Twin Domains in Organometallic Halide Perovskite Thin-Films

Abstract

The perovskite is a class of material with crystalline structure similar to CaTiO 3. In recent years, the organic-inorganic hybrid metallic halide perovskite has been widely investigated as a promising material for a new generation photovoltaic device, whose power conversion efficiency (PCE) record reaches 22.7%. One of its underlying morphological characteristics is the twin domain within those sub-micron sized crystal grains in perovskite thin films. This is important for discussion since it could be the key for understanding the fundamental mechanism of the device's high performance, such as long diffusion distance and low recombination rate. This review aims to summarize studies on twin domains in perovskite thin films, in order to figure out its importance, guide the current studies on mechanism, and design new devices. Firstly, we introduce the research history and characteristics of widely known twin domains in inorganic perovskite BaTiO 3. We then focus on the impact of the domain structure emerging in hybrid metallic halide perovskite thin films, including the observation and discussion on ferroelectricity/ferroelasity. The theoretical analysis is also presented in this review. Finally, we present a spectroscopic method, which can reveal the generality of twin domains within perovskite thin films. We anticipate that this summary on the structural and physical properties of organometallic halide perovskite will help to understand and improve the high-performance of photovoltaic devices.

Role of the Iodide–Methylammonium Interaction in the Ferroelectricity of CH3NH3PbI3

“…In fact, several recent studies have observed twin domains in CH 3 NH 3 PbI 3 crystals, both in thin films and bulk crystals. [29][30][31] Using the space group I4cm instead of I4/mcm during the crystal-structure refinement yielded a refinement that is very similar to the structures reported in the literature, without a clear shift of the relevant atoms (see the Supporting Information). However, two features are distinctly different to the refinements in I4/mcm: 1) the orientation of the molecular cation is less disordered than in previous studies [10,20,32] and only shows two distinct molecule directions with four orientations, as two N positions lie close to each other (Figure 2 a,b).…”

“…We did not, however, directly observe the breaking of Friedel's law, but this is most likely an effect of inversion twinning in the crystals under consideration, rather than a clear indication of centrosymmetry. In fact, several recent studies have observed twin domains in CH 3 NH 3 PbI 3 crystals, both in thin films and bulk crystals …”

Zur Rolle der Iodid‐Methylammonium‐Interaktion in der Ferroelektrizität in CH3NH3PbI3

“…Entsprechende Zwillingsdomänen wurden kürzlich sowohl an Dünnschichten als auch an Einkristallen nachgewiesen. [29][30][31] Durch Verwendung der Raumgruppe I4cm statt I4/mcm in der Kristallstrukturverfeinerung veränderte sich das Modell nicht signifikant, vor allem konnte keine klare Verschiebung der relevanten Atome beobachtet werden (siehe Hintergrundinformationen). Zwei Punkte sind allerdings im Vergleich zu den Verfeinerungen in I4/mcm verschieden: 1) die Molekülkation-Orientierung ist weniger stark fehlgeordnet als in vorherigen Studien [10,20,32] und besitzt nur zwei unterschiedliche Richtungen mit vier Orientierungen, da zwei N-Positionen nah beieinander liegen (Abbildung 2 a,b).…”

“…Allerdings haben wir in unserer Studie keine direkte Verletzung des Friedel'schen Gesetzes beobachten können, was allerdings eher eine Konsequenz der Verzwillingung der untersuchten Kristalle ist als ein direkter Nachweis von Zentrosymmetrie. Entsprechende Zwillingsdomänen wurden kürzlich sowohl an Dünnschichten als auch an Einkristallen nachgewiesen …”

Twinning in MAPbI3 at room temperature uncovered through Laue neutron diffraction

“…The crystal structure of methylammonium lead iodide (MAPbI 3 ), for instance, has been a matter of debate and several, partially contradicting studies since its introduction into solar cells, although MAPbI 3 clearly is the signature compound of this family of materials 5 – 11 . One striking discrepancy in the structural characterisation of MAPbI 3 is the question as to whether it is ferroelectric at room temperature or not 12 – 20 . The generally accepted space group of MAPbI 3 at room temperature is I 4/ mcm 5 , 7 , 9 , 11 , a centrosymmetric space group that would not allow bulk ferroelectricity or further non-linear effects.…”