Films Deposited via Chemical Vapor Deposition - ACS Publications

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Predominant Stable MAPbI3 Films Deposited via Chemical Vapor Deposition: Stability Studies in Illuminated and Darkened States Coupled with Temperature Under an Open-Air Atmosphere S.V.N. Pammi, Hae Lee, Ji-Ho Eom, and Soon-Gil Yoon ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00505 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 11, 2018

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ACS Applied Energy Materials

Predominant Stable MAPbI3 Films Deposited via Chemical Vapor Deposition: Stability Studies in Illuminated and Darkened States Coupled with Temperature under an OpenAir Atmosphere S. V. N. Pammi, Hae-Won Lee, Ji-Ho Eom, and Soon-Gil Yoon* Department of Materials Science and Engineering, Chungnam National University, 34134, Daejeon, Republic of Korea *Corresponding author: [email protected] Abstract Although the use of perovskite materials in solar cells has drawn a tremendous amount of attention due to a rewarding power-conversion efficiency of 22.1%, concerns persist that poor stability will hinder the commercial viability of perovskite-based photovoltaic devices. Chemical vapor deposition (CVD) is the most suitable technology for commercialization and could potentially provide a future for stable and efficient perovskite-based electronic devices. Until now, systematic investigation has addressed neither the preparation nor the intrinsic stability of these materials under both dark and illuminated conditions given the high humidity of perovskite films deposited by CVD. In the present study, we investigated the stability of perovskite films under dark and illuminated conditions coupled with temperature under an open-air atmosphere via the tuning of various deposition parameters. CVDdeposited MAPbI3 films with no additives showed structural and optical properties at a predominant level of stability for ~2,000 h in dark and ~1,000 h under ambient room lighting, which achieved a significantly higher level of stability than that for either solution-processed perovskite films with or without additives, or for composites with polymers. The outstanding stability of CVD-deposited MAPbI3 films is attributed to their large grain sizes with high levels of crystalline quality and chemical purity.

Keywords: CVD-deposited MAPbI3 films; long-term stability; ambient moisture; light; illumination; large-grain size; high-crystallinity 1

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Introduction Organic/inorganic hybrid perovskite (OIHP) films have been both highly anticipated and desired due to dramatic improvements in the performance of perovskite solar cells (PSCs) of from ~3% to ~22% (certified 21.02%) in a short span of less than 10 years with the knock-out of photovoltaic technologies.1-4 OIHP materials have attracted the attention of researchers because of their outstanding properties that include a large absorption coefficient, a long carrier diffusion length, and ambipolar charge transport with high-charge mobility.5-7 Further, the projected future for perovskite materials now includes not only solar cell technology but also use in many electronic devices such as light emitting diodes (LEDs), photo detectors, piezoelectric nanogenerators, lasers, and light emitting transistors.8-12 However, OIHP materials tend to degrade under outdoor conditions such as humidity, oxygen, temperature, UV light, intense lighting irradiation, and also when they are under the effect of an electric field.13-19 Many articles have been published on the use of OIHP, but most of them were dedicated to enhancing the performance of solar cell devices. On the other hand, there have been many approaches to overcoming instability due to humidity: device encapsulation, compositional engineering, halide substitution, assistance of cross-linking additives or polymer additives, usage of passivation layers, device architecture, cation cascade techniques, coating devices with a water-proof fluorinated polymer, and incorporation of UV-filters.20-28 However, such strategies have led to increases in device fabrication costs and have made device fabrication more difficult. In addition, most of these studies use solution-processed, perovskite-based electronic devices that encompass simple and economic strategies to realize efficient lab-scale photovoltaic electronic devices that are not directly applicable to a larger scale and volume preparation, which hinders market entry.29 Another complication is that controlling the thickness and obtaining uniform and homogeneous films without pinholes is problematic when using solution-based techniques. For large-scale and industrial scalable 2


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growth, CVD remains the predominant technology among various vapor-based methods that are used to attain a large grain size, high crystallinity, intrinsic purity, control over film thickness, and low substrate-fabrication temperatures. In addition, avoiding toxic solvents during the preparation of perovskite films is a major benefit.30 Unfortunately, there have been no systematic studies on the preparation of perovskite films by controlling various parameters to tune the structural and optical properties and the intrinsic stability of vapor-deposited perovskite films under outdoor conditions such as humidity, oxygen, variations in temperature, and UV light. Perovskite films must endure structural and morphological changes that affect their optical properties and result in decreases in device performance over time when exposed to outdoor conditions. Herein, we report the results from a systematic study of the structural and morphological properties of MAPbI3 films via tuning the various deposition parameters of CVD. Also, the stability of perovskite films was investigated under conditions of darkness and illumination coupled with varying temperatures under an open-air ambient atmosphere. Results and Discussion For in-situ, high-quality, annealment-free MAPbI3 films, substrate temperature plays a key role in transforming the MAI and PbI2 into MAPbI3, for which a minimum substrate temperature of 80 oC is crucial and instant decomposition occurs at temperatures higher than 150 oC. In addition, post annealment is not mandatory in order to achieve highly crystalline and uniform MAPbI3 films via evaporation techniques.31 Based on these facts, annealmentfree 300 nm MAPbI3 films were fabricated at substrate temperatures of 80-160 oC using a one-step method. Based on the XRD pattern and SEM surface images shown in Figure S1, we found that a deposition temperature of 120 oC is suitable to achieve in-situ and annealment-free MAPbI3 films with a large grain size and high crystallinity. At higher 3



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temperatures (≥ 140 oC), decomposition reflected the coexistence of PbI2 and perovskite phases due to an endothermic reaction that ended with components of PbI2, CH3NH2, and HI.17, 32 In this study, therefore, we maintained the substrate temperature at 120 oC to further optimize the conditions. For an extension of our study, we considered various deposition conditions of MAPbI3 films such as one- and two-step methods (for 300 and 500 nm thicknesses), the working pressure (0.6 - 6.6 × 102 Pa for 150 nm thickness), controlling the thickness (150-750 nm), and the use of different carrier gases (for 300 and 500 nm under N2 and Ar) to optimize the process. It is well known that fine-tuning the structural and optoelectronic properties of thin films is particularly crucial in vapor-deposited techniques by controlling various parameters closely related to the device performance and requirements. For the one-step method, all precursors were mixed prior to film deposition (either in solution or in vapor), whereas the precursors were sequentially deposited in the two-step method.33 However, two-step sequential deposition has opened new doors to a planar architecture for high-performance, flexible, perovskite-based electronic devices such as solar cells. In addition, the controlling the amount of organic and inorganic gas like vapors is difficult in CVD system due to very distinct properties of the two precursors by one-step method.

In contrast, in two-step

deposition process, it is easy to control the amount of organic and inorganic gas like vapors.33 In this study, we fabricated 300 and 500 nm MAPbI3 films using both one- and two-step methods. The XRD patterns of 300 and 500 nm MAPbI3 films processed via both one- and two-step methods exhibited a complete reaction of the precursors during the deposition process in CVD. As shown in Fig.1(a), a set of pronounced peaks at 14.10, 23.47, 28.42, and 30.89° corresponded to (110), (211), (220), and (213) planes in the tetragonal phase of perovskite, and the lack of additional peaks is indicative of the high purity of these films. The 4


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diffraction data showed that the crystallinity and crystalline domain size of the films was increased with increases in grain size. The large grain size and crystallinity gained by using the two-step method is attributed to the reconstruction of grain boundaries during the sequential deposition process, as shown in the SEM surface images. The absorption spectra of the perovskite films are shown in Fig. 1 (b). All spectra revealed that the typical absorption of MAPbI3 perovskite covers the entire visible wavelength; however, the absorption profile slightly red-shifts with increases in grain size in the cases of both 300 and 500 nm-thick films.34 The SEM images displayed in Fig. 1(c-f) reveal that the grain size of MAPbI3 films can be tuned using the proposed deposition process. Both 300 and 500 nm films processed with two-step deposition exhibited an enhanced grain size whereas the one-step process produced a dense packing of crystals with an average domain size ≤ 300 nm. It is interesting that the 300-nm-thick films developed a grain size that was larger than the thickness of perovskite films, particularly when fabricated via the two-step process. However, both the 300 and 500 nm films were pinhole free, homogeneous, and possessed a smooth surface (Fig.1 (g-j). The films that were pinhole free with a smooth surface had levels of rms roughness of ≤ 13 and 20 nm for 300 and 500 nm films, respectively. It is noteworthy that, with increases in the thickness of MAPbI3 films of from 300 to 500 nm, the surface morphology was changed due to a transition of the grain growth direction from that of a lateral grain domain to cube-like structures, which is typical of MAPbI3 due to a high concentration of precursors and reflects an increase in the surface roughness. 35 The monitoring and control of organic halide reagents in the vacuum process has long been the subject of much inquiry. The vapor of the organic halide precursors can be easily monitored and manipulated by controlling the working pressure in a chamber. In the present study we investigated the effect of working pressure (0.6 - 6.6 × 102 Pa) by maintaining the 5



same thickness (150 ± 20 nm) of MAPbI3 by changing the deposition time. Other parameters were kept constant, and a schematic of the CVD process and the experimental details are described at experimental details. The MAPbI3 films deposited under different working pressures exhibited a complete perovskite phase via XRD patterns (Fig. 2(a)). The diffraction patterns of MAPbI3 films showed that the crystallinity increases with increases in the working pressure (0.6 - 1.3 × 102 Pa), but decreases with further increases (> 1.3 × 102 Pa). The absorbance spectra (Fig. 2(b)) followed the same trend as crystallinity and grain size, which was confirmed by XRD and SEM analysis. Later, SEM (Fig. 2(c-f)) investigations revealed apparent differences in the grain domain sizes between the perovskite films fabricated under various working pressures. The films fabricated at 1.3 × 102 Pa exhibited very large micrometer-sized grains (≥ 800 nm) embedded with moderate-sized grains (~500 nm), which together were much larger than the thickness of perovskite films. Films fabricated at low working pressure (0.6 × 102 Pa) with densely packed smaller grains had an average size of 250 nm. With increases in the working pressure (3.3 × 102 Pa), the size of the grains was not increased. Instead, the size of grains had uniformly decreased by as much as 400 nm. Further increases in the working pressure to 6.6 × 102 Pa loosened the texture of the grain boundaries to create crevices and a high level of defective interior grains (marked in circles). It is interesting to note that the rms of the films fabricated under different working pressures was very smooth, ≤ 12 nm, as shown in Fig. 2(gj)). The deposition rate was decreased by 12.5 - 2.83 nm/min with increases in the working pressure (0.6 - 6.6 × 102 Pa). The grain size that is created by different working pressures is related to radical energy and to the mean of the free path during the deposition process. Based on these results, a working pressure of ≤ 3.3 × 102 Pa is suitable for most photovoltaic device applications. Films fabricated at 70% in darkness. Based on the literature, stability was improved by tailoring the compositional purity, morphology, and incorporating polymer. 16, 21, 38, 39 The intrinsic thermal stability of MAPbI3 films had not been properly addressed for vapordeposited films. According to the International Standards (IEC 61646 climatic chamber tests), long-term stability at 85 °C (estimated hot summer temperature on a roof) is required in order to bring perovskite solar cells to market. In addition, thermal energy of 85 oC represents 0.093 eV, which approximates the formation energy (0.11–0.14 eV) of MAPbI3. 17 Few studies have focused on the intrinsic thermal stability of MAPbI3 films fabricated by a solution process that involves annealment. The present study was focused on the intrinsic thermal stability (with different temperatures (85-140 oC) and time intervals) of in-situ CVDMAPbI3 films formed under air without annealment via storage in a hot and darkened oven. The XRD pattern shown in Figure 5(b) exhibits the thermal stability of MAPbI3 films at 85 oC (from 24 to 500 h) and a diffraction peak from PbI2 that increased substantially after 96 h, which indicates that the perovskite decomposition has begun. With further increases in the storage time, the intensities of PbI2 and MAPbI3 were similar to those at 360 h. The MAPbI3 phase had vanished after 500 h, which was revealed by variations in color (Figure S3(b)). A notable drop in the absorption spectra (Fig. 5(d)) of MAPbI3 films was noted as the absorbance features of perovskite were maintained following 360 h at 85 oC. The traces of a PbI2 peak appeared in the absorbance spectra around a wavelength of 500 nm. By contrast, at 9



elevated temperatures of 120 oC (for 12 h) and 140 oC (for 6 h) (higher than deposition temperature of MAPbI3 films), the decomposition of the MAPbI3 films had begun, and the intensity of PbI2 was significantly increased with further increases in storage time. Furthermore, the MAPbI3 phase had completely vanished after 96 and 24 h at 120 and 140 oC, respectively, as evidenced by the color changes in the films (Figure S3(c)). The absorption spectra (Fig. 5(e)) were suppressed at elevated temperatures after 48 and 12 h for 120 oC and 140 oC, respectively. The beginning of severe degradation appeared at temperatures higher than that for deposition.40 Maximum intrinsic thermal stability at 85, 120, and 140 oC was apparent at 36, 18 and 6 h, respectively.41 Our CVD fabricated films, however, seemed even more stable compared with similar materials tested around the operation temperature (85-140 o

C), and this was particularly true at 85 oC, which is lower than the deposition temperature.17,

41

However, to enhance thermal stability (≥ 300 oC) along with moisture stability of lead

based perovskite films by replacing MAI with CsI, CsI has an appropriate ionic radius for the perovskite 3D structure. 42 Investigation into stability under illumination is essential for insight into underlying degradations in addition to the effects of oxygen and moisture. Moisture coupled with illumination was directly associated with the decomposition of MAPbI3 into PbI2, which was an irreversible reaction. 14 For stability of MAPbI3 films under illumination, we tested under room light (~5 mW cm-2) and temperature at 85 - 140 oC using different storage durations and solar light with intensity (~100 mW cm-2) at room temperature (R. H ≥ 80%). The XRD patterns and absorbance spectra of CVD-deposited MAPbI3 films under room light at room temperature in ambient air are shown in Figs. 6(a) and 6(d), respectively. Based on the XRD pattern and absorbance spectra, PbI2 was not observed at 500 h. Further increases in the storage time to 1,000 h revealed the PbI2 content. Perovskite, however, persisted as the main 10


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phase of the films. The intensity of the perovskite phase was decreased with increases in the storage time, and the color of the films was changed from dark brown to a shade of yellow due to degradation (Figure S3(d)), which also suppressed the absorbance spectra following 1,000 h of storage time under illumination in air. The faster degradation rate under illumination in the presence of moisture might be attributed to the incorporation of oxygen, because only oxygen decomposes perovskite under exposure to light. Manshor et al. demonstrated intrinsic stability for ~ 800 h under room light by incorporating the MAPbI3 films in a PVP matrix. 16 CVD-deposited MAPbI3 films are more stable under room light and temperature in the absence of additives. The effect of illumination coupled with temperature under air was apparent when the temperature was increased from room temperature to 85 oC. The degradation that began within 48 h was reflected by changes in the sample color (Figure S3(e)) and by an increase in the intensity of the PbI2 peak, as shown in the XRD pattern (Fig. 6(b)). The perovskite phase had almost vanished when the storage duration was increased to 200 h. Figure 6(e) shows the presence of PbI2 in the absorbance spectra for 48 h, and the absorbance features of perovskite were suppressed with further increases in the storage duration to 200 h. By contrast, at elevated temperatures of 120 °C (for 48 h) and 140 °C (for 12 h), it was the PbI2 phase (Fig. 6(b)) that dominated rather than the MAPbI3 phase, which was verified by the sample colors (Figure S3(f)) and the absorbance spectra (Fig. 6(f)). Furthermore, we studied the stability of perovskite films under 100 mWcm-2 of sunlight at room temperature in ambient air. Based on the XRD pattern (Fig. 6(c)) and absorbance spectra (Fig. 6(g)), the perovskite phase was maintained for 12 h, although the PbI2 phase appeared for 6 h. With increases in the storage time to 12 h, however, the PbI2 content dominated, which was evident in the sample color (Figure S3(g)). The intrinsic stability of CVD-deposited MAPbI3 films under room light (~5 mW cm-2) and 100 mWcm-2 of sunlight 11



showed predominant stability compared with the results from previous reports showing degradation following 60 min of exposure to 100 mWcm-2 of sunlight.16,43,44 Recently, stability measurement protocols have been developed by studying the effect of operation conditions on degradation behaviors of solar cell devices.45 In contrast, in the present manuscript, we focused on long-term intrinsic stability of CVD-deposited MAPbI3 films under both dark and illuminated conditions coupled with temperature under an air atmosphere at R.H ≥ 80%. However, it is difficult to compare the stability behavior with stability measurement protocols described. The most important factors promoting long-term stability under dark and illuminated conditions could be large grain size, high crystallinity, and chemical purity that are all promoted by the sequential CVD process. In addition, we have fabricated perovskite solar cell using single-walled carbon nanotubes counter electrodes, and self-powered pressure and light sensitive bimodal sensors using CVD fabricated MAPbI3 films. These primary results are encouraging in terms of film quality and stability, and there is scope for further enhancement in terms of efficiency when compared to spin coated films.46, 47

X-ray photoelectron spectroscopy (XPS) analysis was performed to investigate the chemical composition, chemical bonding features, and the surface stability of CVD-deposited MAPbI3 films. For reference, we analyzed spin-coated MAPbI3 films along with CVD films before and after etching for 15 min to evaluate the chemical bonding features at the surface and sub-surface levels. The XPS spectra were calibrated according to the C 1s binding energy of adventitious (aliphatic) carbon (284.8 eV), which was used as the reference binding energy. An XPS survey spectra (Fig. 7(a)) for typical MAPbI3 perovskite films exhibited binding energies of N 1s, C 1s, and doublet peaks of Pb and I located at 401, 284.4, 137, and 618 eV, respectively. 48-51 The core level spectra of Pb 4f, I 3d, C 1s, and N 1s are shown in Fig. 7 (b-e, 12


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respectively). It is noteworthy that there was no significant oxygen component in the CVD films. By contrast, the surface of spin-coated MAPbI3 films contained an oxygen concentration of approximately 29 atomic %, which was then decreased to 11 atomic % following etching for 15 min. The binding energy of metallic Pb is typically observed at lower energies (136.3–136.9 eV). 48 However, during formation of the perovskite layer, there was a substantial alteration in the Pb 4f core level due to the re-structuring of surface states to transform Pb atoms into Pb ions, which exhibited symmetrical peaks at 137.6 and 142.6 eV corresponding to the spin-orbit splitting of the Pb 4f7/2 and 4f5/2 components. In the present study, Pb 4f7/2 and 4f5/2 components in CVD-deposited films were observed at 137.6 eV and 142.5 eV, respectively, which confirms the existence of Pb2+ cations in the MAPbI3 structure (Fig. 7(b)). By contrast, the Pb 4f7/2 and 4f5/2 components in the spin-coated films were located at 138.4 and 143.3 eV, respectively, and the shift towards higher binding energies could be attributed to the PbI2 component. In the case of the thinner spin-coated MAPbI3 films (≤ 500 nm), there was a chance for the formation of surface intermediate MAPbI2+x phases to finally form a PbI2 phase on the surface. 51 This could be attributed to the components that appeared in the Pb 4f spectra for spin-coated films (Fig. 7(b)). Based on previous studies, the binding energies for PbO/(OH)2 and PbI2 ranged from 137.7 to 138.5 eV and from 138.3 to 138.9 eV, respectively. It is noteworthy that the binding energies of Pb 4f components at the sub-surface level (after etching for 15 min) were shifted to a higher level in the spin-coated samples, while no significant shift was detected in CVD-deposited films. These results suggest that the spincoated films were degraded during the etching process. By contrast, after the etching of CVD-deposited MAPbI3 films, small traces exhibited a metallic Pb component with negligible intensity at 136.2 eV. 49 The XPS core spectra of I 3d exhibited components of I 13



3d5/2 and I 3d3/2 with binding energies of 618 and 630 eV, respectively. The CVD-deposited films exhibited I 3d peaks at 618.8 and 630.3 eV, which were assigned to triiodide I3- (Fig. 7 (c)).50 On the other hand, I 3d components of spin-coated films usually are located at higher binding energies such as 619.4 and 630.9 eV, which are associated with the loss of an iodide ion in MAPbI3 films via MAI evaporation (Fig. 7(c)). During the etching process, however, the binding energies of I 3d components were shifted to higher energy levels in the cases of the spin-coated films, but CVD-deposited films showed no significant change. These results suggest predominant stability for the CVD-deposited films with high crystallinity following etching. As shown in Fig. 7(d), the C 1s core level includes two peaks for both CVD-deposited and spin-coated MAPbI3 films. CVD-deposited films exhibited a main peak located at 284.7 eV and a shoulder peak at 285.9 eV, which was similar to solution-processed MAPbI3 films. 51 It was interesting that spin-coated samples had a main peak at 286.33 eV with a shoulder peak at 284.8 eV. The peak found at 284.8 eV is commonly used for calibration and is known to result from adventitious carbon or adsorbed surface hydrocarbon species from the atmosphere. The peak located at 285.9 eV can be attributed to methyl carbons within the MAPbI3 films or additionally adsorbed surface carbon bonded only to a hydroxyl group.48 The core level spectra of N 1s (Fig. 7(e)) showed peaks at 401.0 and 402.43 eV for CVD-deposited and spin-coated MAPbI3 films, respectively, which represented the NHx and NH2 components.50 The destruction of the organic core (CH3-NH3) in MAPbI3 films by NHx bonds (at approximately 401 eV) was absent on the surface of spin-coated MAPbI3 films, which indicated a decomposing of the organic core (CH3NH3) due to weak hydrogen bonds.50 These results were attributed to the decomposition of NH3 to NH2 and/or beam damage to the samples during the measurements. Based on the results from solution-processed MAPbI3 14


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films, however, a similar trend appeared with spin-coated films in the present study, which could be attributed to a lower level of thickness that would reflect lower levels of surface stability, although the XRD patterns confirmed a complete MAPbI3 structure with neither impurities nor PbI2 components.48-51 XPS analysis revealed predominant surface stability for the CVD-deposited MAPbI3 films, which reflected long-term stability in either dark or illuminated conditions coupled with temperature under an air atmosphere. These results point out the necessity of stability studies using CVD-deposited perovskite films that mix halides (Br & Cl) and organic cations (MAI & FAI) in order to enhance the intrinsic thermal stability at elevated temperatures (≥ 85 oC) under conditions of both darkness and illumination. Conclusions We systematically investigated the structural and morphological properties of MAPbI3 films via tuning the various parameters of CVD. The stability of the deposited films was investigated under conditions of both darkness and illumination coupled with temperature under ambient air. The 300-nm-thick MAPbI3 films deposited via two-step CVD showed intrinsic moisture stability for 2,400 h (100 days) under an open air atmosphere with ≥ 80% RH at room temperature under darkness. When the films were exposed to light at room temperature, the degradation was increased ~2 times faster. In stability studies coupled with temperature, degradation is much faster under illumination compared with darkness in the presence of oxygen from moisture. By contrast, the degradation under solar light at room temperature was more severe than that from a lamp. Compared with pure and polymer matrix composite-films fabricated via solution process and hybrid deposition techniques, however, CVD-deposited MAPbI3 films showed predominant stability. The results of this study suggest that high-quality and air-stable perovskite films without the need for additives can provide a potential road map for the mass production of perovskite-based electronic devices. 15



Experimental details Deposition of MAPbI3 films via CVD In order to deposit the MAPbI3 films on glass substrates, a quartz tube furnace with a twoinch diameter was placed in a vacuum furnace that possessed two separate heating zones. Custom-made quartz crucibles were used for CH3NH3I (MAI) and PbI2 powders, which were placed inside the quartz tubes to evaporate the sources. In a CVD chamber, we used two 2and 1-inch quartz tubes for MAI and PbI2 vaporization, respectively, in order to avoid the reaction of sources from the two zones. The PbI2 and MAI vapors were carried by nitrogen to react in the zone-2 region, and the MAPbI3 films were deposited on preheated substrates. Levels for the working pressure and N2 gas-flow rate were maintained at 0.6 - 6.6 × 102 Pa, 200-400 (PbI2), and 50-100 (MAI) standard cc min-1 (sccm), respectively. The samples of MAI (300 mg) and PbI2 (100 mg) were kept in separate crucibles (Figure S4). The glass and ITO substrates were placed at the center of zone 2 at 120 °C. The temperature of the two sources was gradually ramped up to 160 and 400 °C for MAI and PbI2, respectively. The working pressure of organic halide gas was controlled by a tunable vacuum gate valve. The experimental conditions for optimized parameters are summarized in Table S1. Preparation of spin-coated MAPbI3 films The spin-coated samples were fabricated to compare the surface chemical composition and chemical bonding states of CVD-deposited MAPbI3 films. A perovskite solution (CH3NH3PbI3) was prepared by mixing MAI and lead (II) Iodide at a 1:1 molar ratio with a concentration of 50 wt% in an anhydrous N, Ndimethylformamide (DMF) solution. The solution was filtered via a 0.45 µm syringe filter before use. The perovskite solution (50 wt%) was dispensed onto the glass substrate and spin-coated at 1,000 revolutions per minute

16


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(RPM) for 30 s under air with a ramping rate of 200 RPM s−1, followed by annealing at 100 °C for 120 min. Characterization of the MAPbI3 films The surface and cross-sectional images of the films were obtained by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800). Atomic-force microscopy (AFM, Auto Probe CP) was used to analyze the surface morphology and the roughness of the MAPbI3 films. The absorbance spectra of the films were measured using S-3100 UV-Vis spectroscopy. The crystalline structure of the films was characterized by X-ray diffraction (XRD, Rigaku D/MAX-RC) using Cu Kα radiation and a nickel filter, and the surface chemical compositions and chemical bonding states of the MAPbI3 films were evaluated by X-ray photoelectron spectroscopy (XPS, model ESCALAB 210) using an X-ray source (Al Kα at 1486.6 eV). The results via XPS were acquired with levels of current and voltage of 10 mA and 15 kV, respectively, at a base pressure of (1–5) × 10−9 mbar. Narrow scans were performed at a resolution of 0.05 eV with analyzer-pass energy of 20 eV. The analyzed X-ray spot size was 500 µm2 with a penetration depth of approximately 10-20 nm. Characterizing the stability of MAPbI3 films To investigate the stability of thin films, we prepared 300-nm-thick MAPbI3 films deposited using a two-step process, which produces the optimum thickness for photovoltaic applications and results in a large grain size and a high level of crystallinity. The deposited perovskite films were stored either in (i) darkness (open-air atmosphere, ≥ 80% RH, room temperature (~27 oC) and 85- 140 oC (hot air oven)), or (ii) under illumination (room light with intensity ~5 mW cm-2 and solar light (IVIUMSTAT under a Sun 3000 solar simulator composed of 1,000 W mercury-based Xe arc lamps and AM 1.5-G filters)) with an intensity of ~100 mW cm-2, under an open-air atmosphere, ≥ 80% RH and room temperature (~27 oC), 17



and at 85 to 140 oC (hot plate under room lighting). The stability of MAPbI3 films was monitored by tracking each sample, which was confirmed based on (i) changes in color, (ii) relative changes in the absorbance, and (iii) XRD patterns. References (1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. (2) Park, N. G. Perovskite Solar Cells: An Emerging Photovoltaic Technology. Materials Today 2015, 18, 65-72. (3) Hodes, G. Perovskite-based Solar Cells. Science 2013, 342, 317-318. (4) Stranks S. D.; Snaith, H. J. Metal-Halide Perovskites for Photovoltaic and Light-Emitting Devices. Nat. Nanotechnol. 2015, 10, 391-402. (5) Lin, Q.; Armin, A.; Burn P.; Meredith, P. Organohalide Perovskites for Solar Energy Conversion. Acc. Chem. Res. 2016, 49, 545-553. (6) Manser, J.; Christians J.; Kamat, P. Intriguing Optoelectronic Properties of Metal Halide Perovskites. Chem. Rev. 2016, 116, 12956-13008. (7) Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; Losovyj, Y.; Zhang, X.; Dowben, P.; Mohammed, O.; Sargent E.; Bakr, O. Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519-22. (8) Kim, Y. H.; Cho, H.; Heo, J. H.; Kim, T. S.; Myoung, N.; Lee, C. L.; Im S. H.; Lee, T. W. Multicolored Organic/Inorganic Hybrid Perovskite Light- Emitting Diodes. Adv. Mater. 2015, 27, 1248-1254. (9) Dou, L.; Yang, Y.; You, J.; Hong, Z.; Chang, W. H.; Li G.; Yang, Y. Solution-Processed Hybrid Perovskite Photodetectors with High Detectivity. Nat. Commun. 2014, 5, 5404. 18


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Perovskite Films. J. Phys. Chem. C. 2016, 120(1), 215–220. (50) Kumar, G. R.; Savariraj, A.D.; Karthick, S. N.; Selvam, S.; Balamuralitharan, B.; Kim, H. J.; Viswanathan, K. K.; Vijaykumar M.; Prabakar, K. Phase Transition Kinetics and Surface Binding States of Methylammonium Lead Iodide Perovskite. Phys. Chem. Chem. Phys. 2016, 18(10), 7284–7292. (51) Rocks, C.; Svrcek, V.; Maguirea P.; Mariotti, D. Understanding Surface Chemistry During MAPbI3 Spray Deposition and Its Effect on Photovoltaic Performance. J. Mater. Chem. C 2017, 5, 902-916. Acknowledgements This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (No. NRF-2015R1A2A1A01003867, NRF–Basic ScienceYoung Researcher Program NRF 2017R1D1A1B03035399). Figure Captions Figure 1 Effect of deposition parameters on CVD-deposited MAPbI3 films (a) XRD patterns, (b) UV-vis spectra; SEM (c, e); and, AFM (g, i) images of the 300- and 500 nm-thick films deposited via one step. SEM (d, f) and AFM (h, j) images of the 300- and 500-nm-thick films deposited via 2-step. Scale bar at 1 µm. Figure 2 Effect of working pressure on CVD-deposited 150 nm-thick MAPbI3 films (a) XRD pattern; (b) UV-vis spectra; and, SEM (c-f) and AFM (g-j) images of the films deposited under different working pressures ranging from 0.6 to 6.6 × 102 Pa. Scale bar at 1 µm. Figure 3 Effect of thickness on CVD-deposited MAPbI3 films (a) XRD pattern; (b) UV-vis spectra; and, SEM (c-f) and AFM (g-j) images of the films with thicknesses ranging from 150 to 750 nm, respectively. Scale bar at 1 µm. 24


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Figure 4 Effect of carrier gas on CVD-deposited MAPbI3 films (a) XRD pattern; (b) UV-vis spectra; and, SEM (c, e) and AFM (g, i) images of the films using argon (Ar: 99.9999%) as a carrier gas with thicknesses of 300 and 500 nm. SEM (d, f) and AFM (h, j) images of films using nitrogen (N2: 99.9999%) as a carrier gas with thicknesses of 300 and 500 nm. Scale bar at 1 µm. Figure 5

Stability studies of CVD-deposited MAPbI3 films under darkness at room

temperature and 85-140 oC in an open-air atmosphere (R. H ≥ 80%) (a) XRD patterns; (c) UV-vis Spectra of the MAPbI3 films kept in darkness at room temperature; (b) XRD pattern; and, (d, e) UV-Vis Spectra of the MAPbI3 films kept under darkness at elevated temperatures ranging from 85 to 140 oC. Figure 6 Stability studies of CVD-deposited MAPbI3 films exposed to room light (~5 mW cm-2) at room temperature and temperatures ranging from 85 to 140 oC, and solar light with intensity (~100 mW cm-2) at room temperature (R. H ≥ 80%) (a) XRD pattern; (d) UV-vis spectra of the MAPbI3 films exposed to room light at room temperature; (b) XRD pattern; (e, f) UV-vis spectra of the MAPbI3 films exposed to room light at elevated temperatures of 85-140 oC; (c) XRD pattern; and, (g) UV-vis spectra of the MAPbI3 films exposed to 100 mW cm-2 of sunlight at room temperature. Figure 7 XPS analysis of spin-coated and CVD-deposited MAPbI3 films before and after etching. (a) Survey spectrum and XPS core level spectra of (b) Pb 4 f, (c) I 3d, (d) C 1s, and (e) N 1 s.

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Fig. 1

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Fig. 2 27



Fig. 3

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Fig. 4 29



Fig. 5

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Fig. 6 31



Fig. 7

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Table of Contents (TOC)

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Films Deposited via Chemical Vapor Deposition - ACS Publications

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