Curing of degraded MAPbI₃ perovskite films

Abstract

Perovskite films were prepared using one-step solution deposition. The prepared films were degraded for 14 days under ambient atmosphere with controlled humidity of 60 ± 5%. The degraded films showed separated regions of PbI₂-rich and methylammonium iodide (MAI)-rich phases. Through X-ray, UV and electron beam irradiation, a fast recovery of perovskite was initiated in the MAI regions and the photoproduct further spread into the PbI₂ regions over a time frame of minutes. Ultraviolet and electron beam were also explored as irradiation sources and the same photoinduced perovskite formation was observed in each case. XRD results proved that the photoproduct is indeed tetragonal perovskite MAPbI₃. A slower conversion process can be achieved by storing the degraded samples at 25% ± 5% humidity. It is also observed that samples degraded at high humidity can be recovered by thermal annealing at a temperature as low as 35 °C. In this work, recovery of the aged perovskite films from decomposed phases (intermediate phase and PbI₂) is shown to be feasible, which paves a new way for sustainable perovskite-based solar cells.

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Perovskite based materials offer the possibility to fabricate low-cost and high efficiency solar cells that provide the potential for large-scale production without the need for complex processing.^1–5 Such materials can be processed from simple solution deposition techniques and moderate temperature annealing.^2,5 Solar cells based on these compounds possess several advantages such as a wide light-absorption range that is tunable by managing chemical composition, ambipolar charge transport and very long electron–hole diffusion lengths.^6–13 The electronic band structure and lattice dynamics of halide perovskites are also well-known.^14,15 Perovskites have been applied as light absorbers onto thin compact or mesoscopic metal oxide layers in solar devices.^16–19 Although the organic–inorganic hybrid semiconductor can be fabricated by multiple or single solution processes,^20,21 the materials suffer from problems such as a lack of long-term stability and degrading conversion efficiency due to fundamental excitation energy losses associated with the interface.^22–28 Relatively little is known about the degradation of organic–inorganic hybrid perovskites under controlled conditions and their influence on the photovoltaic (PV) properties. There are only few reports on the recovery of the perovskite crystal structure under controlled relative humidities.^29,30 However, the curing of degraded perovskite by photo-irradiation could make a critical difference for their optical properties and PV performance. The degradation through the intermediate state with the incorporation of solvent or H₂O molecules as long as it is not exhausted to an extreme extend can be recovered back by the elimination of extra coordinated molecules in the perovskite compound. This work investigates the photo-regeneration and recovery of degraded samples under the irradiation with X-ray, UV and electron beam. Under controlled conditions, the recovery could be observed by low humidity exposure and thermal annealing. The XRD analysis confirmed that the phase degradation is reversible and it can possibly be practical to recover degraded perovskite materials, and potentially even aged perovskite solar cells. This opens up a new path to the recovery of the microstructure of organic–inorganic hybrid perovskites and reusable perovskite-based solar cells.

The perovskite films were prepared by drop casting and sintered at 70 °C for 45 min under reduced atmospheric pressure of 12.7 torr. Then, the films were degraded at a controlled relative humidity level of 60 ± 5% for 14 days. The reason for choosing the humidity level of 60 ± 5% is to select a humidity level that is close to the ambient moisture atmosphere in summer which is most commonly encountered for outdoor solar cells. We selected for 14 days because we found experimentally that the drop-casted sample can be degraded almost completely with very few portions of the perovskite remaining and thus the sample was ready to use for recovery experiments. The X-ray recovery was realized using the X-ray beam of our XRD instrument. To monitor the phases inside the degraded film, fast XRD scans (4.5 min per scan) were employed. All the scans were added on the same sample leading to increased scan numbers. The samples were then recovered by exposing the degraded perovskite films to X-ray irradiation for 45 min (10 scan cycles). The humidity level of the XRD room was also maintained at 60 ± 5% to avoid any effects due to changes in R.H. The resulting XRD patterns of the samples during X-ray recovery are shown in Fig. 1. It can be observed that the black 70 °C_45 min sample were decomposed into yellow and light yellow phases after 14 days of degradation at R.H. 60 ± 5% at room temperature. However, through the consecutive XRD scans, the degraded phase was gradually consumed and the tetragonal phase of perovskite was recovered. To confirm that the degraded perovskite was recovered, in Fig. 1 the XRD patterns of sample 70 °C_45 min at 6 different stages of recovery were labeled with the calculated XRD patterns using MDI Jade software. From the comparison in Fig. 1, the peaks of the tetragonal perovskite phase were indexed; the unidentified peaks that did not match either the pure PbI₂ or MAI tetragonal phase were assigned to an intermediate phase. This intermediate phase might be a complex of PbI₂–MAI–DMF (the residual DMF from the fabrication), a hydrate complex of MAPbI₃ or non-stoichiometric PbI₂-rich/MAI-rich phase arising from the degradation of the sample.

Fig. 1 The XRD patterns of a MAPbI₃ perovskite film degraded under R.H. 60% ± 5% for 14 days with the increasing number of scans. The film was gradually recovered from degraded phases.

During the recovery process of the degraded sample, the peaks of the intermediate phase decreased while the peaks of the tetragonal perovskite phase increased. While the transition was in progress during the 10 scans of X-ray irradiation both the original peaks of decomposed phases and small peaks of newly recovered perovskite can be observed. From the comparison of the peak intensities between the patterns respectively recovered after 1 scan and 10 scans of X-ray radiation, it can be inferred that a fast recovery can be completed within 45 min. The XRD patterns of a sample from fresh to recovered stages are shown in Fig. S1.† The recovered sample displayed slightly different orientation from the fresh sample and residual intermediate phases were observed. A control experiment for the recovery process was made using a spin-coated sample degraded under the same condition for 3 days. However, recovery of the spin-coated sample could not be achieved by consecutive X-ray irradiation. Unlike the drop casted sample, a large portion of the degraded spin-coated sample is PbI₂ instead of the intermediate phase as shown in Fig. S2.† The MAI in the spin-coated samples may decompose into CH₃NH₂ and HI that slowly evaporate as gases, eventually leaving behind PbI₂. To prevent complete degradation of the spin-coated perovskite film, the sample was degraded for only 3 h under R.H. 60%. Recovery is found to occur within the first 5 min, with the fast reduction of monohydrate phase due to exposure to a low humidity level of 25% in Fig. S3† and further recovery by X-ray irradiation for 45 min (even 72 min) didn't come into effect as the dihydrate phase remained. This sample was then stored for 3 days under R.H. 25% and the XRD pattern reveals a recovered perovskite phase with a different crystal orientation. The sharp (202) peak from the fresh sample decreases significantly in the recovered sample, which may imply a diffusion path of water molecules along <202> direction. The photograph shows a color change for the sample from dark brown to white and back to black with some luster and the SEM images in Fig. S4† manifest the corresponding morphology change from short-rod clusters to the aligned stripes with larger grain size. The average grain size estimated from XRD patterns by the Williamson–Hall method is also shown in Fig. S5.† The recovered sample has increased grain size along with reduced lattice strain in contrast to the fresh sample. Indeed we also analyzed the grain size of drop-casted film before degradation and after recovery using the Williamson–Hall method, as shown in Fig. S5,† and by SEM (S7–10). No principle trend of both grain size and microstrain was found that might be due to the variation in the solution processing, i.e. the grain size obtained by solution deposition through the same heat treatment might be varied mainly due to the evaporation rate of the solvent, the surface conditions of the substrate (heterogeneous nucleation), and distribution of the solute in the drops.

To investigate the morphology and composition of the degraded film, new samples were prepared by drop casting with sintering at 70 °C for 45 min under reduced pressure and then degraded at R.H. 60 ± 5% for 14 days. The SEM images and EDX spectra of degraded films are shown in Fig. 2.

Fig. 2 The SEM images and EDX spectra of degraded films prepared by drop casting and sintering at 70 °C for 45 min, (A) PbI₂-rich section; (B) MAI-rich section.

It can be observed that the decomposed part has small rod-like grains and is composed of PbI₂ as a main phase and a minor amount of MAI while the white part has grains of a lath shape and contains a relatively high percentage of MAI and a small amount of PbI₂ relative to the fresh film based on EDX. This is the result of phase separation of PbI₂ and MAI. To make sure that the EDX results are as close to the actual composition of degraded films as possible, we performed the EDX scan as quickly as possible to the degraded parts of the film by using the optical microscope integrated in our SEM (degraded parts appear yellowish). The most possible care was taken to produce the best possible measurements of the film compositions.

Surprisingly, the degraded sample used for EDX analysis were also recovered after 45 min exposure to electron beam, as shown in Fig. 3. The XRD pattern of the recovered sample agreed well with the tetragonal perovskite phase and only a minor amount of intermediate phase still existed. Although the sample was exposed under the electron beam for 45 min, we can expect that overall a shorter time would be needed for the same extend of recovery since the electron beam has a higher photon energy (15 keV) over X-ray (8 keV) for Cu Kα. For 4.5 min, the larger portion of the degraded film should be recovered in contrast to X-ray recovery.

Fig. 3 The XRD pattern of the recovered perovskite film after electron beam exposure. The inset shows photographs of the sample degraded 14 days at R.H. 60 ± 5% and recovered, after 45 min exposure to a 15 kV electron beam.

The photon energy and intensity of the irradiation sources seem to be the key parameters affecting the observed recovery time. It is not easy to tell exactly how long it would take for the whole film under the direct exposure to either X-ray (8 keV) or electron beam (15 keV) since the scanning method was used for XRD that is described in the experimental part of the manuscript. The following difficulties are encountered in such experiments. For X-ray, the illumination area is changing all the time despite the interaction volume is constant. For the electron beam in the SEM, it is difficult to determine the exact time intervals of interaction in the SEM. Therefore, our experimentally determined time intervals are not very fine tuned, but we expect that increased photon energy and intensity might lead to reduced recovery time.

To test the recovery of perovskites using UV irradiation, a perovskite film was drop casted and sintered at 70 °C for 90 min under reduced pressure. The longer annealing time was needed to remove the residual DMF and eliminate possible intermediate phase. The photographs of freshly prepared perovskite and the sample degraded at R.H. 60 ± 5% for 14 days are shown in Fig. S6.† The recovery of degraded film was initiated using 390 nm UV light irradiation. The difference between the UV irradiation and X-ray scanning to accomplish recovery is not only manifested in the photo energy (X-ray, λ = 1.54 Å and UV, λ = 3900 Å) but also in the scanning method. The irradiation area and penetration depth are changing with the rotating sample holder during the X-ray scanning at a fixed speed. The static irradiation using UV beam has the constant area and stationary penetration depth. The degraded sample after 15 minutes UV irradiation and stored at 25 ± 5% for 1 day are shown as insets in Fig. S6,† respectively.

It can be observed that the black perovskite sample was decomposed into yellow and white phases after 14 days degradation at humidity level of 60 ± 5%. Through the 15 min UV irradiation, the white region (MAI-rich) phase transformed first. To avoid X-ray induced recovery, the XRD patterns were only obtained before degradation and after recovery shown in Fig. S6.† By analyzing the XRD patterns, it can be found that the peaks of tetragonal perovskite after recovery are well matched to the pattern of non-degraded perovskite although PbI₂ and intermediate phases still exist in a small proportion. A fast transition process occurs when the sample is excited with UV irradiation that allows for partial recovery. Secondly, placing the film into a 25 ± 5% R.H. allows for a slow transition converting most of the remaining degraded phase. This result implies that a low humidity level can also aid in the healing process of perovskite films from degraded phases.

To verify the influence of the low R.H., another degraded perovskite film was placed under R.H. 25 ± 5% for recovery, as shown in Fig. 4. It can be seen that the degraded sample became dark with the transformation of light yellow phases after 3 hours of exposure to the atmosphere of lower R.H. at 25 ± 5%. We propose that humidity may play a role in activating the diffusion of the charged methylammonium (MA⁺) inside the film. The larger portion of transformation could be achieved with the aid of low humidity atmosphere. The XRD patterns of the sample after recovery are shown in Fig. 4. It can be confirmed that the peaks of this sample after recovery agree well with the XRD peaks of tetragonal perovskite phase.

Fig. 4 The XRD pattern of degraded perovskite film after 3 h storage at 25 ± 5% humidity. The inset shows photographs of a perovskite film degraded 14 days at 60 ± 5% humidity and stored 3 h at R.H. 25 ± 5%.

Since the ionic diffusion of the MA⁺ might be the reason for the recovery of perovskite from the degraded PbI₂ rich or MAI rich phases, the heat treatment that can provide the activation energy for MA⁺ diffusion act as a healing process.

It can be observed in Fig. 5 that the degraded sample recovered after annealing at 35 °C for 45 min and the XRD spectrum confirmed the formation of newly generated tetragonal perovskite. This implies the thermal activation energy could be estimated by kΔT to be about ∼0.86 meV which could be the binding energy of hydrogen bond between MA⁺ and inorganic [PbI₆]⁴⁻ cage under photo-illumination.

Fig. 5 The XRD pattern of a degraded perovskite film after heat treatment at 35 °C for 45 min. The inset shows photographs of perovskite film degraded 14 days at 60 ± 5% humidity and heat treated at 35 °C for 45 min.

The synthesis of the perovskite films used for X-ray and electron beam recovery was completed by sintering at 70 °C for 45 min under reduced pressure. The perovskite film for UV-irradiation were prepared by sintering at 70 °C for 90 min under vacuum respectively. The sample used for the water recovery and heat treated recovery was also prepared at 70 °C for 45 min under reduced pressure. All the prepared films were degraded in a sealed container with a controlled humidity level of 60 ± 5% for 14 days. The degraded films were then recovered using photo irradiation of X-ray, UV, and electron beam, respectively at room temperature. The SEM images and EDX spectra of fresh and recovered films using different method were presented in Fig. S7–S11.† The non-stoichiometric atomic ratio comes from the rough surface of the sample, the structurally changing, and surface degradation by electron beam exposure during the EDX measurement. The fan-shaped morphology formed in the fresh sample prepared by drop-casted under vacuum. After the recovery, the long lamina become short irregular lobes. In the sample recovered by X-ray irradiation, the formation of stoichiometric perovskite grains nucleated from the PbI₂ rich bulk body is observed while in the sample recovered by electron beam, the formation of stoichiometric perovskite grains nucleated from the MAI rich bulk body. The formation of MAI rich perovskite grains nucleated from the PbI₂ rich bulk body is found in the samples recovered by low moisture and thermal annealing respectively.

The fast transition progress of fast transition in the recovery by X-ray and electron beam can be observed in the XRD patterns. The conversion of degraded components can be quickly finished in about 45 min. A slow transition can occur after storing the degraded sample at low humidity level of 25 ± 5%. It is proposed that the irradiation intensity, higher photon energy of irradiation, lower moisture level and higher thermal annealing temperature would result in reduced recovery times for the same and the spin-coated perovskite samples. The recovery for the spin-coated sample however was most successful with low-moisture treatment.

From the similar phenomena observed, we hypothesize that the photo recovery by X-ray, electron beam and UV is nearly the same. Due to the high energy of the photons or electrons, electrons in the valence band of perovskite could be excited into the conduction band which reduces the polarity of the [PbI₃]⁻ lattice. According to work reported by Zaban et al.,³¹ there should be a decrease in the binding energy of the MA ions to the inorganic cage when going from the ground to the excited state. The binding energy reduction originated by the I → Pb charge-transfer depleting the negative charge carried by the iodine atoms along with the in-plane lattice expansion could allow the MA cations to rotate more freely or even diffuse inside the PbI₂ cage upon light excitation shown in Fig. 6.

Fig. 6 The schematic of hypothesized mechanism for photo-induced structural change according to ref. 31, indicating the reorientation of MA⁺ ion and changes of lattice constants between the dark structure (left) and light excited structure (right) with an expansion in a–b plane and a shrinkage along c axis.

Sargent et al.³² estimated the formation energy of the CH₃NH₃PbI₃ perovskite in terms of its decomposed pure PbI₂ and MAI phases by DFT calculations. The result indicated that the difference between the formation energy of the perovskite and its decomposed PbI₂ and MAI phases has a low value of about −0.1 eV, demonstrated as a schematic in Fig. 7. This suggests that the material and its precursors are close to the coexistence state of MAI and PbI₂ phases and the reversible process could be facilitated by means of small energy stimuli regardless of any forms.

Fig. 7 Schematic of the hypothesized mechanism for reversible formation of CH₃NH₃PbI₃ perovskite (tetragonal structure) from its bulk precursors CH₃NH₃I and PbI₂, with a kinetic transition energy, calculated about −0.1 eV difference according to ref. 32.

Petrozza et al.³³ have also demonstrated that in ambient air, the moisture-facilitated ion motion occurs via a hydrated phase, in which the organic methylammonium cation is loosely bound and can drift in response to an electric field, finally leading to the field-induced degradation. Liu et al.^34,35 have shown that the water molecules can easily adhere to the surface of the perovskite crystals and penetrate into the inner region due to the larger interspaces in the crystal structure. The adsorbed water molecules forming hydrogen bonds with MA⁺ moieties are more likely to move and escape after placing the sample under the low moisture condition and also promote the rearrangement and motion of MA molecules inside the bulk of the degraded film. This could be the possible reason that the white region changes first and then the slow transition involving PbI₂ occurs after 1 day storage at 25% ± 5% humidity. The interaction between the photons and perovskite components could induce the transformation of perovskite by charge transfer and ionic diffusion with increased efficiency. Finally, the degraded sample can also be recovered by controlled heat treatment at 35 °C for 45 min indicating the diffusion of MA cations can be thermally activated with low thermal energy.

In summary, we demonstrated that the degraded perovskite film prepared by drop casting and sintered at a humidity level 60 ± 5% can be recovered by irradiation. Excitation sources such as XRD, UV, and an electron beam can be employed to induce transformation of degraded constituents of either MAI rich or PbI₂ rich phases. Minor amounts of intermediate phase due to different degrees of intercalations between organic MA⁺ and inorganic framework may exist in the recovered perovskite film. The lower humidity 25 ± 5% favors further recovery of the perovskite while a R.H. 60 ± 5% would be detrimental and can promote the degradation of the perovskite film. Bulk recovery of degraded perovskite film following the presented protocol was demonstrated to be feasible. This provides the potential of recovery by exposure to controlled atmosphere. We expect that additional optimization of the recovered MAPbI₃ film can be achieved at longer storage time in controlled atmosphere with a humidity level of 25 ± 5% which can enable the rearrangement of MAI molecules and reduce the number of defects in the material. We have also shown that samples degraded at medium-to-high R.H. can also be recovered by thermal annealing.

Acknowledgements

We gratefully acknowledge the assistance of Weiyin Gao from Xi'an Jiaotong University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The calculated XRD data of perovskite and hydrates using the Jade MDI. The control of recovery experiment for spin-coated samples. The SEM images and EDX spectra of fresh and recovered films. See DOI: 10.1039/c6ra09714f

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