Synthesis And Photovoltaic Performance Of Benzothiadiazole-based Acceptors And Quinoxaline-based Polymer Donors

In recent years,the appearance of star acceptor Y6 made a series of electron-withdrawing unites become hotspots,such as benzothiadiazole（BT）,benzotriazole（BTA）,benzoselenadiazole and quinoxaline（Qx）.By designing new materials,optimizing device interfaces and improving the morphology of active layers,researchers have made a great achievements in organic solar cells and further pushed power conversion efficiencies（PCEs）over 19%.In this contribution,focusing on the improvement of open-circuit voltage(V_oc),we designed synthesized electron acceptor as well as electron donor using BT or quinoxaline（Qx）as building units,respectively.The specific work was as follows:（1）To investigate the influence of thiophene-fused indanone or thieno[3,2-b]thiophene-fused indanone on acceptors,we designed and synthesized three A-DA′D-A type molecules BTP-CC,BTP-TTC and BTP-TC.Impressively,a V_oc of 0.98 V was achieved in BTP-CC based device,which is one of the highest values to date.The smallΔE_HOMO（0.05 e V）between PBDB-T and BTP-TTC resulted in the inefficient exciton dissociation and charge transfer.Despite obtaining a relatively high V_oc of 0.95 V,BTP-TTC based device displayed the poor short-circuit current density(J_sc)and power conversion efficiency（PCE）.Compared with BTP-CC and BTP-TTC,BTP-TC exhibited red-shifted absorption and down-shifted energy levels.Benefiting from the increased crystallinity and the stronger intermolecular interaction,the device of PBDB-T:BTP-TC achieved improved charge dissociation,enhanced charge transport and suppressed charge recombination.Eventually,BTP-TC based device realized a higher efficiency of 13.57%.（2）Intensive research has indicated that alkoxy modification can not only change the photoelectrical properties of molecule,but also improved the morphology of active layers.In order to explore the effect of alkoxy chains on photovoltaic materiel and device performance,we designed and synthesized two asymmetric acceptors ABT4F and ABT4F-1O.The ABT4F presented extremely strong crystallinity,resulting in severe self-aggregation and large phase separation.By blending with PM6,ABT4F-based device obtained low efficiency of 2.46%.Relative to ABT4F,the alkoxy-substituted ABT4F-1O showed blue-shift absorption,up-shifted level and good solubility.The blend of PM6:ABT4F-1O exhibited high phase purity and appropriate phase separation,which were beneficial for improving exciton dissociation and surpressing charge recombination.As a result,ABT4F-1O based OSC displayed a PCE of14.62%.（3）Due to small steric resistance and strong electronegativity,fluorine atom is considered as effective electron-withdrawing unit.In deed,introducing fluorine atom into the donor can not only down-shift the energy level,but also improve backbone planarity and molecular stacking through non-covalent interactions.In this chapter,two donors ABQx-2F and ABQx-6F were designed and synthesized by changing the number of fluorine atoms of Qx monomer.The more fluorine-substutited ABQx-6F exhibited red-shifted absorption and down-shifted energy levels,which matched better with the acceptor Y6.Consequently,the device of ABQx-2F:Y6 only obtained an efficiency of 10.56%,while ABQx-6F based OSC achieved a satisfactory PCE of 15.70%with prominent improvement of V_oc,J_sc and fill factor（FF）.