Design And Stability Study Of Fullerene-based Electron Transport Layers For Perovskite Solar Cells

Perovskite solar cells(PSCs)evolved from dye-sensitized solar cells have been considered as one of the most popular research fields nowadays.Though huge enhancements have been achieved in increasing power conversion efficiency(PCE)from 3.8%to over 25%,the stability issues still limited the commercialization of PSCs.In this work,we fabricated the NiOx HTLs via sol-gel methods to replace the traditional poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS)HTLs,resulting in an inverted structure(p-i-n)of fluorine-doped tin oxide(FTO)/NiOx/perovskite/PC61BM/Ag,which possessed a champion PCE of?15%as well as high reproducibility.Based on the model devices,we subsequently focused on designing and optimizing fullerene-based ETLs,with the purpose of solving key problems in the p-i-n PSCs.Firstly,in order to restrict the UV-induced polymerization of fullerenes,we fabricated a PC61BM/CeOx double ETL,which exhibited a champion PCE of 17.35%and negligible degradation after light soaking for 1000 minutes under continuous AM 1.5 G illumination without UV filters in an ambient environment.Secondly,in order to retard thermal induced crystallization of fullerenes,we designed and incorporated a new fullerene derivative,[6,6]-phenyl-propylbenzene(PCPB),exhibiting a champion PCE of 18.48%and excellent thermal stability under continuous 85? thermal aging under N2 atmosphere.Based on computational calculations,the successful cultivation of co-crystals of PCPB and PC61BM demonstrated the strong intermolecular interactions between the two fullerenes for the first time,which could be the reason for improvement of PCE and thermal stability.Thirdly,in order to prevent charge accumulation within or on the surface of perovskite,we synthesized a fullerene multiadducted electron extractor,hexakis[di(ethoxycarbonyl)methano]-C60(HEMC),and fabricated the triple ETL.The multiadducted ester groups could form Lewis base-acid interactions with uncoordinated Pb2+of perovskite as well as ensuring enough solubility in common polar solvent for the PSCs such as dimethyl formamide or isopropanol.The introduction of HEMC strengthened charge extraction,resulting in limiting ion mitigation caused by charge accumulation,thus increased PCE to?20%as well as operational stability.Based on the experiences in designing fullerene electron transport materials(ETMs)and fabricating fullerene-based ETLs,we designed and synthesized series of electronic acceptors and characterized their crystal structures,electronic properties and device performance in order to summarize the rules for designing fullerene ETMs and components of mixed-fullerene ETLs.Though there still exists a long way to go,it is undeniable that the current experiences lay the foundation for designing better fullerene ETMs in the future.