Design And Synthesis Of Conjugated Polymers For Photovoltaic Materials And Fabrication And Optimization Of Photovoltaic Devices

In the development process of polymer solar cells (PSCs), design and synthesis of conjugated polymer photovoltaic materials as well as fabrication and optimization of photovoltaic devices is always a hot research topic in the field of organic photovoltaics. By rational design of the molecular structures of polymers, the physical and chemical properties of polymers can be well tuned. By optimizing the technology of device fabrication, the morphology of photactive layers can be effectively controlled to improve the photovoltaic performance of the devices. In this dissertation, the design and synthesis of novel high-performance conjugated polymer photovoltaic materials and the optimization of the technology of photovoltaic device fabrication are included.1. As a complementary light-harvesting unit in short wavelength region, quinoxalinoporphyrin (QP) was first introduced into the donor-acceptor conjugated polymers. A series of conjugated polymers based on QP, bithiophene (BT) and diketopyrrolopyrrole (DPP) were designed and synthesized by Stille-coupling polymerization. The incorporation of QP remarkably broadened and enhanced the absorptions of short wavelength photons as well as kept the well-tuned electronic energy levels and bandgap of the pristine polymer, which circumvented the trade-off between open-circuit voltage (Voc) and short-circuit current density (Jsc) that often occurs in traditional donor-acceptor polymers. The photovoltaic device based on P(QP1-BT-DPP4) showed a highest power conversion efficiency (PCE) of 5.07% and an increased Jsc of 11.85 mA/cm2 without sacrificing Voc or fill factor. The PCE of 5.07%was 3 times higher than that of the photovoltaic device fabricated from the pristine donor-acceptor polymer.2. Donor-acceptor conjugated polymers based on terthiophene as donor unit and QP as acceptor unit were designed and synthesized by Stille-coupling polymerization. Bulk heterojunction (BHJ) photovoltaic devices were fabricated and the effects of optimization approaches of adjusting the weight ratio of donor/acceptor, using additive, thermal annealing, and solvent treatment on the photovoltaic performance were systematically investigated. Especially, a ligand additive,4,4’-bipyridine (Bipy), was used for the first time to effectively control the morphology of the photoactive layer of the PSCs fabricated from QP-based polymers. The results turned out that after adding 1%Bipy to the blend solution of P(QP-TT-Zn):PC71BM, the blend film was induced to form some ordered packing and the morphology was largely improved mainly because of the coordination effect between the nitrogen of Bipy and the central zinc of P(QP-TT-Zn). The PCE of the device increased from 0.85%without Bipy processing to 2.17%with 1%Bipy processing. Following optimization methods including thermal annealing and solvent treatment further optimized the morphology and resulted in an increased PCE of 3.51%, which was the highest value among PSCs based on porphyrin-based conjugated polymers.3. An approach using multifunctional end caps to modify the donor-acceptor conjugated polymers was raised. Donor-acceptor conjugated polymers based on BT and DPP end-capped by thiophene, QP and quinoxalinoporphyrinatozinc (QPZn) unit were designed and synthesized by a one-step strategy relying on Stille-coupling polymerization. The intruduction of end caps effectively eliminated undesirable structural defects caused by residual unreacted functionalities that deteriorate the overall device performance. Especially, the end caps of QP and QPZn, which showed extremely strong absorption in the range of ca.400-500 nm, not only eliminated structural defects of the polymers but also remarkably enhanced the light absorption of polymers in short wavelength region, which facilitated to increase Jsc of the devices. With the comprehensive benefits of optimization strategies such as using solvent additive DIO, ligand additive Bipy, and thermal annealing, the BHJ photovoltaic device based on P(BT-DPP)-QPZn showed significantly increased PCE from 2.92%to 4.45%. It was demonstrated that the coordination effect between ligand additive and metalloporphyrin optimized and fixed the morphology of the photoactive layer so as to improve the stability of the photovoltaic performance. After thermal aging for a certain time, the retained PCE of the photovoltaic device based on P(BT-DPP)-QPZn increased from 61%for device processed without Bipy to 76%for device processed with Bipy.4. A series of conjugated polymers based on diketopyrrolopyrrole-fumaronitrile and isoindigo-fumaronitrile were designed and synthesized by Suzuki-coupling polymerization for application in front cells of tandem solar cells. These polymers were developed to break the restriction of P3HT that showed too high HOMO energy level and to explore novel structures for materials applied in front cells of tandem solar cells. All the resulted polymers showed deep HOMO energy levels. Especially, il-based polymers, P(iI-C24-DTV) and P(iI-C24-DPV), with longer alkyl chains showed deep HOMO energy levels of-5.92 eV, which were even 1.16 eV deeper than that of P3HT, implying that these polymers have promising application in tandem solar cells.