The Study Of Defect Passivation In The CsPbI₂Br Solar Cells

Metal halide perovskite materials have advantages of high extinction coefficient,long carrier diffusion length,and solution-processable.Thus,they have been widely used in optoelectrical devices such as solar cells and light-emitting diodes in the past ten years.Among metal halide perovskite materials,CsPbI₂Br has suitable optical band gap and excellent thermal stability,which make it a promising material for top cells in tandem solar cells.Recently,solar cells based on CsPbI₂Br perovskite materials have achieved a power conversion efficiency of 17.46%.However,the voltage loss in the device is still significant because of the existence of abundant defects in the CsPbI₂Br film and at the interface/surface.Herein,the concentration of precursor solution was firstly optimized to obtain CsPbI₂Br films with sufficient thickness and high quality.Subsequently,silane modification and step-wise cooling were adopted to reduce the defect density in CsPbI₂Br films.The main outcomes of this thesis are listed as follows:(1)Effects of the concentration of precursor solution(0.6 M-1.1 M)on film thickness,morphology,crystallinity,and photovoltaic performance were studied.As for the CsPbI₂Br film obtained from 0.6 M solution,relatively small grains and pinholes were observed,along with a thickness of only 260 nm.Upon increasing the concentration of precursor solution,improvements in film thickness,grain size,compactness,and crystallinity were observed.However,increasing the concentration up to 1.1 M led to worse crystallinity.Finally,solar cells obtained from 1.0 M precursor solution exhibited the highest power conversion efficiency(8.78%).(2)A silane molecule-propyltrimethoxysilane(NPTMS)was used to modify the surface of titanium dioxide(TiO₂)and the surface of perovskite.Measurements indicated that the modification of TiO₂ with NPTMS resulted in worse film quality and retarded electron extraction at the TiO₂/CsPbI₂Br interface.In contrast,modification of CsPbI₂Br surface with NPTMS led to an increase in power conversion efficiency from 8.78% to 10.22%.A series of measurements suggested that the improvements originated from the interaction between O atoms in the silane molecules and Pb atom and Br atom on the surface of CsPbI₂Br,which resulted in the passivation of surface defects.(3)Generally,perovskite films will be quenched to ambient temperature after being annealed at high temperatures.Consequently,the obtained film may contain large amount of defects.Therefore,a step-wise cooling(SWC)process was proposed to replace the quenching(Que)process.In the SWC process,annealed films will be transferred onto a hot plate with a temperature lower than the annealing temperature for 1 min.Then,perovskite films will be quenched to ambient temperature.First,effects of the temperature on the device performance were investigated,and the best performance was obtained from SWC-140 °C devices.Further measurements suggested that defects in the film and at the TiO₂/ CsPbI₂Br interface can be partially healed during the heat holding stage on the second hotplate.As a result,the highest PCE of 12.59% was obtained via the step-wising cooling strategy,which is much higher than the traditional quenching one(11.42%).