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Flexible all-organic photorefractive devices Kenji Kinashi, Mei Matsumura, Wataru Sakai, and Naoto Tsutsumi ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00075 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Applied Electronic Materials

Flexible all-organic photorefractive devices Kenji Kinashi,1 Mei Matsumura,2 Wataru Sakai,1 Naoto Tsutsumi,1* 1 Faculty of Materials Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 6068585, Japan 2 Master’s Program of Innovative Materials, Graduate School of Science and Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan [email protected] Keywords: Photorefractive polymer, All-organic, Flexible, PEDOT:PSS, PET substrate, ITO-free device Abstract The objective of the present study is to demonstrate and evaluate the photorefractive (PR) performance of an allorganic PR device with a self-assembled monolayer (SAM)-modified organic conductive electrode of PEDOT:PSS coated on polyethylene terephthalate (PET). The PR composite consisted of a triphenylamine-based photoconductive polymer: poly(4-(diphenylamino)benzylacrylate) (PDAA), triphenylamine photoconductive plasticizer: (4-(diphenylamino)phenyl)methanol (TPAOH), nonlinear optical dye based on aminocyanostyrene: (4asacycloheptylbenzylidenemalononitrile) (7-DCST), and soluble fullerene: [6, 6]-phenyl C61 butyric acid-methyl ester (PCBM). For comparison with the all-organic PR device, the PR performances using PET/ITO, glass/ITO, and glass/PEDOT:PSS substrates were also evaluated. The PR performance at an applied electric field of 40 V μm-1: diffraction efficiency and the response time of the PR device using PET/PEDOT:PSS-SAM substrate were 21.9%, and 390 ms, respectively. As a result of repeating bending test on this all-organic PR device, we found that the flexible PR device with PET/PEDOT:PSS-SAM substrate had a potential to withstand bending 10,000 times and revealed that the change in the haze value strongly influenced the degradation of PR performance.

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1. Introduction Organic photorefractive (PR) polymers have wide potential for updatable holographic imaging and dynamic holographic displays. Dynamic holography is the ultimate in three-dimensional (3D) imaging and is expected to be key technology for a next-generation display system.1 – 3 Photorefractive phenomena are based on refractive index modulation derived from the distributed space-charge field through the Pockels (first-order optical nonlinearity) effect.4

– 8

Since the first study of organic photorefractive polymers was reported,9 numerous research has

investigated PR polymers.1 – 3, 10 – 34 Organic light-emitting diodes, touch panels, solar cell devices, electrochromic devices and organic photorefractive devices have commonly used indium tin oxide (ITO) deposited on a transparent glass substrate because of its good transparency and stable conductivity and the flatness of a glass substrate with a smoothed surface.35 ITO-free electronic device is one target for fabricating rare-metal-free devices. Furthermore, glass substrates have low flexibility for bending stress. Polyethylene terephthalate (PET) substrates are highly flexible to bending stress and highly transparent. Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) has attracted attention as a replacement for ITO electrodes because of its transparency and potential as a flexible conductor.36, 37 PEDOT:PSS has widely been used as a conductive coating,38 transparent conductive film with high stability and water resistance,39 hole injection layer for polymer light-emitting diodes,40 photolithographic patterned conductive organic layer for light-emitting diodes,41 modified buffer layer for polymer photovoltaic devices,42 conductive electrode for ITO-free organic solar cells,43, 44 and transparent conductive electrode for electrochromic devices.45, 46 PR devices based on light-weight and flexible substrate and ITO-free electrode have not been reported in the past. In this study, we investigated PR polymer devices sandwiched between PET substrates with a PEDOT:PSS electrode to fabricate ITO-free all-organic flexible PR polymer devices. To evaluate the flexibility of the PR polymer devices, the PR properties of flat and bent devices were compared and the repetitive bending test of PR polymer device was performed. 2. Experimental Section 2.1. Materials Poly(4-(diphenylamino)benzylacrylate) (PDAA) is a photorefractive polymer. 2-(4-azepan-1-yl-benzylidene)malononitrile

(4-asacycloheptylbenzylidenemalononitrile)

(7-DCST)

is

a

nonlinear

optical

dye.

(4-

(Diphenylamino)phenyl)methanol (TPAOH) is a photoconductive plasticizer. [6, 6]-Phenyl C61 butyric acid-methyl ester (PCBM) (Sigma-Aldrich Co.) is a sensitizer. 3-Aminopropyl trimethoxy-silane (APTMS) (Tokyo Kasei Co.) was used to modify the transparent polymer electrode. Transparent organic conductive polymer poly(3,4ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) (1.0 wt% in H2O, high conductivity grade, SigmaAldrich Co.) was used for the organic conductive layer. PDAA, 7-DCST, and TPAOH were synthesized in our laboratory.47 Figure 1 summarizes the structural formulae of the materials used in the present study.


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Figure 1. Structural formulae of the materials used in this study. 2.2. Fabrication of Photorefractive Devices A given ratio of PDAA, 7-DCST, TPAOH, and PCBM were dissolved in tetrahydrofuran (THF) at ambient conditions for 24 h to prepare the PR polymer solution. After filtering the PR polymer solution with a syringe filter (PTFE 0.2 µm), it was stirred for 1 h. The PR polymer was cast on a Teflon sheet and dried at room temperature in a vacuum for 24 h to obtain PR polymer bulk. The PR polymer bulk was melt-pressed between two substrates with electrodes at 145 C. Four types of transparent substrate electrodes were used to fabricate PR devices: glass/ITO, PET/ITO, glass/PEDOT:PSS, and PET/PEDOT:PSS substrates. Commercially available glass/ITO substrate was used. After wet etching using a hydrochloric acid solution for 50 min, the conductive electrode with a keyhole pattern was fabricated. The PR sample was sandwiched between the two etched glass/ITO substrates with a polyimide spacer (thickness: 80 m) under pressure at an elevated temperature to fabricate the PR device. Commercially available flexible ITO electrode deposited on a PET substrate (Sigma-Aldrich, USA) was used. The thickness of the PET substrate was 127 m. The PR sample was sandwiched between two PET/ITO substrates with a polyimide spacer (thickness: 80 m) under pressure at an elevated temperature. To maintain the flatness of the PET/ITO substrates, they were sandwiched between two glass plates. After making the PR device, the two glass plates were removed. Glass substrate (Matsunami Ind., Japan) and PET film substrate with a thickness of 188 m (Teijin Tetron film, Japan) were cleaned with a soap solution, acetone, and 2-propanol in an ultrasonic washer for 10 min. Keyholeshaped polyimide tape was attached to the glass and PET substrates. A PEDOT:PSS aqueous suspension was spincoated on the glass and PET substrates with a polyimide tape with a keyhole pattern at 2800 rpm for 30 s. After spin-coating, the keyhole-shaped polyimide tape was removed and the glass/PEDOT:PSS and PET/PEDOT:PSS 3 ACS Paragon Plus Environment


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substrates were thermally annealed at 130 C for 15 min. The PR sample was sandwiched between the two spincoated PEDOT:PSS substrates with a polyimide spacer under pressure at an elevated temperature. To maintain the flatness of the PR samples with PET/PEDOT:PSS substrates, they were sandwiched between two glass plates. After making the PR device, the two glass plates were removed. 2.3. 　Surface Modification of PEDOT:PSS Electrodes The PEDOT:PSS electrode was modified by a self-assembled monolayer (SAM) using 3-aminopropyl trimethoxysilane (APTMS) to control its work function. Depending on the substrate, two types of hydrophilic treatments were employed. The glass/PEDOT:PSS substrate was soaked in a solution of H2O/NH3/H2O2 (5/1/1 by volume) for 15 min to form a hydrophilized surface of the PEDOT:PSS electrode. The PET/PEDOT:PSS substrate was hydrophilized for 20 min using an ozone cleaner (UV253V8, Filgen, Japan). Hydrophilization of the electrode surface was confirmed using static contact angle measurement. The hydrophilized PEDOT:PSS electrode was soaked in an APTMS methanol solution (methanol/APTMS = 100/1 by volume) for 30 min and washed with 2propanol to form the PEDOT:PSS electrode with an APTMS SAM layer. The modification of the electrode surface with APTMS is shown in Figure 2.

Figure 2. Schematic illustration of the preparation of PR devices with six different electrode substrates: glass/ITO, PET/ITO, glass/PEDOT:PSS(-SAM), and PET/PEDOT:PSS(-SAM). 2.4. 　Curved Sample Holder and Photorefractive Measurement A sample holder for the curved PR device was designed and fabricated to investigate and demonstrate the 4 ACS Paragon Plus Environment

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photorefractive performance of the flexible PR devices. ABS filament was used for 3D printing. The 3D architecture for a bent (curved) sample holder was designed using Shade 3D and is illustrated in Figures 3a and 3b. The sample holder (35 mm long, 30 mm wide, 25 mm depth) had a linear groove (35 mm long, 1 mm wide, 8 mm depth) and curved groove (30 mm radius, 1 mm wide, 8 mm depth) on the front side and a 5 mmφ hole in the backside, as shown in Figure 3c. The bent (curved) sample holder and bent (curved) cover top were fabricated using a 3D printer (da Vince 1.0, XYZ printing). A photograph of a PR device set on the curved sample holder is shown in Figure 3d.

Figure 3. 3D digital models of the sample holder for 3D printing, (a) top view; (b) bottom view of the sample holder. (c) 3D-printed curved sample holder and cover tops. (d) Photograph of a PR device set on the curved sample holder. The degenerated four-wave mixing (DFWM) method shown in Figure 4 was used to evaluate the diffraction response of the curved and flat PR devices. The light source was a diode-pumped solid-state (DPSS) laser SambaTM at 532 nm (Cobolt, Sweden). A laser beam was divided into two beams using a polarized beam splitter. The spolarized two beams were interfered in the PR device positively biased to form the PR grating. The biased electric field ranged from 20 to 40 V m-1. The angle between the two beams was 15 in air. The PR device was tilted 50 to the device normal. A simultaneously oppositely propagated weak p-polarized probe beam was diffracted by the PR grating. The internal diffraction efficiency was determined by the ratio between the intensity of the diffracted probe beam (Id) and the total intensity of the diffracted probe beam and transmitted probe beam (It) in equation (1):

% 

Id 100 It  I d

(1)

To estimate the time required for grating formation (response time) τ, the time profile of η was fitted using the 5 ACS Paragon Plus Environment


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Kohlrausch-Williams-Watts (KWW) stretched exponential function in Equation (2):24

 

  t   

 %   0 1  exp              

(2)

where t is the time, η0 is the diffraction efficiency at the steady condition, and β (0 < β ≤ 1) is the dispersion parameter related to the distribution of the release times from the traps.

Figure 4. Schematic illustration of the degenerated four-wave mixing measurement for the bent PR device. λ/2, half wave plate; L, lens; M, mirror; PBS, polarizing beam splitter; BS, beam splitter; G, glass plate; V: high voltage supply; D1, photodiode, transmitted component; D2, photodiode, diffracted component. 2.5. 　Repetitive Bending Apparatus The repetitive bending property of the flexible PR devices was tested using a homemade repetitive bending apparatus with an actuator (Actuonix L12 actuator). A photograph of the homemade repetitive bending apparatus system and top views of a flexible PR device before and after bending are shown in Figures 5a-c. Figures 5d and 5e show schematic illustrations of the flexible PR device before and after bending, respectively. The stroke of the actuator and bending frequency were controlled using an Arduino UNO. The flexible PR device with 30 mm fixed in the bending apparatus was pushed 2 mm and curved with a 20-mm radius of curvature using the actuator ca. 0.67 Hz (1.5 s per cycle).


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Figure 5. (a) Photograph of the homemade repetitive bending apparatus with an actuator. (b) Top view of a flexible PR device before bending. (c) Side view of a flexible PR device after bending. (d) and (e) schematic illustrations corresponding to (b) and (c), respectively. 2.6. Measurements The highest occupied molecular orbital energies (EHOMO) and Fermi energies (EF) were measured with a photoelectron yield spectrometer (BIP-KV201, Bunkoukeiki Co.) in a vacuum. A deuterium lamp (30 W) was used as the light source. The self-assembly process of the deposited APTMS layer was estimated using static water contact angle measurements. The haze was measured to evaluate the transparency and scattering properties of the PR devices using an integrating sphere. The haze (%) was calculated as the total light intensity of scattered light divided by the total light intensity of the scattered and transmitted light. The surfaces of the ITO and PEDOT:PSS electrodes were observed using an atomic force microscope (AFM, Pacific Technology Nano-R). The electrode resistance was measured using a digital multimeter (Custom, CDM-6000). The photocurrent was measured using the current monitor of a Trek 610E high voltage amplifier when DFWM measurement was performed. Cracks and microcracks in the PR devices were observed using a digital optical microscope (KH-8700, Hirox Co.).



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3．Results and Discussion The PDAA/7-DCST/TPAOH/PCBM PR composite with a composition of 45/30/24/1 by wt was used for all the PR devices with four types of electrode substrates: all-organic PET/PEDOT:PSS, PET/ITO, glass/PEDOT:PSS, and glass/ITO. Commercially available glass/ITO and PET/ITO substrates were used for comparison with the PEDOT:PSS substrates. A PEDOT:PSS thin layer was deposited onto the PET and glass substrates. The details of the fabrication method for the PR devices with each electrode are described in the Experimental section. For application to dynamic holographic displays, it is critical to understand and control the electronic properties, i.e., the work functions of the materials. The EHOMO and EF determined using the photoelectron yield spectrometer were in agreement with the literature:27, 47 EHOMO = ‒5.60 eV (PDAA), ‒5.71 eV (7-DCST), ‒5.60 eV (TPAOH), ‒6.20 eV (PCBM), ‒5.20 eV (PEDOT:PSS) and EF = ‒4.80 eV (ITO), and ‒4.30 eV (SAM-ITO). Unfortunately, the EHOMO of the PEDOT:PSS-SAM electrode could not be accurately measured using the photoelectron yield spectrometer in this study; however, the value would be higher than that of the SAM-ITO electrode. The SAM-ITO electrodes performed a dark current suppression role, which prevents dielectric breakdown. The potential barrier ΔE between EF (ITO) and EHOMO (PDAA) was 0.89 eV whereas that between EF (PEDOT:PSS) and EHOMO (PDAA) was 0.49 eV. The potential barrier ΔE of 0.49 eV would be small for hole distribution; electrons can thermally activate through the potential barrier for hole carriers to drift into the PDAA hole manifold. In this case, dark current flow reduces the effective space-charge field in the PR device. In our previous study for a poly(triarylamine) (PTAA)-based PR device with an EHOMO (PTAA) of ‒5.2 eV, the APTMS SAM interlayer between the ITO and the PR device largely reduced the dark current, contributing to the effective space charge field.27 The energy diagrams of EHOMO and EF for the PR devices are shown in Figure 6.

Figure 6. Energy diagram of the HOMO of the PR components and the EF of the electrodes. The diffraction efficiencies for the PR devices increase with increasing applied electric field because the refractive index amplitude of the photorefractive effect is proportional to the product of the photorefractive space-charge field and applied electric field. At an applied electric field of 40 V μm-1, the PR device with glass/ITO substrate had a

diffraction efficiency of  = 47.5% and response time of τ = 226 ms. The PR device with glass/PEDOT:PSS substrate had a diffraction efficiency of  = 44.7% and response time of τ = 185 ms. These diffraction quantities are the same as those reported previously.30 There is no significant difference in the PR aspects between the ITO 8 ACS Paragon Plus Environment

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and PEDOT:PSS electrodes on a glass substrate, which indicates that PEDOT:PSS can be used as an electrode for PR devices. The PR device with PET/ITO substrate had a diffraction efficiency of  = 25.4% and response time of τ = 208 ms at an applied electric field of 40 V μm-1, a slightly lower diffraction efficiency than but nearly the same response time as the PR device with glass substrates. The photocurrent characteristics of the PR devices with glass/ITO and PET/ITO substrates were 0.5 μA, and 6.5 μA, respectively, at an applied electric field of 40 V μm-1. The results imply that the ITO electrode grown on the PET substrate has an amorphous structure whereas that the grown on the glass substrate has a microcrystalline structure.48 Therefore, the difference in the photocurrents is due to the crystal structures of the thin films deposited on the glass and PET substrates.49 The PR device with PET/PEDOT:PSS substrate had a diffraction efficiency of  = 5.3% and response time of τ = 2773 ms at an applied electric field of 40 V μm-1. The diffraction efficiency and response time of the PR device with PET/PEDOT:PSS substrate were ~1/10 of those with glass/ITO and glass/PEDOT/PSS substrates. Organic electric devices can be improved by forming an SAM on the electrode substrate. It has been reported that the effective voltage of devices modified with SAM is lower than without modification, which is attributable to a decrease in the work function of the electrode surface due to modification and a decrease in the electron injection barrier.27 In the case of PR devices, the SAM–ITO electrode effectively reduced the dark current, which led to improvement in the photorefractive performance.27, 29 When PEDOT:PSS is used as an electrode, modification with an SAM can have the same effect as for

the glass/ITO substrate. As a result, the PR device with glass/PEDOT:PSS-SAM substrate had a diffraction efficiency of  = 76.4% and response time of τ = 472 ms at an applied electric field of 40 V μm-1. Figure 7a shows the comparison of diffraction efficiency trace as a function of time at 40 V μm-1 for the flexible PR devices with

PET/ITO, PET/PEDOT:PSS, PET/PEDOT:PSS-SAM and hard PR devices with Glass/ITO, Glass/PEDOT:PSS, and Glass/PEDOT:PSS-SAM. Notably, the SAM layer apparently improves the photorefractive performance of PR devices with glass and PET substrates; the PR device with PET/PEDOT:PSS-SAM substrate is remarkable, with an improved diffraction efficiency from 5.3% to 21.9% and response time from 2773 ms to 390 ms. A series of recent studies indicated that the response time is inversely proportional to the product of the mobility and the optical intensity, and the mobility in materials. Therefore, the response rate, which is the reciprocal of the response time, is related to the photoconductivity and dielectric constant.29 Table 1 summarizes the diffraction efficiency and response time of the PR devices, measured using the DFWM and photocurrent. The method for the repetitive bending test has widely been used for evaluation of flexible solar cells and electrochromic devices.42-46 The PR performance in the bending condition of the flexible PR devices with PET/PEDOT:PSS-SAM substrate were evaluated. The PR performance of the flexible PR device with a sample thickness of 61 μm were measured at an applied electric field of 40 V μm-1. The PR performance of the flexible devices was first measured in the flat condition and then in the bending condition (using the DFWM method). This measurement was repeated on the same PR device. In addition, the resistance and haze value of the PET/PEDOT:PSS-SAM substrate were measured during the repetitive bending test. The PR diffraction efficiency and response time and the photocurrent as a function of the bending cycle are shown in Figures 7b-d. The PR data were collected from six PR devices. The resistance of the PET/ITO electrode substrate rapidly increased from 142 9 ACS Paragon Plus Environment


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Ohm to 418 Ohm with increasing bending cycles and that of the PET/PEDOT:PSS-SAM electrode substrate was almost constant at 11.1 k Ohm. The PR device with PET/ITO failed at approximately 50 bending cycles because the resistance of the PET/ITO substrate drastically changed (Figure 7e) and fatal cracks appeared on the ITO electrode. The PEDOT-PSS-SAM substrate shows a constant resistance change during 10,000 bending cycles (Figure 7e). The results indicate that the repetitive bending tests for the PET/ITO and PET/PEDOT:PSS-SAM substrates confirmed that the PET/PEDOT:PSS-SAM substrate had better flexibility and fatigue resistance than the PET/ITO substrate. The PR device with PET/PEDOT:PSS-SAM substrate in the flat condition had a normalized diffraction efficiency of /0 = 1.0 and that in the bent condition decreased to approximately half ~ 0.5 because the effective space-charge field is weakened at the illuminated position far from the centerline position of the two-beam interference exposure. However, when /0 was measured again in the flat condition, it almost recovered to the original value. The normalized diffraction efficiencies /0 for the for the PR device with PET/PEDOT:PSS-SAM substrate were 0.8 at 10 bending cycles, 0.7 at 100 bending cycles, 0.6 at 1,000 bending cycles, and was almost constant at 0.5 after 2,000 bending cycles. The response times show a change comparable to the diffraction efficiency. The photocurrents were simultaneously measured and are plotted as a function of bending cycles in Figure 7d. No significant change in photocurrent was observed upon increasing bending cycles. Interestingly, the results indicate that the decrease in diffraction efficiency up to 500 bending cycles is not attributed to carrier photogeneration or related space-charge field formation, which are strongly associated with photocurrent but is due to other physical factors causing significant PR performance degradation. The PET/PEDOT-PSS-SAM substrate shows a constant resistance change during 10,000 bending cycles 10,000; however, the haze value of the PET/PEDOT:PSS-SAM substrate rapidly increased in 70 bending cycles due to microcracks occurring around microbubbles, and delamination of the cracked PR composite film from the PET substrates occurred at approximately 5,000 to 10,000 bending cycles, as shown in Figure 7f. The changes in the PR aspects of the PET/PEDOT:PSS-SAM substrate are in good agreement with the change in the haze value. The change in the intensity of the two-beam interference exposure rapidly decreases with the number of bending cycles. Tt can be concluded that the PR characteristics of the flexible PR device with PET/PEDOT:PSS-SAM are mainly due to sample scattering rather than photodynamics such as photocurrent. Furthermore, the results also indicate the possibility for application of flexible PR devices if the bubbles in the PR composite film can be removed during sample preparation, as the PR performance of the flexible PR devices will be remarkably improved and allow for the development of next-generation 3D displays. Microscope images of the flexible PR device with PET/PEDOT:PSS-SAM after the bending cycles are shown in Figure 7g. The lower photographs show the respective microscope images inverted to emphasize the microcracks and microbubbles. Fifty bending cycles clearly caused a large crack in the upper part; however, the crack does not remarkably degrade the PR performance because the probe light in the DFWM measurement does not directly pass through the big crack. Numerous minute crazes that cannot be observed

with the optical microscope are distributed on the sample surface from the initial bending cycle to < 50 times, which 10 ACS Paragon Plus Environment

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would contribute to the rapid increase in the haze value that leads to degradation of the PR performance. The numerous minute crazes gradually increase with the number of bending cycles and eventually become visible crazes, as seen in the inversion images at > 5,000 cycles. This result reveals a significant issue that should be considered in the future. To solve this issue, a so-called phase separation type polymer alloy in which rubber particles or other polymer components are dispersed could be effective.50, 51 Table 1. Diffraction efficiency, response time, and photocurrent of PR devices for each electrode substrate. Electrode substrate

Thickness (m)

Glass/ITO

68.0

PET/ITO

64.5

Glass/PEDOT:PSS

62.5

Glass/PEDOT:PSSSAM

80.0

PET/PEDOT:PSS

72.5

PET/PEDOT:PSSSAM

48.0

Applied electric field (V m-1) 20 30 40 20 30 40 20 30 40 20 30 40 20 30 40 20 30 40

Diffraction efficiency  (%) 4.2 18.4 47.5 2.7 13.7 25.4 3.7 14.4 44.7 9.5 30.3 76.4 0.7 2.3 5.3 4.7 12.1 21.9

Response time τ (ms) 316 385 226 187 467 208 355 280 185 483 570 472 1811 1388 2773 587 660 390

Photocurrent (A) 0 0.3 0.5 0.7 2.5 6.5 0.1 0.3 0.6 0.3 0.8 1.7 0.2 0.7 1.6 1.0 3.6 9.9



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Figure 7. (a) Comparison of diffraction efficiency trace as a function of time at 40 V μm-1 for the flexible PR devices with

PET/ITO,

PET/PEDOT:PSS,

PET/PEDOT:PSS-SAM

and

hard

PR

devices

with

Glass/ITO,

Glass/PEDOT:PSS, and Glass/PEDOT:PSS-SAM. (b) Normalized diffraction efficiency, (c) response time, and (d) photocurrent as a function of bending cycles for the flexible PR device with PEDOT:PSS-SAM. (e) Resistance changes as a function of bending cycles for the PET/PEDOT:PSS-SAM and PET/ITO substrates. (f) Haze values as a function of bending cycles for the flexible PR device with PEDOT:PSS-SAM and PET/ITO substrates. (g) Microscope images of the PR device with PET/PEDOT:PSS-SAM electrode under bending cycles; the lower images show an inverted image to illustrate the micro crazes and bubbles. Conclusions We demonstrated novel photorefractive performance and bending fatigue properties for all-organic flexible PR devices based on PDAA/7-DCST/TPAOH/PCBM with a PET/PEDOT:PSS-SAM substrate. The flexible PR device showed a diffraction efficiency of 21.9% and a response time of 390 ms at an applied electric field of 40 V μm-1. The repetitive bending test revealed that the PR performance of the PR device with PET/PEDOT:PSS-SAM substrate was not fundamentally influenced by cyclic bending because no significant change in the photocurrent was observed up to 10,000 bending cycles. However, the diffraction efficiency of the flexible PR device with PET/PEDOT:PSS-SAM substrate decreased in the first 500 bending cycles due to the remarkable increase in haze accompanying the increase in micro crazes. Noting the initial reduction in the diffraction efficiency, the flexible PR device with PET/PEDOT:PSS-SAM substrate has the potential to withstand 10,000 bending cycles.


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Flexible all-organic photorefractive devices - ACS Applied Electronic

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