We study the influence of a transient dark current on the buildup dynamics of photorefractive index gratings, which are excited right after the onset of a pulsed voltage, by using low glass-transition temperature photorefractive polymers under a heat-assisted condition. We conclude that the development of photorefractive index gratings is majorly controlled by changes in the transient dark current, through the suppression of a saturated photo-induced space-charge electric field. We also show that this influence can be estimated reasonably well by a simple analysis of the measured transient current traces.
©2013 Optical Society of America
Application of low glass-transition temperature photorefractive (Low-Tg PR) polymers for dynamic image signal processing has been extensively investigated because these polymers have exhibited a refractive index change on the order of 10−3 with a response time of a few tens of milliseconds and they also show suitability for use in large area film devices [1–3]. However, low-Tg PR polymers are basically multicomponent polymer systems and frequently possess ionic mobile charges. Such charges can constitute dark current flow in materials. This is known to degrade the performance of PR index changes, e.g., reduction in the response speed and decrease in the amplitude of the PR index change. Furthermore, PR polymers generally require a high electric field. This can assist charge injection from electrodes to enhance the dark current in materials, leading to degradation of the PR performance [4–6]. It is also known that for PR polymers, the increase in operating temperature is responsible for the acceleration in response speed. However, in this case dark currents should increase and result in a reduction in the response amplitude [7,8].
As mentioned earlier, low-Tg PR polymers essentially need an external electric field during operation. This field maintains the ordering of chromophores and induces photoconductivity in the material, enabling the PR index change. It is known that high electric fields are effective for the enhancement of both the magnitude of PR index changes and the response speed. However, the continuous application of a high electric field increases the probability of electric breakdown and seriously damages materials. In contrast, a pulsed voltage allows the temporal application of a very high electric field to materials and boosts the PR performance. The pulsed voltage enhancement may be used for the period when a quick PR index change or a fast writing process is required. Such a concept was realized recently by the voltage kick-off technique , in which a pulsed voltage with relatively a long period (2 s) is used. A pulsed voltage generally brings a transient dark current on the voltage onset: an electric current rapidly dropping from a high intensity level to a stable low level. In addition, if such a pulsed voltage were used many times continuously, the mobile charges in the materials would gradually accumulate near an electrode, altering the change in the transient dark current from pulse to pulse. If the transient dark current has a transitional period that is comparable with the response time of a PR process, the buildup dynamics of PR index changes would be affected considerably. As a result, the development of PR index changes would differ depending on the pulse timing and the operation duration. Moreover, if the operation temperature of the materials were elevated to obtain the best PR performance [7,8], such effects would be enhanced owing to the increased dark currents. Thus, to optimize the pulsed voltage enhancement, it is crucial to investigate the effects of the transient change of the dark current on the dynamics of PR index changes.
In this paper, we study the effects of transient dark current on the buildup dynamics of PR index changes. In order to enhance the transient dark current effects, we excite the formation of PR index gratings right after a pulsed voltage application, using a low-Tg PR polymer operated under the heat-assisted condition. The voltage pulse was applied repeatedly to materials in order to vary the transient dark current flow, and the effect on the corresponding PR index evolution was examined. We also used a hole-injection blocking electrode to vary the transient dark current.
In the experiments, a low-Tg PR polymer composite of PVK (39.4 wt.%), AODCST (30 wt.%), EHCz (30 wt.%), and C60 (0.6 wt.%), a mixture of typical functional molecules (Fig. 1 ), was examined [9,10]. The polymer composite was fixed as a thin plate in a sandwich cell made of two ITO glass plates and was used in the following optical and electrical experiments. The thickness of the polymer thin plate in the cell structure was 50 μm. Tg of the polymer composite, without application of any electric field , was evaluated to be −30.1 °C by DSC measurements (Seiko, DSC6220). Only fresh samples were used in each measurement described below.
As mentioned earlier, dark currents caused by mobile charges such as ionic ones and electrode-injected ones can deteriorate the light-induced refractive index changes. Further, it has been observed that light irradiation can also affect the dynamics of the refractive index change through the increase of deep traps . Therefore, to observe the direct effects brought about by the dark current on the PR index change, it is necessary to exclude this light irradiation effect.
We prepared four-wave mixing (FWM) experiments to investigate the buildup dynamics of PR index changes induced by a pulsed voltage, in which a diffraction power from a PR index grating formed by two writing beams was measured (Fig. 2(a) ). Timing charts for irradiating the writing beams (I1 and I2) and applying an electric field (E) are shown in Fig. 2(b). An electric field of 30 V/μm (1.5 kV) was first applied for 10 ms to the simultaneous irradiation of two writing beams (λ = 633 nm) on samples with an equal intensity of 60 mW/cm2. The electric field was switched ON or OFF within 5 μs using a fast high power amplifier (Matsusada, HEOPT-5B20), and the writing beams were switched ON or OFF within 200 μs using an optical shutter (Thorlabs, SH05). The sample was light-excited by the two writing beams for 600 ms, and the diffracted beam power (Id) was monitored using a probe laser beam (λ = 808 nm), which had no PR effect on the samples. The writing beam irradiation was limited to minimize the aforementioned light irradiation effect. The electric field was applied for 10 s from the switching (ON state) and then reset to 0 V/μm for 5 s (OFF state). This process for the writing beam and the electric field was repeated. During the ON state, we expected an accumulation of charges near the electrodes in the materials. This would, in turn, alter the transient dark current in the next sequence. The subsequent OFF state was expected to completely remove the PR index grating formed by the two-beam exposure. It should be noted that the chromophore orientation due to the electric field in the sample was completed in a period much lesser than 1 ms. This was confirmed by transient Mach–Zehnder experiments. Thus, the development of the PR index grating measured here should dominantly reflect the growth of the photo-induced space charge electric field. In addition to this experiment, we also measured the temporal change in dark currents (idark) during the ON state in the samples with a picoammeter (Keithley, 6485), by applying the same schedule of the electric field, as shown in Fig. 2(b). It should be noted that during this experiment, the samples were kept in dark in the absence of any light irradiations.
We carried out these measurements on two types of sandwich cells: one made of two bare ITO glass plates (normal cell) and the other of a bare ITO glass plate as a cathode and a thin SiO2 layer (30 nm)-covered ITO glass plate as an anode (SiO2-coated cell). The SiO2-coated cell blocks a hole injection from the anode to the sample polymer, in contrast to the normal cell, and thus decreases the dark current level. In addition, we kept the sample temperature slightly higher than the room temperature (33 °C) in order to operate under the heat-assisted condition.
3. Results and discussions
3.1 Results of the FWM experiments
The normal cell and the SiO2-coated cell show similar trends for the temporal evolution of diffraction power on each pulsed voltage excitation. However, the buildup speeds and the amplitudes reached are different, as will be discussed later. Figure 3(a) shows the typical diffraction power traces obtained from the normal cell, which are extracted from the first 3 s during the ON state in each voltage pulse (only the first 4 pulses data are shown for convenience). The traces from the 2nd voltage pulse show similar temporal evolutions; they abruptly increase on the two-beam exposure and gradually approach stable levels, and then, they drop quickly at the end of the exposure, followed by gradual decay. Only the trace of the 1st voltage pulse shows an apparently different evolution. The trace increases abruptly on the beam exposure but soon reaches a plateau, staying the same for ~300 ms, and increases again. The trace then decays steeply at the end of the exposure like the other voltage pulse cases, but this decay is followed by a slight increase and then a gradual decay. The maximum diffraction power obtained at each end of the beam exposure increases in the order of voltage pulses. Figure 3(b) shows the evolution of the maximum diffraction power (Idmax), where the diffraction power from the 1st voltage pulse is that measured after 400 ms from the writing beam exposure, corresponding to the period before the diffraction increased. Diffraction powers obtained from both cells stabilize over the 9th voltage pulse, and the values, which correspond to the diffraction efficiency of ~2%, are more than twice the values obtained from the 1st pulse. In addition, the maximum diffraction powers from the SiO2-coated cell are always slightly larger than those from the normal cell. The inset of Fig. 3(b) shows the superimposed curves of the diffraction power evolution obtained from the first 4 voltage pulses (SiO2-coated cell). The increase in the maximum level with the voltage pulse is clearly shown. Moreover, interestingly, all the curves trace almost the same buildup line, suggesting that each PR grating grows under the same process. We find that the temporal increase in the diffraction power during the two-beam exposure follows the double exponential function:13]. Figure 3(c) shows the plots of faster response speed (1/ τ1) obtained by the curve fitting for each two-beam exposure. All the response speeds were obtained for a weight factor (m) of around 0.8. The weight factor was slightly higher for the SiO2-coated cell from the 4th voltage pulse (inset of Fig. 3(c)). It should be noted that the diffraction power trace from the first voltage pulse was curve fitted only by the data obtained in the first 400 ms. It should also be noted that the response speed under the heated condition was accelerated to twice the response speed obtained under room temperature. For both cells, the response speed reduces with voltage pulses before reaching a stable value. The 1st voltage pulse results in a faster response speed for each cell. The SiO2-coated cell shows a slightly faster speed. From the 2nd voltage pulse, response speed is reduced by more than 30% in both cells. Response speed for the SiO2-coated cell is seen to gradually decrease until around the 5th voltage pulse, after which it stabilizes, resulting in a slower speed than that of the normal cell.
3.2 Results of the transient dark current measurements
Figure 4 shows the temporal changes in the dark current. The horizontal axis represents the elapsed time from the onset of the pulsed voltage. The period from 0.01 s to 0.6 s (yellow shaded region) corresponds to the two-beam exposure term. For convenience, the figure shows data measured for the first three voltage pulses. The data is extended through the time of 2 s to explain a trend difference between the two cells. The changes in the current from the 4th pulse, indicated by solid lines, are drawn based on a fitting equation described later.
All the currents decrease with time, with an abrupt drop in the initial period until ~0.02 s, followed by a gradual reduction to a stable or semi-stable level at ~0.6 s. The currents also reduce the magnitude in the order of voltage pulses in each cell. The former is due to the drift of mobile charges toward an electrode. It should be noted that the contribution of the polarization current due to the chromophore orientation  would be very small due to the short response time of the orientation (1 ms) as described earlier. The latter is due to the trapping of charges near electrodes. During the ON state, mobile charges drift toward the electrodes and accumulate near them. This results in a reduction in the dark currents with time. During the subsequent OFF state, some of these trapped charges move back to electrically neutral positions in the medium whereas some remain near the electrode. This results in a decrease in the number of mobile charges, which in turn decreases the dark current at the beginning of the ON state in the next voltage pulse. Such effects due to the accumulated charges are also observed in the multiple polings of nonlinear optical polymers . As the voltage pulsing continues, only charges that could migrate remain in the medium at the end and, as a result, show the similar trace curves of dark currents with the pulses like the 6th to 9th pulses for each cell. Thus, the initial current at 0.01 s should reflect the amount of the trapped charges near electrodes. The SiO2-coated cell shows a gradual increase in the trapped charges with the voltage pulse, whereas the normal cell shows a large increase after the 1st voltage pulse, after which there are almost no changes.
In the case of the normal cell, after the initial rapid decrease, the dark current maintains a stable level (1st to 3rd voltage pulses). Such a trend was also found in non-purified PVK-based PR composites and is explained by rich mobile ionic charges . The result obtained here is attributed to the increased mobile charges owing to the elevated temperature and to the injected charges from the electrode. In contrast, in the case of SiO2-coated cell, the stable current is only seen in the 1st voltage pulse. From the 2nd voltage pulse, the current continues decreasing gradually after the initial drop. This is due to less mobile charges as compared to the normal cell case. In addition, from the 4th voltage pulse, the decreasing currents reach stable levels from around 0.3 s. However, in the normal cell case, the currents from the 4th voltage pulse continue decreasing gradually after the initial drop, without reaching an apparent plateau. This is due to a large amount of charges provided by the injection from the electrode.
Such trends in the dark currents for a period of 0.6 s are fitted well by the following equation:15], because of a broad distribution and superposition of relaxation time constants.
3.3 Influence of dark currents on the development of PR gratings
Dark currents from ionic mobile charges and electrode injected mobile charges deteriorate PR index changes through (i) the recombination with photo-charges , (ii) the migration through the sample to weaken the effective electric field by the local field effect [7,17], and (iii) the recombination with trapped charges  or the reduction in photo-to-dark conductivity contrast . As seen earlier, the dark current level decreases with the voltage pulse in each cell case (Fig. 4). However, the PR response speed decreases with the pulses (Fig. 3(c)). Hence, (i) and (ii) would not be a dominant effect, since they should increase the response speed through the raised photoconductivity and effective electric field, respectively, with the decrease in dark currents and thus the voltage pulse. Factor (iii), however, suggests an increase in the saturated number of trapped charges with the voltage pulse; a lower dark current should allow a larger amount of charges to be trapped. This point can be supported by the result of Idmax in Fig. 3(b). Since Idmax is close to the steady state level of the diffraction power, it should reflect the saturated amplitude of the photo-induced space-charge electric field or an effective trap density . Moreover, it would be reasonable to consider that the amount of the saturated trapped charges should be controlled by the temporally (semi-)stable level of the dark current, because this can provide a steady effect on the trapped charges. As pointed out earlier, in each cell, all the gratings formed by the voltage pulse should develop under almost the same growing process. Therefore, it can be said that each grating grows under the same process, but the growth is terminated by the temporally stable dark current, which would limit the effective trap density of materials. Thus, the onset of the stable current flow would determine the buildup properties of the PR grating.
In order to support this hypothesis, the changes in the response speed and the saturated amplitude of the photo-induced space-charge electric field on the voltage pulse were plotted in terms of the onset of the stable dark current flow. The onset was obtained from the simple analysis of the shape of the fitted curve, as shown in the inset of Fig. 5(a) ; the analysis of the intersection (P) of two asymptotes: one to the curve at t = 0.01 s (start of the two-beam exposure) and the other to that at t = 0.6 s (the end of the exposure). The onset time (tp) and the onset level of the stable dark current (ip) were obtained from the intersection. The inverse of the onset time (1/tp) should correspond to the response speed of PR gratings. The onset level of the stable dark current should correlate with the amplitude of the saturated photo-induced space-charge electric field.
Figure 5(a) shows the change in the inverse of the onset time on the voltage pulse for each cell. Compared to the measured result shown in Fig. 3(c), this result represents the trend of the response speed curves very well. Thus, it indicates that a change in the dark current, especially the current levels and the reduction rates at the beginning (t = 0.01 s; initial drop part) and the end (t = 0.6 s; final stabilizing part) of the writing beam exposure, should crucially affect the response speed. It reveals that a faster response speed for the SiO2-coated cell at the 1st voltage pulse is mainly due to a steeper reduction rate of the dark current at t = 0.01 s. The initial steeper reduction for the SiO2-coated cell would be due to the hole-injection block by the SiO2 layer. However, faster response speeds for the normal cell from the 4th voltage pulse are due to slightly larger reduction rates of the dark current at t = 0.6 s, which can start the termination of the PR process early and lead to a faster response speed. Moreover, such a termination would be gradual in time, compared to the termination from the SiO2-coated cell case, resulting in more distributed time constants and the small weight factor (m), as shown in the inset of Fig. 3(c). Figure 5(b) shows the dependence of the onset of the stable dark current level on the voltage pulse. It represents the trend for each cell reasonably well, compared to the trend obtained from Fig. 3(b), and it also shows a smaller onset level of the dark current for the SiO2-coated cell from the 4th voltage pulse, which agrees with the result from Fig. 3(b). However, in Fig. 5(b), the onset levels of the dark current from the 2nd and 3rd voltage pulses do not match the result shown in Fig. 3(b). We attribute this deviation to a larger space-charge electric field formed in the normal cell by the charges accumulated near electrodes. Such a space-charge electric field would act as a screening field and reduce the effective electric field to suppress the saturated photo-induced space-charge field . In fact, the amount of accumulated charges, or the decrease in the dark current level, at t = 0.01 s for the 2nd and 3rd voltage pulses are larger for the normal cell than that for the SiO2-coated cell (Fig. 4), suggesting that the saturated photo-induced space-charge field for the normal cell can be smaller than that indicated in Fig. 5(b).
We studied the influence of a transient dark current on the buildup dynamics of PR index gratings, which were excited soon after the onset of a pulsed voltage, using a low-Tg PR polymer under the heat-assisted condition. We varied the temporal change of the transient dark current by the continuous application of voltage pulses to materials and also by adding hole-injection block layers on electrodes. We then compared the temporal evolution of the corresponding diffraction power obtained from the FWM experiments. It is observed that the evolution of the diffraction power or the development of PR index gratings is dominantly controlled by the transient dark current changes. The semi-stable dark current restricts the saturated amplitude of the photo-induced space-charge electric field. The onset of the stable dark current determines both the buildup speed and modulation amplitude of PR gratings, which can be estimated reasonably well by the simple analysis of the transient current traces. In order to achieve both faster and larger buildup of PR gratings with voltage pulses, the onset of the stable current should be controlled to have shorter time and lower level. This would be realized by the reduction of mobile charges such as ionic mobile charges and electrode injected charges in materials, by which a stable performance from pulse to pulse would be also given. Thus, we show that the transient dark current flow is a very important factor to optimize conditions for pulsed voltage-assisted PR effects. Finally, we point out that a screening electric field formed by accumulated charges near electrodes would also be a factor that degrades the buildup dynamics of PR index changes.
This work has been supported by the program for S-innovation (Strategic Promotion of innovative Research and Development), Japan Science and Technology (JST).
References and links
1. S. Köber, M. Salvador, and K. Meerholz, “Organic photorefractive materials and applications,” Adv. Mater. 23(41), 4725–4763 (2011). [CrossRef]
2. S. Tay, P.-A. Blanche, R. Voorakaranam, A. V. Tunç, W. Lin, S. Rokutanda, T. Gu, D. Flores, P. Wang, G. Li, P. St Hilaire, J. Thomas, R. A. Norwood, M. Yamamoto, and N. Peyghambarian, “An updatable holographic three-dimensional display,” Nature 451(7179), 694–698 (2008). [CrossRef] [PubMed]
3. M. Salvador, J. Prauzner, S. Köber, K. Meerholz, J. J. Turek, K. Jeong, and D. D. Nolte, “Three-dimensional holographic imaging of living tissue using a highly sensitive photorefractive polymer device,” Opt. Express 17(14), 11834–11849 (2009). [CrossRef] [PubMed]
4. J. A. Quintana, J. M. Villalvilla, P. G. Boj, L. Martín-Gomis, J. Ortiz, F. Fernández-Lázaro, Á. Sastre-Santos, and M. A. D. García, “Enhanced photorefractivity of poly(N-vinylcarbazole)-based composites through electric-field treatments and ionic liquid doping,” Adv. Funct. Mater. 19(3), 428–437 (2009). [CrossRef]
5. W. Lv, Z. Chen, and Q. Gong, “Improvement on the photorefractive performance by the insertion of a SiO2 blocking layer,” J. Opt. A, Pure Appl. Opt. 9(5), 486–489 (2007). [CrossRef]
6. H. Zhao, C. Lian, X. Sun, and J. W. Zhang, “Nanoscale interlayer that raises response rate in photorefractive liquid crystal polymer composites,” Opt. Express 19(13), 12496–12502 (2011). [CrossRef] [PubMed]
8. J.-W. Oh, W.-J. Joo, I. K. Moon, C.-S. Choi, and N. Kim, “Temperature dependence on the grating formation in a low-Tg polymeric photorefractive composite,” J. Phys. Chem. B 113(6), 1592–1597 (2009). [CrossRef] [PubMed]
9. W. E. Moerner, A. Grunnet-Jepsen, and C. L. Thompson, “Photorefractive polymers,” Annu. Rev. Mater. Sci. 27(1), 585–623 (1997). [CrossRef]
10. J.-C. Ribierre, T. Aoyama, T. Muto, Y. Imase, and T. Wada, “Charge transport properties in liquid carbazole,” Org. Electron. 9(3), 396–400 (2008). [CrossRef]
11. J. A. Quintana, P. G. Boj, J. M. Villalvilla, M. A. Díaz-García, J. Ortiz, L. Martín-Gomis, F. Fernández-Lázaro, and Á. Sastre-Santos, “Determination of glass transition temperature of photorefractive polymer composites from photoconductivity measurements,” Appl. Phys. Lett. 92(4), 041101 (2008). [CrossRef]
13. R. Bittner and K. Meerholz, “Amorphous organic photorefractive materials,” in Photorefractive Materials and Their Applications, P. Gunter and J.-P. Huignard, eds. (Springer, 2007), Vol. 2.
14. M. A. Pauley, H. W. Guan, and C. H. Wang, “Poling dynamics and investigation into the behavior of trapped charge in poled polymer films for nonlinear optical applications,” J. Chem. Phys. 104(17), 6834–6842 (1996). [CrossRef]
15. K. Okamoto, S. Kusabayashi, and H. Mikawa, “The photoconductivity of poly (N-vinylcarbazole). II. Dark conductivity in a sandwich-type cell,” Bull. Chem. Soc. Jpn. 46(7), 1953–1959 (1973). [CrossRef]
16. D. J. Wehenkel, L. J. A. Koster, M. M. Wienk, and R. A. J. Janssen, “Influence of injected charge carriers on photocurrents in polymer solar cells,” Phys. Rev. B 85(12), 125203 (2012). [CrossRef]
17. S. Schuessler, R. Richert, and H. Baessler, “Relaxation of second-harmonic generation in guest/host polymers poled by indium-tin oxide sandwich electrodes,” Macromolecules 27(15), 4318–4326 (1994). [CrossRef]
18. O. Ostroverkhova, A. Stickrath, and K. D. Singer, “Electric filed-induced second harmonic generation studies of chromophore orientational dynamics in photorefractive polymers,” J. Appl. Phys. 91(12), 9481–9486 (2002). [CrossRef]