Benzodithiophene-based Conjugated Copolymers:Synthesis And Application In Solar Cells

The traditional petrochemical resource is now facing the exhaustion of energy crisis, and it may cause great environmental pollution in the application at the same time. In order to meet the growing energy demand, the need of the development and utilization of some rich and renewable resources is urgent. In a variety of renewable energy resources, the unlimited, inexhaustible solar energy resource belongs to renewable green energy resource. The application of solar energy is one of the best ways for human to obtain clean energy. Among the approaches of solar energy application, the solar cells application is efficient and convenient. Compared to other traditional inorganic solar cells, organic solar cells possess the advantages of low cost, light weight, easy molecular design and potential application in flexible devices with large surface areas, and thus have aroused the interest of scientists and companies.Based on the donor-acceptor (D-A) designing concept, a series of copolymers are synthesized, and the photovoltaic properties are studied when they are as donor materials for BHJ organic solar cells in this thesis.In the second chapter, copolymers based on benzodithiophene and quinoxaline, represented by 4,8-bis(5-(3,4,5-tris(octyloxy)phenyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene (TOBDT) and 2,3-diphenyl-5,8-di(thiophen-2-yl)quinoxaline (TQ1) or 10,13-bis(4-(2-ethylhexyl)thiophen-2-yl)dibenzo[a,c]phenazine (TQ2), were synthesized via a Stille coupling reaction. By increasing the conjugation in the TQ2 unit, the polymer based on TQ2 showed a narrower band gap (Eg), a lower highest occupied molecular orbital energy level and enhanced interchain π-π interactions. Polymer solar cells based on TQ2 showed a simultaneous enhancement of the open-circuit voltage (Voc), short-circuit current density (Jsc) and fill factor (FF) compared with polymer solar cells based on the TQ1 polymer. A good power conversion efficiency of 4.24% was achieved by solar cells based on the TQ2 polymer and [6,6]-phenyl-C71-butyric acid methyl ester composite. These preliminary results indicate that increasing the acceptor unit (quinoxaline) conjugation is an effective way of improving the performance of polymer solar cells.In the third chapter, taking into account the planarity, alkyl chain steric hindrance and molecular weight towards high performance polymer solar cells, a "planar" semiconducting polymer PTOBDTDTffBT is designed. Minimal intra- and inter-molecular steric hindrances are realized by removing the alkyl chains at the 4,4’-positions of DTffBT monomer and introducing shorter octoxyl chains on the BDT unit, respectively. Such steric minimization strategy endows PTOBDTDTffBT with high number average molecular weight of 343.37 kg/mol, a planar conjugated backbone and strong inter-molecular aggregation characteristic as well. The optical property indicates the aggregation can only be partially broken even at 160 ℃, and theoretical calculations provide insightful information that the polymer backbone has small distortion and the thiophene-phenyl bridge can move the octoxyl chains 6.77 A range outward the polymer backbone, both of which ensure the strong interchain aggregation. In spite of the high planarity and strong aggregation of the polymer, PTOBDTDTffBT is relatively amorphous in its film indicated by grazing incidence X-ray diffraction analysis. PTOBDTDTffBT exhibits a narrow bandgap of 1.71 eV together with a deep lying HOMO energy level of -5.46 eV. Because of the strong aggregation of PTOBDTDTffBT, the PTOBDTDTffBT/PC71BM active layer exhibits temperature dependent photovoltaic performances:when the active layer is spin coated at a relative low temperature bellow 100 ℃, PTOBDTDTffBT/PC71BM exhibits stable power conversion efficiency above 6.6%; however, when the temperature is elevated from 120 ℃ to 160 ℃, the power conversion efficiency decreased gradually to 3.58% together with decreased short circuit current density and fill factor. The temperature dependent photovoltaic is explained that the PC71BM is easily to be spun out from the solution at higher temperature after the UV-vis absorption and X-ray photoelectron spectroscopy analysis. High temperature can also lead to coarse morphology and large phase separation in the active layer seen from the atom force microscopy and transmission electron microscopy images. Finally, maximum power conversion efficiency of 7.68% is realized when the active layer is spin coated at 80 ℃, which is a dramatic breakthrough of the power conversion efficiency for "planar" PBDTDTffBTs.