Studies On The Interfacial Structures In Perovskite Solar Cells By In-situ Photoemission Spectroscopies

CH3NH3PbX3(X=Cl,Br,I)based perovskite solar cells(PVSCs)have recently attracted plenty of attention due to their good properties,such as high absorption coefficient,long carrier diffusion length and low exciton binding energy.Up to now,the performance of the PVSCs has been greatly improved.However,the low stability issues of PVSCs are still the largest problems restricting their commercialization.Apart from the low stability of PVSCs caused by the external environment,some interfaces inside PVSCs also greatly affect its stability.These include the interface between the electrode and perovskite,the interface between the transport layer and the electrode,the interface between transport layer and perovskite in the PVSCs device.However,the corresponding studies into the properties of these interfaces have still been limited.Understanding the interfacial structures can not only help us in solving the interface stability issues,it can also help us to guide the selection of interface layer matching the energy level in the device to improve the power conversion efficiency(PCE)of devices.Hence,we adopted in situ photoemission spectroscopy(PES)(including synchrotron radiation photoemission spectroscopy(SRPES)and X-ray photoemission spectroscopy(XPS))to study the main interfaces in the PVSCs.Since metal electrodes often react with perovskite and destroy its structure,hence a comprehensive understanding of specific reaction information is necessary to guide the selection of insertion layer for improving the stability of PVSCs.Moreover,we can find a perovskite-friendly electrode by studying the properties of the electrode/perovskite interface.This is very helpful to optimize the electronic energy level structure in the PVSCs device by inserting a suitable transport layer.This thesis mainly includes the following three parts.1.An in-situ PES investigation of the interfacial structure between perovskite and Al electrode in ultrahigh vacuum was carried out.It was found that Al electrode can react with the perovskite layers,leading to the formation of aluminum iodide species and the bonding between Al and N,as well as the reduction of Pb2+ ions to metallic Pb species at the interface.For a deeper understanding of reaction at the Al/CH3NH3PbI3 interface,interface elemental depth distribution of the sample was investigated by SRPES with different photon energies.It was found that iodide ions can migrate from the CH3NH3PbI3 subsurface to the Al electrode.It reveals that the reaction depth between Al and CH3NH3PbI3 should be larger than 3.5 nm.In addition,the energy level alignment of Al/CH3NH3PbI3 interface was obtained,it was found that threre are 0.3 eV electron transport barrier and 1.3 eV hole transport barrier at the interface.To prevent the reaction at the Al/CH3NH3PbI3 interface,MoOx was chosen as transport layer inserted between Al and CH3NH3PbI3.The MoOx layer was obtained by thermal deposition.It was found that 40 A thick MoOx could effectively prevent the reaction between Al and CH3NH3PbI3.Moreover,the matching of electron energy levels at the interface is also very suitable.Due to the broad band gap of MoOx,its valence band maximum(VBM)is far lower than the VBM of CH3NH3PbI3,thus,forming a large hole potential barrier to prevent the hole from transferring to Al.The conduction band minimum(CBM)of MoOx is 0.1 eV lower than the CBM of CH3NH3PbI3,which can enhance the transport of electron from CH3NH3PbI3 to MoOx.At the MoOx/Al interface,the CBM of MoOx is 0.2 eV higher than the Fermi level of Al,which is suitable for electron transmission to the electrode Al.2.The interfacial structure of Cu/CH3NH3PbI3 was investigated by PES measurements.The results reveal that no chemical reaction occurs between Cu and CH3NH3PbI3.A time-dependent XPS,further provides a direct evidence of the high stability of Cu/CH3NH3PbI3 interface.Furthermore,0.15 eV upward band bending and 0.45 eV interfacial dipole were observed at Cu/CH3NH3PbI3 interface.Even though Cu is in direct contact with the CH3NH3PbI3 film,the PCE of the proto-device(Cu/CH3NH3PbI3/NiOx/ITO)equals or even exceeds the initial PCE(9.99%)after 49 days.This is attributed to the superb stability of the Cu/CH3NH3PbI3 interface.To adjust the interfacial structure of Cu/CH3NH3PbI3,bisfulleropyrrolidinium tris(methoxyethoxy)phenyl iodide(Bis-FIMG)was used as electron transport layer between Cu and CH3NH3PbI3.It was found that the insertion of Bis-FIMG can promote the extraction of electron and reduce the carrier recombination at the interface.3.The interfacial structures of the interfaces between Bis-FIMG and the common metal electrodes(Al,Ag and Au)were studied via PES method.These three metal electrodes were deposited on Bis-FIMG surface by thermal evaporation,respectively.The investigation of the Al/Bis-FIMG interface reveals that Al not only reacts with I but also with O from the Bis-FIMG.An upward band bending of 0.3 eV at the Al/Bis-FIMG interface was found.The interfacial energy level alignment proves that the contact between Al and Bis-FIMG would increase the electron extraction barriers.In situ PES studies on the Ag/Bis-FIMG interface show that Ag also interacts with I from Bis-FIMG.However,the electron energy level alignment of the Ag/Bis-FIMG interface proves that only 0.1 eV electron transport barrier and 0.1 eV interfacial dipole appear at the interface,which is suitable for the transport of electron from Bis-FIMG to Ag.The study on Au/Bis-FIMG interface proves that even Au with relatively high chemical stability would also change the chemical state of I from Bis-FIMG.About 0.4 eV band bending,0.45 eV electron transport barrier and 0.7 eV interfacial dipole were found at the Au/Bis-FIMG interface,which blocks the transport of electron form Bis-FIMG to Au.