Design,Synthesis And Characterization Of A-DA’D-A Benzazole-based Organic Photovoltaic Materials

As an important green renewable energy source,organic solar cells（OSCs）have attracted much attention because of their advantages of light weight,flexibility,low cost,processed in solution and large-area printing fabrication.The design and synthesis of high-efficiency active layer materials and the optimization of photovoltaic device fabrication are the keys to obtaining high power conversion efficiency（PCE）.In this thesis,asymmetric six-membered fused ring acceptors and medium bandgap symmetric fused ring small molecule acceptors based on the electron-deficient units such as benzothiadiazole and benzotriazole were designed and synthesized.Besides,the influence of molecular structure and optoelectronic properties of acceptors on the OSCs was explored,and the working mechanism of the third component on the photovoltaic performance of ternary devices was investigated.The details are as follows:（1）Selecting benzothiadiazole as the electron-deficient central core,an asymmetric six-membered fused ring small molecule acceptor,BP4F-HU,was developed,by conjugating a thiophene unit and thieno[3.2-b]thiophene（TT）moiety on each side of the central core.The length of the alkyl side chain at the end of the fused ring backbone was further changed to obtain a new small molecule acceptor,named BP4F-UU,with an asymmetric backbone but symmetric side chains.The asymmetric fused ring skeleton increases the intramolecular dipole moment and enhances the intermolecularπ-πinteraction,while extending the alkyl side chain reduces the energy disorder of acceptors.When blended with PM6 as the donor,OSC devices achieved higher carrier mobility with optimized morphology.Finally,binary device based on PM6:BP4F-HU obtained a PCE of 14.72%,while PM6:BP4F-UU reached 15.31%.（2）In order to further improve the PCE of binary OSCs,a medium bandgap small molecule acceptor,named BTP-TA,with a similar skeleton but different end group to BP4F-UU,was designed and synthesized.With a weak electron-withdrawing end group 1,3-diethyl-2-thiobarbituric acid（TA）to replace the 2-（5,6-difluoro-3-oxo-2,3-dihydro-1H-inden-1-ylidene）malononitrile（DFIC）,BTP-TA exhibits a complementary absorption spectrum to the PM6:BP4F-UU blend film,which improves the photon utilization.Furthermore,the similar central framework results in good compatibility between BP4F-UU and BTP-TA,making it easier to form an alloy,which helps to maintain relatively uniform nanoscale phase separation in the active layer.Therefore,the ternary device based on PM6:BP4F-UU:BTP-TA achieves an optimum PCE of 16.19%.（3）With the aiming of further improving the performance of classic binary PM6:Y6 based devices,an A-DA’DA non-fullerene small molecule acceptor,named BZ-TA was designed and synthesized with benzotriazole as the electron-deficient core.Ternary devices based on PM6:Y6:BZ-TA were fabricated when BZ-TA as the third component was added to the binary device.Compared with the corresponding binary devices,the performance of the ternary devices improved in overall three parameters.The open circuit voltage(V_OC)of the ternary device was enhanced due to the increased lowest unoccupied molecular orbital（LUMO）energy level of BZ-TA.The absorption spectrum of BZ-TA was complementary to the binary blend film PM6:Y6,which broaden the absorption range and was beneficial for improving the short-circuit current density(J_SC)of the ternary devices.Moreover,with a similar A-DA’DA structure to the host acceptor Y6,BZ-TA shows good compatibility and miscibility with the binary devices,which improves the morphology of the active layer and promotes the realization of more balanced mobility.Therefore,the ternary devices based on PM6:Y6:BZ-TA obtained an optimal PCE of 16.18%（the corresponding binary was 15.35%）.