The Design, Synthesis And Optoelectronic Properties Of D-A Polymers Containing Dithiophene-based Fused-rings

One of the main problems of the world facing today is the energy crisis. With thedevelopment of industry and agriculture, the world has consumed a great amount of energy.Among the numerous new energy technologies, solar cells occupy the irreplaceable position.Organic solar cells underline the ingenuity of those engaged in finding new sources of energy.Due to a low cost and roll-to-roll printing process, polymer solar cells have attracted muchattention. From the view of material scientists, the development of polymer donors withsuitable energy level, a broad absorption spectrum to solar light, and high carrier mobilityrepresents the “Holy Grail”.First, we summarized star polymer donors whose power conversion efficience were above5%, and more than150polymers could be included. Based on these analysis, the relationshipbetween structure and properties were discussed.Aromatic heterocyles based on thiophene are very important to construct many polymerdonors. In chapter3, two polymers based on anthradithiophene (ADT) were designed andsyntheszied. We introduced the alkyl chains into the ADT unit to improve the solubility.Polymers PADT-DPP and PADT-FDPP exhibited broad absorption bands and their opticalband gaps are1.44and1.50eV, respectively. In polymer solar cells, PADT-DPP andPADT-FDPP showed power conversion efficiency (PCE) of3.44%and0.29%, respectively.Atomic force microscopy revealed that the poor efficiency of PADT-FDPP should be relatedto the large two-phase separation in its active layer. If1,8-diiodooctane(DIO)was used as thesolvent additive, the PCE of PADT-DPP remained almost unchanged due to very limitedmorphology variation. However, the addition of DIO could remarkably elevate the PCE ofPADT-FDPP to2.62%because of the greatly improved morphology.In chapter4, dithieno[3’,2’:3,4;2’’,3’’:5,6]benzo[1,2-c][1,2,5]thiadiazole (DTBT), a largecoplanar acceptor, was successfully synthesized. Combined with four selected donors, fourmedium-bandgap polymers were obtained. Cyclic voltammetry revealed the polymers hadsimilar HOMO levels. Polymer DTBT-Th3in a solution showed very strong interchainaggregation at room temperature or elevated temperature, which could be benefit foroptoelectronic devices. Using cholrobenzene as the processing solvent, the best efficiency of6.81%was achieved for the DTBT-Th3-based polymer solar cells. The organic field-effecttransistors of DTBT-Th4gave the highest hole mobility of0.74cm2/Vs. Relatively, polymersDTBT-Th4, DTBT-TT, and DTBT-Th5did not show ideal photovoltaic performances. Theirmorphologies in blend films as measured by atomic force microscopy helped us to understand the existing difference.Isomerism can result in great influenece on the opotoelectronic property, thus an isormer ofDTBT, named as iso-DTBT, was sysnthesized for comparison study. We selected terthiopheneand quarterthiophene as the electron-donating units, from which two alternating copolymersiso-DTBT-Th3and iso-DBBT-Th4were prepared. To our surprise, polymers iso-DTBT-Th3and iso-DBBT-Th4showed totally different behaviors in absorption and energy level. Inpolymer solar cells, only poor photovoltaic performances could be exhibited. In order tounderstand these differences, we selected Density Functional Theory and atomic forcemicroscopy for further studies.In addition, another large coplanar heterocycle, dithieno[3,2-a:2’,3’-c]phenazine, wassynthesized. In combination with terthiophene and quarterthiophene, two alternatingcopolymers DTQ-Th3and DTQ-Th4were prepared. However, the two polymers showed weakintrachain charge transfer. In polymer solar cells, the two polymers displayed efficiencies of1.87%and0.60%, respectively.