First Principles Study Of Benzodithiophene And Dibenzothiophene Based D-A Type Copolymers

The bulk heterojunction organic solar cell is focused in current research. The D-A type copolymer, as the electron donor material of the cell, has attracted wide attention in recent years. The design and optimization of the active layer material in the D-A type copolymer are the key step to further improve the photoelectric conversion efficiency of the organic photovoltaic cell. This thesis uses benzodithiophene(BDT) as an electron donor unit, and dibenzothiophene(BT) as an electron acceptor unit to form the D-A type copolymer, uses a thiophene ring as a πbridge to construct a D-π-A(PBDT-DTBX, X=O, S, Se, Te) structure, employs the density functional theory to systematically calculate corresponding electronic structure and light absorption spectrum, and compares the influences of different oxygen family elements and N atom on the light absorption spectrum of the copolymer during acceptor replacement.(1) In virtue of the calculation software VASP, simulation computations are performed on the three isomers of the BDT, results consistent with experimental values. The calculated energy gap values are 2.38 eV, 2.46 eV, 2.42 eV, while the corresponding experimental values are 2.17 eV, 2.49 eV, 2.26 eV. The consistent results show that the calculation method selected in this thesis is feasible. And then the structure with smallest band gap is selected to be a donor, in combination with different acceptors, to construct active layer material and optimize structure.(2) Element replacement is performed at the acceptor X-position of the D-A type copolymer. The calculation shows that when the acceptor X-position is replaced with O, S, Se and Te respectively, the HOMO energy level barely changes, while LUMO energy level gradually draws close to Fermi level, and the band gap decreases gradually. Two relatively strong absorption peaks reveal in the visible light area. With the increase of the atomic number of the element at the X position, the light absorption peak at about 4.0eV basically remains unchanged, while the intensity of the other light absorption peak is obviously increased and red shift arises.(3) This thesis selects the thiophene ring as a π-bond bridge. The calculation shows that the band gaps of D-π-A structures, compared with D-A structure, are all decreased at different degrees, proving that π-bond bridge can increase the conjugacy of polymer structure, decrease steric hindrance, and widen absorption spectrum.Wherein, the band gap is the minimum when X is Te, and with the increase of the atomic number of the oxygen family elements, the intensity of the light absorption peak is increased and red shift arises.(4) In order to further analyze the relationship between structure and absorption spectrum, this thesis calculates the partial density of states(PDOS) of the PBDT and absorption spectrum of PBDT-BT、P-BDT and P-BT, with the result that the 4.0eV light absorption peak is mainly generated by BDT unit, and the change of oxygen family elements mainly influences the light absorption in the range of 519.4-703.9nm.(5) Section IV simulates the influence on material property after the C atom on the acceptor is replaced with N atom, with the result that the replacement of N atom can decrease the structure band gap of the polymer, and widen absorption range.The conclusion of this thesis can provide beneficial reference for the structure optimization of organic polymers.