Fine-Tuning On Photoelectric Properties Of Fused-Ring Small-Molecules Via Side-Chain Structures

With the coming of the information age,organic optoelectronic materials have been widely applied in various fields such as technology,military and economy.Recently,the invention of A-D-A(A and D:electron-donating and accepting units,respectively)and A-DA’D-A type non-fullerene small-molecule acceptors has pushed rapid advances in the field of organic photovoltaics(OPVs)with the power conversion efficiencies(PCEs)rapidly reaching over 19%.The energy levels,bandgaps and film-morphologies can be finely tuned by changing the fused-ring structures,side-chains,and/or ending electron-accepting units,which improves charge separation and transport,leading to improvement in solar cell performance.Specially,the properties of materials and devices such as solubility,aggregation,charge separation and transportation,electronic luminescence property and non-radiation recombination loss can be modulated via the side-chain engineering.This thesis investigates the fine-tuning on optoelectronic properties of benzotriazole(BTA)based,bulk orthogonal side-chain decorated fused-ring small-molecules via their side-chain structures,e.g.tuning the solar cell performance via side chain lengths and tuning the solid photo-luminescence via the branching sites of side-chains.Following results are obtained:1.Fused-ring small-molecules of BTA-EHBO and BTA-HDHD were synthesized by modulating the lengths of side chains.Compared with BTAHDHD,BTA-EHBO decorated with shorter alkyl side-chains had an upshifted HOMO energy level and exhibited a narrower bandgap.BTAEHBO gave a PCE of 13.21% when blended with polymer donor PBDBT,while BTA-HDHD with longer alkyl side-chains offered an efficiency of 10.74%.The PBDB-T:BTA-EHBO device exhibited better performance,attributed to its increased carrier mobility and reduced monomolecular recombination loss.By adding BTA-EHBO and BTA-HDHD,respectively,as the third component into the PBDB-TF:Y6 system,the ternary organic solar cell achieved PCEs of 14.00% and 14.18%,respectively.2.Eleven-ring fused small-molecules of IDTT-2EH and IDTT-3EH were synthesized by fusing 5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3-d:2’,3’-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene(IDTT)with 3,4-dichlorobenzene at the two ends,both through a pyrrole ring.Then on the pyrrolo-N sites,2-ethylhexyl(2-EH)and 3-ethylheptyl(3-EH)were decorated.IDTT-2EH and IDTT-3EH in dilute chloroform solution exhibited similar fluorescence lifetimes and photo-luminescence quantum yields(PLQYs).Solid powders of both molecules showed similar fluorescence lifetimes,1.07 ns and 1.00 ns,respectively.Interestingly,the solid PLQY of IDTT-3EH was 17.4%,which was more than twice higher than the solid PLQY(8.8%)of IDTT-2EH.This suggests that the branching sites on the ending pyrrolo-N side chains can effectively modulate the solid state PLQYs although the fused-ring small-molecules have been decorated with bulky orthogonal side chains.