Photocarrier Generation in Smectic Phenylnaphthalene Liquid

The intrinsic hole photogeneration process in different phases of a smectic liquid crystalline photoconductor,. 2-(4′-octylphenyl)-6-dodecyloxynapht...
0 downloads 0 Views 69KB Size
Download PDF
J. Phys. Chem. B 1999, 103, 7429-7434

7429

Photocarrier Generation in Smectic Phenylnaphthalene Liquid Crystalline Photoconductor Hong Zhang and Junichi Hanna* Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta Midori-ku, Yokohama 226-8503, Japan ReceiVed: March 24, 1999; In Final Form: June 10, 1999

The intrinsic hole photogeneration process in different phases of a smectic liquid crystalline photoconductor, 2-(4′-octylphenyl)-6-dodecyloxynaphthalene (8-PNP-O12) was investigated as a function of electric field and light intensity by using the time-of-flight transient photocurrent technique. The collected photogeneration charges were found to be approximately proportional to the square of the light intensity irrespective of the phases. A model assuming Onsager type of carrier generation involving double-exciton interaction process, gave a good agreement with the obtained experimental results. The molecular ordering is found to promote the dissociation of the electron-hole pairs. The thermalization distance, r0, was very large and comparable with those in molecular crystals such as anthracene, indicating that the carrier generation process in smectic mesophases is analogous to that in organic molecular crystals rather than that in organic liquids.

I. Introduction A photoexcited molecule relaxes its excess energy through either radiative or nonradiative processes. In condensed organic materials, nonradiative processes often induce the photogeneration of free carriers under the electric field. Owing to a large size of a molecule as the structural element of the condensed materials and its weak van der Waals interaction, this ionization process of molecules is dominated by autoionizing transitions rather than band-to-band transitions. Therefore, the intrinsic photogeneration quantum yield for free carriers is much smaller than unity and depends strongly on the external electric field. It is generally accepted that photocarrier generation in condensed molecular systems occurs in two steps: the formation of a Coulombically bound electron-hole pair, which originates from photoexcited molecules in the first step, and dissociation of this bound ion pair into a free electron and hole with the aid of the electric field in a second step.1,2 The dissociation and recombination of ionic pairs is basically described by the Onsager theory, which originally comes from the solution of an isotropic diffusion, i.e., three-dimension Brownian motion, equation of ions in a liquid in the presence of Coulomb attraction and an applied electric field.3,4 In the Onsager model, the photocarrier generation is characterized by the generation efficiency of the initial bounded ion pairs, Φ0, and the thermalization length of the bounded pair, r0 at a given electric field and temperature.5 In the well-studied conventional organic photoconductive materials including molecular crystals and amorphous organic materials such as conjugated polymers, molecularly doped polymers and vacuum-evaporated thin films, intrinsic photogeneration processes have been testified to accord with the traditional Onsager model6-8 or the modified Onsager model9,10 put forward by Noolandi and Hong in 1979.11 The difference of these two photogeneration processes is whether the electronhole pairs generate in the precursor state or in the first excited singlet state.12 In the former, the photogeneration depends on the excitation wavelength, while in the latter, the photogenera* Corresponding author. E-mail: [email protected]. Fax: (0081)045-924-5175.

tion is wavelength independent. Besides the chemical properties of the molecular material itself, parameters Φ0 and r0 also have some relationship with molecular architecture.1 For example, in single molecular crystals, although r0 is large, which can be up to 10 nm, Φ0 is usually very low on the order of 10-3 to 10-5 because of the fast energy relaxation for excitons to the states below the geminate carrier pairs. Thus, the total photocarrier generation efficiency can hardly exceed this range.6,13 On the other hand, amorphous organic solids such as molecularly doped polymers (MDP) with photoconductive small molecules, and microcrystalline or amorphous vacuum-evaporated thin films, have a smaller r0 in the range of 2-5 nm, but often a larger Φ0 up to the order of magnitude of 10-1, so a high photocarrier generation efficiency can be obtained at a high electric field, and therefore these materials have been used for photogeneration materials in photoelectric devices.14-16 Recently, it was discovered that discotic and smectic type of liquid crystals,17-20 i.e., in triphenylene derivatives and 2-phenyl benzothiazole and naphthalene derivatives, exhibit a photoconductive behavior superior to that of the conventional amorphous materials. They are characterized by a very large mobility of >10-3 cm2 V s independent of electric field and temperature. These materials are promising as a new class of electronic materials in practical applications, because their liquid-like nature enables us to prepare uniform large-area materials, and the crystal-like one provides us with superior carrier transport properties as described above. The characterization and understanding of photoelectrical properties in these liquid crystalline materials provides us with a new insight into understanding those in amorphous or disordered materials including MDP. This is because, in particular, molecular alignments in the calamitic (rodlike) liquid crystals have a broad variety of molecular architectures from one-dimensional molecular orientation without positional order, i.e., “nematic” to three-dimensional crystal-like molecular orientation, i.e., “plastic crystal”. These studies have just begun because of the very recent discovery of these materials. In this work, we have characterized the intrinsic photocarrier generation in the different phases of 2-(4′-octylpheny)-6dodecyloxynaphthalene (8-PNP-O12) (Figure 1), which is a

10.1021/jp9910082 CCC: $18.00 © 1999 American Chemical Society Published on Web 08/13/1999

7430 J. Phys. Chem. B, Vol. 103, No. 35, 1999

Zhang and Hanna

Figure 1. Chemical structure of 2-(4′-octylpheny)-6-dodecyloxynaphthalene (8-PNP-O12).

typical smectic liquid crystalline photoconductor, on the basis of the collected charges in time-of-flight measurements at different electric fields and light intensities. We will describe the general view of the photocarrier generation in its different phases and discuss the effect molecular alignment of the present material system on the photocarrier generation. II. Experiment The synthesis of the calamitic liquid crystal, 8-PNP-O12, which exhibits the phase transition from crystal to SmB at 79 °C, from SmB to SmA at 101 °C, and from SmA to isotropic phase at 120 °C, has been described elsewhere.19,20 The liquid crystal cells were prepared with two glass plates which evaporated semitransparent Al electrode with about 30% transmittance, which were used to establish blocking contact in order to avoid the carrier injection from the electrodes. 8-PNPO12 was capillary-filled into the cells in isotropic phase. The thickness of the cells was adjusted by silica or polymer film spacers and was controlled from 9-15 µm. The real thickness was determined by measuring the capacitances of cells. Usually the error is not larger than 10%. To reduce the influence of O2 absorbed in the cells, the liquid crystal layer was melted on a hot stage under vacuum and cooled under an Ar atmosphere when prepared. The transient photocurrent was measured by a conventional time-of-flight setup.21,22 In this method, the sample, in the presence of electric field, was exposed to a short light pulse from an N2-laser (λ ) 337 nm, pulse width ) 600 ps), which was sufficiently short compared with the transit time of carriers. In this measurement, the illuminated electrode was positively biased for the present limited study of the hole generation process. The one-carrier condition was well established due to the short penetration depth less than 1 µm at 337 nm illumination. To establish the uniform electric field across the liquid crystal layer, the light intensity was limited so as to ensure that the total photogenerated charges were less than one-tenth of the geometrical capacitance, CV of the samples, where C is the capacitance of sample cell and V the applied voltage. The energy of incident laser pulses was adjusted by the ND filter up to about 13 µJ/pulse, which met this requirement. After every measurement, the sample was shorted to discharge for several minutes in order to exclude the influence of remaining space charges. III. Result and Discussion Sample Thickness Dependence. A typical transient photocurrent of 8-PNP-O12, for example, in the SmB phase, is shown in Figure 2. The very beautiful nondispersive transient photocurrents of 8-PNP-O12 indicate that the dispersion of hopping times during the transit is very small, in other words, the density of states below the hopping level is low and distributed in a narrow energetic range. Before counting photogenerated charges from the transient photocurrent, we have to evaluate the loss of charges during the transit, which can be attributed to deep traps in the bulk. The collected charge, Qc, can be estimated by integrating a transient photocurrent with time. The collected charges of the

Figure 2. Typical transient photocurrent signals for positive carriers in SmB phase (90 °C) of 8-PNP-O12. Sample thickness was 13 µm; light intensity was 3.4 µJ/pulse of 337 nm; vacuum-evaporated Al was used as electrodes.

Figure 3. Cell thickness dependence of collected charges in different phases of 8-PNP-O12. Solid circles indicate SmB phase at the electric filed of 7 × 104 V/cm, open circles and squares indicate SmA and isotropic phase at the electric field of 1 × 105 V/cm, respectively.

samples with different thickness are shown in Figure 3. With conventional amorphous photoconductors, where deep traps in the bulk dominate the carrier lifetime, the collected charge Qc is often limited by the carrier range. The Qc depends on the thickness of cells as described by the Hecht rule23 shown in eq 1,

µτE -L Qc(E,L) ) Q0 1 - exp L µτE

[

( )]

(1)

where E is a given applied electric field, L the thickness of sample, µ a carrier mobility, and τ a carrier lifetime. However, as shown in Figure 3, the collected charge Qc was almost independent of cell thickness, indicating that the transient photocurrent was emission-limited, i.e., the range, µτE, of carriers is long enough so that all the photogenerated carriers are transported to the counter electrode and contribute to the transient photocurrent, where the µτ was estimated to be larger than 10-7 cm2/V by comparing the data of Qc vs L with eq 1. This was true irrespective of different phases of 8-PNP-O12. This indicates that the collected charge, Qc, is a good basis to estimate photogenerated carriers from the transient photocurrents. All of our present experiments described in this paper meet this requirement. Thus, the yield of photocarrier generation can be calculated by eq 2.8,22

Qe )

∫0TITOF(t)dt

hν

eI0Ttr

)

hνQc eI0Ttr

(2)

Photocarrier Generation in Smectic Phenylnaphthalene

J. Phys. Chem. B, Vol. 103, No. 35, 1999 7431

Figure 4. Light intensity dependence of collected charges: (a) SmB phase; (b) SmA phase; (c) isotropic phase. One intensity unit corresponds to a photon flux of 2 × 1013 photons/pulse.

where ITOF(t) is the transient photocurrent, Qc the collected charges, I0 the laser energy per pulse, Ttr the transmittance of Al electrode (30%), ν the frequency of used laser light; e the unit charge, h Planck’s constant, and T the end time of measurement at which the photocurrent becomes zero. Light Intensity Dependence. The dependence of the photocarrier generation on the light intensity can provide us important information concerning the formation and relaxation of exciton. Figure 4 shows the collected charges as a function of the light intensity in different phases of 8-PNP-O12. The collected charge was found to be proportional to the square of the light intensity, see eq 3, irrespective of the phases. This obviously differs from the usual linear relation of the Onsager model and from the square root dependence based on the bimolecular recombination of carriers.24,25

Qc ) AI0β; A ) a constant, β ≈ 2.0

(3)

This dependence of photogenerated charge upon light intensity in 8-PNP-O12 perhaps indicates that the exciton or photon interactions, such as a two-photon process or a two-exciton process, was involved in the photocarrier generation processes.2,26 In the steady-state conditions without the bimolecular recombination, the photogeneration rate of carriers governed by kinetically bimolecular mechanisms can be expressed as2

dn ) Rss[S]2 + σs[S]I + k2I2 dt ) Rssk2τs2I2 + σskτsI2 + k2I2

(4)

≈ Rssk2τs2I2 ) γssfφ(E)

(5)

Where Rss is the bimolecular carrier-generation rate constant by exciton-exciton fusion, σs the cross section for the photoionization of the exciton, I the light intensity, k2 the two-photon absorption coefficient for carrier generation, [S] the concentration for the short-lived singlet excitons, τs the lifetime of singlet exciton, and k is absorption coefficient. Here, considering the short-lived excitons and the low faction of excited molecules, the change of τs with the light intensity was neglected. The two-photon process often occurs in a low absorbance region at very high light intensities. According to the absorption spectrum of 8-PNP-O12, as shown in Figure 5, the light absorption and the photon energy at 337 nm are high enough to lead to singlet exciton formation. Thus, the third item of eq 4, i.e., the two-photon process, can be ruled out, i.e., k2 ) 0. The direct photoionization of the exciton, i.e., the second term of the eq 4, can be neglected also due to the large singlet exciton

Figure 5. Light absorption spectrum of 8-PNP-O12. The thickness of the cell is 2 µm: (a) SmA (the solid line) and SmB (the dashed line) phase; (b) isotropic phase. Dotted line represents two possible inhomogeneously broadened singlet-singlet excited transition.

absorption coefficient of k ≈ 104 cm-1 (.1 cm-1) at 337 nm. On the other hand, the double-exciton collision process is usually observed for highly absorbed low-energy light at fairly high light intensity.2 Here, for this liquid crystal system, we assume that there are no deep exciton traps and excitons can be mobile enough.2,27-29 In our experiment, due to the high light intensity of about 1 × 1012 ∼1.4 × 1013 photons/cm2 per pulse and a large exciton density up to 1018 excitons/cm3 per pulse can be produced in the excited region of the sample near the electrode surface. In addition to the large singlet exciton absorption coefficient, it is reasonable to consider that the singlet exciton-singlet exciton interaction, i.e., the double-exciton fusion, dominates the photogeneration of electron-hole pairs in 8-PNP-O12 for the present experimental conditions. Here, eq 4 can be approximated by eq 5, where γss is the rate constant for singlet-exciton fusion, f is the probability that the collision produces a pair of carriers, and Φ(E) is the field-dependent probability of the free carriers generation, which will be discussed later. In fact, a small negative deviation of β from 2 was found as shown Figure 4. We attribute it to a space charge which enhances the recombination of carriers within the penetration depth. This can no longer be neglected because of the high carrier concentration at the high light intensity or high electric field.30 It should be pointed out that at very high electric fields and at a very weak light intensity, the collected charge tended to be a linear function of the light intensity, suggesting that the exciton-exciton interaction disappears. In fact, in the SmB phase, the small negative deviation from quadratic light intensity dependence may be explained by considering that the density of excitons at a low light intensity drops into a regime where the single-exciton and exciton-exciton interaction process coexist. Electric Field Dependence. As for the photocarrier generation, the Onsager theory has been successfully applied to molecular crystals,30,31 molecularly doped polymers,32 and amorphous chalcogenide galsses.5 According to the Onsager model, at first, electrons in the HOMO are promoted to excited states by the incident photons and dissipate their excess kinetic energy or are thermalized by phonon emission. The bound ion pairs which are called geminate pairs are separated to a certain distance r0 and produced with a primary quantum yield Φ0. This electron-hole pair can either recombine or dissociate with probability f in the course of a temperature and field assisted diffusion process. The photogeneration yield based on the Onsager model is given by5

7432 J. Phys. Chem. B, Vol. 103, No. 35, 1999

Zhang and Hanna

∫Ωf(r0,E,T) g(r0,θ) dΩ

Φ ) Φ0

(6)

where Φ0 represents the primary quantum yield of the ionization process, i.e., the number of charge pairs per absorbed photon. The distribution function g(r,θ) is the initial distribution of the thermalized charge pair configurations. In the usual case, it is the simplest to assumed that g(r,θ) is the δ function

g(r,θ) ) (4πr0)-1δ(r - r0)

(7)

then ∞

Am

∞

∑ ∑ m)0m!n)0

Φ(r0,E) ) Φ0B-1 exp(-A)

m+n

[1 - exp(-B)

Bl/l!] ∑ t)0

Figure 6. Electric field dependence of photohole generation yield: (a) SmB phase, r0 ) 8 nm; (b) SmA phase, r0 ) 3.5 nm; (c) isotropic phase, r0 ) 2 nm.

(8)

where A ) rc/r0, rc ) e2/4π0kT, which is the Coulomb capture radius, and B ) eEr0/kT. Figure 6 shows the electric field dependence of the photogeneration yield at different light intensities and the results simulated by the Onsager model for different phases of 8-PNPO12. While the Onsager formulation is for isotropic materials with dielectric constant , the mesophases in 8-PNP-O12 are actually anisotropic because of smectic molecular alignment; that is, it is the quasi-two-dimensional system. Thus, for adaptation of the Onsager model, we used an average value of the dielectric constant, which is selected to be the mean value of the principal components of the dielectric tensor, resembling the method used in molecular crystals.6 From the capacitance measurements, the approximate dielectric constant perpendicular and parallel to the smectic layer at a low frequency was obtained, and the mean value was estimated to be about 2.3 ( 0.10, 2.3 ( 0.10, and 2.4 ( 0.10, for SmB, SmA, and isotropic phase, respectively. As shown in Figure 6, the experimental results for these phases of 8-PNP-O12 were in a good agreement with the calculated curves, whose parameters are summarized in Table 1. As shown in the Figure 7, for the same liquid crystal phase, the fitting result of thermalization length r0 is independent of the number of incident photons, but the Φ0 increases linearly. If normalizing the photoconductivity data to the square of light intensity (the inset of Figure 7), the data is also independent of the number of incident photons, where the pulse photocurrent is proportional to the Qc. This is due to the exciton-exciton interaction for generating the geminate ion pairs. Therefore, it is reasonable that there is no effect of the light intensity on the dissociation of the geminate ion pair under electric yield described by the Onsager model with eq 8, in other words, the thermalization length, r0, was constant. However, although the molecular alignment is not so perfect as in molecular crystal and subject to thermally fluctuation, due to the fusion of two excitons, a dissipation of a large excess energy is expected to result in a large thermalization length r0 comparable to that of molecular crystals such as anthracene.13 In addition, it should be noted that both the thermalization distance r0 and the absolute photogeneration yield increase according to molecular ordering from the isotropic phase to the smectic B phase in the condition of our experiments. These results suggest that the molecular orientation or ordering enhances the dissociation of the electron-hole pairs. At the same time, concerning Φ0, though it is not a constant with the influence of exciton-exciton interaction, suggesting that it is not an essential physical quantity of materials, the relative value can be compared in the different phases by normalizing Φ0 in

Figure 7. Field dependence of photohole generation yield with different light intensity (r0 ) 8 nm): (a) 1.4 × 1012 photons per pulse; (b) 2.39 × 1012 photons per pulse; (c) 6.1 × 1012 photons per pulse. Inset: the photoconductivity data normalized to the square of light intensity.

TABLE 1: Dielectric Constant and Parameters of Onsager Model in Different Phases of 8-PNP-O12  r0 (nm) Φ0

SmB phase

SmA phase

isotropic phase

2.3 ( 0.10 8 2.2 × 10-3

2.3 ( 0.10 3.5 9 × 10-3

2.4 ( 0.10 2 4 × 10-2

Figure 6 to the same light intensity. For example, normalizing to the number of photons illuminating the SmB phase, the Φ0 values in the SmB, SmA, and isotropic phases are 2.2 × 10-3, 9 × 10-3, and 4 × 10-2, respectively, suggesting that the more ordering phase of 8-PNP-O12 leads to the lower primary efficiency of electron-hole pairs. Just as observed for the other organic photoconductor materials such as molecular crystals, π-electron conjugated polymers, and molecularly doped polymers, the absolute values of the carrier-generation yield of this photoconductive liquid crystal are very low. For example, at a medium electric field of about 105 V/cm, the obtained quantum yield of all of above materials is only about 10-5-10-3.2 But using the Onsager model, it can be found that the reasons for such a low quantum yield in ordered small molecular materials and in polymers are completely different. For the former, the low primary quantum yield Φ0 of about 10-3-10-5, but for the latter, in which the Φ0 is 2 orders of magnitude larger than that in the former, the small thermalization length r0 of less than 3 nm,33,34 is the dominating factor in the photogeneration process, respectively. Moreover, in anisotropic molecular crystals, the quantum yield is different along the different crystalline orientation because of the difference in the molecular distance. Since the transport of carriers follows the hopping model in such an organic system, the shorter the distance, the larger r0 becomes because of the

Photocarrier Generation in Smectic Phenylnaphthalene high electron migration rate during the formation of e-h pairs, which is similar to the case in which the r0 varies with the transition of the phase, i.e., the transition of the order parameter in 8-PNP-O12. As shown in Figure 6 and Table 1, in the mesophase, especially in the SmB phase, the parameters are close to the molecular crystals, but in the isotropic phase, the electric field dependence of photogeneration yield is more close to that in the amorphous polymer systems. On the basis of the present results, it is concluded that for this hopping transport material, the sophistication of molecular ordering in 8-PNP-O12 promotes the electron migration or transport process because of lower energetic and positional disorder,35,36 which produces a high thermalization length and high carrier mobility. On the other hand, it may also reduce the primary efficiency of electron-hole pair generation, which remains to be clarified. In the strict sense, the traditional Onsager model can only be used in three-dimensional (3D) isotropic systems as mentioned above. Its application to the one-dimensional (1D) system37,38 also need 1D homogeneous medium approximation. According to the result of Monte Carlo simulations on the geminate pair dissociation in discrete anisotropic lattices by Ries, Schonherr, and Bassler,39 the degrees of anisotropy have a profound effect on the zero-field limit independent of the dimensionality, but the high-field behavior should become almost independent. In fact, the essential difference between 3D and 1D Onsager models is that at the zero field; there is still a finite chance for free carriers to escape from the recombination in 3D case, but in the 1D case, the bound ion pairs result in the complete recombination. However, according to the simulated result of a quasi-1D polymer system, for a finite sample, even though the ratio between the mobility parallel and perpendicular to the electric field is 103, there is still a small number of free carriers that will survive. Here, the smectic liquid crystal 8-PNP-O12 is a quasi-2D molecular system, in which the distance between smectic layers is 36 Å and about 8 times larger than the average distance between molecules in the layer. Therefore, the electronic hopping of carriers should exhibit high anisotropy of 104 in terms of the drift mobility.40 In the homogeneous alignment of 8-PNP-O12, the carriers are transported within a smectic layer along with the electric field. In the smectic layer, however, carriers can diffuse and escape from the recombination in the two directions in the layer when generated. So it can be considered that the probability of escaping from the recombination in this 2D system is larger than that in a quasi-1D system, such as PDA-TS single crystals.41 That is, the photocarrier generation yield at the zero-field limit may have a finite yield, larger than zero. Therefore, the electric field dependence of photogeneration yield in 8-PNP-O12 is closer to that in the 3D system rather than that in the 1D system. In addition, the thermal fluctuation of the smectic molecular alignment can decrease the degree of anisotropy. These are a good basis of why the 3D Onsager model still gives a good approximation for the analysis of the photocarrier generation in this quasi-2D system of 8-PNPO12. It should be noted that the thermalization distance r0 estimated from the 3D Onsager model should be smaller than the true distance of a 2D system in the mesophases. IV. Summary Photocarrier generation in a photoconductive liquid crystal of 8-PNP-O12 is investigated by using the time-of-flight technique. Although some errors produced by the space charge

J. Phys. Chem. B, Vol. 103, No. 35, 1999 7433 effect exits, the collected photogenerated charges are found to be proportional to the square of the light intensity at fairly high light intensity of 1012-1013 photons/cm2. Considering the absorption spectrum of 8-PNP-O12 and the illumination conditions of N2 laser for the excitation, we conclude that the doublesinglet exciton interaction is the main process to produce the electron-hole pairs. The electric field dependence of hole photogeneration yield is analyzed by the 3D Onsager model in different phases: the experimental results were in a good agreement with the simulated curves using the mean dielectric constant of the each phase in 8-PNP-O12. The second parameter of the Onsager model, r0 is found to be fairly large, up to 8 nm, independent of the light intensity, while Φ0 is proportional to the light intensity in the case of high exciton density in our experiment. In addition, it was confirmed that the molecular ordering promotes the dissociation of the electron-hole pairs into free carriers, i.e., increases the thermalization length r0, but the more highly the molecular orientation is sophisticated, the lower the generation efficiency from the excitons to the geminate electron-hole pairs becomes. But in the low and medium electric field range, it is sure that this ordered alignment of molecules is favorable to photogeneration of carriers. In these points, the carrier generation process in the mesophase can be considered to be analogous to that in organic molecular crystals rather than that in organic liquids. At the same time, it must be noted that the 2D Onsager model is needed for further accurate and detail analysis for the photocarrier generation process in this quasi-2D system. Acknowledgment. We thank M. Funahashi for guiding material preparation and TOF measurements and for fruitful discussion. References and Notes (1) Law, K.-Y. Chem. ReV. 1993, 93, 449-486. (2) Pope, M.; Swenberg, C. E. Electronic Processes in Organic Crystals; Oxford: New York, 1982; Chapter III. (3) Onsager, L. Phys. ReV. 1938, 54, 554. (4) Onsager, L. J. Chem. Phys. 1934, 2, 599. (5) Pai, D. M.; Enck, R. C. Phys. ReV. 1975, B11, 5163. (6) Chance, R. R.; Braun, C. L. J. Chem. Phys. 1973, 59, 2269. (7) Gailberger, M.; Bassler, H. Phys. ReV. 1991, B44, 8643. (8) Lin, L. B.; Jenekhe, S. A.; Borsenberger, P. M. J. Chem. Phys. 1996, 105 (19), 8490. (9) Khan, M. I.; Bazan, G. C.; Popovic, Z. D. Chem. Phys. Lett. 1998, 298, 309. (10) Popovic, Z. D. J. Chem. Phys. 1984, 86, 311. (11) Noolandi, J.; Hong, K. M. J. Chem. Phys. 1979, 70, 3230. (12) Law, K.-Y. Chem. ReV. 1993, 93, 449-486. (13) Batt, R. H.; Braun, C. L.; Hornig, J. F. J. Chem. Phys. 1968, 49, 1967. (14) Kitamura, K., et al. J. Electrophotography Soc. Jpn. 1989, 28, 32. (15) Umeda, M. Proceeding of Japan Hardcopy, 1988; p 39. (16) Borsenberger, P. M., et al. J. Appl. Phys. 1978, 49, 5555. (17) Boden, N.; Bushby, R. J.; Clements, J.; Jesudason, M. V.; Knowles, P. F.; Williams, G. Chem. Plhys. Lett. 1988, 152, 94. (18) Adam, D.; Schuhmacher, P.; Simmerer, J.; Haussling, L.; Siemensmeyer, K.; Etzbach, K. J.; Ringsdorf, H.; Haarer, D. Nature 1994, 371, 141. (19) Funahashi, M.; Hanna, J. Phys. ReV. Lett. 1997, 78, 2184. (20) Funahashi, M.; Hanna, J. Appl. Phys. Lett. 1997, 71 (5), 602. (21) Kepler, R. G. Phys. ReV. 1960, 119, 1226. Blanc, O. H. J. Chem. Phys. 1960, 33, 626. (22) Rommens, J.; Van der Auweraer, M.; De Schryver, F. C.; Terrell, D.; De Meutter, S. J. Phys. Chem. 1996, 100, 10673. (23) Kanemitsu, Y.; Imamura, S. J. Appl. Phys. 1990, 67 (8), 3729. (24) Kepler, R. G.; Coppage, F. N. Phys. ReV. 1966, 151, 610. (25) Takai, R. Y.; Mizutani, T.; Ieda, M. Jpn. J. Appl. Phys. 1983, 22, 1388. (26) Helfrich, W.; Schneider, W. G. J. Chem. Phys. 1966, 44, 2902. (27) Kerp, H. T.; Donker, H., et al. Chem. Phys. Lett. 1998, 298, 302. (28) Sigal, H.; Markovitsi, D., et al. J. Phys. Chem. 1996, 100, 10999.

7434 J. Phys. Chem. B, Vol. 103, No. 35, 1999 (29) Barth, S.; Bassler, H. Phys. ReV. 1997, B56, 3844. (30) Gibbons, D. J.; Papadakis, A. C. J. Phys. Chem. Solids 1968, 29, 115. (31) Chance, R. R.; Braun, C. L. J. Chem. Phys. 1976, 64, 3573. (32) Etemad, S.; Mitani, T.; Ozaki, M.; Chung, T. C.; Heeger, A. J.; Macdiarmid, A. G. Solid State Commun. 1981, 40, 75. (33) Okamoto, K., et al. Bull. Chem. Soc. Jpn. 1984, 57, 1626. (34) Barth, S.; Bassler, H. Phys. ReV. 1997, B56, 3844. (35) Funahashi, M.; Hanna, J. Mol. Cryst. Liq. Cryst., in press.

Zhang and Hanna (36) Bassler, H. Phys. Stat. Sol. 1993, B175, 15. (37) Haberkorn, R.; Michel-Beyerle, M. E. Chem. Phys. Lett. 1973, 23, 128. (38) Donovan, K. J.; Wilson, E. G. Philos. Mag. 1981, B44, 31. (39) Ries, B.; Schonherr, G.; Bassler, H. Philos. Mag. 1983, B48, 87. (40) Mort, J.; Pfister, G. Electronic Properties of Polymers; WileyInterscience: New York, 1982; Chapter 6. (41) Lochner, K.; Reimer, B.; Bassler, H. Phys. Status Solidi B 1976, 76, 533.