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Abstract
The effects of a second electron trap, tris-(8-hydroxyquinoline)aluminum (Alq3), 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7), or bathophenanthroline (BPhen), on the photorefractive (PR) sensitivity, response time, and diffraction efficiency are investigated for a methyl-substituted poly(triarylamine) (PTAA)-based PR composite. An extremely high PR sensitivity of 1145 cm2 J−1 with a sub-millisecond response time of 397 μs with an internal diffraction efficiency of 73.3% is achieved at an electric field of 60 V μm−1 for the PTAA-based PR composite containing BPhen. The fast, sub-millisecond response time and fourth-order sensitivity are mainly contributed to the reduction of the photocurrent due to the formation of a charge-transfer complex between PTAA and BPhen. Furthermore, the obtained results estimate the response time to be 97 μs for the PTAA-based PR composite containing an appropriate second electron trap with a photoconductivity of 3.3 nS cm−1.
© 2018 Optical Society of America
1. Introduction
Holographic displays using the photorefractive (PR) effect have received a great deal of attention because they can essentially provide three-dimensional images without special eyeglasses [1–3]. The PR phenomenon in PR polymeric composites consists of the following processes: (1) charge carriers are formed by photon excitation in the bright region of the interference pattern, (2) the produced charges (mainly holes) in the bright region are transported through the material along an external applied electric field and are trapped in the dark region of the interference pattern, (3) a space-charge field is formed by the periodic distribution of charges, and (4) the refractive index modulation is formed by both the electro-optic (Pockels) effect and the reorientation of the nonlinear optical dye (orientational enhancement effect) [4–6]. Normal photorefractive crystal has weak diffraction, whereas the photorefractive polymer composites commonly give high diffraction efficiency due to the orientational enhancement effect. Therefore, the holographic characteristics of a high diffraction efficiency and a fast response time for the photorefractive polymer composites are attractive for realistic 3D displays.
PR polymer composites have response times from seconds to milliseconds [7–11]. The effect of a wide range of pulse durations on the photorefractive diffraction properties has also been investigated [12]. In previous reports [13, 14], we discussed the PR performance induced by the high photoconductive properties of poly[bis(2,4,6-trimethylpheneyl)amine] (PTAA). The high photoconductivity of the PTAA-based PR composite provided the fast response time of 10 ms [13]. However, its low space-charge field of less than 1 V μm−1 limited the PR response. After the study, an improved PR response time of 860 μs with space-charge field of 2 V μm−1 was observed by adding a second electron-trapping agent, tris-(8-hydroxyquinoline)aluminum (Alq3), to control the photocurrent flow through the charge transfer (CT) complex between PTAA and Alq3 [14]. In the paper [14], we discussed the estimated response time to build up the steady-state space-charge field under the assumption that all photogenerated charge carriers contribute to trap filling; however, the estimated response time from the photoconductivity and the measured response time for diffraction have a difference of 2 - 3 orders of magnitude. It is unknown how to reach the photoconduction-based limit. In this study, we investigate the relation between the PR response and the photoconductivity by changing the second electron trap in the PTAA-based PR composite. 1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7), bathophenanthroline (BPhen), and Alq3 are used as second electron trap agents in the PTAA-based PR composite. Using BPhen as the second electron trap leads to a more than two-fold faster response time over that of the Alq3-incorporated PTAA-based PR composite. The effect of photoconductivity on the PR response time is discussed.
2. Experimental sections
2.1. Materials and composite preparation
The photoconductive polymer PTAA (Sigma-Aldrich Co.) was reprecipitated with chloroform (as a good solvent) and hexane (as a poor solvent). PTAA as a pale-yellow powder (95.1% yield, Mw: 22,000 g mol−1, Mw/Mn: 1.8, Tg: 60°C) was collected by centrifugation (4000 rpm, 20 min). Piperidinodicyanostyrene (PDCST) as a nonlinear optical (NLO) chromophore and (2,4,6-trimethylphenyl)diphenylamine (TAA) as a plasticizer were synthesized in our laboratory using the procedure described in a previous report [13]. Phenyl-C61-butyric acid methyl ester (PCBM) (Tokyo Kasei Co., Japan) was used as a sensitizer. Alq3 (Tokyo Kasei Co., Japan), OXD-7 (Tokyo Kasei Co., Japan) or BPhen (Tokyo Kasei Co., Japan) were used as second electron traps. The structural formulae of these materials are shown in Fig. 1.
The composition ratio of PDCST, TAA and PCBM was fixed at 35, 20, and 0.5 by weight, respectively. On the other hand, the composition of PTAA and each second electron trap were changed to 43.5/1 and 44.5/0 (as a blank) by weight. The mixtures of PTAA/PDCST/TAA/PCBM (44.5/35/20/0.5) and PTAA/PDCST/TAA/PCBM/second electron trap (43.5/35/20/0.5/1) were stirred in tetrahydrofuran (THF) for 24 h and then cast on a hot plate at 70°C for 24 h. The resulting composites were sandwiched between two ITO electrodes modified with a self-assembled monolayer (SAM) of aminopropyltrimethoxysilane (APTMS) on hot plate at 160°C. The SAM-ITO electrodes reduced the dark current, which causes dielectric breakdown under very low electric field. The SAM-ITO preparation method and details of the effectiveness are described in a previous reference [15]. The film thickness of each PR composite was adjusted to 50 μm with using Teflon spacers.
2.2. Measurements
The UV-Vis spectrum of each PR composite was measured using a Lambda1050 UV/Vis/NIR spectrophotometer (Perkin-Elmer Co., USA). The absorption coefficient α was calculated by α = Aln(10)/L, where A is the absorbance and L is the thickness of the PR composite. The film thickness was adjusted to 20 - 30 μm so as not to exceed an A of 1 in the short wavelength region. The glass transition temperature Tg was measured using a DSC 2920 (TA Instruments Co.) at a heating rate of 10°C min−1.
The diffraction efficiency and PR response time of the PR device were measured using a degenerated four-wave mixing (DFWM) technique. PR gratings were written in the PR device by the intersected s-polarized beams of a diode-pumped solid-state (DPSS) laser with λ = 532 nm (25 mW, 0.534 W cm−2, Cobolt AB, Sweden) that were incident to the positively biased electrode at an angle of 42.5° (57.5°) for the writing beam relative to the normal angle of the device, which corresponds to an internal angle of θA = 21.94° (θB = 29.67°) according to Snell’s law using a refractive index of n = 1.7. A much weaker intensity p-polarized probe beam from the same laser source was propagated in the direction opposite to the writing beam and diffracted by the refractive index gratings in the PR device. A rectangular high voltage with a 100 Hz frequency was applied to the PR device using a high-voltage amplifier (Trek 10/10E, USA). The diffracted and transmitted signals were detected by two photodiode detectors. We used the intensities of the diffracted beam (Id) and transmitted beam (It) to calculate the internal diffraction efficiency (η) using the Eq. (1) [14]:
The PR response time (τ) was estimated using a stretched exponential function of Kohlrausch-Williams-Watts (KWW), as shown in the Eq. (2) [14]:
where t is the time, η0 is the steady-state diffraction efficiency, and β (0 < β ≤ 1) is a parameter related to the dispersion of the release time from the traps.The external diffraction efficiency ηext was calculated by the Eq. (3) [14]:
where α is the absorption coefficient and θA is the internal angle of beam A. The empirical sensitivity S was defined by the Eq. (4) [14]:where I is the intensity of the illuminated laser. In this case, I = 0.534 Wcm−2.The asymmetric energy transfer was measured using a two-beam coupling (TBC) technique. The laser intensity of two beams crossing through the sample was measured with photodiodes to evaluate the optical gain coefficient Γ [14],
where L is the thickness of the sample; θA and θB are the internal angles between the normal to the sample surface and the recording beams A and B, respectively; and I1 and I2 are the transmitted intensities of the respective beams.We recorded a steady-state photocurrent using a current monitor equipped in a Trek 610E high-voltage amplifier when we measured a steady-state DFWM signal.
3. Results and discussion
3.1. Effect of second electron traps
In our previous report [14] on a PTAA-based PR composite, by adding the second electron trap, Alq3, we achieved a sub-millisecond photorefractive response of 860 μs and a diffraction efficiency of 83.0% due to the reduction of the photocurrent. The photocurrent is plotted as a function of the electric field for the PTAA-based PR composites with a second electron trap and without a second electron trap (blank) in Fig. 2. We measured the photocurrent to be over 100 μA and found a dielectric breakdown below 40 V μm−1 for the PR composite without a second electron trap (blank), whereas the photocurrent is reduced over the entire electric field for the PTAA-based PR composites with second electron traps, almost leveling off at an electric field above 40 V μm−1, which reduces the risk of dielectric breakdown at high electric fields up to 60 V μm−1. The addition of BPhen reduces the photocurrent to 53 μA at 60 V μm−1.
Fig. 2 Plots of the photocurrent as a function of the applied electric field for PTAA-based PR composites with and without second electron traps.
The UV-Vis spectra of these PTAA-based composites are shown in Fig. 3. Compared with the spectrum for the blank, the addition of Alq3 and BPhen gives a broad absorption spectrum with a peak at approximately 560 nm, attributed to the CT complex between PTAA and Alq3 or BPhen. The intensity of the CT complex is higher for BPhen than for Alq3. On the other hand, the addition of OXD-7 leads to a decrease in the absorption coefficient and no CT complex. It is interesting to note that the addition of OXD-7 leads to a lower absorption compared with that of the blank. HOMO and LUMO levels are listed for electron traps and PTAA [16] in Table 1. The LUMO level of PTAA is calculated from the HOMO level and the absorption edge of PTAA. There is not so much differences of HOMO and LUMO levels between three electron traps In the present case, the ability of CT complex formation is significantly related to the reduction of photocurrent.
Fig. 3 UV-Vis spectra of PTAA-based PR composites with and without second electron traps. The dashed curves are the spectra due to charge transfer between PTAA and the second electron trap.
Table 1. HOMO and LUMO levels for three electron traps and PTAA.
Figure 4 shows the energy level diagram of the HOMO-LUMO levels for the PTAA-based PR composites with BPhen. The absorption of the photon energy by a CT complex between PTAA and BPhen competes with that by PCBM, and therefore, the photocurrent resulting from the photoexcitation of PCBM is drastically reduced.
Fig. 4 Energy level diagram of the PTAA-based PR composite containing BPhen and the related potential energies of the ITO substrate, SAM-modified ITO electrode, PCBM, PTAA, BPhen, PDCST and TAA.
3.2. Photorefractive dynamics
Photorefractive response is significantly related to the photorefractive dynamics of the space charge formation under a light illumination with an appropriate electric field applied. To investigate the photorefractive dynamics, the profile between diffraction efficiency and time after the light is turned on or the electric field is turned on. In our previous study, both response times were compared [14]. The rise time of 1.1 ms was measured when the writing beam was turned on under the electric field of 60 V μm−1 and that of 0.86 ms was measured when the electric field of 60 V μm−1 was turned on under the light illumination for PTAA/PDCST/TAA/PCBM/Alq3 [14]. Generally, the former rise time is considered to be the response time including space charge formation. Both rise times are almost the same order within 30% difference and the latter is also reasonably considered to be a measure of the space charge formation dynamics. We measured the sequence response of the diffraction efficiency when a rectangular field with a frequency of 100 Hz (from 0 to 60 V μm−1) is applied to the PR devices. The diffraction efficiency is plotted as a function of time for PTAA/PDCST/TAA/PCBM/OXD-7 (43.5/35/20/0.5/1) in Fig. 5. The diffraction efficiency rise and decay correspond to the switching on and off of the applied rectangular field. The rise profile of the diffraction efficiency is shown on a logarithmic time scale in Fig. 6. The best-fit curve (solid line), calculated with a stretched exponential function of KWW, is well correlated with the plots. We measured the diffraction efficiency to be 62.6% with a response time of 997 μs (β = 0.77) and a decay time of 213 μs (β = 0.81). The decay profile is not shown in the figure. The decay time is much faster than the response time because the space-charge, which induces refractive index modulation, rapidly disappeared upon turning off the applied field.
Fig. 5 Left: sequence response of the diffraction efficiency for a PR device with a composition PTAA/PDCST/TAA/PCBM/OXD-7 (43.5/35/20/0.5/1) under a rectangular applied field at a frequency of 100 Hz (from 0 to 60 V μm−1). Right: one cycle response.
Fig. 6 The same PR response for a shorter time on a logarithmic time scale. The fitting parameter is included.
The diffraction efficiency of PTAA/PDCST/TAA/PCBM/BPhen (43.5/35/20/0.5/1) is plotted as a function of time in Fig. 7. We measured the diffraction efficiency to be 75.0% with a response time of 397 μs (β = 0.54) and a decay time of 1294 μs (β = 0.85). The fast response time of 397 μs is measured for the PR polymer composites. It is noted here that the decay time is much longer than the response time. This phenomenon is related to the over-modulation of the refractive index. The diffraction efficiency reached a maximum within 2 ms, and then, the diffraction signal decayed from 75% to 69% until the applied electric field was turned off. Upon turning off the applied electric field, a quick and sharp increase in the diffraction efficiency was measured, indicating the recovery of the diffraction efficiency, which assists the slow dissipation of the diffraction efficiency.
Fig. 7 Left: sequence response of the diffraction efficiency for a PR device with a composition PTAA/PDCST/TAA/PCBM/BPhen (43.5/35/20/0.5/1) under a rectangular applied field at a frequency of 100 Hz (from 0 to 60 V μm−1). Right: one cycle response. Response time: 397 μs (β = 0.54). Decay time: 1294 μs (β = 0.85).
The PR parameters of the PTAA-based PR composites are listed in Table 2. The sensitivity S of the PR device is an indicator of the performance of real-time 3D holographic displays. The PTAA-based PR composite containing BPhen was calculated to have a sensitivity of 1145 cm2 J−1, which is almost two times larger than that of the other composites. This extremely high sensitivity is promising for obtaining high-performance PR devices as real-time 3D holographic displays.
Table 2. PR quantities and parameters in the PTAA-based PR composites with second electron traps and without second electron traps (blank).
Why BPhen works as better second electron acceptor? As shown in Fig. 2 and discussed above, the addition of BPhen significantly reduced the photocurrent at 60 V μm−1, which is due to the reduction of photoexcitation of PCBM by the formation of CT complex between PTAA and BPhen. In the next section, we discuss the effect of photocurrent on the response time.
3.3. Photorefractive response and photocurrent
The photoconductive properties related to the photocurrent in the PTAA-based PR composites with second electron traps listed in Table 3 were calculated using the same process as in the previous report [14].
Table 3. Photocurrent and related quantities in the PTAA-based PR composites with second electron traps.
The internal photocurrent efficiency φph (E) is related to the photocurrent per unit area Jph by the Eq. (6) [14, 17, 18]:
where σph is the photoconductivity, E0 is the applied field, h is Planck’s constant, ν is the light frequency, e is the elemental charge constant, I0 is the intensity of light, α is the absorption coefficient, and L is the thickness of the PR composite film. The value of φph evaluated using an electric field of 60 V μm−1 is listed in Table 3.The internal photocurrent efficiency φph (E) is related to the photocarrier charge generation efficiency ηp by the Eq. (7) [14, 18]:
where G is the photoconductivity gain factor and Ti is the initial trap density in Schildkraut’s model [19]. We used a dielectric constant εr of 3.5, determined through capacitance measurements employing a charge amplifier.From the aspect of the photocurrent efficiency, φph(E) in Eq. (7), we can estimate the effect of the second electron trap on the values of ηp and Ti. In Eq. (7), φph(E) is parallel to the ratio of ηp/Ti. As listed in Table 3, φph(E) increases in the order of BPhen < Alq3 < OXD-7, which follows the increase of the response time for the optical diffraction. Namely, smaller φph(E) or ηp/Ti leads to the faster response time. Thus for BPhen samples, smaller ηp or larger Ti is reasonably evaluated. Smaller ηp is ascribed to lower efficiency of photocarrier generation via CT complex between PTAA and BPhen.
We first calculate ηp for PR composite with Alq3 using Ti value of 7.00 × 1014 cm−3 reported in our previous paper [14]. In this calculation, we assume that the trap density is not affected by the applying electric field. Next, for the calculation of ηp of each composite, we assume that ηp is parallel to Ip (Jph) and calculated. Finally we estimate the trap density of Ti, 5.65 × 1014 cm−3 for PR composite with OXD-7 and 8.00 × 1014 cm−3 for that with BPhen. Here we noted that Ti is parallel to α in the present estimation. The values of ηp, G and Ti are listed in Table 3. The trap-limited space-charge field Eq is evaluated by the Eq. (8) [14, 18]:
where KG is the grating vector (KG = 2π/Δ) and Δ is the grating period. The Kukhtarev model predicts the space-charge field ESC [20]:with Eq, the diffusion field ED (ED = KGkT/e, where k is Boltzmann’s constant, and T is temperature), and the projection of the electric field on to the grating vector Ep. The Eq and ESC values listed in Table 3 are still lower than those of other PR composites, for example, by one or two orders of magnitude compared with a PVCz-based PR composite [20].The response time (τG) to build up the steady-state space-charge field, under the assumption that all photogenerated charge carriers contribute to trap filling is given by the Eq. (10) [14, 21]:
Using Eqs. (6), (7), and (10), τG is related to the photoconductivity σph by the Eq. (11) [14]:In Fig. 8, τ (closed circles) and τG (open circles) are plotted as a function of the inverse of photoconductivity, σph−1, for the PTAA-based PR composites with second electron traps. The solid line in Fig. 8 indicates the plots based on Eq. (11), and the dashed line is the predicted line from the measured values of τ. Theoretical response time τG is increased with the inverse of photoconductivity, σph−1, but the measured response time τ is decreased with the inverse of photoconductivity σph−1. Here, τG is faster than τ by one order of magnitude. For the calculation of τG, we assume that all the photogenerated charge carriers contribute to trap filling. However,the result of τ suggests that not all the traps are filled by the photogenerated charge carriers in the PR composites. Here we define τ aswhere Nc (s−1 cm−3) is the total number of charge carriers trapped per unit volume and per unit time. Ideally, when all the photogenerated charge carriers are trapped, and . From the obtained results of τ, the relation of NC between three second electron traps, ΝC (OXD-7) < Νc (Alq3) < Νc (BPhen), is evaluated. This relation is consistent with the photocurrent relation between these traps, Jph (OXD-7) > Jph (Alq3) > Jph (BPhen). Larger trap density and lower photoconductivity provide the appropriate response time for the optical diffraction. An intersection point between the two straight lines gives an estimated response time of 94 μs with a photoconductivity of 3.3 nS cm−1 for the PTAA-based PR composite.Fig. 8 Effect of the photocurrent on the response times τ (closed circles) and τG (open circles) for the PTAA-based PR composites.
4. Conclusion
The addition of BPhen to a PTAA-based PR composite provides high PR performance with a fast response time of 397 μs, an extremely high sensitivity of 1145 cm2 J−1, and a diffraction efficiency of 73.3% at 60 V μm−1. The reduction of the photocurrent due to the formation of a CT complex between PTAA and BPhen significantly contributes to the appropriate formation of the space-charge field and thus the fast response time. We estimated the response time to be 94 μs with a photoconductivity of 3.3 nS cm−1 for the PTAA-based PR composite containing an appropriate second electron trap.
Funding
Strategic Promotion of Innovative Research and Development (S-innovation), Japan Science and Technology Agency (JST).
Acknowledgements
K. M. thanks Mr. Ikumi Nakanishi for the measurement of absorption spectra.
References and links
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