Color tuning of doped PLEDs by energy transfer from PVK to dopant constituted with biphenyl and butadienyl unit

Article abstract of DOI:10.1080/15421400701497829

A dopant, 4,4'-bis(dimethyl[4-(2-phenyl-buta-1,3-dienyl)phenyl]amine) biphenyl (BPAB), was synthesized, and single layered polymer light emitting diode (PLED)s, ITO/Host:Dopant/Al, were fabricated with blends of poly(vinyl carbazole) (PVK) as host and BPAB as dopant. The electroluminescence (EL) was observed in doped PLEDs. From the EL and photoluminescence (PL) spectra, the existence of energy transfer from host to dopant was verified. With increasing content of dopant (BPAB), blue emission originated from PVK reduced, but didn't completely disappear, suggesting inefficient energy transfer from PVK to BPAB. When the content of BPAB was 15 wt%, yellow green emission from PLED was observed due to partial energy transfer from PVK to BPAB.

Full text of DOI:10.1080/15421400701497829

This article was downloaded by: [University of Toronto Libraries]
On: 07 October 2014, At: 22:26
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Molecular Crystals and Liquid
Crystals
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gmcl20
Color Tuning of Doped PLEDs
by Energy Transfer from PVK
to Dopant Constituted with
Biphenyl and Butadienyl Unit
Yoon Soo Han a , Hoyoung Kim a , Byeong Dae Choi a
, Younghwan Kwon b , Youngjune Hur c , Giseop Kwak
d , Soon Hak Kim d , Lee Soon Park d & Byeong-Dae
Choi e
a Department of Nano Technology , Daegu
Gyeongbuk Institute of Science & Technology ,
Daegu, Korea
b Department of Chemical Engineering , Daegu
University , Gyeongsan, Gyeongbuk, Korea
c Advanced Display Manufacturing Research Center,
Kyungpook National University , Daegu, Korea
d Department of Polymer Science , Kyungpook
National University , Daegu, Korea
e DGIST , Daegu, Korea
Published online: 22 Sep 2010.
To cite this article: Yoon Soo Han , Hoyoung Kim , Byeong Dae Choi , Younghwan
Kwon , Youngjune Hur , Giseop Kwak , Soon Hak Kim , Lee Soon Park & Byeong-Dae
Choi (2007) Color Tuning of Doped PLEDs by Energy Transfer from PVK to Dopant
Constituted with Biphenyl and Butadienyl Unit, Molecular Crystals and Liquid Crystals,
470:1, 341-351, DOI: 10.1080/15421400701497829
To link to this article: http://dx.doi.org/10.1080/15421400701497829

PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Mol. Cryst. Liq. Cryst., Vol. 470, pp. 341–351, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701497829
Color Tuning of Doped PLEDs by Energy Transfer
from PVK to Dopant Constituted with Biphenyl
and Butadienyl Unit
Yoon Soo Han
Hoyoung Kim
Byeong Dae Choi
Department of Nano Technology, Daegu Gyeongbuk
Institute of Science & Technology, Daegu, Korea
Younghwan Kwon
Department of Chemical Engineering, Daegu University,
Gyeongsan, Gyeongbuk, Korea
Youngjune Hur
Advanced Display Manufacturing Research Center, Kyungpook
National University, Daegu, Korea
Giseop Kwak
Soon Hak Kim
Lee Soon Park
Department of Polymer Science, Kyungpook National University,
Daegu, Korea
Byeong-Dae Choi
DGIST, Daegu, Korea
A dopant, 4,40-bis(dimethyl[4-(2-phenyl-buta-1,3-dienyl)phenyl]amine) biphenyl
(BPAB), was synthesized, and single layered polymer light emitting diode
(PLED)s, ITO=Host:Dopant=Al, were fabricated with blends of poly(vinyl carba-
zole) (PVK) as host and BPAB as dopant. The electroluminescence (EL) was
observed in doped PLEDs. From the EL and photoluminescence (PL) spectra,
the existence of energy transfer from host to dopant was verified. With increasing
content of dopant (BPAB), blue emission originated from PVK reduced, but didn’t
This work was supported by the DGIST Basic Research Program of the MOST,
and the Regional Innovation Center (ADMRC) Program of the Ministry of Commerce,
Industry, and Energy of Korea.
Address correspondence to Lee Soon Park, Department of Polymer Science, Kyungpook
National University, 1370 Sankyuk-Dong, Daegu 702-701, Korea. E-mail: lspark@knu.ac.kr
341

342
Y. S. Han et al.
completely disappear, suggesting inefficient energy transfer from PVK to BPAB.
When the content of BPAB was 15 wt%, yellow green emission from PLED was
observed due to partial energy transfer from PVK to BPAB.
Keywords: dopant; energy transfer; host; polymer light emitting diode (PLED)
INTRODUCTION
Semiconducting small molecules and polymers are currently the focus
of industrial interest for flat panel display. Since the first report of the
polymer light emitting diode s (PLEDs) from poly(p-phenylene vinyl-
ene) (PPV) by Cambridge group, a number of different polymers have
been synthesized for use as emitting layer materials [1–4]. PLEDs
have merit of large panel fabrication with low production cost by
utilizing simple inkjet printing process, but the luminance and color
purity are not as high as small molecular OLEDs which have adopted
a doping system in emissive layer to increase luminance. As draw-
backs of OLEDs using low molecular weight emitting materials, each
organic layer is generally fabricated by vacuum deposition method.
This method is cost ineffective, and the performance of OLEDs was
greatly affected by thermal properties of organic materials and depo-
sition conditions employed.
Recently, the efforts to enhance quantum efficiency by adopting
doping system in PLEDs have been conducted by many researchers
[5–8]. To understand energy transfer from host to dopant, they have
chosen PVK as host material, which has wide band gap energy and
good hole transporting property [9–13].
In this work, we synthesized a dopant, BPAB, containing biphenyl
and butadienyl unit. Doped PLEDs were fabricated with different
ratio of dopant to the host polymer, poly(vinyl carbazole), and evalu-
ated in terms of color change and luminance efficiency.
EXPERIMENTAL
Materials
Benzyl chloride, triethylphosphite, 4-(dimethylamino)benzaldehyde,
phosphoryl chloride, and potassium tert-butoxide were purchased from
Aldrich Chemical Co., and used as received. Solvents such as N,
N-dimethylformamide (DMF) and tetrahydrofurane (THF) were reagent
grades and purified prior to use. Indium-tin oxide (ITO) coated glass with
a sheet resistance of 30 X=& was obtained from Samsung Corning Co.

Color Tuning of Doped PLEDs by Energy Transfer from PVK
343
Aluminum (99.99%, CERAC, USA) was used to deposit cathode
electrode. Poly(vinyl carbazole) (PVK) with Mn ¼ 48,800 and
Mw=Mn ¼ 1.68 used as a host material was purchased from Aldrich
Chemical Co.
Synthesis of Dopant
The synthetic route to the dopant studied in this work is shown in
Figure 1.
FIGURE 1 Synthetic route to dopant BPAB.

344
Y. S. Han et al.
4-Dimethylaminostilbene (DMAS)
Firstly diethoxyphosphinyl methyl benzene (DPMB) was synthesized
from benzyl chloride (3.7 g, 0.03 mol) and triethylphosphite (26 g, 0.17 mol)
at 120ꢀC. This reaction mixture was cooled at room temperature after
stirring 14 hr, and then was stirred with 4-(dimethylamino)benzaldehyde
(3 g, 0.02 mol) in the presence of t-BuOK (19 g) in THF (100 mL) for 10 hr to
produce DMAS. THF and unreacted triethylphosphite were removed
using vacuum, and the reaction mixture was thoroughly washed with
water. The product, DMAS, was obtained by vacuum drying. Yield:
1
=
74%, H-NMR (CDCl3), d: 7.02–7.48 (m, 9H, Ar-H), 6.75 (s, 2H, CHꢁ),
2.98 (s, 6H, ꢁCH3), IR (KBr), cmꢁ1: 3052 (Ar CH), 3020 (aliphatic
=
=
=
CHꢁ), 2884 (ꢁCH3), 1590, 1478 (Ar C C), 962 (trans ꢁCH CHꢁ).
Elemental Anal, Calcd for C16H17N: C (86.05), H (7.67), N (6.27); Found:
C (86.07), H (7.94), N (6.30). Molecular weight, Calcd. for C16H17N:
223.31, Found: 223.20.
4-Dimethylamino-a-phenyl cinnamaldehyde (DPCA)
The synthesized DMAS (4.98 g, 0.022 mol) was dissolved in 40 mL of
DMF and then cooled to 0ꢀC. The 3.06 g of phosphoryl chloride solved
in 10 mL of DMF was added dropwise for 10 min, and the reaction mix-
ture was stirred at 65ꢀC. After stirring for 6 hr, the reaction mixture
was treated with 10 wt% of sodium hydroxide solution to adjust
pH ¼ 7. The product was precipitated. The filtered product was
washed with methanol solution (MeOH:H2O ¼ 50:50 w:w). The pure
DPCA was obtained by recrystallization from cyclohexane. Yield:
28.9%, 1H-NMR (CDCl3), d: 9.66 (s, 1H, aldehydic hydrogen), 7.38
=
(d, 1H, CHꢁ), 7.21-7.38 (m, 9H, Ar-H), 2.97 (s, 6H, ꢁCH3), IR
(KBr), cmꢁ1: 3052 (Ar CH, weak signal), 2919 (ꢁCH3), 2729 (CHO),
=
=
=
1656 (C O), 1603 (C C), 1496 (Ar C C). Elemental Anal, Calcd. for
C17H17NO: C (81.24), H (6.82), N (5.57); Found: C (81.13), H (6.91),
N (5.42). Molecular weight, Calcd. for C16H17N: 251.32, Found: 251.20.
4,40-Bis(diethoxyphosphinyl methyl)-1,10-biphenyl (PHBP)
The procedures used for the preparation of PHBP were the same as
reported [14].
4,40-Bis(dimethyl[4-(2-pheny-buta-1,3-
dienylphenyl]amine)biphenyl (BPAB)
A dopant, BPAB, was prepared by condensation reaction of DPCA
and PHBP. The DPCA (1 g, 0.004 mol) and PHBP (0.78 g, 0.002 mol)
were dissolved in THF, and then potassium tert-butoxide (1.5 g) was
added. After stirring at room temperature for 12 hr, THF was removed
under reduced pressure. The pure product was obtained by successive

Color Tuning of Doped PLEDs by Energy Transfer from PVK
345
washing with water and hot methanol, followed by filtration. Yield:
88%, 1H-NMR (CDCl3), d: 7.52, 7.36 (m, 8H, Ar H), 4.04 (m, 8H,
PꢁOCH2ꢁ), 3.20 (d, 4H, ArꢁCH2ꢁP), 1.27 (t, 12H, ꢁCH3), IR (KBr),
cmꢁ1: 3030 (Ar CH), 2987 (aliphatic CH), 1238 (P O).
=
Fabrication of PLEDs
Doped single layer PLEDs were fabricated using PVK as host and
BPAB as dopant. For comparison undoped single layer PLED was also
fabricated. Indium-tin oxide (ITO) coated glass with a sheet resistance
of 30 X=& was cut into 2.0 ꢂ 2.0 cm2, and electrode area was prepared
by photoetching technique. It was sequentially cleaned with acetone,
methanol, and mixture of isopropyl alcohol and water (1:1 by vol.) in
an ultrasonic bath. A solution of PVK:BPAB in tetrachloroethane
(TCE) after filtration using MFS filter (0.45 mm pore size) was spin-
coated on the ultrasonically cleaned ITO glass at 2800 rpm for 25 s
and dried at 80ꢀC for 1 hr to give an emissive layer with a thickness
˚
of about 600–800 A. After drying the emission layer Al electrode was
˚
deposited at a rate of 20 A=sec by thermal evaporation to give single
layer [ITO=PVK:BPAB (100:0 ꢃ 85:15 w:w)=Al] PLED. The area of
deposited Mg was approximately 0.4 cm2.
Measurements
The 1H-NMR and FT-IR spectra of synthesized dopant were taken on a
Varian Unity Plus 300 and Jasco FT=IR-620 spectrometer, respectively.
UV-visible absorption spectrum of synthesized BPAB was obtained by
Shimadzu UV-2100. PL spectra were obtained by optical multichannel
analyzer (OMA system, laser photonics, USA). Excitation source was
He-Cd laser. EL spectra were recorded by Spectroscan PR 704 (Photo-
research Inc.) at 2 nm resolution and color coordinates were determined
by the same instrument. Current and luminance vs. voltage profiles
were obtained by using a dc power supply connected with Model
8092 A Digital Multimeter (Hyun Chang Product Co. Ltd) and lumi-
nance meter (Minolta LS-100) equipped with close-up lens (No. 110, U
40.5 mm) at room temperature, respectively.
RESULTS AND DISCUSSION
Synthesis and Characterization of Dopant
The dopant, BPAB, was synthesized to investigate energy transfer
from blue emitting PVK to yellow green emitting BPAB. As pre-
sented in experimental section, all the intermediates to synthesize

346
Y. S. Han et al.
FIGURE 2 FT-IR spectra of (a) PHBP and (b) BPAB.
BPAB were characterized with 1H-NMR and FT-IR. As shown in
Figure 2, FT-IR spectra of PHBP and BPAB, the absorption peak
(ꢃ1200 cmꢁ1) of P O stretching mode in PHBP disappeared after
=
=
Honer-Emmons condensation, and the C C stretching mode
(ꢃ1600 cmꢁ1) appeared in FT-IR spectrum of BPAB.
Optical Properties of PVK and BPAB
Energy transfer can be achieved from the excited state of host to the
ground state of dopant due to good spectral overlap between the emis-
sion of host and the absorption of the dopant.[8] In order to recognize
the possibility of energy transfer from PVK to BPAB, UV-Visible
absorption and PL emission spectra of PVK and BPAB were measured,
as can be seen in Figure 3. The absorption maxima (kmax,UV) were mea-
sured to be 330 and 344 nm for PVK, and 404 nm for BPAB, respect-
ively. PL maximum (kmax,PL) values of PVK and BPAB were
observed at 367 and 511 nm, when excited at their own kmax,UV
,
respectively. BPAB exhibited kmax,UV at 404 nm, which was overlapped
to a relatively large extent with PL peaks of PVK. BPAB showed
kmax,PL at 511 nm, but PL was not observed when excited at 330 nm,
kmax,UV of PVK.
PL spectra of PVK=BPAB mixtures, excited at 330 nm, were mea-
sured as a function of the BPAB content in mixtures (Fig.4). As the

Color Tuning of Doped PLEDs by Energy Transfer from PVK
347
FIGURE 3 UV-visible absorption spectra of (a) PVK and (b) BPAB, and PL
spectra of (c) PVK and (d) BPAB.
content of BPAB dopant was increased in the BPAB=PVK mixture
solution, the PL intensity of BPAB at 511 nm steadily increased while
PL intensity of PVK decreased, suggesting energy transfer between
BPAB dopant and PVK host polymer.
Electro-Optical Properties of PLEDs
Figure 5 shows EL spectra of single layer PLEDs made with
PVK=BPAB blend as an emitting layer. Similar to the PL spectra,
PVK emission fraction decreased with increasing BPAB content.
Although the emission from host PVK was reduced and emission from
dopant BPAB became more dominant with increasing dopant concen-
tration, the emission from PVK still appeared. This result suggests
inefficient energy transfer from PVK to BPAB.
Color coordinates of single layer PLEDs were compared with those
of standard red, green, blue, and white color established by NTSC
(national television system committee) [15] in Figure 6. The emission
color could be determined from these color coordinates as measured by

348
Y. S. Han et al.
FIGURE 4 PL spectra of (a) 0, (b) 1, (c) 2, (d) 5, (e) 7, (f) 10 and (g) 15 wt% of
BPAB in PVK:BPAB mixture solved in TCE. Zero wt% of BPAB means to use
only PVK.
FIGURE 5 EL spectra of PLEDs made with (a) 0, (b) 1, (c) 5, (d) 10 and (e)
15 wt% of BPAB in PVK:BPAB mixture.

Color Tuning of Doped PLEDs by Energy Transfer from PVK
349
FIGURE 6 CIE 1931 chromaticity diagram showing color coordinates of
PLEDs fabricated with (a) 0, (b) 1, (c) 5, (d) 10 and (e) 15 wt% of BPAB in
PVK:BPAB mixture.
FIGURE 7 Luminance-voltage profiles of PLEDs fabricated with (a) 0, (b) 1,
(c) 5, (d) 10 and (e) 15 wt% of BPAB in PVK:BPAB mixture.

350
Y. S. Han et al.
Spectroscan PR-704 (Photoresearch Inc.) and Kelly’s map [16] in CIE
(Commission Internationale de l’Eclairage) 1931 chromaticity diagram
[17]. The EL light of PVK-based PLED(ITO=PVK=Al) showed blue
emission with color coordinates of x ¼ 0.19 and y ¼ 0.19, and the EL
colors from PLEDs with 1, 5, 10 and 15 wt% of BPAB in PVK=BPAB
BPAB blend were greenish blue (CIE coordinates of x ¼ 0.19 and
y ¼ 0.25), bluish green (CIE coordinates of x ¼ 0.12 and y ¼ 0.35),
yellow-green (CIE coordinates of x ¼ 0.38 and y ¼ 0.58), and yellow
green (CIE coordinates of x ¼ 0.41 and y ¼ 0.55), respectively.
The luminance-voltage profiles are shown in Figure 7. PLED fabri-
cated with PVK as emitting layer showed lower luminance than those
of PLEDs made with PVK=BPAB blend due to energy transfer from
blue emitting PVK to green emitting BPAB. The highest luminance
was 240 cd=m2 at 18 V from ITO=PVK:BPAB(85:15 w:w)=Al device.
CONCLUSIONS
Yellow green emitting dopant, BPAB, was synthesized to investigate
energy transfer from host PVK and dopant BPAB in PLED. From the
PL and EL studies, we could identify the existence of energy transfer,
and evaluate the degree of energy transfer qualitatively. In the PVK=
BPAB system, the energy transfer was observed, but incomplete. It
was also found that while undoped ITO=PVK=Al device showed blue
emission of high color purity, doped ITO=PVK:BPAB(85:15 w:w)=Al
device exhibited yellow green emission, implying energy transfer from
PVK to BPAB dopant.
REFERENCES
[1] Burroushes, J. H., Bradley, D. D. C., Brown, A. R., Narks, R. N., Mackay, K.,
Friend, R. H., Burns, P. L., & Holmes, A. B. (1990). Nature, 347, 539.
[2] Askari, S. H., Rughooputh, S. D., & Wudl, F. (1989). Synth. Met., 29, E129.
[3] Gurge, R. M., Sarker, A. M., Lahti, P. M., Hu, B., & Karasz, F. E. (1997). Macromo-
lecules, 30, 8286.
[4] Bradley, D. D. C. (1993). Synth. Met., 54, 401.
[5] Hino, Y., Kajii, H., & Ohmori, Y. (2006). Thin Solid Films, 499, 359.
[6] Xu, Z., Liu, M., Chen, X., Hou, Y., Teng, F., Meng, L., & Xu, X. (2006). Displays,
27, 45.
[7] Zhen, H., Jiang, C., Yang, W., Zeng, X., Zhang, C., & Cao, Y. (2005). Synth. Met.,
155, 196.
¨
[8] Wang, X., Andersson, M. R., Thomson, M. E., & Inganas, O. (2004). Thin Solid
Films, 468, 226.
[9] Lee, C.-L., Lee, K. B., & Kim, J.-J. (2000). Appl. Phys. Lett., 77, 2280.
[10] Berkovich, E., Klein, J., Sheradsky, T., Silcoff, E. R., Ranjit, K. T., Willner, I.,
Nakhmanovich, G., Gorelik, V., & Eichen, Y. (1999). Synth. Met., 107, 85.

Color Tuning of Doped PLEDs by Energy Transfer from PVK
351
[11] Kwok, C. C. & Wong, M. S. (2001). Mate. Sci. Eng. B, 85, 126.
[12] Duan, L., Qui, Y., He, Q., Bai, F., Wang, L., & Hong, X. (2001). Synth. Met., 124,
373.
[13] Huang, J., Zhang, H., Tian, W., Hou, J., Ma, Y., Shen, J. & Liu, S. (1997). Synth.
Met., 87, 105.
[14] Han, Y. S., Kim, S. D., Park, L. S., Kim, D. U., & Kwon, Y. (2003). J. Polym. Sci.:
Part A: Polym. Chem., 41, 2502.
[15] Ozawa, L. (1994). Application of Cathodoluminescence to Display Devices,
Kodansha, Tokyo, 18–30.
[16] Agoston, G. A. (1979). Springer series in optical sciences. In: Color Theory and Its
Application in Art and Design, MacAdam, D. L., (Ed.), Vol. 19, Springer-Verlag:
Berlin, 57–59.
[17] MacAdom, D. L. (1981). Springer series in optical sciences. In: Color Measurement;
Theme and Variations, MacAdam, D. L., (Ed.), Vol. 27, Springer-Verlag: Berlin,
17–25.