J. Phys. Chem. B 2000, 104, 3887-3891
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Correlation between Dispersivity of Charge Transport and Holographic Response Time in an Organic Photorefractive Glass† U. Hofmann,‡ M. Grasruck,‡ A. Leopold,‡ A. Schreiber,‡ S. Schloter,‡ C. Hohle,§ P. Strohriegl,§ D. Haarer,‡ and S. J. Zilker*,‡ Physikalisches Institut and Bayreuther Institut fu¨ r Makromoleku¨ lforschung (BIMF), UniVersita¨ t Bayreuth, 95440 Bayreuth, Germany, and Makromolekulare Chemie I and Bayreuther Institut fu¨ r Makromoleku¨ lforschung (BIMF), UniVersita¨ t Bayreuth, 95440 Bayreuth, Germany ReceiVed: October 5, 1999; In Final Form: January 24, 2000
We report on photoelectric and holographic investigations of an organic photorefractive material based on a low molar mass glass with both photoconductive and nonlinear optical properties. By implementing a suitable plasticizer we obtained a composite system which shows extremely fast initial response times down to 450 µs at writing beam intensities of Iwrite ) 10.8 W/cm2 and 2.5 ms at the canonical intensity of Iwrite ) 1 W/cm2. Furthermore, high refractive index modulations up to ∆n ) 6.1 × 10-3, long lifetimes and high optical quality of the samples are observed. In comparison to a second similar composite system, which was plasticized by a more polar dopand, we demonstrate the crucial role of this functional constituent on the photoelectric properties. Time-of-flight measurements show a major impact of the plasticizer on the dispersivity of chargecarrier transport. Subsequently the buildup and decay dynamics of the photorefractive grating are substantially affected. Holographic time-of-flight measurements confirm these observations.
I. Introduction Photorefractivity in organic systems has received a great deal of attention in the past few years. High diffraction efficiencies were readily obtained within this material class.1-3 Therefore, one of the primary aims in material development has been to reduce the response times of organic systems. The latter were much slower as compared to those observed in inorganic photorefractive crystals. Several months ago, a material was published by Moerner et al., which showed response times of 4 ms, the fastest value so far reported in a cw-experiment.4 Although these results have to be considered a great success, very little is known about the microscopic processes which limit the speed of grating formation. In this paper we present a composite material which shows excellent overall performance, including high diffraction efficiencies of up to 33% with samples of only 22.3 µm thickness, a maximum PR gain coefficient of Γ) 138 cm-1, high charge carrier mobility of µ ) 9.5 × 10-6 cm2/Vs and short initial response times down to 450 µs at writing beam intensities of Iwrite W/cm2. We demonstrate the importance of the choice of plasticizer on the photoelectric properties by comparing two materials which differ only in the plasticizing agent. The plasticizer affects the dispersivity of charge transport, which has a major impact on the speed of PR grating formation but not on the hole mobility. II. Material The material is a composite system (Figure 1) consisting of the bifunctional molecule 4,4'-di(N-carbazolyl)-4''-(2 -N-ethyl† Part of the special issue “Harvey Scher Festschrift”. Dedicated to H. Scher on the occasion of his 60th birthday and of the 25th anniversary of the Scher-Montroll model. * Corresponding author. ‡ Physikalisches Institut and Bayreuther Institut fu ¨ r Makromoleku¨lforschung (BIMF). § Makromolekulare Chemie I and Bayreuther Institut fu ¨ r Makromoleku¨lforschung (BIMF).
Figure 1. The bifunctional, low molar mass glass DRDCTA and the plasticizers EHMPA and DOP.
4-[ 2-(4-nitrophenyl)-1-azo]anilinoethoxy)-triphenylamine (DRDCTA)5 which shows charge transport as well as nonlinear optical properties, the plasticizer N-(2-ethylhexyl)-N-(3-methylphenyl)aniline (EHMPA), and C60 for the required charge carrier generation. For a mass ratio of DRDCTA:EHMPA:C60 of 69: 30:1 a glass transition temperature (Tg) of 22 °C was measured by differential scanning calorimetry (DSC) with a Perkin-Elmer DSC-7. This Tg allows for rotational orientation of the highly polar bifunctional molecules at room temperature (25 °C). The experimental results obtained with this system are compared to data from measurements performed on the slightly different
10.1021/jp9935283 CCC: $19.00 © 2000 American Chemical Society Published on Web 03/22/2000
3888 J. Phys. Chem. B, Vol. 104, No. 16, 2000
Hofmann et al.
system DRDCTA:DOP:C60 (71:28:1) which was published previously.6 Here the plasticizer DOP was used (see Figure 1). We had to vary the plasticizer concentration by 2% in order to obtain an almost identical Tg of 21 °C. The refractive indices have been measured ellipsometrically yielding 1.80 at 670 nm and 1.81 at 645 nm for both materials. The absorption coefficient of DRDCTA:EHMPA:C60 was measured to be 42 cm-1 at 670 nm and 93 cm-1 at 645 nm. The samples show no signs of degradation until now. This means that minimum shelf lifetimes of more than one year have been observed for both composite systems. Furthermore, the materials show no signs of photoinduced degradation even for writing beam intensities of up to Iwrite ) 10.8 W/cm2. Electric breakdown usually occurred at external fields between 110 V/µm and 140 V/µm. III. Experimental Setups We used two different setups for degenerate four-wave mixing (DFWM) experiments. One of those has been described previously.7 To achieve higher writing intensities, we used a second setup with a dye ring laser (Coherent 899-21) operating at a wavelength of λwrite ) 645 nm. A photodiode was used for observation of the diode laser read-out beam. An angle of 17° between the writing beams outside the sample and a tilt angle of the sample of 56° with respect to the direction of incidence resulted in a grating spacing of 3.5 µm. Furthermore, we performed time-of-flight (TOF) experiments for the investigation of the charge-carrier mobility as, for instance, described by Adam et al.8 A nitrogen laser was used for irradiation of the samples with pulses of 10 ns duration at a wavelength of 337 nm. The holographic time-of-flight (HTOF) technique is a complementary method to study the buildup of the internal space-charge field.9 Hereby, the grating is written by a pulsed laser, and its dynamics monitored with a cw probe beam. A parametric oscillator (Lambda Physik OPPO) provides 2 ns pulses at a wavelength of λwrite ) 645 nm. The tilt angle of the sample was 30° and the angle between the writing beams was 20°. The drift length LDR, which is the distance the free holes would theoretically travel to reach a point of anticoincidence,
LDR )
Λ 2cosγ
(1)
with Λ being the grating spacing and γ the tilt angle, was LDR ) 3.8 µm for this configuration. To investigate the dynamics of the chromophore orientation we performed transient ellipsometric measurements with a setup described by Hoechstetter et al.10 IV. Results and Discussion First, we will summarize the most important characteristics of DRDCTA:EHMPA:C60 concerning the amplitude of the refractive index grating. Second, the dynamical properties of this material are presented, and finally we will discuss the differences in the speed of holographic response between the two materials DRDCTA:EHMPA:C60 and DRDCTA:DOP:C60. A. Holographic Performance. We determined the diffraction efficiency η as a function of the external field Eext for DRDCTA: EHMPA:C60 by DFWM. Diffraction efficiencies of up to 33% with a sample of 22.3 µm thickness have been obtained at Eext ) 110 V/µm. This corresponds to a refractive index modulation of ∆n ) 6.1 × 10-3. B. Grating Dynamics of DRDCTA:EHMPA:C60. The response times were determined by biexponential fits to the rise
Figure 2. The complex dynamics of the light-induced grating decay at Iwrite ) 10.8 W/cm2 and Eext ) 92 V/µm. The inset shows the dynamics of the chromophore orientation of a poling procedure at Epol = 79 V/µm.
and decay of the refractive index modulation when one writing beam was switched on and off. As can be clearly seen in Figure 2, the dynamic behavior of the grating erasure is very complex. This is also true for the buildup process. To obtain the time constant belonging to the fastest process involved (τ1) one could, for example, apply a single exponential fit function to the very early part of the investigated process. In this case, there is usually a critical dependence of τ1 on the chosen cutoff time. Therefore, we defined a cutoff time for which a biexponential function was sufficient to match the experimental data over a longer time scale. The second (slower) time constant obtained from these fits is very much dependent on the chosen cutoff time, whereas the shorter value is not. Using the biexponential fit, the error of τ1 introduced by the cutoff time is much smaller. This method is not completely satisfying, but as the resulting values are correlated with the one particular part of the buildup process, which is of interest here, we decided to use this approach. We expect this value to represent the erasure (and buildup) of the internal space-charge field. As shown by Cui et al.,11 the erasure of the space-charge field has a biexponential character. The processes which are responsible for the dynamics at longer time scales in our PR glasses are therefore probably the slower of the two time constants reported by Cui et al., as well as the rotational orientation of the bifunctional molecules in the local electric field. The latter was investigated by means of transient ellipsometry. The inset of Figure 2 shows the phase shift φ between s- and p-polarized beams caused by the electricfield-induced birefringence. Obviously the dynamics of the poling procedure is very slow in comparison with the fast processes observed in the holographic experiment. Therefore, one can safely rule out an impact of the orientational diffusion of the chromophores on τ1. As can be seen in Figure 3, fits to the buildup and erasure of the PR grating yield almost identical results. Response times τ1 of about 10 ms have been observed for writing intensities of Iwrite ) 0.18 W/cm2, a wavelength of λwrite ) 670 nm, and electric fields of Eext ) 92 V/µm. The response time decreased by more than an order of magnitude when writing beam intensities of 10.8 W/cm2 at a writing wavelength of λwrite ) 645 nm were used. At Eext ) 92 V/µm the fast component of the decay reaches 450 µs. Figure 2 shows this transient in a semilogarithmic plot, which demonstrates that the first part of the decay is actually finished after 1.5 ms.
Charge Transport and Holographic Response Time
Figure 3. Electric field dependence of the fast component τ1 as obtained from biexponential fits to the initial light-induced decay (filled symbols) and the initial rise (open symbols) using small writing intensity (Iwrite ) 0.18 W/cm2, λwrite ) 670 nm: diamonds) and high writing intensity (Iwrite ) 10.8 W/cm2, λwrite ) 645 nm: circles). The lines are fits to a power law dependence. The inset shows τ1 as a function of the writing beam intensity Iwrite obtained from the erasure process at Eext ) 92 V/µm. The line is a fit to a power law dependence.
The inset of Figure 3 shows experimental data for τ1 in dependence of the writing beam intensity. According to earlier studies,11-13 we fitted a power law and found a sublinear dependence on Iwrite as well. To allow for a comparison with other organic PR systems, an interpolation yielded a value of τ1 ) 2.5 ms for Eext ) 92 V/µm at the canonical writing intensity of Iwrite ) 1 W/cm2. At Iwrite ) 1.2 W/cm2 we obtained τ1 ) 2.2 ms. The latter value is important, because we investigated the dynamics of DRDCTA:DOP:C60 by using this writing beam intensity in an earlier study.6 C. Comparison with DRDCTA:DOP:C60. Although only the plasticizer has been varied, there is a very interesting difference in the holographic speed of DRDCTA:EHMPA:C60 and DRDCTA:DOP:C60. At writing beam intensities of 1.2 W/cm2, response times of τ1 ) 30 ms have been observed in the DOP doped material,6 as compared to 2.2 ms for EHMPA. This means that the response times τ1 of both systems differ by 1 order of magnitude. The fast time constant is related to the formation of the space-charge field. Accordingly, photoelectric properties are expected to dominate the dynamic behavior of the initial rise and decay and are therefore responsible for this deviation in response times between both materials. We may safely rule out that the differences mentioned above are mainly due to a change in the charge generation efficiency, because DRDCTA:DOP:C60 already shows a high primary quantum yield Φ0 of 40%.6 So even if this value should vary with the choice of the plasticizer it could not explain the difference of 1 order of magnitude. To understand the mechanisms involved, we investigated the charge transport properties of the two systems via the TOF technique. The results are depicted in Figure 4. The inset shows that the charge-carrier mobility differs only slightly for both materials. The mobility of the holographically faster material is even somewhat smaller. This means that the charge carrier mobility itself does not limit the holographic response of DRDCTA:DOP:C60. Taking a closer look at the shape of the current transients obtained with comparable electric fields (70 V/µm, see Figure 4), one recognizes a fundamental difference: The upper curve (belonging to DRDCTA:EHMPA:C60) clearly exhibits a monotonic decrease in the pretransit regime; according
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Figure 4. Two TOF transients obtained with DRDCTA:EHMPA:C60 (upper curve) and DRDCTA:DOP:C60 (lower curve) at 70 V/µm (sample thickness d ≈ 10 µm). The inset shows the hole mobility at room temperature as a function of the electric field for both composite systems (triangles with DOP; circles with EHMPA).
to Scher/Montroll, an unambiguous characteristic of dispersive charge transport.14 The lower curve (belonging to DRDCTA: DOP:C60) shows a pronounced plateau indicating nondispersive charge transport.15 This means charge transport in the system which has been plasticized with EHMPA is much more dispersive. According to an energetic interpretation for dispersive charge transport by Tiedje and Rose,16 dispersivity signifies a relaxation of the mean energy of the charge carriers from the conduction band mobility edge down to lower energies. This leads to higher thermal release times and therefore an increased immobilization of the charge carriers. The source of the buildup of a space-charge grating in a PR material is an immobilization of the holes in the dark regions, so a correlation between dispersivity and PR response times, as recently demonstrated by Grasruck et al.,17 is compelling. This is well confirmed by our measurements. To explain nondispersive charge transport in the picture of Tiedje and Rose, one has to assume a finite trap depth, which is equivalent to a minimal release rate for the charge carriers in a trapping site.18 If the charge carriers reach the bottom of the trap distribution during the relevant length scale, one can observe nondispersive (trap-free) transport, otherwise dispersive transport. This means that the occurrence of dispersive transport depends on the length scale on which charge transport takes place. The latter is the sample thickness in the TOF experiment and the grating spacing in the holographic experiment. So the observation of a plateau with the TOF technique does not necessarily mean that there is nondispersive transport in the holographic experiment as well. The comparison of the photocurrents of two samples of similar thickness (10 µm and 11 µm) is significant though. As a consequence, we expect the energetic relaxation of the charge carriers in the distribution of trapping sites to take much longer in the system DRDCTA: EHMPA:C60, which corresponds to deeper trapping sites and faster immobilization of the holes. Consequently, the average distance the charge carriers travel is probably smaller in this system. This conclusion is confirmed by results from HTOF experiments, where both materials have been studied under identical conditions. The light intensity grating is generated with a laser pulse, which means that the charge carrier generation process is restricted to a few nanoseconds. The generated holes will drift in the external electric field and will be eventually trapped.
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Figure 5. Holographic time-of-flight transients for DRDCTA:EHMPA: C60 and DRDCTA:DOP:C60 (inset) at 65 V/µm and 68 V/µm, respectively.
Hofmann et al. sivity prohibits such investigations for DRDCTA:EHMPA:C60. A decrease of the temperature by only a few degrees leads to almost perfectly dispersive transients where no transit time can be determined. For a more detailed study of the charge transport properties of DRDCTA:DOP:C60 refer to the paper from Schloter et al.,6 where the Gaussian disorder formalism has been applied,20 or to Grasruck et al.17 This work discusses the charge transport properties of bifunctional glasses with different chromophores on the basis of the multiple trapping model21 and the Scher-Montroll formalism. The latter provides an intuitive picture of the length scale dependence of charge transport in organic photorefractive systems. A thickness-dependent mobility is one of the core results of Scher-Montroll, whereas a sequence of steps is required for its derivation in the frame of the Gaussian disorder model. Grasruck et al. also report a dependence of the holographic response time on the dispersivity of charge transport. This is another strong confirmation of our results. Conclusion In conclusion, we presented investigations on the dynamics of two similar composite systems, which differ only in the choice of the plasticizer. Data from TOF measurements suggest that the differences in the holographic response times observed are due to the dispersivity of charge transport. This clearly demonstrates that the buildup of the space-charge field is determined not only by the speed of the charge carriers, but also by the trapping rate. HTOF measurements back these observations. Both high mobilities and a high concentration of deep traps are essential for obtaining a rapid photorefractive response.
Figure 6. Electric field dependence of tmax in a holographic time-offlight experiment for DRDCTA:DOP:C60 and DRDCTA:EHMPA:C60. Dashed lines are guide to the eye.
In organic systems, the mean drift length of the holes is significantly smaller than the theoretical drift length LDR (eq. 1).19 Therefore, when the majority of the holes has been trapped and thermal diffusion takes over, a maximum in the diffraction efficiency will appear. Figure 5 shows typical HTOF transients for DRDCTA:EHMPA:C60 and DRDCTA:DOP:C60 at an external electric field of 65 V/µm and 68 V/µm, respectively. Here, tmax is the time corresponding to the maximum of the transients. As can be seen in Figure 6, the time scales for tmax are even smaller than for the cw experiment. Thus, we can again neglect chromophore reorientation. Consequently, the position of tmax is determined solely by the hole mobility and the trapping dynamics. As we have already shown, the hole mobilities of DRDCTA:EHMPA:C60 and DRDCTA:DOP:C60 are quite similar. The tmax for DRDCTA:EHMPA:C60 is obviously shorter than for the DOP doped material. This difference in tmax in the HTOF experiment can be due only to the trapping characteristics of both materials. So the holes are immobilized more quickly in DRDCTA:EHMPA:C60 and therefore tmax is observed on smaller time scales. Obviously the plasticizer EHMPA seems to act as a trapping site, but until now we are unable to explain this effect. In the measurements of Grasruck et al.,17 the chromophore played a similar role. Temperature dependent TOF measurements could provide more insight into the nature of the traps, which are induced by EHMPA. Unfortunately, the high degree of disper-
Acknowledgment. The authors thank I. Otto and A. Lang for the preparation of the samples, the “Bayerische Forschungsstiftung” and the “Deutsche Forschungsgemeinschaft” (SFB 481) for financial support. References and Notes (1) Meerholz, K.; Volodin, B. L.; Sandolphon; Kippelen, B.; Peyghambarian, N. Nature 1994, 371, 497. (2) Moerner, W. E.; Grunnet-Jepsen, A.; Thompson, C. L. Annu. ReV. Sci. 1997, 27, 585. (3) Zobel, O.; Eckl, M.; Strohriegl, P.; Haarer, D. AdV. Mater. 1995, 7, 911. (4) Wright, D.; Diaz-Garcia, M. A.; Casperson, J. D.; DeClue, M.; Moerner, W. E.; Twieg, R. J. Appl. Phys. Lett. 1998, 73, 1490. (5) Hohle, C.; Strohriegl, P.; Schloter, S.; Hofmann, U.; Haarer, D. Proc. SPIE 1998, 3471, 29. (6) Schloter, S.; Schreiber, A.; Grasruck, M.; Leopold, A.; Kol’chenko, M.; Pan, J.; Hohle, C.; Strohriegl, P.; Zilker, S. J.; Haarer, D. Appl. Phys. B 1999, 68, 899. (7) Schloter, S.; Ewert, K.; Hofmann, U.; Ba¨uml, G.; Hoechstetter, K.; Eisenbach, C.-D.; Haarer, D. J. Opt. Soc. Am. B 1998, 15, 2560. (8) Adam, D.; Closs, F.; Frey, T.; Funhoff, D.; Haarer, D.; Ringsdorf, H.; Schuhmacher, P.; Siemensmeyer, K. Phys. ReV. Lett. 1993, 70, 457. (9) Malliaras, G. G.; Krasnikov, V. V.; Bolink, H. J.; Hadziioannou, G. Phys. ReV. B 1995, 52, R14324. (10) Hoechstetter, K.; Schloter, S.; Hofmann, U.; Haarer, D. J. Chem. Phys. 1999, 110, 4944. (11) Cui, Y.; Swedek, B.; Cheng, N.; Zieba, J.; Prasad, P. N. J. Appl. Phys. 1999, 85, 38. (12) Silence, S. M.; Walsh, C. A.; Scott, J. C.; Matray, T. J.; Twieg, R. J.; Hache, F.; Bjorklund, G. C.; Moerner, W. E. Opt. Lett. 1992, 17, 1107. (13) Orczyk, M. E.; Swedek, B.; Zieba, J; Prasad, P. N. J. Appl. Phys. 1994, 76, 4995. (14) Scher, H.; Montroll, E. W. Phys. ReV. B 1975, 12, 2455.
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