Synthesis And Optoelectronic Properties Of Conjugated Polymers Based On Fluorinated Benzoheterocyclics

Conjugated polymers have attracted tremendous academic and industrial interests in recent years because of their particular advantages in terms of light-weight, mechanical flexibility as well as low-cost fabrication of large-area devices. In the past two decades, conjugated polymers have been widely used for fabrication of electronic devices including organic photovoltaics(OPVs), organic light-emitting diodes(OLED), organic field-effect transistors(OFETs) and so forth. Despite OPVs have achieved the milestone with power conversion efficiency(PCE) of more than 11%, many research efforts still focus on the development of narrow-bandgap conjugated polymers, while media- or wide-bandgap polymers are less concerned. Additionally, recent development in materials design demonstrated that regior-chemistry is of vital importance for higher absorption, better packing as well as excellent device performance, since the regioregular conjugated polymers follow a strict orientation of alternating repeating units.Conjugated polymers with alternating donor(D) and acceptor(A) units along the backbone are therefore developed D-A copolymers for high performance devices. Among the available acceptors moieties, 5,6-difluorobenzo[c][1,2,5]thiadiazole(FBT), a famous derivative of benzo[c][1,2,5]thiadiazole unit, has attracted much attention due to the ―fluorine(F) effect‖, including better intermolecular stacking, lower molecular orbitals, and higher charge mobilities. Two derivatives of ff BT, 5-fluorobenzo[c][1,2,5]thiadiazole(f BT) and 5,6-difluorobenzo[d][1,2,3]triazole(FBTz), were developed. The sp3-hybrid nitrogen atom in the FBTz unit can allow for the incorporation of solubilizing alkyl side chain without sacrificing the planarity of the backbone compared with FBT. As for f BT unit, it should be noted that the bromo-substituent in the meta-position relative to the fluorine atom is more reactive than the ortho-Br in palladium-catalyst coupling reaction, which provide the probability to synthesis high performance regioregular conjugated polymers.In chapter 2, two novel conjugated polymer donor materials PICz-TFBTz, PICz-SFBTz based on indolo[3,2-b]carbazole and FBTz were synthesized by Suzuki polymerization. Polymer solar cells based on the copolymers were fabricated with the configuration of ITO/PEDOT: PSS/Polymer: PC61BM/PFN/Al. The highest power conversion efficiency(PCE) with polymer: PC61BM(1:2, wt:wt) as photoactive layer, showed PCE of 1.01% and 2.39%, respectively. OFETs with the architecture of Au/polymer/CYTOP/Al were fabricated to evaluate the charge mobility of the two polymers. The hole mobility of PICz-SFBTZ and PICz-TFBTZ were calculated to be 0.01 cm2 V-1 s-1 and 0.0014 cm2 V-1 s-1, respectively. These results demonstrated that the selection of appropriate π-bridge is crucial for the enhancement of the device performance of π-conjugated semiconductors.In chapter 3, wide-bandgap donor materials PDTBDT-FBT, PDTBDT-FBTz based on benzo[1,2-b:4,5-b’]diithieno[3,2-b]thiophene and FBT or FBTz were synthesized by Stille polymerization. Polymer solar cells based on the copolymers were fabricated with the configuration of ITO/PEDOT:PSS/Polymer:PC71BM/Ca/Al. The PCEs of the OPVs based on PDTBDT-FBTz and PDTBDT-FBT were 5.55%(VOC = 0.95 V, JSC = 9.32 m A cm-2, FF = 62.7%) and PCE = 7.45%(VOC = 0.98 V, JSC = 12.76 m A cm-2, FF = 59.6%). These results demonstrated that the constructed wide-bandgap π-conjugated polymers can be promising candidates for the fabrication of high-performance solar cells with multi-junction architectures.In chapter 4, the effect of fluorine substitutions on the molecular geometry and optoelectronic properties were investigated based on the conjugated molecular framework consisting of mono-fluorinated benzo[c][1,2,5]thiadiazole unit. The two-dimensional nuclear magnetic resonance spectroscopy of model compounds indicated that the fluorine-substitution can significantly change the chemical environment of the adjacent hydrogen-atoms, which was in consistence with the changed molecular geometry as indicated by single crystal diffractions and theoretical calculations. Based on an AB-type of monomer, the regioregular conjugated polymer with specifically oriented fluorine along the vector of the polymer backbone was constructed. The UV-vis absorption, photoluminescent and electroluminescent spectra demonstrated that the slightly hypsochromic shift of the copolymer comprising of the fluorine substitution relative to the non-fluorinated counterpart. Of particular interest is that the copolymer based on fluorinated benzothiadiazole unit exhibited electron-dominant charge transportation, for which the electron mobility is about one order of magnitude higher than that of hole mobility.