THz‐Wave Absorption Properties of Organic–Inorganic Hybrid Perovskite Materials: A New Candidate for THz Sensors

Over the past two decades, organic–inorganic hybrid perovskite materials (OHP) have been extensively explored across various scientific disciplines, including physics, chemistry, and materials science, with a primary focus on solar cells. Building on numerous studies, the development of OHP‐based solar cells has transitioned into practical product realization, instilling the anticipation of novel solar cell advancements. Notably, OHP demonstrates versatility beyond its conventional application in solar cell materials. The physical properties of OHP materials exhibit a unique signature, thereby underscoring their potential utility as innovative functional materials, encompassing light‐emitting diodes, lasers, and photodetectors. Recent reports on terahertz (THz)‐wave absorption properties of OHP materials indicate a high possibility of their potential application as THz sensors. From the viewpoint of medical devices, which hold the most promising application potential, the exploration of optical phonon vibrational modes in the 0.5–3 THz frequency range is important. Moreover, understanding the correlations between atomic structure and lattice vibration modes is indispensable. In this concise review, the THz‐wave absorption properties exhibited by 3D OHP materials are meticulously explored. Furthermore, future research directions for THz sensors using OHP materials are suggested.


I. Maeng YUHS-KRIBB Medical Convergence Research Institute College of Medicine Yonsei University Seoul 03722, Republic of Korea
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smsc.202300186.
Over the past two decades, organic-inorganic hybrid perovskite materials (OHP) have been extensively explored across various scientific disciplines, including physics, chemistry, and materials science, with a primary focus on solar cells.Building on numerous studies, the development of OHP-based solar cells has transitioned into practical product realization, instilling the anticipation of novel solar cell advancements.Notably, OHP demonstrates versatility beyond its conventional application in solar cell materials.The physical properties of OHP materials exhibit a unique signature, thereby underscoring their potential utility as innovative functional materials, encompassing light-emitting diodes, lasers, and photodetectors.Recent reports on terahertz (THz)-wave absorption properties of OHP materials indicate a high possibility of their potential application as THz sensors.From the viewpoint of medical devices, which hold the most promising application potential, the exploration of optical phonon vibrational modes in the 0.5-3 THz frequency range is important.Moreover, understanding the correlations between atomic structure and lattice vibration modes is indispensable.In this concise review, the THz-wave absorption properties exhibited by 3D OHP materials are meticulously explored.Furthermore, future research directions for THz sensors using OHP materials are suggested.
addressed.The pursuit of material stability remains a crucial focus, particularly regarding device lifetime for LED applications.
][55][56][57][58][59][60] Theoretical analyses of the atomic structure of OHP materials predict phonon vibrational modes encompassing frequencies that are suitable for lattice vibration (1-100 THz), molecular vibration (2 THz), and molecular rotation (0.3 THz). [61]The 1-100 THz range associated with lattice vibrations is suitable for typical THz sensor applications.][65][66][67][68][69][70][71] Due to the fundamental physical properties of OHP materials, with a typical AMX 3 crystal structure, the ease of substitution among organic (or inorganic) cations (A: ), and halogen anions (X: Cl À , Br À , or I À ) enables the creation of a variety of phonon vibrational modes within the 0.3-100 THz range.Furthermore, the investigation of phonon vibration modes of OHP materials within the 0.5-3.0THz range which is sensitive to molecules in the human body should be conducted (Figure 1).This review aims to consolidate the reported research from this perspective, discuss the intriguing THz absorption properties at room temperature (RT), and suggest pathways for realizing THz sensors using OHP materials.In 2016, La-O-Vorakiat et al. performed THz-time-domain spectroscopy (THz-TDS) experiments using a representative OHP thin film composed of MAPbI 3 . [72]They observed two phonon modes at 1 and 2 THz originated from the buckling of Pb-I-Pb angles and the Pb-I length vibrations, respectively (Figure 2a).In the 0.5-2.5 THz, notably, there is no report for the phonon mode originating from the CH 3 NH 3 cation.Consistently, A. M. A. Leguy, et al. found no molecular-based phonon mode confirmed by Raman spectroscopy experiment  [87] Copyright 2023, published by Optica Publishing Group.Images for "Breast cancer": Reproduced with permission. [88]Copyright 2009, Optica Publishing Group.Images for "Teeth": Reproduced with permission. [89]Copyright 2022, Optica Publishing Group.Images for "Feet of a diabetic": Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [90]Copyright 2022, The Authors, published by Springer Nature.Images for "Colon cancer": Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [91]Copyright 2015, The Authors, published by SPIE.Images for "Gastric cancer": Reproduced with permission. [92]Copyright 2015, Optical Society of America.Images for "Brain cancer": Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [93]Copyright 2014, The Authors, published by Springer Nature.Images for "Otoscope": Reproduced with permission. [94]Copyright 2016, Optical Society of America.and theoretical calculation. [73]Their results suggested the absence of phonon modes originating from the organic cations in the 0.5-3.0THz range, a key area for the THz sensor.

THz-Wave Properties in MAPbI
In 2019, Maeng, et al. reported additional THz absorption features in MAPbI 3 (Figure 2b). [74]A THz absorption peak was observed at 1.58 THz with a significant absorption coefficient of %50%.Interestingly, the different observations between two studies were attributed to the difference in fabrication methods.][77] These two methods exhibit significant differences in the properties of thin films.First, thin-film fabrication using the SVE method shows smaller grain sizes compared to the solution-prepared methods, therefore resulting in higher grain boundary densities.Second, the increase in grain size results in an increased density of molecular defects, such as CH 3 NH 2 molecules (Figure 2c).The high density of CH 3 NH 2 molecular defects contributes to the formation of CH 3 NH 2 -incorporated perovskite structure and leads to the giant 1.58 THz absorption with an absorption coefficient exceeding 10 000 cm À1 (Figure 2d).
Interestingly, the CH 3 NH 2 molecular defect was first discovered through synchrotron radiation-based experiments in MAPbI 3 thin films fabricated using a solution-prepared method (Figure 3a). [24]The chemical states of Pb and I with CH 3 NH 2 molecular defects did not differ from the original chemical state of the OHP, which demonstrates the equivalence of the chemical states of Pb and I with CH 3 NH 3 þ .Based on these results, CH 3 NH 2 molecular defects were determined to be located at the grain boundaries.Importantly, these molecular defects induced a unique phonon mode at 1.58 THz, exhibiting significant THz absorption properties (Figure 3b,c).
To further demonstrate this significant absorption property, Maeng et al. conducted a study in 2020 to investigate the correlation between the density of CH 3 NH 2 defects and the 1.58 THz absorption peak. [57](Figure 3c) Under various postannealing conditions, the density of CH 3 NH 2 molecular defects obtained from the C 1s core-level spectra showed correlation with the THz absorption (Figure 4a).A linear correlation between the density of the CH 3 NH 2 molecular defects and the THz oscillator strength at 1.58 THz was confirmed (Figure 4b), revealing that higher densities of the CH 3 NH 2 -incorporated perovskite structures led to significantly increased oscillator strengths (Figure 4c).Paradoxically, in the context of THz sensor applications, controlling defect densities is pivotal for inducing THz absorption.However, the high density of defects negatively affects material stability and presents challenges to the lifetime of a device.opyright 2016, American Chemical Society.b) THz-wave absorption properties of MAPbI 3 (vacuum evaporated) at RT. c) Different absorption peaks for various fabrication methods.d) In the MAPbI3 fabricated using the SVE method, a remarkable absorption coefficient exceeding 10 000 cm À1 is observed.b-d) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/ licenses/by/4.0). [74]Copyright 2019, The Authors, published by Springer Nature.

Nonaffected Defect Structure and Its THz Absorption Property (MAPbBr 3 )
The lattice constant of OHP materials can be tuned through the substitution of halogen elements.The focus of this review is the 0.5-3.0THz phonon vibrational modes in OHP materials, which are attributed to the lattice constant between the metal cation and halogen anion.Elements such as I, Br, and Cl, contained within the same group in the periodic table, can be easily substituted through fabrication processes to yield OHP materials like  [24] Copyright 2016, The Authors, published by AIP Publishing.b) The differences in the surface morphology and C 1s chemical state are dependent on to the fabrication method.c) MAPbI 3 fabricated using the SVE method shows a new vibrational mode at 1.58 THz which originated from CH 3 NH 2 defect-incorporated perovskite structure.b,c) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [74]Copyright 2020, The Authors, published by Springer Nature.MAPbI 3 (Cubic: 6.31 Å), MAPbBr 3 (5.99Å), and MAPbCl 3 (5.67Å).80] Thin films of MAPbBr 3 fabricated via the SVE method exhibit distinctly different THz absorption properties compared to MAPbI 3 thin films (Figure 5). [55]The observed chemical states at the C 1s core level appear not only as CH 3 NH 2 molecular defects but also as C-O contaminants (Figure 5a).These different chemical states could provide a variety of phonon modes in THz absorption property.Unfortunately, these expected phonon modes are not observed to be the origin of any THz absorption (Figure 5b).Despite various postannealing treatments, three phonon vibration modes at 0.8, 1.4, and 2.0 THz were consistently observed; however, the maximum power absorption (PA) did not exceed 2000 cm À1 (Figure 5b).The modes at 0.8, 1.4, and 2.0 THz confirmed transverse, longitudinal optical, and Br self-vibrational modes, respectively (Figure 5c).Remarkably, the Br self-vibrational mode was observed in MAPbBr 3 , but not in the I halogen of MAPbI 3 .
From these results, we can determine the distinct effects of Br compared to I on the THz absorption property.First, studies with Br indicated that defects within the thin film have almost no impact on the THz absorption properties.Specifically, the CH 3 NH 2 molecular defect located at the grain boundaries and C-O contaminants on the surface of MAPbBr 3 thin film do not influence its THz absorption properties.Second, the introduction of Br results in a new self-vibrational mode, thus demonstrating that the vibrational effect of the Br halogen itself is greater than that of the defect-incorporated perovskite structure with the I halogen elements.Third, the insertion of the Br halogen element leads to a lattice constant reduction of %0.32 Å, resulting in absorption peaks at a lower frequency relative to the THz absorption in MAPbI 3 .For instance, because of the same underlying causes (buckling and translating in Pb-X-Pb: X = halogen such as I or Br), the 1.0 and 2.0 THz absorption peaks observed in MAPbI 3 manifest as 0.8 and 1.4 THz absorption peaks in MAPbBr 3 , respectively.From the perspective of the exchange of organic cations, one can intuitively expect that formamidinium (FA)-based hybrid perovskites, such as α-FAPbI 3 (black perovskite, cubic structure) with a lattice constant of %6.36 Å, may yield THz absorption at even higher frequencies compared to those observed in MAPbI 3 . [81]This comparison referenced by the lattice constant will be discussed later.The THz absorption property observed in MAPbBr 3 thin films shows fixed THz absorptions, irrespective of the presence of defects.However, the PA threshold of 2000 cm À1 is a limiting factor.
In addition to the unique THz absorption property in MAPbBr 3 , this study presents the possibility for flexible and stable THz sensor fabrication through the incorporation an ultrathin protection layer using a polymer [poly[bis(4-phenyl)(2,4,6trimethylphenyl)amine]] (PTAA) and a flexible substrate (PET) to achieve THz transmittance exceeding 90%. [58]The introduction of a protective layer and flexible substrate may be suitable applications as medical devices.

THz-Wave Absorption Properties in FAPbI 3 Thin Film and Single Crystal
In 2019, Lee et al. reported on the THz absorption property of FAPbI 3 thin films fabricated using the SVE method. [75]They observed significant THz absorption peaks when the mixed phase with two atomic phases (δ-and α-phases) was formed (Figure 6a).Before and after annealing, the δand α-phase comprised the atomic phase of FAPbI 3 , respectively (Figure 6b).Interestingly, after being subjected to a 10 min.postannealing period, the sample exhibited a significant THz absorption peak at 1.62 THz which was characterized by transmittance exceeding 40%.Unfortunately, this report does not mention the detailed discussion of the origin of the δ/α-mixed phase.This is because of the limitations of thin films (<300 nm), including grains, which can provide many factors such as defects, and the use  [75] Copyright 2019, The Japan Society of Applied Physics.
of simplistic chemical state analysis to understand the origin of phonon vibration modes.The origin of this unusual THz absorption property in the δ/α-mixed FAPbI 3 can be assumed in two ways: first, it could be due to defect-incorporated perovskite structure like those observed in MAPbI 3 ; second, it could arise from a unique structure at the interface between δand α-phases.To elucidate the origin of this unusual THz absorption property, physical properties associated with thin films, such as grain and grain boundary, must be eliminated by transitioning from thin films to single crystals.
In 2021, Maeng, et al. prepared single crystal FAPbI 3 and generated δ-, δ/α-mixed, and α-phases through simple postannealing (Figure 7a,b). [58]Similar to previous research, they observed the unusual THz absorption property in the δ/α-mixed phase through a THz-TDS experiment (Figure 7c).The consistent observation of the unusual THz absorption peak provides clear evidence that the phonon vibrational mode does not originate from the defect-incorporated perovskite structure, thus indicating a different origin from that observed for MAPbI 3 .Because they selected the single crystal which confirmed no  [58] Copyright 2021, The Authors, published by Springer Nature.
defect-incorporated structure.Interestingly, unusual THz absorption peaks at 2.0 and 2.2 THz unique to the δ/α-mixed phase were observed; these contained peaks present in both the δand α-phase (Figure 7c).The theoretical simulation confirms that these two unusual THz absorptions originated from the interface of two different atomic phases (Figure 8).Consequently, the unusual THz absorption property observed in the δ/α-mixed phase of FAPbI 3 arises from interfacial phonon vibrations unique to the interface of the δand α-phase.
Unfortunately, this study did not quantitatively explore the density of interfaces between the δand α-phase that would optimize the interfacial phonon vibrational modes.If subsequent research is performed with a different ratios of 1:3, 1:1, and 3:1 of the δ/α-mixed phase, it would confirm a trend of interfacial phonon vibration modes.
Initially, the motivation of this study was to explore the possibility of modulating THz absorption frequencies by fine tuning the lattice constant (6.31! 6.36 Å) through the substitution of the organic cation (MA !FA) to investigate the influence of the atomic phase of FAPbI 3 on unusual THz absorption.However, the presence of unusual THz absorption features originating from the mixed phase in FAPbI 3 suggests the potential presence of a unique THz absorption property present not only in typical 3D hybrid perovskites, but also in 2D hybrid perovskites (Pb free) such as MA 3 Sb 2 I 9 and MA 3 Bi 2 I 9 .
4. THz-Wave Properties in Solution-Prepared γand δ-CsPbI 3 Finally, we discuss the THz absorption property of all-inorganic perovskite CsPbI 3 , where the organic cation is replaced by an inorganic cation (Cs).A recent report investigated the THz absorption properties of γ-CsPbI 3 , fabricated by the solutionprepared method, with various grain boundary sizes created using various postannealing conditions (Figure 9a). [60]nterestingly, γ-CsPbI 3 samples fabricated with average grain sizes of 275, 349, and 483 nm, each, exhibit consistent optical bandgaps and chemical states, including the Cs 3d, Pb 4f, and I 3d core levels (Figure 9b).This shows the same atomic and electronic structures of the films without any defects.
Remarkably, THz absorption peaks are observed across the entire range of 0.5-3 THz, with a relative conductivity that approaches a maximum of 40 S cm À1 (Figure 10a).Furthermore, samples with different grain sizes show no significant differences in THz absorption properties.In the case of CsPbI 3 , this is because of the absence of defects and the substitution of organic cations with inorganic Cs cations, which induce THz absorption throughout the 0.5-3 THz range (Figure 10a).From the theoretical simulation, they confirmed that the observed 0.9, 1.5, and 1.8 THz originated from the transverse I-Pb-I frame, Cs-I-Cs optical vibration, and longitudinal I-Pb-I frame, respectively (Figure 10b).While organic cations such as MA and FA in MAPbI 3 , MAPbBr 3 , and FAPbI 3 did not show phonon vibrational modes interacting with the surrounding metal cations or halogen anions, the inorganic Cs cation led to Cs-I-Cs optical vibration.Additionally, the absorption peak appearance at 1.5 THz covers the intermediate 0.5-3 THz range, which presents a significant advantage over typical OHPs containing organic cations.
In Supporting Information, this study confirms the phase change from γ-CsPbI 3 into δ-CsPbI 3 over time caused by the structural degradation in ambient conditions and discusses the THz-TDS experimental results for altered samples (Figure 10c). [60]In conclusion, δ-CsPbI 3 demonstrates THz absorption despite exhibiting lower real conductivity (maximum of 35 S cm À1 ) compared to γ-CsPbI 3 , We can expect that CsPbI 3based THz sensors can maintain sufficient THz absorption despite the internal structural degradation.

Conclusion
We have investigated the THz absorption properties of the representative 3D OHP (AMX 3 ) materials at RT.The consistent phonon vibrational modes observed in MAPbI 3 , MAPbBr 3 , FAPbI 3 , and CsPbI 3 comprise Pb-I(or Br)-Pb buckling and Pb-I(Br)-Pb translational vibrations.However, the significant THz absorption property for each OHP material is caused by the different nature of each OHP material (Figure 11).Furthermore, the absorption power of each OHP can be assessed through its absorption coefficient (cm À1 ) and real conductivity (S cm À1 ) (Table 1).
Based on these findings, we discuss the direction for future research on THz sensor fabrication using OHP materials.
First, achieving coverage of the entire core frequency range of 0.5-3.0THz with a single type of thin film is impractical.Therefore, future studies should create multilayer structures with different 3D OHPs to cover each desired frequency range.Notably, the investigated samples were fabricated using SVE, which facilitates multilayer fabrication.In the case of the solution-prepared method, it is difficult to fabricate a multilayer structure because of the presence of solvents.On the other hand, the SVE method is based on the vacuum evaporation method which is more convenient for multilayer formation.However, it requires detailed postannealing conditions to avoid the formation of mixed-hybrid perovskite structures (e.g., (MA, FA)(Pb, Sn)(Br, I) 3 ) that have no significant THz absorption property. [82,83] [60]Copyright 2023, The Authors, published by Elsevier.
Finally, the investigation of material stabilities of the suggested candidates, such as multilayers using 3D and 2D OHPs, is necessary.THz sensors are operated at RT, and since we can assume a lack of temperature-dependent problems in the materials, only the material itself poses a critical problem.In a multilayer structure, additionally, the interface stability must also be considered.
In summary, we have explored THz absorption properties in 3D OHP materials.From this short review, we found several issues such as increasing PA, solving Pb exclusion (Pb free), and improving material stability.Finally, we expect THz sensors based on OHP materials sooner that have various merits such as low unit cost, easy fabrication, high PA (<4000 cm À1 at 0.5-3.0THz), and flexibility of device.

Figure 2 .
Figure2.a) THz-wave absorption properties of MAPbI 3 thin film (solution prepared) at various temperatures.Reproduced with permission.[72]Copyright 2016, American Chemical Society.b) THz-wave absorption properties of MAPbI 3 (vacuum evaporated) at RT. c) Different absorption peaks for various fabrication methods.d) In the MAPbI3 fabricated using the SVE method, a remarkable absorption coefficient exceeding 10 000 cm À1 is observed.b-d) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/ licenses/by/4.0).[74]Copyright 2019, The Authors, published by Springer Nature.

Figure 3 .
Figure 3. a) C K-edge absorption and core-level spectra of Pb 4f and I 4d measured by near-edge X-Ray absorption of fine structure and high-resolution XPS using synchrotron radiation.The chemical state of the CH 3 NH 2 defect is observed in the C K-edge in both partial electron yield and total electron yield modes.Notably, the chemical states of the Pb 4f and I 4d core-levels are unchanged.Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[24]Copyright 2016, The Authors, published by AIP Publishing.b) The differences in the surface morphology and C 1s chemical state are dependent on to the fabrication method.c) MAPbI 3 fabricated using the SVE method shows a new vibrational mode at 1.58 THz which originated from CH 3 NH 2 defect-incorporated perovskite structure.b,c) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[74]Copyright 2019, The Authors, published by Springer Nature.

Figure 4 .
Figure 4. a) Carbon 1s core-level spectra and the relative intensity area (%) of CH 3 NH 2 defect.b) THz-wave absorptions and the oscillator strength (Ω À1 cm À1 ) at 1.58 THz.c) Linear correlation can be established between the concentration of CH 3 NH 2 defects and oscillator strength at 1.58 THz.a-c) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/ 4.0).[57]Copyright 2020, The Authors, published by MDPI.

Figure 6 .
Figure 6.a) The unusual THz-wave absorption at 1.62 THz is observed in the sample after postannealing for 10 min.b) This sample has the δ/α-mixed phase, which was confirmed by X-ray diffraction (XRD) measurements.a,b) Reproduced with permission.[75]Copyright 2019, The Japan Society of Applied Physics.

Figure 9 .
Figure 9. a) The atomic structures and surface morphologies measured by XRD and atomic force microscopy are shown in γ-CsPbI 3 of different grain sizes.b) Interestingly, there is no change in the chemical states of Cs 3d, Pb 4f, and I 3d core-level spectra.a,b) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[60]Copyright 2023, The Authors, published by Elsevier.

Table 1 .
Maximum values of absorption coefficients and real conductivity in the range of 0.5-3.0THz.-Absorption coefficient [cm À1 ]