r a c t In this work, a bilayer cathode, composed by alcohol-soluble poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9bis(3 -(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN) and high work-function Al, was utilized for efficient electron injection in non-doped white light-emitting diodes (WPLEDs) based on an electroluminescent single conjugated polymer PFO-R005-G010 that could show red, green, and blue (RGB) emissions simultaneously. The non-doped WPLEDs with the bilayer cathode could display higher luminous efficiency (LE) and better white light CIE coordinates, than those of well-known Ba/Al cathode. A pure white light (CIE coordinates of (0.33, 0.33)) containing spectral ranges of both deep blue and deep red as well as a good forward-viewing maximum LE of 6.62 cd/A at luminance of 200 cd/m2 could be achieved for a device with a PFN thickness of 10 nm and a thermal annealing process before Al deposition. The WPLED could show a maximum luminance of 9500 cd/m2 and good spectral stability at high luminance range. The results suggest that material systems for high efficiency three color WPLEDs, already realized by Ca/Al or Ba/Al cathode, could be the promising candidates for WPLEDs with a bilayer cathode. © 2011 Elsevier B.V. All rights reserved.

1. Introduction During the past decade, polymer light-emitting diodes (PLEDs) have undergone significant improvements in efficiency, brightness, and drive voltage toward efficient full-color display panels [1–3]. PLEDs are carrier injection devices, whose balanced hole injection from the anode and electron injection from the cathode and their fast transports and recombination in the emissive layer are the basic requirements [4]. Low work-function metals such as Ba and Ca are the widely utilized cathode materials to realize efficient electron injection. However, the low work-function metals are environmentally unstable, which can greatly affect the lifetime of PLEDs. Thus it is desirable to use high work-function metals (Al, Ag, Au, etc.) as the cathode. Tremendous efforts have been paid to improve electron injection from high-work-function stable metals into the emissive layer. Inserting a thin layer of LiF, CsF or some surfactants, between Al and the emissive layer, could significantly improve the electron injection [5–8]. However, all these methods showed cathode dependence and they were not effective for other high work-function metals such as Ag or Au. Recently, some alcohol/water-soluble polymers have been found excellent

∗ Corresponding authors. Fax: +86 20 8711 0606. E-mail addresses: [email protected] (J. Chen), [email protected] (Y. Cao). 0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2011.07.006

electron injection abilities in PLEDs [9–12]. It is the special solubility of the polyfluorenes in alcohol or water that makes sure the successful fabrications of PLEDs consisting multilayer polymers. For example, with poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3 -(N,Ndimethylamino)propyl)-2,7-fluorene)] (PFN, Fig. 1) as a cathode interlayer, 80 and 200 times increases of luminous efficiency for Al and Ag, respectively, were reported for green PLEDs with P-PPV as the emissive layer [13]. A PLED with Au cathode almost did not emit detectable light, however, inserting PFN interlayer between PPPV and Au could boost luminous efficiency to 11.6 cd/A [13]. The PFN interlayer also established the feasibility of all-solution processed PLEDs with silver paste cathode to emit efficient red, green, and blue lights [14]. A PFN/high work-function metal cathode had been called as a type of bilayer cathode [12]. Other research groups also have strong interest to introduce different bilayer cathodes in PLEDs, from which highly efficient PLEDs were demonstrated [15–20]. It was proposed that dipole formation or ionic charge redistribution might contribute to the distinguished electron injection properties of the interlayer [12,20]. It should be pointed that PFN/Al bilayer cathode has shown remarkably improvements of energy conversion efficiency in bulk-heterojunction photovoltaic cells [21,22]. Recently, white PLEDs (WPLEDs) have also found promising applications in full-color displays coupled with color filters, backlighting sources for liquid–crystal displays, and solid-state lighting

J. Luo et al. / Synthetic Metals 161 (2011) 1982–1986

S

1-2x-2y C8H17

C8H17

C8H17

S N

C8H17

S

PFO-R005-G010

C8H17

1983

x

N

C8H17

C8H17

N

S

N

y

x=0.00005, y=0.0001

n C8H17 N

N

PFN Fig. 1. Chemical structures of PFO-R005-G010 and PFN.

sources [9,23–26]. A bilayer cathode would supply a big potential to achieve low cost WPLEDs through all-solution processing. So far, most of the WPLEDs with a bilayer cathode comprised highly efficient phosphorescent complexes as the dopants in polymer hosts, and the white spectra were composed by blue and orange lights [18,19,27–29]. Red-green-blue (RGB) three color WPLEDs, with well-resolved RGB peaks, are more ideal in some practical applications [25,30]. However, RGB three color WPLEDs with a bilayer cathode are seldomly investigated because of possible difficulty in control of an ideal white spectrum [19]. Thermal annealing process and using bilayer cathode to replace Ca/Al or Ba/Al all might result in some changing of white spectra [19]. It is strongly desired to know that three color white electroluminescent polymers, developed in many reports, can be applicable in WPLEDs with a bilayer cathode. In this paper, we report three color WPLEDs with PFN/Al as the bilayer cathode, based on PFO-R005-G010 (Fig. 1), a single polymer showing RGB three color electroluminescence when using a Ba/Al cathode in our previous work [30]. The non-doped WPLEDs with the bilayer cathode could display higher luminous efficiency (LE) and better white light CIE coordinates, than those of well-known Ba/Al cathode. A pure white light (CIE coordinates of (0.33, 0.33)), good forward-viewing luminous efficiency (LE) of 6.62 cd/A at luminance (L) of 200 cd/m2 , and a maximum L (Lmax ) of 9500 cd/m2 could be achieved for a non-doped device with a PFN thickness of 10 nm and a thermal annealing process before Al deposition. The results suggest that material systems for high efficiency three color WPLEDs, already realized by Ca/Al or Ba/Al cathode, could be the promising candidates for WPLEDs with a bilayer cathode.

(vacuum atmospheres). The film thickness of the PFO-R005-G010 was around 75 nm, as measured with an Alfa Step 500 surface profiler (Tencor). We note that since PVK is not soluble in toluene, PVK remains untouched during spinning of the top copolymer layer. The PFN layer was spin-coated from methanol solution onto the emissive layer. Determination of the thickness of the PFN layer followed a previously published paper [12]. Finally, a 100 nm aluminum layer was thermally evaporated with a shadow mask at a base pressure of 3 × 10−4 Pa. The overlapping area between the cathode and anode defined a pixel size of 0.15 cm2 . The thickness of the evaporated cathodes was monitored by a quartz crystal thickness/ratio monitor (Model: STM-100/MF, Sycon). Except for the deposition of the PEDOT:PSS layers, all the fabrication processes were carried out inside a controlled atmosphere of nitrogen drybox (Vacuum Atmosphere Co.) containing less than 10 ppm oxygen and moisture. The current–luminance–voltage (I–L–V) characteristics were recorded with a Keithley 236 source meter and a calibrated silicon photodiode. Luminance was calibrated by using a PR-705 SpectraScan spectrophotometer and the forward-viewing LE was calculated accordingly. The external quantum efficiency was verified by measurement in the integrating sphere (IS-080, Labsphere). The electroluminescent spectra were collected by a PR-705 photometer. For photocurrent versus voltage characteristics measurements, the I–V characteristics under illumination were measured with a Keithley 236 source meter. The photocurrent was measured under solar simulator with AM 1.5G illumination (78.2 mW/cm2 ).

3. Results and discussion 2. Experimental

3.1. Comparison of WPLEDs with different cathodes

The synthesis procedures of PFO-R005-G010 and PFN were described elsewhere [30,31]. PFO-R005-G010 was dissolved in toluene and filtered through a 0.45 ␮m filter. Indium-tin oxide (ITO; ∼15 /square) substrates were cleaned by routine cleaning procedure, which included sonication in detergent followed by repeated rinsing in distilled water, acetone, and isopropanol, subsequently. After treatment with oxygen plasma, 50 nm of poly-(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic acid) (PEDOT:PSS) (Baytron P 4083, Bayer AG) was spin-coated onto the ITO substrate followed by drying in a vacuum oven at 80 ◦ C for 8 h. Poly(vinylcarbazole) (PVK, from Aldrich) from 1,1,2,2-tetrachloroethane solution was coated on top of a dried PEDOT:PSS layer subsequently. The thickness of PVK was 40 nm. Then a thin film of PFO-R005-G010 was coated by spin-casting of its toluene solution inside a nitrogen-filled drybox

WPLEDs with device configurations of ITO/PEDOT:PSS (50 nm)/PVK (40 nm)/PFO-R005-G010 (75 nm)/PFN (20 nm) or Ba (4 nm)/Al (100 nm) were fabricated to compare the difference between PFN/Al bilayer and the well-known Ba/Al cathode. The energy level diagrams of the devices are shown in Fig. 2. The selection of the PVK layer in the device is to enhance hole transport. The EL spectra of the WPLEDs are shown in Fig. 3. The two devices all displayed white light emissions through partial energy-transfer from the blue-emitting fluorene segments to green-emitting benzothiadiazole units (at 523 nm) and redemitting 4,7-dithienyl-benzothiadiazole (at 605 nm) [30]. The similar EL spectra obtained from the two types of cathodes imply that the recombination zones for the devices are located in the bulk of the emissive polymer. The CIE coordinates (0.30, 0.31) of the WPLED with PFN/Al cathode are closer to (0.33, 0.33) of pure

J. Luo et al. / Synthetic Metals 161 (2011) 1982–1986

Vacuum level 2.12 eV 2.14 eV

2. 2 eV G2. 22eV

2.7 eV

R2. 72 eV 4.9 eV

5.2 eV 5.8 eV

ITO

G5.72 eV R5.65 eV

PEDOT PVK

PFO

4.27 eV 5.61 eV 5.8 eV PFN

Al

Ba

Fig. 2. Energy level diagrams of the WPLEDs.

Luminous efficiency (cd/A)

6

8000 Ba/Al PFN/Al

5

6000 4

3

4000

2 2000

Luminance (cd cm-2)

1984

1

Normalized EL intensity (au)

1.0

Ba/Al (0.29, 0.29) PFN/Al (0.30, 0.31)

0.8

0 300

0 0

100

200

Current density (mA/cm2) Fig. 5. LE–J–L characteristics of WPLEDs by using different cathodes.

0.6

0.4

0.2

0.0 400

500

600

700

800

Wavelength (nm) Fig. 3. Normalized EL spectra of WPLEDs by using different cathodes.

white light, in comparison to the (0.29, 0.29) for Ba/Al cathode. The slight deviations of the EL spectra and the CIE coordinates with the PFN/Al cathode might be ascribed to microcavity effect due to the inserting of the PFN layer. The I–V curves of the WPLEDs with Ba/Al and PFN/Al cathodes are shown in Fig. 4. The WPLEDs with Ba/Al and PFN/Al cathodes showed turn-on voltages (Von ) of 6.0 and 6.2 V, respectively. From 6 to 9 V, the device with Ba/Al cathode could display slightly higher current in comparison to PFN/Al. The two devices showed comparable currents between 9 and 10 V. Generally, the slightly higher Von and the slight lower current between 6 and 9 V with the PFN/Al

cathode could be attributed to the decreased field strength at a certain voltage by the PFN layer with the 20 nm thickness. LE–current density (J)–L characteristics of the two WPLEDs are shown in Fig. 5. For a given J up to 300 mA/cm2 , the device with PFN/Al cathode could show higher luminance than that of Ba/Al cathode. Therefore the device with PFN/Al cathode could show higher efficiency than that of Ba/Al cathode. The forward-viewing Lmax values of PFN/Al and Ba/Al cathodes were close to 8000 and 6000 cd/m2 , respectively. The WPLED with Ba/Al cathode displayed a forward-viewing LEmax of 2.9 cd/A while a LEmax of 4.0 cd/A was found for PFN/Al. The LE values of the two WPLEDs showed small roll-off in the J range. The higher LE value of the PFN/Al cathode than that of the Ba/Al cathode at a given J in the wide J range demonstrates that the PFN/Al cathode is an outstanding cathode for WPLEDs, which can show better device performances in WPLEDs. It was reported before that the decreased electron injection barrier from the bilayer cathode to the emissive layer could be ascribed to the formation of positive interfacial dipole between the PFN interlayer and the Al electrode [12]. The polar amine groups on the ends of the side chains of PFN should generate strong interactions with the high work function Al electrode. Open-circuit voltage, an important parameter relevant to the built-in potential across the junction and the electrodes, would reflect the interfacial characteristics of the different cathodes [12]. Fig. 6 shows the photovoltaic 1E-3

10

Ba/Al PFN/Al

Current (mA)

Current (mA)

1 0.1 0.01

1E-4

1E-5 Ba/Al PFN/Al

1E-3 1E-6

1E-4 0

3

6

9

12

Voltage (V) Fig. 4. I–V curves of WPLEDs by using different cathodes.

-1

0

1

2

3

Voltage (V) Fig. 6. Photovoltaic characteristics of WPLEDs under white light illumination with different cathodes.

J. Luo et al. / Synthetic Metals 161 (2011) 1982–1986

1985

Table 1 Device performances of WPLEDs with different thickness of PFN (annealed before deposition of Al at 150 ◦ C for 10 min). No.

PFN (nm)

Voltage (V)

Current density (mA/cm2 )

L (cd/m2 )

LE (cd/A)

CIEa

1 2 3 4 5 6

1 5 10 12 23 29

9.4 7.2 6.2 7.2 8.1 8.5

11.9 11.5 7.60 11.5 9.20 7.93

51 589 487 637 486 461

0.43 5.11 6.38 5.55 5.28 5.79

(0.31,0.30) (0.33,0.33) (0.33,0.33) (0.33,0.34) (0.34,0.34) (0.35,0.36)

Obtained at a current density of 13.3 mA/cm2 .

behaviors of the two WPLEDs with the different cathodes. The device with Ba/Al cathode showed an open-circuit voltage (Voc ) of 1.4 V. The Voc value increased to 2.4 V for the PFN/Al bilayer cathode. The results indicate that inserting the polar interfacial layer between Al electrode and the PFO-R005-G010 emissive layer can decrease the barrier height for electron injection to the emissive layer. Based on the higher device performances with the PFN/Al bilayer cathode, the enhanced electron injection with the PFN/Al cathode would supply more balanced injections of holes and electrons, in comparison to the Ba/Al cathode. 3.2. Film thickness effect of PFN

8

12000

6

9000

4

6000

2

3000

0 0

100

200

Luminance (cd cm-2)

Luminous efficiency (cd/A)

Thermal annealing has been revealed to be an important method to elevate device performances in PLEDs and PVCs [19,30,32]. Considering the important role of PFN in the enhancement of electron injection from the high work-function Al, we anticipated that the thickness of the PFN layer would have some influences on the device performances. Here, thermal annealing before deposition of Al, at a temperature of 150 ◦ C for 10 min, was carried out in the fabrications of WPLEDs whose PFN thickness for a bilayer cathode was varied from 1 nm to 29 nm (Table 1). The annealing condition was previously introduced in a related report on WPLEDs with Ba/Al cathode [30]. For a very thin PFN layer of 1 nm, the WPLED displayed a much poor LE of 0.43 cd/A (Table 1, no. 1). With a PFN thickness of 5 nm, the WPLED under the annealing process showed obvious increasing of device performance (Table 1, no. 2). The LE value of the device was 5.11 cd/A for a L of 589 cd/m2 . Moreover, the white light spectrum was also improved. The CIE coordinates of the device at a current density of 13.3 mA/cm2 were located at pure white light region (0.33, 0.33). A thicker PFN layer of 10 nm showed the best

0 300

Current density (mA/cm2) Fig. 7. LE–J–L characteristics of the WPLED with 10 nm PFN (annealed before deposition of Al at 150 ◦ C for 10 min).

Normalized EL intensity (au)

a

800 cd/m 2 (0.33, 0.33) 2300 cd/m 2 (0.31, 0.31) 3200 cd/m 2 (0.30, 0.31) 6000 cd/m 2 (0.30, 0.32)

1.0 0.8 0.6 0.4 0.2 0.0 400

500

600

700

800

Wavelength (nm) Fig. 8. EL spectra stability of the WPLED with 10 nm PFN (annealed before deposition of Al at 150 ◦ C for 10 min).

performance, with LE of 6.38 cd/A for a L of 487 cd/m2 (Table 1, no. 3). Further increasing PFN thickness from 12 nm to 29 nm did not achieve a more optimal LE. The CIE coordinates of WPLEDs with PFN thickness between 5 and 23 nm were almost comparable, which were all close to pure white light region. The LE–J–L characteristics of the WPLED with 10 nm PFN are shown in Fig. 7. The forward-viewing LEmax of the device was 6.62 cd/A at a L of 200 cd/m2 . The WPLED possessed LE higher than 5 cd/A for L up to 4000 cd/m2 and LE higher than 4 cd/A for L up to 8000 cd/m2 . Thus the roll-off of efficiency of the WPLED for high brightness is not high. The forward-viewing Lmax of the device was 9500 cd/m2 . The EL spectra of the device at a practical brightness range between 800 and 6000 cd/m2 are shown in Fig. 8. From L of 800 to 2300 cd/m2 , the CIE coordinates of the WPLED moved from (0.33, 0.33) to (0.31, 0.31), showing slight decreasing of green and red emissions. Further increasing luminance up to 6000 cd/m2 caused very limited change of the EL spectra. 4. Conclusion In this article, a PFN/Al bilayer cathode was utilized for efficient electron injection in non-doped WPLEDs based on single conjugated polymer PFO-R005-G010 that could show three primary color emissions simultaneously. WPLEDs with the bilayer cathode could display higher LE and better white light CIE coordinates, than those of well-known Ba/Al cathode. A pure white light (0.33, 0.33) and a good forward-viewing LEmax of 6.62 cd/A at L of 200 cd/m2 could be achieved for a device with a PFN thickness of 10 nm and a thermal annealing process before Al deposition. The WPLED could show a forward-viewing Lmax of 9500 cd/m2 and good spectral stability at high L range. The results suggest that material systems for high efficiency three color WPLEDs, already realized by Ca/Al or Ba/Al cathode, could be the promising candidates for WPLEDs with a bilayer cathode.

1986

J. Luo et al. / Synthetic Metals 161 (2011) 1982–1986

Acknowledgments We gratefully acknowledge the financial support of National Natural Science Foundation of China (Nos. 50773023, 50990065, 51010003), Ministry of Science and Technology of China (Nos. 2009CB623600, 2011AA03A110), and SCUT Grant (No. 2009ZZ0003). References [1] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539. [2] A.C. Grimsdale, K.L. Chan, R.E. Martin, P.G. Jokisz, A.B. Holmes, Chem. Rev. 109 (2009) 897. [3] H.B. Wu, L. Ying, W. Yang, Y. Cao, Chem. Soc. Rev. 38 (2009) 3391. [4] Y.Z. Lee, X. Chen, S.A. Chen, P.K. Wei, W.S. Fann, J. Am. Chem. Soc. 123 (2001) 2296. [5] L.S. Hung, C.W. Tang, M.G. Mason, Appl. Phys. Lett. 70 (1997) 152. [6] P. Piromerium, H. Oh, Y. Shen, G.G. Malliaras, J.C. Scott, P.J. Brock, Appl. Phys. Lett. 77 (2000) 2403. [7] Y. Cao, G. Yu, A.J. Heeger, Adv. Mater. 10 (1998) 917. [8] X. Yang, Y. Mo, W. Yang, G. Yu, Y. Cao, Appl. Phys. Lett. 79 (2001) 563. [9] F. Huang, H.B. Wu, Y. Cao, Chem. Soc. Rev. 39 (2010) 2500. [10] C.V. Hoven, A. Garcia, G.C. Bazan, T.Q. Nguyen, Adv. Mater. 20 (2008) 3793. [11] H. Jiang, P. Taranekar, J.R. Reynolds, K.S. Schanze, Angew. Chem. Int. Ed. 48 (2009) 4300. [12] H.B. Wu, F. Huang, Y. Mo, W. Yang, D. Wang, J.B. Peng, Y. Cao, Adv. Mater. 16 (2004) 1826.

[13] H.B. Wu, F. Huang, J.B. Peng, Y. Cao, Org. Electron. 6 (2005) 118. [14] W.J. Zeng, H.B. Wu, C. Zhang, F. Huang, J.B. Peng, W. Yang, Y. Cao, Adv. Mater. 19 (2007) 810. [15] W.L. Ma, P.K. Iyer, X. Gong, B. Liu, D. Moses, G.C. Bazan, A.J. Heeger, Adv. Mater. 17 (2005) 274. [16] F. Huang, Y.H. Niu, Y. Zhang, J.W. Ka, M.S. Liu, A.K.Y. Jen, Adv. Mater. 19 (2007) 2010. [17] F. Huang, Y. Zhang, M.S. Liu, A.K.Y. Jen, Adv. Funct. Mater. 19 (2009) 2457. [18] X. Guo, C.J. Qin, Y.X. Cheng, Z.Y. Xie, Y.H. Geng, X.B. Jing, F.S. Wang, L.X. Wang, Adv. Mater. 21 (2009) 3682. [19] Y.H. Xu, R.Q. Yang, J.B. Peng, A.A. Mikhailovsky, Y. Cao, T.Q. Nguyen, G.C. Bazan, Adv. Mater. 21 (2009) 584. [20] R.Q. Yang, H.B. Wu, Y. Cao, G.C. Bazan, J. Am. Chem. Soc. 128 (2006) 14422. [21] L.J. Zhang, C. He, J.W. Chen, P. Yuan, L. Huang, C. Zhang, W. Cai, Z.T. Liu, Y. Cao, Macromolecules 43 (2010) 9771. [22] Z.C. He, C. Zhang, X.F. Xu, L.J. Zhang, L. Huang, J.W. Chen, H.B. Wu, Y. Cao, Adv. Mater. 23 (2011) 3086. [23] J. Kido, M. Kimura, K. Nagai, Science 267 (1995) 1332. [24] X. Gong, W. Ma, J.C. Ostrowski, G.C. Bazan, D. Moses, A.J. Heeger, Adv. Mater. 16 (2004) 615. [25] J. Liu, Q. Zhou, Y. Cheng, Y. Geng, L. Wang, D. Ma, X. Jing, F. Wang, Adv. Mater. 17 (2005) 2974. [26] K.T. Kamtekar, A.P. Monkman, M.R. Bryce, Adv. Mater. 22 (2010) 572. [27] Y. Zhang, F. Huang, Y. Chi, A.K.Y. Jen, Adv. Mater. 20 (2008) 1565. [28] F. Huang, P.I. Shih, C.F. Shu, Y. Chi, A.K.Y. Jen, Adv. Mater. 21 (2009) 361. [29] D. An, J.H. Zou, H.B. Wu, J.B. Peng, W. Yang, Y. Cao, Org. Electron. 10 (2009) 299. [30] J. Luo, X.Z. Li, Q. Hou, J.B. Peng, W. Yang, Y. Cao, Adv. Mater. 19 (2007) 1113. [31] F. Huang, H.B. Wu, D.L. Wang, W. Yang, Y. Cao, Chem. Mater. 16 (2004) 708. [32] W.L. Ma, C.Y. Yang, X. Gong, K. Lee, A.J. Heeger, Adv. Funct. Mater. 15 (2005) 1617.