9226
J. Phys. Chem. B 2005, 109, 9226-9230
Charge Carrier Transport Properties in Polymer Liquid Crystals Containing Oxadiazole and Amine Moieties in the Same Side Chain Masuki Kawamoto,†,§ Hiroyuki Mochizuki,† Tomiki Ikeda,*,† Hiroaki Iino,‡ and Jun-ichi Hanna‡ Chemical Resources Laboratory and Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, R1-11, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ReceiVed: October 10, 2004; In Final Form: March 18, 2005
Steady-state and transient photocurrent measurements were carried out to study the charge carrier transport properties of polymer liquid crystal (LC) containing oxadiazole (OXD) and amine moieties in the same side chain. The steady-state photocurrent measurement with asymmetric electrodes of ITO and Al and a short penetration depth of the illumination light indicated that both electrons and holes can be transported in this film. The transient hole photocurrent observed by time-of-flight (TOF) experiments was dispersive at room temperature. The hole drift mobility significantly depended on temperature and electric field and was determined to be 6.1 × 10-8 cm2/Vs at a field of 9.1 × 105 V/cm. According to the disorder formalism, the Gaussian width of the density of states was determined to be 170 meV for holes. Despite the indication of possible electron transport in this film, we could not determine the electron mobility by TOF experiments due to strong dispersive photocurrent. We discuss the present charge transport properties of the film in relation to a large dipole attributed to an electrical push-pull structure of p-dimethylaminophenyl-substitited OXD moiety in polymer LC and its electroluminescent properties.
Introduction Polymer light-emitting diodes (LEDs) have made rapid progress since the original work was reported by Burroughes et al.,1 and their commercial applications in lighting and displays now look viable.2 Among the factors that determine electroluminescent (EL) device performance, carrier mobility is one of the most important material issues because of its decisive role for a current density and a charge balance for injection.3 Recently, much attention has been paid to liquid-crystalline (LC) materials for their fast charge carrier transport properties. An electronic conduction characterized by a high mobility of 10-3∼10-1 cm2/Vs taking place in the discotic (D) and smectic (Sm) mesophases with π-conjugated aromatic moieties has been disclosed in these 10 years: Haarer et al. reported fast drift mobility in triphenylene LC derivatives by time-of-flight (TOF) measurement, in which the hole mobility was in the order of ∼10-3 cm2/Vs and was independent of temperature and electric field;4 Hanna et al. reported high carrier mobility in the Sm phases of phenyl benzothiazole,5a,b 2-phenylnaphthalene,5c,d and terthiophene5e derivatives. This indicated that the LCs could be an organic semiconductor. In fact, an EL device containing homogeneously aligned 2-phenylnaphthalene derivative of (4octylphenyl)-6-dodecyloxynaphthalene doped with a coumarin dye was fabricated, which emitted polarized green light.6 More recently, this idea is extended to polymer LCs, leading to the discovery of a nematic (N) mesophase in poly(9,9-dialkylfluorene). This material has good processability and easy fabrication * Corresponding author. Phone: +81-45-924-5240; FAX: +81-45-9245275. E-mail:
[email protected]; URL: http://www.res.titech.ac.jp/ polymer † Chemical Resources Laboratory. ‡ Imaging Science and Engineering Laboratory. § Present Address: RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan.
by solution process,7 and its charge carrier mobility was achieved up to 10-3 cm2/Vs and was independent of the electric field.8 A bipolar carrier-transporting material is a good candidate for a single-layer LED because it combines electron-transporting and hole-transporting properties. However, the bipolar charge carrier transport seldom happens in organic materials. The best known material is a binary system of poly(vinylcarbazole) and 2,4,7-trinitrofluorenone. Although there have been many studies of hole or electron transport in molecularly doped polymers (MDPs), which are the polymers with low molecular weight carrier transport materials and practically used as indispensable components of the photoreceptor for xerographic copiers and laser beam printers,9 few studies on bipolar-transport behavior have been reported: Yokoyama et al.10 and Borsenberger et al.11 investigated the carrier-transport properties of the MDPs containing both hole- and electron-transporting materials. This approach, however, is limited by the maximum concentration that can be used in the binary homogeneous mixtures due to crystallization that causes serious degradation of carrier transport properties. We have investigated the hole drift mobility in sidechain homopolymers and copolymers exhibiting mesophases such as carbazole-containing acrylate,12 acrylates with phenylbenzoate or cyanobiphenyl,12 and acrylate with 1,2-trans-bis(9H-carbazol-9-yl)cyclobutane (DCZB).12b Incorporation of the DCZB moiety into copolymers resulted in homogeneous dispersion of the carrier transport molecules, but with destabilization to a great extent of the LC phase of the resulting copolymers.12 Light emitted from polymer LEDs is produced via recombination of electrons and holes injected through electrodes. It is known that balanced and efficient charge transport for both electron and hole is essential for high device efficiency. Unfortunately, most of the polymers developed so far transport electrons much less efficiently than holes, which significantly decreases the efficiency of the devices. To solve this problem, we investigated a novel polymer material, PM6OXDMA, for
10.1021/jp0453820 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/15/2005
Charge Carrier Transport Properties of PM6OXDMA
J. Phys. Chem. B, Vol. 109, No. 19, 2005 9227 R8340). The excitation light source, the source measurement, and an electromagnetic shutter were controlled by a personal computer.5a For transient photocurrent measurements, a conventional TOF setup was used with a N2 laser (337 nm; 40 µJ/ pulse; pulse width, 600 ps). The signals were amplified and recorded with a digital oscilloscope (Nicolet, Model Pro92).5b
Figure 1. Chemical structure and properties of PM6OXDMA used in this study. G, glassy; N, nematic; I, isotropic; Mn, number-average molecular weight; and Mw, weight-average molecular weight.
single-layer LEDs, because it contains an oxadiazole (OXD) moiety as an electron-transporting unit and an amine moiety as a hole-transporting unit. This polymer exhibited LC behavior and strong blue fluorescence.13a We also explored electrochemical and EL properties of PM6OXDMA. It has been found that a polarized EL emission was observed due to self-organized mesogenic chromophores.13b From a molecular design aspect, PM6OXDMA possesses an electron-withdrawing moiety (OXD) and an electron-donating one (dimethylaminophenyl group) in the same side chain. Thus, we have investigated charge carrier transport properties of PM6OXDMA films by steady-state and transient photocurrent measurements, and we discuss the charge transport properties in relation to LC behavior, molecular structure and EL properties. Experimental Section Material and Sample Preparation. The chemical structure of PM6OXDMA is illustrated in Figure 1. We observed two endothermic events via differential scanning calorimeter on heating: one is due to the glass transition temperature at 97 °C and the other corresponds to a nematic (N) to isotropic (I) phase transition at 211 °C.13 The sample was prepared by casting the polymer solution in 1,2-dichloroethane (2 wt %) onto ITO coated glass substrates without an LC alignment layer. The film thickness was 3.3 µm, measured with a Dectak surface profiler. The film was annealed at 150 °C for 1 h under argon, and then cooled to a glassy state. Finally, a semitransparent aluminum electrode of 20 nm thickness was deposited by thermal evaporation at 2 × 10-6 Torr through a shadow mask onto the film yielding sandwich structures with an active area, defined by the electrode overlap, of 40 mm2. Steady-State and Transient Photocurrent Measurements. Steady-state and transient photocurrent measurements were carried out to study the carrier transport properties in a film of PM6OXDMA. A Xe lamp (500 W) equipped with a band-pass filter (Toshiba, UV33DS) was used for UV irradiation from 380 to 300 nm. Light intensity was 2.5 mW/cm2. The photocurrent was recorded with a source measurement unit (Advantest,
Results and Discussion Steady-State Photocurrent. The steady-state photocurrent involves photogeneration of charge carriers and their transport, which is governed by carrier lifetime and mobility in the film and often by the contact with electrode materials. To establish a single carrier condition in the transport, we selected the irradiation light wavelength shorter than 400 nm, where the penetration depth is estimated to be sufficiently smaller than the film thickness. According to the sign of the applied bias to the illuminated electrode, either photogenerated electrons or holes drift under an external bias to the collecting electrode and contribute mainly to the resulting photocurrent that is monitored with a source measurement unit. The dark current, however, is governed by thermally injected carriers from the electrodes and is proportional to the applied voltage, because the present applied electric field, i.e., 3 × 105 V/cm at most, is much smaller than that for EL and results in no significant reduction of the energy barrier for the injection. Thus, the steady-state photocurrents provide us with information useful to understand the factors that determine characteristics of PM6OXDMA. Figure 2 exhibits the photocurrent response under UV light irradiation in the film at room temperature. A clear response of the photocurrent was observed according to the applied bias: the photocurrent increased immediately on photoirradiation and returned to the initial value when irradiation was turned off, which was not affected by repeating the turn-on and turn-off cycles of photoirradiation. The photo- and dark current ratio of photoconductivity was 50 and 40 for a given positive and negative bias of 100 V, respectively. Taking account of a short penetration depth of illuminated light of 300-380 nm, this result indicates that both electrons and holes can be transported in the PM6OXDMA film. Figure 3 shows current-voltage characteristics of photo and dark currents in a film at room temperature. We checked that the smaller photocurrent obtained by photoirradiation through the aluminum electrode came from its smaller optical transmittance compared with that of the ITO electrode. Therefore, we concluded that there is little difference in the photocurrent as a function of the applied bias irrespective of its sign despite asymmetric electrodes (Figure 3). This fact also supports that the electrons and holes can transport in PM6OXDMA.
Figure 2. Photocurrent response under steady-state UV irradiation in a film at 100 V. (A) Photoirradiation through an ITO electrode; (B) through an aluminum electrode. (I) Hole; (II) electron. Measurements were performed at room temperature. Sample thickness: 3.3 µm.
9228 J. Phys. Chem. B, Vol. 109, No. 19, 2005
Figure 3. Current-voltage characteristics of photo and dark currents in PM6OXDMA at room temperature. (square) Dark current. Photocurrent was obtained by photoirradiation through an ITO electrode (triangle) and an aluminum electrode (circle). The values of photocurrent through the aluminum electrode have been compensated for the difference in the transmittance of the electrodes.
Figure 4. Optical setup for TOF measurement.
Figure 5. Transient photocurrent curves of PM6OXDMA in a film at various voltages. Measurements were performed at 28 °C.
Transient Photocurrent. To evaluate the carrier transport properties of the PM6OXDMA films quantitatively, we studied the carrier mobility of the films by TOF method. Figure 4 shows the optical setup for TOF measurement. Optical excitation of a carrier packet was achieved by illumination through the ITO electrode by using a short duration light pulse from a N2 laser.
Kawamoto et al. The penetration depth of the absorbed light and the instantaneous extent of the optically generated charge packet were very small compared to the film thickness. Transient times were determined from double logarithmic plots of transient photocurrent curves, because the resulting photocurrents were very dispersive. The carrier mobility was calculated by the relation between the transient time of tT, a film thickness of d, and a given applied bias of V, µ ) d2/VtT. The film exhibited a clear hole transient photocurrent even at the ambient temperature, which was dispersive as shown in Figure 5. The hole mobility, µp, could be determined to be 6.1 × 10-8 cm2/Vs at a field of 9.1 × 105 V/cm from transient photocurrent curves. We observed nondispersive hole transport at the elevated temperature of 130 °C, whose mobility was determined to be µp ) 2.0 × 10-6 cm2/Vs at a field of 9.1 × 105 V/cm (Figure 6A). The mobility depended on temperature rather significantly and on the electric field as well. This result suggests a rather broad distribution of the density of states (DOS) in PM6OXDMA. On the other hand, the materials containing the OXD moiety such as 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) are well known to be an electron transport material for the LEDs.14 Their mobility was determined to be in a range of 10-5 cm2/Vs.15 However, we could not observe any clear electron transient photocurrent, either at ambient temperature or at 130 °C as shown in Figure 6B. As we previously discussed, the film exhibits electron transport in the steady-state photocurrent. As we described above, LCs exhibit fast charge carrier transport in D and Sm mesophases,4,5 which is characterized by temperature-and-field independent mobility. The present polymer LC shows an N phase between 97 °C and 211 °C (Figure 1).13 The N LCs are characterized by a long-range orientational order and no positional order of individual molecules, so the average molecular distance is not so different from that of the isotropic phase and the molecular alignment is not sophisticated much. Therefore, the charge carrier transport of this polymer looks similar to those of amorphous phases rather than those in the Sm and D mesophases. This can be the reason we observed such a slow charge carrier transport that depended on temperature and the electric field in the film. Analysis of the Drift Mobilities. The charge transport process in the disordered molecular systems assumes that charge transport is governed by hopping through a manifold of Gaussian distributed localized states. This model describes the disordered carrier transport by the following equation,16
[ (32 kTσ ) ]exp{C[(kTσ ) - Σ ]xE}
µ ) µ0 exp -
2
2
2
(1)
where σ and Σ are the parameters that characterize the degree
Figure 6. Transient photocurrent curves of PM6OXDMA in a film at 130 °C. (A) Hole; (B) electron.
Charge Carrier Transport Properties of PM6OXDMA
J. Phys. Chem. B, Vol. 109, No. 19, 2005 9229
Figure 7. Electric-field dependence of the hole mobilities in PM6OXDMA at various temperatures.
Figure 9. β versus (σ/kT)2 for hole mobilities. β ) ∂lnµ/∂E1/2, where µ is the hole mobility, and E is the electric field.
TABLE 1: Hole Transport Parameters in Terms of the Disorder Formalism for PM6OXDMA, O1C10-PPV, and PFO material
µ0 (cm2/Vs)
σ (meV)
Σ
C (cm/V)1/2
PM6OXDMA O1C10-PPVb PFOc
4.4 × 4.0 × 10-6 4.9 × 10-2
170 120 100
1.6 -a 2.8
2.2 × 10-4 4.3 × 10-5 2.9 × 10-4
a
Figure 8. Temperature dependence of hole mobilities at zero-electric field in PM6OXDMA.
of energetic disorder and positional disorder, respectively, µ0 is a hypothetical mobility in the energetic disorder-free system, E is the electric field, T is the temperature, and C is an empirical parameter, typically 2.9 × 10-4 (cm/V)1/2. In fact, this formalism can explain the carrier transport properties in various organic amorphous thin films reported previously and give an insight into the relation between the carrier transport properties and the molecular structure of the materials. Figure 7 shows the electric-field dependence of the hole mobility of PM6OXDMA at various temperatures. The hole mobility increases with increasing electric field, following the electric-field dependence of exp(βE1/2), where E represents the electric field and β is a coefficient. The hole mobility at zero electric field, µE)0, and the coefficient β were obtained from an intercept and a slope, respectively, of the linear plot of log µE)0 versus E1/2 extrapolated to zero electric field. These values are used for the determination of the charge-transport parameters in eq 1. Figure 8 shows the temperature dependences of µE)0. The hole-transport parameters, µ0 and σ, were determined from the intercept and the slope, respectively, of the linear plot of log µE)0 versus T-2 extrapolated to T f ∞ in Figure 8. Figure 9 shows plots of β versus (σ/kT)2 for hole mobility, where β is described by eq 2.
β)
∂ ln µ σ 2 - Σ2 )C kT ∂xE
[( )
]
(2)
The value Σ was determined from the intercept at β ) 0 in the linear plot of β versus (σ/kT)2, where (σ/kT)2 ) Σ2 holds. The charge-transport parameters, µ0, σ, Σ, and C in eq 1, obtained for the film are summarized in Table 1. The charge-transport parameters of poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4phenylenevinylene) (OC1C10-PPV)17 and poly(9,9-dioctylfluo-
10-3
Not estimated. b From ref 17. c From ref 8d.
rene) (PFO)8d as typical light-emitting polymers are also included in Table 1 for comparison. The term σ, i.e., a Gaussian width of the DOS, for a hole in the film was 170 meV, which is very large compared with ∼50 meV of the D and Sm mesophases reported, and even larger than 100∼120 meV of typical π-conjugated polymers, O1C10PPV and PFO as listed in Table 1. This DOS reflects the energetic spread in the charge-transport levels due to fluctuations in structural disorder.9 Furthermore, it was clarified that charge carrier in MDPs depends on the permanent dipole of the doped materials and polymer matrices experimentally.18-21 In this case, σ is the result of different electrostatic potentials at various sites due to distribution in orientation of the dipole moments of the molecules. Therefore, it is very likely that the dipole affects σ and so the carrier transport in the disordered systems. Tazuke,18 Ba¨ssler,19 Nishikawa,20 and Dunlap21 investigated the relation between σ and the dipole. In fact, this was experimentally confirmed as well. For example, a typical hole-transporting material, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD), possesses a relative small dipole moment (1.52 D). The hole-transport parameters of µ0, σ, Σ, and C are 3.9 × 10-2 cm2/Vs, 80 meV, 1.9, and 2.8 × 10-4 (cm/V)1/2.22 On the other hand, Borsenberger et al. reported the hole-transporting properties of a low molecular weight OXD, 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole, with a large dipole moment (5.56 D) in a vapor deposited film (µ0 ) 4.1 × 10-3 cm2/Vs; σ ) 158 meV; Σ ) 2.0).23 Thus, the very large σ of 170 meV obtained for PM6OXDMA could be interpreted in terms of its large dipole moment of 5.41 D, which is as large as that of the low molecular weight OXD (5.56 D).23 It also explains the observation that the carrier transport of PM6OXDMA is sensitive to temperature and electric field. Furthermore, the heavy dispersion of transient photocurrents observed for electron can also be explained by results from a considerable concentration of deep states in the fairly extended DOS for electron transport. The basic concept for the material design in PM6OXDMA contains two aspects: one is the side-chain polymer containing the charge-transporting chromophore with a donor- and an acceptor-like moiety for bipolar charge transport; the other is the LC polymer for enhancing the charge-transporting properties. According to the results, the bipolar charge transport was
9230 J. Phys. Chem. B, Vol. 109, No. 19, 2005 confirmed experimentally as we expected from the EL performance in a single layer device,13b although the electron mobility could not be determined. However, the mobility was rather small, i.e., on the order of 10-8 cm2/Vs for hole and probably far smaller for electron at room temperature. The analysis based on the disorder formalism implies that the major problem on the charge transport comes from the large dipole over 5 D in PM6OXDMA, resulting in a large Gaussian width of 170 meV, which is responsible for inferior charge transport properties, e.g., small and temperature and electric field-dependent mobility and heavily dispersive transport. Therefore, we have to pay attention to the chemical structure of the charge-transporting chromophore for improving the bipolar charge transport in the side chain, because the large dipole in PM6OXDMA is originated from the donor-acceptor-like chemical structure of OXD in the charge-transporting chromophore. PM6OXDMA shows the N phase, which is a one-dimensionally ordered elastic fluid. The molecules in the N phase tend to align parallel to each other with their long axes in the same direction. In fact, it was demonstrated that the N alignment of the charge-transporting chromophore does contribute to the enhancement of the charge transport in the π-conjugated mainchain polymers such as PFO, but it is not certain in the side chain LC polymer with a small charge-transporting mesogen, as is the case of PM6OXDMA, because of less effective overlap of molecular orbitals responsible for charge transfer. Conclusions In this article, charge carrier transport in a PM6OXDMA film was studied by steady-state and transient photocurrent measurements. In steady-state photocurrent measurements, it was found that a clear response of the photocurrent occurs according to the applied bias, irrespective of the sign of bias, indicating bipolar charge carrier transport in the PM6OXDMA film. Furthermore, we estimated the carrier mobility of the polymer by using TOF method, 6.1 × 10-8 cm2/Vs at a field of 9.1 × 105 V/cm at 28 °C. On the other hand, we could not determine the electron mobility, even though the steady-state photocurrent measurement indicated that electrons could be transported in the film. Despite the N phase of PM6OXDMA, the charge carrier transport was dispersive and depended on temperature and electric field, which can be attributed to the low ordered N phase, whose structure is rather similar to that of the isotropic phase. We analyzed its hole transport properties on the basis of Bassler’s disorder formalism and determined the disorder parameters, i.e., µ0 ) 4.4 × 10-3cm2/Vs, σ ) 170 meV, Σ ) 1.6, and C ) 2.2 × 10-4. We concluded that this large σ is due to a large dipole moment of 5.41 D originating from the dimethylaminooxadiazole moiety. Since electrooptical and LC properties can be tuned by modification of chemical structures in the chromophores and/or by copolymerization of the monomers having either hole- or electron-transporting chromophore, these results will be an interesting subject to be clarified in the future.
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