Synthesis And Photovoltaic Properties Of Small Molecule Acceptors Based On Fused-Ring Molecule Engineering

Organic solar cells（OSCs）have become a hot research spot due to their light weight, excellent solution processability,flexibility and the ability to fabricate semitransparent devices.Fused-ring small molecular acceptors（SMAs）have the advantages of strong light absorption ability,wide light absorption range from visible light to near-infrared region,adjustable energy level,good compatibility with polymer donor materials,the power conversion efficiency（PCE）achieved a huge breakthrough based on SMAs.Structurally,the fused-ring SMAs can be divided into three parts:electron-donating fused-ring central core,electron-withdrawing end-capping groups and the side chain for adjusting solubility.However,the PCE is still lower than that of silicon-based solar cells,which have been commercialized.The main methods to solve this problem are to reduce the energy loss and broaden the absorption spectrum.In this thesis,we hope to redshift the absorption spectrum of SMAs to the near-infrared region by expanding the conjugation length of the central core,enhancing the electron-donating ability of the fused-ring central core and reinforcing the intramolecular charge transfer effect,so as to achieve higher PCE for better utilization of solar spectrum.In Chapter 1,we briefly introduce the development of photovoltaic technology and the device structures,working principles,photovoltaic performance parameters of OSCs,then focus on the development of active layer materials,and finally propose the design idea and the corresponding research contents of this thesis.In Chapter 2,by expanding the conjugation strategy of fused-ring central cores based on fluorene,we designed and synthesized two nonacyclic SMAs,namely FTTCN and FTTCN-M.The maximum molar extinction coefficients(ε_max)of FTTCN and FTTCN-M are 1.4×10⁵ and 1.8×10⁵ M^–1 cm^–1,respectively,and the absorption edges exceed 790 nm.FTTCN and FTTCN-M deliver compatible energy levels with polymer donor PBDB-T.PBDB-T:FTTCN and PBDB-T:FTTCN-M blend films show favorable morphology and beneficial charge transport properties,PCEs of PBDB-T:FTTCN and PBDB-T:FTTCN-M devices are over 10%.In Chapter 3,we adopted an asymmetric central core strategy to enhance the intermolecular interaction,and synthesized two heptacyclic SMAs bridged with carbon/silicon moiety,namely BTC-4F and BTSi-4F,respectively.The absorption edge of BTSi-4F is red-shifted to 824 nm,while the absorption edge of BTC-4F is further red-shifted to 845 nm due to the stronger electron-donating ability of the fused-ring central core.Theε_max of BTC-4F and BTSi-4F are 1.75×10⁵ and 1.89×10⁵ M^–1cm^–1,respectively.Compared with the PCE of PM6:BTSi-4F-based device of 6.51%,the PCE based on PM6:BTC-4F device is over 10%.In Chapter 4,we replaced the benzene unit with a larger planar,stronger electron-donating and better charge transporting thieno[3,2-b]thiophene unit,and synthesized hexacyclic SMA T6Me,heptacyclic SMA T7Me and octacyclic SMA T8Me.Due to the strong intramolecular charge transfer（ICT）effect,the absorption edges of T6Me,T7Me and T8Me are all over 900 nm,withε_max of 1.82×10⁵,1.35×10⁵ and 1.88×10⁵M^–1 cm^–1,respectively.When blending the three SMAs with polymer donor PM6 to fabricate OSCs devices,and the photovoltaic performance is optimized by solvent engineering.Although the HOMO energy offset between PM6 and T6Me is only 0.07eV,the PM6:T6Me device can still achieve a high PCE of 12.09%with a V_OC of 0.87V,a J_SCof 21.33 mA cm^–2,and an energy loss of 0.51 eV.In Chapter 5,we used benzo[1,2-b:4,5-b’]dithiophene as the fused-ring central moiety and adopted the conjugation expansion strategy to construct a nonacyclic S,N heterocycle,namely BDTOT-4F.Owing to the strong ICT effect and the small steric hindrance of N-alkyl chains,the absorption spectrum of BDTOT-4F is red-shifted to940 nm,and theε_max is 2.92×10⁵ M^–1 cm^–1.PM6:BDTOT-4F-based device only presents a PCE of 5.61%,mainly ascribed to PM6 and BDTOT-4F could not form favorable morphology,resulting in incomplete energy transfer between BDTOT-4F and PM6.In Chapter 6,we used the electron-withdrawing benzo[c][1,2,5]thiadiazole unit to construct a DAD-type fused-ring central core,and introduced alkyl side chains to increase the solubility,synthesized two A-DA′D-A-type pentacyclic SMAs,namely BTT-4F and BTTCN-Cl,respectively.Due to the multiple D-A interactions in the molecules,the absorption edges of BTT-4F and BTTCN-Cl are red-shifted to 811 nm and 835 nm,respectively,and theε_max are 2.7×10⁵ and 2.6×10⁵ M^–1 cm^–1,respectively.PM6:BTT-4F blend film can form favorable morphology for charge transport,and the PCE of PM6:BTT-4F-based device exceeds 10%.In Chapter 7,based on BTT-4F in Chapter 6,by adopting the conjugation expansion strategy of the fused-ring central core and chlorination of the end-capping group,we developed a alkylated dithieno[3,2-b:2’,3’-d]thiophene electron donating unit and synthesized asymmetric heptacyclic SMA BP7T-4F and octacyclic SMAs BP8T-4F and BP8T-4Cl;considering the effect of conjugation expansion on molecular packing,we used an alkyl-substituted benzo[d][1,2,3]triazole as electron-withdrawing central moiety,synthesized octacyclic SMAs Bz8T-4F and Bz8T-4Cl,and nonacyclic SMA Bz9T-4F for further increase the conjugation.The absorption edges of BP8T-4F,BP8T-4Cl,Bz8T-4F,Bz8T-4Cl and Bz9T-4F are all beyond 900 nm by conjugation extension and optimization of end-capping groups,among which the absorption edges of Bz8T-4F and Bz8T-4Cl are 944 and 954 nm,respectively,and theε_maxs exceed2.0×10⁵ M^–1 cm^–1.PM6:BP8T-4F and PM6:BP8T-4Cl blend films exhibit clear nanofibrous interpenetrating network,which is beneficial for charge transport,and the PCEs based on PM6:BP8T-4F and PM6:BP8T-4Cl devices are 14.44%and 15.10%,respectively.Moreover,the PCEs of PM6:BP8T-4F and PM6:BP8T-4Cl-based devices are improved to 16.32%and 16.41%by optimizing the ETL layer.