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. 2023 Mar 28;3(3):230–240. doi: 10.1021/acsnanoscienceau.2c00065

Effect of Air Exposure on Electron-Beam-Induced Degradation of Perovskite Films

Romika Sharma †,*, Qiannan Zhang ‡,*, Linh Lan Nguyen †, Teddy Salim †, Yeng Ming Lam †, Tze Chien Sum ‡, Martial Duchamp †,*
PMCID: PMC10288607  PMID: 37360848

Abstract

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Organic–inorganic halide perovskites are interesting candidates for solar cell and optoelectronic applications owing to their advantageous properties such as a tunable band gap, low material cost, and high charge carrier mobilities. Despite making significant progress, concerns about material stability continue to impede the commercialization of perovskite-based technology. In this article, we investigate the impact of environmental parameters on the alteration of structural properties of MAPbI3 (CH3NH3PbI3) thin films using microscopy techniques. These characterizations are performed on MAPbI3 thin films exposed to air, nitrogen, and vacuum environments, the latter being possible by using dedicated air-free transfer setups, after their fabrication into a nitrogen-filled glovebox. We observed that even less than 3 min of air exposure increases the sensitivity to electron beam deterioration and modifies the structural transformation pathway as compared to MAPbI3 thin films which are not exposed to air. Similarly, the time evolution of the optical responses and the defect formation of both air-exposed and non-air-exposed MAPbI3 thin films are measured by time-resolved photoluminescence. The formation of defects in the air-exposed MAPbI3 thin films is first observed by optical techniques at longer timescales, while structural modifications are observed by transmission electron microscopy (TEM) measurements and supported by X-ray photoelectron spectroscopy (XPS) measurements. Based on the complementarity of TEM, XPS, and time-resolved optical measurements, we propose two different degradation mechanism pathways for air-exposed and non-air-exposed MAPbI3 thin films. We find that when exposed to air, the crystalline structure of MAPbI3 shows gradual evolution from its initial tetragonal MAPbI3 structure to PbI2 through three different stages. No significant structural changes over time from the initial structure are observed for the MAPbI3 thin films which are not exposed to air.

Keywords: organic−inorganic halide perovskites, transmission electron microscopy, time-resolved photoluminescence, X-ray photoelectron spectroscopy, microstructural changes

Organic–inorganic halide perovskites have sparked a huge scientific interest for photovoltaics (PV) and optoelectronic applications such as light-emitting diodes (LEDs)1−4 owing to their advantageous properties such as tunable band gap,5 low material cost,6 and high charge carrier mobilities.7 Because of these properties, organic–inorganic halide perovskites are one of the most promising candidates for next-generation low-cost high-efficiency LEDs. Perovskite light-emitting diodes (PeLEDs) with external quantum efficiencies (EQE) above 20% in the visible and 21.6% in the near-infrared regions8 have been demonstrated.

Despite making significant progress, commercialization of the perovskite-based technology is still hampered by material stability concerns. Large-area perovskite deposition frequently results in inconsistent films with poor coverage, holes, and tiny grains. This issue not only makes it difficult to achieve a high-performance device but also accelerates the film decomposition in the environment. Increased moisture,9 oxygen,10 temperature,11 light illumination,12 or a combination of those13,14 have been found to cause faster degradation of organic–inorganic halide perovskites, which is often caused by structural instability resulting from ion migration15 and imperfections in interfacial layers such as lattice distortion and phase disintegration.16,17

To develop devices from organic–inorganic halide perovskites, it is critical to understand the intrinsic structure, structural stability, and decomposition process of these materials at all scales. The understanding of the intrinsic structures of materials can be advanced using electron microscopy. Unfortunately, halide perovskites are electron beam-sensitive, posing a huge challenge for structural characterizations by electron beams. As a result, there is a critical need to develop techniques that limit the harm caused by electron beams to retrieve intrinsic structural information.

Recently, there has been a lot of discussion on the challenges and limitations of electron imaging on beam-sensitive perovskite materials.18−24 Numerous studies have shown that the temperature,24 dose rate,20,24 and total electron dosage19,21,23 all have an impact on the rate at which MAPbI3 perovskite degrades during electron beam experiments.

It is generally known that perovskites are sensitive to air and moisture. Since most techniques for sample preparation and transfer often result in unintended air exposure, it is difficult to perform high-resolution structural and chemical studies on non-air-damaged perovskite films. To overcome this, we employ the air-free transfer transmission electron microscopy (TEM) holder, which permits the air-free transfer of samples from a dry environment (a glovebox) to the transmission electron microscope.

It is observed that MAPbI3 thin films that have not been exposed to air suffer from less electron beam damage as compared to air-exposed films. This implies that characterizations of perovskite films which are not exposed to air will lead to higher total damage-free doses. This could have an impact on electron microscopy research approaches for beam-sensitive perovskites and highlights the increased need for device encapsulation to prevent degradation.

We also compare the microstructural and optical trap density evolution of MAPbI3 layers shortly exposed to a controlled air environment versus films maintained in an air-free atmosphere. It is to be noted that sample transfers from the glovebox to the characterization instruments are done using an air-free transfer shuttle to prevent any unintentional air exposure.

Two distinct degradation paths for the microstructural evolution of MAPbI3 thin films depending on their history of prior exposure to air environments are observed. The two degradation pathways observed by TEM are further supported by X-ray photoelectron spectroscopy (XPS) and time-revolved photoluminescence (TRPL) measurements, albeit through the observation of chemical changes and recombination traps which are created upon air exposure of MAPbI3 films.

Results and Discussion

To image the pristine perovskite structure, we first determine the critical electron dose before any structural changes are visible in the MAPbI3 thin films, for both air-exposed and non-air-exposed thin films. Different reports have shown that both the total electron dose and dose rate have an impact on the degradation of MAPbI3 perovskite.18−24 The dose rates employed in TEM studies for the observation of perovskite materials over the last few years are summarized in Table 1.

Table 1. Representative Works of Electron-Beam-Related Characterizations on MAPbI3/MAPbBr3.

structure method dose rate [eÅ–2 s–1] damage information references
MAPbI3 thin film TEM 200 kV ∼1 twin domains disappear Rothmann et al.18
MAPbI3 thin film TEM 200 kV 2 formation of a √2 × √2 supercell after 2 min of exposure Rothmann et al.19
MAPbI3 nano-crystals HRTEM 80 kV 0.5–4 a two-step degradation process begins with the loss of MA+ Chen et al.20
MAPbI3layers TEM/EDX 200 kV ∼1 degradation starts at 12 s and reaches saturation within 120 s of exposure time Alberti et al.21
MAPbBr3 HRTEM 300 kV low doses atomic structure level details are shown; the normal and parallel arrangements of the CH3NH3 cations (relative to the projection direction) are observed Zhang et al.22
MAPbI3 nanowires cryo-electron microscopy (cryo-EM) 3.8 for MAPbI3 cryogenic electron dose tolerance of MAPbI3 is 12 eÅ–2 Li et al.23
MAPbBr3 nanowires   25.6 for MAPbBr3 MAPbBr3 is 46 eÅ–2  
MAPbI3 thin film liquid cell TEM 50 degradation (coalesce) formation within 25 s Qin et al.24
MAPbI3 thin film HRTEM 200 kV 1–4 in-built strain during preparation causes the degradation Jones et al.25
MAPbI3 thin film TEM 300 kV 4.2 for air exposed sample degradation for the air exposed sample this study
    5.5 for no air exposed little/no degradation for sample not exposed to air (using an air-free transfer holder)  
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The electron beam damage-related observations presented in these research studies can be summarized as follows. Rothmann et al., who acquired selected area electron diffraction (SAED) patterns from the MAPbI3 thin films, reported the disappearance and reappearance of twin domains at a low dose rate of 1 eÅ–2 s–1.18 In another study, Rothmann et al. reported the formation of a √2 × √2 supercell after 2 min of exposure at 2 eÅ–2 s–1 and continued it for 18 min (total dose ≈2000 eÅ–2).19 Chen et al. studied the impact of the dose rates and proposed a disintegration mechanism in the case of single-crystalline MAPbI3.20 Alberti et al. noted that MAPbI3 films when subjected to a dose rate of 1 eÅ–2 s–1 shows Pb clustering at the grain boundaries after 12 s exposures, i.e., for a total dose of 12 eÅ–2. They further noted that after 120 s, i.e., for a total dose of 120 eÅ–2, the MAPbI3 to Pb transition reached saturation.21 Zhang et al. used a direct-detection electron-counting camera to image, in real space, the high-resolution structure of CH3NH3PbBr3 (MAPbBr3) at low doses (below 1 eÅ–2 s–1). They were able to distinguish the two different configurations of the CH3NH3 cations.22 Li et al. used cryo-electron microscopy (cryo-EM) to extract the structure of MAPbI3 and quantified the cryogenic electron dose tolerance of MAPbI3 to be 12 eÅ–2.23 Qin et al. used time-lapse liquid-cell TEM imaging and observed that MAPbI3 nanoparticles exhibited a relatively high endurance ability to electron irradiation. However, a dose rate of less than 90 eÅ–2 s–1 was still required to avoid the beam damage.24 Jones et al. used a low electron dose rate of 1–4 eÅ–2 s–1 to identify that heterogeneity and strain introduced into the film during processing are two of the reasons for the degradation of the perovskite films observed in TEM.25 All these studies emphasize the need for a low electron dose for the characterization of beam-sensitive perovskites. However, fewer studies have looked at the effect of air exposure on MAPbI3 perovskite or its influence on the electron beam-induced irradiation degradation pathways.20,26

Figure 1a–d illustrates a time series of SAED patterns taken from an initially pristine MAPbI3 thin film which was exposed to an air atmosphere for 2 min at 60% relative humidity (RH). A 300 kV electron beam was continuously applied to the film at a dose rate of 4.21 eÅ–2 s–1. To better demonstrate the microstructural evolution as a function of the total electron dose, the rotationally averaged SAED patterns of the time series were plotted versus the two-theta angle for the MAPbI3 film after 2 min of air exposure (Figure 1e). The peaks at the diffraction angles of ∼20.7° correspond to the diffraction from the (020) plane, and the peaks at ∼36.6° correspond to the (Inline graphic05) plane of the [501] zone axis in tetragonal MAPbI3.

Figure 1.

Figure 1

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Time evolution of SAED patterns for MAPbI3 films which are exposed to the atmosphere for 2 min. (a–d) Initial SAED pattern matches the [501] zone axis in tetragonal MAPbI3. Additional spots appear at 10 s (total dose ∼ 42.1 eÅ–2) as marked by yellow circles. After 30 s (total dose ∼ 126.3 eÅ–2), twin peaks are observed as marked by white circles. (e) Time evolution of the rotationally averaged SAED pattern for MAPbI3 observed in TEM for samples which are exposed to air (2 min). The dose rate is ∼4.21 eÅ–2 s–1.

Three phases can be observed in Figure 1e: the initial structure up to ∼10 s exposure, i.e., a total dose of 42.1 eÅ–2; the second phase structure up to 30 s, i.e., a dose of ∼126.3 eÅ–2; and a final phase, where diffraction peaks are hardly visible. After 10 s of irradiation (a total dose of 42.1 eÅ–2 and the start of phase 2), new diffraction spots (as indicated by the yellow circles in Figure 1b) appear, indicating the presence of a new intermediate phase.

Some studies mis-identified and incorrectly classified the air-exposed decomposed structures as pristine MAPbI3.27,28 Kim et al. reported that the origin of new diffraction spots can be due to the coexistence of tetragonal and cubic phases in MAPbI3 perovskite thin films.29 In refs17 and (21), the authors argued that the inadequate reflections in the MAPbI3 perovskite thin films are linked to the presence of PbI2 and further electron beam irradiation breaks down PbI2 into metallic lead (Pb). Rothmann et al.19,30 suggested that additional reflections are consistent with the formation of a √2 × √2 supercell. In perovskite systems, this typically arises from octahedral tilts or rotations. Despite the lack of consensus on the intermediate states involved in the degradation of MAPbI3, most authors agreed that MAPbI3 undergoes continuous structural and compositional changes under TEM, which leads to the formation of lead iodide byproducts.

The new diffraction spot at ∼45.4° which appears at 10 s (42.1 eÅ–2) and diffuses by 30 s (∼126.3 eÅ–2) as illustrated in Figure 1e is ascribed to the (103) plane of PbI2. The different phases observed can be divided into 3 stages. For the sample exposed to air, phase 1 is observed for doses less than 42.1 eÅ–2 (10 s). It is the state initially observed in TEM. Though the diffraction spots match the [501] zone axis, the absence of some spots points out that it may not be the pristine tetragonal MAPbI3 structure. Such an intermediate phase was noted by Chen et al.20,26 to be MA0.5PbI3, MAPbI2.5, or MaxPbI3y due to locally ordered vacancies that exist before the structure collapses. Phase 2 is obtained for 42.1 eÅ–2 (10 s) to 126.3 eÅ–2 (30 s). Phase 2 emerges when the original reflections of phase 1 become more diffuse and new reflections corresponding to PbI2 appear. Phase 3, observed above a dose of ∼126.3 eÅ–2 (>30 s), is the state where all spots diffuse resulting from the decomposed product.

It has previously been reported12,14 that light-induced degradation processes of MAPbI3 are different, depending on whether the thin film has previously been exposed to air. However, such an effect has not yet been reported at a local scale, neither has the evolution of its microstructure when the MAPbI3 thin film is not exposed to the air environment. In this section, we investigate the degradation pathway in MAPbI3 thin films which are not exposed to air. Figure 2 shows no appearance of diffraction spots other than the tetragonal MAPbI3 phase for a cumulative electron dose of 513.3 eÅ–2. Thus, the samples which are not exposed to air do not degrade for a dose below ∼500 eÅ–2. However, the “split spots” characteristic of twin domains30,31 are clearly visible in SAED (indicated as white circles in Figure 2b) which are also observed in the air-exposed sample (indicated as white circles in Figure 1c).

Figure 2.

Figure 2

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Time evolution of SAED patterns for MAPbI3 films which are not exposed to the atmosphere (using an air-free transfer holder). (a–d) SAED pattern matches the [315] zone axis in tetragonal MAPbI3. Twin peaks are observed as marked by white circles. (e) Time evolution of the rotationally averaged SAED pattern for MAPbI3 observed in TEM for samples which are not exposed to air (using an air-free transfer holder). The dose rate is ∼5.58 eÅ–2 s–1.

To avoid exposing the sample to the beams while searching for the suitable zone axis, the acquisition of the TEM data was carried out using the “blanking” technique, which means that the beam is blanked after the initial focusing as a new location is found; thus, the crystal is not necessarily aligned with the zone axis. Consequently, a significant number of diffraction patterns were recorded outside the zone axis. However, these out-of-zone axis diffraction patterns cannot be analyzed reliably due to dynamic scattering events. Therefore, the series that were in different zone axes rather than similar out-of-zone axis diffraction patterns were analyzed.

To support the generalizability of the observations irrespective of the zone axes, diffraction patterns from different zone axes for air- and non-air-exposed samples are presented in the Supporting Information S1 and S2. Similar to the observations in Figures 1 and 2, it is observed that air-exposed samples (Figure S1) undergo structural changes, whereas samples not exposed to air (Figure S2) do not change structurally when observed under an electron beam.

In addition to the total dose, the decomposition of perovskites is also considered to be sensitive to the dose rate.20,24,32 The same experiments were repeated at a dose rate of 0.82 eÅ–2 s–1 to investigate the impact of low dose rate on the structural changes in MAPbI3. Figure S3 shows the time evolution of the rotationally averaged SAED intensity for the two samples, i.e., MAPbI3 exposed to air for 3 min and not exposed to air. For the air-exposed sample, the SAED peaks retreat gradually as seen in Figures S3a, but new peaks at 29 and 30.5° are observed after 65 s (53.4 eÅ–2) and the structure undergoes further transformation at 140 s (115.1 eÅ–2). The total dose required for the formation of phases 1, 2, and 3 is quite similar to the one reported for larger electron dose rates (Figure 1e) where the structural changes, i.e., new peaks, are observed at ∼42 and ∼120 eÅ–2. Moreover, for the film not exposed to air (Figure S3b), similar to the earlier reported results, no new diffraction spots were observed even at this dose rate. Though the intensity of the diffraction spots diminishes within 270 s (i.e., with a total dose of 221.9 eÅ–2), the absence of any new spot suggests that the MAPbI3 structure has not undergone any structural transformation yet.

Time evolution of SAED patterns gives information on the evolution of the crystallographic structure of the thin film but does not describe the morphological evolution of the texture. We investigate the structural evolution of the MAPbI3 thin film exposed to a dose rate of 8.03 eÅ–2 s–1 in a real space. The signal-to-noise ratio of the images is further increased by applying denoising algorithms principal components analysis (PCA) within an open-source software suite.33Figure 3a1–a5 shows bright-field (BF) TEM images of the MAPbI3 layer not exposed to air. There is no significant change observed in the grain morphology for the sample not exposed to air. The corresponding FFT patterns are displayed in the bottom row (Figure 3b1–b5). An atomic lattice pattern consistent with a tetragonal [010] MAPbI3 orientation is obtained. No diffraction peaks are observed initially for a time period of 12 s, i.e., a dose of 96.3 eÅ–2; either the film has not yet crystallized, or more likely, the sample is not in the zone axis. Hence, it shows the presence of reflections (indicated by red circles) after an imaging time of ∼18 s (total dose of 144.5 eÅ–2), and the FFT pattern now matches with the [010] MAPbI3 orientation. The diffraction spots match the [010] MAPbI3 tetragonal structure even after a dose exposure of 38 s (total dose ∼305.1 eÅ–2). This shows that the MAPbI3 tetragonal structure has not degraded even after a dose exposure of 305 eÅ–2, though the intensities of the diffraction spots have diminished.

Figure 3.

Figure 3

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(a1–a5) Time evolution of BF TEM image of the MAPbI3 layer not exposed to air. (b1–b5) Corresponding FFT patterns at different time stamps, i.e., with increasing electron dose exposure. The dose rate is ≈8.03 eÅ–2 s–1.

A series of BF TEM pictures of the MAPbI3 layer pre-exposed to air for 2 min is shown in Figure 4c1–c5. The equivalent FFT patterns are shown in the second row (Figure 4d1–d5). New diffraction spots corresponding to PbI2 (yellow circles in Figure 4d5) are noticed after imaging for ∼24 s (total dose of 192.7 eÅ–2). This demonstrates that air-exposed MAPbI3 thin films degrade within a dosage exposure of ∼192 eÅ–2. These results are consistent with the time-dependent SAED evolutions depicted in Figures 1 and 2.

Figure 4.

Figure 4

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(c1–c5) Time evolution of BF TEM image of the MAPbI3 layer exposed to air for 2 min. (d1–d5) Corresponding FFT patterns at different time stamps, i.e., with increasing electron dose exposure. The dose rate is ≈8.03 eÅ–2 s–1.

Results for Optical Measurements

Air-induced microstructural changes in MAPbI3 thin films also have an influence on the optical properties, yet the correlation is not clearly established. To do so, we study the evolution of the optical response using TRPL experiments to examine the influence of air exposure on free carrier recombination lifetimes. A comparison with TEM experiments presented in the previous section will allow us to establish the relation between the microstructural evolution and the optical response. The optical response of the sample exposed to different air-exposure times is presented below, starting with the non-air-exposed sample (0 min) to 2 min air exposure. Figure 5a presents TRPL decay curves at 766 nm before and after 2 min exposure to air with RH 58%. The corresponding PL spectra are shown in Figure 5b. By comparison, 2 min exposure to air resulted in PL quenching, indicating suppressed radiative recombination as well as enhanced nonradiative recombination. To assess carrier lifetime changes, TRPL kinetics were fitted in a biexponential decay function

graphic file with name ng2c00065_m002.jpg 1

where I stands for PL intensity, Inline graphic refers to time decay, A1 and A2 are weight factors for each decay species, and τ1 and τ2 stand for PL fast and slow decay lifetime. The fitting results are summarized in Table 2. The average lifetime is calculated based on

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Figure 5.

Figure 5

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Effect of air exposure on the PL lifetime of MAPbI3 thin films. (a) Time-resolved PL kinetics at 766 nm before and after 2 min exposure to air (RH 58%, 19.2 °C) under excitation of a 400 nm pulsed laser beam (fluence 2.3 μJ/cm2). (b) PL spectra before and after 2 min exposure to air (RH 58%, 19.2 °C). (c) PL lifetime as a function of time for samples exposed to air (RH 58%, 19.2 °C). (d) PL lifetime as a function of time for samples under an air-free environment (vacuum).

Table 2. Fitting Results of TRPL Kinetics at 766 nm by the Biexponential Decay Functiona.

air exposure time
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0 0.2 ± 0.1 1.7 ± 0.1 1.3 ± 0.1
2 0.2 ± 0.1 1.5 ± 0.1 1.1 ± 0.1
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a

Instrument resolution is 0.1 ns. The uncertainty for both lifetimes is instrument resolution. The uncertainty for average lifetime is calculated based on error propagation.

The slow decay lifetime τ2 consistent with previously reported values34 is the monomolecular recombination lifetime dominated by trap states, and τ1 is the bimolecular recombination lifetime due to the radiative recombination of electrons and holes in the MAPbI3 thin film. On comparison we see that as the air exposure time is increased from 0 to 2 min, the change of τ1 is negligible and within system resolution (0.1 ns). However, the reduced radiative free carrier recombination is evident in the PL intensity drop in Figure 5b. At the same time, τ2 decreases, which means that the trap-assisted recombination rate is faster and more traps gradually form. These TRPL data were collected when samples were in vacuum after exposure to air. As a reference, TRPL experiments were also conducted in vacuum without air exposure. Figure 5c shows the decreasing average radiative lifetime of MAPbI3 for a period of 220 min exposure to air due to gradual degradation. However, no significant change with time is observed under vacuum as shown in Figure 5d. Figures S4 and S5 show the SEM and XRD patterns for samples subjected to air for a prolonged period. These results are consistent with the TEM and TRPL observations.

Results for XPS Measurements

To assess the chemical changes that the exposure has caused to the films, the photoelectron spectra of MAPbI3 thin film samples both before and after air exposure were measured. The core-level shifts in XPS can be associated with changes in the chemical state. For this purpose, the samples were exposed to air for a longer period of 220 min to advance the degradation process which facilitates the observation of the final decomposed products.

Figure 6 demonstrates the narrow-scan XPS spectra of C 1s, N 1s, Pb 4f, and I 3d from MAPbI3 samples before and after air exposure.

Figure 6.

Figure 6

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XPS spectra of the MAPbI3 films with and without air exposure: (a) C 1s, (b) N 1s, (c) Pb 4f, and (d) I 3d.

As shown in Figure 6a, the C 1s spectra of both MAPbI3 films can be deconvoluted into two peaks at ∼284.8 and ∼286.4 eV, corresponding to the C–C and C–N bonds, respectively. The C–C peak is attributed to the adventitious carbon,35 which arises from the adsorption of hydrocarbon species from the atmosphere. In the pristine sample, the amount of both carbon groups appeared to be comparable to each other. Upon air exposure, however, the intensity of the C–N peak was reduced to approximately half that of the C–C peak. This indicates that the air exposure triggers the release of the volatile amine group (MA+) from the organic salt in the perovskite.36,37 Additionally, it should be highlighted that it is plausible that the air exposure has also caused a widespread carbon contamination, resulting in an increase in the aliphatic adventitious carbon.35 The elemental atomic ratio of C–N and C–C bonds in the non-air-exposed and air-exposed MAPbI3 is shown in Table S1a.

The N 1s core level at ∼402.3 eV also showed an apparent decrease in the peak intensity on air exposure (Figure 6b). The reduced intensity of the N 1s peak correlates well with the loss in the C–N group as shown earlier in the C 1s spectra.

Figure 6c shows the Pb 4f spectra for MAPbI3 films. On air exposure, additional peaks are observed at 136.8 and 141.6 eV, which correspond to spin–orbit components of the metallic (Pb0) peaks, in addition to the Pb2+ peaks corresponding to the perovskite phase, which were also observed in the non-air-exposed MAPbI3 film. The elemental atomic ratio of Pb2+ and Pb0 in both MAPbI3 films, summarized in Table S1b, shows that the air exposure has induced significant formation of the metallic Pb phase. Previous studies36−39 have also observed the presence of metallic lead in MAPbI3, which is related to the excessive precursor PbI2 in the MAPbI3 or loss of I2. The presence of the Pb0 peak only in the air-exposed MAPbI3 film indicates that the Pb0 peaks are likely to be the byproducts of the decomposition of PbI2 based on the following reaction: PbI2 → Pb(s) + I2(g). Also, a decrease in intensity along with a small negative shift from 619.25 to 619.18 eV was detected for I 3d5/2 on air exposure (Figure 6d), which indicates the escape of iodine.38

The wide-scan XPS spectra of both films, i.e., before and after air exposure, are shown in Figure S6. The observed peak positions (Table 3) of non-air-exposed samples, which were determined from the associated high-resolution narrow-scan spectra, are consistent with earlier MAPbI3 XPS studies.38,40,41 The change in the compositions (relative atomic concentration) of Pb, I, N, and C of the MAPbI3 film on air exposure, which is derived from the background-subtracted peak integrals from the respective XPS spectra, is shown in Table 4.

Table 3. XPS Peak Position for MAPbI3 Thin Films with and without Air Exposurea.

  Pb 4f (Pb2+) Pb 4f (Pb0) I 3d or Br 3d* C 1s (C–C) C 1s (C–N) N 1s
not exposed to air, MAPbI3 138.50 136.8 619.25 285.0 286.4 402.39
exposed to air, MAPbI3 138.50 136.7 619.18 284.8 286.6 402.24
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a

For the doublets, only the main peak is listed.

Table 4. Atomic Concentration (at. %) of the MAPbI3 Thin Film.

details Pb I N C I/Pb
not exposed to air, MAPbI3 13.2 36.1 12.2 38.5 2.73
exposed to air, MAPbI3 25.3 37.6 4.4 32.7 1.48
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Non-air-exposed MAPbI3 has an initial elemental ratio (N/Pb/I) of 1:1.08:2.95, which is close to the initial stoichiometric value of CH3NH3PbI3. On air exposure, the I/Pb atomic ratio falls from 2.73 to 1.48 along with a significant drop in the N/Pb ratio from 0.92 to 0.17.

This indicates the decomposition of MAPbI3 into methylamine and lead iodide (PbI2), followed by further decomposition of the lead iodide phase into Pb and I2.

Discussion

The TEM, XPS, and optical measurements show that the degradation pathway of MAPbI3 thin films varies depending on whether the samples are exposed to air or not. The schematic representation of the different phases of degradation for the MAPbI3 thin film exposed to (i) 2–3 min in air and (ii) only in nitrogen and vacuum environments is shown in Figure 7.

Figure 7.

Figure 7

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Schematic representation of different phases of MAPbI3 thin film degradation, for the sample exposed to air and not exposed to air.

No degradation was observed for the sample not exposed to air. However, the MAPbI3 thin film exposed to air underwent different phase changes as observed in electron microscopy which can be summarized as follows:

Phase 1: distorted tetragonal MAPbI3 (likely MA0.5PbI3, MAPbI2.5, or MAxPbI3–y) for doses less than ∼40 eÅ–2 in our air-exposed sample. During this phase, the main component in the thin film structure is still MAPbI3.

Phase 2: this phase occurs between a dose of ∼40 and 120 eÅ–2 for our air-exposed sample. The components in this phase are MAxPbI3–y + PbI2. This phase is marked by the appearance of PbI2 peaks.

Phase 3: this phase exists above a dose of ∼120 eÅ–2 for our air-exposed sample. In this phase, the diffraction spots diffuse as the electron beam breaks the bonds between Pb and I along with MAxPbI3–y intermediate products.

Many decomposition pathways in air have been suggested.41−43 However, the most accepted one according to our findings can be written as

Phases 1 and 2

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The organic residue is

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Phase 3

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Moisture and oxygen have been shown to limit MAPbI3’s long-term stability and drastically reduce the device performance.13,41−43 In a study by Philippe et al., one perovskite sample is maintained in ambient air and the other in an argon-filled chamber with low oxygen content. Both air and argon exposures showed degradation, but argon degradation was slower.41 In another study by Wang et al., PbI2 structures are clearly visible in the perovskite film exposed to ambient-air environments, but are less visible in nitrogen-rich environments.42 This suggests that MAPbI3 degradation accelerates in the presence of water/oxygen. The surface of MAPbI3 nanowires eroded even after only 10 s of moisture contact.23 Li et al. suggested that iodide is oxidized to create iodide anions in the presence of light and air, which causes deterioration.23 According to Deretzis et al.,43 moisture (60% RH) and temperature stress (150 °C) significantly reduce the lifespan of MAPbI3 thin films.

As shown in this study, even a brief air exposure for a few minutes changes the structure of the MAPbI3 film, and these changes become the precursors that further accelerate the degradation processes under electron-beam illumination. This study highlights the need to consider the air-exposure history of beam-sensitive materials for electron microscopy studies.

Conclusions

In this study, we explore the impact of air exposure on the structural stability of the MAPbI3 thin film using TEM, XPS, and TRPL measurements. Based on combined optical, structural, and chemical characterizations, we establish different phase transformation pathways based on whether MAPbI3 has any prior air exposure. Even when exposed to air for a few minutes, MAPbI3 thin films undergo substantial structural degradation. On the contrary, the deterioration kinetics are substantially slower for MAPbI3 films that are not exposed to air. The structural degradation also leads to the formation of more traps and compositional changes in the films.

This implies that large damage-free doses can be used to characterize perovskite thin films in electron microscopy if the films are not exposed to air. Also, perovskites with better air stability may sustain higher doses and could be less susceptible to beam-induced degradation. Our findings could have an impact on electron-microscopy research on beam-sensitive perovskites and highlight the need for device encapsulation to prevent degradation.

Methods

Material Preparation

The MAPbI3 thin film was prepared by a standard solution processing procedure. The 0.2 M precursor solution was synthesized by mixing equal ratios of CH3NH3I (Greatcell Solar materials) and PbI2 (TCI) in DMF (Sigma-Aldrich) and stirring for 2 h at 80 °C in a nitrogen glovebox. For the spin-coating process, a copper (Cu) grid with lacey carbon was fixed onto a clean glass substrate. Then, a filtered precursor was dropped onto the substrate and spin-coated at 4000 rpm for 30 s. 80 μL of toluene was added as an antisolvent at the 5th s. Then, it was annealed for 30 min at 100 °C.

Transmission Electron Microscopy

The degradation of the MAPbI3 thin film under air-free vacuum conditions versus under air exposure was investigated using a JEM-ARM300F, a 300 kV double-aberration-corrected transmission electron microscope. The air-free atmosphere was maintained by using the Mel-Build air-free transfer TEM Specimen Holder.

X-ray Photoelectron Spectroscopy

XPS spectra were collected directly from the Cu grid with lacey carbon samples using an AXIS Supra spectrometer (Kratos Analytical Inc., UK) equipped with a hemispherical analyzer and a monochromatic Al Kα source (1.487 keV) operated at 15 mA and 15 kV. The spectra were acquired from an area of 700 × 300 μm2 with a take-off angle of 90°. Pass energies of 160 and 20 eV were used for survey and high-resolution scans, respectively.

Time-Resolved Photoluminescence

For optical measurements, the Cu grid with lacey carbon was transferred onto a clean quartz after the MAPbI3 thin film deposition. The Cu grid with the lacey carbon grid was used to ensure that the same structure is characterized by TEM and TRPL. Then it was transferred into a cryostat to control the moisture and maintain vacuum conditions. TRPL data were collected at a backscattered angle with 400 nm pulsed laser excitation with a fluence of 2.3 μJ/cm2 by using a Optronis streak camera system. To match with the TEM data acquisition condition, the data collection condition for TRPL was also in vacuum.

X-ray Diffraction

A Shimadzu XRD-6000 was used for obtaining the XRD spectra.

Scanning Electron Microscopy

A ZEISS crossbeam 450 was used to obtain the SEM images.

Acknowledgments

Financial support from the Nanyang Technological University and the Ministry of Education Tier 2 grant MOE2019-T2-2-066 is gratefully acknowledged. Q.Z. and T.C.S. also acknowledge the support from the AcRF Tier 2 grant MOE2019-T2-1-006. We would like to acknowledge the Facility for Analysis, Characterization, Testing and Simulation, Nanyang Technological University, Singapore, for use of their electron microscopy/X-ray facilities.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsnanoscienceau.2c00065.

  • Time evolution of SAED patterns for MAPbI3 films exposed to atmosphere and not exposed to atmosphere; time evolution of rotationally averaged SAED patterns (versus intensity) for MAPbI3 films observed in TEM exposed to air and not exposed to air; SEM images of MAPbI3 thin films in vacuum and after exposure to air; XRD spectra of MAPbI3 thin films; elemental atomic ratio of C–C/C–N and Pb2+/Pb0 in MAPbI3 films; XPS survey spectra of the MAPbI3 thin film with and without air exposure; and simulations of different zone axes using JEMS (PDF)

Author Contributions

Microscopy experiments and characterizations were done by R.S., L.L.N., and M.D. Optical measurements were done by Q.Z. and T.C.S. XPS measurements were done by T.S. and Y.L.M.

The authors declare no competing financial interest.

Supplementary Material

ng2c00065_si_001.pdf (956KB, pdf)

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Associated Data

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Supplementary Materials

ng2c00065_si_001.pdf (956KB, pdf)

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