Study On The Design,Synthesis,Aggregation Structures And Optoelectronic Properties Of A-D-A Type Molecules

Donor-acceptor(D-A)type conjugated molecules show versatile optoelectronic functions that attracted more and more attentions in the research areas of light emitting diodes,photovoltaics and sensors.Charge transfer excitation in such D-A type molecules is usually strongly dependent on the linkage between D unit and A unit,and the twisting between D and A are used to control the intersystem crossing processes in recent years to achieve high performance light emitting materials.In addition,molecular stacking of D-A type molecules plays very important role on the optoelectronic properties in condensed state.Due to the relatively strong intermolecular charge transfer interaction between D unit and A unit from adjacent molecules,the planar D-A molecules tends to form reversed face to face stacking,that means H-type stacking formed easily.Such charge transfer interactions have been widely studied to construct supramolecular architectures.The introduction of twisted units in molecular structure may reduce such kind of charge transfer interaction,and thus it is possible to achieve strong ?-? interactions between two D units or two A units from adjacent molecules.For example,in non-fullerene acceptor materials strong ?-? interactions between A units are highly expected for high electron mobility.However,till now it is still rare for the detailed discussion of the relationship between the stacking of the twisted D-A molecules on the optoelectronic properties.In this dissertation,we design and investigated a serious of A-D-A molecules and investigated the influence of A-D-A molecule ?-? interaction on the optoelectronic function.By changing the molecule donor moieties and acceptor moieties,we successfully synthesized planar A-D-A molecules based on indacenodithiophene(IDT)or indacenodithieno[3,2-b] thiophene(IDTT)electron pushing donor core and benzimide(BI)or naphthalimide(NI)electron withdrawing acceptor end group.The spectral study indicated that all BI based molecules formed J-aggregate while the NI based molecules formed less ordered stacking.The single-crystal structure analysis revealed that the planar molecules exhibited stronger ?-? interactions with overlap coverage of donor molecule orbitals and acceptor molecule orbitals,which indicate low electron mobilities and low fluorescence quantum efficiency while the more twisted molucules showed strong ?-? interactions between acceptor NI molecule orbitals,but due to the small ?-? stacking distribution,the electon mobilities were low.By introducing thiophene unit between donor and acceptor,we synthesized IDTT-NI-iC3 which showed cross linked molecule packing with NI acceptor group as the ?-? stacking point,the very strong ?-? interaction and large ?-? stacking distribution guarantee the high electron mobility which was 10 times that of other molecules.Hydrogen bond was introduced in our A-D-A system with hydrogen atom replacing the alkyl chain of BI or NI acceptor groups(BI-H or NI-H).A serial of conjugated molecules containing IDT/IDTT core and BI-H/NI-H end groups,which tend to self-assemble to supramolecular J-aggregates either directed by intermolecular multiple hydrogen bondings(IDT-BI-H,IDT-NI-H,IDTT-BI-H)or promoted by ?-? interactions between the end groups of the adjacent molecules(IDTT-NI-H),as revealed by single-crystal structure analysis combined with spectroscopy studies.The J-aggregates absorb high energy visible lights and convert to low energy delocallized excitons due to the coherent resonance of the transition dipoles of the monomers.The thin films of these J-aggregates are used as interlayers in inverted polymer solar cells between the ZnO layer and the active layers.The delocallized excitons in the J-aggregates penetrate more energy to the active layer that compensate the energy loss caused by the interlayer absorption,which results in quite similar external quantum efficiency(EQE)in the absorption region of the J-aggregates(400~550 nm)showing solar concentrating properties.Furthermore,some of these J-aggregates show enhanced electron collecting and transporting properties that result in obviously increased EQE in 550~750 nm,which might be attributed to the improved contact between the interlayers and electron acceptor ITIC for the similar A-D-A type molecular stuctures.The results clearly indicate the promising strategy for high performance interlayers by designing J-aggregates of dye molecules.We further introduced metal chemical bond into our A-D-A molecule system by doping carboxyl based A-D-A molecules(IDT-BA and IDTT-BA)with ZnO.It was usually believed that the introduction of carboxyl groups to ZnO interlayer would lead searious damge causing massive defects which could trap eletrons and decrease electon mobilities.But in our study,the introduction of carboxyl A-D-A molecules only provided a way for doping with ZnO.The doping of IDT-BA and IDTT-BA with ZnO improved the ZnO interlayer with the same electron mobilities and less charge recombination rate.Though there exist light absorbing for IDT-BA and IDTT-BA,the EQE curves showed no obvious difference in the absorbing region(370~500 nm),which indicated the absorbing lights could be transformed to currents with 14.6% the highest enhancement in PCEs.