Perovskite Cluster-Contained Solution for Scalable D-bar Coating

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Perovskite Cluster-Contained Solution for Scalable D-bar Coating Toward High Throughput Perovskite Solar Cells Dong-Nyuk Jeong, Do-Kyoung Lee, Seongrok Seo, Soo Yeon Lim, Yong Zhang, Hyunjung Shin, Hyeonsik Cheong, and Nam-Gyu Park ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00042 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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ACS Energy Letters

Perovskite Cluster-Contained Solution for Scalable D-bar Coating Toward High Throughput Perovskite Solar Cells Dong-Nyuk Jeong,a Do-Kyoung Lee,a Seongrok Seo,b Soo Yeon Lim,c Yong Zhang,a Hyunjung Shin,b Hyeonsik Cheong,c and Nam-Gyu Parka* a

School of Chemical Engineering, Sungkyunkwan University, Suwon 16419, Korea b

Department of Energy Science, Sungkyunkwan University, Suwon 16419, Korea c

Department of Physics, Sogang University, Seoul 04107, Korea *Corresponding authors (E-mail: [email protected])

Abstract For scalable perovskite solar cell (PSC), deposition of homogeneous and high-quality perovskite film on large area (>100 cm2) is prerequisite. Conventional solutions for spincoating on small area contain usually polar aprotic solvents with high boiling point, which is hard to be adopted for large-area bar coating because of uncontrollable and slow drying process due to strong interaction between polar aprotic solvent and Lewis acidic PbI2 or perovskite. Thus, precursor solution plays a vital role in the success of large-area coating. Here we report a coating solution suitable for large-area perovskite films. The coating solutions prepared via gas-mediated solid-liquid conversion contain pre-formed perovskite clusters as confirmed by rotational mode of methylammonium cation in the PbI3- framework from Raman spectroscopy. CH3NH3PbI3 (MAPbI3) films formed by D-bar coating within 20 s on the area over 100 cm2 exhibits tetragonal/cubic superlattice structure with highly preferred orientation in the entire film, which results in average PCE of 17.01% and best PCE of 17.82%.

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Since the first report on the solid-state perovskite solar cell (PSC) with power conversion efficiency (PCE) of 9.7% and stability for 500 h without encapsulation,1 following two root researches on perovskite-sensitized solar cells,2,3 works on PSC have been intensively performed as a result PCE of 23.7% was recorded in a best research-cell efficiency chart provided by national renewable energy laboratory.4 This achievement is approaching theoretical PCE of 30.5% for the perovskite with bandgap of around 1.6 eV.5 PCEs of smallarea PSCs strongly depends on precursor solution, coating procedure and quality of resulting perovskite films,6,7 where a polar-aprotic-solvent based precursor solution has been generally used to form high quality perovskite film via spin-coating technique. In spin-coating process, physical property of solvent in the precursor solution may not be critical because solvent is mostly removed by centrifugal force while the substrate is rotated at high speed. However, physical property of solvent becomes important when enlarging the coating area and changing the coating procedure from spin-coating method to non-rotating coating techniques

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ACS Energy Letters

using such as slot-die or doctor-blade. Thus, development of precursor solution is a key to the success of producing reproducibly high quality large-area perovskite films. For scalable fabrication of perovskite film, slot-die coating is one of feasible methods. However, the slot-die-coated perovskite film using the precursor solution similar to those used for spin-coating demonstrated PCE as low as ~12%,8 which indicates that development of a new precursor solution for large-are perovskite coating is highly required. Recently, large-area perovskite film was fabricated using a viscous liquid precursor based on amine complexes of CH3NH3I·3CH3NH2 (MAI·3MA) and PbI2·CH3NH2 formed by reaction of MAI or PbI2 powder with MA gas,9 which however requires rather complicated steps of adding the viscous precursor, covering with polyimide sheet, applying pressure, heating and then peeling off the polyimide film. Although a PCE of the module with aperture area of 36 cm2 was verified to be 12.1% using pressure processing method, uncontrollable solution viscosity along with the complicated process may be drawback to scaling up reproducibly. Nevertheless, change in state between solid and liquid by amine gas has been regarded as useful methodology to enhance perovskite film morphology and thereby photovoltaic performance.10-15 In addition, gas-assisted solid-liquid phase transition can be beneficial to making precursor solution for large-area coating because the precursors can dissolve without using polar aprotic solvents. Development of an eminently suitable coating solution combined with a simple and fast coating process is a key to success of high efficiency largearea PSCs. We report here a new coating solution for a large-area (>100 cm2) perovskite film based on MA-assisted MAPbI3 solution. The solutions enable the fabrication of highly crystalline MAPbI3 film within 20 s (see video clip1 and 2 in Supporting Information) at a room 3



temperature on either rigid glass substrate or flexible polymer substrate using a wire-bar (Dbar) or analogues coating technique. D-bar coating method has been known as a simple and efficient coating technology for large-area thin films as opto-electronic devices.16-19 Chemical species in the coating solution are investigated by Raman spectroscopy and crystal structure of the coated perovskite film is studied by transmission electron microscopy combined with selected area electro diffraction. We first investigate D-bar coated perovskite film quality using a conventional precursor solution containing polar aprotic solvents. As mentioned previously, drying process is expected to be the rate-determining step in large-area coating (Figure 1a). Thus, not only physical property such as boiling point but also chemical interaction between solvent and precursors can affect the final film quality. Since most of polar aprotic solvents have Lewis base characteristics, chemical interaction between polar aprotic solvent and Lewis acid PbI2 or methylammonium cation is readily made to from adducts,20 which can hinder fast evaporation of solvent. High boiling point (bp) of polar aprotic solvents can retard evaporation of solvent, which may also affect the resulting morphology of perovskite film. The dried large-area MAPbI3 films show different morphology depending on polar aprotic solvent used in the precursor solution (Figure 1b-e), where dimethylformamide (DMF, bp ~ 153 oC) and dimethyl sulfoxide (DMSO, 189 oC) produce rod shaped MAPbI3, while mixed morphology of sphere and needle is formed after evaporation of dimethylacetamide (DMA, bp ~ 165 oC) and small and large spherical shapes are found from γ-butyrolactone (GBL, bp ~ 204 oC) based precursor solution. Moreover, large-area substrate is not fully covered with perovskite film. Since no systematic evolution of the final film morphology is, chemical interaction is expected to be strongly involved in drying process rather than a simple 4


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ACS Energy Letters

difference in bp. This experimental result is clearly indicative of importance of precursor solution and ill-influence of polar aprotic solvent for large-area perovskite film coating.

Figure 1. MAPbI3 film morphologies formed by D-bar deposition technique using conventional MAPbI3 solutions. a, Schematic illustration of large-area MAPbI3 thin film coating process using conventional precursor solutions via D-bar. Optical microscopic images of the dried MAPbI3 films depending on different polar aprotic solvent of b, DMF. c, DMA. d, DMSO, and e, GBL (scale bar is 35 µm).

To prepare large-area coating solution single crystal (SC) or powdered MAPbI3 is put in a small vial which is placed in the closed container together with a beaker containing MA solution at room temperature (Figure 2a). The solid MAPbI3 is changed to yellowish liquid phase upon exposure to MA vapor at room temperature (Figure 2b). It was first reported that MA gas was used for defect-healing the coated MAPbI3 film, where solid-liquid phase transition was found to be reversible.10 However, it is still questioned how solid MAPbI3 can be dissolved by MA gas. It was argued that electron donating nitrogen in MA molecule was interacted with cations (Pb2+ and MA+) in MAPbI3 leading to disruption of MAPbI3 lattice and formation of complex between cation and MA,9 like cation-Lewis base adduct, which 5



may not explain dissolution of MAPbI3 in gas. In order to investigate mechanism involved in solid-liquid phase transition via MA gas, we study Raman spectra that are obtained from solution samples. The Raman spectrum of the viscous liquid obtained by reaction of solid MAPbI3 with MA gas is different from those of the GBL or DMF solution in which MAPbI3 is dissolved (Figure 2c and d). The overall Raman spectral feature for the viscous liquid is almost similar to that for the MAPbI3 single crystal.21 It is noted from Figure 2c that the broad peak centered at around 288 cm-1 is attributed to the restricted rotation of MA+ in the PbI3- inorganic framework. In fact, the rotational mode of the isolated MA+ ion is Raman inactive, but the local symmetry lowering in the PbI3- inorganic framework makes it partially Raman active22,23 as demonstrated in the comparison of the Raman spectra of MAPbI3 and MAI.21 Our Raman data strongly indicate that the MA+ ions in GBL or DMF solution are not restricted inside the PbI3- inorganic framework but those in MAPbI3 in viscous liquid are still captured inside the framework preserving its perovskite structure to a certain extent instead of breaking-down to form cation-MA complexes. A strong peak at 95 cm-1 along with weak shoulder at 160 cm-1 is observed for the viscous liquid, which is related to (Pb-I) species,24 while a sharp peak at 120 cm-1 with shoulder at 80 cm-1 is observed for both GBL and DMF solutions. This suggests that (Pb-I) iodoplumbate species in the viscous liquid are quite different from those in GBL and DMF solutions. Based on the Raman study, here, we propose that solid MAPbI3 can be dissolved because gaseous MA will be pseudo-liquefaction via physisorption. Gas cannot be thermodynamically and spontaneously transformed to liquid due to unfavorable entropy change. However, gaseous MA will be first physisorbed on crystalline MAPbI3 because of adsorption is more exothermic than liquefaction. Adsorbed MA will be less disordered due to enhanced chemical interaction between MA and behavior like liquid, which is followed by dissolving surface of MAPbI3 crystals (Figure 2e). Since 6


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ACS Energy Letters

perovskite structure still exists in the viscous liquid as confirmed by Raman spectroscopic study, cluster-like MAPbI3 might be formed in the viscous solution. In D-bar coating, film thickness is affected by viscosity and concentration of the coating solution, coating speed and coating gap (distance between bar & substrate).16 In order to deposit MAPbI3 film uniformly with desired thickness (400~600 nm) using the as-prepared viscous solution via D-bar coating process, the coating should be performed very slowly with a thin coating gap because of high viscosity. At very slow coating rates (< 5 mm/s), the steady state coating is not possible because evaporation rate of MA gas is faster than the coating speed. Moreover a flat and uniform surface may not be expected by using highly viscous solution. Thus solution viscosity is required to be lowered by dilution, where the diluting solvent should not interact with MA in the viscous solution to keep MAPbI3 dissolved. We have first tried to dilute the viscous solution using common polar solvents such as methanol, ethanol and water. However, precipitation occurs because those solvents are miscible with the mother viscous solution. Although acetonitrile (ACN) and tetrahydrofuran (THF) do not seem to be good solvent because MAPbI3 cannot dissolve in ACN and THF, those solvents might work for diluting the viscous solution because MA(g)-dissolved ACN was found to dissolve perovskite precursor.25 Other solvents with low boiling points such as methyl acetate, methyl formate, propionaldehyde, methyl ethyl ketone (MEK), 3-methyl-2-butanone (3MB), propionitrile (PN), acrylonitrile and 2-methyltetrahydrofuran have been tested for dilution. MEK, 3MB and PN are miscible but perovskite phase is hard to form after coating. The viscous solution is successfully diluted with ACN and THF. As one can expect, polar aprotic solvent such as DMSO and DMF is one of potential candidates but as mentioned previously slow evaporation due to strong Lewis acid-base interaction is obstacle to obtain high quality 7



film. D-bar coating results in highly compact perovskite films with smooth surface from the solution diluted with ACN and THF but poor film quality with coarse surface from the solution diluted with DMSO and DMF (Figure S1 and S2). The film quality reflects well photovoltaic performance, where D-bar coated MAPbI3 using an ACN-diluted viscous solution exhibits best performance amongst the studied solvents for dilution (Figure S2). As confirmed from Raman spectra in Figure 2d, chemical species is hard to change even after dilution with ACN. This indicates that less interaction between diluting solvent and MA in the viscous solution leads to better photovoltaic performance.

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ACS Energy Letters

Figure 2. Preparation procedure of large-area coating solution via MA gas-assisted solid-liquid transition and Raman spectra. a, Schematic illustration of conversion of MAPbI3 solid into liquid by MA gas in a closed system. b, Single crystal (SC) or powder MAPbI3 was used as a starting precursor, which was first converted to viscous liquid (VL) in MA gas filled closed vessel and then diluted with acetonitrile (ACN). c, Raman spectrum of VL, which was compared with those of MAPbI3 solution dissolved in GBL and DMF. d, Raman spectra of the coating solution prepared by diluting VL with ACN, which was compared with those of VL and solvent ACN. Asterisks represent Raman peaks corresponding to ACN. e, Schematic illustration of explaining how to MAPbI3 dissolves in MA(g): Gaseous MA is first physisorbed on crystalline MAPbI3 and then MA(g) is converted to liquid MA due to enhanced chemical interaction between MA. MAPbI3 dissolves in liquefied MA, where cluster-like MAPbI3 exist in the viscous solution. Weight ratio of MAPbI3 to MA (MAPbI3 : MA) in the final solution was found to be about 80 : 20

Scanning electron microscopy (SEM) and transmission microscopy (TEM) images are measured to understand the resulting MAPbI3 film morphology, along with X-ray diffraction (XRD) patterns. As shown in Figure 3a and 3b, average grain size (estimated by the intercept method using a straight-line) of the D-bar coated MAPbI3 film is about 435 nm (largest is over 1000 nm) which is much larger than that of the spin-coated one (ca. 190 nm). In addition, surface and cross-sectional morphology of the D-bar coated MAPbI3 is quite 9



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different from the normally observed morphology in the spin-coated MAPbI3. Since the Dbar coated MAPbI3 using the ACN-diluted viscous solution has quite different morphology, TEM study is further conducted. Figure 3c shows bright-field TEM image of the whole device, together with selected area electron diffraction (SAED) pattern. It is noted that cubic and tetragonal phase coexists in the grain, which indicates that MAPbI3 made from the ACNdiluted viscous solution is composed of superlattices. It was reported that superlattice is beneficial to reduction of defect in perovskite.26 High resolution TEM images (Figure 3d) show clearly lattice fringes, where the fast Fourier transform (FFT) patterns confirm highly oriented lattice planes regardless of positions of perovskite layer scanned from bottom (near FTO) to top (near spiro-MeOTAD). This indicates that the primary grain is an individual single grain, which is confirmed by FFT patterns obtained from different parts in a grain, while the primary grain of MAPbI3 obtained from spin-coating the conventional precursor solution in DMSO/DMF is composed of numerous secondary grains having different lattice planes (Figure S3). The larger primary grain size with highly oriented crystal structure is responsible for highly intense (hk0) XRD peaks showing preferred orientation (Figure 3e). In fact, photoluminescence (PL) life time of the D-bar-coated MAPbI3 film is increased by 13.7% from 168 ns (spin-coated MAPbI3) to 191 ns (Table S1). Defect density (nt) measured by space charge limited current (SCLC) method is 21.7% lower for the D-bar coated MAPbI3 (nt = 1.19×1016 cm-3) than for the spin-coated one (nt = 1.52×1016 cm-3) (Figure S4 and Table S2). Longer carrier life time along with reduced defect density is responsible for the higher open-circuit voltage (Voc) as compared to the MAPbI3 film prepared by spin-coating the conventional precursor solution (Figure S5 and Table S3). The D-bar coated film shows slightly higher absorbance near bandgap illumination (Figure S4), which is indicative of more radiative recombination and well consistent with the increased carrier life time. Average 10


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ACS Energy Letters

PCE of 17.37% is observed from the D-bar coated MAPbI3, which is higher than that of 16.69% from the spin-coated MAPbI3 using a conventional DMSO/DMF based precursor solution (Table S3).

Figure 3. SEM and TEM images and XRD data of MAPbI3 thin films formed by D-bar coating using a viscous solution diluted with ACN, those which are compared with spincoated MAPbI3 using a DMSO/DMF based solution and anti-solvent dripping process. Top-view and cross-sectional SEM images of MAPbI3 thin films formed by a, D-bar coating using a viscous solution diluted with ACN and b, spin-coating using a DMSO/DMF solution with stoichiometric MAI and PbI2. c, Cross-sectional TEM images of D-bar coated MAPbI3 film. Inset is SAED pattern showing superlattices of cubic and tetragonal phases. Samples were prepared by focused ion beam (FIB). d, High magnification TEM images of area scanned from near FTO to near HTL (hole transporting layer, here spiro-MeOTAD was used). Fast Fourier Transform (FFT) patterns are shown in inset. e, XRD data of two different MAPbI3 thin films prepared by (1) D-bar coating using a viscous solution diluted with ACN and (2) spin-coating using a conventional precursor dissolved in DMSO/DMF solvent. Samples for SEM, TEM and XRD were prepared by annealing MAPbI3 films at 150 oC for 5 min.

Photovoltaic performance and opto-electronic property are investigated for PSCs based on 11



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the D-bar coated MAPbI3 films, where D-bar coating is conducted with two different coating solutions based on single crystal and powder MAPbI3 precursor. Figure 4a-d shows that photovoltaic parameters are better for the single crystal MAPbI3 based D-bar coating solution than for the powder MAPbI3 based one. At the same experimental batch, average PCE of 16.6% is observed from the D-bar coating solution based on single crystal MAPbI3, whereas lower PCE of 14.55% is obtained based on powder MAPbI3. The highest PCE of 17.8% is able to be achieved by the D-bar coating using a solution based on single crystal MAPbI3. The lower PCE in case of using powder MAPbI3 might be attributed to trace impurity. The higher PCE for single crystal MAPbI3 is mainly due to higher Jsc, which is related to higher absorption coefficient (Figure 4e). In addition, higher external quantum efficiency (EQE) in the wavelength ranging between 440 nm and 760 nm is attributed to higher Jsc. (Figure 4f). TRPL data showing more than two times longer carrier life time for the MAPbI3 film coated using single crystal MAPbI3 precursor reflects higher Voc. (Figure 4g and Table S1). Little change in defect density is observed and both D-bar coated films have lower defect density than spin-coated one using DMSO/DMF based solution (Table S2), which indicates that physical property of the resulting perovskite film depends on coating solution and process.

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Figure 4. Comparison of photovoltaic parameters, EQE, PL decay and absorption coefficient (α) between single crystal and powder MAPbI3 as precursor for the ACNbased viscous solution. a-d, Photovoltaic parameters of Jsc, Voc, FF and PCE for the planar PSC devices with D-bar coated MAPbI3 films formed from the single crystal or powder MAPbI3 dissolved viscous solution diluted with ACN. An electron transporting SnO2 layer and a hole transporting spiro-MeOTAD layer were used. e, Absorption coefficient of D-bar coated MAPbI3 films formed from the single crystal or powder MAPbI3 dissolved viscous solution diluted with ACN. f, External quantum efficiency (EQE) for the planar PSC devices. g, Time-resolved PL, where PL data were fit with bi-exponential decay equation.

Based on the developed coating solution made from MAPbI3 single crystals, a large-area MAPbI3 film (10×10 cm2) is formed on a SnO2-coated FTO glass substrate by D-bar coating (Figure 5a). MAPbI3 film with mirror-like surface is immediately formed by D-bar coating within 20 s at ambient temperature (see video clip1). The large-area film is divided into 16 pieces with dimension of 2.5×2.5 cm2 to investigate homogeneity of photovoltaic performance of PCSs. Photovoltaic performance for the as-coated non-annealed perovskite film is compared with those for the annealed films. Temperature increases from 50 oC to 200 o

C with interval of 25 oC. Mean Jsc increases from 17.75 mA/cm2 to 20.89 mA/cm2 as 13



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annealing temperature increases from room temperature to 150 oC, and then slightly decreases to 20.61 mA/cm2 at 175 oC and 18.67 mA/cm2 at 200 oC (Figure S6a). Deterioration of Jsc at temperature higher than 175

o

C is probably due to partial

decomposition of MAPbI3. Highest Jsc of 22.39 mA/cm2 is observed at annealing temperature of 150 oC. Mean Voc does not seem to depend on annealing temperature but annealing at 150 o

C leads to higher Voc of 1.10 V (highest one is 1.14 V) (Figure S6b). FF depends strongly

on annealing temperature, where mean FF increases from 0.55 to 0.70 as temperature increases from room temperature to 125-150 oC (highest FF is 0.79 at 125 oC), and then declines to 0.62 at 175 oC and 0.42 at 200 oC (Figure S6c). As a consequence, mean PCE increases linearly as annealing temperature increases up to 150 oC, resulting in best PCE of 17.82% (Figure S6d). Although XRD shows (hk0) oriented crystalline phase for the nonannealed MAPbI3, smaller grain size along with lower absorbance is responsible for the lower PCE (Figure S7). However, it is noted that over 10% PCE could be obtained from nonannealed MAPbI3 by using D-bar coating the ACN-diluted viscous liquid, which is beneficial for flexible PSCs (see video clip2). With the optimal annealing condition, we achieve average PCE of 17.01% and best PCE of 17.82% from the 16 subdivided PCSs from a largearea MAPbI3 film (Figure 5b and Table S4). In Figure 5b, there is tendency in distribution of PCE in terms of coating direction. As the coating time increases along the coating direction, PCE tends to decrease, which is due to the same tendency in Jsc (Figure 5c). Decrease in Jsc along the coating direction is attributed to decrease in film thickness (Figure S8), which is because the loaded solution on D-bar is gradually consumed as the coating progresses. Little change in Voc along the coating direction is indicative of no change in film quality (Figure 5d). However, FF tends to increase with coating progresses because of reduced resistance, associated with decreased film thickness (Figure 5e). The thickness variation can be fixed if 14


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ACS Energy Letters

a consistent amount of loading is automatically fed rather than manual loading. The developed D-bar coating solution is confirmed to extend the deposition size to 20×20 cm2 (Figure S9), which is beneficial for scale-up production of PCSs.

Figure 5. Photovoltaic performances of D-bar coated large-area MAPbI3 film. a, Digital photo of a MAPbI3 film coated on a SnO2-coated FTO substrate (10 cm wide and 12 cm long. after D-bar coating upper 1 cm and lower 1 cm were cut off). After annealing at 150 oC for 5 min, the substrate was divided into 16 pieces to make PSCs with dimension of 2.5×2.5 cm2. One device with 2.5×2.5 cm2 contains 5 cells. b, PCEs of best performing PSC in each device. c-e, Jsc, Voc and FF of best performing PSC in each device. Arrow indicates coating direction. Photovoltaic parameters displayed here were based on the reverse scanned J-V data.

In conclusion, for scaling PSCs up, homogeneous and high quality perovskite film should be instantly formed on large area so that PCSs with a high throughput are available. Precursor 15



solution suitable for large-area coating was proved to be more important than coating methods. We have successfully demonstrated that MAPbI3 films were rapidly (within 20 s) deposited on a FTO substrate with area larger than 100 cm2 by a simple D-bar coating technique using a coating solution prepared via solid-liquid phase transition and dilution with ACN. Raman spectroscopic studies confirmed that colloids having perovskite structure already existed not only in the vicious liquid but also in the diluted solution with ACN, which was quite different from chemical species such as iodoplumbate ions usually found from the conventional polar aprotic solvent based coating solutions. Moreover, the D-bar coated MAPbI3 was highly (hk0) oriented crystalline in the entire film together with tetragonal/cubic superlattice structure, which was also clearly distinguished from randomly oriented structure observed in spin-coated MAPbI3 using DMSO/DMF based coating solution. Such a unique morphology resulted in long carrier life time and low defect density. Average PCE exceeding 17% and best PCE approaching 18% were demonstrated with a 100 cm2-area MAPbI3 film. Since the diluted viscous solution for large-area coating does not give rise to a waste of solution and a high speed deposition, the developed coating solution is proposed to be highly adaptable to scalable PSCs for commercialization. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:xxxx Experimental details of D-bar coating solution preparation, coating procedure, solar cell fabrication and characterizations (EQE, XRD, TRPL, TEM, FIB, SCLC and UV-vis); additional figures and tables showing high resolution TEM with FFT patterns, SCLC 16


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details (dark I-V data with capacitance), dependency of annealing temperature on photovoltaic parameters, SEM images and XRD patterns of D-bar coated MAPbI3 films before and after annealing; two supporting videos on real-time MAPbI3 D-bar coating process at room temperature on glass and flexible substrates. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT Future Planning (MSIP) of Korea under contracts NRF2012M3A6A7054861 and NRF-2014M3A6A7060583 (Global Frontier R&D Program on Center

for

Multiscale

Energy

System),

NRF-2016M3D1A1027663

and

NRF-

2016M3D1A1027664 (Future Materials Discovery Program) and NRF-2015M1A2A2053004 (Climate Change Management Program).

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