Preparation Of Large-Area High-Efficiency Formamidine-Based Perovskite Solar Cells By A Vapor-Solid Reaction Method

In recent years,the efficiency of perovskite solar cells(PSCs)has grown rapidly,from 3.8%in 2009 to more than 25%in 2020,and its stability has also been greatly improved.However,most of the existing preparation methods of PSC are spin coating processes.Although spin coating can achieve high efficiency,it is unsuitable for large-scale preparation due to the existence of edge effects and the serious waste of materials.The vapor-solid reaction method is one of the mature processes that can be prepared in a large area.So far,the efficiency of devices prepared using this method has reached about 20%.In this thesis,we have studied how to prepare high-efficiency formamidinium-based PSCs using the two-step vapor-solid reaction method.Main contents are as follows.We first studied how to prepare amorphous precursor films.The key step in the preparation of formamidinium-based PSC via vapor-solid reaction is to prepare an amorphous lead halide precursor film.In order to stabilize the black phase formamidinium-based perovskite,we pre-doped Cs I into the precursor film.However,if only Cs I was added to the precursor film,it is difficult for FAI to penetrate the lead halide film,making the reaction at the film bottom incomplete.To overcome this problem,we simultaneously added Cs I-DMSO,FAPb Br₃,and MACl to the Pb I₂-DMI-DMF precursor solution.Cs I successfully suppressed the appearance of yellow phase FAPb I₃ in the as-prepared perovskite films,FAPb Br₃ increased the reaction rate of FAI and Pb I₂.Adding suitable amounts of MACl further accelerated the reaction rate.We used XRD and SEM to track the reaction process,and fixed the amounts of additives to be 140?L and 80?L for FAPb Br₃-DMF and Cs I-DMSO,respectively.The crystal grains of the perovskite thin film obtained via vapor-solid reaction could penetrate vertically,and no Pb I₂ remained.After that,the precursor films prepared by the above method was used for the optimization of the vapor-solid reaction process(vacuum degree,reaction time,FAI dosage,etc.).It was found that the reaction of precursor films could be completed in 70minutes under a vacuum of 500 Pa.According to the evolution of texture and morphology of the film,the vapor-solid reaction process was divided into three steps:diffusion,reaction,and crystallization.We found that slowly heating the upper substrate during the diffusion stage,slowing the sublimation and deposition rates of FAI,can prevent excessive FAI from accumulating on the surface of the film,thereby avoiding subsequent incomplete reactions.Meanwhile,the crystallization stage also requires the participation of enough FAI to obtain vertically monolithic perovskite grains.Based on these results,we optimized the hole transport layer and the electron transport layer.Finally,a PSC with an area of 0.1125 cm~2 and an efficiency of 20.15%was prepared.The best efficiency of PSC with a 1 cm~2 area was 20.27%.The device efficiency also reached 19%based on a similar blade coating-vapor process.Passivating the defects in perovskite materials can effectively improve the efficiency and stability of the device.Therefore,we tried to passivate defects in the above-mentioned perovskite film using a variety of passivation materials and methods,including solution engineering,surface modification,and evaporation.However,by comparing the device efficiency(short circuit current density*open circuit voltage*fill factor)between reference and the passivated device,we found that although some materials can slightly increase the open circuit voltage,from the perspective of device efficiency,these films prepared via the vapor process is hard to be passivated using normal solution processes.On the other hand,vapor deposition is more suitable for passivating perovskite films prepared via VSR.