Photovoltaic spatial light modulators (PSLMs) form a new type of optically addressed liquid crystal (LC) devices, capable of adapting its optical properties to incident light in less than a second, without external power supply. Since the photo-induced voltage generated by a PSLM is continuous, the presence of residual ions in the LC may interfere with the device operation and compromise the performances. Here, we investigate the influence of the alignment layers (AL) in a PSLM on ion accumulation/neutralization at the LC/AL interfaces by probing the optical and dielectric response under DC bias of LC light valves based on Poly(3-hexylthiophene-2,5-diyl) (P3HT) and/or poly(vinyl-alcohol) (PVA) as ALs. We find that charge injection into the P3HT layer allows neutralization of ions accumulated at LC/ALs interfaces thereby avoiding the screening of the electric field in the LC.
Photovoltaic spatial light modulators (PSLM) are self-activated optical devices that can be used as dynamic glazing or as optically addressable spatial light modulator. The range of potential applications of this new kind of optical device is highly dependent on its clear-state transmittance, spectral distribution of transmittance modulation, as well as on its response time, stability, and spatial resolution. These features are in turn mainly determined by the materials used for the various parts making up a PSLM, namely the photovoltaic unit, the liquid crystal layer, the liquid crystal alignment layers, and the polarizers.
This contribution will focus on the multiple links between material properties and device performance and present our recent results on the design and processing of organic semiconductor materials designed to broaden the field of applications of PSLMs.
We present a versatile characterisation of liquid crystal devices, including those integrated with organic photovoltaics. Photovoltaic thin film serves as an alignment layer and also generates an electric field under illumination that reorients liquid crystals for self-activated or autonomous operation. Apart polar alignment angle, anchoring energy, thickness uniformity, the photovoltaic properties, such as the photovoltage generated and photoconductivity, are captured and the map of the spatial changes of the parameters can be created. The method is applicable to other liquid crystal systems, such as doped liquid crystals and optically thin cells, with phase lag as small as π.
Photovoltaic spatial light modulators combine liquid crystals with organic photovoltaic layers to achieve self-activated transmittance modulation. Short response times, energy-efficient operation, and user-controllable sensitivity make these devices attractive for many applications. We will show that the transmittance modulation is highly reversible and can be stable for hours under light exposure. Modulators based on new organic materials, selected to enhance the transmittance of the clear state and device sensitivity, will be presented. Results illustrating selective transmittance modulation in the near-infrared to control solar heating, while harvesting near-UV light will be shown. Remaining challenges and development possibilities will be outlined.
Photovoltaic spatial light modulators form a new class of dynamic glazing that could be of interest to smart windows applications. The structure of the modulators includes a twisted nematic liquid crystal layer and an organic donor-acceptor bulk heterojunction. The latter is in contact with the liquid crystal and is used as a molecular alignment layer. In addition, under illumination, the bulk heterojunction generates an electric field that can be strong enough to orient the liquid crystal molecules homeotropically and change the device optical transmittance, without requiring an external power source. The transmittance of this hybrid device adjusts spontaneously to ambient light within less than a second, with a sensitivity that can be tuned by a passive resistor. While this unique combination of features is desirable for smart windows, the device maximum transmittance in the clear state is currently limiting the possible scope of application.
In this contribution, we will firstly present the detailed structure, elaboration procedure and optical properties of a first generation of photovoltaic spatial light modulators that are based on commercially available polymer:fullerene blends and liquid crystals. The physical mechanism underlying the device operation will be demonstrated by crossed-polarizer intensity measurements as a function of incident light intensity and applied voltages. Furthermore, the time-dependent transmittance of a device that is exposed to a pulsed light source will be presented in order to assess its response time and reversibility.
In the second part we will describe various routes that we are following to improve the device optical properties in terms of maximum transmittance and sensitivity to ambient light. In particular, a new high band-gap semiconducting molecule that has been designed to achieve a highly transparent bulk heterojunction layer and increase the photo-induced electric field will be presented and its utilization in photovoltaic spatial light modulators will be shown.
Organic semiconductor materials such as planar conjugated small molecules are of great interest to the photovoltaic community. In thin films, the exciton and charge carrier dynamics, which are crucial to photovoltaic device operation, depend in a non-trivial way on the organic molecular structure and on the molecular organization in the solid state. Recently, the exciton diffusion has been found to strongly depend on the crystalline order of the organic thin films. This work presents the study of the exciton lifetime in an innovative class of molecular semiconductors able to present different crystalline order. This family of molecules has a “dumbbell-shaped” structure based on triazatruxene units that act as a π-stacking platform. Such molecules with different side-chains have been found to self-assemble into various crystalline and liquid crystalline phases. We have studied the steady-state photoluminescence and the exciton lifetime for several triazatruxene-based derivatives with different side-chains, in solution and in thin films for different solid state phases. In solution, the fluorescence lifetime corresponds to the reference value that can be obtained without intermolecular interaction. In thin films, we measured the exciton lifetime for different molecular structures in order to correlate the exciton dynamics with the molecular stacking. The results reveal a significant increase in the exciton lifetime with the enhancement of the structural order.
KEYWORDS: Liquid crystals, Photovoltaics, Modulators, Transmittance, Measurement devices, Tandem solar cells, Solar energy, Power supplies, Polarizers, Photonic crystals
Photo-voltaic light modulators consist of a liquid crystal layer integrated with an organic photovoltaic structure. Addressing them with light produces an internal voltage that changes the liquid crystal orientation and the optical transmission properties of the device. They offer an exciting prospect for autonomous, light controlled smart displays and visors.
Herein we report the development of self-activated light modulators, whose transmittance drops with increasing light intensity without applying an external power supply. This could be achieved by introducing a tandem photovoltaic structure that allows to produce larger voltages. Crossed polarized intensity measurements on devices based on different liquid crystals and photovoltaic layers are presented to clarify the physical mechanisms underlying self-activation.
Photovoltaic light modulators integrate liquid crystals and solar cells and offer an exciting prospect for autonomous, smart displays and visors. Illumination produces a photovoltage that modifies the liquid crystal alignment and light transmission. However, determining their properties, for example, the voltage dropped across the liquid crystal, different pretilts or anchoring energies, inherent to asymmetric cell designs, poses significant challenges. We have successfully applied to such photovoltaic modulators a new measurement methodology based on wide-area cross-polarized intensity measurements, coupled to an Ericksen-Leslie model. We have implemented it in a versatile optical analyzer, driven by a Matlab graphical user interface.
Organic semiconductor materials such as planar conjugated small molecules are of great interest to the photovoltaic community [1, 2]. In thin films, the exciton and charge carrier dynamics, which are crucial to photovoltaic device operation, depend in a non-trivial way on the organic molecular structure and on the molecular organization in the solid state. [3, 4] Several recent studies have established that the exciton diffusion strongly depends on the crystalline order of the organic films. [5, 6] Our work presents the study of the exciton dynamics in an innovative class of molecular semiconductors able to present high crystalline order. This family of molecules has a “dumbbell-shaped” structure based on triazatruxene units that act as a π-stacking platform. [7] Such molecules with different side-chain have been found to self-assemble into various crystalline and liquid crystalline phases. We have studied exciton dynamics of molecular with different side-chain: in solution and in thin films for different solid state phases. In solution, the lifetime corresponds to the reference value that can be obtained without intermolecular interaction. In thin films, we measured the exciton lifetime for different molecular structures in order to correlate the exciton diffusion length with the intermolecular stacks. The results reveal a significant increase in exciton lifetime with structural order.
[1] Y. Lin et al., Chem. Soc. Rev. 2012, 41, 4245.
[2] S. D. Collins et al., Adv. Energy Mater. 2017, 7, 1602242.
[3] J. A. Bartelt et al., Adv. Energy Mater. 2013 , 3 , 364.
[4] J. T. Bloking et al., Adv. Energy Mater. 2014, 4, 201301426.
[5] R. R. Lunt et al., Adv. Mater., 2010, 22, 11, 1233-1236.
[6] A. T. Haedler et al., Nature, 2015, 523,196–199.
[7] I. Bulut et al., J. Mater. Chem. A, 2015, 3, 6620.
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.
Experimental results concerning the influence of plasmon effect from silver nanoparticles on the organic photovoltaic
device performance are presented. The metallic nanoparticles (NPs) are placed on top of ITO layer using a physical
vapor deposition technique. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) followed by an
interpenetrated poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) blend layer are then
spin-coated. The aluminum electrode is finally evaporated on. Photovoltaic properties compared to devices without NPs
are shown. A spectrophotometric characterization is carried on. Moreover, a ToF-SIMS measurement is performed in
order to obtain the depth chemical profiles of solar cell containing such NPs. Silver NPs diffusion inside other layers of
the cell is investigated.
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