Transmission Electron Microscopy For Organic-inorganic Hybrid Perovskite Solar Cell:Sample Preparation And Microstructural Study

Organic-inorganic hybrid perovskite solar cell has attracted much attention due to their unusual properties and great potentials in fabricating photovoltaic devices,and therefore has achieved tremendous progress in the past few years.Current research mostly focus on promoting its efficiency and stability,while little research is being conducted on its microstructure.The properties and performance of perovskite solar cell are closely related to their structure and microstructure,and Transmission electron microscopy?TEM?is ideal in the study of their microstructure.However,its application in the investigation of perovskite solar cell is still limited,mainly due to the fact that organic-inorganic hybrid perovskite materials are beam-sensitive and susceptible to electron/ion beam irradiation.In order to study the structure of perovskite solar cell devices by TEM,cross-section samples have to be prepared.Focused Ion Beam?FIB?remains the most efficient method in preparing TEM lamellae,although it exposes perovskite structure to ion irradiation,where the beam damage during FIB sample preparation remains unclear and hence requires further study.This thesis is dedicated to study of the degradation mechanism of organic-inorganic hybrid perovskite solar cell when exposed to ion beam.Combining FIB and Nanomill,series of cross section have been prepared and their microstructures have been investigated using TEM.Damage induced during sample preparation have been discussed in details,a degradation mechanism has been proposed,and an optimized procedure in preparing damage-reduced or damage-free TEM lamellae is suggested.The thesis are mainly focused on the following parts:1.Series of cross-sections have been prepared using FIB by varying ion beam conditions,including accelerating voltage and beam current.The microstructure and the damage induced by FIB sample preparation at different conditions has been investigated by TEM.We found out that:?1?Adapting beam current with regard to lamellae's thickness at different stages is crucial in controlling beam damage.A thickness of 250nm before switching from FIB milling to cleaning can significantly minimize beam damage.?2?Inappropriate FIB milling could induce unwanted damage into the sample.On one hand,voids can be generated in FIB lamellae.On the other hand,organic-inorganic hybrid perovskite can decompose into PbI₂ nanoparticles or even Pb nanoparticles.2.The removal of surface damage or contamination layer has been conducted using Nanomill.Microstructures at different stages of nanomill has been investigated by TEM to reveal the degradation mechanism when perovskite structure interacts with ion beam.We found out that:?1?Nanomill could effectively remove the residual damage from FIB milling and prepare a thin lamella suitable for High Resolution TEM?HRTEM?study.However,over-cleaning using Nanomill could induce severe damages including voids in the lamella as well by comparing Nanomill of different parameters.?2?HRTEM study of the lamellae reveals that the PbI₂ from the FIB-induced degradation could be removed by Nanomill,which means that PbI₂ stays as surface contamination layer.We therefore propose that the degradation mechanism of ion-sample interaction is essentially a“layer-by-layer degradation”.3.An optimized sample preparation procedure combining FIB and Nanomill is suggested.We demonstrated the application of this procedure on perovskite solar cell employing MAPbI₃,FAMAPbI₃,Cs⁺-doped FAMAPbI₃,and MAPbI₃ doped with CsPbBr₃ quantum dots,separately.TEM lamellae from these structures have been successfully prepared and microstructures have been studied.This research has important contributions to the TEM characterization of organic-inorganic hybrid perovskite solar cell.An optimized sample preparation procedure combining FIB and Nanomill to prepare a damage-minimized TEM lamella is proposed for the first time.Meanwhile,detailed TEM study at different stages of sample preparation reveals the degradation mechanism of perovskite solar cell materials interacted with ion beam.