| Polymer solar cells (PSCs) have drawn much attention for their light-weight, low-cost and potential utility in the production of large-area and flexible photovoltaic devices via roll-to-roll film-deposition processes as well as easily structural tailor and modification for organic molecules used in PSCs. It has been proven that combination of an electron-rich donor (D) unit and an electron-deficient acceptor (A) unit is a most powerful strategy in designing narrow band gap conjugated polymers. Based on this strategy, a series of novel acceptors that fusing a thiophene ring on the benzothiadiazole, quinoxaline and benzotriazole with strong electron-deficient ability and more stable quinoid structure have been designed and synthesized for the first time in this thesis. Moreover, sevral new series of D-A copolymers based on these acceptors have been synthesized and their optical and electrical characteristics as well as the device performances have also been investigated systematically to explore the relationships between the structures and properties.The main contents of this thesis are displayed as below:In Chapter 1, the types of solar cells such as inorganic silicon solar cells, CIGS thin film solar cells, organic solar cells were briefly depicted and then compared for their advantages and disadvantages. Then the principle, development and current situation of PSCs were introduced including the materials used for active layer of the PSC devices. At last, the design strategies and reseach contents of this thesis were outlined.In Chapter 2, two different schemes have been designed for the first time to synthesize a new kind of thiophene-fused benzothiadiazole (BTT) acceptor to construct D-A conjugated polymers for PSCs. The advantages and disadvantages of the two synthetic routes were discussed according to the experimental research. By introducing two thiophene spacers adjacent to the benzene ring of BTT, the monomer BTT-T was formed. Then the photophysical as well as the electrochemical characteristics of BTT-T and its anologue, a classic monomer BT-T, were carefully investigated. Compared to BT, BTT unit possesses deeper HOMO and LUMO energy levels, narrower band gap and strong ICT effect and electron withdrawing ability. It can be used as an acceptor for D-A copolymers, and is expected to get better device performance.In Chapter 3, four low band-gap D-A conjugated copolymers (P1-P4) with 2-ethylhexyl substituted BTT as an electron acceptor combined with bithiophene (DT), benzo[1,2-b:4,5-b']dithiophene (BDT) or dithieno[3,2-b:2',3'-d]silole (DTS) as electron donors were first designed and synthesized, The differences of the structure and characteristics among them were discussed. It has been found that BDT and DTS seem to be more appropriate for the BTT acceptor. The polymers P2 and P4 with the composition of BDT-BTT and DTS-BTT reached higher power conversion efficiency (PCE) of 2.65% and 2.30% individually than that of DT-BTT.In Chapter 4, by adjusting the alkyl side chains of the donor and acceptor in P4 with the composition of DTS-BTT, a series of polymers with improved solubility and enhanced molecular weight were obtained. OPV device based on the higher molecular-weight polymer P7 was fabricated. It has been found that though the molecular weight and solubility of P7 were improved and the absorption spetra were broadened, the device performance was not so well. The PCE was just improved by 0.51% compared to its analogical polymer P4, indicating that DTS donor showed much limited potential when combined with BTT acceptor.In Chapter 5, BTT unit as an acceptor and BDT as a donor were combined to build a series of polymer P9 with different molecular weight. By fusing a thiophene ring into 2,1,3-benzothiadiazole unit and adjusting the side chains of the donor and acceptor units in polymers composed of BDT-BTT, P9 exhibited broader absorption, deeper HOMO energy level and better solubility when compared with the traditional 2,1,3-benzothiadiazole based analogue polymer P12. The best PCE of the device based on P9 reached 6.07% with enhanced circuit voltage, short circuitcurrent density and fill factor which was increased by 90% compared to the polymer P12. Moreover, the influence of various side groups in BDT donor was studied. It has been found that 2D conjugated polymers exhibited red-shifted absorption spectra, deeper HOMO energy level as well as improved device performance. The effect of molecular weight on the PSC device performance was also discussed based on P9.In Chapter 6, by adjusting the side chains of the BDT donor and BTT acceptor in P9, three new conjugated D-A copolymers (P13-P15) with impoved solubility and enhanced molecular weight were designed and synthesized. Compared to P9, P13 with alkylcarbonyl-substituted BTT as the acceptor exhibited better solubility, lower HOMO energy level, improved solubility and relatively higher circuit voltage. Through modifying the alkyl side chains in BDT units, P15 was obtained. The PCE of P15-based device reached 4.14%, which was relatively lower than P9 mainly due to the obvious deacrease of the short circuitcurrent density in the PSC device. Nevertheless, the circuit voltage was successfully improved, and this provided reasonable references for molecular design in the future.In Chapter 7, a new kind of thiophene-fused quinoxaline (QxT) electron acceptor was firstly designed and synthesized to construct D-A copolymers P16 and P17 for PSCs. The effect of structural changes of thiadiazole and carbonyl side chain in QxT on the PSC device performance was discussed systematically. It has been found that the replacement of thiadiazole with quinoxaline resulted in the decrease of HOMO energy level of the polymer and the enhancement of circuit voltage of the OPV device. P17 with alkylcarbonyl-substituted QxT as the acceptor exhibited lower absorption coefficient and poorer device performance than P16. The best PCE of P16 device reached 4.50%.In Chapter 8, a new kind of thiophene-fused benzotriazole (BTzT) electron acceptor was firstly designed and synthesized to construct D-A copolymers P18 and P19 for PSCs. The effect of structural change from thiadiazole to 1,2,3-triazole as well as the difference of side-chain with ester group or carbonyl group in BTzT unit on the PSC device performance was discussed systematically. It has been found that the replacement of thiadiazole with 1,2,3-triazole can adjust the band gap for us to obtain the D-A polymers with medium band gap. P19 with alkylcarbonyl-substituted BTzT as the acceptor exhibited higher circuit voltage. The effect of DIO and NMP additives on the PSC device performance was also clarified. DIO additive was effective for P18 while NMP worked on P19. In this work, PDIN and PDINO were also used to modify the cathode interlayer to improve the photovoltaic perfromaces. The best PCE of approximately 7% in P18 based device was obtained. Especially, its largest short circuitcurrent density reached 14.32 mA cm-2 which is one of the best performances ever reported in benzotriazole based PSCs. The PCE of polymer P19 reached 6.16%. |
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