Improvement on the photorefractive performance of a monolithic molecular material by introducing electron traps

Article abstract of DOI:10.1063/1.1784522

The ehancement in photorefractive performance was observed by introducing tris(8-hydroxyquinoline) aluminum (Alq₃) as electron traps. A small amount of Alq₃ formed quantum dots in (9-ethyl-9H-carbazol-3- ethylmetjylent)(4-nitrophenyl)-amine (ECYENPA), serving as electron traps. Alq₃ prevented the the electron-hoe recombination and accelerate the electron-hole separation, benefiting the buildup of internal space-charge electric field. The analysis show that for a sample doped with 2 wt% of Alq ₃ produced large two-beam coupling coefficient of 424 cm^-1 in contrast to a value of 232 cm^-1 for a sample without Alq ₃.

Full text of DOI:10.1063/1.1784522

Improvement on the photorefractive performance of a monolithic molecular material by
introducing electron traps
Jie Zhang, Zhijian Chen, Yihong Liu, Maomao Huang, Qun Wei, and Qihuang Gong
Citation: Applied Physics Letters 85, 1323 (2004); doi: 10.1063/1.1784522
View online: http://dx.doi.org/10.1063/1.1784522
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129.12.234.185 On: Sun, 23 Nov 2014 21:50:49

APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 8
23 AUGUST 2004
Improvement on the photorefractive performance of a monolithic molecular
material by introducing electron traps
a)
b)
Jie Zhang, Zhijian Chen, Yihong Liu, Maomao Huang, Qun Wei, and Qihuang Gong
State Key Laboratory for Mesoscopic Physics and Department of Physics, Peking University,
Beijing 100871, People’s Republic of China
(
Received 1 March 2004; accepted 21 June 2004)
The electron-injecting material, tris(8-hydroxyquinoline) aluminum ͑Alq ͒, was introduced into
3
photorefractive
(PR)
samples
containing
the
(9-ethyl-9H-carbazol-3-ylmethylent)-
(
4-nitrophenyl)-amine (ECYENPA) molecule, which has been proved to possess the fine functions
of both electro-optic effect and hole transportation. A small amount of Alq forms quantum dots in
3
ECYENPA, serving as electron traps and leading to a great enhancement of the PR effect. A large
two-beam coupling coefficient of 424 cm−1 (at 44.6 V/␮m) was observed in a sample doped with
−
1
2
wt% of Alq , in contrast to a value of 232 cm for a sample without Alq . Moreover, the
3
3
measured response time was one order of magnitude shorter than that of the undoped sample.
2004 American Institute of Physics. [DOI: 10.1063/1.1784522]
©
Photorefractive (PR) materials possess great potentia1l
The all-functional PR small molecule ECYENPA was
applications in optical information memory and processing.
synthesized via the reaction of N-ethyl-3-carbazole-
2
10–12
Since the report on PR polymers in 1991, intense research
carboxaldehyde and P-nitroaniline in pyridine.
The glass
transition temperature ͑T ͒ of molecule was −5°C, as mea-
on organic PR materials has contributed to a significant im-
provement in the PR performance. The PR effect in organic
materials comprises four processes: Charge carriers are pho-
togenerated by a spatially modulated light intensity and sepa-
rated through drift by an electric field and diffusion by a
gradient of carrier density. Then, charges are trapped at new
sites by impurities, defects, or different energetic levels, pro-
ducing a nonuniform space-charge distribution that results in
g
sured by differential scanning calorimetry (DSC2010). Alq3
was synthesized via the reaction of 8-hydroxyquinoline and
aluminium chloride. Alq is a good electron-injecting and
3
transporting material, which is often used in the technology
1
3–15
of organic light-emitting devices (OLEDs).
The chemi-
cal structures of these compound are shown in Fig. 1.
ECYENPA doped with 0,1,2, and 3 wt% of Alq were
3
dissolved in chloroform for uniform mixing. After evaporat-
ing the solvent, the four residuals were respectively sand-
wiched between two indium-tin-oxide (ITO) glass plates at a
temperature of 120°C, and then cooled rapidly. The thick-
ness of the samples was controlled at 80 ␮m by using spac-
ers. The films were uniform and transparent, as shown by
x-ray scattering and grazing-incidence small-angle x-ray
scattering measurement. No scattering peaks were detected,
indicating that no observable crytalline domains are present
in the samples.
an internal space-charge electric field ͑E ͒. Finally a refrac-
sc
tive index grating is formed through an electro-optical (EO)
effect. These processes require that the PR sample possesses
both an EO effect and a charge transporting property. Ac-
cording to this requirement, many approaches have been
adopted to prepare organic PR samples, such as composite
materials, full-functional polymers, monolithic small mol-
1
–6
ecules, etc. More recently, full-functional PR monolithic
molecules have attracted much attention due to the merits of
no phase separation, high chromophore concentration, low
The PR performances of the four samples were investi-
gated by TBC experiments employing a typical tilted
5
,6
Tg, and ease of film fabrication. In this letter, an all-
functional PR small molecule (9-ethyl-9H-carbazol-3-
ylmethylent)-(4-nitrophenyl)-amine (ECYENPA) was syn-
thesized. It has been reported that introducing traps to PR
1
6,17
configuration.
Two mutually coherent p-polarized
He-Ne laser beams at a wavelength of 632.8 nm and powers
2
of 0.22 and 0.33 W/cm were overlapped in the sample film.
7
–9
The tilted angle was 35° and the angle between the two
incident beams was 20° in air. The dependence of the TBC
materials can improve the PR performance. Therefore, the
electron-injecting material tris(8-hydroxyquinoline) alumi-
num ͑Alq ͒ was deliberately introduced to the ECYENPA.
3
Alq acts as an electron trap that prevents the electron-hole
3
recombination, thus accelerating electron-hole separation
and benefiting the buildup of the E . As a result, the two-
sc
−
1
beam coupling (TBC) coefficient increases up to 424 cm at
an applied electric field of 44.6 V/␮m and the response time
becomes one order of magnitude shorter than that of the PR
sample without Alq3.
a)
Author to whom correspondence should be addressed; electronic mail:
zjchen@pku.edu.cn
FIG. 1. Chemical structures of the components of the PR samples: (9-ethyl-
b)
Author to whom correspondence should be addressed; electronic mail:
9H-carbazol-3-ylmethylene)-(4-nitro-phenyl)-amine
tris(8-hydroxyquinoline) aluminum ͑Alq3͒.
(ECYENPA)
and
qhgong@pku.edu.cn
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003-6951/2004/85(8)/1323/3/$20.00 1323 © 2004 American Institute of Physics
29.12.234.185 On: Sun, 23 Nov 2014 21:50:49
0
1

1324
Appl. Phys. Lett., Vol. 85, No. 8, 23 August 2004
Zhang et al.
FIG. 2. Dependence of the TBC coefficients on the applied electric field for
FIG. 3. Response time for different samples vs concentration of Alq3.
samples with different concentrations of Alq . The solid lines represent the-
3
oretical fits of the TBC coefficients. Inset: TBC coefficients at E0
=
39.4 V/␮m vs concentration of Alq . The solid line is a guide for the eye.
3
optical gap ͑E ͒ from the E
. E was measured from the
g
g
HOMO
1
8
optical absorption curve, shown in the inset of Fig. 4. Alq3
is a good electron-injecting material, with the LUMO energy
level ͑−3.1 eV͒ lower than that of ECYENPA by 0.6 eV.
coefficient on the applied electric field for the four samples is
illustrated in Fig. 2. Solid lines are the theoretical fits accord-
ing to the equation,
1
7
Therefore, electrons are easily injected into the Alq from the
3
ECYENPA. Figure 5 gives the HOMO and LUMO energy
2
0
2
1/2
2
0
E
+ E
G
D
levels of ECYENPA, Alq , and ITO, and the proposed pro-
3
⌫
= CEqE0
ͩ
ͪ
2
2
cesses of charge-carrier transportation and trapping. Charge
carriers (electrons and holes) are photogenerated in the
ECYENPA. Electrons are transported from the LUMO of
͑
E + E ͒ + E
D
q
0G
2
2
D
E
+ E + E E
0G
D
q
−
1
ϫsin
ͫ
tan
ͩ
ͪ
ͬ
,
E E
0
ECYENPA to that of Alq , and then trapped in the LUMO of
G
q
3
Alq , due to the difference of LUMO energy levels. More-
3
where C is a constant, E the external electric field, E the
component of the external electric field along the grating
wave vector, ED the drift field, and E the limiting space-
charge field. The inset shows the dependence of the TBC
coefficient on the concentration of Alq at the same applied
electric field of 39.4 V/␮m. The TBC coefficient of the
sample containing 2 wt%Alq was much higher than that of
other samples in the whole range of the applied electric field,
reaching a maximum value of 424 cm at an applied electric
field of 44.6 V/␮m. The response time for the PR grating
erasure was also measured. When the stable coupling signal
was achieved, one of the writing beams was blocked, so the
other beam would read and erase the internal grating simul-
taneously. This diffracted reading beam was monitored. The
measured experimental curves were appropriately fitted by
using a double exponential form. Figure 3 shows the fast
0
0G
over, Alq3 molecules are surrounded by ECYENPA for
minute content, thus electrons can hardly detrap and continue
q
transporting in the Alq . Contrary to electrons, holes are
3
transported along the HOMO of ECYENPA and cannot be
trapped by Alq , because the HOMO energy of Alq is lower
3
3
3
7,21,22
than that of ECYENPA.
Therefore, electrons are
3
trapped and holes are transported normally, so photogener-
ated charge carriers are separated much easier and more rap-
−
1
idly, and the formation of E gets stronger and quicker. This
sc
indicates that a proper percentage of Alq may enhance the
3
TBC coefficient and shorten the PR response time. When the
concentration of Alq is further increased, electrons can hop
3
response time as a function of the concentrationof Alq . The
3
response time of the doped samples become one order of
magnitude shorter than that of the undoped sample. The fol-
lowing model is proposed in order to explain this improve-
ment on PR performance.
Electrons move away, along the lowest unoccupied mo-
lecular orbitals (LUMO), while holes tend to transfer to the
highest occupied molecular orbitals (HOMO), of the mol-
ecules at the applied electric field. The energy level of the
HOMO ͑EHOMO͒ of the monolithic molecule ECYENPA was
estimated to be −5.2 eV under the vacuum level by cyclic
1
8,19
voltammetry (CV),
using platinum disks as the working
anode and counterelectrode, and a saturated calomel elec-
trode (SCE) as the reference. Figure 4 shows the CV spec-
trum of ECYENPA. The energy level of the LUMO ͑E
͒
LUMO
FIG. 4. Voltammograms of the monolithic molecule ECYENPA. Inset: UV-
visible absorption spectra of ECYENPA.
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was determined to be about −2.5 eV by the subtraction of the
129.12.234.185 On: Sun, 23 Nov 2014 21:50:49

Appl. Phys. Lett., Vol. 85, No. 8, 23 August 2004
Zhang et al.
1325
National Key Basic Research Special Foundation (NKBRSF)
Grant No. TG1999075207) supported this work. The au-
(
thors are grateful to Professor D. S. Xu and F. Xu of the
college of Chemistry and Molecule Engineering for their
kind assistance.
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was achieved by introducing Alq as electron traps. A large
3
−
1
two-beam coupling coefficient of 424 cm at an electric
16
17
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−
1
2
wt% of Alq , as compared to 232 cm measured in un-
3
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1