Yann Kervella

Conjugated alternating copolymer of dialkylquaterthiophene and fluorenone: synthesis, characterisation and photovoltaic properties

New π-conjugated alternating copolymers containing thienylene and fluorenone units, namely poly[(5,5‴-dioctyl-[2,2′;5′,2″;5″,2‴]quaterthiophene)-alt-(2,7-fluoren-9-one)] (PQTF8), as well as its analogue without fluorenone groups (PQT8), have been synthesized. Absorption studies carried out both in solution and in thin films indicate that the presence of fluorenone chromophores in PQTF8 leads to a significant extension of the absorption spectrum in the visible range as compared to PQT8. The redox properties of both polymers, in particular their LUMO and HOMO levels, have been characterized by cyclic voltammetry and have been found suitable for potential use of these systems in organic solar cells. Finally, these materials have been tested as donor components in bulk-heterojunction-type photovoltaic cells using PCBM as an electron acceptor. We demonstrate a strong effect of the polymers molecular weight on crucial cell parameters, such as the short-circuit current density Jsc, and therefore on the overall cell efficiency. This effect is particularly pronounced for PQTF8-based cells leading to power conversion efficiencies up to 1.5% for the highest molecular weights.

Side chain engineering of organic sensitizers for dye-sensitized solar cells: a strategy to improve performances and stability

Four organic dyes specifically designed for application in dye-sensitized solar cells are reported. These dyes are based on a perfectly identical π-conjugated backbone and only differ by the number and the nature of the alkyl chain substituents employed as solubilizing groups. These sensitizers have outstanding light absorption properties in the visible range up to 750 nm on TiO2 and they exhibit quite similar energy level positions. Solar cells fabricated and characterized under exactly the same conditions with iodine-based liquid electrolytes show performances ranging from 6.53% to 9.05%. We highlight that the nature and the number of solubilizing groups have a tremendous impact on the performances of solar cells. The differences in the performance of the four sensitizers can be correlated with the current generation, the formation of aggregates and the dye-loading on the electrodes. We also report solar cells with ionic liquid electrolytes that demonstrate power conversion efficiencies up to 7.81% and a good stability of the performances under accelerated ageing conditions for 7300 hours. We establish that the number and the type of solubilizing groups attached on the π-conjugated backbone of the dyes have a strong influence not only on the performances but also on the lifetimes of the devices; i.e. the dyes that contain lower ratios of alkyl groups lead to the more stable cells.

S-Shaped Conformation of the Quaterthiophene Molecular Backbone in Two-Dimensional Bisterpyridine-Derivative Self-Assembled Nanoarchitecture

The conformation and the two-dimensional self-assembly of 4-(3,4″-dihexyloxy-5,2:5,2″:5″,2‴-quaterthien-2,5‴-diyl)-bis(2,2:6,2″-terpyridine) molecules are theoretically and experimentally investigated. This molecular building block forms a hydrogen-bonded chiral supramolecular nanoarchitecture on graphite at the solid/liquid interface. Scanning tunneling microscopy (STM) shows that the molecule adopts an S-shaped conformation in this structure. DFTB+ calculations reveal that this conformation is not the lowest-energy conformation. The molecular nanoarchitecture appears to be stabilized by hydrogen bonding as well as van der Waals interactions. I-, L-, and D-shaped molecular conformations are, however, locally observed at the domain boundary, but these conformations do not self-assemble into organized 2D structures.

Visible and near-infrared organic photosensitizers comprising isoindigo derivatives as chromophores: synthesis, optoelectronic properties and factors limiting their efficiency in dye solar cells

The development of ruthenium-free organic photosensitizers showing panchromatic absorption up to the near-infrared (NIR) region for application in dye-sensitized solar cells (DSSCs) is still scarce. Among the sensitizers with absorption beyond 700 nm and developed for DSSCs, only zinc-phthalocyanine and boron-dibenzopyrromethene-based dyes have been able to reach efficiencies as high as 6%. Here we report metal-free organic dyes based on isoindigo, thieno-isoindigo or benzo-thieno-isoindigo chromophores that absorb in the UV-visible and NIR spectral range up to 900 nm. These molecules, that exhibit purple, blue, or green hues, were used to sensitize TiO2 mesoporous electrodes in order to fabricate DSSCs with an iodide/triiodide-based electrolyte. Advanced photophysical characterizations, including charge extraction, transient photovoltage, and laser transient absorption spectroscopy experiments, combined with density functional theory modeling and computational investigations allow us to fully unravel the interfacial processes at the origin of the solar cell performances and to identify the limiting factors. A power conversion efficiency as high as 7% associated with a Jsc close to 19 mA cm−2 was obtained with one of the dyes, which is comparable to those of the best panchromatic organic dyes reported so far. We also demonstrate in this work that the Voc of the solar cells is linearly correlated to the dipolar moments of the oxidized dyes, the molecules possessing larger dipoles leading to the highest Voc values.