Narrow Bandgap Polymers Constructed From Donating Fused Ring Sturctures And Their Photovoltaic Characterization

Organic polymers semi-conductors as new materials have attracted greatly interests. Due to outstanding their properties, many new materials have been applied in optoelectronic devices, such as Organic Field-Effect Transistors (OTETs) and polymer solar cells (PSCs). Compared to inorganic materials, the structure of polymers semi-conductors can be adjusted by chemical methods and has further effect on optical properties and electrochemical properties of polymers. The work of this dissertation is mainly focused on design and synthesis of a series of fused-ring donor and "pull-push" low bandgap polymers. Furthermore, these materials will be applied in OTFTs or PSCs to study the relationship between structure and photovoltaic properties.In chapter2, Two-dimensional conjugated polymers have been prepared by grafting pendent electron-accepting side chains onto poly-[fluorene-alt-bithiophene] backbone. Key reactions include Suzuki coupling, Vilsmeier-Haack formylation, and Knoevenagel condensation. The absorption spectra and band gaps of the resulting2-D polymers were adjusted by attaching accepting side chains while maintaining the high-lying HOMO levels of the backbone. The charge-transport property of OPVs resulting from the blending of the as-prepared polymers with PCBM was greatly improved by using MoO3as the interfacial layer. The ITO/MoO3/polymer:PCBM(1:3)/Al devices demonstrated a PCE of3.13%and1.72%for PF-BTDTA and PF-BTDCN, respectively. Both of them obtained nearly1Voc-The device based on PF-BTDTA obtained the highest Jsc of6.41mA/cm2. Nanoscale phase separation was also found for the active layer. High-efficiency OPVs may be achieved by rational adjustment of the electron-accepting strength of the side chain and the hole-mobility of the conjugated backbone when developing2-D narrow bangap polymers.In chapter3, we have presented here a versatile method for the4,8-functionalization of benzo[1,2-b:4,5-b’]dithiophene via palladium-catalyzed Suzuki and Sonogashira couplings as well as carbon-sulfur bond formation reaction. Using this methodology, a library of4,8-functionalized benzo[1,2-b:4,5-b’]dithiophene oligomers have been developed with aryl, ethynyl, arylethynyl, alkylthio and arylthio substituents. The absorption and energy levels of BDT oligomers can be fine-tuned by adjusting the donating strength of the incorporated substituents. The UV-vis absorption spectra of the derivatives showed notably red-shifted with the elongation of π-conjugation in the solution and film. The HOMO energy levels decreased as the electron-deficient abilities of aryl substituents increased. The as-prepared BDT oligomers exhibit low HOMO energy levels from-5.38to-5.58eV, which were nearly the ideal HOMO energy level (-5.40eV). The oligomers as p-type organic materials in FETs obtained the highest mobility of6×10-2cm2V-1s-1.In chapter4, forth new polymers based on TIPS-substituted benzo[1,2-b:4,5-b’]dithiophene (TIPSBDT) donor unit and TTz, BO, TPD or DTQX acceptor have been synthesized. The solubility, optical, and electrochemical properties of resulted polymers were studied in detail. The ITO/PEDOT:PSS/Pl:PC71BM(1:1, wt%)/PFN/Al devices demonstrated a PCE of1%. The BHJ solar cell devices based on these polymers are further studied.In chapter5, a new electron-donating unit,4,4’-biphenyl-dithieno[3,2-b:2’,3’-d]silole (DTS-Ph), was developed and used for the design of D-A polymer for PSCs application for the first time. By alternating DTS-Ph or4,4’-octyl-dithieno[3,2-b:2’,3’-d]silole with thiazolo[5,4-d]thiazole (TTz), we obtained two low bandgap polymers(P1and P2). The solubility, optical, electrochemical and photovoltaic properties of these polymers was studied. Compared to P2, the decomposition temperature of P1improved35℃; The UV-vis absorption spectra P1in solution exhibits a red-shift of40nm; The HOMO energy level of P1decreased about0.14eV. The BHJ solar cell devices based on these polymers are still studied.In chapter6, we have developed three novel low-bandgap polymers PDTT-PDPP, PDTT-TTz, and PDTT-TPD by alternating DTT with PDPP unit, TTz and TPD units through a Suzuki-Miyaura coupling reaction and non-poisonous direct arylation polycondensation. The polymers exhibited strong absorption in the range of300-650nm and reasonable bandgap of1.75-1.87eV. The preliminary investigation on BHJ solar cells showed a highest PCE of0.73%from PDTT-TPD:PC61BM (1:3) blends. The accepting TPD unit demonstrated as a good candidate in constructing narrow-bandgap "pull-push" alternating polymers. This study may provide guidance for screening suitable accepting units for the development of DTT-based polymers for high-efficiency solar cells.