Design,Synthesis And Photovoltaic Performance Of Fused-Ring Electron Acceptors With New Donating Cores

Recently,the power conversion efficiencies(PCEs)of the single-junction organic solar cell(OSCs)were boosted over 18%,manily benefiting from the rapid progress of fused-ring electron acceptors(FREAs).However,the development of novel photovoltaic materials is still the key to the commercialization of OSCs.In this dissertation,several series of FREAs were developed with strategies including the terminal block modification,introduction of bridging unit and side-chain engineering.These strategies were employed to solve some drawbacks of FREAs,such as poor absorption in the visible and near-infrared regions,unbalanced electron and hole transport in the active layer,difficulty in purification and synthesis,and poor film morphology.Based on these FREAs,their optical and electrochemical properties and photovoltaic performances were systematically studied.The correlation between the molecular structure of acceptor materials and photovolatic performance has been further explored.The main contents are summarized as follow:1.An electron-donating core 6,12-dihydropyrene[1,2-b:6,7-b']dithiophene(DPT)was synthesized through modulation of molecular backbones on the existing FREAs.Based on DPT core,NSIC,NSIC-2F and NSIC-4F were developed with different electron-withdrawing units as the end-groups.Fluorination of the terminal unit can effectively increase the intermolecular charge transfer(ICT)of acceptor molecules,thus leading to higher charge mobility in the corresponding active layer.Owing to unbalanced electron and hole transport in the active layer,NSIC-2F and NSIC-4F based decices show lower PCEs.To further improve the ability to capture photons in the near-infrared region,NST-4F,NSTO-4F and NSTDO-4F were synthesized by introducing different bridging units into NSIC-4F.NSTO-4F features the alkoxy substituted thiophene bridge,and presents a more red-shifted absorption spectrum with absorption edge extending to 897 nm.The short circuit current J_sc of NSTO-4F based device is enhanced to 17.58 m A cm^-2,and the PCE reaches 7.35%.The introduction of 3,4-ethylenedioxythiophene bridging unit can not only increase the LUMO energy level of NSTDO-4F,but also induce the aggregation of NSTDO-4F molecules.Consequently,the open circuit voltage(V_oc)of NSTDO-4F based device is up to 0.92 V,and its PCE is improved to8.35%.2.A novel central building block 6,12-dihydropyrene[1,2-b:6,7-b']dithieno[3,2-b]thiophene(DPTT)was synthesized through extending the conjugation of the DPT core.The electron acceptor NTTC6-4F end-capped with 2F-IC has been constructed,and the PCE of NTTC6-4F based device was only 5.60%due to its too strong crystallinity.To increase the solubility of NTTC6-4F,two derivative acceptors NOC6TT-4F and NTTm OC6-4F were developed by introducing alkoxy groups into the backbone and side chains of the DPTT units,respectively.By the introduction of alkoxy group into the backbone,NOC6TT-4F exhibits a stronger near-infrared absorption with absorption edge extending to 848 nm,and higher LUMO energy level.Owing to better compatibility of NOC6TT-4F and the donor material,the corresponding active layer achieves a smooth surface with moderate phase separation,which is benefical for exciton dissociation and reducing recombination loss.The NOC6TT-4F based device shows outstanding photovoltaic performance,and the PCE is enhanced to 10.38%.3.A rigid fused-ring core indacenobis(dithieno[3,2-b:2?,3?-d]pyrrol)(INP)was developed based on previous work.Three narrow-band-gap acceptors end-capped with different terminal units,IPIC,IPIC-4F and IPIC-4Cl have been rationally designed.The influence of terminal block modification is systematically investigated.Compared with its counterparts,IPIC-4Cl exhibits enhanced ICT effect and obtains a narrower optical bandgap.Meanwhile,PBDB-T:IPIC-4Cl blend possesses efficient exciton dissociation and charge collection.Consequently,the OSCs constructed by PBDB-T:IPIC-4Cl obtain a champion power conversion efficiency(PCE)of 13.4%with an extremely low energy loss of 0.51 e V.More encouragingly,we achieve a higher photovoltaic performance of 14.3%for ternary solar cells by combining an optimal amount of PC₇₁BM with PBDB-T:IPIC-4Cl blend.This efficiency was also the highest performance value of device based on chlorinated FREAs at that time.4.Different series of side-chain molecular engineering were performed on the basis of INP core:three acceptors INPC8-4Cl,INPC12-4Cl and INPC4C8-4Cl were synthesized by altering the alkyl chains on dithiophenopyrrole(DTP)unit.Alkyl chain modification on DTP unit has little impact on optical and electrochemical properties,but has great influence in charge mobility and film morphology.Longer or branched alkyl chains can balance the solubility and crystallinity of acceptor materials,especially the steric hindrance effect of branched alkyl chains can effectively inhibit the excessive aggregation of molecules.INPC12-4Cl and INPC4C8-4Cl exhibit more balanced charge mobility and appropriate phase separation,and these devices obtain the PCEs of 11.14%and 12.12%,respectively;Seveal novel FREAs MF-IC,MF-4F,MF48-IC and MF48-4F were developed with fluorine atom substituting in the meta-alkoxyphenyl side-chains.The effect of fluorinated side-chains on the photovoltaic performance and active layer morphology was systematically evaluated.Consequently,the PCE of PBDB-T:MF-4F devices is 12.32%,while that of PBDB-T:MF48-4F enhancd to 13.20%.The study of the active layer morphology reveals that the introduction of fluorine atoms into the side-chain can enhance intermolecular interaction,and regulate aggregation behavior,thus facilitate charge transfer.