Research On The Interface Stability Of Nonfullerene Organic Solar Cells

Organic solar cells have attracted much attention because of their advantages of light weight,good mechanical flexibility,and solution-processed.At present,the power conversion efficiency of nonfullerene organic solar cells has exceeded 18%,showing great industrial application prospects.In addition to efficiency,the stability of organic solar cells is also a key factor that determines their industrial applications.So far,the research on the stability of nonfullerene organic solar cells is still in infancy.Understanding the degradation mechanism and finding strategies to improve the stability of the devices are the focuses of research.The chemical and photochemical interaction between the interface layers and the nonfullerene acceptors are important factors leading to the degradation of organic solar cells.This thesis focuses on the influence of interfacial reactions on the stability of nonfullerene organic solar cells,including interfacial chemical stability and interfacial illumination stability.The main contents are as follows:(1)Explore the influence of the structure of nonfullerene acceptors on interfacial chemical stability.It is found that terminal fluorine-substitution would increase the chemical reactivity and reduce interfacial chemical stability.Device with fluorine-substituted nonfullerene acceptor shows more severe“S”shape in the current density-voltage(J-V)characteristics under illumination when polyethylenimine(PEI)is used as the interfacial layer and larger efficiency difference comparing with the reference device with ZnO as interfacial layer.Furthermore,the fluorine-substituted nonfullerene acceptor shows higher reaction rate with monoethanolamine model compound.Mass spectrometry on the products of the nonfullerene acceptors added with monoethanolamine indicates that the reaction is more complete for the nonfullerene acceptors with fluorine substitution.In addition to the acceptor moiety,the donor moiety also affects the interfacial chemical stability of the nonfullerene acceptors.For example,the high-performance nonfullerene acceptor Y6 with terminal fluorine-substitution shows good chemical stability with amines.(2)The photocatalytic activity of ZnO on decomposition of nonfullerene acceptors is discovered,and Sn O₂ was used to replace ZnO to achieve better interfacial illumination stability of the devices.ZnO is the commonly used electron transporting materials in inverted nonfullerene organic solar cells.Here,it is reported that nonfullerene acceptors tend to decompose by ZnO due to its photocatalytic activity under UV illumination.That leads to poor device stability of nonfullerene solar cells under AM1.5(air mass 1.5)solar light illumination.Sn O₂ is used as the electron transporting layer that has wide band gap and is almost irresponsive to solar spectrum,which can inhibit the photocatalytic decomposition of nonfullerene acceptors.Nonfullerene solar cells with Sn O₂(including Sn O₂ nanocrystal and modified aqueous Sn O₂)show better device interfacial illumination stability than the reference cells based on ZnO.(3)Further explore the mechanism of ZnO catalytic decomposition of nonfullerene acceptors,and modify ZnO by aqueous polyethylenimine(a-PEI)to improve the stability of nonfullerene organic solar cells under AM1.5 illumination.After 1000-hour AM1.5illumination,efficiency of the cell without a-PEI modification degrades to 43%of its initial value.For the cell with a-PEI modification,the efficiency could maintain 75%after such illumination.The a-PEI modification can reduce the number of surface defects as suggested by different measurements:reduced adsorbed oxygen on ZnO surface;faster work function recovery kinetics after UV irradiation;and suppressed defect peak of ZnO(oxygen vacancies and interstitial zinc).The reduction of ZnO surface defects would inhibit the catalytic photodecomposition of nonfullerene acceptors,thereby improving the interfacial illumination stability of the device.