2797
J. Phys. Chem. 1995,99, 2797-2802
Detailed Studies on a New Conjugated Photorefractive Polymer Luping Yu," Yong Ming Chen, and Wai Kin Chan Department of Chemistry, University of Chicago, 5735 S. Ellis Avenue, Chicago, Illinois 60637 Received: May 12, 1994; In Final Form: August 1, 1994@
A novel photorefractive polymer containing a conjugated backbone and a second-order nonlinear optical chromophore has been investigated. Detailed studies including photoconductivity, carrier mobility, and the grating formation demonstrated the photorefractive nature of the polymer. A large optical gain of 5.9 cm-' was observed under a zero-external field. Investigations of the grating phase in this polymer revealed features of the refractive index grating and the absorption grating. Several experimental results suggested that an internal field existed in the PR polymer. This internal field assisted charge separation and enhanced the photorefractivity .
Charge Generator
Introduction Photorefractive (PR) materials are multifunctional materials which combine photoconductivity and electrooptic (E-0) activity to manifest a photorefractive effect.'-3 In these materials, the indexes of refraction can be modulated by light via their E - 0 effect (Pockel effect) and a photoinduced space charge field. The unique feature of the photorefractive effect is that it is a nonlocal effect and creates large index changes at relatively low light Some well-known phenomena such as twobeam coupling, dynamic holography, and real time holography result from the photorefractive effect.' Since the discovery of the PR effect in LiNb03: PR materials have attracted considerable interests due to their potential applications in electrooptic (E-0) devices, such as optical interconnects, optical data processing, image amplification, and optical phase conjugation and so Many inorganic materials, such as inorganic single crystals (BaTiO3, KNbO3, L i m o 3 ) and semiconductors (GaAs,CdTe, CdF2) and sillenites (Bi12Si020), have been found photorefractive.1-6 Recently, research progress has been made in the area of the organic materials. Observations of the photorefractive effect in organic single-crystal and polymer composites have been r e p ~ r t e d . ~ - ' ~ Multifunctional polymers with both E - 0 activity and photoconductivity were also reported and demonstrated photorefractivity.' 1-14 Compared with inorganic crystals, organic materials offer advantages of lower dielectric constants, larger E-0 coefficients and are more versatile in choosing charge carrier generators and trappers. They are easily fabricated into thin films and wave-guide forms as required by applications. We recently have developed a new polymer system, containing a second-order nonlinear optical (NLO) chromophore and a conjugated backbone. l5 We expected that the conjugated backbone would absorb photons in the visible region and play a triple role of a charge generator, charge transporter, and charge trapper (due to the defects in the backbone). Detailed studies revealed several unique features for this system. First of all, it is a single-component polymer system that can form uniform amorphous films. Phase separation was not observed. Second, the polymer system has a high level nonlinear optical chromophore density and the electrooptic effect is large. Third, the polymer system has a relative high glass transition temperature, the dipole orientations induced by the external field were fairly stable at room temperature, which enabled us to study the @
Abstract published in Advance ACS Absrracts, September 1, 1994.
0022-365419512099-2797$09.00/0
f
A CgH13
Figure 1. Structure of the photorefractive polymer.
photorefractive effect under a zero-field condition. Finally, the approach to synthesize this polymer was versatile and many new structures have been ~ynthesized.'~ In this paper we report the detailed physical studies for one such polymer. The results provided us with much insight into a series of other systems.
Structural Information of the Polymer The structure of the polymer is shown in Figure 1. The detailed synthesis and structural characterizationsof this polymer were reported in our previous paper.15 This section reviews some of the crucial data. The glass transition temperature of the polymer was found to be 110 "C (with a heating rate of 10 "Clmin) using a TA Instrument DSC-10 and a TGA-50. Gel permeation chromatography (GPC) studies showed that it has a weight-averaged molecular weight of 21 000 with a polydispersity of 2.00 (against polystyrene standards). The refractive indexes of the film, measured using a Rudolph 43603-200E ellipsometer, were 1.787, 1.722, 1.699, and 1.638 at 532, 633, 690, and 1064 nm, respectively. The UV/vis spectrum of the polymer film (Figure 2) indicated the absorption peaks of the charge generator at 560 nm and the NLO chromophore at 380 nm. The absorption coefficient, a, of the polymer film at 690 nm was determined to be 2011cm.
Experimental Section Sample Preparation. To prepare films for physical characterizations, the polymer was dissolved in chloroform. The filtered solution was cast onto I T 0 conductive glass slides and the resultant films were baked overnight at 90 "C under a vacuum. The thicknesses of the films were determined by a Dektak profiler. To determine the photoconductivity, the polymer film on an I T 0 glass slide was coated with gold
0 1995 American Chemical Society
2198 J. Phys. Chem., Vol. 99, No. 9, 1995
Yu et al. "I
1 beam1
300
400
so0
600
t
HLOCK-IN AMP.^
PC
beam2 air
!
700
Wavelength (nm) Figure 2. UV/vis spectrum of the conjugated polymer and (0). Photocurrent measured as a function of the wavelength at laser intensity of 48 mW/cm2with an Arf laser, a He-Ne laser (1= 632 nm) and a diode laser (A = 690 nm) under a field strength of 400 kV/cm.
electrodes. To measure the electrooptic effects, the films on I T 0 glass were corona-poled before coating them with gold electrodes. For the two-beam coupling experiments and additional E-0 measurements the sandwich-like polymer films were prepared using two I T 0 glass slides. The resulting film thickness was about 12 pM,which was confirmed by profile measurements and by measuring the absorbance. To break the centrosymmetry, the polymer samples were poled at 120 "C with a field strength of 167 kV/cm. Electrooptic Measurements. The linear E-0 coefficient 133 was determined utilizing a simple reflection technique.I6 A value of 4 p m N (12 pm) was observed at a wavelength of 690 nm.15 It is known that a figure of merit for photorefractive materials is n3r/c,where n is the index of refraction, E is the dc dielectric constant, and r is an E-0 coefficient.' For this polymer, a dielectric constant of 4.85 was determined by measuring the capacitance of the gold-polymer-IT0 structure. It was then estimated that the limited figure of merit for this polymer was ca 4.2 pmN. Photoconductivity Studies. The photoconductivity was studied by measuring the voltage drop resulting from the photocurrent through the polymer film coated with the gold electrode and a 10 kQ resistor." A diode laser (Power Technology, 690 nm) with an intensity of 187 mW/cm2 was used as the light source. The charge carrier's mobility was characterized by a conventional time-of-flight (TOF) method. 12*18 The polymer films (about 1.5 pm) were cast onto IT0 glass slides. Semitransparent gold layers were thermally evaporated onto the polymer surface under a high vacuum. A 337 nm nitrogen laser (Laser Science, Inc., Model 337, pulse width, 3 ns) was shown onto the gold electrode and a sheet of charge carriers was generated near the electrode. According to eq 1, the charge carriers' mobility was determined from the transient time, 't, required for the carrier sheet to exit the counterelectrode: z =L / , E
where L is the thickness of the sample and E is the field strength. Two-Beam Coupling. The experimental arrangement for the two-beam coupling is shown in Figure 3. To utilize the largest component of the electrooptic tensor, 133, for poled PR polymer film, the sample was tilted about 30" as indicated in Figure 3a. A diode laser (690 nm, s polarized) was used as the laser source which was split into two beams with equal intensities (224 mW/
Figure 3. (a, top) Diagram of the experimental arrangement for the
two-coupling experiments. (b) Grating formation for the two-beam coupling. K (4n sin 8/L)is the grating wave vector; 8 is the titled angle of the sample; 81 and 82 are the incident angles of the two beams with respect to the normal of the film plane. cm2 for each). The incident angles in the air were about 11.6" and 48.4" with respect to the normal of the sample. The resulting grating spacing (A) was about 1 pm unless otherwise stated. From the two-beam coupling experiments, an important parameter: optical gain (r)can be deduced. According to the photorefractive theory developed by Kukhtarev et al.,19 the optical gain can be calculated:
where a = A&(r)/I,,Is is the intensity of the transmitted signal, AI,, the intensity change of the signal beam before and after the two beam coupled to each other, $ , (=Is(0)/Ip(O)) is the intensity ratio of the signal beam and the pump beam before entering the polymer, r is the optical path length. Since the polymer absorbs at the working laser wavelength, the written grating is a combination of an index grating and an absorption grating:20,22
n(r) = no + An cos(K*r - 4p) a ( r ) = a,
+ A a cos(K*r - 4,)
(3)
where no is the refractive index, is the field amplitude absorption coefficient, r is a vector in space, K is the grating wave vector, and @p and @a are the phase shifts of the index and absorption gratings relative to the interference pattern. It is known that diffraction by a holographic grating can be analyzed by using the general coupled wave theory.21 For a low diffraction efficiency and ignoring the square terms of the diffraction amplitudes, the output powers of two beams are given by20.22
A New Conjugated Photorefractive Polymer
J. Phys. Chem., Vol. 99, No. 9, 1995 2799
r"' = (l12)no(~o)1~2 cos e,(T,)2[(E,2T,E,E,
COS
q c o s e,)lI2(u
COS
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. I
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-
where E1 and E2 are the initial field amplitudes, and 81 and 62 are defined in Figure 3b. The diffraction amplitudes of P and A are defined as
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:
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,
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P = JtdAn/A(cos 8, cos €J2)'l2, A = ~ A ~ / ~ ( e, c oCOS s e2)1/2( 5 ) T , = exp(-qd/2 cos e,),
(6) If the written gratings are moved, the diffraction of the two beams by the gratings will be changed and so will the transmitted beams' intensity. Since the translation time of the sample was much less than the grating growth or decay time, the sum and difference of the two transmitted powers as a function of the grating displacement x can be expressed as22
+ 2Jtx cos e/A)] I'-'(x) = Z0[0.08 + 2 P sin(@, + 2zx cos &A)]
I'+'(X)
5 10'
0
T2 = exp(-qdI2 cos e,)
= Z0[1.21 - 2'4 cos(@,
(7)
where A is the grating space. The values of fi+) and fi-) as a function of the displacement of the grating space were measured by using lock-in amplifiers. The results were fitted by using the nonlinear least-squares method to obtain P, A, 4p,and q5,, from which the An and ha can be calculated.
Results and Discussion There are four processes involved in the photorefractive effect, namely, charge generation, charge transporting, charge trapping, and grating formation. In the present polymer system, the role of charge trapping was played by the defects existing in the polymer. Detailed studies about the trapping are very difficult before the exact nature of the trapping centers is known. However, from our measurements, we can indeed deduce the concentration of the effective charge trapping centers. Charge Generation. When a polymer absorbs a photon, a bound electron-hole pair is formed. This excited state can decay in several possible mechanisms.18 One possibility is that the pair further dissociates into free charge carriers under the assistance of thermal motion or an external field. Therefore, the dissociation has a strong dependence on the external field strength. For this polymer, the electric field dependence of the photocurrent is shown in the inset of Figure 4 . A typical photocurrent response time of ca. 100 ms was estimated. A photoconductivity of 1.8 x lo-" C2-l cm-' was obtained under a field strength of 1500 kV/cm and a laser intensity of 3 11 mW/ cm2. This value was relatively large and comparable to wellknown photoconductive polymers.18 The dark current was very small and difficult to measure (< S ' - l cm-') because the polymer is a very good insulator. The photocurrent was also measured as a function of the several wavelengths of excitation at the same laser intensity (48 mW/cm2) and electric field (400 kV/cm, Figure 3). The spectral dependence of the photocurrent had a shape similar to the absorption spectrum of the conjugated PR polymer in this limited wavelength range. This seemed to indicate that the optical excitation of the conjugated backbone was the origin of the photocharge generation. However, more
1 lo8
1.5
Id
2 lo8
Electric field (V/m) Figure 4. Field dependence of the quantum yield for the photocharge carrier generation. The solid line is the fitting curve based on the Onsager model. The inset is the photocurrent as a function of the applied electric field at wavelength 690 nm.
extensive measurements in a wider spectral range are in progress to further confirm this conclusion. From the photocurrent results, the quantum yield of the photogeneration of charge carriers can be evaluated. The electric field dependence of the quantum yield is illustrated in Figure 4. It was known that in many photoconductive polymer systems, the field dependence of the quantum yield of photocarrier generation can be rationalized by Onsager's model of the geminate-pair d i s s ~ c i a t i o n .The ~ ~ essential assumption of Onsager's model was that the photogeneration of free caniers involves the dissociation of a bound electron-hole pair. The quantum efficiency q(E,T) can be described as18.23
vEZ? = voJP(r,B,E,T) G(r,@ d3r
(8)
where q o is the yield of the thermalized bound pairs and is field independent, P(r,B,E,T)is the probability of dissociation of the thermalized pairs as a function of the initial separation, field and temperature; G(r,B)is the initial pair distribution. Assume that the G(r,8) is isotropic and that all pairs have the same thermalization distance ro, the quantum efficiency q can be approximately expressed as23
where 5 = rJr0, 6 = eErdkT, e is the elementary charge, k is the Boltzmann constant, ro = e2/4m@kTis the Onsager distance, fo(f) = 1 - exp(-Q and the recursion relationfn+l(
