Synthesis, characterization and optoelectronic devices of new conjugated polymers : from 1D-linear polymers to 2D-conjugated networks
Yemene, Amsalu Efrem
Date of Issue2017-02-28
School of Chemical and Biomedical Engineering
Conjugated polymer semiconductors have gained a tremendous development in terms of their design and synthesis diversity for applications in different optoelectronic devices such as organic field-effect transistors (OFETs), organic photovoltaics (OPVs), and organic light emitting diodes (OLEDs). Recent studies demonstrated that the design strategy of combining an electron donor (D) and acceptor (A) moieties into the polymer backbone results in high-performance organic semiconductors, as it gives a means to tune physicoelectrochemical properties of the resulting conjugated polymer. D-A conjugated polymers are mainly synthesized via traditional coupling techniques (such as Stille and Suzuki couplings) and also by using a highly anticipated greener and economical method, direct C-H arylation polymerization (DAP). In this research work, new electron acceptor moieties, 2,1,3-dithienobenzo-chalcogenodiazole (2,1,3-dithienobenzo-thiadiazole and 2,1,3-dithienobenzo-oxadiazole) and -lactone-Pechmann dye were synthesized and incorporated into the construction of D-A conjugated polymers by using traditional coupling methods (such as Stille and Suzuki couplings). Detailed systematic investigations were carried out to study the effects of chalcogen atoms and different electron donor moieties on the optical and electrochemical properties, as well as on OPV and OFET performance of their corresponding D-A conjugated polymers. Thermogravimetric analysis results show the good thermal stability of all polymers with decomposition temperature (Td) at 5% weight loss above 300 oC. The UV-vis-NIR spectra of -lactone-Pechmann dye based D-A polymers shows a broad absorption band extended to 1000 nm and narrow band gaps with deep LUMO energy levels. D-A polymers containing 2,1,3-benzooxadiazole moiety shows deeper LUMO level as compared to their 2,1,3-benzothiadiazole-containing analogs, whereas polymers containing weak electron donor moiety (alkylidenefluorene) exhibited deeper HOMO levels than polymers with stronger electron donors. Photovoltaic power conversion efficiency of over 2%, hole field-effect mobility of 2.6 x 10 -2 cm2V-1s-1 and on/off ratio of over 105 were obtained. Another D−A conjugated polymers based on tert-butoxycarbonyl (t-Boc) substituted isoindigo as electron acceptor moiety and dithiophene-benzodithiophene derivative as electron donor moieties designed and synthesized via Stille polymerization method. The elimination of bulky t-Boc side groups resulted in the emergence of N−H···O=C hydrogen bonding interactions. It results in an increased field-effect mobility of the polymer after chemical or thermal treatment, which may arise from the enhanced planarity and intermolecular ordering of the isoindigo units. Direct C-H arylation polymerization (DAP) as a facile, cheap, and atom economy synthesis protocol was applied for the synthesis of an alternating 5,6-difluoro-2,1,3-benzothiadiazole based low bandgap D-A conjugated polymers. The DAP reaction conditions were optimized, and a polymer with number-average molecular weight (Mn) of 27 kDa was obtained under the optimal catalytic condition of Pd(OAc)2/(o-MeOPh)3P/PivOH/K2CO3 in o-xylene. UV-vis absorption spectra of the polymers indicated strong interchain aggregation in films. While the C-H selectivity and the alternating polymer structure of the polymers synthesized via DAP are comparable to those of the same type polymers synthesized via Stille coupling. The batch of D-A polymer synthesized via optimal DAP, despite its lower Mn, showed higher hole mobility of 7.5 × 10-3 cm2V1s1 in OFETs and larger power conversion efficiency of 4% in OPVs. The versatility of DAP for synthesizing other types of conjugated polymer networks was investigated by applying it to the synthesis of conjugated microporous polymers (CMPs) based on a single monomer unit, 8,11-dibromodithieno [3,2-a:2',3'-c] phenazine. The resulting polymers exhibit hierarchical porous structures and a narrow bandgap of 1.5 eV. The findings in this research work could help to the future developments of conjugated polymers, in their design, synthesis, and application for organic electronics devices (OFETs and OPVs).