Rony Bechara

A deep-purple-grey thiophene–benzothiadiazole–thiophene BODIPY dye for solution-processed solar cells

In this work we explore the synthesis of an extended 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene dye (BODIPY) engineered from thiophene–benzothiadiazole–thiophene modules linked in the 3,5-substitution positions. We found that this highly soluble dye absorbs up to 800 nm in solution and up to 900 nm in thin films. An effective charge transfer absorption band was found at around 479 nm. The hybrid dye emits at 778 nm with a quantum yield of about 6%. Similar electrochemical and optical gaps were determined about 1.36 eV. When deposited in thin films the dye exhibits an ambipolar nature with well-balanced hole and electron mobilities. Bulk heterojunction solar cells based upon this dye blended with [6,6]phenylC61or71butyricacid methylester (PC61BM or PC71BM) provide a power conversion efficiency of about 1.26% upon a mild thermal annealing.

LUMO's modulation by electron withdrawing unit modification in amorphous TAT dumbbell-shaped molecules

The synthesis and characterization of a series of dumbbell-shaped molecules constructed from paraffin-free central dyes linked to two triazatruxene (TAT) units are described. The photovoltaic properties of these new glassy electron-donating molecules in a bulk heterojunction with fullerene derivatives have been studied. For this series of molecules, the local-range molecular packing in the amorphous solid state is mainly influenced by the TAT units bearing bulky side-chains, while the optical properties and energy levels are tuned by the choice of the central core. Therefore, TAT acts as a structuration platform in thin films for pure or blended active layers and independently, the optoelectronic properties are driven by the choice of the central dye. Photovoltaic cells based on these materials exhibited power conversion efficiencies of up to 3.5%, among the top values reported so far for soluble amorphous small molecule based organic photovoltaic devices. These TAT-based derivatives are a promising platform to reach high solar cell efficiencies as well as to understand in detail the impact of the chemical structure on the optoelectronic properties.

Impact of the arrangement of functional moieties within small molecular systems for solution processable bulk heterojunction solar cells

A series of molecules based on thienofluorene derivatives as electron donating (D) and benzothiadiazole as electron accepting (A) moieties have been assembled into DAD and ADA architectures in order to investigate the influence of the way of assembling the D and A units along the conjugated backbone, on the photovoltaic performances of bulk heterojunction solar cells. It was found that the major difference in going from ADA to DAD architecture is the change in molecular organization (nematic to crystalline), which increases the charge transport mobility and probably also affects the structural organization in blends with PCBM that ultimately leads to higher photovoltaic performances.