Synthesis of push-pull compounds for the sensitization of p-type semi-conducting oxides

Romain Brisse 1
1 LICSEN - Laboratoire Innovation en Chimie des Surfaces et NanoSciences
NIMBE UMR 3685 - Nanosciences et Innovation pour les Matériaux, la Biomédecine et l'Energie (ex SIS2M)
Abstract : Tandem Dye Sensitized Solar Cells (T-DSSCs) represent a promising, low cost, way for light conversion to electricity. Actually the theoretical yield of such device overstep the one of classical, one photo-electrode, DSSCs. However, due to a still irreplaceable, but not optimal, photocathode material: NiO, the yields of tandem DSSCs are still very limited (around 2%). This is due to the rapid geminate charge recombination that happens with NiO. In order to fight against charge recombination, the colorant employed is of paramount importance. For this, in this thesis, four new dyes, « push-pull » type, are presented. They have been synthesized and implemented into p-type DSSCs. These colorants are all triphenyl-amine (TPA) bithiophen based, for the electron rich part, and bear four different acceptors of different strenght. Two carboxylic acids permit dye anchoring at the NiO surface. An efficient synthetic method of TPA, in one step and through a Buchwald-Hartwig amination reaction, has first been developed. Then a microwave assisted Suzuki cross-coupling permitted to graft all the different acceptors to the donor TPA part. The dyes’ physical properties (electrochemistry, UV/Visible) were assessed and precisely correlated to theoretical DFT/TDDFT results. The colorants possess very high molar extinction coefficients, and adapted redox levels for integration into NiO based device. Besides, they possess a wide and intense charge transfer band, which maximum is gradually shifted toward high wavelengths when the strength of the acceptor is increased. This allows the photocathode to absorb light from the near UV to the red part of the solar spectrum. Interestingly, one of the colorant absorbs between 450 and 700 nm and then represents a good candidate for a synergistic integration into tandem DSSC. In this work, a new way to synthesize NiO is also presented. It consists of ink-jet printing a sol-gel ink, containing NiCl2 salt, as the NiO precursors and a triblock copolymer F127. This family of ink has already been employed for NiO synthesis and has given the actual highest photo-currents. However, until now, doctor-blade is mandatory for the ink deposition. This doctor blade step is source of important irreproducibility. Indeed, ink-jet printing permits to solve this issue. Firstly, the rheological parameters of the ink were adapted for ink-jet printing. Then, after optimization of the printing parameters, NiO films were realized and analysed (MEB, XPS). The obtained films possess a quasi-uniform thickness, with no cracking and are constituted of 15 to 20 nm densely packed nanoparticles. Interlayer drying conditions’ control permitted to form two types of NiO films, thicker than one micrometer. Sintering the printed film at high temperatures between each layer permitted to obtain compact films of NiO nanoparticles, more adapted to p-type DSSCs. After optimization of the sensitization conditions of NiO, the photocathodes were assembled into devices and tested. Their performance challenge the one of the most efficient photocathodes found in the literature, in a very reproducible way for the first time. Then ink-jet printing is an adapted way for NiO synthesis for p-type DSSCs. This new method allows a good level of control of the film formation conditions and then may a deeper study of the synthesis parameters is possible. This offers interesting prospects for future more performing tandem DSSCs.
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Romain Brisse. Synthesis of push-pull compounds for the sensitization of p-type semi-conducting oxides. Material chemistry. Ecole Polytechnique (EDX), 2015. English. ⟨tel-01870923⟩

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