Grain Boundary Healing of OrganicâInorganic Halide Perovskites for

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Grain Boundary Healing of Organic-Inorganic Halide Perovskites for Moisture Stability Do Hyung Chun, Sungsoon Kim, Sung Uk Chai, Wook Kim, Wanjung Kim, Jung Hwan Lee, Ryan Rhee, Dukhyun Choi, Jung Kyu Kim, Hyunjung Shin, and Jong Hyeok Park Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b02721 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 14, 2019

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Grain Boundary Healing of Organic-Inorganic Halide Perovskites for Moisture Stability

Do Hyung Chun†, Sungsoon Kim†, Sung Uk Chai‡, Wook Kim§, Wanjung Kim†, Jung Hwan Lee†, Ryan Rhee†, Dukhyun Choi§, Jung Kyu Kim∥, Hyunjung Shin⊥ and Jong Hyeok Park*,† †Department

of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea ‡Photo-electronic

Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul, 02792, Republic of Korea §Department

of Mechanical Engineering, Kyung Hee University, 1732, Deogyeongdaero, Giheung-gu, Yongin-si, Gyeonggi-do 17104, Republic of Korea ∥Department

of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of

Korea ⊥ SKKU

Advanced Institute of Technology, Department of Nano Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea *Correspondence to Prof. J. H. Park (email: [email protected])

ABSTRACT

Although organic-inorganic halide perovskite (OIHP)-based photovoltaics have high photoconversion efficiency (PCE), their poor humidity stability prevents commercialization. To overcome this critical hurdle, focusing on the grain boundary (GB) of OIHPs, which is the main humidity penetration channel, is crucial. Herein, pressure-induced crystallization of OIHP films prepared with controlled mold geometries is demonstrated as a GB-healing technique to obtain

ACS Paragon Plus Environment

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high moisture stability. When exposed to 85% RH at 30 ℃, OIHP films fabricated by pressureinduced crystallization have enhanced moisture stability due to the enlarged OIHP grain size and low-angle GBs. The crystallographic and optical properties indicate the effect of applying pressure onto OIHP films in terms of moisture stability. The photovoltaic devices with pressure-induced crystallization exhibited dramatically stabilized performance and sustained over 0.95 normalized PCE after 200 h at 40% RH and 30 ℃. KEYWORD: Halide perovskites, Pressure-induced crystallization, Grain boundary healing, Moisture stability


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Organic-inorganic halide perovskites (OIHPs) with ABX3 structure (A is an organic cation, B is usually lead or tin, and X is a halide ion) have attracted attention as promising next-generation photovoltaic materials to replace existing silicon-based photovoltaics due to their high light absorption coefficient, long carrier diffusion length and high carrier mobility.1-4 Many studies have focused on improving the photoconversion efficiency (PCE) of OIHP-based photovoltaics via developing deposition methods and composition engineering, and these methods have led to PCE values exceeding 24%.5-9 However, the low moisture stability of OIHP materials is still a major hurdle to the commercialization of OIHP-based photovoltaics.10,11 To solve this inherent drawback, encapsulation and passivation of OIHP photovoltaics have been reported; these methods conceal the intrinsic instability of the perovskite films to sustain device performance over 500 h under ambient conditions.12,13 From a material point of view, even though it has been recognized that the grain and grain boundary (GB) of methylammonium lead iodide (CH3NH3PbI3, denoted as MAPbI3) is beneficial for charge carrier transport, several studies have also suggested the GB in OIHP film has negative aspects, such as defects and ion migration channels.8,

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Additionally, environmental degradation of OIHP films is mainly initiated by GBs, which can act as moisture penetration channels.14-18 For instance, Wang et al.16 observed that moisture penetrates OIHP films through GBs rather than the grain surface by utilizing precise transmission electron microscopy (TEM) measurements. Additionally, Yun et al.18 compared the electrical properties between GBs and internal grains by utilizing Kelvin prove force microscopy (KPFM) and confirmed the recovery of moisture-degraded OIHP films with post-thermal treatment. Based on previous research, GB-free single-crystal OIHP thin films are the best option to minimize environmental degradation. However, it is extremely challenging to obtain single-crystal OIHP thin films from solution coating and thermal annealing because polycrystalline OIHPs with GBs


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have low formation energy; hence, polycrystalline OIHP films with many GBs are the thermodynamically stable phase. While enlarging the size of the OIHP via solvent engineering/annealing, gas blowing, and additive engineering and defect passivation methods have been widely reported as strategies to enhance moisture stability, the formation of sustainable, moisture-resistant OIHP thin films is still challenging.19-24 Herein, we report pressure-induced crystallization of an OIHP film using MAPbI3 as a model system with the aim of obtaining low-angle GB and larger grain size to minimize water penetration. During the solvent evaporation and thermal annealing steps to create the MAPbI3 thin film, a small amount of pressure was exerted on the MAPbI3 via using three different embossing polymer molds (flat, rectangular and hexagonal25). After a few seconds of pressure-induced crystallization, moisture penetration through the GB of the OIHP was largely prevented by the enlarged grain size and collapsed GB with an ultralow GB angle. Presumably, the external pressure exerted on the MAPbI3 thin film during crystallization may influence the thermodynamic equilibrium of the MAPbI3 thin film. Due to this, the degradation rate of the MAPbI3 thin film in the presence of moisture was greatly suppressed, and the resultant photovoltaic devices maintained over 80% of their initial performance after 200 h under ambient conditions without any encapsulation. OIHP films were prepared on a substrate by a spin-coating method, the so-called antisolvent washing method, which is shown in Figure 1a.5 For precise GB healing, various polyurethane acrylate (PUA) polymeric molds were placed on an intermediate OIHP film that was partially wetted, and then, the pressure of 4.3 kgf was applied onto the films with hands for a few seconds during the annealing process. For guaranteeing experimental credibility, the applied pressure was precisely controlled by measuring average weight during the process. (Figure S1)


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The photographic images also support the reproducibility of experiments in terms of film quality. (Figure S2) Previous work reported that a compression force during the annealing step positively influences the intrinsic crystallography of OIHPs, which results in enhanced optoelectronic properties and solar cell efficiency.24-26 To investigate the morphology evolution and GB healing from pressure-induced crystallization and their impact on the moisture stability of OIHPs, imprinting polymer molds with various shapes were prepared by the conventional transfer method reported elsewhere.25 Using the process shown in Figure 1a, pristine and compressed OIHP films were successfully fabricated using three different PUA molds, flat, rectangular dot pattern and hexagonal dot pattern (denoted as fp-, rp- and hp-, respectively). The scanning electron microscopy (SEM) images clearly show the OIHPs have controlled nanostructures based on the PUA master molds. As shown in Figure 1b-e, a flat surface and uniform nanohole arrays were observed. Compared to the pristine OIHP film, the compressed OIHP films exhibit larger grain sizes, and this observation is supported by the grain size distribution data in Figure 1f-i. The average grain size of the OIHP film subjected to pressure during thermal annealing increased compared to that of the pristine film. (Size of 14876, 170593, 69150 and 44524 nm2 for the pristine, fp-, rp- and hpOIHP films, as listed in Table 1.) Interestingly, the OIHP film compressed by the flat PUA mold had the largest grain size, which was over 10 times larger than that of the pristine film. The SEM images also confirm that the crystallized rp- and hp-OIHP films appear to have more closely packed grains. Because of the different grain sizes and GB morphologies, as shown in Figure 1a, OIHP films subjected to compression might prevent H2O and O2 penetration, which will ultimately lead to OIHP film stability against a H2O/O2 environment. To investigate the morphology evolution of the OIHP films due to pressure exertion with different PUA molds, a mechanical simulation was conducted by calculating the effective stress


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applied during the process with Midas NFX software (Figure 2). The simulation results show the in-plane effective stress (σeff) applied to the OIHP films. The applied pressure was set to be 0.14 MPA considering the weight averagely applied during process at the area of 3 cm2 is 4.3 kgf. Figures 2a-c show the top view of the 3-dimensional images of the compressed OIHP films, and Figures 2d-f show the top view of the cross-sectional images taken at a height of 200 nm. When the films were compressed by the flat PUA mold, a small stress, i.e., less than 0.14 MPa on the overall film, was applied because of the high effective contact area between the OIHP film and the polymer mold. In addition, the compression pressure from the top to bottom (z-direction) was spread in the x-y direction, which results in the small effective stress on the OIHP film. However, the PUA molds with a nanopillar structure reduce the effective contact area between the OIHP film and the imprinting PUA mold; thus, the hexagonal and rectangular nanopillar-arrayed molds applied higher in-plane effective stresses on the OIHP films. Furthermore, Figures 2e and f distinctly support the difference in the applied stress due to the density of the pillar structure. The OIHP film prepared by the hexagonal PUA mold experienced a higher in-plane stress due to the higher density of nanopillars compared to that of the rectangular PUA mold. To investigate the positive effects of the enlarged grain size and boundary healing against moisture-induced degradation, the OIHP films on glass substrates were stored in a container with a controlled environment of 85% RH and 30 ℃ for 50 h. Photographic images of the OIHP films as a function of the moisture-exposure time are shown in Figure 3a. Before moisture exposure, the MAPbI3 films with nanodot arrays exhibited bright colors (purple and green for the rp- and hpOIHP films, respectively) owing to their photonic crystal effects, and the reference and fp-OIHP films exhibited a typical black color. The pristine OIHP film completely changed into a yellow non-perovskite PbI2 phase after 50 h. As the grain size increased due to application of a uniaxial


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compression force using the flat polymeric mold, as shown in Figure 1c, degradation slowed compared to that of the pristine film because of the enlarged grain size. The enlarged grain size appeared to have reduced the GB number. Interestingly, the rp- and hp-OIHP films, which were also formed with a uniaxial compression force during the solid film formation, maintained their original colors, indicating enhanced moisture stability relative to that of the OIHP film created with a flat polymeric mold. The optical microscopy (OM) images (Figure 3b-e) of the OIHP films after exposure to 85% RH for 9 h more clearly show less degradation in the rp- and hp-OIHP films. Previous reports have shown that a damaged OIHP film exhibits dark colors in OM images,28 and the pristine OIHP film was nearly black after 9 h (Figure 3b). Slower degradation was observed with the fp-, rp- and hp-OIHP films (Figure 3c-e). Although the fp-OIHP film had the largest average grain size, the OM image color of this film was also black after 9 h, while the rp- and hpOIHP films maintained their original color. Even after 50 h, the rp-OIHP film nearly maintained its original color (Figure S3). This result shows that the rp-OIHP film has the best morphology among the 4 samples in terms of moisture stability. According to the SEM images, the grains in the rp- and hp-OIHP films are more closely packed than those in the pristine and fp-films to minimize the boundary volume (Figure 1b-e). Figure S4 clearly shows that OIHP degradation begins at the GBs in the pristine OIHP films. Therefore, the fp-OIHP films with the largest grain size should have the highest moisture resistance, but the small pressure exerted during crystallization limits the GB healing. As expected, Figure 1 shows that the rp-OIHP films have relatively large grains and collapsed GBs, resulting in moisture stability superior to that of the hpOIHP films. To clarify the degradation of OIHP films under high humidity conditions, crystallographic and photophysical analyses were conducted. To observe crystallographic changes after


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degradation, X-ray diffraction (XRD) measurements were performed, and the pristine, fp-, rp- and hp-OIHP films clearly exhibited the tetragonal MAPbI3 diffraction peaks (Figure 4a, S5). In the patterns for the rp- and hp-OIHP films, the intensity of the peak at 13.8°, which represents the (110) tetragonal structure of MAPbI3, increased, implying the crystallographic orientation was enhanced after the pressure-induced crystallization step.22,26 After exposure to a high humidity environment (RH 85%, 30 ℃), the PbI2 diffraction peak was significantly more intense in the pristine MAPbI3 film than the rp-OIHP film (Figure 4b). The difference in the intensity of the impurity diffraction peak confirmed the enhanced moisture resistance of the OIHP film with a lower GB angle and enlarged grain size. The other minor peaks that indicate degraded OIHP phases were prominently observed in the pattern of the pristine OIHP film after degradation (Figure 4b, S6). In addition to the crystallographic analysis, UV-vis absorption spectroscopy was also used to examine film durability. Figures 4c and d show the UV-vis absorption spectra of pristine and rpOIHP films as a specific comparison. Figure 4d shows that the rp-OIHP film maintained its absorption spectrum, while the pristine OIHP film lost its main absorption after 50 h of exposure to humidity. As shown in Figure 4e and S7, the normalized UV-vis absorbance values of the main peak at 550 nm were calculated. For rp-OIHP, the normalized absorption peak value was still over 80% of the original peak value after high humidity exposure, while the pristine film lost more than 60% of its original light absorption ability. Figures S8a and b show the absorption spectra of the 4 OIHP films before (Figure S8a) and after (Figure S8 b) humidity exposure. As shown in Figure S8b, compared to the pristine film, the rp- and hp-OIHP films exhibited more stable UV-vis absorption spectra, which corresponds well with the XRD results. Furthermore, space-chargelimited current (SCLC) measurement was conducted for evaluating defect densities (ntrap) of OIHP/ITO devices in the dark condition to compare the emergence of defect after degradation


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(Figure S9). Derived from the formulation, (VTFL=entrapd2/2εε0, where VTFL=Trap-filled limited voltage, ntrap is the trap density, d is the thickness of perovskite film, ε0 is the vacuum permittivity, and ε is the relative dielectric constant of the perovskite layer), ntrap of rp-OIHP film is lower than that of pristine OIHP film whose value well corresponds to conventional polycrystalline MAPbI3 (8.55  1016 cm-3 and 2.17  1016 cm-3 for pristine and rp-OIHP film, respectively)3. The VTFL was 6.87 V and 1.74 V for the pristine and rp-OIHP films, respectively. The reduced ntrap can bring positive effect in terms of alleviating hysteresis behavior of devices. Despite 9 h degradation under RH 85% and 30 ℃, the increase ntrap of rp-OIHP film was suppressed to 9.55  1016 cm-3 while an increase by an order was observed at pristine OIHP film, 2.15  1017 cm-3. According to the intrinsic property analysis, it is revealed that the rp-OIHP film is the optimal film to prevent moisture degradation. Finally, we applied pristine, rp- and hp-OIHP films to mesoscopic perovskite solar cells (PSCs) to compare their performance and moisture stability. The schematic image of the device shows the structure of the mesoscopic PSCs (Figure 5a). The 3 types of OIHP films were fabricated on the mesoporous TiO2-deposited fluorine-doped tin oxide (FTO) anode with compact TiO2 layers, and 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiroOMeTAD) was used as the hole-transport layer (HTL). Figure 5b and S10 presents the J-V curves of champion devices and histogram of photoconversion efficiency of pristine, rp- and hp-OIHP based PSCs respectively, and their specific photovoltaic parameters are listed in Table 2. Hysteresis behavior of 3 types of devices are also demonstrated in Figure S11a-c and reduced ntrap contributed to resolving device hysteresis. After the pressure-induced crystallization process, the average PCE of the mesoscopic PSCs increased from 16.48±0.58 to 17.46±0.50 (rp-OIHP) and 16.70±0.80 (hp-OIHP), which agrees with our previous reports on the benefits of applying pressure


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during crystallization, such as enhanced crystallographic orientation and conductivity.21 The improvement of PCE is clearly shown in statistical distribution of device performance measured with 15 devices of each samples. (Figure 5c and S12) The specific photovoltaic parameters of 3 types of devices are illustrated in the Table S1-3 in detail. The processed device exhibits improved PCE and better distribution of PCE thanks to enhancement in Voc, Jsc and FF, which are caused by enlarged contact area between OIHP and spiro-OMeTAD and GB shrinkage which contribute to reducing series resistance of devices.29-31 To examine device stability, 15 devices based on each 3 types of OIHP were stored in a chamber under the condition of 30 ℃ and 40% RH following the moisture stability test protocol reported previously.17,32 After 200 h of exposure, the pristine OIHP-based PSC was severely degraded and had a PCE of 9.06% on average, some of which are even less than half of nondegraded devices since the moisture penetrate through the GBs of the pristine OIHP film (Figure 5d, Table 3). The normalized photovoltaic parameters of PSCs until 200 h moisture degradation is demonstrated in Figure 5e and S13. Although devices were damaged by moisture, rp-OIHP based PSCs sustained its original photovoltaic performance including specific factors of Voc, Jsc, and FF compared to the pristine one due to the harmony between grain size enlargement and reduced GB angle. (Figure S13, S14) The stabilized current density output of devices is also measured to be improved after the process exhibiting enhanced stability under AM 1.5 one sun illumination at rpand hp-OIHP based ones. (Figure S15) To confirm the stability under harsh humidity condition, device stability under the condition of 30 ℃ and 60% RH was also investigated. (Figure S16) The result of device stability was consistent with the trend observed in the film stability test. Although the grain size enlargement and GB close-packing effects are most prominent in the fp- and hpOIHP films, respectively, the rp-OIHP film is revealed to be the most moisture-resistant film.


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Moreover, some rp-OIHP based devices even exhibited improved PCE and Voc even after exposure to air and humidity since spiro-OMeTAD undergoes oxidation during storage.33 Therefore, we can conclude that the moisture stability of OIHP films can be strategically achieved by optimizing the grain size increase and GB angle shrinkage, ultimately resulting in GB healing. In this study, we observed the advantages of a pressure-induced crystallization process, which was used as a strategy to improve the stability of device performance. Although encapsulation and passivation methods can increase stability, a technique for improving film stability by modifying the characteristics of an OIHP film is crucial from an economic perspective for future commercialization.34 Furthermore, analyzing the moisture-induced degradation process of OIHP films with various GB characteristics clarified the role of GBs as moisture penetration channels, and water degradation can be suppressed by directly controlling the GB angle of OIHP films.


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EXPERIMENTAL SECTION Device fabrication The FTO glass (TEC, Pilkington) was cleaned with detergent, acetone and ethanol in sequence with sonication. A 0.2 M titanium diisopropoxide bis(acetylacetonate) (75 wt % in isopropanol, Sigma-Aldrich) solution in n-butanol (Sigma-Aldrich) was spin-coated on precleaned FTO substrates at 2000 rpm for 30 s after 30 min of UV-ozone treatment. The annealing process (125 ℃ and 500 ℃ for 10 and 30 min) was followed by drying and crystallization of the compact TiO2 hole-blocking layer. Next, 60 mg of commercial TiO2 paste (18NR-T, Dysol) was diluted in 10 mg of absolute ethanol (Merck) to prepare the mesoporous TiO2 precursor. On the compact TiO2 film, a mesoporous TiO2 film was fabricated by spin-coating the diluted mesoporous TiO2 paste at 4000 rpm for 30 s and annealing at 500 ℃ for 30 min. The fabricated TiO2 film was treated with a 20 mM aqueous TiCl4 (99.8%, Sigma-Aldrich) solution at 80 ℃ for 30 min and annealed at 500 ℃ for 30 min. For the MAPbI3 precursor preparation, 0.8 M MAI (Great Cell Solar) and PbI2 (TCI) were dissolved stoichiometrically in a solution of dimethylformamide (DMF)/ dimethylsulphoxide (DMSO) (9:1, volume ratio) at 70 ℃. Thereafter, a hydrophobic filter (Advantec, JP050AN) was used to filter the prepared MAPbI3 precursor. The MAPbI3 film was fabricated with the so-called anti-solvent washing method, i.e., spin-coating the precursor at 4000 rpm for 30 s with anhydrous toluene casting (Sigma-Aldrich) after 10 s. The spin-coated film was baked to crystallize MAPbI3 at 100 ℃ for 10 min. For fp-, rp- and hp- MAPbI3 film, the stamping process is conducted right after spin-coating during annealing process for 10 sec with PUA molds exhibiting flat, rectangular and hexagonal nanodot arrays. The spiro-OMeTAD precursor solution was prepared with 72.3 mg of spiro-OMeTAD (Merck), 32.5 mg of 4-tert-bytyl pyridine (TBP) and 9.2 mg of Li-bis(trifluoromethanesulfonyl) imide (Li-TFSI, Sigma-Aldrich) in chlorobenzene


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(Sigma-Aldrich). The spiro-OMeTAD HTL was coated on the substrate by spin-coating at 4000 rpm for 30 s, and a 120 nm Au cathode was formed by thermal evaporation. Characterization The surface morphology of the OIHP films was observed with scanning electron microscopy (SEM, JSM-7001, JEOL) and optical microscopy (OM, BA310 MET, Motic) measurements. The grain size distribution of the OIHPs was calculated with ImageJ software. The effective stresses and uniaxial stresses applied during the compression process were simulated with Midas NFX, a commercially available finite element method (FEM) software. The crystallographic information of the OIHP films was obtained with X-ray diffraction spectroscopy (XRD, SmartLab, Rigaku). UV-visible absorption spectroscopy was performed with a Cary-5000 instrument (Agilent), and the normalized value was calculated at 550 nm. The current densityvoltage (J-V) characteristics were measured under the condition of 1.5 G air mass one sun (100 mW/cm2) illumination with a solar simulator (Sol 3A, Class AAA, Newport) utilizing a filtered 450 W xenon lamp (6279NS, Newport). Before measurement, the light intensity was calibrated with a silicon reference solar cell. Stability evaluation The high humidity chamber was prepared in a convection oven at the desired humidity (85% RH for film stability observations and 40% RH for device performance measurements) and temperature (30 ℃) without any encapsulation. The device performance was periodically measured to determine the photovoltaic parameters.


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ASSOCIATED CONTENT Supporting Information Available: [Figure S1-S16 include distribution of pressure during process, photographs of perovskite films after process, supplementary OM and SEM images, XRD spectra, UV absorption spectra, dark I-V curves of the ETL only devices with perovskite films and extra device performances including parameter distribution, hysteresis, current density output measurement at max power point and normalized PCE under the condition of 30 ℃, RH 60%. Table S1-S3 include detailed information on the individual device for each batch.] This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Correspondence to Prof. J. H. Park (email: [email protected]) ORCID Dukhyun Choi: 0000-0002-4788-0215 Jung Kyu Kim: 0000-0002-8218-0062 Hyunjung Shin: 0000-0003-1284-9098 Jong Hyeok Park: 0000-0002-6629-3147 Notes The authors declare no competing financial interests Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT Prof. J. H. Park acknowledges the support from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20163010012450, 20173010013340). D. H. C. was supported by National Research Foundation of Korea (NRF) Grant funded by the Korean Government (NRF-2018-Global Ph.D. Fellowship Program, 2018H1A2A1063511).


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Table 1. Average grain size of OIHP films according to morphology.

Average grain size (nm2)

Pristine

fp-

rp-

hp-

14876

170593

69150

44524


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Table 2. Photovoltaic performance of mesoscopic PSCs with pristine, rp-, hp-OIHP films

Pristine

rp-

hp-

PCE

Voc

Jsc

FF

[%]

[V]

[mA/cm2]

[%]

best

17.37

1.01

23.10

74.61

average

16.48

1.00

22.56

72.47

best

18.29

1.02

23.50

76.31

average

17.46

1.00

23.08

75.33

best

18.06

1.02

23.24

75.84

average

16.70

0.99

22.65

74.22


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Table 3. Photovoltaic performance of mesoscopic PSCs with pristine, rp-, hp-OIHP films after 200 h degradation under 30℃ RH 40%.

Pristine rphp-

PCE

Voc

Jsc

FF

[%]

[V]

[mA/cm2]

[%]

Average

8.17

0.85

20.20

46.12

Average

16.58

1.00

23.02

71.95

Average

13.79

0.95

22.23

65.20


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Figure 1. (a) Schematic illustration of the pressing process with polymeric imprinting master molds for the fabrication of GB-engineered OIHPs. The GB healing process induced enlargement of the grain size and formed closely packed and low-angle GBs. (b-e) SEM images of (b) pristine, (c) fp-, (d) rp- and (e) hp-OIHP films. The scale bar is 500 nm. (f-i) Grain size distribution of (f) pristine, (g) fp-, (h) rp- and (i) hp-OIHP films. The compressed OIHP films have larger grain sizes than the pristine OIHP films; the largest grain sizes are in fp-OIHP.


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Figure 2. Top view of compressed OIHP films observed at (a-c) 400 nm and (d-f) 200 nm exhibiting in-plane effective stress during compression. The mechanical simulation results show the (a,d) fp- (b,e) rp- and (c,f) hp-OIHP films, respectively, and the highest pressure was exerted on the rp- and hp-OIHP films.


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Figure 3. (a) Photographic images of the OIHP films according to moisture exposure time. (b-e) Optical microscopy images of (b) pristine, (c) fp-, (d) rp- and (e) hp-OIHP films after 9 h of exposure. The rp-OIHP film shows enhanced stability compared to the other films. The scale bar is 125 μm. All OIHP films were exposed to RH conditions of 85% at 30 °C.


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Figure 4. (a-b) XRD patterns of rp- (red) and pristine (black) OIHP films (a) before and (b) after 9 h of exposure to humidity. The rp-OIPH film exhibited enhanced intensity tetragonal MAPbI3 peaks (asterisk, *) and a lower intensity PbI2 peak after degradation. (c-d) UV-vis absorption spectra (c) initially and (d) after exposure to humidity for 50 h for the pristine (black line) and rp-


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OIHP films (red line). (e) Normalized UV-vis absorption at 550 nm of the rp- (red) and pristine (black) OIHP films.


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Figure 5. (a) Schematic images of mesoscopic PSC devices. (b) J-V curves of mesoscopic PSCs employing pristine (black) and rp- (red) OIHP films. (c) Device distribution chart with respect to the PCE for the pristine (black) and rp- (red)OIHP based PSCs. (d) PCE distribution chart of pristine and rp-OIHP based PSCs after 200 h exposure under 30 ℃, RH 40%. (e) Normalized PCE data under and 30 ℃, RH 40% of PSCs with rp-(red) and pristine (black) OIHP films. The PCE of rp-OIHP based PSCs sustained over 95% after 200 h degradation under and 30 ℃, RH 40%.


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Grain Boundary Healing of OrganicâInorganic Halide Perovskites for

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