Series of Selected Papers from Chun-Tsung Scholars, Peking University (2003)
Improvement of composites doped with Alq3 in PR
performance
Qun Wei and Yihong Liu
Department of Physics, Peking University
Abstract
The photorefractive composites composed of 8-pertyloxy-4’-cyanobiphenyl (8OCB) /
N,N’-diphenyl-N,N’-bis(3-methylphenyl)-[1,1’-biphenyl]-4,4’-diamine
(TPD)
/
2,4,7-trinitro-9-fluorenone (TNF) / 8-hydroxyquinoline aluminum (Alq3) / polycarbonate
plastics were fabricated. The influence of Alq3 on the photorefractivity was studied,
indicating that the presence of Alq3 leads to a larger two-beam coupling coefficient  and
shorter response time.  over 330cm-1 at an applied electric field of 26V/μm has been
measured while  of the sample without Alq3 is 210cm-1. It is presumed that the
electron-injecting material Alq3 and charge transporting material TPD can form more
effective traps in the composites and the PR performance is improved consequently.
I. Introduction.
Since the photorefractive (PR) effect was observed in LiNbO3 for the first time in
1967[1], numerous electro-optic applications have been predicted based on high qualified
PR materials, such as high density holographic data storage, phase conjugation, optical
computing, and pattern recognition
[2-6]
. These potential applications
have attracted many researchers to
this field. Especially after J.
Ducharme discovered PR effect in a
polymer system in 1991[7], plenty of
organic photorefractive materials
have been investigated due to the
merits of organic materials compared
with inorganic ones, such as low
dielectric constant, high impurity
doping, composition flexibility, and
easily fabricated complex structures.
Liquid crystals are one important
type of the PR materials with long
rod-shaped molecules that can
produce large birefringence, which Figure 1. Structures of the components of the
photorefractive composite: 8-pertyloxy-4’-cyanobiphenyl
would maximize the orientational (8OCB), 2,4,7-trinitro-9-fluorenone (TNF), N,N’enhancement effect.[8,9] Furthermore, diphenyl-N,N’-bis(3-methylphenyl)-[1,1’-biphenyl]-4,4’-d
the low glass temperature (Tg) of iamine (TPD) and 8-hydroxyquinoline aluminum (Alq3).
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Series of Selected Papers from Chun-Tsung Scholars, Peking University (2003)
some liquid crystal results in easy orientation at low external electric field at the room
temperature.
Some kinds of composites with n-pertyloxy-4’-cyanobiphenyl (nOCB) as the
chromophore have been studied and showed excellent PR performance[10,11]. In this study,
an electron-injecting material of Alq3[12,13] was introduced to liquid crystal composites
based on hole transporting system. It is assumed that Alq3 may serve as traps for electrons
in the hole transporting materials and improve the PR performance. So Alq3 was doped
into the sample. A much higher two-beam-coupling coefficient and shorter response time
were obtained.
II. Experiments and results.
Figure 1 shows the structures of the compounds used in this study. The composites
were added to the matrix of polycarbonate plastics and the proportions are shown in
Table 1. To prepare the samples, all the compounds were dissolved in chloroform and
mixed sufficiently and equably. Then the solution was dipped onto two glass plates
coated with indium tin oxide (ITO) at the temperature of 100°C and sandwiched after
chloroform had been evaporated. And the samples were cooled rapidly in order to keep
the plastic transparent and get good optical qualities. The thickness of the film was
controlled to be 80m by spacers.
Table 1. The compositions of two sample.
Sample
1
2
8OCB
40
40
TPD
20
20
Composition (wt.%)
Plastic
39
37
TNF
1
1
Alq3
0
2
To evaluate the photorefractive effect of
the two samples, we carried out the typical
two-beam-coupling
(TBC)
experiment
employing a typical tilted geometry shown in
Figure 2. The tilt angle is φext=35° in air and
two mutually coherent p-polarized He–Ne
laser beams at wavelength of 633nm and
powers of 10 and 17mW intersected in the
sample at angel 2θext=20° in air. The external
electric field was pre-applied on the sample
for 10 minutes before the measurements.
Strong photorefractive effect was observed
in both Sample 1 and Sample 2 at electric field Figure 2. Experimental geometry for the
TBC experiments.
from 6 to 26V/μm. Figure 3 shows the applied
electric field dependence of TBC coefficient and inset shows the typical TBC curves of
sample 2. The TBC coefficients are calculated by[14]:
I 2 I12
cos

ln(
)，
(1)
d
I1 I 2  I1 I12  I12
where d is the thickness of the film, θ is the tilting angle within the sample, I1 and I2 are
the transmitted intensities without coupling, I12 and I21 are the transmitted intensities with
coupling, and the index of refraction of the samples is assumed to be 1.6. As what is
shown in Figure 3, the TBC coefficient of Sample 2 was much larger than that of sample
223
i n t e n s i t y ( a . u . )
Series of Selected Papers from Chun-Tsung Scholars, Peking University (2003)
350
B e a m
1 . 6
2
B e a m
B e a m
o n
1
2
S a m p l e
S a m p l e
1 . 4
1
2
1 . 2
B e a m
-1
 (cm
)
1 . 0
300
0 . 8
0 . 6
0 . 4
250
0 . 2
0 . 0
200
0
2
4
6
8
10
12
14
T i m e ( s )
150
100
50
0
0
5 0 0
1 0 0 0
1 5 0 0
2 0 0 0
2 5 0 0
V o l ta g e (V )
Figure 3. The dependence of TBC coefficient on applied electric field, inset is the typical
TBC curve.
160
150
Sample 1
Sample 2
140
130
TBC Buildup Time (s)
120
110
100
90
80
70
60
50
40
30
20
10
0
-10
600
800
1000
1200
1400
1600
1800
2000
2200
Voltage (V)
Figure 4. The dependence of response time on applied electric field.
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Series of Selected Papers from Chun-Tsung Scholars, Peking University (2003)
1 at higher applied electric field.  was measured to be as high as 336cm-1 and 213cm-1 at
26 V/μm for two samples, respectively. And the response time of PR grating-buildup is
depicted in Figure 4. The response time of sample 2 is much shorter than that of sample
1.
Energy level (eV)
III. Discussion.
The improvement of sample 2 in TBC coefficient and response time possibly results
from the increase of trap density caused by presence of Alq3. Figure 5 shows the energy
levels of the compounds of samples. The LUMO (Lowest Unoccupied Molecular Orbital)
level of Alq3 is lower than that of TPD, and Alq3 is a good electron-injecting material.
Therefore, the photo-generated electrons are easier to inject to Alq3’s LUMO level. Alq3
has a little content in composites and
is surrounded by TPD molecules.
0
0
Thus, the electrons can hardly
Alq
TPD
ITO
TNF
continue transporting in Alq3. In
-1
-1
3
other words, the electrons were
-2
-2
LUMO
trapped by quantum dots which are
-3
-3
formed by the difference of LUMO
-4
-4
level between Alq3 and TPD. And
the HOMO (Highest Occupied
-5
-5
HOMO
Molecular Orbital) level of Alq3 is a
-6
-6
little lower than that of TPD, so Alq3
-7
-7
has no big influence on the
-8
-8
hole-transportation. Therefore, the
separation of charge carriers
Figure 5. The energy levels of TNF, TPD, Alq3
becomes easier and quicker. And a and ITO[15,16].
stronger internal space-charge field is
formed in shorter time which results in improvement of PR performance[17-19]. In order to
confirm this point, some more experiments have been made and two of them are worth
mentioning here.
Current-voltage characteristics were measured in the dark and at 633nm He-Ne
laser’s illumination, respectively. The photocurrent was given by subtracting dark current
from light current (jphoto=jlight-jdark)[20]. In sample 1, jphoto=jephoto+jhphoto (where jephoto is the
photoelectron current, jhphoto photohole current and electron-hole combination current is
neglected). According to the analysis above, the quantum dots which are able to trap the
photo-generated electrons will lead to a decrease in the number of free carriers in sample
2, so the photocurrent should be smaller than that across sample 1. Meanwhile, the dark
currents of two samples should have no big difference since there are no photo-generated
electrons and the current is caused by the hole-injection from ITO to HOMO level of
TPD. The result is shown in Figure 6. Considering the error in the measurement, the dark
currents of two samples have no evident difference while it is obviously that the
photocurrent of sample 1 is much larger than that of sample 2. This result is completely
consistent with the theoretical analysis.
The decay time of photocurrent was measured. The experimental set-up is shown in
Figure 7. An optic parameter amplifier ns-laser pulse with width of 10 nm was
illuminated on the sample in the direction of the sample normal while the external electric
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Series of Selected Papers from Chun-Tsung Scholars, Peking University (2003)
field was applied. The dependence of current on time was recorded by an oscillograph.
Because of the different mobility of electrons and holes, there should be a long tail in the
current-time curve. In accordance with the theory above, some photo-generated electrons
are trapped by the quantum dots and the current decay of sample 2 is chiefly determined
by the mobility of holes. Hole mobility is larger than electron mobility in TPD[21].
Therefore, the tail of Sample 2 should be shorter than that of Sample 1. The prediction
agrees with the experimental results, which are shown in Figure 8.
dark current of sample 2
dark current of sample 1
photocurrent of sample 2
photocurrent of sample 1
250
Current(A)
200
150
100
50
0
0
500
1000
1500
2000
2500
Voltage(V)
Figure 6. The dependence of dark current and photocurrent on applied electric field.
Figure 7. The set-up measuring the samples’ response to the ns-laser pulse.
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Series of Selected Papers from Chun-Tsung Scholars, Peking University (2003)
14
Sample 1
Sample 2
Laser pulse signal
Current (a.u.)
12
10
8
6
4
2
0
500
1000
1500
2000
2500
T(nm)
Figure 8.The dependence of current on time decay.
The two experiments above authenticate the view that the remarkable improvements
both in TBC coefficient and response time in sample 2 mainly come from the effect Alq3
having on the increase of density of trapping sites.
V. Conclusion.
Electron-injecting material of Alq3 has been introduced to hole-transporting based PR
composites to improve PR performance. The TBC coefficients and response time of the
samples with and without Alq3 were measured and compared. It is obviously that the
presence of Alq3 leads to remarkable improvements in sample’s photorefractive
characteristics. And the explanation has been made and validated by experiments. In
addition, in the process of measurement, the samples could last for more than two months,
which indicates that the samples have great stabilities. We also found that the
transparence of the sample has noticeable influence on the PR performance. The further
work about the composites is still carrying on.
Acknowledgement.
We acknowledge Hui-Chun Chin and Tsung-Dao Lee Chinese Undergraduate
Research Endowment (CURE), the National Science Foundation of China under grant
Nos. 10104003, 90206003 and 90101027, and the National Key Basic Research Special
Foundation (NKBRSF) under grant No TG1999075207 for the support of the work. And
we are very grateful to the guidance of our instructors Associate Prof. Chen Zhijian and
Prof. Gong Qihuang in out research. We also appreciate the help of Huang Maomao,
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Series of Selected Papers from Chun-Tsung Scholars, Peking University (2003)
Zhang Jie and Li Fushan in the experiment. And we thank Associate Prof. Xun Kun for
providing the equipment for fabrication of OLED films.
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作者简介：
刘轶鸿，女，1982 年 8 月出生于天津市，2000 年从天津南开中学毕业，获第十
六届全国中学生物理竞赛三等奖保送进入北京大学物理学院物理专业学习。2000 学
年获得新生奖学金，2001 年获得 IET 奖学金。
魏群，男，1982 年 6 月出生于山西省长治市，2000 年从长治第二中学毕业，获
第十六届全国中学生物理竞赛三等奖保送进入北京大学物理学院物理专业学习。
2000 学年获得新生奖学金。
在校期间，两人刻苦勤奋，积极参与课内外各种活动。2002 年两人受“ 政基
金”资助，进入物理学院人工微结构与介观物理国家重点实验室，在龚旗煌和陈志
坚两位老师的指导下，进行有机光电材料的研究。
感悟与寄语：
进入大学后，对于学以致用的欲望愈加强烈，而“ 政基金”无疑是一个非常
228
Series of Selected Papers from Chun-Tsung Scholars, Peking University (2003)
好的机会。我们研究的课题是有机光电材料，正是一个理论与实用相结合的课题。
科研同日常的上课是很不同的，很多问题在理论上没有直接对应，大胆合理的猜想
和尝试是必要的。通过一年多的科研工作，我们受到了很有效的锻炼，在思维方式
和动手能力方面都有了很大的提高，而且对于整个科研活动的流程有了切身的体验，
两位老师的耐心指导也给了我们莫大的帮助。参与“ 政基金”对于我们未来的学
习和工作都是一次宝贵的经历。
指导老师简介：
龚旗煌：男，长江特聘教授，博士生导师。 北京大学人工微结构和介观物理国
家重点实验室主任、北京大学现代光学研究所所长、中科院--北京大学联合超快光
科学和激光物理中心常务副主任。1964年8月出生于福建。主要研究方向为：飞秒超
快超强光物理、新型有机非线性光学材料和光子学材料及器件研究。
陈志坚：男，副教授，1971年11月出生于山东。主要在新型有机电致发光和非
线性光学材料方面开展研究工作。
229