Synthesis And Study On The Properties Of Polymer Photovoltaic Materials Containing Benzodithiophene And Fluorene

The study progress of the bulk heterojunction polymer solar cells(BHJ–PSCs), work principle and photosensitive materials of BHJ–PSCs were systematically introduced in this dissertation. A number of electron donor polymer materials used in BHJ–PSCs and their current developments were reviewed. In BHJ–PSCs, the molecular energy levels, absorption spectra, solubility, hole mobilities and microstructure of the polymer materials were important factors to affect the power conversion efficiency(PCE). Hence, the dissertation focused on the design and synthesis of main-chains polymers, side-chains polymers and a combination of both which showed broader absorption, higher hole mobilities and better solubility for improving the PCE. The as-synthesized intermediates and target polymers were proved by GC–MS, MALDI–TOF MS, 1H NMR and 13 C NMR. The photophysical and electrochemical properties of the polymers had been studied by UV–Vis and cyclic voltammetry(CV). Finally, the photovoltaic properties of BHJ–PSCs based on target polymers were also investigated. The main study content was listed below.1. Two new polymers, PSOBDT–DPP and PDSBDT–DPP, were designed and synthesized, by copolymerizing an electron-deficient diketopyrolo[3,4-c]pyrrole(DPP) and an electron-donating building-block asymmetric 4-(2-ethylhexyloxy)-8-(2-ethylhexylthio)benzo[1,2-b:4,5-b′]dithiophene(SOBDT) or symmetric 4,8-bis(2-ethylhexylthio)benzo[1,2-b:4,5-b′]dithiophene(DSBDT). We investigated how the symmetry of the molecular and different numbers of alkylthio affected the photophysical, electrochemical and photovoltaic properties of the polymers. The study results indicated that PDSBDT–DPP with the symmetric DSBDT units showed deeper HOMO energy level, which was beneficial for improving the Voc. Because of the poor solubility of the polymers, the PCEs of BHJ–PSCs based on PSOBDT–DPP and PDSBDT–DPP exhibited 0.42% and 0.73%, respectively.2. Two novel polymers(PSOBDT–FBT and PSOBDT–TID) containing an asymmetric 4-(2-ethylhexyloxy)-8-(2-ethylhexylthio)benzo[1,2-b:4,5-b′]dithiophene(SOBDT) units and 5,6-difluoro-4,7-bis(4-(2-ethylhexyl)thiophen-2-yl) benzo[c][1,2,5]thiadiazole(DTff BT) or 6-(thiohen-2-yl)-di(2-ethylhexy)isoindigo(TID) conjugated side groups, were designed and synthesized to avoid the poor solubility and high HOMO level of PSOBDT–DPP. The effect of different conjugated side groups on the solubility, photophysical properties, electrochemical properties and photovoltaic properties of the target polymers were investigated. The results showed that the two polymers with conjugated side chains exhibited good solubility in common organic solvents, good thermal stability, good film-forming ability and deep HOMO level. Compared with PSOBDT–TID, PSOBDT–FBT containing DTff BT showed deeper HOMO level and better miscibility with PCBM in the blend films. The BHJ–PSCs based on PSOBDT–FBT and PSOBDT–TID/PC61BM(1/2, w/w) exhibited the PCEs of 4.4% and 2.2%, respectively. For the PSOBDT–FBT/PC71 BM cells, combined with an increase of Voc and Jsc, the resulting PCE was raised to 5.0%. The preliminary results showed that the asymmetric SOBDT could be used as an electron-rich building block in the development of a new D–A type low band gap conjugated polymer for BHJ–PSCs and exhibited good photovoltaic performance.3. By introducing electron-deficient units benzo[c][1,2,5]thiadiazole(BT) or quinoxaline(Q) into polymer main chain, Two double-acceptor side-chain polymers(PBDT–BT and PBDT–Q) were designed and synthesized to overcome the disadvantages of side chain polymers in narrow absorption spectra. The results indicated that the two polymers showed a little broader absorption than PSOBDT–FBT, which was beneficial for getting higher Jsc. The PCEs of BHJ–PSCs based on PBDT–BT and PBDT–Q exhibited 1.70% and 0.42%, respectively. For the PBDT–BT/PC71 BM cells, with increasing the FF and Jsc, the PCE was raised to 2.42%.4. We designed and synthesized three polymers(PFDT–BT, PFDT–DTBT and PFDT–TTBT) containing electron-donor 9-(bis(2-(2-ethylhexyl)thiophene-5-yl)-methylene)-9H-fluorene(FDT), electron-acceptor BT and different numbers of thiophene–bridge. The results showed that the absorption red shifted and HOMO level deepened, when the numbers of thiophene–bridge increased. The BHJ–PSCs based on PBDT–BT, PFDT–DTBT and PFDT–TTBT/PC61 BM exhibited 0.16%, 1.26% and 1.37%, respectively.5. Three new random conjugated terpolymers(P1, P2 and P3) were designed and synthesized by copolymerizing an electron donor benzo[1,2-b:4,5-b′]dithiophene(BDT) with two electron acceptors diketopyrrolo[3,4-c]pyrrole(DPP) unit and athiophene-vinylene-dithienyl-benzothiadiazole(TVDTBT) side groups for broadening absorption spectra. The results indicated that all terpolymers showed broad absorption and good thermal stability. From P1 to P3, as the TVDTBT content increased, the HOMO level deepened. Because P3 exhibited strong and broad absorption, deep-lying HOMO level(–5.47 e V), high hole mobility and good morphology, BHJ–PSCs based on P3 exhibited a high PCE of 5.29% without any processing additives. This good result indicated that the conjugated random terpolymer had great potential for increasing Voc and Jsc at the same time in bulk heterojunction photovoltaic devices. This synthetic strategy provided ideas and methods for BHJ–PSCs of high photovoltaic performance.