Atomic Force Microscopy Study of Cross-Sections of Perovskite Layers

Д М Мигунов ,D Gudkov,A M Koz’min ,Inga Ermanova ,Danila Saranin ,K B Eidelman ,А В Алексеев

doi:10.18321/ectj795

Abstract

Improvement of methods for imaging of the volume structure of photoactive layers is one of the important directions towards development of highly efficient solar cells. In particular, volume structure of photoactive layer has critical influence on perovskite solar cell performance and life time. In this study, a perovskite photoactive layer cross-section was prepared by using Focused Ion Beam (FIB) and imaged by Atomic Force Microscopy (AFM) methods. The proposed approach allows using advances of AFM for imaging structure of perovskites in volume. Two different types of perovskite layers was investigated: FAPbBr3 and MAPbBr3. The heterogeneous structure inside film, which consist of large crystals penetrating the film as well as small particles with sizes of several tens nanometers, is typical for FAPbBr3. The ordered nanocrystalline structure with nanocrystals oriented at 45 degree to film surface is observed in MAPbBr3. An optimized sample preparation route, which includes FIB surface polishing by low energy Ga ions at the angles around 10 degree to surface plane, is described and optimal parameters of surface treatment are discussed. Use of AFM phase contrast method provides high contrast imaging of perovskite structure due to strong dependence of phase shift of oscillating probe on materials properties. The described method of imaging can be used for controllable tuning of perovskite structure by changes of the sample preparation routes.

Highlights

Number of papers related to development of organic-inorganic perovskite solar cells (PSC) has been increasing in geometric progression since 2009 [1]
Material FAPbBr3 with its band gap of 2.23 eV is a promising candidate for using it in tandem solar cells
It was found that additional treatment of surface with cross-section of perovskite layer by low energy Ga ions (2 kV) at angles around or less than 10 degree is necessary to obtain not modified structure

Summary

Introduction

Number of papers related to development of organic-inorganic perovskite solar cells (PSC) has been increasing in geometric progression since 2009 [1]. The highest thermal instability is observed in most promising cation MA Perovskites with this cation are crystallized at temperature 100 °C and under working conditions of PSC are decomposed into initial precursors MAI – PbI2 at temperatures 60‒80 °C. The most widespread method used for imaging PSC cross-section is mechanical cleavage and following SEM imaging of obtained cross-section [9‒10]. We describe and apply another combination of methods for imaging of perovskite cross-section, which is based on the use of FIB milling for cross-section preparation and following AFM measurements of it. This method was used for investigations of polymer solar cells [12]. In this article the sample preparation route is described and two different perovskite samples FAPbBr3В and MAPbBr3 were imaged by the proposed method

Material synthesis

Characterization

Results and discussion

Conclusions