Design And Synthesis Of High-Performance Polymer Donors Based On 3-Cyanothiophene For Organic Solar Cells

Organic solar cells（OSCs）have broad commercial application prospects in the fields of wearable devices and building integration,on account of their advantages including low cost,light-weight,flexibility,and semitransparency.Donor polymers are crucial for high-efficiency OSCs.Nevertheless,high-performance donor polymers suffer from high production cost and batch-to-batch variation of donor polymers.This drawback greatly limits the up-scalability of polymer synthesis and the fabrication of large-area OSC modules.Therefore,it is imperative to develop high performance donors with simple structure for large-scale manufacturing of OSC modules.In this thesis,a series of high performance donors based on 3-cyanothiophene（CT）was developed,and the crucial correlation between molecular structures,aggregation characteristics,bulk-heterojunction morphology,and photovoltaic performance was revealed.The specific contents are as follows.1.A new building block,3-cyanothiophene（CT）,which features very simple asymmetric structure and facile synthesis was reported.The donor polymer（PBCT-2F）based on CT unit was developed to investigate the influence of CT on photovoltaic performance of PBCT-2F.PBCT-2F realized a higher open-circuit voltage(V_oc)and remarkable power conversion efficiency（PCE）of 17.1%,which is consistent with the down-shifting energy levels of frontier molecular orbitals caused by cynao group.More importantly,PBCT-2F exhibited excellent batch-to-batch reproducibility.The six polymer batches with the molecular weight ranging from18 to 74 k Da produced very similar PCEs:15.9–17.1%and 12.7–13.2%when the polymer blended with Y6 and IT-4F,respectively.And we have revealed that the similar aggregation degree in solution of the polymer aggregation degree and the similar BHJ morphology are the reasons for excellent batch-to batch reproducibility of PBCT-2F with different molecular weights.This feature may stem from the random structure of PBCT-2F due to the asymmetric nature of the CT unit.This work demonstrates the bright future of CT unit for constructing high-performance donor polymers.2.To improve the photovoltaic performance of the donors based on CT,two polymers were developed by copolymerizing the CT unit and benzo[1,2-b:4,5-b’]difuran（BDF）unit with powerfulπ-πinterchain interaction,and the relationship between the polymer aggregation behavior and the photovoltaic performance was investigated.For PFCT-2F,the self-aggregation of the polymer in solution occurred very quickly,which provided very short crystalline growth time,thereby inducing less developed polymer stacking ordering in the film.Benefitted from the larger steric hindrance of chlorine atom on BDF unit,PFCT-2Cl exhibited more adjustable aggregation,higherπ-πstacking ordering and appropriate miscibility with electron acceptor.Benefited from the decreased energy loss(E_loss)and optimal blend morphology,a high PCE of17.5%has been achieved by the OSC based on PFCT-2Cl,which is the new efficiency record for BDF-based polymers.This success proves the critical role of aggregation control on BDF-based polymers and bright future of constructing high-performance OSCs from bio-renewable sources.3.We reported a new polythiophene（PT）,which is abbreviated as P5TCN-2F and features cyano-group substituents for high-efficiency OSCs.The cyano-group endows P5TCN-2F with a deep-lying highest occupied molecular orbital energy level,which thereby contributed to high open-circuit voltage in OSCs as a result of reduced non-radiative recombination energy loss.Moreover,cyano-group has led to strong interchain interaction,improved polymer crystallinity,and appropriate miscibility with the prevailing non-fullerene acceptors.As a result,the OSC based on P5TCN-2F:Y6 yielded a high PCE of 16.1%due to the largely decreased E_loss and the formation of favorable bulk-heterojunction morphology.These results revealed the critical role of CT unit in PT-based OSCs.4.In chapter 5,we report a set of new PTs（P5TCN-Fx）with varied fluorination degree to achieve high-efficiency OSCs.The increasing backbone fluorination leads to strong interchain interaction,improved polymer crystallinity,and decreased thermodynamic miscibility with the prevailing acceptor Y6 for the new PTs.For P5TCN-F0,the hyper-solubility in chloroform caused liquid-liquid phase separation during spin coating of the P5TCN-F0:Y6 blend solution,which,in turn,causes large phase separation.In contrast,large phase separation has been formed for the blend of P5TCN-F50:Y6,which could be caused by the lowest thermodynamic miscibility between P5TCN-F50 and Y6 and the lowest polymer solubility.Favorable morphology with suitable phase separation and interpenetrating networks was thus formed for the blend based on P5TCN-F25 and P5TCN-F35 as a result of the moderate polymer solubility and appropriate thermodynamic miscibility.As a result,a prominent PCE of 17.2%has been obtained by P5TCN-F25,which is the new efficiency record and represents a big breakthrough for PT-based OSCs.This work reveals the key role of aggregation regulation in determining the photovoltaic performance of polythiophene.