Self-Assembling of Novel Fullerene-Grafted Donor–Acceptor Rod−Coil Block Copolymers

Abstract

Fullerene-grafted rod−coil block copolymers, designed for being used as active polymer layers in donor–acceptor bulk heterojunction photovoltaic devices, have been synthesized. The copolymer synthesis was monitored by 1H nuclear magnetic resonance, size exclusion chromatography and infrared absorption spectroscopy, while the material properties were explored by X-ray diffraction, atomic force microscopy, UV–vis absorption, and photoluminescence spectroscopy. Structural properties of bulk materials and thin films were investigated to study the influence of the coil block molecular weight as well as that of the grafted fullerenes on the molecular self-assembling process. The results show that the rod homopolymer is in the crystalline state at ambient temperature and undergoes a phase transition into a smectic-type liquid-crystalline phase at 55 °C. The rod−coil microphase separation enhances interlamellar ordering but destabilizes the intralamellar order, leading to the appearance of a liquid-crystalline phase for the non-fullerene-grafted copolymer at ambient temperature. Adding the fullerene moieties considerably affects the polymer assembling through the growth of fullerene nanocrystals. The latter hinder the formation of the lamellar phase by pinning the coil segments. The identification of the major driving forces that control the molecular self-assembling process allows us to suggest different alternative strategies that can be used as guidelines for the design of new photovoltaic polymer self-assembling materials.

Block copolymer strategies for solar cell technology

“…82 To rectify one of the problems with PCBM (namely its tendency to undergo excessive crystallization), research has effectively demonstrated the use of cross-linking groups. 83,84 Of similar interest are polymers carrying pendent fullerenes, 85 although they tend to suffer from aggregative effects of C 60 rather than self-assembly, 86 or linear main-chain polymers based on C 60 as a monomer, which seem to have displayed morphologies more appropriate to charge collection and transfer. 87 Further work on other acceptors such as perylene, notably perylene tetracarboxydiimide (PDI), 88,89 the poly(benzimidazobenzophenanthroline) ladder (BBL), 90 and units based on 9,9'-bifluorenylidene, 91 conversely, have shown that fullerene may not be necessary.…”

Block Copolymer Nanostructures for Technology

“…Phase separation of a copolymer consisting of a donor block and an acceptor block may be a straightforward way to generate an ideal OPV morphology. Reported energy conversion efficiencies using this approach have not been high: Hazdiioannou's group has several reports on synthesizing BCPs containing PPV and C 60 , with the latter grafted onto polystyrene [76]. A small photovoltaic effect under monochromatic illumination was demonstrated [77].…”

A New Supramolecular Route for Using Rod‐Coil Block Copolymers in Photovoltaic Applications

“…[14][15][16][17][18][19] Block copolymers composed of an electron-donating and an electronaccepting block are therefore particularly interesting for PV applications and are presently studied worldwide by several research groups. [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] Particularly, rod-coil block copolymers using poly[(2,5-di(2-ethyl)hexyloxy)-1,4-phenylenevinylene] (DEH-PPV) as electron donor and various coil blocks (such as polystyrene or polybutylacrylate) with covalently linked fullerene moieties as electron acceptor have been investigated intensively. [23][24][25][26][27][28] Although these studies have given considerable insight into the physics of copolymer self-assembly, their efficient utilization as the active layer in PV devices has not yet been fully demonstrated.…”

“…Also, in the case of block copolymer/fullerene blends, the copolymer microphase separation has been found to be less affected by the C 60 crystallization than for fullerene-grafted block copolymers. [34,39] In particular, the pristine DEH-PPV-b-P4VP nanostructure was shown to be preserved when blended with 10% C 60 and to be thermally stable up to at least 16 h at 180 8C. [39] Finally the PCBM fullerene derivative was chosen for its high solubility in common solvents.…”

“…This behavior contrasts with that of previously reported fullerene-grafted block copolymer self-assembly. [34] Furthermore, from the pronounced thermal stability of the http://doc.rero.ch copolymer morphology at 150 8C, it may be anticipated that devices made thereof will experience a significantly enhanced lifetime, in comparison to P3HT:PCBM reference devices, when used in typical solar cell operating conditions (temperature $60 8C).…”