Preparation And Performance Of Electron Transport Layers To Inhibite Degradation Of Non-fullerene Acceptor Molecules

Over the past few decades,organic solar cells(OSCs)have attracted much attention due to their excellent intrinsic properties.These features include low cost,light weight,solution processability and mechanical flexibility.Recently,with the rapid development of non-fullerene acceptors(NFAs),the power conversion efficiency(PCE)of single-junction organic solar cells has reached more than 19%,which is getting closer to the requirement of commercialization.Interface engineering also plays an important role to improve the performance of OSCs.At present,there are many studies on the application of amine-based ETLs in non-fullerene OSCs.In this dissertation,the polyelectrolytes based on quaternary ammonium salt groups as ETLs in OSCs,the degradation of non-fullerene molecules induce tertiary amine groups and their inhibition methods were investigated.The details are as follows:We used triethylamine as the representative tertiary amine molecule to investigate the reaction between of tertiary amine and a representative non-fullerene acceptor ITIC.The results showed that triethylamine would lead to the degradation of ITIC.That is to say,the tertiary amine residuals in ETLs also lead to the degradation of non-fullerene acceptor molecules,which also results in a reduced performance in OSCs.Therefore,several kinds of organic acids with different alkyl chain lengths,such as acetic acid(AA),hexanoic acid(HA),hexadecanoic acid(PA)and eicosanoic acid(EA)were chosed to add into PEDETA-DBO during the preparation of ETLs.The organic acids reacted with the residual tertiary amine groups in ETLs to form organic acid ammonium salts,which would inhibite the degradation of non-fullerene acceptor molecules caused by tertiary amine groups.The results show that the ETLs present different surface energy when different organic acids are added,which affects the surface morphology of ETLs and the interface between ETLs and the active layer.Among them,the ETL PEDETA-DBO:PA prepared by adding hexadecanoic acid possesses the best performance.The inverted device prepared by using PEDETA-DBO:PA with a thickness of 30 nm as ETL and PM6:Y6 blend film as active layer achieved a PCE of 15.42%.However,the corresponding device prepared by PEDETA-DBO with the same thickness only presented a PCE of13.10%.The results indicate that the addition of organic acid can effectively shield tertiary amine groups and inhibit the degradation of non-fullerene acceptor molecules,which improves the thickness tolerance of ETLs and the photovoltaic performance of the OSCs.Hexahydroxyethyl melamine(HTHE)was synthesized from 2,4,6-trichloro-1,3,5-triazine and diethanolamine,and the HTHE was quaternized with1,8-dibromooctane(DBO)to prepare the ETL HTHE-DBO.The conventional OSCs were fabricated by using PM6:Y6 as the active layer as well as,HTHE-DBO and PEDETA-DBO as ETLs,and the influence of the structure of ETLs on the device performance was studied.The result shows that the OSCs based on HTHE-DBO achieved a high PCE of 17.23%,while the PEDETA-DBO-based OSCs only showd a PCE of 15.13%.In other words,HTHE-DBO possesses better performance than PEDETA-DBO,which are attributed to two cause.On the one hand,HTHE-DBO can effectively inhibit the degradation of non-fullerene acceptor molecules because the electron-deficient triazine ring bonded to the tertiary amine groups lead to less basicity and nucleophilicity than the alkyl tertiary amine.On the other hand,the triazine ring can further improve the electron mobility of ETLs.Moreover,the OSCs based on HTHE-DBO also showed better storage stability than the OSCs based on PEDETA-DBO.The device based on HTHE-DBO remained 84% of the PCE values after being stored in the glove box for 700 hours,while the device based on PEDETA-DBO only remained 78% of the PCE valuess after being stored for 480 hours.This study shows that the triazine ring can reduce the basicity and nucleophilicity of the tertiary amine groups,so HTHE-DBO can effectively inhibit the degradation of non-fullerene acceptor molecules and improve the device performance.Our work provides a new strategy to prepare high-performance electron transport materials for non-fullerene OSCs.