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.
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.
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