Study On Fabrication And Stress Engineering Of High Performance Perovskite Solar Cells

In recent years,organic-inorganic hybrid halide perovskites have attracted widespread attention due to their remarkable optical and electronic properties,such as double carrier transport,defect tolerance and so on.Since the first report in the 2009,the power conversion efficiency(PCE)of perovskite solar cells(PSCs)has been improved rapidly from an initial 3.8%to 25.2%,suggesting its potential commercial application in the future.However,the stability problem is still the major problems for PSCs further development.Therefore,our works aim to the instability problem of high efficiency PSCs.We start from building high efficiency PSCs by solvent engineering and inorganic metal salt doping.Then in suit tracing the cells'performances and defect evolutions while the cells were under working situation.The interface stress accumulation caused by ionic migration is found to be the one origin of instability.Finally,by focusing on the stress engineering,the high efficiency and long stability PSCs were built.Our specific achievements are summarized as following:First,a sequential interdiffusion method which have high repeatability is developed.This method can control the perovskite conversion step by step,which is beneficial for the mechanism study of perovskite forming.The process of organic cation(like formamidine(FA))entering the PbI2 lattice was studied.DMF was found to be a useful additive in interdiffusion method,which can help open the PbI2 lattice and make organic cation diffusion easily.Finally,high efficiency PSCs have been built and as high as 20.1%PCE has been achieved.Our work offers a simple method to prepare high-quality perovskite films for high-performance PSCs and also helps further understand the perovskite crystallization process.Second,Cs-based triple-cation mixed-perovskite photovoltaic device was developed by introducing the CsI into the PbI2-DMSO precursors,followed by the sequential introduction of MA/FA cations with the interdiffusion method.The effects of Cs+ on the structure and properties of PbI2-DMSO precursor films have been well investigated,and a new triplecoordination intermediate phase among Pb2+,DMSO,and Cs+ was formed.The presence of this coordination phase efficiently retarded the crystallization of PbI2 in the precursor films resulting in perovskite film with fewer defects,larger grains,and a more uniform morphology.A PCE of 20.3%was achieved for the champion PSCs.This work provides a practical and effective method for high efficiency and reproducibility PSCs.Third,the defects were separated and quantitatively extracted by admittance spectrum through an extended equivalent circuit.The defect evolutions are traced in situ for the first time when the cells decay under illumination or voltage in order to reveal their impact on device performance and especially stability.We find that the electric field induced interface defect rather than the bulk defect is the major direct determinant factor for stability.Electric field will lead to ionic migration,which results in lattice strain increase in the interface.That causes interface lattice broken.Introducing fullerene derivative(PCBA)is an effective route to decrease interface defect.Because PCBA can suppress lattice strain and improve stability.Overall,unraveling the inherent correlation between the electric field,interface defects and cell stability has important implications for ongoing device stability engineering.Finally,polystyrene(PS),as a stress-buffer-layer,was introduced into the SnO2-based PSCs.The inherent correlation between stress and device performance was study deeply.Stress can cause the lattice transform,which increased the defect density and limited the carrier lifetime.Because of the low glass transition temperature,PS can stay soft glass state and release the residual stress in the perovskite during the film annealing,which can significantly reduce interface defect and improve cell performance and device stability.Furthermore,PS was also introduced as a capping layer on the top of the perovskite film to build the inner-encapsulation PSCs.These inner-encapsulated PSCs can improve long-term device stability by preventing moisture invasion.In the end,as high as a 21.89%PCE with a steady-state PCE of 21.5%has been achieved,and the cell can retain almost 97%of its initial efficiency after 5 days of "day cycle" stability testing.Our work offers a simple,repeatable method to enhance both the efficiency and stability of PSC.