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The photorefractive (PR) response and dynamics are investigated in a methyl-substituted poly(triarylamine) (PTAA)-based PR composite. The charge transfer complex between PTAA and an added small amount of second acceptor, (tris(8-hydroxyquinolinato)aluminium) Alq3, effectively suppresses the photoconductivity, and thus the sample is able to withstand the dielectric breakdown at a high electric field. The resulting PR response is enhanced at a higher electric field. Sub-millisecond PR response times were observed for both optical diffraction and optical amplification: i.e., 350 μs for optical amplification and 860 μs for optical diffraction observed under 532 nm illumination (0.534 W cm−2) at 60 V μm−1. The response time of optical amplification followed the photocurrent response time of 367 μs.
© 2015 Optical Society of America
1. Introduction
Organic electro-optical materials have attracted attention in the fields of telecommunications, energy, and displays. Organic light-emitting diodes (OLED) [1–3] are used as flexible display panels and flat-panel light sources. Organic photovoltaic cells (OPC) [4–6] are expected to be used as flexible solar cells. The organic photorefractive (PR) effect based on photoconductive properties and optical nonlinearity exhibits optical diffraction and optical amplification. These phenomena result from the spatially phase-shifted refractive index modulation due to the redistribution of the space-charge field, which is caused by the interference of beams under an applied electric field.
Organic PR materials have the advantages of a low dielectric constant, high optical nonlinearity, and easy processing. Thus, organic PR polymer composites are promising candidates for potential applications such as updatable three-dimensional (3D) holographic displays. In particular, research on updatable 3D holographic displays using the PR effect has recently received considerable attention. The high diffraction efficiency and fast response of the PR polymer composites are attractive features for updatable, dynamic hologram applications [7]. The high diffraction efficiency is directly related to the brightness of the hologram, and the fast response time is related to the smoothness of the holographic movie images. The fast optical amplification of the PR polymer composite is also attractive for use in the field of telecommunications.
In a previous report, we demonstrated that a PR composite based on poly[bis(2,4,6-trimethylphenyl)amine] (PTAA) exhibited a diffraction efficiency of 6.4% at an applied field of 25 V μm−1 and a response time of 11.3 ms at an applied field of 20 V μm−1 under illumination at 632.8 nm [8]. We improved upon this PR performance by the modification of components: the nonlinear optical dye and the plasticizer were changed from 4-azacycloheptylbenzylidenemalononitrile (7-DCST) to piperidinodicyanostyrene (PDCST) and from N-ethylcarbazole (ECZ) to (2,4,6-trimethylphenyl)diphenylamine (TTA), respectively, and the concentration of the PCBM sensitizer was changed from 1 wt% to 0.5 wt% [9]. The resultant diffraction efficiency of 16.6% and response time of 5 ms were achieved at 632.8 nm under an applied field of 45 V μm−1 [9]. A large photoconductivity related to the fast response for optical diffraction and amplification was also reported [9]. A drastic improvement in the PR response was reported by adding a second electron acceptor, (tris(8-hydroxyquinolinato)aluminium) (Alq3), which acts as another trapping sites for electrons and thus forms a competing gratings via separate transport and trapping for holes and electrons [10].
In this study, we investigated the PR response and dynamics of a PTAA-based PR polymer composite with a small amount of a second electron acceptor, Alq3 in addition to primary electron acceptor, PCBM. A high diffraction efficiency up to 83%, an optical gain coefficient of 109 cm−1, and a response time on the sub-millisecond time scale are achieved with 0.534 W cm−2 at 532 nm. The key point for the improvement of the PR response is the control of the photocurrent flow through the charge transfer (CT) complex between PTAA and Alq3. The role of Alq3 in the present PR composite is clarified and discussed.
2. Experimental sections
2.1. Materials
Photoconductive PTAA (Sigma-Aldrich, St. Louis, MO, USA) was reprecipitated with chloroform (a good solvent) and hexane (a poor solvent). The polymer precipitate was collected by centrifugation (4000 rpm, 20 min) to give PTAA as a pale yellow powder (95.1% yield; Mw: 22,000 g mol−1; Mw/Mn: 1.8; Tg: 60.7 °C). PDCST and TAA were used as a nonlinear optical (NLO) dye and a plasticizer, respectively, and were synthesized in our laboratory. The details of the synthesis have already shown in previous report [9]. [6,6]-Phenyl-C61-butyric acid methyl ester (PCBM) (Tokyo Chem. Industry Co., Tokyo, Japan) was used as a sensitizer. Alq3 (Tokyo Chem. Industry Co., Tokyo, Japan) was used as a second electron-trapping agent. The structural formulae of these materials are shown in Fig. 1. The composition of PTAA/PDCST/TAA/PCBM/Alq3 was 43.5/35/20/0.5/1 by weight. The mixtures of PTAA, PDCST, TAA, PCBM, and Alq3 were stirred in tetrahydrofuran (THF) for 24 h and were then cast on a hot plate at 70 °C for 24 h. We sandwiched the resulting PR polymer composites between two self-assembled monolayer (SAM)-coated indium-tin-oxide (ITO) glass plates to create the PR composite at 160 °C. The thickness of the PR composite was controlled by Teflon spacers. We used an ITO electrode modified with aminopropyltrimethoxysilane (APTMS), i.e. SAM-ITO, for the PR devices. The SAM-ITO electrode effectively reduced the dark current, which led to suppression of the dielectric breakdown.
2.2. Measuremens
The UV-vis absorption spectra of each PTAA PR composite were measured using a Lambda 1050 UV/Vis/NIR spectrophotometer (Perkin-Elmer Inc., Waltham, MA, USA). The absorption coefficient α is given by α = Aln(10)/d, where A is the measured absorbance, and d is the thickness of the PR layer. The glass transition temperature Tg was recorded with a DSC 2920 (TA Instruments, New Castle, DE, USA) at a heating rate of 10 °C min−1. The highest occupied molecular orbital energies (EHOMO) and Fermi energies (EF) were measured with an AC-3 photoyield spectroscope (Riken Keiki Co., Tokyo, Japan). The diffraction efficiency η and response time τ (inverse of response rate τ−1) of the PR composite were measured using a degenerate four-wave mixing (DFWM) technique. The build-up time of the PR gratings was recorded using 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, Solna, Sweden) that were incident onto the positively biased electrode at an angle of 42.5° (57.5°) for writing beam A (B) relative to the device's normal angle, which corresponds to an internal angle of θA = 23.42° (θB = 29.74°) according to Snell's law, with an index of refraction n = 1.7. A p-polarized reading beam with weaker intensity from the same source that propagates in the direction opposite the writing beam was diffracted by the refractive index gratings in the PR composite. We measured the intensities of the transmitted beam (It) and the diffracted beam (Id) to calculate the internal diffraction efficiency η with Eq. (1):
The optical gain coefficient (Γ) due to the asymmetric energy transfer in the PR composite was determined using a two-beam coupling (TBC) technique. The experimental setup for the TBC technique was essentially the same as that of DFWM except that no probe beam was used. The TBC measurement was performed by two p-polarized beams with the same intensity. The optical gain coefficient Γ is given by:
where IA and IB are the transmitted intensities of the writing beams A and B, respectively.To estimate the build-up time of each grating formation (response time) τ, we fitted the time traces of η and Γ using the stretched exponential function of Kohlrausch-Williams-Watts (KWW) in Eq. (3):
where t is the time, η0 is the steady-state diffraction efficiency, Γ0 is the steady-state optical gain, and β (0 < β ≤ 1) is the dispersion parameter.The external diffraction efficiency ηext is given by Eq. (4):
where α is the absorption coefficient, and θA is the internal angle of beam A. An empirical sensitivity S is defined as:where ηext is the external diffraction efficiency at a certain exposure time (corresponding to the response time τ), and I is the intensity of the illuminating laser per unit area.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. The rectangular shape of the light illumination was achieved using a Thorlabs MC2000 Optical Chopper System with an MC1F2 chopper wheel (1 - 99 Hz). The rise time of the optical chopper is 110 μs. A rectangular high voltage with a 100-Hz frequency was applied to the sample using a Trek 10/10E high-voltage amplifier with a high slew rate of 700 V μs−1. Optical diffraction and amplification were recorded on a digital oscilloscope (Tektronix DPO3054). The high-frequency noise in the data obtained by the oscilloscope was removed by Savitzky-Golay filtering.
An AO modulator (Neos Technologies model 35085 with 72005, A Gooch & Housego Co., Ilminster, UK) was used for light chopping for a measurement of the rectangular photocurrent. The rectangular photocurrent response under a constant voltage was recorded on a digital oscilloscope through a low-noise current amplifier (Femto DLPCA-200 with a bandwidth: 500 kHz, Berlin, Germany). A constant voltage was applied using a Trek 610E high voltage amplifier.
3. Results and discussion
3.1. Effect of Alq3
In a previous report on PTAA-based PR polymer composites [8], we succeeded in suppressing the dark conductivity by using a self-assembled monolayer (SAM)-modified indium tin oxide (ITO) electrode. The cause for this success is an increase in the Fermi level of the ITO electrode from ‒4.8 to ‒4.3 eV by introducing an SAM interlayer. An increase in the Fermi level of the ITO electrode suppresses the dark current flow to PTAA, with a HOMO level of ‒5.3 eV. Furthermore, the change of the plasticizer and nonlinear optical dye improved the photorefractive performance [9]. The optimum composition of PTAA/PDCST/TAA/PCBM was determined to be 44.5/35/20/0.5 [9]. In the present paper, by adding the second electron acceptor Alq3, a drastic improvement in the photorefractive response can be achieved. The diffraction efficiency is plotted as a function of time in Fig. 2. A logarithmic time scale is employed to clearly show the initial rise of the signal. The solid line in the figure is a best-fit curve with a response time of 2.2 ms (β = 0.99), and a diffraction efficiency η of 59.0% is demonstrating using a KWW plot. The beam intensity is 0.534 W cm−2 at 532 nm, and the applied field is 60 V μm−1.
Fig. 2 Plot of the diffraction efficiency as a function of time for a PR device with a composition of PTAA/PDCST/TAA/PCBM/Alq3 (43.5/35/20/0.5/1 wt%) (thickness: 64 μm) at an applied field of 60 V μm−1. The writing beam of 532 nm light (I = 0.534 W cm−2) was turned on at 0 s.
In our previous report [9], a response time of 10.2 ms and a diffraction efficiency of 47.0% were observed for PTAA/PDCST/TAA/PCBM (44.5/35/20/0.5) without Alq3 under an applied field of 45 V μm−1 and 532 nm illumination with an intensity of 0.427 W cm−2. The present photorefractive composite has a response time almost five times faster than that of the previous composite. The electric field dependence of the photocurrent is investigated for the sample with and without Alq3. The plots of the photocurrent of the PTAA PR composite with and without Alq3 as functions of the applied field are in Fig. 3. A drastic improvement in the photocurrent is observed by adding Alq3. Without Alq3, a large photocurrent exceeding hundreds of μA was observed, and the sample frequently suffered from dielectric breakdown at applied fields above 40 V μm−1. On the other hand, adding a small amount of Alq3 drastically depressed the photocurrent flow and suppressed the dielectric breakdown up to 80 V μm−1. Furthermore, the photocurrent flow was almost constant at an applied field above 60 V μm−1. The suppression of the excess photocurrent improves the photorefractive response.
Fig. 3 Plots of the photocurrent as a function of the applied field for PTAA PR composites with and without Alq3.
We measured the photocurrent as a function of the Alq3 content, as shown in Fig. 4. The minimum photocurrent is measured at 1 wt% Alq3, and the photocurrent is much higher for other Alq3 contents. Why does an Alq3 content of 1 wt% suppress the photocurrent? Figure 5 shows the UV-vis absorption spectrum for a PTAA PR composite with and without Alq3. A broad absorption spectrum with a peak at approximately 560 nm is observed upon the addition of Alq3. Because Alq3 itself does not have an absorption band in the region of these wavelengths, we attribute this spectrum to the CT complex between PTAA and Alq3. The HOMO and LUMO levels of the components are shown in Fig. 6. We measured EHOMO and EF with AC-3: EHOMO = −5.3 eV (PTAA), −5.9 eV (PDCST), −5.9 eV (TAA), −6.2 eV (PCBM), −6.0 eV (Alq3) and EF = −4.8 eV (ITO). The EF of SAM-ITO is −4.3 eV (SAM-ITO) [11]. The LUMO levels of the materials can be estimated from a report in which a theoretical calculation of the HOMO-LUMO bandgap is shown [12]: ELUMO = −3.1 eV (PTAA), −3.2 eV (PDCST), −2.2 eV (TAA), −3.8 eV (PCBM), and −3.3 eV (Alq3). ELUMO of −3.8 eV in PCBM is considerably lower due to the high electron affinity, and thus a large photocurrent was measured in the PR composite of PTAA with a higher EHOMO of −5.3 eV. As shown in Fig. 6, the absorption of photon energy by PCBM competes with that by the CT complex formed between PTAA and Alq3. Thus the carrier photogeneration with PCBM competes with that via CT complex. Carrier photogenration efficiency via PCBM is higher than that via CT complex of PTAA and Alq3, and thus the addition of 1 wt% Alq3 significantly suppresses the photocurrent. Whereas at higher content of Alq3 than 1.5 wt%, CT complex significantly contributes to the carrier photogeneration and the large photocurrent was observed. In the previous report [10], the addition of Alq3 led to the development of a competing grating in the PR composite, which directly improved the steady-state diffraction efficiency and rise time. Thus, the role of Alq3 is here quite different from that in the previous report [10].
Fig. 4 Photocurrent as a function of the Alq3 content in a PTAA PR composite. Applied field: 60 V μm−1. Solid curve is a guide for eye.
Fig. 5 UV-vis spectra of PTAA PR composites with and without Alq3. Dashed curve (difference) is the spectrum due to the charge transfer between PTAA and Alq3.
Fig. 6 Energy-level diagram of the PTAA PR composite related to the potential energies of the ITO substrate, SAM-ITO electrode, PCBM, PTAA, PDCST, Alq3, and TAA.
3.2. Photorefractive Dynamics
The photorefractive dynamics of the optical diffraction and amplification for the present PTAA-based PR polymer composite (film thickness: d = 55 μm) is investigated using both a rectangular laser illumination and a rectangular applied electric field. The sequence response was measured for the asymmetric energy transfer when a continuous rectangular field with a frequency of 100 Hz (0 to peak field: 60 V μm−1) was applied to the PR composite (film thickness is d = 55 μm). The resultant response of the optical gain is shown in Fig. 7. One beam signal increases, and a counter-beam signal simultaneously decreases, corresponding to an applied rectangular field. We measured an optical gain of 109 cm−1. A response time of 350 μs (β = 0.74) and a decay time of 200 μs (β = 0.44) were measured for the optical amplification due to the asymmetric energy transfer. To compare with the responses of the optical amplification, we evaluated the photocurrent response and decay. A rectangular response of the photocurrent is shown in Fig. 8 when the laser beam is turned on and off at a frequency of 100 Hz using an AO modulator under an applied field of 60 V μm−1. A response time of 367 μs (β = 0.66) and a decay time of 213 μs (β = 0.49) were measured. The rise and decay time for the rectangular voltage applied to the sample using a Trek 10/10E high-voltage amplifier with a high slew rate of 700 V μs−1 are on the order of tens of μs, and those for rectangular light illumination using an AO modulator are on the order of hundreds of ns. The time constant for the experimental setup is much faster than the obtained response time. Thus, the obtained response time reflects the photorefractive and photoconduction events in the material. The response time and the decay time of the photocurrent coincide well with the response and decay behaviour of the optical amplification. The good coincidence of the response behaviour for the optical amplification and the photocurrent shows that the photorefractive response of the optical amplification is directly governed by the photoconduction mechanism, the photogeneration of charge carriers and the hopping of holes through the hopping manifold.
Fig. 7 Left: sequence response of the optical gain for a PTAA PR composite upon applying a rectangular field at a frequency of 100 Hz. Right: one cycle response with a response time of 350 μs and a decay time of 200 μs.
Fig. 8 Left: sequence response of the photocurrent for a PTAA PR composite when the laser beam was turned on and off at a frequency of 100 Hz under a constant field of 60 V μm−1. Right: one cycle response with a response time of 367 μs and a decay time of 213 μs.
Figure 9 shows the resultant sequence response of the optical diffraction when the laser light is chopped using an optical chopper at 99 Hz under a constant electric field of 60 V μm−1. The diffraction response is fitted using KWW plots. A response time of 1.1 ms (β = 0.99) and a decay time of 0.83 ms (β = 0.99) with a diffraction efficiency of 63.2% are measured. The rise and decay times for light illumination using an optical chopper are faster than the response and decay times, and these values are therefore valid for the evaluation of the photorefractive dynamics.
Fig. 9 Sequence response of the diffraction efficiency for a PR device with the composition PTAA/PDCST/TAA/PCBM/Alq3 (43.5/35/20/0.5/1 wt%) with a chopping frequency of 99 Hz. Applied electric field is 60 V μm−1. Response time: 1.1 ms; decay time: 0.831 ms.
Figure 10 shows the sequence response of the diffraction efficiency when a rectangular field with a frequency of 100 Hz (0 to peak field: 60 V μm−1) is applied to the PR composite under a constant illumination of the interference writing beams. We measured a diffraction efficiency of 83% with a response time of 860 μs (β = 0.85) and a decay time of 105 μs (β = 0.45). The response times in Figs. 9 and 10 are similar, within experimental error. The response time for the optical diffraction is three or four times slower than those for the optical amplification and photocurrent. The chromophore orientation will additionally contribute to the optical diffraction. Just after the applying field is turned off in Fig. 10, the space-charge is disappeared and very short decay time of 105 μs is observed. Whereas the illumination light is turned off with a constant electric field, the decay time of the space-chage field obey the reorientation of NLO chromohpore and longer decay time is observed.
Fig. 10 Left: sequence response of the diffraction efficiency for a PTAA PR composite under a rectangular applied field at a frequency of 100 Hz. Right: one cycle response. Response time: 0.86 ms; decay time: 0.105 ms.
3.3. Photorefractive Response and Photoconductivity
To evaluate the space-charge field effective for the grating formation, we employed the same procedure as in the previous report [9]. We measured a photocurrent Iph of 50 - 87 μA at 532 nm (photocurrent per unit area Jph is 2.3 - 4.0 mA cm−2 with a beam area of 0.0219 cm2) and of 17 μA at 632.8 nm (photocurrent per unit area Jph is 0.69 mA cm−2 with a beam area of 0.0245 cm2), which result in photoconductivity values σph of 5 – 8.9 and 1.5 nS cm−1, respectively. The photocurrent and related quantities at both 532 and 632.8 nm are summarized in Table 1. The corresponding data without Alq3, as reported in the previous paper, are also shown in Table 1.
Table 1. Photocurrent (Iph) and related quantities in the PTAA PR composites with and without Alq3. I0 is 0.534 W cm−2 at 532 nm and 0.221 W cm−2 at 632.8 nm.
The internal photocurrent efficiency φph (E) is related to the photocurrent per unit area Jph by the equation [13, 14]:
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. Absorption coefficients α of 404 and 64 cm−1 were measured at 532 and 632.8 nm, respectively. The φph value evaluated at an applied field of 45 V μm−1 is listed in Table 1.The internal photocurrent efficiency φph (E) is related to the photocarrier generation efficiency ηp by the equation [14]:
where G is the photoconductivity gain factor, and Ti is the initial trap density in Schildkraut’s model [15]. We used the dielectric constant εr of 3.5 obtained from a capacitance measurement employing a charge amplifier in the previous paper [9]. Based on the relation of and previous data without Alq3, the decreased with Alq3 was estimated with an appropriate Ti value, and the G value was evaluated with Eq. (7). The , G and Ti values are listed in Table 1. In the present case, a Ti value of 7 - 9 × 1014 and a G value of 0.15 - 0.19 are determined.The trap-limited space-charge field Eq is evaluated by the equation [14]:
where KG is the grating vector (), and Λ is a grating period. An Eq value of 2.1 V μm−1 is evaluated with a Ti value of 9 × 1014 cm−3 and a dielectric constant εr of 3.5.Assuming that all of the photogenerated charge carriers contribute to trap filling, the response time (τG) required to fill the traps by the photogenerated holes is given by the equation [16]:
Here, there is a fundamental limit for the grating formation given by the time required to generate the space-charge density that builds up the steady-state space-charge field across the grating period. All other processes, including charge transport, charge trapping and chromophore orientation, are assumed to occur instantaneously after the photogeneration of the charge carriers, as any finite time involved in these processes can only lengthen the formation time of the grating [16].
Using Eqs. (6), (7), and (9), we can obtain a simple Eq. (10):
The response rate τG−1 (the inverse of response time) is simply related to the photoconductivity σph and the dielectric constant. We can see that the photorefractive response is directly correlated to the photoconductivity in Eq. (10). Using a dielectric constant εr of 3.5 [9] and a photoconductivity σph of 5 – 8.9 nS cm−1, a response time τG of 35 - 61 μs is predicted. This is orders of magnitude faster than the response time observed for the optical diffraction and the optical gain. Similar phenomena were reported in previous reports [14, 17]. This can occur for several reasons. Either the photocurrent does not directly contribute to the photorefractive response, or the large photocurrent limits the formation of an effective space-charge field. Further reduction of the photocurrent is required to achieve a faster response time.
The sensitivity S of a PR device is a measure of the performance required to realize real-time 3D displays. An η value of 52% was measured at a response time τ of 860 μs; thus, the sensitivity S of the PR device is evaluated to be 437 cm2 J−1 according to Eq. (5) for a 532 nm writing beam (0.534 Wcm−2) writing at an applied field of 60 V μm−1. This achievement is of considerable interest, as it is a sign of the extremely highly efficient performance of the PR device.
4.Conclusion
We have successfully achieved a sub-millisecond photorefractive response in a PTAA-based PR composite. In addition to the use of SAM-ITO electrodes, the second acceptor Alq3 plays a significant role in achieving a high PR performance in the PTAA-based PR composite. In terms of an appropriate suppression of the photocurrent through the CT complex of PTAA and Alq3, a high diffraction efficiency of 83% with a response time of 860 μs was successfully achieved. A resulting sensitivity of up to 437 cm2 J−1 at 532 nm was obtained. The rectangular response of the optical amplification with a response time of 350 μs and a decay time of 200 μs was successfully measured when a rectangular electric field was applied to the sample at a frequency of 100 Hz. The fast response corresponds well to the rectangular response of the photocurrent with a response time of 367 μs and a decay time of 213 μs. These fast responses are attributed to the control of the photocurrent flow through the CT complex of PTAA and Alq3 in the PTAA PR composite.
Acknowledgments
N. T. and K. K. thank Dr. Wataru Sakai for fruitful discussions. This work was supported by the program for Strategic Promotion of Innovative Research and Development (S-Innovation), Japan Science and Technology Agency (JST).
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