Christine Dagron-Lartigau

Enhanced thermal stability of organic solar cells by using photolinkable end-capped polythiophenes

The use of poly(3-hexylthiophene) (P3HT) end-capped with anthracene (P3HT-A) in a blend with [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is demonstrated to physically stabilize bulk-heterojunction photovoltaic solar cells. Bulk heterojunction-based devices are known to undergo phase separation of donor and acceptor materials during operation resulting in the formation of large (μm scale) PCBM crystals that dramatically decrease the photovoltaic characteristics of the cell. By way of a facile UV-curing step, the P3HT-A chain most likely reacts with PCBM fullerene via a [2+2] cyclo-addition to stabilize the blend. Photovoltaic devices based on P3HT-A and PCBM have been optimised in terms of thermal annealing to obtain initial devices to determine the UV-curing protocols. UV-exposure was found to improve device stability while simultaneously having a minimal effect on device efficiency. Optical microscopy demonstrates that the few reactions of P3HT-A with PCBM are efficient enough to prevent the formation of micro-sized PCBM crystals responsible for the failure of solar cells.

Facile synthesis of poly(3-hexylthiophene)- block -poly(ethylene oxide) copolymers via Steglich esterification

Poly(ethylene oxide) has been coupled to poly(3-hexylthiophene) using esterification to produce pure diblock copolymers, highly relevant for use in organic electronic devices. The new synthetic route described herein uses a metal-free coupling step, for the first time, to afford well-defined polymers in high yields following facile purification.

Design, synthesis and thermal behaviour of a series of well-defined clickable and triggerable sulfonate polymers

In the printing industry, the exploitation of triggerable materials that can have their surface properties altered on application of a post-deposition external stimulus has been crucial for the production of robust layers and patterns. To this end, herein, a series of clickable poly(R-alkyl p-styrene sulfonate) homopolymers, with systematically varied thermally-labile protecting groups, has been synthesised via reversible addition-fragmentation chain transfer (RAFT) polymerisation. The polymer range has been designed to offer varied post-deposition thermal treatment to switch them from hydrophobic to hydrophilic. Suitable RAFT conditions have been identified to produce well-defined homopolymers (Đ, Mw/Mn 80% for all but one monomer) with controllable molar mass. Poly(p-styrene sulfonate) with an isobutyl protecting group has been shown to be the most readily thermolysed polymer that remains stable at room temperature, and was thus investigated further by incorporation into a diblock copolymer, P3HT-b-PiBSS, by click chemistry. The strategy for preparation of thermal modifiable block copolymers exploiting R-protected p-styrene sulfonates and azide-alkyne click chemistry presented herein allows the design of new, roll-to-roll processable materials for potential application in the printing industry, particularly organic electronics.

Surface-initiated polymerization of A–A/B–B type conjugated monomers by palladium-catalyzed Stille polycondensation: towards low band gap polymer brushes

A surface-initiated polycondensation which enables access to well-defined core–shell nanoparticles is described. Stille polymerization was initiated from the surface of palladium catalyst-immobilized zinc oxide nanorods. For the first time, a low band gap polymer, poly[(4,4′-bis(2-ethylhexyl)dithieno-[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PSBTBT), was anchored on zinc oxide nanorods to create hybrid materials with tunable photophysical properties.

Graphene-based acceptor molecules for organic photovoltaic cells: a predictive study identifying high modularity and morphological stability

Ideal donor:acceptor pairs are required to improve organic photovoltaic lifetimes and efficiencies. Here an idealized donor:acceptor for organic solar cells is predicted. Circular and triangular graphene-based hexabenzocoronenes are shown, in conjunction with the energetically well-placed poly(3-oxypentylthiophene), to provide requisite HOMO/LUMO levels for efficient charge transfer through columnar structures and hole/electron extraction. Circular hexabenzocoronenes are furthermore adaptable to energy-level modulation by targeted substitutions.

Synthesis and photovoltaic properties of a new donor-acceptor conjugated polymer based on fluorinated benzothiadiazole units

A novel fluorinated copolymer (PDTSffBT) based on dithienosilole donor unit and fluorinated benzothiadiazole acceptor unit has been synthesized for polymer solar cells. This copolymer exhibits a deeper HOMO energy level compared to that of hydrogenated benzothiadiazole-based copolymer (PDTSBT), resulting in high open-circuit voltages well exceeding 0.75 V. The introduction of the two electron-withdrawing fluorine atoms on the benzothiadiazole unit also leads to an increase in the molar extinction coefficient as well as the interchain interaction in the PDTSffBT copolymer with respect to its analogue PDTSBT copolymer. Power conversion efficiency (PCE) of the device using PDTSffBT:PC71BM as active layer (2.93 %) is much higher than of a PDTSBT:PC71BM (1.64 %) counterpart under identical conditions.