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Abstract
Black-phase formamide lead iodide (α-FAPbI3), considered one of the most important materials for a solar cell application, is generally poorly stable in air, which leads to not only the inability of photovoltaic devices but also the lack of its fundamental optics research. In this paper, we synthesized a stable α-FAPbI3 film by modifying a previously reported method and investigated its temperature- and excitation intensity-dependent photoluminescences (PLs). It is found that at low temperatures, the crystal phase competition process is unusually complicated and out of order. The temperature range of the biphasic coexistence is at least 30 K, and during this process, the PL intensity of either the high-temperature phase (cubic) or low-temperature phase (tetragonal) changes chaotically. After the complete transition to the tetragonal phase, compared with the cubic monophase, PL resulting from the crystal defects is obviously enhanced. Our findings provide a deeper understanding of the complex structural phase transition of halide perovskite and valuable insights into the fundamental optics of α-FAPbI3.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Halide perovskite materials are attracting attention for their excellent performance in optoelectronic devices [1–5]. This material has excellent properties such as high absorption coefficient [6], low recombination rate [7–10], direct interband gap [11,12], long diffusion length [13,14], and high photoluminescence quantum yield [15,16]. Previous studies have demonstrated the photoluminescence property and mechanism of some typical perovskite materials such as CsPbBr3 and FAPbBr3 nanocrystals (NCs) [17–20], while the photoluminescence properties of α-FAPbI3, so far has not been well studied. As known, such black-phase α-FAPbI3 has the narrowest band gap (1.45-1.51 eV) in the halide perovskite family [21,22], together with its very high carrier mobility and long diffusion length, thus has attracted much attention in solar cell application. However, α-FAPbI3 synthesized by the conventional method using PbI2@DMF: DMSO film as precursor followed by FAI intercalation is extremely unstable and very easy to completely degrade into a waste phase, namely, δ-FAPbI3, in several tens of minutes when stored in ambient conditions [23]. Recently, Huang et al. developed a modified method to vertically align PbI2 thin film grown from methylamine formate (MAFA) and obtained highly stable α-FAPbI3, with a photoelectric conversion efficiency as high as 24.1% [24]. The successful acquisition of stable α-FAPbI3 also make it possible to study its optical properties in detail.
Because of its loosely arranged structure, the structural, electronic and optical properties of FAPbI3 are sensitive to external modulations. Previous studies have calculated the strain effect of FAPbI3 using the first principles and described the effect of strain effect on its structural phase [25], but real experimental data are lacking and no study has ever shown the effect of temperature, an important factor to induce structural phase transition, on α-FAPbI3. In this paper, the photoluminescence (PL) spectral data of α-FAPbI3 at 100–300 K were investigated on the phase transition-related optical properties. When α-FAPbI3 is stabilized in a monophase, the obtained PL spectra were fitted with split peaks to make a logarithmic plot of the excitation versus emission intensity to determine the PL type.
At room temperature, the absorption as well as PL spectra of the fabricated α-FAPbI3 thin film are in close agreement with the previous report [24], which indicates that we have indeed obtained such stable α-FAPbI3 material. From 300 to 160 K, α-FAPbI3 is in pure cubic phase and below 160 K, the tetragonal phase appears. Such two phases can coexist to at least 130 K, or sometimes even to below 100 K. By performing excitation intensity-dependent PL measurements, we found that α-FAPbI3 in cubic monophase is a perfect single-exciton or free e-h recombination emissive, while in tetragonal monophase it obviously has a defect-related PL increasement. We ascribe it to the disordered competition of the two phases, which leads to a large number of defects in the phase transition process. The phase competition disordering also can be observed from the PL intensity ratio of the two phases during the biphasic coexistence, since it goes irregularly up and down with decreasing temperature, and no facts were found that the PL intensity of the low-temperature phase keeps increasing all time or the high-temperature phase weight goes monotonously decreasing. Our findings not only fill the research gap of α-FAPbI3 in the field of low-temperature phase conversion, but also helps deeply understand the fundamental optics of halide perovskites.
2. Experimental details
2.1 Material preparation
The stable α-FAPbI3 was synthesized via modifying the previously reported method [24]. Specifically, (i) 15 mL of formic acid was placed in a beaker and then diluted with 62.5 mL of anhydrous methanol to make solution A. Put 62.5 mL of methylamine (33% ethanol solution) into a flask to stir at –16°C, and then dilute it with 25 mL absolute ethanol to make solution B. After the temperature of solution B is stabilized, solution A was slowly dropwise added into solution B. Note that solution B must be stirred continuously and maintained at –16 °C. After 2 h rotary evaporation at 55 °C of the mixture, MAFA solvent was obtained. (ii) Dissolved 1.5 mmol PbI2 powder in 1 mL MAFA solvent to stir for 2 h at 60°C in a nitrogen-filled glove box, then PbI2@MAFA precursor was successfully obtained. (iii) The glass substrate was ultrasonically washed for 15 min in turn with detergent, acetone, isopropanol and ethanol, and then dried by high-pressure nitrogen gas. The MAFA precursor was spin-coated onto the glass substrate followed by annealing at 120 °C for 30 min. Finally, it was stored in a drying oven at 80 °C for 8 h to obtain stable α-FAPbI3 crystalline films.
2.2 Characterization and test
Scanning electron microscopy (SEM, TESCAN MIRA LMS) and transmission electron microscopy (TEM, JEM-2100F, Jeol) were used to observe the morphologies of the samples. The crystalline diffraction patterns were identified using an X-ray diffractometer (XRD, Mini Flex 600, Rigaku and In-situ XRD Bruker D8 discover). The elementary composition and chemical bonding were characterized via X-ray photoelectron spectroscopy (XPS, ESCALAB-250XI, ThermoFisher Scientific).
PL spectra of the sample were tested via a Laser Raman spectrometer (LABRAM HR EVO, Horiba), and the film was mounted in a cryostat fed by a continuous flow of liquid nitrogen to control the temperature. The excitation intensity was altered through a rotary attenuator (GCT-060101, Daheng Optics) and measured by a light power meter (PM16-140, Thorlabs). The absorption spectra were conducted on a UV-visible spectrophotometer (UV-2700, Shimadzu).
It is worth mentioning that the temperature-dependent experiments in PL and XRD measurements were carried out after the temperature is stabilized for 120 s.
3. Results
3.1 Characterization of α-FAPbI3
The conventional method of preparing α-FAPbI3 films usually involves first forming pin-like initial grains to provide nucleation sites [26], while the preparation method used in this paper directly forms uniform, large grain PbI2 film without pin holes, which virtually isolates the degradation caused by the disturbance of water in the environment.
Figure 1 (a) shows the SEM image of the synthesized α-FAPbI3 film after annealing and placing it in air for 2 hours. The characteristics of large crystal grains can be clearly seen, indicating that the film has no pinholes. This is because the MAFA solution evaporates during annealing, the PbI2 grains have a strong vertically aligned orientation, which can help to form a uniform film. At the same time, the nanometer-scale gap between the vertically arranged PbI2 grains is filled with MAFA, providing direct reaction channels for the cations to form a pinhole-free film [24]. Figure 1(b) shows the SEM image of the sample placed in air for 50 h, and it can be seen that the originally formed large grains are gradually getting smaller, which leads to some holes on the surface of the film, but it still demonstrates typical α-type grains. After 200 h [Fig. 1(c)], the sample turns into typical δ-type FAPbI3, characteristic as holey long stripe-like morphology. Namely, such sample can keep its α-phase structure at least for several tens of hours, guaranteeing that we can conduct its intrinsic property measurement in ambient conditions. The transmission electron microscopic (TEM) image [Fig. 1(d)] clearly shows the lattice stripes, and the neighboring space distance is ∼ 0.34 nm, corresponding to (001) planes of α-FAPbI3. Figure 1(e) shows the strong X-ray diffraction (XRD) signal of α-FAPbI3 at around 14°, which corresponds to the position of the characteristic peak of the α phase. There is no characteristic peak at 11.8° corresponding to the δ phase, indicating that the film is well α-type crystallized. At the same time, the XRD patterns of α-FAPbI3 thin films after being placed in air for 2, 10, 20 and 50 h are also provided [Fig. 1(e)]. The XRD pattern of the sample after 10 h is basically consistent with that of the 2 h. After 20 h, the PbI2 and impurity peaks appear, but the peak intensity representing α phase is still very high even after 50 h. This shows that the prepared film can keep its α phase for at least tens of hours.
Fig. 1. SEM images of α-FAPbI3 after being placed in air for (a) 2 h, (b) 50 h and (c) 200 h, (d) the typical TEM image, (e) typical XRD signal of α-FAPbI3 in air at different times.
Meanwhile, we used X-ray photoelectron spectroscopy for the bonding analyses of the film surface [ Fig. 2(a)]. Peaks at ∼ 618.5 and 629.3 eV are attributed to I 3d, ∼ 284.3 eV is C 1s, and ∼ 142.3 and 137.3 eV are Pb 4f. The presence of the O 1s peak indicates that the film has been partly oxidized by exposure to air. The quantitative elemental analysis yielded 16.37% of I, 10.09% of Pb, and 8.95% of O, respectively. After deducting the amount of Pb in PbO2, the I content is about three times of Pb, which is following the proportion of elements in the chemical formula of α-FAPbI3 and indicates that, except for the oxidized part of PbO2, the fabricated film can be regarded as pure α-FAPbI3.
Fig. 2. (a) XPS survey spectrum of α-FAPbI3 film, fine XPS spectra of (b) Pb 4f and (c) I 3d. (d) In situ XRD patterns of α-FAPbI3 film at 300 K, 160 K, and 100 K.
High-resolution XPS analyses of Pb and I were done and shown in Figs. 2(b) and 2(c), respectively. The two subpeaks in Pb 4f5/2 are the characteristic peaks of the Pb-C (∼ 142.5 eV) and Pb-O (∼ 143.5 eV) bonds, and the Pb 4f7/2 peak at ∼ 138.0 eV is ascribed to the Pb-I bond. The peak position of I 3d [Fig. (2(c))] shows the I-bonds in FAPbI3. To demonstrate its phase transition with the temperature, we performed the in situ XRD tests at different temperatures [Fig. 2(d)]. At 300 K, it demonstrates typical signals of the cubic phase and such crystal structure can maintain to 160 K, while at 100 K the characteristic peaks are slightly shifted and the tetragonal signals appears [27]. The lattice constant of the cubic phase is calculated to be 5.58 Å from the XRD data, and when the temperature drops to 113 K, a = b = 5.55 Å, c = 6.21 Å. The optical results show the complexity of phase transition process of such α-FAPbI3.
3.2 Optical properties of α-FAPbI3
We performed absorption and temperature-dependent PL measurements under 532 nm laser excitation of α- FAPbI3. As shown in Fig. 3(a), at room temperature, the UV-vis absorption spectrum shows the typical absorption shoulder of α-FAPbI3 at ∼ 710 nm with a long tail, and its PL peak is centered at ∼ 780 nm (1.58 eV). Via Tauc method conducted on the absorption spectrum, the band gap of the sample is calculated as narrow as 1.58 eV [the inset in Fig. 3(a)], which is consistent with the study of the typical α-FAPbI3 perovskite film materials [21,28]. The PL center is very close to the bandgap, implying at room temperature, there is no identical exciton behavior in α-FAPbI3. Namely, at room temperature, the PL type of α-FAPbI3 can be deemed as free e-h recombination. Can we clearly see from Fig. 3(b) that, the PL peak has a red shift with temperature decreasing, which is consistent with previous studies of the Pb-based samples without phase transition [29]. When the temperature continued to decrease to 220 K, the intensity of the PL peak weakened instead of enhancing continuously. The appearance of this anomaly is speculated to be caused by the thermal equilibrium disruption, but is not the main argument of this paper. Afterward, double peaks appear under 160 K, obviously due to the phase transition from the cubic to the tetragonal phase [30,31]. We conducted the tests on different spots of the same film, and found that this transition all happened at just below 160 K. It is reasonable to draw a conclusion that ∼160 K is the cubic → tetragonal transition temperature. The truly abnormal is the PL variation during the coexistence of the two phases. Among more than 10 tests on the same film at different experimental spots, we found two typical phenomena: One is that the two phases coexist all the way even down to 100 K, and the other is that the complete phase transition comes up at ∼ 130 K. Here we choose two representative spots (named as spot 1 and spot 2) to clarify it in detail.
Fig. 3. (a) UV-vis absorption and PL spectrum of α-FAPbI3 perovskite thin film, the inset is the bandgap diagram calculated by the Tauc method. (b) Temperature-dependent mapping PL spectra of α-FAPbI3 under 532 nm excitation with the intensity of 1 µW/cm2 on spot 1. (c) PL spectra of monophase cubic α-FAPbI3 under different excitation intensities at 160 K. (d) Log-Log function of the PL emission dependent on the excitation intensity of cubic α-FAPbI3 at 160 K.
Figure 3(b) shows the biphasic coexistence situation with decreasing the temperature down to 100 K. The excitation intensity dependent PL was performed at 160 K on spot 1. In general, the integrated intensity of free e-h or exciton emission increases linearly with the excitation power (slope α = 1 in log-log dependence), if the emission comes from a localized state, namely, defect, it will show a sublinear behavior (slope α < 0.8) because the defect-induced emission will approach saturation under high excitation power [32]. From Fig. 3(c) it is seen that the PL curve keeps perfect symmetrical Gaussian shape, indicating there is no noteworthy defects in mono-cubic phase of α-FAPbI3. The log-log dependence between emission and excitation intensities [Fig. 3(d)] shows a slope of α ∼ 1, also confirmed at 160 K it is a complete free e-h or exciton emission.
3.3 PL resulted from phase transition of α-FAPbI3
As well it can be seen from Fig. 3(b), when the tetragonal phase appears at below 160 K, the PL from the cubic phase turns from red shifting to blue shifting, indicating the tetragonal nucleation “drag” cubic atoms towards tetragonal to show a biphasic competence. The blueshift degree is as large as 40 nm, and at 100 K the PL peaks from these two phases is very close (770 nm for cubic and 743 nm for tetragonal), maybe it is better seen as a “half-cubic and half-tetragonal” phase. For the tetragonal part, its PL demonstrates a typical Pb-based red shift as decreasing temperature. It should be noticed that with decreasing temperature, the PL intensity from the cubic part is not going monotonically decreasing, further implying the complexity of the structural phase competence of α-FAPbI3.
The other representative PL results conducted on another spot (spot 2) on the same α-FAPbI3 film were shown in Fig. 4. The PL behaviors are totally similar to the above discussions on spot 1 from 300 to 160 K, namely, α-FAPbI3 is a monophasic cubic crystal and shows a symmetric Gaussian-shape PL [Fig. 3(c)]. The PL of tetragonal phase arises just below 160 K and however, the PL of cubic phase is completely disappeared below 130 K [Fig. 4(a)]. During the coexistence of the two phases, PL of tetragonal phase also shows a typical Pb-based redshift (peak II), while the cubic part exhibits a blueshift (peak I) similar to Fig. 3(b). The PL plots of peak I and peak II were integrated to obtain the areas of the two peaks, i.e., the PL intensities of the two phases at 1 µW/cm2 light intensity. The intensity ratio of peak II to peak I is used to observe the phase transition process of the sample in the interval of 130–160 K. The sample has only a cubic phase at 160 K and only a tetragonal phase at 130 K. However, in this temperature interval, the PL ratio of cubic to tetragonal phase keeps jumping without any pattern, and this indicates that the phase transition of α-FAPbI3 from cubic to tetragonal is indeed non-monotonic and varies chaotically.
Fig. 4. PL results of α-FAPbI3 on spot 2. (a) Normalized temperature-dependent PL intensities under 532 nm excitation with an intensity of 1 µW/cm2. Peak I and peak II are ascribed to the cubic and tetragonal phase, respectively. (b) Intensity ratio of peak II to peak I under different temperatures. (c) PLs of monophasic tetragonal α-FAPbI3 with different pump intensities at 100 K. (d) Log-Log dependences of the emission on the excitation intensities of peak A and B in (c).
The excitation intensity dependent PL of α-FAPbI3 is shown in Fig. 4(c) after the complete tetragonal transition at 100 K. A remarkable asymmetric shape unlike Fig. 3(c) is observed that the peak can be convoluted into two subpeaks, peak A and B. By fitting its log-log emission-excitation intensity dependences, the slope α of peak A is ∼ 0.8, and peak B is ∼ 1 [Fig. 4(d)], indicating noteworthy defects appear in the tetragonal α-FAPbI3 after transition from the cubic phase. As mentioned above, the pure cubic α-FAPbI3 has no remarkable defect-induced PL, which means that such defects are mainly generated during the structural phase transition, i.e., the competition of one phase will lead to the disorder of the other phase, even after its PL is completely unobservable. The creation of the defects again demonstrates the chaos and disorder of two-phase competition, which leads to the distortion of some α-FAPbI3 crystals.
4. Discussion
Halide perovskites are easily affected by temperature to produce phase transitions. For example, MAPbI3 undergoes a phase transition from cubic to tetragonal at ∼ 315 K, and from tetragonal to orthorhombic at ∼ 160 K [31]. CsPbI3 undergoes a phase transition from cubic to tetragonal in the range from 398 to 598 K depending on the different ligands [33]. At room temperature, FA(1-x)MAxPbI3 leads to its crystal phase transition from tetragonal to cubic with the increase of FA content [27]. The structural change of the [PbI6]4- octahedron in perovskites is the main cause of the phase transition, and those with an organic group at the A-site is softer and more prone to deformation [34].
Stress is an important factor leading to deformation and distortion in organic-inorganic hybrid perovskites [28,35,36]. At room temperature, FAPbI3 is a cubic perovskite [37], however, due to the distortion of the ideal structure, including tilting, deformation, expansion and contraction of the octahedral network, the local strain increases [36,38,25]. When two phases coexist, previous study has shown that strain is introduced into the material during phase transformation [39]. The two coexisting phases observed in α-FAPbI3 exert stress on each other by tearing the other phase, thus exhibiting out-of-order competing behaviors, even to the point that after complete transformation to the low-temperature phase, the residual stress still produces many defects. Owning to the biphasic competence and phase transition-induced residual stress, α-FAPbI3 exhibits such abnormal PL behaviors. Our research provides valuable insights into the study of perovskite phase transition.
5. Conclusion
In summary, we have produced stable α-FAPbI3 via a modified method, and investigated its temperature- and excitation intensity-dependent PLs. Abnormal PLs due to the structural phase competence or transition-induced disorder were observed. We attribute them to the impact of stress induced by the phase distortion. During the biphasic coexistence, the deformation of one phase exerts influence on its own PL, as well puts stress on the other phase to affect its PL, eventually disordered and complex PL behaviors were shown up. Our results provide a better understanding of the structural phase transition of halide perovskite.
Funding
National Natural Science Foundation of China (11604061, 11664003, 11664007, 12164005); Guangxi Provincial Natural Science Foundation Project (2018GXNSFAA050014, 2021GXNSFAA220044).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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