Improving performances of self-powered liquid crystal devices by doping and photovoltaic layers engineering

Abstract

Liquid crystal devices with an integrated organic solar cell are semi-transparent light valves that can modulate light upon illumination. The modulation and light sensitivity of such devices are widely tuneable as the choice of the polarisers used in the liquid crystal device will define the modulated wavelength range, while the organic semiconductors, used to build the solar cell photoactive layers will constrain the portion of the spectrum absorbed to produce a voltage and address the liquid crystal material. This technology has a wide range of application, from optical sensor protection and light intensity modulator to energy saving in building (e.g., by modulating the near-infrared transmission through windows). Despite these promising uses, the technology still suffers from two major issues: it needs a sufficient voltage generation to address the liquid crystal part of the device, and we cannot yet reliably monitor the aging of the photovoltaic components due to the loss of their electrical contacts because of their integrated nature.
This thesis will describe both liquid crystal materials and organic photovoltaic materials and structures to then present the photovoltaic spatial-light modulators, that can be self-powered. These are liquid crystal devices that use organic solar cells as an internal voltage source. In this study, the use of doped liquid crystal materials to modify the liquid crystal material intrinsic properties will be explored. Different nanoparticles such as ferroelectric nanoparticles (SPS and BTO), quantum dots (ZnO) and functionalised metallic nanoparticles (Azo-thiol gold nanoparticles) will be used as dopants to influence their host properties. The focus will be made on the reduction of the liquid crystal driving voltage and a reduction of their viscosity for faster response time. These mixtures will be characterised using an optical, non-invasive method. Different concentrations will be explored, and the impact of the possible aggregation will be shown. Then, the discussion will be focused on the photovoltaic part of the device, in particular on the efforts to increase its efficiency and spatial resolution by working on the alignment layers, the interfacial layers between the liquid crystal material and the photovoltaic unit. The use of P3TH as an alignment layer will be explored to allow the device to drive the liquid crystal layer with the DC voltage generated by the organic solar cell. Finally, a study of working self-powered devices will be carried out using an optical non-invasive method, allowing us to characterise both liquid crystal and integrated solar cell properties without the need of direct electrical contacts to each component. This technique permits us to study the aging and the stability of the devices over several months and monitor both the solar cell and liquid crystal layer evolution over time.

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Identifiers

ORCID for Giampaolo D'alessandro: orcid.org/0000-0001-9166-9356

ORCID for Ondrej Hovorka: orcid.org/0000-0002-6707-4325

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