Synthesis and Characterization of Donor-Acceptor Conjugated Polymers for Bulk Heterojunction Solar Cell Applications

• Main Authors: Yuan, Mao-Chuan, 袁茂川
• Other Authors: Wei, Kung-Hwa
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/64074637541032761509

博士 === 國立交通大學 === 材料科學與工程學系 === 99 === The objective of this thesis is to develop new donor–acceptor (D–A) conjugated polymers for bulkheterojunction (BHJ) solar cell applications. First of all, we have used Suzuki coupling to prepare a series of donor–bridge–accptor copolymers, comprising the conjugated cyclopentadithiophene-carbazole main chains and the pendent perylene diimide (PDI) side chain. We observed quenching in the photoluminescence spectra of the copolymers after incorporating the electron-deficient PDI moieties on the polymer side chains, indicating the occurrence of efficient photoinduced electron transfer. The photovoltaic devices based on the PDI-containing polymers revealed significantly enhanced photocurrents as a result of the presence of PDI moieties. Second, we have used Stille polymerization to prepare a series of low-bandgap copolymers, P1–P4, by conjugating the electron-withdrawing pyrido[3,4-b]pyrazine (PP) moieties with the electron-rich benzo[1,2-b:3,4-b´]dithiophene (BDT) or cyclopentadithiophene (CPDT) units. The two donors possessing different electron-donating abilities were used to tune the optical bandgaps and energy levels of the resultant polymers. The three-component copolymers P3 and P4 containing the thiophene and bithiophene segments, respectively, can provide not only broad spectral absorption ranges from 350 to 800 nm but also homogeneous morphologies when blended with [6,6]-phenyl-C70-butyric acid methyl ester (PC70BM). An optimal photovoltaic device incorporating the P4:PC70BM blend at a weight ratio of 1:4 displayed a short-circuit current (Jsc) of 10.85 mA cm–2 and a power conversion efficiency (PCE) of 3.15%. Subsequently, we prepared a new donor–acceptor polymer PBTTPD through conjugating the electron-rich bithiophene donor with the electron-deficient thieno[3,4-c]pyrrole-4,6-dione (TPD) acceptor. PBTTPD exhibits high crystallinity and a low-lying highest occupied molecular orbital (HOMO). An optimal photovoltaic device incorporating a PBTTPD/[6,6]-phenyl-C61-butyric acid methyl ester blend at a weight ratio of 1:1.5 displayed an large open circuit voltage (Voc) of 0.95 V and an excellent PCE of 4.7%. Finally, we synthesized two new low-bandgap polymers PCPDTTPD and PDTSTPD through combining the electron-deficient TPD moieties with the coplanar, electron-rich cyclopenta[2,1-b:3,4-b´]dithiophene (CPDT) or dithieno[3,2-b:2´,3´-d]silole (DTS) units, respectively, to reach narrow optical bandgaps. PCPDTTPD and PDTSTPD differ merely in terms of their bridging atom—carbon and silicon on their corresponding CPDT and DTS donors. Both polymers exhibited excellent thermal stabilities, crystalline characteristics, broad spectral absorptions, and low-lying HOMO energy levels. Notably, the presence of silicon atoms along the chain of PDTSTPD resulted in that having a lower-lying HOMO energy level and a higher hole mobility relative to those of PCPDTTPD. A device incorporating the PCPDTTPD:PC71BM blend (1:4, w/w) provided a power conversion efficiency of 2.15%, whereas that for a device incorporating the PDTSTPD:PC71BM blend (1:1, w/w) was higher at 3.42%.