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Florent Caffy

High band gap copolymers based on quinoxaline units: New materials for the heterojunction organic solar cells

Published on 30 March 2016
Thesis presented March 30, 2016

An alternative to fossil fuels are the organic photovoltaic cells which have recently started their technological transfer from research laboratories to industry. Many research efforts have been made on the modification of materials and processes to increase the performance of organic solar cells. In this context, this work presents a comprehensive study from the design of new electron-donor high band gap polymers to their characterisation in photovoltaic devices. The main requirement was to decrease the HOMO energy level of the polymers in order to increase the open circuit voltage of the solar cells. The "push-pull" approach was used to obtain the desired properties. Polymers with quinoxaline or dithienoquinoxaline as electron-deficient units and dibenzosilole or carbazole as electron-rich units were synthesized by Suzuki coupling or by direct heteroarylation. Molecular weights up to 56 kg.mol-1 were obtained. The electron-withdrawing unit quinoxaline was substituted by fluorine atoms on the benzene moiety and by thiophene, bithiophene and terthiophene group on the pyrazine moiety. Thiophenes or thiazoles were used as spacers to link the electron-donating and the electron-withdrawing units. The relationship between the structural modification of the polymers and their optoelectronic properties were analysed. The optical properties were studied by UV-visible spectroscopy and fluorescence spectroscopy. Whereby it appears that polymers with dithienoquinoxaline-dibenzosilole units showed an absorption up to 550 nm and polymers with both quinoxaline-dibenzosilole units and quinoxaline-carbazole units showed an absorption up to 650-700 nm respectively. The corresponding optical band gaps were found to range from 1.8 eV to 2.3 eV. The HOMO and LUMO energy levels of the polymers were determined by electrochemistry. All polymers exhibited HOMO energy levels below -5.0 eV. Fluorine atoms and thiazole spacers significantly lowered the HOMO energy levels of the polymers up to -5.69 eV. DFT was used to model the polymer structures. X-ray diffraction was used to analyse the distances between the polymer chains. Hole mobilities were measured in organic field effect transistors and values of up to 9.0 x 10 3 cm2.V-1.s-1 were obtained. The polymers were tested in organic photovoltaic devices according to a standard bulk heterojunction structure in binary and ternary mixtures. In a blend with PC71BM or IC61BA, these polymers have led to open circuit voltages ranging from 0.65 V to 1.05 V and to power conversion efficiencies of up to 5.14 % on a surface area of 0.28 cm2. The active layer morphologies were studied by AFM. The polymers presented in this work were used in ternary blend solar cells. Some polymers were tested in photocathodes for hydrogen evolution and showed an improvement of the reduction potential compared to that of the photocathodes based on P3HT. Owing to their optoelectronic properties and their photovoltaic properties in standard device configurations, some of the materials developed in this study appear as valuable materials for future developments of organic tandem solar cells.

Quinoxaline, Organic solar cells, Polymers, Materials, High band gap

On-line thesis.