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Enhanced photorefractive (PR) performances are reported for a PR composite of perylene bisimide (PBI)–sensitized poly(4-(diphenylamino)benzyl acrylate) (PDAA), 2-(4-(azepan-1-yl)benzylidene)malononitrile (7-DCST), and (4-(diphenylamino)phenyl)methanol (TPAOH). The addition of a large amount of the TPAOH photoconductive plasticizer was found to produce a preferred hole manifold that reduces the disordered state. A photorefractive performance with a minimum response time of 11 ms, a maximum external diffraction efficiency of 41.6%, a maximum sensitivity of 117 cm2 J−1, and a maximum optical gain of 296 cm−1 were obtained with an external electric field of 55 V μm−1 for PDAA/7-DCST/TPAOH/PBI (30/30/39.9/0.1 by wt.).
© 2016 Optical Society of America
1. Introduction
Large optical amplifications, high diffraction efficiencies and fast response times are attractive features of photorefractive (PR) polymer composites when used in optical applications, such as image amplification [1]; updatable and dynamic three-dimensional (3D) displays; optical phase conjugation; and edge enhancement [2,3]. Organic PR materials have the advantages of high optical nonlinearity, suitability for image updating and easy processing [4–7]. The organic PR effect involves both photoconductive and electro-optic effects, which induce optical diffraction and amplification [8]. These phenomena result from the spatially phase-shifted refractive index modulation caused by the first-order electro-optic effect and the optical anisotropy of the reoriented nonlinear chromophores in the presence of the space-charge field caused by the charge redistribution along the interference of light beams under an applied electric field.
Poly(4-(diphenylamino)benzyl acrylate) (PDAA) exhibits features of a relatively low glass transition temperature, easy synthesis and processability to a large size device, which are promising for its application in the updatable recording of holographic displays [5]. Perylene bisimide (PBI) has emerged as an important n-type organic semiconductor material due to its many outstanding photophysical properties that are caused by its strong absorption of visible light due to its large π-conjugated structure [9], its high fluorescence quantum yield [10] and its excellent photo stability [11]. Additionally, PBI also exhibits a high electron affinity and an excellent electron mobility [12,13]. In a recent study, we reported a PR response time of 47 ms using a perylene bisimide-sensitized PDAA composite [14]. The PBI-PDAA PR composite shows promising PR features, such as a fast response time, due to the formation of a charge-transfer complex between the photoconducting polymer and the sensitizer.
In this study, we aimed to improve the photorefractivity of these materials by optimizing the concentration of the PDAA polymer composite. The resulting PR performance was improved with a diffraction efficiency of up to 77%, a fast response time near 10 ms, and a large optical gain of up to 296 cm−1. These enhancements were evaluated by an approach based on the density of the states of the photoconducting polymer and the photoconductive properties.
2. Experimental sections
We used photoconductive PDAA as a host photoconductive polymer matrix (Mw = 26 000 g mol−1, Mw/Mn = 3.1, Tg = 80 °C), 2-(4-(azepan-1-yl)benzylidene)malononitrile (7-DCST) as a nonlinear optical (NLO) chromophore, (4-(diphenylamino)phenyl)methanol (TPAOH) as a plasticizer, and N,N’-diisopropylphenyl-1,6,7,12-tetrachloroperylene-3,4,9,10-tetracarboxyl bisimide (PBI) as a sensitizer. The PBI sensitizer was synthesized using a method described in a previous report and was found to be highly soluble in common solvents, such as chloroform, acetone, tetrahydrofuran, and toluene, due to the presence of the solubilizing 2,6-diisopropylphenyl groups [15]. The structural formulae of PDAA, 7-DCST, TPAOH, and PBI were shown in Fig. 1.
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 on the positively biased electrode at an angle of 42.5° (57.5°) for writing beam A (B) relative to the device’s normal angle with an index of refraction n = 1.7. A p-polarized reading beam with a weaker intensity from the same source that propagates in the direction opposite to that of the writing beam was diffracted by the refractive index gratings in the PR composite. The diffracted and transmitted signals were detected by two photodiode detectors. Based on the intensities of the transmitted beam It and the diffracted beam Id, the internal diffraction efficiency ηint was calculated as:
and the external diffraction efficiency ηext was given by:where θA is the internal angle, d is the thickness of the PR composite film, and α is the absorption coefficient, which is defined by α = Aln(10)/d, where A is the measured absorbance. To estimate the response time τ, we fitted the time trace of the internal diffraction efficiency ηint using the Kohlrausch-Williams-Watts (KWW) stretched exponential function:where t is the time, η0 is the steady-state diffraction efficiency, and β (0 < β ≤ 1) is the dispersion parameter.An empirical PR sensitivity S is defined as:
where ηext is the external diffraction efficiency at a certain exposure time that corresponds to the response time, and I is the unit intensity of the illuminating laser. While measuring a steady-state DFWM signal, we simultaneously recorded a steady-state photocurrent using the current monitor in a Trek 610E high-voltage amplifier.The asymmetric energy transfer in the PR composite was measured using a two-beam coupling (TBC) technique. The intensities of the two beams transmitted through the PR composite were measured with photodiode detectors to evaluate the optical gain coefficient Γ based on the following equation:
where d is the thickness of the PR composite; θA and θB are the internal angles between the normal to the sample surface and the recording beams A and B, respectively; and I1 and I2 are the transmitted intensities of the respective beams.The refractive index modulation amplitudes Δn were calculated using Kogelnik’s expression for the diffraction efficiency in the thick transmission holograms:
where , and λ is the laser beam’s wavelength.The phase shift Φ is calculated from the following equation:
where and are the polarization unit vectors of the two recording beams.The trap-limited space-charge field Eq is evaluated using the Kukhtarev model [16]:
where ED is the diffusion field and is defined by ED = KGkBT/e, where KG is the grating wave-vector , kB is Boltzmann’s constant, T is the absolute temperature, and e is the electronic charge; and E0 is the projection of the applied field onto the grating wave vector.The Kukhtarev model also predicts the space-charge field ESC, which is given by:
The initial trap density Ti in Schildkraut’s model [17] is calculated as:
The dielectric constant εr was set equal to 3.5 based on a capacitance measurement using the same procedure with a charge amplifier, which was reported in a previous study [18].
The photoconductivity σph is calculated as:
where Iph is the photocurrent, and s is the illuminated area.The internal photocurrent efficiency φph is related to the photoconductivity σph by the following equation [19, 20]:
where h is Planck’s constant, ν is the light frequency, and I0 is the unit intensity of light.The photocarrier generation efficiency ηp is also related to the internal photocurrent efficiency φph by the following equation:
The emission electron yield spectrum was measured using photoelectron yield spectroscopy (PYS, BIP-KV201, Bunkoukeiki, Japan) in a vacuum. A deuterium lamp was used as the light source for this experiment. Calculations based on semi-empirical molecular orbital theory AM1 for 3D-optimized structures and dipole moment were carried out using WinMOPAC 3.0 software.
3. Results and discussion
The thermal properties of PBI were investigated via thermo-gravimetric analysis (TGA) with a heating rate of 10 °C/min, as shown in Fig. 2.
As shown in Fig. 2, PBI exhibits considerable thermal stability. Examining the TGA curve shows that the perylene bisimide derivative used in this study can be stable up to 375 °C and exhibits a 5% weight loss at 412°C. These results are comparable to those obtained for other perylene derivatives used in organic solar cells and organic transistors [21, 22] and are important for practical use in severe environmental conditions.
UV-Vis absorption spectra were used to investigate the optical properties of PBI in dilute chloroform and as a solid film. The absorption spectra obtained in solution and as a solid film are shown in Fig. 3. In solution, PBI shows a broader absorption range from 400 to 580 nm with two peaks at 490 and 520 nm. Compared to the absorption in solution, the solid film of PBI shows a broader absorption with a similar profile. The 2-nm redshift of the absorption peak from the solution to the film suggests that the intermolecular interactions and molecular aggregation in the film are weak.
To understand the photorefractive response of PDAA/7-DCST/TPAOH/PBI composites with various photoconducting polymer concentrations, we measured the glass transition temperatures Tg, the absorption coefficients α, the PR quantities of internal diffraction efficiency ηint, the external diffraction efficiency ηext, the response time τ, the photocurrent Iph, the sensitivity S, the optical gain Γ, the refractive index modulation amplitude Δn, and the phase shift Φ in an electric field of 40 V μm−1. These results are summarized in Table 1. The composition of the NLO dye, 7-DCST was fixed at 30 wt% in all samples.
Table 1. PR quantities, glass transition temperatures and absorption coefficients of the PR polymer composites at the electric field of 40 V μm−1
The well-known, non-zero TBC gain coefficient feature proves that the PR effect is present in this system due to the nonlocal nature of this effect. Using TBC measurements, optical gains were determined based on the asymmetric energy transfer for the PR composites at 40 V μm−1. The magnitude of the energy exchange is expressed in the form of the gain coefficient, Γ, which depends on the magnitude of the space-charge field and the phase shift between the light interference pattern and the index modulation [23]. The nature of the diffraction efficiency growth is primarily affected by the growth of the refractive index modulation Δn [5]. In this study, a small difference in Δn values can provide an explanation for the same trend in diffraction efficiencies.
The choice of plasticizer is important due to its significant influence on the PR performance. In this study, we chose TPAOH as a plasticizer because TPAOH has a structure similar to that of the PDAA polymer, thus improving the compatibility between the plasticizer and the host matrix. TPAOH and PDAA contain triphenylamine [24]; thus, the transit points of electrical charges can be considered to be the same for both materials. Examining Table 1 shows that the Tg values of all compositions are below room temperature; thus, the 7-DCST chromophore could be easily reoriented, and the photorefractive response time will be dominated by the formation speed of the space-charge field, which is primarily affected by the photoconductivity [8].
It is well-known that changing the electronic structure of semiconducting polymer films with various degrees of structural order (i.e., amorphous, partially ordered, and highly ordered) can significantly affect the carrier mobility and the device performance [25–27]. The emission electron yield spectrum is a tool that can be used to measure the ionization potential of PR bulk material. In our study, the electronic density of states (DOS) was estimated by differentiating the measured photoemission yield spectrum as a function of the incident photon flux. DOS was plotted as a function of photon energy in Fig. 4(a). In an organic disordered medium, the DOS and DOS width are significantly related to the charge carriers’ transport mechanism [28–30]. The peak separation from an original DOS profile and curve fitting were performed using a Peak FitTM (ver.4.0) program. The separated curves are summarized in Fig. 4(b). The onset and offset of each PDAA DOS bandwidth were determined using the tangent lines of the curves. Consequently, the couples of (onset, offset) were determined to be (‒5.6 eV; ‒6.4 eV), (‒5.7 eV; ‒6.4 eV), (‒5.75 eV; ‒6.4 eV), and (‒5.8 eV; ‒6.4 eV) for the composites with PDAA contents of 45, 40, 35, and 30 wt%, respectively. Figure 4(b) shows that the normalized DOS becomes narrower as the polymer content decreases. The DOS widths are 330, 320, 310 and 290 meV at the PDAA contents of 45, 40, 35 and 30 wt%, respectively. The dipole moment of 1.774 D for TPAOH and that of 2.256 D for PDAA monomer were also calculated using a WinMOPAC. Thus the decrease of PDAA concentration leads to the lower dipole moment, and reasonably narrower DOS bandwidth. A narrower DOS distribution typically indicates a faster charge transport; thus, the hole mobility, which is defined as the drifting velocity versus the electric field, increases when a smaller amount of polymer and a larger content of photoconductive plasticizer are present. The photoconductive plasticizer functions as an effective hole manifold that promotes intermolecular hole hopping. As a result, the values of the photocurrent Iph have been measured to be 5.2, 9.2, 9.2, 9.4 μA for samples with PDAA concentrations of 45, 40, 35, and 30 wt%, respectively. In previous study reported by Borsenberger et al. [31], faster hole transport occurred by hopping through a triphenylamine manifold with smaller dipole moment i.e. smaller DOS bandwidth, which can be explained by the Bässeler formalism of localized states with superimposed energetic and positional disorder [32].
Fig. 4 Electronic band structures of composites: a) DOS. b) Extracted DOS curves from (a). Curve fitting was performed in the photon energy region between the peak (6.0 and 6.1 eV) and lower energy tail toward 5.6 eV or smaller.
As shown in Table 1, the response time decreases with decreasing photoconductive polymer content and increasing photoconductive plasticizer content. The origin of this effect can be described by considering the hole-hopping mechanism on the highest occupied bands that dominate the transport properties based on the energy diagrams of each component summarized in Fig. 5. The neutral CT complexes between the PDAA and PBI absorb photons to separate the charge carrier (i.e., hole) through the ion-pair mechanism. Then, the holes are injected into the transport manifold of PDAA and TPAOH. As estimated from the DOS width in Fig. 4(b), a broader distribution of DOS is obtained when a larger photoconductive polymer content and a smaller photoconductive plasticizer content are present. When the DOS distribution is broader, charge carriers frequently hop in and out of sites with larger energies, leading to a longer transit time.
PR properties are known to depend on the formation of the space-charge field, which is a complex process that includes the photo-generation of charge carriers, the migration of more mobile charges and a trapping process in the dark area of an interference pattern [10]. To describe the effect of polymer content on the photoconducting properties, we calculated the values of photoconductivity σph, internal photocurrent efficiency φph, initial trap density Ti, the trap-limited space-charge field Eq, the space-charge field Esc, and the photocarrier generation efficiency ηp, producing the results shown in Table 2.
Table 2. Photoconductivity and related quantities as well as the space-charge field of the PR polymer composites at the electric field of 40 V μm−1.
As expected, the content of the PDAA photoconducting polymer directly affected the photoconductive properties. Based on Table 2, Ti decreases as the PDAA content decreases; this is reasonable because structural disorder is linked to electronic localization within molecular stacks and can produce electronic traps. Additionally, the trap-limited space-charge field Eq value shows the same trend as the Ti values. However, the obtained space-charge field is of the same order of magnitude for all of the sample compositions investigated in this study. The resulting carrier generation efficiency is also of the same order of magnitude for all of the sample compositions investigated in this study. Thus, the differences in the photocurrent values presented in Table 1 are due to the differences in the hole mobility, which agree with the effects of the larger DOS width. Additionally, with the small concentration of traps caused by the lower PDAA content, charge carriers could migrate over a long distance to produce the large phase shift Φ shown in Table 1.
A smaller PBI contributes to a lower absorbance and thus higher external diffraction efficiency. Additionally, the PR performance is particularly sensitive to the application of an electric field. A composite with a PBI sensitizer content of 0.1 wt% was characterized as the applied electric field increased. The resultant PR parameters are shown in Table 3.
Table 3. Photorefractive parameters of PDAA/7-DCST/TPAOH/PBI (30/30/39.9/0.1 wt%, d = 46 μm) at various electric fields.
As shown in Table 3, a sensitivity of 12 cm2 J−1 with a response time of 27 ms and an external diffraction efficiency of 3.2% were observed with an applied electric field of 40 V μm−1. Conversely, a minimum response time of 11 ms, a maximum external diffraction efficiency of 41.6%, a maximum sensitivity of 117 cm2 J−1, and a maximum optical gain of 296 cm−1 were obtained with an external electric field of 55 V μm−1. The value of the photocarrier generation efficiency ηp was found to approach 0.5. Such a high quantum efficiency, high external diffraction efficiency, large gain and fast response time are advantageous for applications in optical devices, such as real-time image amplifiers and accurate measurement devices [33]. Consequently, from the viewpoint of PR optical applications, PDAA/7-DCST/TPAOH/PBI (30/30/39.9/0.1 by wt.) can be a particularly useful composition.
4. Conclusions
The electronic structures of a hole transport manifold can be controlled by adding a photoconductive plasticizer. DOS measurements support a higher hole mobility for a PR composite with a higher content of photoconductive plasticizer. The resulting high external diffraction efficiency of 41.6%, the fast response time of 11 ms, the largest sensitivity of 117 cm2 J−1, and the highest optical gain of 296 cm−1 were obtained with an applied electric field of 55 V μm−1 for the PR composite of PDAA/7-DCST/TPAOH/PBI (30/30/39.9/0.1 by wt.).
Acknowledgments
This research was supported by the Strategic Promotion of Innovative Research and Development (S-Innovation) program of the Japan Science and Technology Agency (JST), Japan.
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