Research On Synthesis And Photovoltaic Properties Of Wide Band Gap Polymers Based On Dithiazobenziophene

The rapid development of non-fullerene small molecule acceptors in recent years has greatly improved the power conversion efficiency of non-fullerene solar cells,and the efficiency of single-section cell devices has now exceeded 18%.Wide band gap polymers with better matching absorption spectra as well as energy levels of non-fullerene small molecule acceptors have received great attention.In this thesis,dithieno[2:3-d:2’,3’-d’]benzo[1,2-b:4,5-b’]dithiophene（DTBDT）,or benzo[1,2-b:4,5-b’]dithiophene（BDT）are used as electron-rich structural units,and dithiadiazolo[3’,2’:3,4;2’’,3’’:5,6]benzo[1,2-c][1,2,5]thiadiazole（DTBT）or benzo[1,2-c:4,5-c’]dithiophene-4,8-dione（BDD）-based derivatives as electron-deficient structural units to construct wide-bandgap polymers and modulate their light absorption properties and molecular energy levels by side chain modification or alteration ofπ-bridges.It is expected that the molecular design strategies of wide-bandgap polymer donor materials for high efficiency organic solar cells will be clarified through the exploration of the conformational relationships of wide-bandgap polymer donor materials.In the second Chapter,the first copolymerization of the electron-donating structural unit DTBDT containing the 2-（2-butyloctyl）-3,4-difluorothiophene side chain with the electron-deficient structural unit DTBT was performed to obtain the wide-bandgap polymer PD1.The results show that PD1 has a wide optical band gap（1.98 e V）and a deep HOMO energy level（-5.48 e V）.The pure film of polymer PD1 exhibited a smaller interchain stacking distance compared to polymer D18.This could be attributed to the introduction of DTBDT containing doubly fluorinated side chains,which increases the planarity of the polymer backbone.The addition of 1%1-chloronaphthalene（CN）to the photovoltaic device with PD1:Y6 as the active layer greatly increased the FF of the device,and the final optimal device obtained an energy conversion efficiency（PCE）of 12.86%with a corresponding open-circuit voltage(V_OC)of 0.87 V,a short-circuit current density(J_SC)of 22.27 m A cm^-2,and a fill factor（FF）of 66.31%.In the third section of the work,two wide band gap polymers PD2 and PD3 were constructed by introducing thiophene（T）and thieno[3,2-b]thiophene（TT）as conjugatedπ-bridges in the DTBDT structural unit modified with 2-（2-butyloctyl）-3,4-difluorothiophene side chains and BDD structural unit,respectively.The results show that both polymers have strong absorption in 450-650 nm,and the polymer films have similar HOMO energy levels with a more matching absorption spectrum with the acceptor material Y6.Compared with the polymer PD2 containing Tπbridges,the introduction of TTπbridges resulted in a red-shifted absorption and better photostability of the polymer PD3 films,but also led to a larger stacking distance between polymer chains,a lower dissociation efficiency of excitons and an increased complexation probability.Finally,the photovoltaic device based on polymer PD2 containing Tπ-bridges obtained a PCE of 12.68%,corresponding to a V_OC of 0.84 V,a J_SC of 22.28 m A cm^-2,and a FF of 67.64%,which is slightly higher than the PCE of the photovoltaic device with polymer PD3 containing TTπ-bridges（12.12%）.In the fourth chapters,an asymmetric wide band gap polymer PD4 was designed and synthesized by introducing a fluorinated alkylthiophene side chain and a non-fluorinated alkylthiophene side chain to the BDT structural unit and copolymerizing with the BDD structural unit through a"symmetry breaking"molecular design strategy.The results show that the polymer PD4 films have strong absorption at 400-650 nm and exhibit strong aggregation in solution.Both the symmetric polymer PBDB-T with a similar structure without fluorine in the side chain and the symmetric polymer PM6 with a double fluorine in the side chain exhibit similar characteristics.It is shown that the HOMO energy levels of the polymers PBDB-T,PD4 and PM6 are-5.21,-5.49 and-5.54 e V,respectively,and the HOMO energy levels of the polymers decrease gradually with the increase of the number of fluorine atoms.The best PCE of 13.16%was achieved for the PD4:Y6-based device compared to the PCE of PBDB-T:Y6-based device（8.60%）and the PCE of PM6:Y6-based device（15.72%）reported in the literature.With the increase of fluorine atoms in the WBG polymer donors,the V_OC,J_SC and FF of the photovoltaic devices with Y6 as the acceptor material showed an increasing trend,indicating that the increase in the number of fluorine atoms in this polymer system effectively modulates the performance of the polymer donor material and facilitates the performance of its photovoltaic devices.