Materials Today
ResearchMethodologies for structural investigations of organic lead halide perovskites
Graphical abstract
Introduction
Since the emergence of solid-state perovskite solar cells (PSCs) in 2012 (Fig. 1) [1], [2] following attempts which tried to utilize the perovskite materials as a sensitizer for liquid junction solar cells [3], [4], organic lead halide perovskites (OLHPs) have attracted enormous attention [5], [6], [7], [8] as the leader of the next generation photovoltaic materials. This great attention is based on the remarkable accomplishments in their solar cell application [9], [10], [11], [12], [13], [14], [15]. Consequently, the PSCs have achieved a certified power conversion efficiency (PCE) of 25.2% exceeding top records of some conventional solar cells [16]. The unprecedented achievements are generated from the superb characteristics of OLHPs [17], [18], [19], [20], [21], [22]. As the special properties of OLHPs originated from their unique crystallographic characteristics based on the hybrid of organic and inorganic components which are different from conventional perovskite materials, the structural investigation of OLHPs has been considered to be one of the most critical points for deep understanding of them. For this reason, there have been various structural investigations of OLHPs by using diverse analysis approaches such as X-ray diffraction (XRD) and transmission electron microscopy (TEM). As a result, various important structural information of OLHPs including phase transformation has been reported. Even though multiple structural investigations of OLHPs have been reported, details of their structural characteristics are still disputed due to various obstacles caused by their vulnerability during structural investigation. Therefore, advanced structural analysis of OLHPs is strongly needed.
In this article, the structural analysis approaches, which have been used for the understanding of OLHPs, will be reviewed from macro- to micro-scale. In the macro-scale structural investigation, the progress of XRD analysis, photoluminescence spectroscopy (PL), and optical spectroscopy is discussed based on the development of OLHPs. For the micro-scale structural investigation methods, atomic force microscopy (AFM) and electron microscopies are treated. Moreover, the limits in the electron microscopies caused by electron beam damage on the specimen are described to provide a careful and precise analysis.
Section snippets
Macro-scale structure analysis for OLHPs
Perovskite is the name of the crystal structure originating from calcium titanium oxide (CaTiO3). CaTiO3 consists of TiO6 and Ca which occupy octahedra and cubo-octahedral cavity, respectively. Based on CaTiO3, the perovskite structure refers to the crystal structure of the materials with an ABX3 formula such as SrTiO3 and BaTiO3. The structure of OLHP is also constructed as ABX3 stoichiometry, where A is an organic cation [e.g., CH3NH3+ (MA+)], B is a metal ion (e.g., Pb2+), and X is a halogen
Atomic force microscopy (AFM)
AFM is an analysis approach which visualizes the surface of the objects with the information induced by Van der Waals interaction between a cantilever of the AFM and the surface of the specimen. In the PSCs field, the AFM has been utilized for the investigation of the surface morphology [51], [58], [59] or roughness [54], [92], [93], [94] (Fig. 8a–e). The AFM has been used not only for the investigation of morphology but also for further investigation such as conductivity via its modification.
Conclusions and perspective
OLHPs have shown remarkable performances in photovoltaic field and have become one of the leading materials for next generation energy conversion technologies and opto-electronics. Interestingly, OLHPs have demonstrated somewhat unique characteristics different from the conventional semiconducting materials. As they are the materials constructed on the basis of the “perovskite” crystal structure by “hybridizing” the organic and inorganic components, the unique characteristics must be treated
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT Future Planning (MSIP) of Korea under contracts NRF-2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System), NRF-2016M3D1A1027663 and NRF-2016M3D1A1027664 (Future Materials Discovery Program), and NRF-2015M1A2A2053004 (Climate Change Management Program).
References (152)
Mater. Today
(2015)J. Phys. Chem. Solids
(1992)Nano Energy
(2016)Sci. Rep.
(2012)Science
(2012)J. Am. Chem. Soc.
(2009)Nanoscale
(2011)J. Phys. Chem. Lett.
(2013)Small
(2015)Chem. Rev.
(2019)