Design Of D-A-A Type Organic Small-molecule Photovoltaic Materials And Modulation On Photoelectric Properties

In the past decade,organic solar cells（OSCs）as alternatives to silicon-based solar cells have been investigated.And small-molecule（SMs）photovoltaic materials are developing rapidly due to its small molecular weight,a well-defined chemical structure and easy purification etc.With the significant achievements in new materials and device optimization,the PCEs over 16%have been achieved in single-junction OSCs.Currently,the reported molecular configurations are mostly symmetrical,such as A-D-A,D-A-D,or extended A-D-A’-D-A,which are also the typical configurations with high efficiency.Relatively,there are few reports on asymmetric SMs,such as D-D’-A,D-A-A’etc.In this work,eight novel asymmetric D-A-A’type SMs were designed and synthesized with tuned terminal groups and central core.The specific contents are as follows:（1）Triphenylamine（TPA）unit was selected was chosen as the electron-donating（D）unit because of its electron-rich character and the three-dimensional structure5,6-difluoro-2,1,3-benzothiadiazole（BT）unit or diketopyrrolopyrrole（DPP）unit were selected as the central electron-withdrawing unit（A）respectively.The molecular properties are optimized by extending the conjugated skeleton or regulating the end group.We use quantum chemical calculations to predict ground-state geometries,energy levels,electron cloud distribution and absorption properties of the target compounds,and characterize the molecular photoelectric properties by uv-vis and CV measurements.（2）Through the test can be found that all target molecules possess good planar backbone except for triphenylamine（TPA）unit,the BT-based molecules showed wide bandgaps,through the introduction of acetylene in molecular structure and ethinyl faculties of thiophene groups or adjust the terminal group can reduce the bandgaps,achieve the goal of optimized molecular light absorption properties.（3）Compared with the BT-based molecules,the HOMO level of the DPP-based molecules increased significantly due to the bigger conjugate structure of DPP unit.And the DPP-based molecules showed a narrow band gap and good optical absorption properties.It is worth noting that TPADPPIO exhibits an absorption edge over 850 nm in film with a narrow bandgap of 1.35 eV.（4）Photovoltaic devices based on the above mentioned molecules as donor materials and PC₆₁BM as acceptor materials were made in this paper.By testing,devices based on the BT-based molecules had relatively high V_occ in their parameters,and devices based on TPABTThRD had the highest V_oc value of 1.03V.Due to the superior optical absorption properties of the DPP-based molecules,the photovoltaic devices based on the DPP-based molecules exhibit relatively high J_sc,among which the TPADPPPI-based devices exhibit the highest J_sc value of 9.31 mA·cm^-2 and the highest PCE（4.21%）..（5）The bulk heterojunction organic solar cell in solution process was fabricated by mixing the BT-based molecules with P3HT.The test results show that the BT-based molecules have the potential to act as acceptor materials,showing the bifunctional characteristics.