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Integrated organic donor-acceptorbulk heterojunctions for self-activatedliquid crystal light modulators.

Thomas Regrettier, Malgosia Kaczmarek, GiampaoloD'Alessandro, Thomas Heiser

Thomas Regrettier, Malgosia Kaczmarek, Giampaolo D'Alessandro, ThomasHeiser, "Integrated organic donor-acceptor bulk heterojunctions for self-activated liquid crystal light modulators.," Proc. SPIE 10735, Liquid CrystalsXXII, 1073514 (14 September 2018); doi: 10.1117/12.2320692

Event: SPIE Organic Photonics + Electronics, 2018, San Diego, California,United States

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Integrated organic donor-acceptor bulk heterojunctions for self-activated liquid crystal light modulators

Thomas Regrettiera, Malgosia Kaczmarekb, Giampaolo D'Alessandroc, Thomas Heiser*a aLaboratoire ICube, Université de Strasbourg, CNRS, Strasbourg, France; bPhysics and Astronomy, University of Southampton, Southampton, UK; cMathematical Sciences, University of Southampton,

Southampton, UK

ABSTRACT

Optically addressed spatial light modulators (OASLM) are promising for holographic applications and optical limiters. Self-activated OASLMs can operate as autonomous energy devices, opening the route to stand-alone laser protection devices and smart windows. They are mainly based on liquid crystal (LC) devices, integrated with inorganic photovoltaic substrates such as LiNbO3:Fe or dye-sensitized TiO2. While robust, they also suffer from several restrictions such as high costs, low performances and/or small device area.

In this work, we propose a new type of an autonomous modulator. Blends of electron-donating (D) conjugated polymers and electron-accepting (A) fullerene molecules were used as a photovoltaic thin films and integrated into liquid crystal device. Such D/A bulk heterojunctions are the major building block of solution-processed organic solar cells and are known to convert incident light into electrical energy. In our case, the organic layer generates a photovoltaic field that is used to control the LC alignment under illumination. We carried out cross-polarized intensity measurements on this photovoltaic-LC device to demonstrate the expected occurrence of a light–dependent birefringence change, without an applied voltage. In this way, by combining solution-processed organic photovoltaic thin films with optical responsive liquid crystals, our work paves the way to low cost and large area self-sustained optical devices.

Keywords: liquid crystals, optically addressed spatial light modulator, P3HT, PCBM, photovoltaics

1. INTRODUCTION Liquid crystals (LC) are ideal materials to control the phase, amplitude or polarization of light waves, due to their adjustable optical properties with the application of an electric or optical field. Their applications include displays1, optical switches, variable light attenuators and spatial light modulators (SLM). Applied electric field, its intensity, shape or frequency, is the most common method to address and manipulate a LC layer. However, if a LC device includes a photosensitive layer or substrate, the illuminating beam can spatially modulate the electric field within the LC layer as a function of incoming light intensity or wavelength. Most LC devices fall into one of these two categories a common example being SLMs, which can be either electrically addressed (EASLM) or optically addressed (OASLM). Both addressing methods necessitate an external power supply, with the exception of OASLMs based on a particular photoactive materials, such as z-cut LiNbO3:Fe2, CdSe3 or ruthenium dye sensitized TiO2

4 for which an additional physical effect allows stand-alone operation, without an applied electric field.

Self-activated, energetically autonomous liquid crystal devices represent a promising route towards smart glass devices that are able to protect either human eyes or sensors from high intensity light sources (laser, sunlight, arc welding). There are, however, many challenges that need to be addressed for such application to be realized, namely the photoactive material must be affordable, processable on large areas and transparent in the visible range. Inorganic crystalline materials such as LiNbO3:Fe are not suitable as the wafer size limits the device area and makes it expensive. To meet the above requirements, inorganic or organic thin film technologies would be more appropriate. Yet, both thin films reported so far, CdSe and ruthenium dye sensitized TiO2, absorb strongly visible light and have a low sensitivity, thus limiting

* [email protected]; phone: 0033388106233; https://icube.unistra.fr/en/

Liquid Crystals XXII, edited by Iam Choon Khoo, Proc. of SPIE Vol. 10735, 1073514© 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2320692

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their potential for smart glass applications. Using blends of organic electron-donating (D) and electron-accepting (A) molecules in thin films to form a so-called “bulk heterojunction” device represents an interesting alternative. The latter are the key component of organic photovoltaic devices and can be easily processed at low cost from solution or by sublimation under vacuum on large areas. Additionally, their absorption spectrum can be tuned through molecular engineering of the D and A constituents.

At present, PVK blended with either TNF or C60 are the main organic systems tested so far in OASLM devices5,6. They have been used exclusively as photoconductor7 and require the application of an external AC voltage. Moreover, the device operation was found to be limited to very low frequencies (below 10 Hz) at which the LC device flicker. The resulting fluctuating optical response impedes their utilization as smart glass. Considering the large number of organic semiconductors developed for photovoltaic applications, we decided to explore materials that would offer better performance. In this work, we use blends of electron-donating poly(3-hexylthiophene) (or P3HT) and electron-accepting [6,6]-phenyl-C61-butyric acid methyl ester (or PCBM) as photo-sensitive layer in OASLM devices and explore the possibility for self-activation. The optoelectronic properties of P3HT:PCBM blends are very well documented in the literature since these layers have been widely investigated as donor/acceptor bulk heterojunction in organic solar cells. Although they absorb visible light (up to 650 nm), they are nevertheless well suited to be used as a reference bulk heterojunction to explore new modes of operation to benchmark the studies on different organic blends.

The paper is organized as follows: in section 2, we will detail the experimental methodology, device structure and measurement methods. In section 3, we will present the results and show that it is possible to design self-activated liquid crystal devices using P3HT/PCBM blends. Finally, conclusions are outlined in section 4.

2. EXPERIMENTAL METHODOLOGY 2.1 Device fabrication

It is well known that the binding energy of photo-generated excitons in most organic semiconductors is much larger than the thermal energy at room temperature. Therefore, thin films composed of a single organic semiconductor, such as for instance pure P3HT, will exhibit only a negligible photoconductivity. Blending the semiconductor with a second compound (in our case PCBM) whose electron affinity and ionization potential differ significantly (typically by at least 0.3 eV) from those of the host semiconductor, introduces an interface between D and A materials at which the excitons can be dissociated into free charge carriers. This charge transfer at the D/A interface is the major driving force in organic solar cells and relies on the D/A effective interfacial area. For this reason, the influence of the P3HT:PCBM blend mass ratio is an important parameter to consider.

Our study focuses on the electro-optical properties of OASLMs using P3HT:PCBM as photo-sensitive layer. Different P3HT:PCBM mass ratios are used and their impacts on the electro-optical response of OASLMs are investigated. The device structure is given in Figure 1(a). It is composed of two glass/ITO substrates functionalized by different organic thin films and a nematic liquid crystal (E7) layer that is sandwiched between both substrates. The ITO substrates were first cleaned in an ultrasonic bath in the following order: acetone, distilled water and isopropyl alcohol during 15 minutes each. A subsequent UV Ozone treatment was then applied to ensure the removal of organic contaminants. After the cleaning procedure, the organic films were spin coated on top of the ITO substrates. A thin film (30 nm) of PEDOT:PSS (or poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) was spin-coated on one ITO substrate and, annealed at 140°C for 10 minutes under nitrogen atmosphere. On the second substrate a 100-nm-thick P3HT:PCBM was deposited by spin-coating a chlorobenzene solution (25 mg/ml) under nitrogen atmosphere (to avoid air-contamination and minimize photo-oxidation). Both P3HT (with 93-95% regio-regularity and 50 – 70 kg/mol molecular weight) and PCBM were used as received. The samples were then annealed for 10 minutes at 140°C to remove residual solvents. A standard rubbing technique was used to brush superficially the PEDOT:PSS and P3HT:PCBM layers, both layers being applied as LC alignment layer (AL) in the final device. Finally, the two ITO substrates were assembled together in anti-parallel configuration using a UV curable adhesive mixed with 7.75 μm spacers. The cells were filled with E7 by capillarity action and the cell edges were sealed using a 2-component epoxy adhesive. Four different P3HT:PCBM blends with the weight ratios of, respectively, (1:0), (1:0.01), (1:0.5) and (1:1) were used for the photo-sensitive layer. One device having only PEDOT:PSS layers on both substrates was also made and used as a reference sample for comparison.


2.2 Cross polarized intensity (CPI) and Voltage Transfer Function (VTF)

CPI is a common method to measure the birefringence of a LC cell at discrete wavelength if the gap thickness is known8, as a function of applied voltage. The experimental setup is shown in Figure 1(b) and is composed of a 532 nm laser source, a polarizer at 45° with respect to the LC alignment direction, an analyzer at 90° to the first polarizer, a photodiode and a trans-impedance amplifier (TIA). An oscilloscope monitors the TIA voltage output and a voltage generator is used to drive the LC cell placed between the polarizer and analyzer. When passing through the LC cell, the light experiences a phase delay that depends on the LC birefringence. As a result, the output polarization changes as a function of LC alignment, causing the measured intensity to vary as a function of the voltage applied to the cell.

Figure 1. (a) Structure of the LC cell composed of a P3HT:PCBM-coated electrode at different weight ratio and a PEDOT:PSS-coated electrode. (b) CPI experimental setup.

VTF is a convenient and graphical method to determine the electrical behavior of liquid crystal cells from optical measurements. It has been used to estimate the voltage drop variation across the LC layer as a function of light intensity on PVK:C60 based OASLM devices7. In order to obtain the VTF data, several CPI measurements are made over a set range of frequencies and are plotted on a 3D graph. Regardless of the frequency, the VTF and CPI curves always represent the intensity, averaged over several periods of the LC cell driving frequency. An example of such measurement on a PEDOT:PSS-only reference sample is shown Figure 2. A color code is used to depict the intensity values of the light transmitted for different frequencies and applied voltages. For clarity, the measurement made at 1 KHz has been highlighted on the VTF graph (blue dashed line) and the corresponding CPI trace has been plotted on a 2D graph.

Figure 2. VTF curve of a LC cell using PEDOT:PSS coated-electrodes on both side (left) and the corresponding CPI trace at 1 KHz (right).

2.3 Diffraction efficiency

In order to estimate the potential of the P3HT:PCBM OASLMs for holographic applications, we measured its diffraction efficiency in a two beam coupling configuration. The measurement setup is shown in Figure 3. In this experiment, a laser beam (λ = 532 nm, incident intensity of 24 mW/cm2), also referred to as pump beam, is split into two coherent beams that subsequently interfere on the OASLM. The beam interference creates a sinusoidal light intensity grating (Λ = 9.4 μm) on the OASLM that translates into a refractive index grating (periodic change of the liquid crystal birefringence). The resulting sinusoidal refractive grating is probed using a second laser beam of a different wavelength (λ = 633 nm, intensity 13 mW/cm2), referred to as probe beam. When the probe beam encounters the refractive index grating formed


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Figure 3. Diffraction efficiency setup

3. RESULTS 3.1 Cross-polarized intensity measurements

The LC devices with different P3HT:PCBM weight ratios were characterized by CPI using a 10 kHz AC voltage. The incident light intensity was varied logarithmically from 0.0018 mW/cm2 to 49 mW/cm2. The results are depicted in Figure 4.

Figure 4. CPI curves of P3HT:PCBM OASLMs at different weight ratio : (a) 1:0, (b) 1:0.01, (c) 1:0.5 and (d) 1:1 under different light intensities (with λ = 532 nm). The LC cell gaps were estimated from the CPI data at the lowest light intensity assuming a birefringence of 0.235 for E79 and are equal to 7.6 μm (a), 6.4 μm (b), 7.5 μm (c) and 7.3 μm (d).

As can be seen, the P3HT:PCBM weight ratio has a significant impact on the device response as a function of light intensity. As expected, the response of pure P3HT devices (Figure 4(a)) has only a weak dependence on the light intensity. The slight shift towards the lower voltages with increasing light intensity (most pronounced at high voltages) can be attributed to an increase in the voltage potential drop across the LC layer. In other words, the electrical impedance of the P3HT layer varies with light intensity, i.e. it behaves as a photoconductor. The rather weak amplitude of the shift is due to the fact that, in the absence of PCBM, only excitons photo-generated close enough (typically less than a few


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nm) to the ITO/P3HT and LC/P3HT interfaces may undergo a charge transfer and give rise to free charge carriers10. Another possible explanation is that such a thin P3HT layer induces a negligible voltage drop. To understand this, we must assume that the LC cell can be modelled using an equivalent electrical circuit11. The model consists of an association in series of several impedances, each impedance characterizing the electrical properties of a given layer. It is likely that at a frequency of 10 KHz and low light intensity (0.0018 mW/cm2), both the LC and P3HT layer impedance is purely capacitive. Knowing both the dielectric constant and thickness of E7 (5.2≤εE7≤19; d=7.6 μm) and P3HT (εP3HT≈3.5, d=0.1 μm), it is easy to show that the impedance of the LC layer is higher than the P3HT impedance. At low voltages, when the alignment is planar, the LC impedance is 51 times higher than the P3HT impedance and at higher voltages, when the LC alignment is homeotropic, the LC impedance is 14 times higher than the P3HT impedance. Hence, the P3HT layer should induce a negligible voltage drop even at low light intensities. It should be noted that according to this electrical model, the voltage drop induced by the P3HT layer should increase when the LC molecules pass from a planar to a homeotropic alignment, hence with the voltage applied to the LC cell.

Upon addition of PCBM, the device response changes significantly. First of all, the shift towards lower voltages of the CPI curves with increasing light intensity is more pronounced and increases with PCBM content. Most interestingly, the transmittance is influenced by the light intensity even at zero voltage. In other words, the threshold AC voltage needed for changing the LC birefringence appears to be negligible. The aforementioned phenomenon increases with the PCBM loading and is not present for cells based on a pure P3HT layer. Evidently, such a light-induced change in birefringence at 0V cannot be explained in terms of a simple change in electrical impedance. Two different phenomena may in principle be at the origin of this behavior: (1) the pre-tilt angle of the LC molecules at the P3HT:PCBM/LC interface increases with light intensity, or (2) the P3HT:PCBM layer spontaneously generates a photovoltage (VPH) under illumination. The corresponding electric field in the LC layer allows the LC molecules to reorient at 0V. The high pre-tilt angle at the LC/P3HT:PCBM interface would facilitate the response of the LC molecules to VPH. It is possible to get an estimate of the photo-voltage amplitude from the shift value of the CPI curves by recalculating the effective RMS voltage (VRMS) applied to the LC layer. Indeed, in the presence of a continuous VPH that would add to the applied AC voltage (with amplitude VPK), the device can be considered as being driven by an effective VRMS voltage given by:

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Figure 5(a) represents the CPI curves of Figure 4(d) after having added a VPH to VPK according to equation (1). The VPH values were adjusted so as to achieve a good overlap between the different curves at low voltages. Note that this operation assumes that the P3HT:PCBM layer induces a negligible voltage drop (at least at low voltages). As shown in Figure 5(b), VPH increases logarithmically with light intensity and reaches a value of almost 0.6V, which is close to the open-circuit voltage generated by an organic solar cell based on P3HT:PCBM bulk heterojunction under one sun illumination (100mW/cm2)12. Importantly, the logarithmic dependence is a characteristic feature of organic solar cells13. The light intensity dependence and the estimated VPH values strongly support the generation of a photovoltage as the responsible mechanism for the birefringence change at 0V. The precise mechanism that allows photo-generated charges to be separated and induce VPH is still under investigation.

Figure 5. Fitted CPI curves using equation (1) of the P3HT:PCBM sample at a (1:1) weight ratio (a). Photovoltage values (VPH) used for the fit plotted versus light intensity.


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3.2 Voltage Transfer Function measurements

The VTF measurements obtained on the P3HT:PCBM (1:1) device at two different light intensities are shown in Figure 6. The AC voltage frequency was varied from 10 Hz to 100 kHz. The color code depicts the average light intensity measured by the photodiode (from dark, for the lowest intensity, to white for the highest intensity).

Figure 6. VTF graph showing the frequency dependence of P3HT:PCBM based OASLM with a 1:1 ratio, at a light intensity of 0.0018 mW/cm2 (left) and 49 mW/cm2 (right), λ=532 nm.

Again, a significant change in birefringence can be observed at 0V. The change in VRMS voltage needed to achieve the same change in transmittance (or, equivalent, in birefringence) between both light intensities is of the order of 0.6 V, which is close to the value of VPH estimated from the CPI curve corrections discussed above. Note that the light-induced change in VTF is constant over the whole investigated frequency range. This behavior is in-line with the photovoltaic effect, which should not depend on the applied voltage frequency, and rules out photo-conductance as a major driving mechanism (the latter is indeed expected to give rise to a strong frequency dependence). Finally, it is important to point out that each VTF measurement at a fixed light intensity lasts for at least three hours. The well-defined and reproducible VTF signal is therefore a strong indicator of the stability and absence of hysteresis in the response of our device.

3.3 Diffraction Efficiency of P3HT:PCBM at 1:1 weight ratio

The diffraction efficiency (DE) of the LC device with a (1:1) P3HT:PCBM layer was measured by the two beam-coupling set-up described in Figure 3. Figure 7 shows the resulting DE as a function of the applied VRMS (10 kHz).

Figure 7. Diffraction efficiency of the P3HT:PCBM sample at 1:1 weight ratio

A diffraction efficiency of 0.4% is observed already at 0V. Although rather low in comparison to state-of-the-art devices14, it increases with applied AC bias before decreasing at higher voltages. Nevertheless, those preliminary results demonstrate the capability of the photovoltaic-LC system for the autonomous operation, ideal for different optical applications.


4. CONCLUSION In conclusion, we have shown that we were able to control LC orientation of anti-parallel aligned LC cells using an organic P3HT:PCBM blend. Furthermore, we estimated the voltage generated by the organic layer from the CPI curves to demonstrate that the photovoltaic effect was responsible for the autonomous LC reorientation. Finally, we showed that organic photovoltaic-LC devices can spatially modulate light without an applied voltage, opening new prospects in the field of self-activated LC devices such as stand-alone laser protection devices and smart windows.

ACKNOWLEDGMENTS

The authors wish to acknowledge Nina Podoliak for performing the diffraction efficiency experiment as well as Jérémy Bartringer and Matthew Proctor for their valuable advice. We acknowledge financial support from the Defence Science and Technology Laboratory (Dstl) (DSTLX1000087813R, DSTLX1000105958).

REFERENCES

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[6] Ono, H. and Kawatsuki, N., “Orientational holographic grating observed in liquid crystals sandwiched with photoconductive polymer films,” Appl. Phys. Lett. 71(9), 1162–1164 (1997).

[7] Proctor, M., Bateman, J., Daly, K., Herrington, M., Buchnev, O., Podoliak, N., D’Alessandro, G. and Kaczmarek, M., “Light-activated modulation and coupling in integrated polymer–liquid crystal systems,” J. Opt. Soc. Am. B 31(12), 3144 (2014).

[8] Wu, S.-T., Efron, U. and Hess, L. D., “Birefringence measurements of liquid crystals,” Appl. Opt. 23(21), 3911–3915 (1984).

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[10]Wang, H., Wang, H.-Y., Gao, B.-R., Wang, L., Yang, Z.-Y., Du, X.-B., Chen, Q.-D., Song, J.-F. and Sun, H.-B., “Exciton diffusion and charge transfer dynamics in nano phase-separated P3HT/PCBM blend films,” Nanoscale 3(5), 2280 (2011).

[11]Seiberle, H. and Schadt, M., “LC-Conductivity and Cell Parameters; Their Influence on Twisted Nematic and Supertwisted Nematic Liquid Crystal Displays,” Mol. Cryst. Liq. Cryst. Sci. Technol. Sect. Mol. Cryst. Liq. Cryst. 239(1), 229–244 (1994).

[12]Schilinsky, P., Waldauf, C. and Brabec, C. J., “Recombination and loss analysis in polythiophene based bulk heterojunction photodetectors,” Appl. Phys. Lett. 81(20), 3885–3887 (2002).

[13]Chelouche, A., Magnifouet, G., Al Ahmad, A., Leclerc, N., Heiser, T. and Lévêque, P., “Disentangling energetic and charge-carrier dynamic influences on the open-circuit voltage in bulk-heterojunction solar-cells,” J. Appl. Phys. 120(22), 225501 (2016).

[14]Shrestha, P. K., Chun, Y. T. and Chu, D., “A high-resolution optically addressed spatial light modulator based on ZnO nanoparticles,” Light Sci. Appl. 4(3), e259 (2015).


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