Interfacial Electronic Structures And Photoelectrical Modulation In Perovskite Solar Cells

In recent years,perovskite solar cell has become a rising star among all photovoltaic technologies,and has shown potential in challenging the current market leader,crystalline silicon photovoltaics in efficiency,economical large-scale fabrication and portability.However,the relatively low chemical stability is still slowing down their path toward the industry.The rich chemistry and physics at the perovskite surface and the derived device interface deeply control the charge generation,transport and collection process.At the same time,they also implement strong impacts on the device stability.At present,although researchers have good knowledge on the bulk property of perovskite materials,less is known about the evolution of surface/interface properties under extrinsic stress,which is critical in understanding the degradation behavior in long-term working devices.On the other hand,metal halide perovskite is a giant material family.Interfaces in many promising perovskites such as two-dimensional perovskite and all-inorganic perovskite,are very less studied and understood.Therefore,in the spirit of pushing perovskite photovoltaics to market,we focus on recognizing and modulating the interface in perovskite solar cells,and the main results are as follows:A series of in-situ study on the surface property evolution of perovskite films under oxygen and water vapor exposure were performed.Oxygen gas exposure induces an upward shift of vacuum level of the perovskite films,that is,a WF increase,because of the formation of an oxygen-induced surface dipole.Water vapor leads to downshift of vacuum level and the valence band binding energy referenced to the Fermi level simultaneously increases so as to keep the IP of the perovskite films unchanged.There was no saturating trend for the shifts for the exposure times used,which suggests a n-type doping behavior.These fast changes on energetics may contribute to the initial burn-in degradation of perovskite solar cells.Photo-induced degradation processes and mechanisms of three mixed cation perovskites were investigated in UHV condition.The experimental results demonstrate that the three perovskite films undergo chemical decomposition and the products escape from the substrate under continuous white-light illumination.It is attributed to the defect-induced trap states triggering strong coupling between photoexcited carriers and the crystal lattice.The substitution of A-site cations significantly modifies the tolerance factor which stables the crystal structure,and suppress the defect states formation to minimize the charge carriers getting trapped.Ultimately,the photostability of perovskites is improved from these two prospects.Thermal-induced degradation mechanism and process of mixed cation(Cs/FA/MA)mixed halide(I/Br)perovskite under 85?were systematically investigated.It is found the initial dangling bonds and vacancies on the imperfect surfaces decrease the activation energy and cause the perovskite decomposing in a layer-by-layer pathway sequentially from the film surface to bulk.The synergistic effect of organic cation MA⁺and halide anion I^-dominate the degradation,while Cs⁺,FA⁺and Br^-are unaffected under 85?.We have put forward a simple strategy to improve both efficiency and stability of perovskite solar cell by introducing an air-stabile n-type DMBI-doped PCBM as a modification interlayer.Compared to bare TiO₂/perovskite interface,inserting PCBM interlayer reduces the energy level offset with formation of a sequential potential step.Further n-doping of PCBM significantly diminishes the energy level offset to form an ideal electron transport channel,eliminating the potential energy loss at the contact.As a result,a remarkable promotion in PCE from 17.46%to 20.14%is noticed in corresponding devices,together with great improvement in stability.We systematically present experimental evidence and theoretical explanation for the energetics at 2D RPP/PCBM interfaces with n=1,3,5,40,and?.The surface PEAI ligands cause the potential across the ligands near the surface,resulting in the low surface work function and the band edge downshift,leading to formation of n–n junctions at the quasi-2D RPP(n=3,5,and 40)/PCBM interfaces.We demonstrate that the potential gradient promotes the separation of the photogenerated charge carriers with the electron transferring from the perovskite crystal side to the ligand side at the interface,while the holes are blocked.The ligands provide space between free electron-rich region(in PCBM)and free hole-rich region(in perovskite),which further decreases the probability of charge recombination at the 2D RPP/PCBM interfaces.These effects attribute to the smallest energy loss and the highest Voc in devices based on n=5perovskite with the poorest crystallinity.