The electron transporting molecule tris(8-hydroxyquinoline) aluminum (Alq3) was added in low concentrations to a photorefractive polymer composite to provide trapping sites for electrons. This sample exhibited larger two-beam coupling gain, higher diffraction efficiency at lower voltages, and an increased dielectric breakdown strength compared to a control sample. The dynamics also revealed the presence of a competing grating, and a bipolar charge transport model is shown to fit the data. Overall, Alq3 improves the response time, efficiency, and breakdown voltage without a significant increase in absorption or loss of phase stability. This has applications for reflection displays and pulsed writing, where charge trapping and generation are major factors limiting the usefulness of photorefractive polymers.
©2010 Optical Society of America
Since the first discovery of the photorefractive (PR) effect in polymers , a wide range of applications have been studied , such as optical correlation , beam cleanup , communication , tissue imaging , and dynamic holographic displays [7,8]. PR polymers have gained interest due to their high optical nonlinearity compared to inorganic materials, as well as low cost, ease of fabrication, and tunability of optical and electronic properties. Large diffraction efficiency, fast response time, and long persistence [8–11] have all been demonstrated in organic polymer composites.
In the PR effect, a modulation of the refractive index is induced which mimics the interference pattern of two coherent light beams. Charge generation, transport, and trapping lead to the development of space-charge (SC) field, and alignment of nonlinear and/or birefringent molecules in this field creates the macroscopic index modulation.
Many studies have been conducted on the effects of trap density and depth on PR performance [12–14]. In particular, it has been shown [15,16] that the addition of Alq3 increases the two-beam coupling (TBC) gain and response time by acting as a trap for electrons, which increases the magnitude and speed of charge separation. Here, we report that the addition of Alq3 leads to the development of a competing grating, as well as significant improvements in the steady-state diffraction efficiency and rise time. A bipolar charge transport model is shown to fit the data and the physical mechanisms are discussed. The advantages provided by Alq3 are anticipated to be useful for updateable displays, where speed and high efficiency are essential to the success of applications such as medical imaging, data mining, and command-and-control.
The PR composites studied contained the polymer host matrix poly(acrylic tetraphenyldiaminobiphenyl) (PATPD), which has a polyacrylate backbone with the well-known hole-transporting tetraphenyldiaminobiphenyl-type (TPD) pendant group attached through an alkoxy linker . The chromophore 4-homopiperidinobenzylidenemalononitrile (7-DCST) was added to provide a refractive index change, but also acts as a sensitizer at 532nm, though C60 was the primary sensitizer. N-ethyl carbazole (ECZ) was included to act as a plasticizer. In one sample, Alq3 was included at 1 wt%, an approximately optimal concentration for providing trapping sites for electrons generated through the photorefractive process, while still preventing bulk electron transport in the Alq3 itself. Larger concentrations (1.5-2.0 wt%) tended to phase separate quickly, making the measurements unrepeatable. The compositions of the two samples studied are shown in Table 1 . The samples were prepared by melt processing the mixture between two indium-tin-oxide coated glass slides, with 105μm spacer beads used to fix the thickness. The absorption coefficient of each sample at 532nm and 633nm was approximately 160cm−1 and 45cm−1, respectively. Both samples exhibited phase stability for several months at room temperature.
Degenerate four-wave mixing (DFWM) measurements were performed at 532nm to take advantage of the sensitization from 7-DCST. Each writing beam was s-polarized and incident on the sample with 100mW/cm2. The external angle between the beams was 36° and the sample was tilted 55° relative to the bisector. The reading beam was p-polarized with an intensity of 1mW/cm2 and aligned to be counter-propagating. Steady-state efficiency was measured by ramping the voltage up at 25V/sec and continuously monitoring the reading beam. To measure the transient behavior, a constant voltage was applied and the reading beam monitored the grating buildup as the writing beams were turned on and off, the total recording time being about 4 minutes.
TBC measurements were performed in the same geometry as DFWM, except no reading beam was present and the writing beams were p-polarized. The polarity of the electric field was set to minimize beam fanning . 633nm light was used as the increased absorption at 532nm and the polarization introduced a prohibitive amount of beam fanning. This allowed an increase of the intensity to 1W/cm2, since the samples were less vulnerable to high power at 633nm owing to the lower absorption coefficient there.
Photoconductivity was also measured at 532nm. The voltage was applied in the dark for 10-20min until the dark current reached steady-state, then the sample was illuminated until the current again leveled out.
The TBC results are shown in Fig. 1 . The addition of Alq3 improved the magnitude of the gain at all voltages; at 60V/μm there is a 75% increase in the net gain. Additionally, the Alq3 tends to increase the dielectric breakdown strength, allowing larger voltages to be applied. The net gain of Sample B at 70V/μm was 89cm−1, while at 532nm, no net gain was observed up to this voltage due to the large scattering losses. This could be attributed to the increased magnitude of the SC field due to more charge separation, however the TBC gain is affected by both the magnitude and phase of the SC field. DFWM is only directly affected by the former, and those results are shown in Fig. 2 .
The improvement in the material response is significant. While no over-modulation peak was observed in Sample A up to 60V/μm, the addition of Alq3 pushes the over-modulation down at least 20V/μm. Figure 3 shows the transient response for Sample B (70V/μm) and Sample A (60V/μm). Sample A rises to 67% of the steady-state value in about 40s, while it takes Sample B only 200ms, a difference too large to be attributed to the small difference in voltage. Sample B also exhibits recovery after about 30s of decay, which eventually turned around and started to decay again after 10-20min of observation. The decay for Sample A was measured for about 10mins without any sign of recovery.
At Alq3 concentrations of 1.5 wt% and above the composition was not phase stable, or would separate very quickly, precluding the possibility of measurements. Though up to at least 10 wt%, the breakdown strength remained. At 0.1 wt%, there was no observed difference with Sample A. This suggests there is window from above 0.1 to below 1.5 wt% where the dynamics can be altered without affecting the stability.
4. Theory and discussion
Observation of dynamic recovery in other systems has been associated with the formation of a competing grating via separate transport and trapping processes for holes and electrons [19–21]. Note that a grating from a single charge carrier is insufficient to explain the existence of two extrema in the decay without similar behavior also being present during formation. A useful theoretical model is provided by Bashaw et al. , developed for inorganic PR crystals. The SC field transients will rise and decay with two time constants. In general, however, the parameters for each rate depend on the recombination and ionization rates, mobilities, etc. of each charge species, and cannot be attributed separately to electrons and holes, except in the trap-limited case. That solution is
This result is adapted here to include a relative phase shift between the two gratings and to associate two time constants with one of the gratings. This latter modification is added because in PR polymers, both conduction and orientation contribute to the efficiency dynamics on different time scales, so multiple time constants are needed. It is incorporated only for one grating because it is clear from the transients that one grating is much faster than the other, so additional parameters to describe longer term behavior are not necessary. Thus, the equations for the time-dependence of the SC field and efficiency are as follows:
There are seven parameters in this model, which was used to fit the dynamics of Sample B. The phase shift and relative grating magnitudes should be the same for the rise and decay, so both regimes were fit in an iterative fashion. The results are shown in Fig. 4 . For the rise from (2), the parameters are: c1 = 2.10, m = 0.82, Γ1s = 4.5s−1, Γ2s = 0.00085s−1, Γ1l = 0.0095s−1, E2/E1 = 0.22, and ϕ = 0.70π. For the decay from (3), the parameters are: c1 = 2.30, m = 0.62, Γ1s = 0.15s−1, Γ2s = 4x10−5s−1, Γ1l = 0.020s−1, E2/E1 = 0.22, and ϕ = 0.70π.
The physical processes are as follows. During writing, both a strong grating and a weak competing grating are formed. The decay in the signal is due primarily to the fact that the peak efficiency occurs at a lower modulation than what is achieved in the steady-state at 70V/μm. When the writing beams are turned off, the quick increase and decay is caused by the index modulation passing again through the over-modulation point. The stronger grating decays first and eventually becomes equal in magnitude to the weaker competing grating, which results in a minimum of the SC field (not exactly zero due to the phase shift). As the decay continues, the SC field reverses sign and the magnitude increases as the competing grating is slowly revealed. The dashed line in Fig. 4 is the best fit without the competing grating, showing the lack of recovery. Similarly, if only one decay constant is used for grating one, the theory predicts that the revelation of the second grating and recovery of efficiency would happen much faster than is observed, since the first grating decays more quickly. It is not possible to obtain as good a fit.
Based on experimental observations, we propose both hole and electron transport in this system. The HOMO and LUMO energies of the components are shown in Fig. 5 [17,23,24]. Hole transport occurs through the HOMO level of PATPD, and traps are provided by impurities, molecular conformations, and C60 anions. The electrons in the C60 will remain in the LUMO given the energies, but the electrons in 7-DCST can be trapped in either the Alq3 or neutral C60. Given that C60 has a higher molecular weight, smaller loading, and can be reduced via illumination, the number density of Alq3 is at least 4 times larger, and thus Alq3 is the primary electron trap. Electrons can also be trapped in Alq3 without having to transport to a dark region. Thus, there is a higher probability that charge pairs separate before recombination, increasing the magnitude of the primary grating leading to the improved steady-state performance, and allowing the SC field to develop to larger values in a shorter amount of time. This also allows both charge species to transport and trap, leading to the formation of two gratings. It is believed that the electron transport is occurring through the 7-DCST, though further tests are needed to confirm this.
The increased response time and efficiency is beneficial for many real-world applications, such as video rate displays and dynamic image correction. However, the long decay time of the competing grating limits speed without an erasing step. This can be overcome by writing for shorter times, which will still allow for efficient charge separation on short time scales while preventing the competing grating formation on longer time scales. However, in this composition, to make use of the effective increase in persistency the applications are limited to sufficiently long writing times.
The decreased photoconductivity (Fig. 6 ) in Sample B supports the interpretation of Alq3 acting as an electron trap. Under illumination, both holes and electrons contribute to the current. Given the much larger concentration of the energetically favorable PATPD, Alq3 is unlikely to affect the conduction of holes. This is also supported by the fact that Sample B tended to survive higher applied fields for much longer periods of time.
The addition of Alq3 to a photorefractive composite has been shown to increase the magnitude of two-beam coupling, reduce the over-modulation voltage, drastically increase the hologram writing speed, and allow the formation of a second, weaker grating via bipolar charge transport and trapping. In particular, Alq3 doping can increase the sensitivity of PR polymers, improve the trap density necessary for reflection geometries and pulsed laser writing, and increase the dielectric breakdown strength, all of which can be limiting factors for real-world applications. This can have significant impact on several potential applications involving holography, such as telepresence, 3D entertainment, and industrial design.
The authors acknowledge support from the US Air Force Office of Scientific Research, the NSF Science and Technology Center for Materials and Devices for Information Technology Research Grant# - 0120967, and DARPA Phase II STTR.
References and links
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