All-Conjugated Block Copolymers for Organic Photovoltaics
Doctor of Philosophy
Organic photovoltaics (OPVs) are a promising source of alternative energy due to cost effectiveness and process simplicity. However, the performance of OPVs must be improved to produce viable devices. This can be achieved by optimizing the optoelectronic properties of constituent materials, tuning the nanostructures of materials within active layer of OPVs and defining a well-defined interface between electron-donor materials and electron-acceptor materials. The above opportunities can potentially be addressed with using all-conjugated block copolymers in that self-assembly of block copolymers can lead to well-defined nanostructures driven by thermodynamics. The focus of this thesis is on the synthesis and development of all-conjugated block copolymers in which one block is an electron-donor polymer and the other is an electron-acceptor polymer. We focus primarily on poly(3-hexylthiophene) (P3HT)-based block copolymers in which the electron-donor P3HT is made from Grignard metathesis polymerization (GRIM) and the other block is synthesized by Suzuki-Miyaura polycondensation reaction for wide variety of electron-acceptor polymers. Subsequently, the nanostructures of polymers were studied on a model series of all-conjugated block copolymer: poly(3-hexylthiophene)—block—poly[2,7-(9′,9′-dioctyl-fluorene) (P3HT–b–PF) under different processing conditions with using differential scanning calorimetry (DSC) and grazing-incidence X-ray scattering (GIXS). This reveals strong process-structure-property relationships of all-conjugated block copolymers. Furthermore, using our two-step synthetic route, we prepared an all-conjugated block copolymer poly(3-hexylthiophene)—block—poly[2,7-(9′,9′-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′,-benzothiadiazole)] (P3HT–b–PFTBT) that exhibits over 3% PCEs as the active layer in a solution processed OPV due to the formation of lamellae of the block copolymers and preferential π-π stacking direction of the P3HT perpendicular to the substrate. In addition to covalently linked block copolymers, we applied a quadruple hydrogen group, 2-ureido-4[1H]-pyrimidinone (UPy), as polymeric end functionalities to reduce macro-phase separation in polymer blends. In the polymer blends OPVs comprised of P3HT and PFTBT, the UPy hydrogen bonding group reduces macro-phase separation in polymer blends and leads to improved power conversion efficiency of OPVs from 0.43% to 0.77% under 155 oC annealing condition. This thesis demonstrates that both the covalently linked and hydrogen bonding linked all-conjugated block copolymers are potential to enhance performance of OPVs. Furthermore, with the advancement in synthetic techniques and better understandings on structure-processing-property relationships of all-conjugated block copolymers, we are able to apply those into more emerging conjugated polymers and engineer molecules for efficient energy generation in OPVs.
All-conjugated block copolymers; organic photovoltaics; polymer synthesis; X-ray scattering