Design And Sythesis Of Novel Conjugated Polymers For Organic Field-Effect Transistors And Solar Cells

Conjugated polymers combine the optical and electronic properties of metals andsemiconductors with the attractive mechanical and processing properties of traditionalpolymeric materials. The ease of manipulation with the chemical structures of polymersallows the fabrication of functional materials with tailor-made physical and chemicalproperties. As the critical materials for organic photoelectric devices, the design andsynthesis of high-performance π-conjugated polymeric semiconductors has an importantimpact on the device properties. In this thesis, we designed and synthesized a series ofnew conjugated polymers with good solution-processability and stablity for organicfield-effect transistor and bulk heterojunction solar cell applications. The relationshipbetween molecular structures and photoelectric properties were also studied in detail toprovide important insights for further design of excellent polymeric photoelectricmaterials. The main contents are described as follows:1.2,4,6-Tri(2-thienyl)pyridine, had been used to synthesize new conjugated polymerPBDTTPy with4,8-didodecyloxybenzo[1,2-b:4,5-b’]dithiophene. The concept ofintroducing electron deficient pyridine repeating unit and meta-linked structure intoconjugated polymer for reducing its HOMO energy level and thereby increasing ambientstability had been tested. The preliminary field-effect transistor devices based on thispolymer had been reported. The PBDTTPy exhibited p-type transporting performanceunder ambient conditions in bottom-gate, top-contact OFET devices. It was found that theannealing temperatures had signifcant effects on the performances of the OFET devices,although the polymer PBDTTPy showed typical amorphous structures in solid state basedon the information from XRD and AFM studies.2. The bis(2,3-dialkylthienyl)benzo[1,2-b:4,5-b’]-dithiophene (BDTT) building block was copolymerized with the unit of2,2’-bithiophene or thieno[3,2-b]thiophene by Stillecouple reaction to afford new2D conjugated polymers, PBDTT-2T and PBDTT-TT. TheBDTT building block with pendant thienyl moieties on the central phenyl rings providedadditional conjugation. This conjugated side chain system may extend the π-conjugationand thus facilitate the charge carrier transport. Both polymers had deep HOMO energylevels. The field-effect transistor devices based on new polymers displayed p-typetransporting performances under ambient conditions. The best hole-mobility obtainedfrom PBDTT-2T and PBDTT-TT were0.035cm2V-1s-1and0.008cm2V-1s-1,respectively, with the on-off ratios of6.56×105and9.01×104. Structural analysis by XRDand AFM were obtained to investigate the molecular ordering and film morphology ofboth polymers. The crystalline formation and highly ordered structures in films should befavorable for the charge transport, and could help to explain the high performance of theOFETs based on PBDTT-2T.3. Four new D-A alternative polymers, PBDTT-PhI, PBDTT-TPD, PBDTT-ID andPBDTT-DPP, containing BDTT as electron-rich (D) unit and strong electron-withdrawing imide or lactam groups as electron-deficient (A) units, had been synthesizedand characterized. The bandgaps and the energy levels of the polymers can be fine-tunedby introducing different electron-deficient moieties, such as2-dodecylisoindoline-1,3-dione,5-(2-octyldodecyl)-4H-thieno[3,4-c]pyrrole-4,6(5H)-dione,6,6’-di(2-ethylhexyl)-isoindigo and2,5-bis(2-ethylhexyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione. The isoindigo-based polymer PBDTT-ID exhibited strong absorption in thevisible and near-infrared region with an ideal optical bandgap (Eg) of1.52eV, low HOMOenergy level of-5.41eV, and suitable LUMO energy level of-3.89eV. The lactamstructure makes the DPP unit exhibit a strong electron-withdrawing effect. The polymerPBDTT-DPP possessed broad absorption in the visible and near-infrared region with anarrow optical bandgap of1.42eV. The photovoltaic performances of these polymers asdonors and PC61BM as acceptors were investigated with the device configuration of ITO/PEDOT:PSS/polymer:PC61BM/LiF/Al, under simulated air mass (AM)1.5Gillumination at100mW cm-2. The bulk heterojunction solar cell devices based onPBDTT-PhI and PBDTT-TPD displayed power conversion efficiencies (PCE) of0.77%and2.93%, respectively. The devices based on PBDTT-DPP showed a PCE of4.24%anda high short-circuit current density (Jsc) over10mA cm-2. The PBDTT-ID based deviceshad achieved a PCE of4.02%and a open-circuit voltage (Voc) of0.94V without thermalannealing and adding addtives, it is the highest Vocrealized among the isoindigo-basedpolymers.4. A series of modified electron-deficient quinoxaline units, such as2,3-bis(4-(trifluoromethyl)phenyl)quinoxaline (QxF),2,3-bis(4-(trifluoromethyl)phenyl)py-rido[3,4-b]pyrazine (PpF) and pyrido[3,4-b]pyrazine (Pp) were synthesized and used tocopolymerize with electron-rich3,3’-didodecyl-2,2’-bithiophene (DT) or bis(dialkyl-thienyl)benzodithiophene (BDTT) to obtain new D-A alternative polymers, PDT-QxF,PBDTT-QxF, PBDTT-PpF and PBDTT-Pp. Different electron-donating ability of DTand BDTT had an obvious effect on the absorption spectra of the resulting polymers,which can be seen from the fact that PBDTT-QxF with Egof1.72eV showed a visiblered-shifted absorption compared to PDT-QxF with Egof1.92eV. Two ways of modifingthe quinoxaline units by introducing trifluoromethylbenzene substitutes from the top of thepyrazine and/or changing the benzene unit to more electron-deficient pyridine moiety canboth lead to lower HOMO and LUMO energy levels of the polymers. PBDTT-QxF,PBDTT-PpF and PBDTT-Pp exhibited deep HOMO of-5.40to-5.51eV. UV-visabsorption spectra revealed that the electron-withdrawing ability increases in the order ofQxF, Pp, and PpF, which resulted in narrowering Egfrom1.72eV (PBDTT-QxF),1.65eV (PBDTT-Pp) to1.60eV (PBDTT-PpF).5. An efficient procedure towards electron-withdrawing trifluoromethylbenzenesubstituted benzo[1,2-b:4,5-b’]dithiophene (BDTTFB) was developed. A new lowbandgap conjugated polymer PBDTTFB-TTE based on2-heptylundecyl thieno[3,4- b]thiophene-2-carboxylate and BDTTFB as repeating units had been synthesized andcharacterized. The new polymer had a board absorption spectrum in the range from300to1000nm. The introduction of trifluoromethylbenzene lowered both HOMO and LUMOenergy levels, and the PBDTTFB-TTE exhibited much deep HOMO of-5.41eV. A lowPCE of0.23%but a high Vocof0.78V had been achieved in bulk heterojunction polymersolar cell devices with PBDTTFB-TTE as donor and PC61BM as acceptor under theillumination of AM1.5G,100mW cm-2. It is the highest Vocrealized among thealternative polymers of ester-substituted thieno[3,4-b]thiophene and benzo[1,2-b:4,5-b’]dithiophene.