Narrow Bandgap Polymers Based On Fused Thiophene Derivatives:Design And Optoelectronic Properties

The research of narrow bandgap (less than 2.0 eV) polymers, featuring electron donor alternating electron acceptor type structure became the mainstream of polymer solar cells (PSC). It is difficult to balance the bandgap, carrier mobility and energy level for most narrow bandgap polymers. Major work of this dissertation is focused on the design and synthesis of a series of new electron donors, electron acceptors and polymers alternating electron donors and electron acceptors. The spectral and electrochemical properties of the polymers were studied and the BHJ polymer solar cells were also made to explore the optimal conditions for developing high-efficiency PSC materials.In chapter 2, two new 2-D conjugated polymers PTCDPP, PTCTTz by alternating carbazole-substituted triarylamine donor unit with diketopyrrolopyrrole (DPP) and thiazolo[5,4-d]thiazole (TTz) acceptor units have been prepared and characterized. The influence of bearing different electron acceptors on the optical and electrochemical properties of resulted polymers was studied. PTCDPP-based bulk heterojunction solar cells with a structure of FTO/TiO2/PTCDPP:PCBM/MoO3/Ag were fabricated. The devices were optimized by adjusting the composition of the PTCDPP:PCBM active layer, thermal treatment, and addition of processing additives. The device based on PTCDPP:PCBM (1:4, w/w) showed a power conversion efficiency (PCE) of 2.31%. The best cell performance (2.65%, PCE) is achieved by using 1,8-diiodooctane (3%, v/v) as a processing additive and annealing the active layer at 80℃.In chapter 3, four narrow bandgap copolymers PBTTDPP, PBTTTTz, PBTTTPD and PBTTDTBTQx, by alternating benzo[1,2-b:3,4-b’:5,6-d’]-trithiophene (BTT) with DPP, TTz, Thieno[3,4-c]pyrrole-4,6-dione (TPD) and thiadiazolo[3,4-g]quinoxaline (DTBTQx) have been synthesized by Stille reaction, which display the optical bandgap ranging from 1.20 eV～1.80 eV. The solubility, optical, electrochemical and photovoltaic properties of the copolymers were investigated. Bulk heterojunction solar cells with a structure of ITO/PEDOT4083 (40 nm)/PC71BM (80 nm)/Al (80 nm) were studied. Optimizing by adjusting the composition of the polymer:PC71BM active layer, adding the poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3’-(N,N-dimethylamino)-propyl)-2,7-fluorene) (PFN), the best BHJ solar cell device based on PBTTTTz:PC7]BM (1:1, w/w) exhibited a PCE of 2.24%, with a Jsc of 4.17 mAcm-2, Voc of 0.79 V, and FF of 0.35 by adding no PFN layer.In chapter 4, we have developed four novel low-bandgap polymers PNDTDPP, PNDTTTz, PNDTTPD, PNDTDTBTQx by alternating naphtho[2,1-b:3,4-b’]-dithiophene (NDT) and DPP, TTz, TPD and DTBTQx units through a non-poisonous direct arylation polycondensation, which circumvents the use of costly diboronic acid/ester or distannyl monomers. The polymers exhibit a bandgap ranging from 1.20 eV ～1.80 eV with highest occupied molecular orbital (HOMO)-4.94 eV ～-5.28 eV. The BHJ solar cell devices based on these polymers are still studied.In chapter 5, we have presented here a versatile method for the 4,8-functionalization of benzofuro[5,6-b]furan. Via palladium-catalyzed Suzuki and Sonogashira couplings as well as carbon-sulfur bond formation reaction, a series of 4,8-substituted benzofuro[5,6-b]furan units could be obtained which avoid the limit of expanding 4,8-substituted benzodifuran (BDF) derivatives through benzofuro[5,6-b]furan-4,8-dione as the intermediate. Using this methodology, four 4,8-functionalized benzofuro[5,6-b]furan donor units called BDFT, TIPSBDF, BDF-S, BDFT-S have been developed with alkylthienyl, triisopropylsilyl)ethynyl, alkylthio and alkylthiothienyl. A series of copolymers comprised of BDFT, TIPSBDF, BDF-S, BDFT-S as acceptor units and DPP, TTz, DTBTQx as donor units were also synthesized. The solubility, optical, electrochemical properties of resulted polymers were studied in detail. The DTBTQx based polymers show absorption spectrum to the near-infrared region with the optical bandwidth less than 1.13 eV. The polymers exhibited the HOMO level ranging from-4.94～-5.28 eV. The BHJ solar cell devices based on these polymers are still studied.