A Ring-locking Strategy To Enhance The Chemical And Photochemical Stabilities Of Vinyl Bridges In A-D-A-type Non-fullerene Acceptors

Organic solar cells（OSCs）have made outstanding progress in recent years,with the certified power conversion efficiency（PCE）beyond 18%already.This is mainly attributed to the continuous innovation of photovoltaic material design and device structure,especially the successful development of A–D–A-type non-fullerene acceptors（NFAs）,in which the“D”and“A”are the electron-donating and electron-withdrawing units,respectively.Nevertheless,the high-performance NFAs synthesized by Knoevenagel condensation always exhibit strong“push-pull”effect,and thus the exocyclic vinyl bridges between“D”and“A”units have an obvious electron-deficient characteristic,making them easily undergo various chemical and photochemical reactions,which would destruct the conjugated structure of acceptor to degrade the device performance.Thus,it remains a critical challenge for enhancing the chemical and photochemical stabilities of vinyl bridges of A–D–A-type NFAs.In this dissertation,through in-depth understanding the degradation mechanism of NFAs,a novel ring-locking strategy has been successfully developed to enhance the chemical and photochemical stabilities of A–D–A-type NFAs at the molecular level.（1）Based on the Knoevenagel condensation catalyzed by Ti Cl₄—pyridine,two A–D–A-type NFAs,IDT-CR and IDTT-CR,were designed and synthesized by using cyclohexene ring-locked vinyl bridges and dicyanorhodanine end groups.Compared with the acceptors without ring-locked vinyl bridges,the cyclohexene-bridges not only increase the steric hindrance to prevent the nucleophiles attacking,but also lead to intramolecular hydrogen bonds with end groups which enhances the molecular planarity.Therefore,they show enhanced chemical and photochemical stabilities.However,the isomeric structure of rhodanine end group may cause an inefficient electron transfer.With P3HT as the donor,the optimized PCEs of IDT-CR-and IDTT-CR-based devices are 0.99%and 2.86%,respectively.Unfortunately,the intrinsically stable IDTT-CR has poor device stability due to the changed film morphology and increased phase separation scale under irradiation.（2）Furthermore,two new ring-locked acceptors,IDT-CT and IDTT-CT,were successfully synthesized by using structurally symmetrical thiobarbituric acid with higher electron-withdrawing ability as the“A”unit.Both acceptors possess single molecular conformation,reduced energy levels,and red-shifted absorption,as well as enhanced chemical and photochemical stabilities.Adopted PTB7-Th as the donor,the IDT-CT-and IDTT-CT-based devices deliver optimized PCEs of 4.62%and 6.13%,respectively.In particular,the IDTT-CT-based device shows outstanding device photostability,which can retain over 80%of its initial PCE in air after one sun irradiation without UV filter for 200h,far better than that of NFAs without ring-locked vinyl bridges.In addition,it also can retain about 70%of its initial PCE in dry nitrogen after heating at 85°C for 100 h.（3）In view of the better chemical and photochemical stabilities of endocyclic double bonds,two wide-bandgap A–D–A-type NFAs,namely EH-FPCN and O-CPCN,were further synthesized by using the Knoevenagel condensation catalyzed by Ti Cl₄—pyridine,in which the phenalene with largely and rigidly planar structure is used as the ring-locked bridge,while malononitrile as the“A”unit and fluorene/carbazole as the“D”units.The new acceptors exhibit good thermal,chemical,and photochemical stabilities in comparison with the NFAs without ring-locked bridges.However,they also have poor molecular planarity and the malononitrile end groups can not form an efficient electron transfer path,both of which cause low exciton diffusion and separation efficiencies.Thus adopted PTB7-Th as the donor,the EH-FPCN-and O-CPCN-based devices achieve optimized PCEs of0.91%and 1.62%,respectively.（4）To improve the photovoltaic performance of phenalene-locked acceptors,IDT-PR and IDTT-PR were then synthesized by using IDT/IDTT and dicyanorhodanine as“D”and“A”units,respectively.Although such modification red-shifts the absorption and reduce the bandgap,it simultaneously improves the reactive activity of phenalene and decreases the thermal,chemical,and photochemical stabilities.Single crystal structure analysis demonstrates that the poor molecular planarity of IDT-PR would reduce intramolecular charge transfer,while two stacking modes between end groups could be adverse to the electron transfer paths.Adopted PTB7-Th as the donor,the PCEs of IDT-PR-and IDTT-PR-based devices are 3.42%and 2.40%,respectively.Nevertheless,the IDT-PR-based device can still retain 72%of its initial PCE in dry nitrogen after irradiating for 200 h.