Unique phonon modes of a CH3NH3PbBr3 hybrid perovskite film without the influence of defect structures: an attempt toward a novel THz-based application

Abstract

The exploration of new physical properties for various THz-based applications, such as THz-wave sensing, modulation, and imaging devices, is a key challenge in the research on organic–inorganic hybrid perovskite materials. These THz-based applications require satisfactory, sensitive, and stable absorption properties with values between 0.5 and 3 THz. To achieve these properties, candidate materials should possess a purified structure that induces regular and fixed phonon modes without any defects or impurities. CH3NH3PbBr3, an organic–inorganic hybrid perovskite thin film produced by a sequential vacuum evaporation method on a flexible PET substrate, was investigated in this study. Although the thin film contains only molecular defects related to CH3NH2 incorporated into the perovskite structure, our THz-wave absorption measurement and first-principles simulation confirmed that these molecular defects do not influence the three phonon modes originating from the transverse vibration (0.8 THz), the longitudinal optical vibrations (1.4 THz) of the Pb–Br–Pb bonds, and the optical Br vibration (2.0 THz). After spin-casting an ultrathin PTAA polymer protective layer (5 nm) on the hybrid perovskite thin film, it was additionally observed that there was no significant effect on the phonon modes. Thus, this novel flexible organic–inorganic hybrid perovskite material is a potential candidate for THz-based applications.

Terahertz Wave Absorption Property of all Mixed Organic–Inorganic Hybrid Perovskite Thin Film MA(Sn, Pb)(Br, I)3 Fabricated by Sequential Vacuum Evaporation Method

“…Consistently with the XRD results, it shows a degradation of perovskite structure after the 10-min annealing process. According to our previous studies, the 1.0 and 1.7 THz absorptions are assumed to originate from buckling and translation vibrations, respectively, ( Maeng et al, 2019 ; 2020a ). However, we did not observe any additional absorption such as defect-incorporated structure or Br self-vibration.…”

“…To protect the formed thin films against air and water, finally, spin coating was applied at 4 krpm for 1 min to coat all thin films with a PTAA (Sigma-Aldrich) solution containing 5 mg PTAA and 4 ml chlorobenzene. The PTAA solution was sonicated for 1 h ( Lee et al, 2019 ; Maeng et al, 2020a ).…”

“…( Ono and Qi, 2016 ). Also, the grain size does not improve significantly ( Lee et al, 2019 ; Maeng et al, 2020a , 2020b ).…”

“…To realize a THz-based application using OHP materials, two phonon modes such as molecule cation and metal cation–halogen anion vibrations in the THz range from 0.5 to 3 THz are primarily investigated ( La-O-Vorakiat et al, 2016 ; Zhao et al, 2017 ; Cinquanta et al, 2019 ; Lee et al, 2019 ; Maeng et al, 2020b , 2020a ). Further, the molecular motions (rotation and translation) of the CH 3 NH 3 + (MA + ) ion, which were confirmed by powder neutron diffraction, were found to be ordered, whereas the 2D and 3D disorder was dependent on each structure being controlled by temperature ( Weller et al, 2015 ).…”

“…Additionally, a high THz-wave absorption at 1.58 THz was observed in MAPbI 3 thin film fabricated by sequential vacuum evaporation (SVE) method, which incorporated CH 3 NH 2 molecular defects in the synthesized thin film ( Maeng et al, 2019 ; 2020b ). Our recent reports indicate that a vacuum-processed method such as SVE creates a greater number of variable atomic and chemical states than a solution-processed method because vacuum-processed methods produce smaller grains, a higher density of grain boundaries and defects ( Jung et al, 2018 ; Lee et al, 2019 ; Maeng et al, 2020a , 2020b ). Generally, the thin films formed by solution-processed methods such as 2-step and antisolvent methods are shown with two significant features such as large grain size (>500 nm) and low density of grain boundary that are induced with high carrier mobility in a solar-cell application ( Ono and Qi, 2016 , 2018 ).…”