Research on Green-solvent Processing Ability and Stability of Polymer Solar Cells

Author:Xin Yue

Supervisor:yang xiao niu


Degree Year:2019





Given the increasingly serious problems of fossil energy depletion and environmental pollutions,polymer solar cells(PSCs),which convert the sunlight into electricity,featuring light weight,low cost,flexibility as well as solution processing,have obtained widespread attentions.High power conversion efficiency(PCEs),environmentally friendly solvent processing and long-term stability are the three preconditions for the industrialization of PSCs.The PCEs of bulk heterojunctions(BHJs)PSCs have met the application requirement.However,the halogenated solvent with high toxicity can only be used in the film-forming process,and the long-term stability of the device can not meet the application standard,which both seriously hinder the industrialization process of PSCs.Conjugated polymers are difficult to dissolve in non-halogenated solvents due to the rigid structures of their main chains.The morphology deterioration,caused by the accumulated heat from the long-term sunlight irradiation,will bring negative effect on the device efficiency.The crystallization and aggregation of molecules are the main driving forces for the destruction of the optimal morphologies for the active layers.In this work,on the one hand,we modified the backbone of conjugated polymers to improve their solubility in non-halogenated solvents and the resulted polymer based device outperformed its traditional analogues.On the other hand,we introduced non-fullerene small molecules as acceptor materials in PSCs,and studied the relationship between molecule structures and aggregation behaviors of the acceptors,which provides guidance for the development of PSCs suitable for industrial processing.The main innovations are as following:To realize non-halgenated solvent processing of PSCs,we designed and synthsised a novel region-random copolymer,named PIT,with D-π-A or D-A-π as repeat units to realize the non-halengated solvents processing of PSCs.As compared to the traditional polymers featuring D-A or D-π-A-π backbone,PIT exhibited superior solubility in non-halogenated solvent.The device based on PIT outperformed its traditional analogues with the highest performance of 8.53%when processed from non-halgenated solvents.Importantly,the optimal device performance was attained at the thickness of active layer over 200 nm and showed no batch-to-batch difference,implying its compatibility with printing technology.Our results demonstrated this simple chemical modification for copolymers may significantly benefit the development of PSCs using environmentally friendly solvents.To establish the relationship between molecular structure of small molecule acceptors(SMAs)and the thermal stability of PSCs,we synthesized ITIC-4F by introducing fluorine atom into the terminals of ITIC and studied the effect of the fluorine substitution of SMAs on the thermal stability of active layer.As compared to ITIC devices,devices based on ITIC-4F exhibited significantly higher PCEs,but its thermal stability was much worse(after annealing at 150℃ for 24 h,it was found that the PCEs of ITIC and ITIC-4F devices decreased by 4%and 39%respectively),indicating that the introduction of fluorine atoms into the terminal groups of n-SMAs is detrimental to the long-term thermal stability of the devices.To improve the thermal stability of devices based on ITIC,we changed the substitution position of fluorine atom on ITIC,introduced fluorine atom into the phenyl side chains of itic,and regulated the thermal stability of PSCs by changing the substitution position of fluorine atom on the phenyl side chains.The two side chain fluorinated SMAs were named oF-ITIC and mF-ITIC,respectively.When combined with PBTIBDTT,both oF-ITIC and mF-ITIC devices showed enhanced PCEs as compared to ITIC device.Moreover,among the acceptors,mF-ITIC had the best thermal stability,which can retain 92%of its initial device efficiency after thermal annealing for 96 h at 150℃,while ITIC and oF-ITIC devices only preserved 82%and 67%of their initial efficiencies,respectively.Investigation indicated that the miscibility of donor/acceptor and crystallinity of acceptors synergistically affected the thermal stability of active layers,which provided a new molecule design strategy to achieve high efficiency nonfullerene PSCs with outstanding thermal stability.