Electron spin resonance and photoelectron yield spectroscopic studies for photocarrier behavior in photorefractive polymeric composites

Organic Electronics 68 (2019) 248–255

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Electron spin resonance and photoelectron yield spectroscopic studies for photocarrier behavior in photorefractive polymeric composites

T

Yuki Tanakaa, Kenji Kinashib,∗∗, Kenji Konoc, Wataru Sakaib, Naoto Tsutsumib,∗ a

Master's Program of Innovative Materials, Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto, 606-8585, Japan Materials Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto, 606-8585, Japan c Department of Macromolecular Science and Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto, 6068585, Japan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Organic photorefractive materials Electron spin resonance Photoelectron yield spectroscopy Charge generation Charge transport Density of states

An electron spin resonance (ESR) spectroscopy as well as a photoelectron yield spectroscopy (PYS) is used to study the photocarrier behavior (generation and transport) for photorefractivity in poly(N-vinyl carbazole) (PVCz) and poly[bis(2,4,6-trimethylphenyl)amine] (PTAA) based photorefractive (PR) polymer composites. ESR spectra directly catch the radical species in PR composites on the laser illumination under applied electric field; the cation radicals and anion radicals induced under light illumination upon applying an electric field were evaluated. PYS was used to evaluate the energy levels and the width of the electron density of states (DOS) of the carrier transport manifolds. The narrower DOS width of the PTAA-based PR composite was significantly related to the faster response time and broader DOS width for the PVCz-based PR composite to the slower response time. Based on all these results, the comprehensive energy diagrams, including perspectives on the energy level and its distribution, the generated charge carriers with spins, and occupied trap density, are proposed.

1. Introduction Photorefractive (PR) properties involve holographic diffraction as well as asymmetric energy transfers. Asymmetric energy transfers lead to optical amplification [1], and holographic diffraction is the origin of phase-conjugation wave generation and realistic three-dimensional (3D) image recording/reproduction without requiring special eyeglasses [2–6]. PR phenomena in organic polymers are based on the photogeneration of charge carriers followed by charge redistribution, which leads to the phase-shifted refractive index modulation [7–9]. Dynamic measurements of optical diffraction and two-beam coupling, as well as photocurrent measurement, are commonly used to characterize PR performance [10,11]. Electron spin resonance (ESR) spectroscopy is a highly sensitive and powerful tool for analyzing the electronic states of charge carriers with spins in electro-optical devices such as organic field-effect transistors (OFET) [12,13], organic light-emitting diodes (OLED) [14–16], and organic thin-film solar cells (OSC) [17–23]. In particular, the ESR can measure the spin density distribution (wave function) of carriers in the devices. In addition, it is well-known that the carrier transport and trapping (corresponding to the device performance) in the organic

∗

semiconducting polymer-based films are affected by the various states of the bulk, which consists of amorphous, partially ordered, highly ordered structures and dipolar molecules [24–30]. Photoelectron yield spectroscopy (PYS) is a method for obtaining information related to the carrier transport properties of the composite films; PYS is a useful tool that can be used to measure the ionization potential of materials and the electron density of states (DOS) of the carrier transport manifold in organic semiconducting materials. The DOS distribution provides direct information about carrier transport and trapping in PR materials [11]. ESR and PYS are considered useful as other tools for characterizing the photogeneration of charge carriers and their transport in the PR composites. In the present paper, we focus on measurements using ESR and PYS to evaluate and discuss the photogeneration of charge carriers and the transport and trapping of charge carriers in two types of the organic PR polymer devices based on the photoconductive poly(N-vinyl carbazole) (PVCz) and the photoconductive poly[bis(2,4,6-trimethylphenyl) amine] (PTAA). PVCz is a pioneer photoconductor with hole mobility in the order of 10−7 to 10−6 cm2 V−1 s−1, and it is still an attractive material to use as a base polymer for PR composites. A PTAA possessing hole mobility on the order of 10−3 to 10−2 cm2 V−1 s−1 has recently

Corresponding author. Corresponding author. E-mail addresses: [email protected] (K. Kinashi), [email protected] (N. Tsutsumi).

∗∗

https://doi.org/10.1016/j.orgel.2019.02.030 Received 12 November 2018; Received in revised form 20 February 2019; Accepted 24 February 2019 Available online 27 February 2019 1566-1199/ © 2019 Elsevier B.V. All rights reserved.

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been developed in the field of OFET, which is also attractive for organic PR devices [31–33]. In past studies [31–34], both PVCz and PTAA PR composites presented PR quantities of sufficient diffraction efficiency larger than 50% and optical gain coefficient above 100 cm−1 under the irradiation of a 532 nm laser. However, the PR response time of optical diffraction and optical amplification for both PR devices have significant differences; this response time is on the order of hundreds of milliseconds for PVCz PR devices [34], and a couple of milli-seconds to hundreds of microseconds for PTAA PR devices [31–33]. Also, the trap density Ti at the initial state in Schildkraut's model [35], which is related to the trap density for PR characteristics, is evaluated; Ti in the order of 1016 cm−3 was evaluated for the PVCz-based composite [34] and that of 1014 cm−3 for the PTAA-based composite [32,33]. The large difference of response time and Ti value are significantly related to the photocurrent (i.e. carrier behavior) for both PR composites. PR dynamics, in general, are not only limited by the space charge formation dynamics, but also by the orientational dynamics of NLO dyes [8]. When the PR response speed is limited by the build-up rate of the space charge field, the PR response speed is linearly related to the laser intensity and the photoconductivity [8]. Indeed, the PR response speed is linearly proportional to the laser intensity and the photoconductivity for PVCz PR composite [36] and PTAA PR composite [31], thus both PR response speeds for the present PVCz and PTAA PR composites are surely limited by the build-up rate of the space charge field. In such case, the orientational dynamics are faster than the PR dynamics. On the contrary, if the orientational dynamics are the same as or slower than the PR dynamics, in which less or weaker dependence of the laser intensity and the photoconductivity on the PR response speed is measured, the PR response dynamics are orientationally limited. But, this is not the present case. ESR spectroscopy is successfully applied to evaluate the photogeneration of charge carriers for both PR composites during device operation. ESR spectroscopy visualizes the ionic state of species in the PR devices, and PYS visualizes the ionization potential and the DOS of the dispersed ionization states. The key point of this paper is that the ESR and PYS will give us a more in-depth understanding of electronic states through the photogeneration of charge carriers and the transport and trapping of charge carriers in the organic PR polymeric composites, which is consistent with the PR quantities and parameters (such as response time, photocurrent and trap density) evaluated using the standard methods of degenerate four-wave mixing (DFWM) and twobeam coupling (TBC) in our previous studies [32,34]. Thus far, an ESR spin-trapping method has been used to analyze similar types of photopolymers by another group [37]. To the best of our knowledge, however, this is the first report on ESR for applying organic PR polymeric composite during device operation. 2. Experimental sections

Fig. 1. Structural formulae of materials used in the experiments.

2.1. Materials and device preparations

electron-trapping reagent. ECz, TNF, PCBM, and Alq3 were used as received. PVCz (Mw: 870,000 g mol−1), 7-DCST, PDCST, and TAA synthesized in our laboratory [31–34] were used. Device preparations for ESR spectroscopy. We prepared two types of PR polymeric composites, PVCz-based composite and PTAA-based composite. The mixtures PVCz/7-DCST/ECz/TNF (44/35/20/1 by wt.) and PTAA/PDCST/TAA/PCBM/Alq3 (43.5/35/20/0.5/1 by wt.) were stirred in tetrahydrofuran (THF) for 24 h and then cast on a hot plate at 70 °C for 24 h. For ESR spectroscopic measurement, the resulting PR polymeric composites were sandwiched between two rectangular-type indium-tin-oxide (ITO)-coated quartz substrates (30 mm long, 6 mm wide, and 0.5 mm thick) on a hot plate for which the temperature was maintained at a given temperature, and then the samples were rapidly quenched on a cooling plate to obtain PR devices. The temperatures were 180 °C for the PVCz-based device and 160 °C for the PTAA-based device. The thickness of the composites was adjusted to ca. 40–100 μm

Materials. Fig. 1 summarizes the structural formulae of these materials used in the present study. PVCz and PTAA were used as photoconductive polymer matrixes. The PTAA was purchased from SigmaAldrich and was reprecipitated with chloroform (as a good solvent) and hexane (as a poor solvent). The polymer precipitate was collected by centrifugation (4000 rpm, 20 min) to yield PTAA as a pale yellow powder (95.1% yield, Mw: 22,000 g mol−1). 4-Azacycloheptylbenyliedene-malononitrile (7-DCST) and 4-piperidinobenzylidene-malononitrile (PDCST) were used as nonlinear optical (NLO) dyes. NEthylcarbazole (ECz) (Tokyo Chem. Industry Co., Japan) and (2,4,6trimethylphenyl) diphenylamine (TAA) were used as plasticizers. 2,4,7Trinitro-9-fluorenone (TNF) (Tokyo Chem. Industry Co., Japan) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) (Sigma-Aldrich Co., USA) were used as sensitizers. Tris(8-hydroxyquinolinato)aluminum (Alq3) (Tokyo Chem. Industry Co., Japan) was used as a second 249

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by hand. Here, a spacer such as polyimide tape was not used because of detecting ESR signals of the spacer and then crumbling base lines of spectrum. Copper wires adhered to a part of the ITO with the use of a conductive paste, Dotite (Fujikura Kasei Co., Japan), to apply the electric field from the outside of an ESR cavity. For the PTAA-based device, to reduce a dark current and to prevent a dielectric breakdown due to the Schottky barrier junction, the ITO-coated quartz substrates were modified by 3-aminopropyltrimethoxysilane (APTMS) (Tokyo Chem. Industry Co., Japan) to make a self-assembled monolayer (SAM) on ITO surface using the same procedure as that described in our previous report [38]. 2.2. Measurements Light and field-induced electron spin resonance spectroscopy. The PVCz-based and PTAA-based devices were directly set up in the ESR cavity without using an ESR sample tube, and ESR spectra were measured on an ESR spectrometer (JES-TE300, JEOL Ltd., Japan) using an X-band microwave at a frequency of approximately 9.45 GHz, a power of 2.0 or 3.0 mW and R.T. air. The magnetic field was scanned at approximately 336.8 ± 5 mT and the modulation width was 0.2 mT at 100 kHz, with Mn2+/MnO used as a magnetic field standard. The spectra were observed under the on/off-controlled irradiation of an optically pumped semiconductor laser (OPSL) (at 532 nm, 100 mW, 0.534 W cm−2, Coherent Co., USA) at applied electric fields of 0–40 V μm−1 to investigate the dependence of the light and the field on the charge carrier generation in the PR composites. Fig. 2 shows a schematic illustration of the ESR system in the present study. The spectral separation and simulation were performed using commercially

Fig. 3. ESR spectra of the PVCz-based device: PVCz/7-DCST/ECz/TNF (44/35/ 20/1 by wt.) under the irradiation of a 532 nm laser, an applied electric field of 0–40 V μm−1 and room temperature air: (a) measured spectra, (b) enlarged spectra to make it easy to see the dependence on the electric field intensity, and (c) simulated spectra for the differential spectrum between Conditions 2 and 6.

available software (Excel, Microsoft). For the spectral simulation, a common Lorentzian function and a Gaussian function were used to simulate each signal in the ESR spectra. The radical amount was determined by using a benzene solution of 2,2-diphenyl-1-picrylhydrazyl (DPPH) (Tokyo Chem. Industry Co., Japan) as a standard sample. Photoelectron yield spectroscopy. Photoelectron yield spectra of the pristine materials (the PVCz and PTAA powders), the PVCz-based composite film, and the PTAA-based composite film were measured using a PYS (BIP-KV201, Bunkoukeiki Co., Japan) in a vacuum. The composite films were measured on an ITO glass substrate. A deuterium lamp (30 W) was used as the light source for this experiment. 3. Results and discussion Light and field-induced electron spin resonance spectroscopy. ESR spectroscopy is used to discuss the states of the hole carriers in the PR composites. The measured and simulated spectra of the PVCz-based device are shown in Fig. 3. Fig. 3a and b shows the original ESR signals under the on/off-controlled irradiation of a 532 nm laser at different applied electric fields of 0–40 V μm−1 and R.T. air. As shown in Fig. 3a, mainly four signals (named S1, S2, S3, and S4) were observed in addition to the two outer signals of the magnetic field standard (Mn2+). The intensity of the S1 signal, the g-value (a unique value for materials and species), was determined to be 2.0006, and it was stable continuously regardless of the irradiation of the laser and the application of the electric field. The S1 signal was attributed to ITO-coated quartz substrates for the following reason: the only stable signal for the S1 was detected when the ITO-coated substrates were measured without sandwiching the PR composites under the on/off-controlled irradiation of the laser. In accordance with Fig. 3b, the intensities of the S2, S3, and S4 signals were increased significantly by laser irradiation, and then they increased slightly after an increase in the electric field intensity. In the organic PR polymeric composites, charge carrier generation is generally caused by a charge transfer (CT) complex and/or selectively excited sensitizer. The detailed processes of the charge carrier generation are as follows: (1) the positive and negative electron pairs (in this case, the PVCz cation radical and TNF anion radical pairs derived from the CT complex between PVCz and TNF) are generated by photoexcitation, and (2) the electron pairs are separated and become charge carriers (i.e., hole carriers or cation radicals) when applying an electric

Fig. 2. Schematic representation of the ESR measurement under irradiation by a 532 nm laser at the applied electric field and room temperature air. 250

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DCST at 532 nm was studied for the poly(4-(diphenylamino)benzyl acrylate) (PDAA)-based composites with/without Alq3 sensitizer [41]. Holes in highest occupied molecular orbital (HOMO) of 7-DCST are transferred to HOMO of host polymer PDAA and the electrons in lowest unoccupied molecular orbital (LUMO) of 7-DCST are transferred to neighboring 7-DCST to reach the counter electrode for the sample without Alq3. However, the electrons in LUMO of 7-DCST are immediately trapped by Alq3 for the sample in the presence of Alq3. With the similar manner in PDAA with/without Alq3, after the excitation of 7-DCST at 532 nm, the hole carriers are transferred to PVCz hole manifolds and the electrons are hopping through 7-DCST manifolds in the absence of TNF and trapped by TNF in the presence of TNF. Thus, PVCz cation radicals are measured for the either case with/without TNF. In this meaning, 7-DCST significantly contributes to the hole carrier generation. As discussed above, the 7-DCST can be excited efficiently at the wavelength of 532 nm due to a large optical absorption. The overall signals of the spectra in Fig. 5 can be compared with those in Fig. 3. The shape of the spectra shows differences although the S1, S2, S3, and S4 signals commonly exist in the both spectra. The noticed one is the broad signals between 337.2 and 339 mT, which suggests that resonance signals of the photo-excited 7-DCST exist in Fig. 3. It is difficult to get more information about the 7-DCST such as g-value and peak to peak linewidth ΔHpp by the simulation analysis due to the complicated overlap between the signals. To estimate the integral area of the S3 signal (i.e. the PVCz cation radical) related to the charge carrier generation in the PVCz/7-DCST/ ECz/TNF (44/35/20/1 by wt.) composites, a spectral simulation for the difference spectrum between Conditions 2 and 6 was performed, and it is shown in Fig. 3c. Here, the resonance signal S2 at ca. 335 mT observed in the original difference spectrum was excluded because the S2 and S4 signals derived from the TNF anion have an anisotropy. The difference spectrum of the PVCz/ECz/TNF (44/55/1 by wt.), which subtracting the condition of laser OFF, 0 V μm−1 from the condition of laser ON, 40 V μm−1, is shown as Fig. 5c. The findings that signals of the TNF anion consist of the S2 – S2’, which is left-right asymmetric signals, and the center signal of the S4 were confirmed by the simulation. Accordingly, in Fig. 3c, only the S3 signal and center signal of the TNF anion (i.e. the S4 signal) were considered for simulation fitting to get the integral area of the S3. Moreover, the separation of the S3 and S4 signals from the original difference spectrum was difficult due to the overlap of the signals. However, we predicted the parameters (such as g-value and ΔHpp) of the S3 and S4 signals by simulating the difference spectrum between Conditions 1 and 6, which show the notable distinction in the S3 and S4 signals rather than the distinction between Conditions 2 and 6. As shown in Fig. 3c, the g-value and ΔHpp were determined to be 2.0036 and 1.30 mT for the S3 signal, and to be 2.0041 and 0.52 mT for the S4 signal, respectively. We propose that the signal at g = 2.0036 primarily corresponds to the PVCz cation radical, whereas the signal at g = 2.0041 corresponds to the TNF anion radical. These simulation spectra are used to evaluate the carrier density and the effective carrier mobility later. Next, the measured and simulated spectra of the PTAA-based device are shown in Fig. 6. Fig. 6a and b shows the original ESR signals under the same conditions as the PVCz-based device. As shown in Fig. 6a, mainly three signals (named S5, S6, and S7) were observed in addition to the Mn2+ and S1 signals. The intensity of the S5 signal, which showed an approximate ΔHpp of 1 mT and consequently overlapped the S6 and S7 signals, was practically increased by laser irradiation only. The gvalue of the S5 signal was determined to be approximately 2.0030. The S5 signal are observed in PTAA-based composites without including the Alq3 or without the PCBM and Alq3, and its intensity increased drastically as shown in Figs. 7 and 8. It is known that Alq3 forms the CT complex with the PTAA and becomes the Alq3 anion radical in the PTAA-based composite; the addition of 1 wt% Alq3 significantly suppresses the photocurrent [32]. The absorption of photon energy by the PCBM simultaneously competes with that by the CT complex between

Fig. 4. ESR spectra of the PVCz-based device: PVCz/7-DCST/ECz (45/35/20 by wt.) under irradiation by a 532 nm laser at the applied electric field of 0–40 V μm−1 and room temperature air: (a) measured spectra, and (b) enlarged spectra to make it easy to see the dependence on the electric field intensity.

field, which is proportional to the electric field intensity (Onsager model) [39]. Therefore, the analyses of the S2, S3, and S4 signals, which indicated the light-induced formation of the CT complex and the fieldinduced generation of charge carriers, are used to obtain direct information about the electronic states of the charge carriers in the PVCzbased composite. To assign the S2, S3 and S4 signals specifically, the ESR spectra of PVCz-based devices without TNF sensitizer: PVCz/7-DCST/ECz (45/35/ 20 by wt.) and without 7-DCST dye: PVCz/ECz/TNF (44/55/1 by wt.), are shown in Figs. 4 and 5, respectively. No resonance signals corresponding to S2 and S4 signals appeared without TNF sensitizer as shown in Fig. 4. On the other hand, the S2 and S4 signals appeared at 334.7 mT and 337 mT, respectively, with TNF and without 7-DCST as shown in Fig. 5. Both signals have electric field dependences. These results support the assignment of the S2 and S4 signals to the TNF. In addition, the S3 signal, whose intensity increased in association with the laser irradiation and the electric field application, was observed in Figs. 4 and 5. Thus, the S3 signal is assumed to be primarily derived from the PVCz cation radicals, and the S2 and S4 signals were attributed to the TNF anion radicals from the CT complex between the PVCz and TNF. As shown in Fig. 4, the PVCz cation radicals are noticeably generated without TNF sensitizers. One probability is the radical generation via the excitation of 7-DCST at 532 nm [40]. The direct excitation of 7-

Fig. 5. ESR spectra of the PVCz-based device: PVCz/ECz/TNF (44/55/1 by wt.) under irradiation by a 532 nm laser at the applied electric field of 0–40 V μm−1 and room temperature air: (a) measured spectra, (b) enlarged spectra to make it easy to see the dependence on the electric field intensity, and (c) difference spectrum subtracting the condition of laser OFF, 0 V μm−1 from the one of laser ON, 40 V μm−1 to make it to see the anisotropy of the S2 and S4 signals. 251

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Fig. 9. ESR spectra of the PTAA-based device: PTAA/TAA/PCBM/Alq3 (43.5/ 55/0.5/1 by wt.) under irradiation by a 532 nm laser at the applied electric field of 0–40 V μm−1 and room temperature air: (a) measured spectra, and (b) enlarged spectra to make it easy to see the dependence on the electric field intensity.

broadening from fast spin-relaxation time at R.T. air, and/or the presence of the PDCST radicals described later. According to Fig. 6b, it was observed that the irradiation of the laser significantly increases the intensities of the S6 and S7 signals, and then it slightly increased when increasing the electric field intensity, the same as the behavior in the PVCz-based device. In comparison with Figs. 6–8, the S7 signal can be observed in the PTAA-based composites including the PCBM and attributed to the PCBM anion radical. To assign the S6 signal, note that the PDCST dye having the similar structure of the 7DCST are possibly photo-excited. The ESR spectra of PTAA-based device without PDCST dye: PTAA/TAA/PCBM/Alq3 (43.5/55/0.5/1 by wt.), are shown in Fig. 9. The S6 signal appeared in Figs. 6–9 and showed the dependence on the laser irradiation and field application, which leads to assign the S6 signal to the PTAA cation radical for the same reason as the PVCz. Also, comparing with Figs. 6 and 9, the presence of signals of the PDCST is confirmed as with the 7-DCST. Interestingly, as shown in Fig. 9, new signal at ca. 337.4 mT (named S8) is clearly observed by applying the electric field, which cannot be confirmed in the PTAAbased composites without including the Alq3 shown in Figs. 7 and 8. The approximate g-value of the S8 signal were determined to be 2.0024 corresponding to the reference value of the Alq3 anion radical described above [14]. This supports the assignment of the S8 signal to the Alq3 anion radical. The simulated spectra of the difference spectrum between Conditions 2 and 6 are shown in Fig. 6c for the same reason as that of the PVCz-based device. The g-value and ΔHpp were determined to be 2.0035 and 0.60 mT for the S6 signal, and to be 2.0005 and 0.28 mT for the S7 signal, respectively. We consider that the ESR signal at g = 2.0035 primarily correspond to the PTAA cation radical, and at g = 2.0005 correspond to the PCBM anion radical. The ESR signals of the PCBM anion radical have been reported to have g-values of 1.9982–2.00058 [22]. The fact that the calculated g-value of the S7 signal coincides with the reference values assists the assignment of the PCBM anion radical. It should be noted that the common trend has been observed in the both PVCz-based and PTAA-based composites. The trend in which the cation and anion radicals slightly remained after turning off the irradiation of the laser and the electric field is observed. This process can elucidate an important issue in the PR phenomena, that is, the remainder and the disappearance of the space-charge field when the light and voltage are turned off and how they affect these remaining spacecharge fields in the PR response to follow. We discuss the states of charges in the PR polymeric composites regarding the lineshape and the spin density. The shape of the ESR signals for S3 and S6 were simulated as a Gaussian function. This finding is evidence that the generated and injected charge carriers (primarily the PVCz or the PTAA cation radicals) are deeply trapped and localized in the composites without motional narrowing [14]. Thus, we aimed to analyze the S3 and S6 signals further and approximate the effective

Fig. 6. ESR spectra of the PTAA-based device: PTAA/PDCST/TAA/PCBM/Alq3 (43.5/35/20/0.5/1 by wt.) under irradiation by a 532 nm laser at the applied electric field of 0–40 V μm−1 and room temperature air; (a) measured spectra, (b) enlarged spectra to make it easy to see the dependence on the electric field intensity, and (c) simulated spectra for the differential spectrum between Conditions 2 and 6.

Fig. 7. ESR spectra of the PTAA-based device: PTAA/PDCST/TAA/PCBM (44.5/35/20/0.5 by wt.) under irradiation by a 532 nm laser at the applied electric field of 0–40 V μm−1 and room temperature air: (a) measured spectra, and (b) enlarged spectra to make it easy to see the dependence on the electric field intensity.

Fig. 8. ESR spectra of the PTAA-based device: PTAA/PDCST/TAA (45/35/20 by wt.) under irradiation by a 532 nm laser at the applied electric field of 0–40 V μm−1 and room temperature air: (a) measured spectra, and (b) enlarged spectra to make it easy to see the dependence on the electric field intensity.

the PTAA and Alq3. Thus, it is assumed that the S5 signal relates to the photoconductivity, and in-depth understanding the behavior of the S5 signal may lead to get more insight of correlation between the PCBM, Alq3 and photocurrent in future studies. Also, the Alq3 anion radical has three principal g-values of 2.0024, 2.0029 and 2.0032 calculated by density functional theory (DFT) method [14]. In the present study, the absence of Alq3 anion radical in Fig. 6 is probably due to the signal 252

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mobility of carriers in the PVCz-based and PTAA-based composites, respectively. The absolute amount of spin was determined through double integrations of the ESR spectra using the DPPH/benzene solutions at different concentrations as standard samples. Using laser beam diameter 0.7 mm and angle divergence 1.2 mrad and the distance of 1 m between laser and ESR sample, we calculated beam spot area on ESR sample is 0.0283 cm2. Accordingly, the spin density was calculated by dividing by the irradiation volume. The spin densities of the separated S3 and S6 signals shown in Figs. 3 and 6 were determined to be 2.4 × 1016 cm−3 and 3.1 × 1016 cm−3, respectively. Here, the carrier generation efficiency corresponding to the photocurrent slightly decreases under the magnetic field [42]. Therefore, we regard the determined spin densities of the S3 and S6 signals as approximate values of the carrier density in the PVCz-based and the PTAA-based composites under irradiation by the 532 nm laser at the applied electric field of 40 V μm−1 and R.T. air. The effective hole carrier mobility μeff was estimated by using a conductive equation:

Jph = eμeff Nspin E

Fig. 11. DOS profiles and estimated DOS curves of the conduction band for (a) the pristine PVCz and PTAA, and (b) the PVCz-based and PTAA-based composite films.

The peak separation from an original DOS profile was performed using Peak Fit™ (ver.4.0) software. The fitted Gaussian curves are summarized in Fig. 11. The DOS widths of the carrier transport manifold are determined to be 140 meV for the PVCz powder, 42 meV for the PTAA powder, 178 meV for the PVCz-based composite film, and 106 meV for the PTAA-based composite film. In the Bässler's formalism [43,44], diagonal disorder is characterized by a standard deviation of σ, the variance of the Gaussian energy distribution for the hopping site manifold (energetic disorder), and offdiagonal disorder is characterized by the positional disorder described by the parameter Σ. Monte Carlo simulations results in the following universal law:

(1)

where Jph is the photocurrent density, Nspin is the spin density of hole carriers determined from the ESR spectra (nearly equal to the hole carrier density), and E is the intensity of the external applied electric field. In the ESR measurements at the applied electric field of 40 V μm−1, the photocurrent Iph values were monitored to be ≤ 1 μA (regarded as 0.5 μA) of the PVCz-based composite; the Iph value was occasionally run up to 1 μA when monitoring the 0 μA value due to the lower photoconductivity, and 60 μA of the PTAA-based composite. Using the Iph values and beam spot area, the μeff were estimated to be 1.15 × 10−8 cm2 V−1 s−1 and 1.07 × 10−6 cm2 V−1 s−1 in the PVCzbased and the PTAA-based composites, respectively. It is reasonable that the various components, such as NLO dyes with a higher dipole moment, plasticizers, and sensitizers in the PR polymeric composites, lead to decreases in the carrier mobility [24]. The validity of the μeff are verified using DOS width measured in the next section. Photoelectron yield spectroscopy. The photoelectron yield spectra were measured for the PVCz powder, the PTAA powder, the PVCz-based composite film, and the PTAA-based composite film to compare the difference in the electronic structures between the pristine polymer and the polymeric composite film and discuss the correlation between the DOS width and PR performance. First, the obtained photoelectron yield spectra are shown in Fig. 10. The photoelectron yield clearly increased by approximately 5.9 eV for the PVCz powder, 5.2 eV for the PTAA powder, 5.9 eV for the PVCz-based composite film, and 5.4 eV for the PTAA-based film. These spectra were reproduced several times. Here, the increase in the yield for PTAA-based composite slightly shifted the higher energy side compared with that of the PTAA powder. The trace probably reflected the difference in the bulk state. The DOS spectra were estimated by differentiating among the measured photoelectron yield spectra as a function of the incident photon energy, and they are shown in Fig. 11. The edge part of the DOS profile (i.e. the start of HOMO distribution at low energy region) is useful for evaluating the energy dispersion of the carrier hopping sites.

μ (E , T ) = μ0 exp [−(

2 σ 2 σ ) ] exp {C [( )2 − Σ 2 ] E1/2 } 3 kT kT

(2)

where σ is the variance of hopping site energies, Σ is a parameter that describes the degree of positional disorder, μo is a prefactor mobility, C is an empirical constant, k is the Boltzmann constant, and T is the temperature. Equation (2) is valid for high electric field in the order of a few tens V μm−1and Tg > T > Tc, where Tg is the glass transition temperature and Tc, the nondispersive-to-dispersive transition temperature [44]. Using Bässler's formalism of eq. (2), we evaluate the validity of μeff with reasonable parameters of μ0, σ, C, and Σ values. Hole mobility of 1.15 × 10−8 cm2 V−1 s−1 is reproduced for the PVCz-based composite using the measured σ of 0.178 eV, C value of 4.06 × 10−4 cm1/2 V−1/2, and Σ = 3.00, and μ0 = 0.001 cm2 V−1 s−1 [45]. The hole mobility of 3.69 × 10−7 cm2 V−1 s−1 is reasonably evaluated with the σ value of 0.140 eV measured for PVCz powder. The hole mobility of 1.06 × 10−6 cm2 V−1 s−1 is reasonably reproduced for the PTAA-based composite with the σ value of 0.106 eV measured, C value of 6 × 10−4 cm1/2 V−1/2, Σ = 5.22, and μ0 = 0.1 cm2 V−1 s−1. The dipole moments of the major components in the PR composites are 0.665 D for the PVCz monomer, 0.909 D for the PTAA monomer (i.e., TAA), 8.366 D for 7-DCST, and 7.962 D for PDCST, which are calculated using SCIGRESS MO Compact 1.0 software based on semiempirical molecular orbital theory AM1 for 3D-optimized structures. The DOS width is affected by the presence of crystallite [46], and the dipole moments in the PR composites [47]. As shown in Figs. 10 and 11, PR composites give a mild onset for the DOS distribution and the broader DOS width compared with those for the PVCz and PTAA powders. The PR response time under the irradiation of the 532 nm laser at the applied electric field of 40 V μm−1 is plotted as a function of the DOS width as estimated from the PYS in Fig. 12, in which the previous results for the PDAA-based composite [47] are also plotted. Here, response time of the PVCz-based and PTAA-based composites were measured and estimated to be 341 ms and 6 ms by using same methods and conditions in our previous report [47], respectively. Fig. 12 clearly indicates that the PR response time decreases with the narrower DOS width, in which the hole mobility is in the following order: slower one for the PVCz (side chain type-carbazole manifold), middle one for the PDAA (side chain type-triphenylamine manifold),

Fig. 10. Photoelectron yield spectra as a function of the incident photon energy for (a) the pristine PVCz and PTAA, and (b) the PVCz-based and PTAA-based composite films. 253

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photorefractivity in the PTAA-based composite, which can easily result in injected carriers that are quickly filled with the occupied trap sites. This situation will be a better way to design more efficient PR devices with a high PR performance and a long operational life. 4. Conclusions We investigated and discussed the photocarrier behavior (generation and transport) through the studies of ESR spectroscopy and PYS for the photorefractivity in PR polymer composites. The radicals derived from polymer cations and sensitizer anions are strongly depended on the electric field under the light irradiation, which is significantly related to the electric field dependent diffraction efficiency and optical gain for the photorefractivity in both PR polymer composites. The densities of the charge carriers are 2.4 × 1016 cm−3 for the PVCz-based PR composite and 3.1 × 1016 cm−3 for the PTAA-based PR composite. The quantified density of the charge carriers and the measured photocurrent estimates the effective carrier mobility of 1.15 × 10−8 cm2 V−1 s−1 for the PVCz-based composite and that of 1.07 × 10−6 cm2 V−1 s−1 for the PTAA-based composite. The DOS width is evaluated using a PYS for pristine PVCz, PTAA, PVCz-based composite, and PTAA-based composite. The DOS width is reasonably correlated with the PR response time; the narrower DOS width of the carrier transport manifold is related to the faster PR response time for the PTAA-based PR composite, and the broader DOS width of the carrier transport manifold shows the slower response time for the PVCzbased PR composite. The present characterization is consistent with the PR quantities and parameters obtained using standard PR techniques. Thus, with the combined study of the ESR and the PYS analyses with the standard PR characterization measurements, the overall energy level diagrams for the carrier responsivity are proposed for the typical PR composites of PVCz-based and PTAA-based ones. Finally this report presents useful insights for understanding electronic states during device operation and design guidelines for improving device performance.

Fig. 12. Plots of the PR response time as a function of the DOS width. The PR response time is measured under irradiation by the 532 nm laser at an applied electric field of 40 V μm−1.

Fig. 13. Energy level diagram including the energy position, the energy distribution of the carrier transport manifold, and occupied trap density for (a) the PVCz-based composite, and (b) the PTAA-based composite.

Notes and faster one for the PTAA (main chain-type triphenylamine manifold). Thus, from the perspective of the PYS experiment, a narrower DOS distribution for the conduction band in the PR polymeric composites typically induces a faster response time, i.e., a faster charge carrier transport. Based on the trap density, the light and field-induced carrier density, the DOS width of the hole transport manifolds obtained from the photorefractivity, ESR spectroscopy, and PYS, the difference in carrier responsivity are illustrated in Fig. 13 for the PR composites studied in this work. Here, the HOMO energy (EHOMO) of the materials and the Fermi energy (EF) of the ITO were previously measured with photoelectron spectrometers (AC-1, AC-3 or BIP-KV201). The EF of the SAMmodified ITO was referenced in the literature [48]. The LUMO energy (ELUMO) of the materials were noted in the literature [49], estimated by referencing the absorption edges corresponding to the HOMO-LUMO bandgaps at solid states [50,51], or estimated from a study in the literature involving a theoretical calculation of the HOMO-LUMO bandgap [52]. The onset and offset of the DOS widths were estimated by using the tangent lines of the curves shown in Fig. 11. The (onset, offset) couples were determined to be (−5.9 eV, −6.7 eV) and (−5.4 eV, −5.9 eV) for the PVCz-based and the PTAA-based composite films, respectively. Fig. 13 demonstrates the energy level diagrams including the HOMO-LUMO levels of the materials, the occupied trap density, the generated (and injected) carrier density and the DOS distributions of transport manifolds for the PVCz-based and PTAA-based PR composites. The thermal ionization of the sensitizers and any photovoltaic effect are neglected. A noteworthy point is that the injected carrier density estimated from the ESR is two orders of magnitude higher than the occupied trap density calculated from the

The authors declare no competing financial interest. Funding Strategic Promotion of Innovative Research and Development (Sinnovation), Japan Science and Technology Agency (JST). Acknowledgments The authors thank Bunkoukeiki Co. for measuring the PYS spectra. References [1] T. Sasaki, S. Kajikawa, Y. Naka, Dynamic amplification of light signals in photorefractive ferroelectric liquid crystalline mixtures, Faraday Discuss 174 (2014) 203–218 https://doi.org/10.1039/C4FD00068D. [2] J.-L. Maldonado, V.-M. Herrera-Ambriz, M. Rodríguez, G. Ramos-Ortíz, M.A. Meneses-Nava, O. Barbosa-García, R. Santillan, N. Farfán, Reversible holography and optical phase conjugation for image formation/correction using highly efficient organic photorefractive polymers, J. Appl. Res. Technol. 13 (2015) 537–542 https://dx.doi.org/10.1016/j.jart.2015.10.007. [3] 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, N. Peyghambarian, An updatable holographic three-dimensional display, Nature 451 (2008) 694–698 https://doi.org/10.1038/nature06596. [4] P.-A. Blanche, S. Tay, R. Voorakaranam, P. Saint-Hilaire, C. Christenson, T. Gu, W. Lin, D. Flores, P. Wang, M. Yamamoto, J. Thomas, R.A. Norwood, N. Peyghambarian, An updatable holographic display for 3D visualization, J. Disp. Technol. 4 (2008) 424–430 https://doi.org/10.1109/JDT.2008.2001574. [5] P.-A. Blanche, A. Bablumian, R. Voorakaranam, C. Christenson, W. Lin, T. Gu, D. Flores, P. Wang, W.-Y. Hsieh, M. Kathaperumal, B. Rachwal, O. Siddiqui, J. Thomas, R.A. Norwood, M. Yamamoto, N. Peyghambarian, Holographic threedimensional telepresence using large-area photorefractive polymer, Nature 468

254

Organic Electronics 68 (2019) 248–255

Y. Tanaka, et al.

(2010) 80–83 https://doi.org/10.1038/nature09521. [6] N. Tsutsumi, K. Kinashi, A. Nonomura, W. Sakai, Quickly updatable hologram images using poly(N-vinyl carbazole) (PVCz) photorefractive polymer composite, Materials 5 (2012) 1477–1486 https://doi.org/10.3390/ma5081477. [7] W.E. Moerner, S.M. Silence, Polymeric photorefractive materials, Chem. Rev. 94 (1994) 127–155 https://doi.org/10.1021/cr00025a005. [8] O. Ostroverkhova, W.E. Moerner, Organic photorefractives: mechanisms, materials, and applications, Chem. Rev. 104 (2004) 3267–3314 https://doi.org/10.1021/ cr960055c. [9] J. Thomas, R.A. Norwood, N. Peyghambarian, Non-linear optical polymers for photorefractive applications, J. Mater. Chem. 19 (2009) 7476–7489 https://doi. org/10.1039/B908130E. [10] N. Tsutsumi, Photorefractive polymer, in: S. Kobayashi, K. Müllen (Eds.), Encyclopedia of Polymeric Nanomaterials, Springer-Verlag, Berlin, Heidelberg, 2015(Chapter 225). [11] N. Tsutsumi, K. Kinashi, Photorefractive response: an approach from the photoconductive properties, in: Springer Series in Materials Science, in: P.–A. Blanche (Ed.), Photorefractive Organic Materials and Applications, vol. 240, Springer International Publishing, Switzerland, 2016, pp. 129–156 (Chapter 3), https://doi. org/10.1007/978-3-319-29334-9. [12] K. Marumoto, S. Kuroda, T. Takenobu, Y. Iwasa, Spatial extent of wave functions of gate-induced hole carriers in pentacene field-effect devices as investigated by electron spin resonance, Phys. Rev. Lett. 97 (2006) 256603 https://doi.org/10. 1103/PhysRevLett.97.256603. [13] K. Marumoto, N. Arai, H. Goto, M. Kijima, K. Murakami, Y. Tominari, J. Takeya, Y. Shimoi, H. Tanaka, S. Kuroda, T. Kaji, T. Nishikawa, T. Takenobu, Y. Iwasa, Microscopic mechanisms behind the high mobility in rubrene single-crystal transistors as revealed by field-induced electron spin resonance, Phys. Rev. B 83 (2011) 075302 https://doi.org/10.1103/PhysRevB.83.075302. [14] D. Son, K. Marumoto, T. Kizuka, Y. Shimoi, Electron spin resonance of thin films of organic light-emitting material tris(8-hydroxyquinoline) aluminum doped by magnesium, Synth. Met. 162 (2012) 2451–2454 https://doi.org/10.1016/j. synthmet.2012.11.009. [15] K. Azuma, D. Son, K. Marumoto, M. Kijima, Y. Shimoi, Electron spin resonance of thin films of N,N’-di(1-naphthyl)-N,N’-diphenylbenzidine (NPB) doped by iodine vapor, Chem. Lett. 41 (2012) 191–193 https://doi.org/10.1246/cl.2012.191. [16] G. Sato, D. Son, T. Ito, F. Osawa, Y. Cho, K. Marumoto, Direct observation of radical states and the correlation with performance degradation in organic light-emitting diodes during device operation, Phys. Status Solidi 215 (2018) 1700731 https:// doi.org/10.1002/pssa.201870013. [17] T.J. Savenije, A. Sperlich, H. Kraus, O. Poluektov, M. Heeney, V. Dyakonov, Observation of bi-polarons in blends of conjugated copolymers and fullerene derivatives, Phys. Chem. Chem. Phys. 13 (2011) 16579–16584 https://doi.org/10. 1039/C1CP21607D. [18] K. Marumoto, T. Fujimori, M. Ito, T. Mori, Charge formation in pentacene layers during solar-cell fabrication: direct observation by electron spin resonance, Adv. Energy Mater. 2 (2012) 591–597 https://doi.org/10.1002/aenm.201100774. [19] N. Camaioni, F. Tinti, L. Franco, M. Fabris, A. Toffoletti, M. Ruzzi, L. Montanari, L. Bonoldi, A. Pellegrino, A. Calabrese, R. Po, Effect of residual catalyst on solar cells made of a fluorene-thiophene-benzothiadiazole copolymer as electron-donor: a combined electrical and photophysical study, Org. Electron. 13 (2012) 550–559 https://doi.org/10.1016/j.orgel.2011.12.005. [20] T. Nagamori, K. Marumoto, Direct observation of hole accumulation in polymer solar cells during device operation using light-induced electron spin resonance, Adv. Mater. 25 (2013) 2362–2367 https://doi.org/10.1002/adma.201204015. [21] L. Franco, A. Toffoletti, M. Ruzzi, L. Montanari, C. Carati, L. Bonoldi, R. Po, Timeresolved EPR of photoinduced excited states in a semiconducting polymer/PCBM blend, J. Phys. Chem. C 117 (2013) 1554–1560 https://doi.org/10.1021/ jp306278v. [22] D. Liu, T. Nagamori, M. Yabusaki, T. Yasuda, L. Han, K. Marumoto, Dramatic enhancement of fullerene anion formation in polymer solar cells by thermal annealing: direct observation by electron spin resonance, Appl. Phys. Lett. 104 (2014) 243903 https://doi.org/10.1063/1.4883858. [23] A. Lefrançois, B. Luszczynska, B. Pepin-Donat, C. Lombard, B. Bouthinon, J.M. Verilhac, M. Gromova, J. Faure-Vincent, S. Pouget, F. Chandezon, S. Sadki, P. Reiss, Enhanced charge separation in ternary P3HT/PCBM/CuInS2 nanocrystals hybrid solar cells, Sci. Rep. 5 (2015) 7768 https://doi.org/10.1038/srep07768. [24] A. Goonesekera, S. Ducharme, Effect of dipolar molecules on carrier mobilities in photorefractive polymers, J. Appl. Phys. 85 (1999) 6506 https://doi.org/10.1063/ 1.370154. [25] R. Coehoorn, P.A. Bobbert, Effects of Gaussian disorder on charge carrier transport and recombination in organic semiconductors, Phys. Status Solidi 209 (2012) 2354–2377 https://doi.org/10.1002/pssa.201228387. [26] D. Venkateshvaran, M. Nikolka, A. Sadhanala, V. Lemaur, M. Zelazny, M. Kepa, M. Hurhangee, A.J. Kronemeijer, V. Pecunia, I. Nasrallah, I. Romanov, K. Broch, I. McCulloch, D. Emin, Y. Olivier, J. Cornil, D. Beljonne, H. Sirringhaus, Approaching disorder-free transport in high-mobility conjugated polymers, Nature 515 (2014) 384–388 https://doi.org/10.1038/nature13854. [27] H.-R. Tseng, H. Phan, C. Luo, M. Wang, L.A. Perez, S.N. Patel, L. Ying, E.J. Kramer, T.-Q. Nguyen, G.C. Bazan, A.J. Heeger, High-mobility field-effect transistors fabricated with macroscopic aligned semiconducting polymers, Adv. Mater. 26 (2014) 2993–2998 https://doi.org/10.1002/adma.201305084. [28] B.B.-Y. Hsu, C.-M. Cheng, C. Luo, S.N. Patel, C. Zhong, H. Sun, J. Sherman, B.H. Lee, L. Ying, M. Wang, G. Bazan, M. Chabinyc, J.-L. Brédas, A. Heeger, The density of

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39] [40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

255

states and the transport effective mass in a highly oriented semiconducting polymer: electronic delocalization in 1D, Adv. Mater. 27 (2015) 7759–7765 https:// doi.org/10.1002/adma.201502820. C. Liu, K. Huang, W.-T. Park, M. Li, T. Yang, X. Liu, L. Liang, T. Minari, Y.-Y. Noh, A unified understanding of charge transport in organic semiconductors: the importance of attenuated delocalization for the carriers, Mater. Horiz. 4 (2017) 608–618 https://doi.org/10.1039/C7MH00091J. W. Kaiser, T. Albes, A. Gagliardi, Charge carrier mobility of disordered organic semiconductors with correlated energetic and spatial disorder, Phys. Chem. Chem. Phys. 20 (2018) 8897–8908 https://doi.org/10.1039/C8CP00544C. N. Tsutsumi, K. Kinashi, K. Masumura, K. Kono, Photorefractive performance of poly(triarylamine)-based polymer composites: an approach from the photoconductive properties, J. Polym. Sci. B Polym. Phys. 53 (2015) 502–508 https://doi. org/10.1002/polb.23663. N. Tsutsumi, K. Kinashi, K. Masumura, K. Kono, Photorefractive dynamics in poly (triarylamine)-based polymer composites, Optic Express 23 (2015) 25158–25170 https://doi.org/10.1364/OE.23.025158. K. Masumura, T. Oka, K. Kinashi, W. Sakai, N. Tsutsumi, Photorefractive dynamics in poly(triarylamine)-based polymer composite: an approach utilizing a second electron trap to reduce the photoconductivity, Opt. Mater. Express 8 (2018) 401–412 https://doi.org/10.1364/OME.8.000401. K. Kinashi, Y. Wang, W. Sakai, N. Tsutsumi, Optimization of photorefractivity based on poly(N-vinylcarbazole) composites: an approach from the perspectives of chemistry and physics, Macromol. Chem. Phys. 214 (2013) 1789–1797 https://doi. org/10.1002/macp.201300208. J.S. Schildkraut, A.V. Buettner, Theory and simulation of the formation and erasure of space charge gratings in photoconductive polymers, J. Appl. Phys. 72 (1992) 1888–1893 https://doi.org/10.1063/1.351662. D. Wright, M.A. Diaz-Garcia, J.D. Casperson, M. DeClue, W.E. Moerner, R.J. Twieg, High speed photorefractive polymer composites, J. Appl. Phys. Lett. 73 (1998) 1490–1492 https://doi.org/10.1063/1.122182. Qi, H. Li, J.P. Fouassier, J. Lalevée, J.T. Sheridan, Comparison of a new photosensitizer with erythrosine B in an AA/PVA-based photopolymer material, Appl. Opt. 53 (2014) 1052–1062 https://doi.org/10.1364/AO.53.001052. K. Kinashi, H. Shinkai, W. Sakai, N. Tsutsumi, Photorefractive device using selfassembled monolayer coated indium-tin-oxide electrodes, Org. Electron. 14 (2013) 2987–2993 https://doi.org/10.1016/j.orgel.2013.08.024. L. Onsager, Initial recombination of ions, Phys. Rev. 54 (1938) 554–557 https:// doi.org/10.1103/PhysRev.54.554. C.W. Christenson, J. Thomas, P.-A. Blanche, R. Voorakaranam, R.A. Norwood, M. Yamamoto, N. Peyghambarian, Grating dynamics in a photorefractive polymer with Alq3 electron traps, Optic Express 18 (2010) 9358–9365 https://doi.org/10. 1364/OE.18.009358. H.N. Giang, T. Sassa, T. Fujihara, S. Tsujimura, K. Kinashi, W. Sakai, N. Tsutsumi, Electron dominated grating in a triphenylamine based photorefractive composite, J. Mater. Chem. C 4 (2016) 6822 https://doi.org/10.1039/C6TC01609J. F. Ito, T. Ikoma, K. Akiyama, S. Tero-Kubota, Spin dynamic study on the electric field dependence of carrier generation, J. Phys. Chem. B 109 (2005) 7208–7213 https://doi.org/10.1021/jp044475e. P.M. Borsenberger, L. Pautmeier, H. Bässler, Charge transport in disordered molecular solids, J. Chem. Phys. 94 (1991) 5447–5454 https://doi.org/10.1063/1. 460506. H. Bässler, P.M. Borsenberger, The transition from nondispersive to dispersive charge transport in vapor deposited films of 1-phenyl-3-p-diethylamino-styryl-5-pdiethylphenylpyrazoline (DEASP), Chem. Phys. 177 (1993) 763–771 https://doi. org/10.1016/0301-0104(93)85039-B. M. Fujino, H. Mikawa, M. Yokoyama, Charge carrier transport under trap-free condition in PVCz, J. Non-Cryst. Solids 64 (1984) 163–172 https://doi.org/10. 1016/0022-3093(84)90213-8. S. Tsujimura, T. Fujihara, T. Sassa, K. Kinashi, W. Sakai, K. Ishibashi, N. Tsutsumi, Characterization of carrier transport and trapping in photorefractive polymer composites using photoemission yield spectroscopy in air, Macromol. Chem. Phys. 217 (2016) 1785–1791 https://doi.org/10.1002/macp.201600070. T.V. Nguyen, K. Kinashi, W. Sakai, N. Tsutsumi, Enhanced photorefractivity of a perylene bisimide-sensitized poly(4-(diphenylamino) benzyl acrylate) composite, Opt. Mater. Express 6 (2016) 1714–1723 https://doi.org/10.1364/OME.6.001714. X.-D. Dang, A.B. Tamayo, J. Seo, C.V. Hoven, B. Walker, T.-Q. Nguyen, Nanostructure and optoelectronic characterization of small molecule bulk heterojunction solar cells by photoconductive atomic force microscopy, Adv. Funct. Mater. 20 (2010) 3314–3321 https://doi.org/10.1002/adfm.201000799. P. Chantharasupawong, C.W. Christenson, R. Philip, L. Zhai, J. Winiarz, M. Yamamoto, L. Tetard, R.R. Nair, J. Thomas, Photorefractive performances of a graphene-doped PATPD/7-DCST/ECZ composite, J. Mater. Chem. C Mater. Opt. Electron. Devices 2 (2014) 7639–7647 https://doi.org/10.1039/C4TC00782D. J. Sun, H.-J. Jiang, J.-L. Zhang, Y. Tao, R.-F. Chen, Synthesis and characterization of heteroatom substituted carbazole derivatives: potential host materials for phosphorescent organic light-emitting diodes, New J. Chem. 37 (2013) 977–985 https:// doi.org/10.1039/C2NJ40900C. C. Schwarz, F. Milan, T. Hahn, M. Reichenberger, S. Kümmel, A. Köhler, Ground state bleaching at donor–acceptor interfaces, Adv. Funct. Mater. 24 (2014) 6439–6448 https://doi.org/10.1002/adfm.201400297. Y. Shiraishi, K. Adachi, M. Itoh, T. Hirai, Spiropyran as a selective, sensitive, and reproducible cyanide anion receptor, Org. Lett. 11 (2009) 3482–3485 https://doi. org/10.1021/ol901399a.