Temperature Dependence of Photochemical Degradation of MAPbBr₃ Perovskite

Abstract

The experimental results of X-ray diffraction (XRD), optical absorbance, scanning electron microscopy (SEM), and X-ray photoelectron spectra (XPS) of the core levels and valence bands of MAPbBr₃ (MA-CH₃NH₃⁺) perovskite before and after exposure to visible light for 700 h at temperatures of 10 and 60 °C are presented. It reveals that the light soaking at 60 °C induces the decomposition of MAPbBr₃ perovskite accompanied with the decay of organic cation and the release of a PbBr₂ phase as a degradation product whereas the photochemical degradation completely disappears while the aging temperature is decreased to 10 °C.

1. Introduction

Organic−inorganic hybrid perovskites of the ABX₃ type (A–organic cation, B–metal cation, and X–halide anion) have been the subject of numerous studies since 2009 when it was first shown that these materials can be used in photovoltaic applications [1]. After 3 years, the first work on the creation of highly efficient organic−inorganic perovskite solar cells marked a new era in photovoltaic research [2]. Since then, a real boom in these studies has begun, aimed primarily at replacing expensive silicon solar cells, because of the low cost of the technological process (about $2.5/cell) and a fairly high energy conversion coefficient, currently reaching up to 25.8% [3]. The presence of such unique properties, namely high absorption coefficients, low exciton binding energy, high mobility of charge carriers, and lengths of electron−hole diffusion, allow their wide use in optical and electronic fields, including solar cells, photocatalysis, light-emitting diodes (LED), photodetectors, lasers, and so on [4,5,6,7].

Up to the present time, the largest number of studies of hybrid halide perovskites relates to lead triiodide (MAPbI₃). On the other hand, the studies have shown that MAPbBr₃ is another promising perovskite material for optoelectronics and photovoltaics [8,9,10]. Having a higher hole mobility, MAPbBr₃ has certain advantages over MAPbI₃. For example, it has been found that sensitive and inexpensive X-ray detectors made from MAPbBr₃ single crystals have an extremely low surface charge recombination rate and an extended diffusion length [11]. MAPbBr₃ is nonpolar (centrosymmetric) [12], and, therefore, ferroelectricity can be excluded; the influence of grain boundaries can also be ignored. In addition, it is assumed that MAPbBr₃ has better air stability than MAPbI₃ [13,14,15].

The wide practical use of halide perovskites is hindered by instability in the presence of environmental elements such as light, oxygen, heat, and moisture [16,17,18,19]. While the interaction of hybrid perovskites with oxygen and moisture can be prevented by proper encapsulation [20], the question of improving the long-term thermal and photochemical stability is still unresolved. Most of this work is aimed at finding new compositions associated with the replacement or doping of all three components of ABX₃-hybrid perovskites, providing greater stability and a lifetime of devices using these materials. Much less attention is paid to operating conditions, including the influence of the temperature regime. Usually, the photodegradation variables are studied in laboratory experiments under conditions as close as possible to their operating conditions. As shown in Ref. [21], under UV irradiation of glass encapsulated perovskite for 260 s, the temperature of the device is increased from 26 to 48 °C because of the photocuring reaction by its natural exothermic reaction [22]. In this regard, most laboratory experiments that study the photochemical degradation of hybrid perovskites are carried out at a temperature of 45–60 °C. Until now, such studies have been mainly aimed at determining the time resource of these materials while only few works have been devoted to the study of the stability of hybrid perovskites under the light soaking at different temperatures [23,24,25]. It was shown in Ref. [24] that that irradiation of MAPbI₃ films with sunlight (100 Sun) for 60 min at a temperature of ~45–55 °C leads to the decomposition of the hybrid perovskite while degradation was not observed at a lower sample temperature (~25 °C). In our previous work [25], it was found that photolysis of MAPbI₃ has quite high effective activation energies of ∼85 kJ mol⁻¹, and decreasing the temperature from 55 to 30 °C can extend the MAPbI₃ perovskite’s lifetime by factors of >10−100. In this work, we present the results of studying the XRD, optical, and XPS spectra of the core levels and valence bands of MAPbBr₃ perovskite exposed to visible light (700 h) at two temperatures of 10 and 60 °C.

2. Experimental

All aging experiments were carried out in a nitrogen-filled glove box (MBraun, Stratham, NH, USA). The metal halide lamp was used for the aging of the MAPbB₃ films, which gave a good approximation of the AM1.5G solar spectrum. The power of light incident on the samples was 100 mW/cm² while two equilibrium temperatures, 60 and 10 °C, were maintained. Fan cooling was used in the first case whereas aggressive fluidic cooling was employed in the second case to keep the reasonably low sample temperature at high light fluxes. The light-induced decomposition of MAPbBr₃ films was monitored by periodic ultraviolet−visible spectroscopy measurements. The absorption spectra were recorded using a AvaSpec-2048-2 UV-VIS fiber spectrometer (Avantes, Apeldoorn, Netherlands) integrated inside a glove box.

The XRD patterns were obtained using a Aeris diffractometer with a CuKα anode (Malvern PANalytical B.V., Almelo, Netherlands). The measurements were carried out in the fixed illumination mode without sample rotation. Perovskite films were encapsulated prior to the measurements by spin coating a polystyrene solution (50 mg/mL in toluene) on the top of the films inside a glove box at 2000 rpm. The sample exposed to air was limited to approximately 6 min (5 min scans).

Scanning electron microscopy images were obtained on a Zeiss SUPRA 25 instrument (Carl Zeiss Industrielle Messtechnik GmbH, Oberkochen, Germany). The samples were prefixed on a microscope stage inside the glove box to reduce the exposure time in air to ~1 min.

The X-ray photoelectron spectroscopy (XPS) method was used to measure core-level and valence band (VB) excitation with the help of a PHI XPS 5000 VersaProbe spectrometer (ULVAC-Physical Electronics, Chanhassen, MN, USA) with a spherical quartz monochromator and an energy analyzer in the range of binding energies (BE) from 0 to 1500 eV. Electrostatic focusing and magnetic screening was used to achieve an energy resolution of ∆E < 0.5 eV for the Al Kα radiation (1486.6 eV). The XPS spectra were recorded using a 200 µm spot size. The X-ray power delivered at the sample was less than 50 W, and the pass energy was 46.95 eV. XPS spectra were processed using MultiPak 9.9.0.8 software (ULVAC-Physical Electronics, Chanhassen, MN, USA).

3. Results and Discussion

Exposure to light leads to a decrease in the optical density of the spectra and the photobleaching of the perovskite films, as one can conclude from the evolution of the optical spectra presented in Figure 1. It was shown that MAPbBr₃ thin films strongly depredated under light exposure at the temperature of 60 °C (Figure 1b) while a decrease in temperature to 10 °C gives a positive effect, and photochemical degradation is practically absent.

XRD patterns were used to determine the changes in the phase compositions of the films under study. The results of aging in the light at different temperatures show noticeable differences. Although a change in the crystallinity of the film is observed at a temperature of 10 °C, no new phases appear. On the other hand, light aging of perovskite films at 60 °C for 700 h leads to the appearance of new peaks in XRD patterns characteristic of PbBr₂ (Figure 2).

Significant differences in the aging of films at temperatures of 10 and 60 °C are also observed in SEM images (Figure 3a–c). Noticeable changes occur in the film when exposed to light and a temperature of 60 °C. Light aging at this temperature for 700 h leads to the loss of the domain structure and film uniformity. MAPbBr₃ films, on the other hand, do not show any visible changes in the domain microstructure after aging under light exposure at 10 °C for 700 h. However, we note that aging also occurs at 10 °C, as evidenced by the appearance of pinholes (upper right corner in Figure 3b).

Using XPS to study the photochemical stability of hybrid perovskites solves several problems [26,27]. First of all, the measurements of the XPS survey spectra make it possible to obtain the surface composition and to determine the degree of purity of the obtained samples. In this respect, the spectra shown in Figure 4 and the quantitative data determined from them in

Table 1. Surface composition (in at. %).

Sample	C	O	N	Pb	Br	Si	Sb	N:Pb	Br:Pb
MAPbBr3 as prepared	34.9	–	11.6	19.0	33.7	–	0.8	0.61	1.77
photo 10 °C, 700 h	72.4	9.9	2.2	5.7	9.8	–	–	0.38	1.71
photo 60 °C, 700 h	66.9	10.0	1.8	9.0	11.8	0.5	–	0.20	1.31

prove that there are no uncontrolled impurities in the studied samples. Further, the obtained quantitative data given in Table 1 allow us to determine the N:Pb and Br:Pb ratios. The first ratio makes it possible to determine the dynamics of the decomposition of an organic cation, and the second one makes it possible to view the change of the photochemical decomposition of the MAPbBr₃ perovskite with the release of a PbBr₂ phase since the Br:Pb ratios in these two compounds are significantly different.

An analysis of the data obtained shows that when MAPbBr₃ is exposed to visible light for 700 h at a temperature of 60 °C, a sharp drop in both ratios is observed, which indicates both the decomposition of the organic cation (CH₃NH₃⁺) and the release of a PbBr₂ phase as products of photochemical decomposition. On the other hand, a decrease in temperature to 10 °C does not lead to a change in the ratio Br:Pb, and the ratio N:Pb only shows a slight decrease. This undoubtedly indicates at least a significant slowdown, if not the absence, of photochemical degradation at the indicated temperature.

In order to verify these conclusions, the high-energy resolved XPS spectra of core levels (XPS Pb 4f and Br 3d) and valence bands were measured (see Figure 5a–c). For comparison, the XPS spectra of the PbBr₂ compound were measured as a product of the possible photochemical degradation. First of all, we note that, as in the case of XPS survey spectra, the notable changes are observed for the XPS Pb 4f (Figure 5a) and Br 3d (Figure 5b) spectra of MAPbBr₃ light soaked at 60 °C. A high-energy shift and broadening of these spectra in the direction of the spectra of the PbBr₂ reference compound are observed, which undoubtedly confirms the photochemical decomposition of the MAPbBr₃ samples and the separation of the PbBr₂ phase as a degradation product. Finally, the measurements of the XPS valence band spectra fully confirm all these conclusions. As shown in Figure 5c, the spectra were normalized to the intensity of the Pb 5d-spectrum, which made it possible to estimate the degree of perovskite decomposition due to the PbBr₂ phase precipitation, compared to their relative intensities [28]. From the presented spectra, it can be seen that only a slight decrease in the relative intensity of XPS VB is observed upon irradiation at 10 °C while a dramatic decrease in intensity is noted for a temperature of 60 °C and an almost complete coincidence with the spectrum of the reference PbBr₂ compound.

4. Conclusions

In summary, we studied the photochemical stability of MAPbBr₃ perovskite films at a dose of 700 h and temperatures of 10 and 60 °C. It was found that at T = 60 °C, the organic cation methyl ammonium (CH₃NH₃⁺) decomposed, and the PbBr₂ phase separated as products of photochemical decomposition. On the other hand, no degradation was observed after exposing the MAPbBr₃ film at the same light intensity and dose but at a lower aging temperature of 10 °C with up to 600 h of aging, indicating that photoinduced degradation is thermally enhanced. Therefore, one can conclude that the observed photochemical degradation is accelerated by the heat.