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.
The overall performance of liquid crystal devices is determined by a large number of interlinked features. We demonstrate that an easy-to-implement methodology and optical technique can provide a comprehensive characterisation and mapping of liquid crystal systems, capturing their static as well as dynamic properties. It has also been successfully applied to thin liquid crystal cells, planar and twisted cells. The technique is not only a powerful tool for optimising the choice of materials for each specific application, but also offers a great insight in the polarisation dynamics of light propagating in the anisotropic and multi-layer liquid crystal systems.
Current liquid crystal technologies often rely on the use of optically thin cells and new liquid crystals. The characterisation of their parameters, such as elastic constants, including twist elastic constant and pretilt, key to control the liquid crystal response, poses several challenges. We present an optical method that successfully characterises such liquid crystal devices, is relatively simple yet a powerful probe of their static and dynamical properties. The method is demonstrated for the cells with the total phase lag smaller than 2pi and for experimental liquid crystals, where optical and dielectric properties are only partially known and for estimating K2.
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.
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.
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