Heat- and Gas-Induced Transformation in CH3NH3PbI3 Perovskites

Article pubs.acs.org/cm

Heat- and Gas-Induced Transformation in CH3NH3PbI3 Perovskites and Its Effect on the Efficiency of Solar Cells Weixin Huang,†,‡ Subha Sadhu,† and Sylwia Ptasinska*,†,§ †

Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, United States Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States § Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡

S Supporting Information *

ABSTRACT: Following the remarkable success of the application of CH3NH3PbI3 perovskites in photovoltaics, a great focus has been placed on their stability to improve their optoelectronic properties and seek commercial production. To gain a better understanding of their thermal stability, we investigated the chemical, morphological, and photovoltaic transformations of CH3NH3PbI3 perovskites under elevated temperatures and various controlled atmospheric conditions (vacuum, 1 mbar O2, and 1 mbar H2O). A temperature-dependent study showed that CH3NH3PbI3 decomposed to PbI2 with the release of CH3NH2 and HI under low-temperature annealing (25−150 °C). Further annealing resulted in the formation of metallic lead (Pb0) under vacuum and Pb oxides and hydroxides under an O2 or H2O pressure. Moreover, the sublimation of Pb-based compounds occurred at temperatures above 150 °C, causing structural changes, which resulted in a decrease in the power conversion efficiency of the solar cell. A time-dependent study showed that, compared with vacuum conditions, the addition of O2 or H2O accelerated the degradation of the perovskite films. the excess halide-containing precursors.9,19−22 It has been reported that annealing of perovskite films at 100−200 °C led to a decrease in the photovoltaic performance of solar cells.19 Similarly, temperature-dependent morphological studies showed that severe degradation occurred at temperatures above 250 °C.23 Thus, a detailed understanding of the effects of the annealing conditions is essential in respect to the device longevity and material fabrication. In addition, the decomposition of CH3NH3PbI3 perovskites can be triggered by water or oxygen molecules at room temperature.10,24 In this study, we investigated the chemical and morphological changes of CH3NH3PbI3 perovskites during thermal treatment. To obtain a deeper understanding of the role of gaseous environments, we compared the changes in perovskite films annealed under vacuum, O2 or H2O pressure. Furthermore, the treated films were incorporated into solar cells, and their performances were also measured. Temperature- and timedependent studies showed that changes in the surface chemistry, morphology, and photovoltaic efficiency of perovskite solar cells were related directly to the gas environment to which they were exposed. Our study provided a correlation between the chemical changes that CH3NH3PbI3 films undergo

1. INTRODUCTION Over the past several years, the interest in hybrid organic− inorganic lead halide perovskites (CH3NH3PbX3, X = I, Cl, and Br) has surged because of their potential significance in optical and photovoltaic applications.1−4 The power conversion efficiency (PCE) of hybrid perovskite solar cells has doubled to approximately 20% in recent years.5 The high PCE, simple fabrication techniques, low manufacturing costs, and relative insensitivity to defects make perovskite solar cells highly promising in photovoltaic technology. However, despite the impressive achievement in device efficiency, the stability of perovskite materials is a key challenge before they can be applied broadly.6−9 This concern has sparked a series of studies on the influence of external factors (e.g., humidity, oxygen, ultraviolet light, and polar molecules) on the transformation of perovskites.9−16 One of the key requirements, specifically for solar cell applications, is the long-term thermal stability of perovskites. The nominal solar cell operating temperature can reach up to 85 °C or can easily exceed this temperature in extreme environments or if solar radiation concentrators are utilized.17,18 Therefore, exploring the thermal stability of perovskite materials is critical for the development of perovskite solar cells that are suitable for device applications. Additionally, the thermal annealing process is indispensable for perovskites synthesis because it plays a crucial role in the removal of residual solvent, perovskite crystallization, and sublimation of © 2017 American Chemical Society

Received: August 1, 2017 Revised: August 31, 2017 Published: September 1, 2017 8478

DOI: 10.1021/acs.chemmater.7b03243 Chem. Mater. 2017, 29, 8478−8485

Article

Chemistry of Materials

Figure 1. (a) Pb 4f, (b) I 3d5/2, and (c) N 1s spectra of the CH3NH3PbI3 films annealed under vacuum. Peak assignment: (I) 136.9 eV ± 0.1 eV attributed to Pb0; (II) 138.5 eV ± 0.1 eV to Pb2+-I; (III) 619.3 eV to Pb2+-I, and (IV) 402.5 ± 0.1 eV to CH3NH3+. eV). The binding energy (BE) of all the spectra was calibrated with respect to the BE of Au 4f7/2 (84.0 eV). The uncertainty of the BE values for all the assigned spectral peaks was in the range of 0.1−0.2 eV. The ratios of N/Pb, I/Pb, Pb0/Pbtotal, and Au/Pb were estimated N IS using the following relation: N1 = I1 S2 , in which N is the concentration

when exposed to different annealing conditions and the efficiency of solar cells containing the treated films.

2. EXPERIMENTAL SECTION We prepared CH3NH3PbI3 perovskites from methylammonium iodide (CH3NH3I) and lead iodide (PbI2) dissolved in N,N-dimethylformamide (DMF) according to a procedure reported previously.10,20 A Au foil used as a substrate was cleaned by ultrasonication for 30 min in a detergent solution (FisherBrand Versa-Clean from Fisher Scientific), followed by rinsing with deionized (DI) water and ethanol. Then, the substrate was cleaned with plasma in an air atmosphere for 5 min. Prior to deposition, the substrate was loaded into a vacuum chamber with an Ar pressure of 10−5 mbar and sputtered using an Ar+ beam of 1.7 keV for 10 min in order to remove any surface contamination. In a typical synthesis of CH3NH3PbI3, a mixture of 0.32 mM CH3NH3I and 0.32 mM PbI2 was dissolved in 0.5 mL of DMF to form a precursor solution. This solution was stirred for 2 h at 70 °C and then spin-casted onto the substrates at 2000 rpm for 30 s, followed by annealing at 70 °C for 10 min to form a CH3NH3PbI3 film structure. All steps were carried out in a dry nitrogen glovebox. The in situ characterization of the surface chemistry of CH3NH3PbI3 perovskite films was performed with a laboratory-based, ambient pressure X-ray photoelectron spectrometer with a monochromated Al Kα X-ray source (1486.6 eV). Within an analysis chamber, the spectrometer was equipped with an in situ reaction cell to allow the reactant gas to flow through the cell at a fixed temperature during data acquisition. A differentially pumped electrostatic lens system was integrated between the in situ reaction cell (with a capability to sustain pressures of up to 20 mbar) and a hemispherical energy analyzer (PHOIBOS 150, SPECS). The pumping system had three stages in which a quadrupole mass spectrometer was mounted on the second stage to monitor the change in the gas partial pressure during the in situ experiments. Two types of in situ experiments, temperature and time dependence, were performed for the perovskite films in the reaction cell using X-ray photoelectron spectroscopy (XPS). In the temperature-dependent experiments, the sample temperature was varied from 25 to 300 °C under ∼10−8 mbar (referred to as vacuum unless stated otherwise) or under 1 mbar pressure of O2 or H2O. The sample was heated at a rate of 5 °C/min, followed by 15 min of stabilization, before data acquisition. The temperature was measured using a K-type thermocouple with a chromel−alumel junction placed between the sample holder and the sample. In the time-dependent experiments, the samples were annealed to a certain temperature under vacuum or under 1 mbar pressure of O2 or H2O, and the changes in the photoelectron spectra were recorded as a function of time. All elemental spectra were collected at the same pass energy (20

2

2 1

of the atoms, I is the peak area of the element, and S is the atomic sensitivity factor. Under a gaseous pressure, photoelectrons with different kinetic energies exhibit different scattering cross sections and mean free path lengths.25,26 As a consequence, exposure to O2 and H2O led to a higher attenuation of photoelectrons from the I 3d5/2 and N 1s, which have lower kinetic energies (867.3 and 1084.1 eV) than those from Pb 4f7/2 (1348.2 eV). This indicated that the ratios obtained from the experiments in the gaseous environment were not as accurate as those in the vacuum. To determine the accurate ratios of I/Pb and N/Pb, the calibration curve was initially obtained by measuring a series of standard samples in both the vacuum and the gaseous environments. The synthesis of the standard samples followed the same method as described above for the CH3NH3PbI3 perovskites, but different amounts of CH3NH3I and PbI2 were dissolved in 0.5 mL of DMF to form samples with different atomic ratios (Table S1). The I/Pb and N/Pb ratios of the standard samples were measured under vacuum, which represent the actual I/Pb and N/Pb ratios of the samples. Then, these ratios were measured in 1 mbar pressure of O2 or H2O. A pressure of 1 mbar was chosen as a compromise between the gas reaction speed on the perovskite surface (higher gas pressure, more surface reactions) and a sufficient photoelectron signal (lower gas pressure, higher signal). The calibration curves were obtained by plotting the ratios measured under vacuum and the ambient conditions (Figure S1 in the Supporting Information (SI)). From the calibration curves, the I/Pb and N/Pb ratios of the perovskite samples measured under 1 mbar pressure of O2 or H2O were estimated, taking into account photoelectron scattering. Data processing and quantitative analyses of the photoelectron spectra were performed using Casa XPS software and using calibration curves (Figure S1). Perovskite solar cells were fabricated following a method reported previously.27 First, a portion of the fluorine-doped tin oxide (FTO) substrate was etched by depositing a thin, even layer of Zn powder, and then 2 mL of HCl solution was added dropwise and allowed to stand for ∼4 min. After etching, the substrate was sonicated in ethanol for 20 min and then cleaned by plasma for 5 min. To deposit the TiO2 blocking layer, 0.5 mL of 0.15 M titanium diisopropoxide bis(acetylacetonate) (75 wt % in isopropanol, Sigma-Aldrich, Milwaukee, USA) in 1-butanol (99.8%, Sigma-Aldrich) was spin-coated on the etched and cleaned FTO substrate at 700 rpm for 8 s, 1000 rpm for 10 8479


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Figure 2. Pb 4f and O 1s spectra with fitted components obtained for the CH3NH3PbI3 films annealed under 1 mbar pressure of (a) O2 and (b) H2O. Peak assignment: (I) 136.9 ± 0.1 eV attributed to Pb0; (II) 137.5 ± 0.1 eV to Pb4+−O; (III) 137.9 ± 0.1 eV to Pb2+−O; (IV) 138.5 ± 0.1 eV to Pb2+−I; (V) 138.7 ± 0.1 eV to Pb4+−O satellite and Pb2+−OH; (VI) 529.3 ± 0.1 eV to α-Pb2+−O/Pb4+−O; (VII) 531.1 ± 0.1 eV to β-Pb2+−O/ Pb4+−O satellite, and (VIII) 532.3 ± 0.1 eV to Pb2+−OH. s, and 2000 rpm for 40 s, and dried at 125 °C for 5 min. Thereafter, the mesoporous TiO2 layer was deposited on top of the TiO2 blocking layer by spin-coating 1 mL of 30 NR-D TiO2 paste (Dyesol) at 2000 rpm for 20 s, followed by annealing at 550 °C for 1 h. Next, the mesoporous TiO2-coated substrate was dipped in 20 mM aqueous TiCl4 solution at 90 °C for 10 min and then cleaned thoroughly with DI water and heated in an oven at 500 °C for 30 min. Next, 461 mg of PbI2, 159 mg of CH3NH3I, and 78 mg (71 μL) of dimethylsulfoxide (DMSO) (molar ratio 1:1:1) were mixed in 600 mg (631 μL) of DMF solution and stirred for 1 h at room temperature to prepare the perovskite solution for drop-casting on the titania electrode. Fifty microliters of the solution was spin-coated on the substrate at 4000 rpm for 25 s, during which time 0.5 mL of diethyl ether was added dropwise on the spinning substrate for 10 s to fabricate a transparent film. After that, the substrate was heated at 65 and 100 °C for 1 and 2 min, respectively. Then, to deposit the hole transport layer, 30 μL of a spiro-MeOTAD solution consisting of 72.3 mg of spiro-MeOTAD (SHT-263 livilux) (Merck), 28.8 μL of 4-tert-butylpyridine, and 17.5 μL of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg Li-TSFI in 1 mL acetonitrile (Sigma-Aldrich, 99.8%)) in 1 mL of chlorobenzene was spin-coated on the perovskite layer at 3000 rpm for 30 s. The substrate was kept under vacuum overnight and thereafter, 100 nm of a Au layer was deposited at a rate of 10 Å/s in a thermal evaporator under vacuum (here, 10−6 mbar) to form a counter electrode. Fabrication of the film and hole transport layer was carried out in a dry nitrogen glovebox. To investigate the effect of annealing temperature under vacuum and under different ambient conditions, the perovskite-coated TiO2 anode was first treated under specific conditions and then the hole transport layer was deposited on the substrate. Photocurrent−voltage characteristics were measured while the cell was irradiated with 100 mW/cm2 simulated solar irradiation generated by a 300 W xenon lamp with a 1 sun AM 1.5 G filter using a Princeton Applied Research 2273 (PARstat) potentiostat. The active area of the cell was 0.1 cm2. The voltage sweep was maintained at 50 mV/s for both reverse and forward scans. The film morphology was measured using an FEI Magellan-400 field emission scanning electron microscope (FESEM).

located at 138.5 and 143.4 eV, respectively, and attributed to Pb2+ ions.28 New peaks at 136.9 and 141.8 eV corresponding to metallic lead (Pb0) appeared and were dominant at temperatures above 150 °C.29 Thus, the thermal treatment induced the decomposition of perovskite films, in which Pb2+ ions were reduced. The I 3d5/2 and N 1s spectra of the CH3NH3PbI3 film at 25 °C showed peaks at BEs of 619.3 and 402.5 eV, respectively (Figure 1b,c). The intensities of both peaks became attenuated in the low-temperature region of 25−150 °C and further annealing at 200−300 °C resulted in the complete quenching of these peaks, suggesting that CH3NH3+ and I− ions were removed from the perovskite films. Figures 2 and S2 show the evolution of the Pb 4f, O 1s, I 3d5/2, and N 1s spectra of the perovskite films annealed under 1 mbar pressure of O2 or H2O. At annealing temperatures below 200 °C, the Pb 4f spectra showed no distinguishable change except for the appearance of the minor metallic Pb peaks that were also observed after annealing under vacuum. However, distinctive new peaks appeared at annealing temperatures above 200 °C. When the films were treated in the presence of 1 mbar pressure of O2 at temperatures above 200 °C, the three contributions could be fitted in the Pb 4f7/2 spectra, corresponding to Pb2+−O, Pb4+−O, and plasmon satellites of Pb4+−O, at 137.6, 138.0, and 138.7 eV,30−34 respectively (Figure 2a). The new contributions in the O 1s spectrum, located at 529.3 and 531.1 eV, were identified as the tetragonal (α) Pb2+−O/Pb4+−O and orthorhombic (β) Pb2+−O/plasmon satellites of the Pb4+−O structures, respectively.30−34 When annealed at temperatures above 200 °C, the CH3NH3PbI3 films exposed to 1 mbar pressure of H2O also showed the formation of Pb2+−O, Pb4+−O, and Pb4+−O/Pb2+−OH satellites at 137.6, 138.0, and 138.7 eV, respectively. The appearance of an O 1s peak at 532.3 eV confirmed the presence of Pb2+−OH.30,32 The BEs of all components in the Pb 4f7/2 and O 1s spectra for the perovskite films are summarized in Table S2. The I/Pb and N/Pb atomic ratios were calculated from the spectral peak area of the fitted components and by taking into account relative sensitivity factors (Figure 3a,b). Only the Pb2+ and Pb4+ components were considered in calculating the I/Pb and N/Pb ratios; the Pb0 peaks were excluded. As expected

3. RESULTS 3.1. Temperature-Dependent Study. Changes in the chemical composition of CH3NH3PbI3 films under vacuum (∼10−8 mbar) with increasing annealing temperature were recorded in the high-resolution photoelectron spectra (Figure 1). The Pb 4f7/2 and 4f5/2 peaks from the film at 25 °C were 8480


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To observe the morphological changes of the perovskite films treated at increasing temperatures under vacuum and different ambient conditions, we performed scanning electron microscopy (SEM) imaging. Figure 4 shows the SEM images of

Figure 3. Atomic ratios of (a) I/Pb, (b) N/Pb, (c) Pb0/Pbtotal, and (d) Au/Pbtotal for the CH3NH3PbI3 films as a function of annealing temperature under vacuum, and under 1 mbar pressure of O2 and H2O.

from the chemical formula, the ratios of I/Pb and N/Pb for the CH3NH3PbI3 films at 25 °C were found to be 2.9 and 1.1, respectively (Figure 3a,b). Under vacuum as well as ambient conditions, the N/Pb ratio dropped with increasing temperature, reaching 0 at 150 °C. A decline was also observed in the I/Pb ratio, reaching 2.0 at 150 °C. These results indicated the removal of CH3NH3+ and I− ions and the formation of PbI2. Furthermore, the gas products that formed under vacuum conditions were detected using an online mass spectrometer (Figure S3). The formation of methylamine (CH3NH2, m/z = 31) was observed, confirming the production of CH3NH2 during the transformation of CH3NH2PbI3 to PbI2. The constant mass spectrometric signal of HI (m/z = 128) likely resulted from the fact that the effusion rate of a gas is greater for lighter gases than for heavier gases (Graham’s laws).35 The heavier molecule (HI) is less likely to reach the aperture and effuse into the chamber to which the online spectrometer was integrated. At higher temperatures (200−300 °C), the I/Pb ratio decreased continuously and finally reached 0 at 300 °C under vacuum conditions, indicating total degradation of PbI2. However, as mentioned above, Pb0 was the dominant component in the Pb 4f spectra within this temperature range (Figure 1). As shown in Figure 3c, the increase in Pb0/ Pbtotal and the decrease in I/Pb provided clear evidence for the complete decomposition of PbI2 to form Pb0. Similar trends in the change of the I/Pb and N/Pb ratios were observed under O2 and H2O conditions (Figure 3a,b), suggesting the loss of CH3NH3+ and I− from CH3NH3PbI3. However, at 300 °C, the I/Pb ratios of the films annealed under O2 and H2O pressures decreased to 0.6 and 0.3, respectively, indicating the formation of other lead compounds, as assigned in Figure 2a,b. Figure 3c shows that, compared with the results under vacuum, the extent of Pb0 production decreased under the O2 and H2O pressures. According to the reports published earlier, lead oxides and hydroxides are produced from oxidation of metallic lead in the presence of oxygen and water.30,36 Thus, it is reasonable to conclude that the appearance of Pb oxides and Pb hydroxides in the perovskite samples originated from the oxidation of Pb0 under O2 and H2O conditions.

Figure 4. SEM images of the perovskite films annealed under vacuum at different temperatures: (a) 25 °C, (b) 100 °C, (c) 200 °C, and (d) 300 °C. The scale bar is 2 μm.

the perovskite films treated at different temperatures under vacuum. These images showed that the pristine film was coated uniformly and smoothly, and the morphology of the film remained unchanged after treatment at 100 °C for 15 min. The surface of the film became uneven and coarse after it was exposed to 200 °C for 15 min. In addition, a large number of cracks and huge pores were also observed. There was no perovskite residue on top of the FTO substrate after the film was treated at 300 °C for 15 min. Thus, the morphology of the film was the same as that of the bare FTO substrate (Figure S4). Figure S5 shows the morphological changes in the perovskite films treated at increasing temperatures under different ambient conditions. Compared with the film annealed at 100, 200, and 300 °C under vacuum, no significant changes in morphology were observed when the films were annealed at the same temperatures in the presence of 1 mbar pressure of O2 or H2O. Given the distinct heat-induced chemical and morphological transformations of CH3NH3PbI3 perovskites, we sought to understand how these changes can affect the PCE of solar cells. We fabricated a series of CH3NH3PbI3 solar cells according to a previously reported procedure,27 and measured the photovoltaic performance of devices that incorporated perovskite films treated under different temperatures and ambient conditions. To confirm that the changes observed in the photovoltaic efficiency occurred because of the transformation of the perovskite films, first we fabricated the anode by spincoating TiO2 and methyl ammonium lead iodide solution on the FTO substrate and then annealed the anode at different temperatures under vacuum and 1 mbar pressure of O2 or H2O. Next, we spin-coated the hole-transport material and then deposited a 100 nm thick gold layer to complete the fabrication of the solar cell. Figure 5a shows the current density−voltage plots of the pristine and perovskite cells fabricated after 8481


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Figures S8 and S9 show the changes in the Pb 4f, I 3d, N 1s, and O 1s spectra under these different conditions over 6 h. The typical acquisition time for recording one spectrum was shorter than 10 min. The changes in the N/Pb and I/Pb ratios as a function of treatment time were estimated from these spectra (Figure 6). Treatment of the films at 75 °C under vacuum

Figure 5. (a) Current density−potential curves for the perovskite solar cell fabricated using the films annealed at different temperatures under vacuum, measured at air mass 1.5 illumination and 100 mW/cm2 light intensity. RS and FS represent reverse (short circuit current (JSC) to open circuit voltage (VOC)) and forward scans (VOC to JSC), respectively. (b) Trend in the atomic ratio of N/Pb and the average PCE of the cells as a function of the annealing temperature of the perovskite films.

annealing the anode under vacuum at 100 and 200 °C. Both the forward and the reverse scans display the well-known J−V hysteresis of perovskite solar cell devices. For forward scanning, i.e., from a short to an open circuit (JSC to VOC), and reverse scanning, i.e., from an open to a short circuit (VOC to JSC), the scan rate was 50 mV s−1. The average photon-to-electron conversion efficiency of the pristine sample was 8.4%. The average efficiency decreased to 6.5% and 0.2% after the perovskite film was treated at 100 and 200 °C, respectively (Figure S6). Statistical analysis of all of the photovoltaic parameters (VOC, JSC, fill factor (FF), and efficiency) is shown in the SI (Figure S6). Compared with the pristine film, the efficiency of the cell decreased ∼23% and ∼97% after annealing the film under vacuum at 100 and 200 °C, respectively. The reduction in the cell’s efficiency with increased annealing temperatures was attributable to the decrease in JSC, VOC, and fill factor. Figure S7a,b shows the photovoltaic performance of the cells fabricated after annealing the anodes at 100 and 200 °C under 1 mbar of O2 and H2O. The light harvesting efficiency of the cells fabricated after annealing the perovskite film at 100 °C in an O2 or H2O environment decreased by ∼6% and ∼11% in O2 and H2O, respectively. As shown in Figure 5b, the change in chemical composition of the films annealed at different temperatures, as represented here by the atomic ratio of N/Pb, correlated well with the decrease of the average PCE. For instance, at 100 °C, the N/Pb ratio decreased by ∼30% from 1 to 0.7 owing to the removal of CH3NH3+ ions, which caused an ∼23% decrease in the PCE of the cell compared to the pristine sample (Figure 5a). Thus, the change in the chemical composition is one of the essential factors for the sharp drop in the efficiency after annealing. The alteration of the microstructure that occurred upon annealing (Figure 4) is also responsible for the decrease in photovoltaic efficiency. As demonstrated previously, the charge trap states are concentrated at the grain boundaries.37 Therefore, we expect that the coarsening of the crystalline surface and formation of pinholes in the films presented in Figure 4 increase the number of surface trap states, and thus a reduction of the photovoltaic efficiency. 3.2. Time-Dependent Study. To study the effects of two factors, the temperature and ambient gas, on the degradation of the perovskite films, a time-dependent study was performed over 6 h at 75 and 130 °C under vacuum and 1 mbar pressure of O2 or H2O. In this temperature range, these two factors are essential for perovskite film formation and crystal growth.1,38,39

Figure 6. Atomic ratios of I/Pb and N/Pb of the CH3NH3PbI3 films annealed at 75 °C (a and b) and 130 °C (c and d) as a function of annealing time under vacuum and 1 mbar pressure of O2 and H2O.

resulted in a slight decrease in the I/Pb and N/Pb ratios of the perovskite film, from 3.0 to 2.8, and 1.1 to 0.9, respectively, indicating the simultaneous removal of CH3NH3+ and I− from the perovskite structure. However, the perovskite films degraded much faster under the O2 or H2O atmospheres. The I/Pb and N/Pb ratios decreased by 0.4 in the presence of O2 within 6 h. More pronounced changes occurred under H2O conditions, whereby the I/Pb and N/Pb ratios decreased by 0.8 and 1.0, respectively, after annealing for the same amount of time. This indicated that the introduction of gas molecules accelerated the degradation process. When the films were annealed at 130 °C, substantial changes in the I/Pb and N/Pb ratios were observed under all three experimental conditions (vacuum, 1 mbar O2, and 1 mbar H2O). Within 2 h of annealing, both ratios decreased and finally reached 0 for N/Pb and 2 for I/Pb, indicating a total decomposition into PbI2.

4. DISCUSSION The temperature-dependent study revealed the chemical changes of the CH3NH3PbI3 films annealed at temperatures between 25 and 300 °C under three ambient conditions: vacuum, 1 mbar pressure of O2 and H2O. At 150 °C, the decrease in the I/Pb and N/Pb ratios under all three conditions (Figure 3a,b) indicated the degradation of CH3NH3PbI3 into PbI2 through the removal of CH3NH3+ and I− ions from the perovskite structure. The average bond dissociation energies for the dissociation of CH3NH3PbI3 along the N−H (CH3NH3 → CH3NH2 + H) and C−N bonds (CH3NH3 → CH3 + NH3) were calculated to be of 2.03 and 2.66 eV, respectively.40 Therefore, two degradation mechanisms have been proposed, in which CH3NH3PbI3 degrades to form HI, PbI2, and CH3NH2,4 or thermal annealing of CH3NH3PbI3 leads to the formation of NH3, HI, PbI2, and hydrocarbon residue.41 8482


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Figure 7. Schematic representation of the overall processes of thermal annealing of CH3NH3PbI3 perovskite films under vacuum and O2 or H2O conditions.

O2, the degradation appeared to be faster under a H2O pressure (Figure 6a,b). This is because the monohydrated intermediate (CH3NH3PbI3·H2O) can be formed from perovskites exposed to humidity, which has been reported to accelerate the degradation process.10,13 When the temperature was increased to 130 °C, we observed rapid degradation of CH3NH3PbI3 perovskite into PbI2 without any formation of lead oxide or hydroxide compounds. Thus, long-term annealing of perovskite materials at relatively low temperatures can cause a decrease in the PCE of perovskites.

However, the mass spectrometric data in Figure S3 show the appearance of CH3NH2 as the only carbon-containing product. The mass signal of NH3 did not increase at elevated annealing temperatures. These results confirmed that the perovskite structure dissociates to CH3NH2 by cleavage of the N−H bond. As a result, the thermal-induced degradation of CH3NH3PbI3 can be described by the following reaction, (CH3NH3PbI3(s) → PbI2(s) + CH3NH2(g) + HI(g)), in which s and g represent the compound in the solid state and gas phase, respectively. In addition, the intensity of the Pb0 peak at a BE of 136.9 eV in the photoelectron spectra increased after annealing under vacuum (Figure 1). The Pb0 was present as a consequence of the reduction of Pb2+ during the annealing.42−44 To elucidate the formation of Pb0 in the perovskite films, we recorded the photoelectron spectra for the PbI2 films annealed at 100 °C under vacuum (Figure S10). The Pb 4f spectra showed the formation of Pb0 at this temperature and proved that heat reduced the PbI2 into Pb0. In the case of the films annealed in O2 and H2O, the formation of Pb oxides and hydroxides in the perovskite films was observed (Figure 2). The formation of lead oxides and lead hydroxides resulted from the oxidation of metallic lead by O2 or H2O.30,36 As shown in Figure 2, the βPbO peak in the O 1s spectra increased gradually because the formation of β-PbO is thermodynamically more favorable than the formation of α-PbO.30,45 The further oxidation of lead(II) oxide was responsible for the appearance of lead(IV) oxide.45,46 Annealing at higher temperatures (200−300 °C) caused the loss of Pb components, as indicated by the Au/Pb ratio (Figure 3d). The rapid increase of the Au/Pb ratio suggested the sublimation of the Pb components. From the above results, we concluded that the CH3NH3PbI3 films decomposed to PbI2, CH3NH2, and HI when they were annealed at 25−200 °C. There was also a visible change in the color of the film from dark gray to yellow, and its surface structure changed from smooth to coarse (Figure 4). The photovoltaic performances also reduced drastically compared with that for the pristine film (Figure 5a). When the film was annealed further at higher temperatures (200−300 °C), Pb0 and/or other Pb compounds formed and sublimated. A schematic of the mechanisms of the perovskite degradation as a function of annealing temperature is outlined in Figure 7. The time-dependent study showed that the exposure to O2 and H2O led to an accelerated degradation of the perovskite. Figure 6a,b shows that the CH3NH3PbI3 films decomposed over the course of 6 h at 75 °C, and the films degraded more rapidly when they were exposed to O2 or H2O. Under an O2 pressure, the faster degradation rate can be attributed to oxygen diffusion within the perovskite structure,47−50 which expedited the segregation of defects and enhanced the decomposition reaction. Compared with the experiment in both vacuum and

5. CONCLUSIONS In this work, we investigated the transformation of CH3NH3PbI3 films under different annealing environments (vacuum (10−8 mbar), 1 mbar pressure of O2 and H2O) to determine their thermal stability. CH3NH3PbI3 annealed at 25−150 °C led to the formation of PbI2 and the release of CH3 NH2. At higher temperatures (200−300 °C), the appearance of metallic lead (Pb0) was observed under vacuum, whereas the formation of Pb oxides and hydroxides was found in the presence of 1 mbar pressure of O2 and H2O. Moreover, sublimation of Pb compounds into the environment occurred at these temperatures. The decomposition of the perovskite films with increasing annealing temperature led to a significant decline in the photovoltaic efficiency of the solar cells. Furthermore, a time-dependent study demonstrated that the introduction of O2 and H2O to the system accelerated the degradation, as the interaction of water or oxygen molecules with the perovskites accelerated the decomposition of the film. Our results reveal a correlation between the chemical, morphological, and photovoltaic changes of perovskite films under annealing conditions and the reaction pathways of their decomposition. The correlation between the annealing conditions and the chemical composition of the perovskites facilitates the development of techniques for direct synthesis or postconditioning of more resilient perovskites with long-term durability and exceptional device performance.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03243. The calibration curves for the I/Pb and N/Pb ratios, mass spectrometric study, additional XPS spectra along with detail peak fitting, SEM images, and a summary of the photovoltaic parameters of the perovskite solar cells (PDF) 8483


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: + 1 (574) 631-12819. ORCID

Weixin Huang: 0000-0003-1456-4387 Sylwia Ptasinska: 0000-0002-7550-8189 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-FC02-04ER15533. This is contribution number NDRL 5181 from the Notre Dame Radiation Laboratory.

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Heat- and Gas-Induced Transformation in CH3NH3PbI3 Perovskites

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