The Study Of Constructing Novel Conjugated Polymer Donors And Non-fullerene Acceptors Via Main Chain And Side Chain Modification Strategies

Organic photovoltaic cells(OPVs)have attracted many attentions and showed huge potential of applications due to their unique advantages of lightweight,flexibility and capability of low-cost mass-fabricated by roll to roll printing technique.In recent year,with the rapid development of non-fullerene acceptors(NFAs),the power conversion efficiencies(PCEs)of the non-fullerene OPVs have been greatly improved,and a PCE of over 18% has been reported.Generally,the active layer of most high-efficiency OPV is composed of a narrow bandgap(NBG)NFA and a wide bandgap(WBG)donor polymer,due to their complementary absorption,matched energy level and suitable morphology.Therefore,developing new kinds of wide bandgap polymer donors and narrow bandgap NFAs will continue to be the driving force to promote the PCEs of OPVs.In the thesis,a variety of the conjugated polymer donors and NFAs were designed and synthesized based on the main chain and side chain modification strategies.And the relationship between the molecular structure and its photovoltaic performance were systematically studied,which would guide the molecular design of active layer material toward highly efficient OPVs.In chapter 2 and 3,the relationships between the chemical structures and photovoltaic performance of the dithienobenzoxadiazole based polymer were studied.A WBG polymer donor PBODT based on dithienobenzoxadiazole-quarterthiophene(DTf BO-4T)copolymer was designed and synthesized based on the side chain modification strategy.Polymer PBODT have a low-lying HOMO level of-5.44 e V and a wide optical bandgap of 1.88 e V.When using NFAs ITIC and IDIC to construct the OPV devices,the power conversion efficiencies(PCEs)of 7.06% and 9.09% can be achieved,respectively.Moreover,the chemical structure of PBODT was further optimized using main chain modification strategy.Three new WBG polymer PBOff DT,PBOTT and PBOTVT by altering the copolymerization unit.The influence of noncovalent and covalent backbone rigidification strategies on the photoelectric properties of the polymer were studied.Using IT-4Cl as the acceptor to construct the OPV devices,the PCEs of 10.56%,8.25% and 3.12% can be achieved for the PBOff DT,PBOTT,and PBOTVT based devices,respectively.Benefit from the deepest HOMO energy level of PBOff DT and proper compatibility with IT-4Cl as well as favorable morphology,the efficient exciton dissociation and suppressed charge recombination can be achieved for the PBOff DT:IT-4Cl blend,leading to the superior PCE.The result suggested that the photovoltaic performance of the DTf BO based donor polymer could be promoted by rational molecular design and paring with suitable acceptors.It also showed the great potential of DTf BO based donor polymers in non-fullerene OPV devices.In chapter 4 and 5,the relationships between the chemical structures and photovoltaic performance of the fluorobenzotriazole(FTAZ)based polymer were studied.The siloxaneterminated side chains were attached to fluorobenzotriazole-benzodithiophene(FTAZ-BDTT)based polymers and three polymers PBZ-1Si,PBZ-2Si,and PBZ-3Si with different numbers of siloxane-terminated side chains were synthesized.The number of siloxane-terminated side chains has greater impact on the polymer aggregation property and packing behaviors and the interaction with NFA.When using IT-M as the acceptor,PBZ-2Si based device achieved the highest PCE(11.14%),mainly due to the strong crystallinity of the acceptor in the blended film and the appropriate phase separation size and higher domain purity.Relatively,the devices based on PBZ-1Si and PBZ-3Si showed lower PCE of 8.98% and 9.92%,respectively.In addition,the PBZ-2Si:IT-M blend could exhibit good thickness tolerance and excellent shelf stability.Furthermore,we designed and synthesized four polymers PBZ2Si-C4,PBZ2Si-C5,PBZ2Si-C6 and PBZ2Si-C7 with siloxane-terminated side chain branching positions at the fourth,fifth,sixth,and seventh carbon,respectively.The branching position has minor impact on the thermal,electrochemical,and absorption properties of polymers.When i-IEICO-4F was employed as the acceptor to construct photovoltaic devices,the PCEs of 13.47%,13.38%,13.10% and 12.75% can be obtained,respectively.Compared to the varied branched position of alkyl chains,the siloxane-terminated side chains show weaker impact of branching points on the photovoltaic performance.The result suggests that fine-tuning the branching positions of a siloxane-terminated side chain was still helpful in optimizing photovoltaic performance of the resulted polymers.In chapter 6,we incorporated different thiophene π-bridge units to IDTO central unit via the C-H activation cross-coupling reaction,and difluorodicyano indanone(2F-IC)unit was utilized as the end-capped unit to construct novel A-π-D-π-A type NBG NFAs.Five small molecular acceptors 4O4F-C6,4O4F-EH,i-4O4 F,in-2O2 A and out-2O2 A were obtained.The influence including the length of alkoxy chain of the thiophene π-bridge unit(C6,EH),the connection position of the end group(?,?),the type of thiophene side chain(alkyl,alkoxy)and the alkyl chain position(inner,outer)on the photoelectric properties were systematically investigated.Using PBDB-T as the donor,the PCEs of 5.45%,5.73%,11.45%,9.66% and 10.71% were achieved,respectively.The result indicated that tuning the π-bridge units of A-π-D-π-A type NFAs would be an effective way to promote the photovoltaic performance to the resultant NFAs.In chapter 7,the vinyl bifurcated side chains were used to modify the central D units of NFAs and six NFAs containing the vinyl bifurcated side chains namely IVTC,IVTC-4F,FVTC,FVTC-4F,FVTPC and FVTPC-4F were synthesized.The vinyl bifurcated side chains on the D unit could up-shift energy level,broaden absorption range,and induce H-aggregation of molecular of the resulting acceptors.Using PBDB-T as the donor,the PCE of 1.09%,1.55%,7.14%,5.58%,4.81% and 3.21% were achieved,respectively.Therefore,it is still a challenge for the vinyl bifurcated side chains contained NFA to obtain satisfied photovoltaic performance.The chemical structure of relevant materials need to be further optimized.