Study On Defect Control Of FA_0.85MA_0.15PbI_xBr_3-x Perovskite Solar Cells

Since 2009,the photoelectric conversion efficiency（PCE）of perovskite solar cells（PSCs）has achieved from the initial 3.8%to the curent 25.5%in less than 12 years,which indicated that the PSCs displayed a bright application prospect.At present,the commercialization of perovskite solar cells are restricted by various factors,including the inefficiency,poor stability,the difficult preparation of large-area devices,high toxicity of lead,and so on.Among them,the defect control is one of the keys to improve the efficiency of PSCs.This article mainly explained the key scientific problem of defect control of perovskite photovoltaic devices,conducting the following researches:one is that the influence of annealing process on the quality of FA_0.85MA_0.15Pb I_xBr_3-x mixed ion organic-inorganic perovskite film;another is the research on MXene（Ti₃C₂T_x）quantum dot（TQD）/Sn O₂composite electron transport layer（ETL）.（1）Compared with MAPbI₃,the FA_0.85MA_0.15Pb I_xBr_3-x mixed ion perovskite has attracted attention in recent years due to its smaller band gap and better stability.Annealing is a key step in the process of preparation perovskite films by the two-step process.On the one hand,annealing process could increase the diffusion of lead iodide（Pb I₂）and amine salts,promote complete crystallization,and obtain high-quality perovskite films;On the other hand,the perovskite coud be decomposed during this process,forming excessive Pb I₂ and increasing defects.It would lead to increase the recombination probability of photogenerated charge,affecting the performance of devices.However,less studies regarded on the influence of annealing process on the quality of FA_0.85MA_0.15Pb I_xBr_3-x perovskite film.In this work,the annealing temperature and time were changed to effectively control the properties of perovskite films and PSCs.When the annealing temperature and time are lower than 150 ^oC for 15 min,with the increasing of annealing temperature and time,the（110）crystal plane XRD diffraction peak and the grain size of perovskite films were increased,resulting in the enhanced light absorption and the decreased defects.At the same time,part of the perovskite was decomposed to Pb I₂.When the annealing temperature and time was exceeded to 150^oC and 15 min,the decomposition of perovskite films became more serious,and some phenomenon have appeared such as the increase of defect density and the decrease of grain size and light absorption.When perovskite films annealed at 150 ^oC for 15 min,the defect density was decreased from 9.8×10²¹ m^-3 to 2.1×10²¹ m^-3,the average grain size was increased from 350 nm to 700 nm and the PCE was increased from 12.91%to 19.34%.Annealing reduce the number of perovskite nucleation and increase the perovskite decomposition to produce Pb I₂.The Pb I₂ had the following dual functions,on the one hand,a small number of Pb I₂ in perovskite film could passivate the defects;On the other hand,excessive Pb I₂ served as the recombination center of charge increased the defect density.Therefore,the suitable annealing process was conducive to formating a uniform,large grain size,and less defects of perovskite film,in-depth improving PCE of PSCs.（2）Compared to TiO₂,SnO₂ displayed the advantages of lower preparation temperature and higher mobility efficiency of electronic.Recent year,it has been widely used as ETL of PSCs.However,Sn O₂ had a lot of surface defects and undesirability energy levels,which restricted the further improvement of PSCs performance.In this paper,TQD were prepared and incorporated into the Sn O₂nanoparticle films for serving as composite ETL.By changing the concentration of TQD,the performance of ETL and perovskite films were adjusted.When TQD concentration is less than 0.2 mg/m L,with the increasing of TQD concentration,the dispersion of precursor solution meliorated,the conductivity of ETL improved,and the surface roughness increased,led the decreased defect density in the perovskite layer,thereby improving the PCE of PSCs,but the unreacted Pb I₂ increases,which affects the properties of perovskite films.When the TQD concentration was higher than 0.2 mg/m L,the surface roughness and defects of the perovskite layer and the unreacted Pb I₂ were increased,thus contributing to a decreasd the PCE of PSCs.Compared with Sn O₂,the optimized TQD@Sn O₂（0.2 mg/m L）increased surface roughness from 1.25 nm to 1.35 nm and improved conductivity from 0.03 S/m to 0.16 S/m.The surface roughness of the perovskite films was reduced from 33.7 nm to 26.3 nm,the defect density was decreased from 5.2×10²¹m^-3 to 0.64×10²¹ m^-3,and the PCE of the device was increased from 17.32%to 19.70%.By further optimizing the TQD doping process,more than 21%efficient devices were obtained.TQD was possesed with Ti-OH and Ti-F bonds in the activated Ti sites,which demonstrated better adsorption affinity to the metal(Pb²⁺)ions and resulted in that the Pb I₂ was accumulated around to TQD,further inducing the nucleation and growth of perovskite.Therefore,the appropriate concentration of TQD could improve the conductivity of Sn O₂layer,enhance the transport of charge carriers at Sn O₂/perovskite interface,decrease the surface roughness of perovskite film,in-depth reducing the density of defect states and improving the PCE of PSCs.