Synthesis And Photovoltaic Properties Of Conjugated Polymers With Two Electron-withdrawing Units

Photoelectric conversion efficiency of polymer solar cells（PSCs）has been rapidly improved with the development and utilization of new photovoltaic materials and the continuous optimization of photovoltaic devices,which indicates its great potential for commercial application.Electron acceptor materials have been paid a large amount of attention as one of the most important photovoltaic materials in PSCs.Among them,polymeric electron acceptors possess the advantages of easily modificated chemical structure,tuneable HOMO/LUMO energy levels,large molecular weight,good stability in structure and performance,so they have been attached importance by more and more researchers.Accordingly,two series of conjugated polymer materials based on two kinds of electron-withdrawing units（A）are designed,synthesized and used for PSCs in this thesis,and the relationship between their structure and properties have been studied in detail.1. Two conjugated polymers P（DPP-T）and P（DPP-NDI）with A-A’structure were designed and synthesized by using DPP as strong electron-deficient unit（A）and dithiophene-dione（T）as weaker electron-deficient unit（A’）or naphthalene diimide（NDI）as stronger electron-deficient unit（A’）,respectively.The effects of the A’unit on polymer properties have been studied.Two polymers possess a broad absorption spectra between 300 nm and 950 nm.P（DPP-T）shows a absorption peak origined from inthermolecularπ-πstacking in solution and film for the small steric hindrance between DPP and T units,whereas P（DPP-NDI）does not show obvious absorption peak origined from inthermolecularπ-πstacking for the large steric hindrance between DPP and NDI units.P（DPP-T）possesses a high LUMO energy level over-3.57 e V,so it is not an ideal candidate of acceptor in PSCs.The PSCs based on P（DPP-T）as the donor and PC₇₁BM as the acceptor showed a PCE of 3.1%.P（DPP-NDI）possesses a low LUMO energy level of 4.00 e V,so it could act as an acceptor in PSCs.The PSCs based on P（DPP-NDI）as the acceptor and PBDB-T as the donor showed a PCE of 0.20%.The PSCs based on P（DPP-T）and P（DPP-NDI）show low PCE values,the possible reason could be that the low solubility of these polymers in solvent could result in inferior morphology in blend film,which would lead to inferior exciton exciton dissociation,serious charge recombination and low photovoltaic performance.2. Two polymeric nonfullerene acceptors（NFAs）,named as P（IDTIC-DPP）and P（IDTIC-DTDf BT）,with A-D-A-A’were designed and synthesized by using indacenodithiophene（IDT）as the electron-donating（D）unit,dicyano-substituted indenone（IC）as the A units on both ends of IDT unit for its large dipole moment and goodplanarity,diketopyrrolo[3,4-c]pyrrole（DPP）and5,6-difluorobenzo[c][1,2,5]thiadiazole combined with two thiophenes as as the second electron-withdrawing unit（A’）respectively.The results indicate that both of the polymers possess broad absorption spectra,and the absorption edges of the films exceeds 900 nm.The LUMO（lowest occupied molecular orbitals）energy levels of the polymers are lower than-3.80 e V,so they could be used as polymeric acceptors in PSCs.The energy levels of the polymers mathch with the energy levels of classic donor material PBDB-T,and they also present complementary absorption spectra with PBDB-T.Therefore,the nonfullerene PSCs were prepared by using PBDB-T as the donor and two polymers as the acceptors.Among them,the PSC based on P（IDTIC-DTDf BT）shows higher power conversion efficiency（PCE）of 6.05%(J_sc=14.99 m A cm^-2,V_oc=0.80 V,FF=50.43%),which is attributed to its bicontinuous nanophase separation morphology of blend film and higher charge mobilities.However,the PSC based on P（IDTIC-DPP）shows lower PCE value of 0.72%because of its worse morphology of blend film and lower charge mobilities.