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Titre : Oriented semiconducting polymers for photovoltaic spatial light modulators.
Directeur(s) de Thèse : Thomas HEISER (professeur, 1ière classe) Unité(s) d’Accueil(s) : ICube
Établissement de rattachement : Université de Strasbourg
Collaboration(s) (s’il y a lieu) : M. Brinkmann (ICS), D. Ivanov (IS2M), N. Leclerc (ICPEES) Rattachement à un programme (s’il y a lieu) : Projet ANR PSLM
Résumé (1500 caractères au maximum) :
Descriptif du sujet (en complément, au format Word ou pdf)
This project focuses on a new type of dynamic glazing, called PSLM, whose tint adapts spontaneously to ambient light. PSLMs are made of semiconducting polymers and liquid crystals and rely on the photovoltaic effect to control the orientation of the liquid crystals and change the optical transparency. The lack of power supply, low response time and adjustable optical response make PSLMs attractive for "smart"
windows. However, their maximum transparency in the visible range is still insufficient and limits their field of application.
This project aims to improve the optical performance of PSLMs by using mechanically oriented semiconducting polymers. In the first part, thin films of electron-donor and electron-acceptor polymer blends, aligned by brushing at high temperature, will be used to generate a polarized photovoltaic response. This property will avoid the use of an external polarizer and increase the visible transparency of the PSLM. The second part of the project will focus on the use of thin films based on oriented conducting polymers, which thanks to a dichroic absorption in the infrared and a high transmittance in the visible, can be used as an infrared polarizer while ensuring good transparency in the visible.
Description of the project
Context, state-of-the-art, and positioning
There is growing interest in the development of semi-transparent materials whose opacity or colour can be adjusted on demand. Such dynamic glazing can for instance be used in optical protection systems or in smart windows, where the tuneable optical properties help improve the energy efficiency of buildings or vehicles by regulating interior heating and lighting through solar radiation.1,2 Many switchable glazing technologies have been investigated over the past, the most advanced ones being based on electrochromic (EC), photochromic (PC), or thermochromic (TC) materials or on stimuli-sensitive liquid crystals (LC) and their composites, such as Polymer Dispersed Liquid Crystals (PDLC).3,4,5 However, none of these technologies can simultaneously achieve high switching speed, self-powered operation, and easy user control. Self-powered operation simplifies installation, improves the energy efficiency and is thus of particular interest.1 While this is a built-in feature for PC or TC glazing, external power sources are required for EC and LC devices.Significant effort has, therefore, been devoted to coupling EC or LC glazing with energy harvesting devices.6Yet, the manufacturability and scalability of autonomous glazing that combine two independently operating devices, pose significant barriers for large scale integration. Furthermore, the means of controlling the operation of smart windows create another set of constraints that differ from one technology to the other. PC or TC windows for instance do not give users the ability to monitor the tint of the device in accordance with their needs.
We have recently developed a new kind of dynamic glazing, named “Photovoltaic Spatial Light Modulator”
(PSLM), that overcomes some of these bottlenecks.7 Its operation relies on the association of a nematic LC with a donor – acceptor bulk heterojunction (BHJ) in a single hybrid device. The former is used for its ability to be oriented by an electric field, while the BHJ forms a photovoltaic unit (PVU) capable of generating, under illumination, an electric field large enough to orient LC molecules. The PVU is in direct contact with the LC and is also used as the LC alignment layer. This configuration leads to a self-powered photo-responsive device, whose light transmittance adjusts spontaneously to the incident light intensity. The PSLM can be considered as an optically - addressed spatial light modulator8 using a photovoltaic rather than photoconductive material as the photosensitive component. The structure and self-powered operation of a PSLM is illustrated in Figure 1. Under illumination (AM 1.5, 100 mW/cm2), the PSLM transmittance drops from its maximum value (≃ 14%) to its minimum value (< 1%), within seconds after the connector has been closed (Figure 1b). The drop in transmittance depends on the incident light intensity and on the resistor which is bridging both electrodes (Figure 1c).9 While these results point out the self-powered operation, fast response time and easy user- control of a PSLM, they also highlight a major bottleneck: the low transmittance of the device in the OFF state (switch open). The latter is the consequence of the use of polarizers, which are a constituent part of the PSLM, and to the absorption of photons by the photovoltaic layer.
Objectives
The aim of this project is to improve the clear-state transmittance of the PSLM by using aligned semiconducting polymer films as both a component of the BHJ and a polarizer. The optical properties of aligned polymer thin films can be highly anisotropic, allowing these materials to behave as a dichroic polarizer.10 As a consequence, a bulk heterojunction layer that is composed of aligned polymers is expected to exhibit anisotropic optical extinction coefficients. When used as PVU in a PSLM, the aligned polymer film
can therefore fulfil two functions simultaneously: (i) absorb incoming photons to generate the photoelectric field necessary to change the orientation of the liquid-crystal director, and (ii) polarize the transmitted light.
In other words, the PV unit behaves as an internal polarizer of the device. Importantly, unlike the standard configuration (Figure 1a), the polarized light that is transmitted by the internal polarizer will not be absorbed by the bulk heterojunction to generate the photovoltage. Therefore, the transmittance of the PSLM device should increase.
Donor-acceptor bulk heterojunctions composed of blends of aligned electron donor polymers (PBFTZ) and electron acceptor polymers (PNDIT2) have already been reported before. Recently, our investigations on a similar system oriented by high temperature rubbing have led to promising anisotropic absorption coefficients with a dichroic ratio as high as 10 (Figure 2).11 Using polarizers with such a “low” dichroic ratio (in comparison to standard polarizers) would be enough to allow a modulation of the PSLM transmittance by a factor of 3.
However, the utilization of such oriented layers in a PSLM has not yet been achieved, the major challenge being to make them compatible with the operation of the device.
Figure 2: Polarized absorption spectrum of (a) a thin film on an oriented PBFTZ:PNDIT2 blend, and (b) of an oriented doped P3HT film as a function of doping level (concentration of doping solution)12
Part of this project will be devoted to the study of the orientation of similarly rubbed blends and their interaction with the liquid-crystal molecules. A better understanding of the blend/LC interface is indeed necessary to achieve a working PSLM with superior properties.
The second advantage of using aligned polymer films as polarizers for a PSLM is their narrow absorption bands.
For instance, if the absorption band is centred in the near infrared (NIR) region without extending significantly
a) b) c)
Figure 1 : a) Schematic illustration of a PSLM in the ON state (switch closed). b) Change in transmittance of a PSLM upon switching from the OFF (switch open) to the ON state.
c) Normalized transmittance of a PSLM as a function of light intensity with various resistors inserted between both electrodes.
into the visible range, the PSLM will modulate only NIR light, while remaining transparent in the visible range.
Such a behaviour is of particular interest for smart windows applications, as it would allow to attenuate in- door solar heating while providing sufficient visible light transmission for user comfort. Our recent work on doped semiconducting polymers has shown that aligning highly doped poly(3-hexylthiophene) thin films by high-temperature rubbing leads to strongly anisotropic transmittance and reflectance spectra in the NIR while being highly transparent in the visible range (Figure 2b).10 However, the dichroic ratio in the NIR was found to be rather low (≃ 4). As a consequence, the modulation of the transmitted NIR light of a PSLM using such films as external polarizer was small (relative change of less than 10%). Rapid out-diffusion of the dopants is possibly at the origin of this behaviour. The second part of this project will address this issue by studying the dopant diffusion kinetics and investigate means to stabilize the doping level. In particular, Rutherford backscattering will be used as an advanced tool to measure the dopant profile across the film thickness and evaluate the efficiency of barrier films to avoid dopant out-diffusion.
Methodology
To achieve the objectives outlined above, the project will focus on the optoelectronic properties of aligned semiconducting polymers and on their utilisation as either internal or external polarizer in a PSLM device. It involves the controlled deposition and alignment of polymer layers, the advanced structural and optical studies of the oriented polymers, and their integration into a PSLM. The polymers of interest will be either provided by partners involved in a related ANR research project or be commercially available (ex: P3HT).
The work will be organized in two work-packages:
WP1: Elaboration and characterization of oriented polymer films - High temperature rubbing will be the main tool to orient the polymer films. A dedicated equipment is available at ICS (Group of Dr. Martin Brinkmann, partner of the ANR project). For the oriented photovoltaic unit, blends of PFBTZ and PNDIT2 will be considered at a first place, as some preliminary encouraging results (see Figure 2) have already been obtained with these materials. Their utilization as a photovoltaic layer and internal polariser has not yet been accomplished and will be fulfilled within this project.
Oriented thin films of poly(3-hexylthiophene) doped with a tris(4-bromophenyl)ammoniumyl hexachloroantimonate (Magic Blue), will be considered as a first NIR polarizer for PSLM devices. The doping will be done by exposing the polymer films to a solution of the dopant in a polar solvant.12 The stability of the doping concentration will be studied by measuring the depth profile of the antimony nucleus (Sb is a component of magic blue) in the polymer film by Rutherford backscattering (RBS). Previous experiments have shown that the detection limit of Sb in an organic film (i.e. a heavy nucleus embedded in a matrix of light nuclei) by RBS is sufficiently low to detect the dopant with a high signal-to-noise ratio.12 If dopant out-diffusion is confirmed to cause the rapidly decreasing doping level, the doped polymer films will be encapsulated by transparent oxide layers (ex: MoO3 deposited from vapor phase). The efficiency of the encapsulant will be studied by RBS. Finally, doped layers of different thicknesses will be considered to reach an optimum balance between the dichroic ratio and the transparency in the visible.
In addition, polarized UV-vis absorption will be used to evaluate the overall orientation of the polymers. The photovoltaic properties of oriented blends will first be explored by using them as absorber layer of organic photovoltaic devices. The optimized oriented blends will be integrated as the PV unit in a PSLM (WP2).
WP2: PSLM fabrication and characterization - The oriented polymer blends will be used as anisotropic bulk heterojunction (BHJ) of the photovoltaic unit of a PSLM. The BHJ will be embedded between an electron- transporting layer and a hole transporting layer (HTL). The HTL is also used as LC alignment layer and needs to be aligned. This can be achieved either by an additional, light, rubbing step or by spontaneous alignment of the HTL molecules when deposited on the already aligned BHJ. The oriented conducting polymers will be used as external polarizers.
In both cases, the PSLMs will be fully characterized by measuring the spectroscopically resolved transmittance as a function of incident light intensity.
References
(1) Ghosh, A.; Norton, B. Advances in switchable and highly insulating autonomous (self-powered) glazing systems for adaptive low energy buildings. Renew. Energy (2018), 126, 1003-1031.
(2) Casini, M. Active dynamic windows for buildings: A review. Renew. Energy (2018), 119, 923-934.
(3) Kamalisarvestani, M.; Saidur, R.; Mekhilef, S.; Javadi, F. S. Performance, materials and coating technologies of thermochromic thin films on smart windows. Renew. Sustain. Energy Rev. (2013), 26, 353-364.
(4) Granqvist, C.G.; Arvizu, M. A.; Bayrak Pehlivan, I.; Qu, H.-Y.; Wen, R.-T.; Niklasson, G. A. Electrochromic materials and devices for energy efficiency and human comfort in buildings: A critical review. Electrochimi. Acta (2018), 259, 1170-1182
(5) Ke, Y.; Chen, J.; Lin, G.; Wang, S.; Zhou, Y.; Yin, J.; Lee, P. S.; Long, Y. Smart Windows: Electro-, Thermo-, Mechano-, Photochromics, and Beyond. Adv. Energy Mater. (2019), 9, 1902066.
(6) Davy, N. C.; Sezen-Edmonds, M.; Gao, J.; Lin, X.; Liu, A.; Yao, N.; Kahn A.; Loo, Y.-L. Pairing of near-ultraviolet solar cells with electrochromic windows for smart management of the solar spectrum. Nat. Energy (2017), 2, 17104.
(7)Patent “Liquid Crystal spatial light modulator” , 2019, EP3669231A1, US2020/023248A1;
(8) N. Collings, Optically addressed spatial light modulators for 3D display , J. of Nonlinear optical physics & mat. (2011), 20, 453 (9)Link to video of a PSLM demonstrator : https://seafile.unistra.fr/f/46c8d3fdf61540c7bce2/
(10) H.M. Schrickx, et al. Ultra-High Alignment of Polymer Semiconductor Blends Enabling Photodetectors with Exceptional Polarization Sensitivity , Adv. Funct. Mat. (2022), 32, 2105820
(11) S. Fall et l., Self-Powered Dynamic Glazing Based on Nematic Liquid Crystals and Organic Photovoltaic Layers for Smart Window Applications, ACS Appl. Mater. Interfaces (2023) https://doi.org/10.1021/acsami.2c21727
(12) Y. Zhong, et al., « Preferential Location of Dopants in the Amorphous Phase of Oriented Regioregular Poly(3-hexylthiophene- 2,5-diyl) Films Helps Reach Charge Conductivities of 3000 S cm−1 », Adv. Funct. Mat. (2022), 32, 2202075
