Significant THz-wave absorption property in mixed δ- and α-FAPbI₃ hybrid perovskite flexible thin film formed by sequential vacuum evaporation

Organohalide hybrid perovskite (OHP) materials are some of the best candidate materials for solar-cell applications.^1–7) The power conversion efficiency is already over 23.3% with high competition in comparison to that of CdTe (22.1%), CIGS (22.6%), and Si (25.4%).¹⁾ Recently, researchers also started exploring a possibility for new applications using hybrid perovskite materials such as optoelectronic and memory applications.^8,9) To overcome the weaknesses of OHP materials such as material instability, on the other hand, many researchers are focusing on studying instability origins, stable compounds, defect structures, and multi-functional hole-transport layers (good hole mobility and water protection).^10–19) It looks like it is now going towards optimization. However, we still have many unclear physical properties of OHP materials such as thermoelectric and THz absorption properties.^20–24) Studies of these are just beginning and require more experiments and theoretical calculations.

Our previous reports about the sequential vacuum evaporation method (SVE) and the property of THz-wave absorbance in CH₃NH₃PbI₃ (MAPbI₃) show that OHP materials can have THz-wave sensing applications.^25,26) In the simplification, the SVE method for the fabrication of OHP thin film creates the molecular defect-incorporated hybrid perovskite with over 50% absorptance at 1.59 THz (300 nm thick). To be a candidate for THz-wave sensing application, we believe many OHP materials should be tested such as the change in or mixture of halide elements (Cl, Br, and I), metal cations (Pb and Sn), and organic elements (MA and FA) to obtain a wide THz range (0.1–5 THz) and enough THz absorbance. From our last report, the origin of THz-wave absorption in the MAPbI₃ case is the different Pb–I vibration mode originated by the molecular defect CH₃NH₂-incorporated hybrid perovskite structure.²⁶⁾ Unfortunately, we could not find any significant molecular vibration modes in the required THz range such as MA and FA in the typical hybrid perovskite structure. This means that we need to find a way to optimize a different vibration mode from the metal cation and halide anion bonding (Pb/Sn–Cl/Br/I) to make a different chemical environment. The simple approach is to exchange the organic part of MA with FA to change the chemical environment of the metal cation and halide anion bonding. For future applications such as THz-wave sensing and THz-wave imaging devices, besides, a flexible substrate will be one of the possible good approaches.

In this study, we fabricated formamidinium lead iodide (FAPbI₃) thin films on a flexible substrate using the SVE method and then confirmed its THz-wave absorption property. The formed thin films were characterized by atomic force microscopy (AFM), UV–visible spectrometry (UV–vis), X-ray diffraction (XRD), THz time-domain spectroscopy (THz-TDS) with a femtosecond laser (Spitfire ACE, Spectra-physics), and X-ray photoelectron spectroscopy (XPS). Finally, we found the mixed phases with the δ- and α-phases of FAPbI₃ showed approximately 40% absorptance at 1.62 THz. We assume it originates from partial α-FAPbI₃ (such as a defect structure) in the main structure of δ-FAPbI₃.

Prior to the film deposition process, n-type Si(100) and glass substrates were cleaned by (1) sonication in acetone, (2) rinsed in heated acetone, and (3) UV–ozone treatment for 10, 1, and 30 min, respectively. In the case of indium-doped tin oxide (ITO) coated polyethylene naphthalate (PEN) substrates, we performed the cleaning processes in acetone, ethanol, and isopropanol, and subsequently treated by UV–ozone cleaner. For the SVE method for FAPbI₃ films on the cleaned Si, glass, and ITO-coated PEN substrates, PbI₂ (Alfa Aesar, 99.99%) and FAI (Dyesol) precursor powders were placed in two different quartz crucibles inside a chamber. Under the pressure of 1.9 × 10⁻⁴ Pa, PbI₂ films were firstly deposited with the deposition rate of 10 Å s⁻¹, and sequentially FAI films were grown with the deposition rate of 2 Å s⁻¹ as well.²⁵⁾ The thickness of PbI₂ and FAI films was 91 nm and 209 nm, respectively. The as-deposited FAPbI₃ films were further annealed at 140 °C for 10 min.

We observed the optical band gaps with 1.81 and 1.57 eV before and after annealing in UV–vis measurements [Fig. 1(a)]. The sample color had changed from dark-yellow to dark-brownish [Fig. 1(a)]. This color change is very similar to that in a previous report.²⁷⁾ In the XRD measurement, however, we observed a mixed state with δ- and α-FAPbI₃²⁷⁾ [Fig. 1(b)]. Before annealing the FAPbI₃ thin film, the δ-phase was mainly observed. After annealing, we confirmed the α-phase had appeared. However, we still observed mainly the δ-phase in the annealed sample. From these XRD results, we can confirm a mixed state with a major δ-phase and a minor α-phase. We assume that this observed mixed state is due to the relatively low annealing temperature of 140 °C.²⁷⁾ Unfortunately, the melting temperature of the flexible PEN substrate is near at 180 °C which was reported with the annealing temperature.²⁷⁾ To avoid any melting problem, we performed the post-annealing at 140 °C for this experiment. This is the reason why we observed a mixed state in the XRD measurement. Consistently, the increase of grain size observed in the AFM measurement seems to be due to the appearance of α-FAPbI₃.

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From the surface morphologies before and after annealing the FAPbI₃ thin film, we found enhanced grains (from 150 nm to 350 nm on average) that show clearer shapes [Fig. 2(a)]. The surface roughnesses are 15.9 and 13.8 nm before and after annealing, respectively. This observation of enhanced grains is different with MAPbI₃ thin film fabricated by the SVE method.²⁵⁾ In the samples annealed for 20 and 30 min, we confirmed the atomic structures were shown completely with only the α-phase. (See Fig. S1 in the supplementary material available online at stacks.iop.org/APEX/12/051003/mmedia.) Over 60 min of annealing, the δ-phase marginally appears in the main α-phase (Fig. S1).

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Transmitted THz signals are acquired by a standard THz-TDS system. A femtosecond Ti:sapphire laser beam with a center wavelength of 800 nm, a pulse width of 35 fs, and a repetition rate of 1 kHz was employed. We used optical rectification generation and electro-optical sampling detection with 2-mm-thick ZnTe(110) crystals. Before the THz-TDS measurement, we performed the post-annealing processes in a N₂ glovebox to keep the structural phase stability of all the samples.²⁷⁾ The phase delay and reduction of the amplitude in transmitted time-domain THz-pulse signals through the as-received sample (before annealing) are not clear, but in the case of the 10-min-annealed sample, the specific changes are observed. Over 20 min of annealing, we could not observe any significant absorption [Fig. 2(b)]. The significant THz absorption is observed at 1.62 THz only in the sample annealed for 10 min. Therefore, we can assume this THz absorbance originates from the partially occupied structure of α-FAPbI₃ in the main structure of δ-FAPbI₃ that appears after annealing.

To see the possibility of a different chemical state, we performed high-resolution XPS (Fig. 3). In the C 1s core-level spectra of the as-received sample with δ-FAPbI₃, we observed two main peaks with binding energies of 287.9 and 284.7 eV [Fig. 3(a)]. From the chemical formula of FA⁺, we can confirm that the C–N and C=N bondings are 284.7 and 287.9 eV, respectively.²⁸⁾ In the 10-min-annealed sample, two shoulders (indicated by arrows) at the low binding energies referring to the C–N and C=N bondings appear. We can assume that these new chemical states originate from the α-FAPbI₃ that appears after annealing. Besides, we can observe a small shoulder at the high binding energy of 289 eV [indicated by an arrow in Fig. 3(a)]. Interestingly, the N 1s, Pb 4f, and I 4d core-level spectra do not show any changes after annealing [Figs. 3(b)–3(d)]. The N 1s core-level spectra show a shoulder at the high binding energy referring to the maximum intensity position with a binding energy of 400.1 eV [Fig. 3(b)]. We can see there is a different chemical state. In the case of the Pb 4f and I 4d core levels, they have only single chemical states at the binding energies of 4f_7/2 = 137.8 eV and 4d_5/2 = 48.7 eV, and there is no change after annealing. From these results, we can confirm the only different chemical state is in the C 1s core level between the δ- and α-phases in the FAPbI₃ thin film.

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In order to analyze the spectra in more detail, we fitted the C and N 1s core-level spectra of the as-received and 10-min-annealed samples using Doniach–Sŭnjić curves convoluted with Gaussian distribution with a 0.5 eV FWHM²⁹⁾ (Fig. 4). The background due to inelastic scattering was subtracted by the Shirley (or integral) method.³⁰⁾ From these curve fittings, we found C–O and C=O bondings with a constant value²⁸⁾ (see Fig. S2 in the supplementary material). Before and after annealing, the intensities of these chemical states were not changed. And we clearly observed C=N and C–N from δ-FA⁺ (287.9 and 284.7 eV) and α-FA⁺ (286.3 and 284.3 eV) corresponded to hexagonal and cubic structures, respectively³¹⁾ [Fig. 4(a)]. As mentioned in the previous paragraph, interestingly, the N 1s core level in the 10-min-annealed sample had two chemical states with binding energies of 400.4 and 399.9 eV with no different chemical states between δ- and α-FAPbI₃ [Fig. 4(b)]. From the calculation of the relative intensity area of δ-FA⁺ and α-FA⁺ in the C 1s core-level spectrum of the annealed sample, we obtained the ratio between δ-FA⁺ and α-FA⁺ of approximately 5:1.

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In summary, we fabricated flexible and mixed δ- and α-FAPbI₃ hybrid perovskite thin films using the SVE method and then studied the THz-wave absorption property. After annealing for 10 min at 140 °C, we confirmed the mixed state with the main δ-phase and partial α-phase. The THz-wave absorption property at 1.62 THz with 40% absorptance was observed in the mixed-phase FAPbI₃ thin film on a flexible substrate. In the C 1s core-levels after annealing, we found two different chemical states originating from the δ- and α-phases. The origin of the THz-wave absorption property is assumed to be a significant Pb–I vibration mode of the α-phase in the main structure of δ-FAPbI₃.

Acknowledgments