Scalable Ultrasonic Spray Processing Technique for Manufacturing

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Scalable Ultrasonic Spray Processing Technique for Manufacturing Large-Area CH3NH3PbI3 Perovskite Solar Cells Li-Hui Chou, Xiao-Feng Wang, Itaru Osaka, Chun-Guey Wu, and Cheng-Liang Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12463 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Applied Materials & Interfaces

Scalable Ultrasonic Spray Processing Technique for Manufacturing Large-Area CH3NH3PbI3 Perovskite Solar Cells Li-Hui Chou,a,b,c Xiao-Feng Wang,d Itaru Osaka,c Chun-Guey Wub,e,f and Cheng-Liang Liu*,a,b aDepartment

of Chemical and Materials Engineering, National Central University, Taoyuan

32001, Taiwan. E-mail: [email protected] bResearch

Center of New Generation Light Driven Photovoltaic Modules, National Central

University, Taoyuan 32001, Taiwan cDepartment

of Applied Chemistry, Graduate School of Engineering, Hiroshima University,

Higashi-Hiroshima, Hiroshima 739-8527, Japan dKey

Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education),

College of Physics, Jilin University, Changchun 130012, China eDepartment

fResearch

of Chemistry, National Central University, Taoyuan 32001, Taiwan

Center for New Generation Photovoltaics, National Central University, Taoyuan 32001,

Taiwan

ACS Paragon Plus Environment

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ABSTRACT. Organic–inorganic hybrid perovskite solar cells are on the brink of a breakthrough in photovoltaic technology. Scale-up and large-area processing have become the focal points that must be resolved before commercialization. In this study, scalable ultrasonic spray deposition method for high-throughput coating of the perovskite photoactive layer with a large active area up to 3 cm2 is implemented by precisely controlling the concentration of the precursor solution and spray passes. CH3NH3PbI3 films with large crystallites and a suitable thickness of ~350 nm are facilely developed through one-step direct ultrasonic spraying. Hysteresis-less and highly reproducible power-conversion efficiencies (PCEs) up to 12.30% (11.43 ± 0.43% on average for 20 devices) are achieved by an optimized single-junction device with an active area of 1 cm2, along with good ambient stability. The device retained ~80% and ~65% of the initial PCE after 60 and 105 days sitting in ambient, respectively. The ultrasonic spray-coated perovskite solar cells can be further scaled to larger areas of 2 and 3 cm2 and exhibit PCEs of 10.18% and 7.01%, respectively. The reliable scale-up process for manufacturing atmospheric wet-coated perovskite film is available in cost-effective and easily operated benchtop variants to bridge the interconnection between applied research and industry. KEYWORDS. perovskite, spray-coating, large-area, scalable, solar cells


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INTRODUCTION Recent research on perovskite solar cells, which use the organometal halide perovskite, has aimed at an urgently needed breakthrough in the emerging field of photovoltaic technologies.1-9 Solution processability offers the possibility for mass production while delivering high efficiencies. Tremendous progress has already been made in terms of enhancing the efficiency of small-area cells (typically ≤ 0.1 cm2). However, many of the processing techniques (such as spin-coating and drop-casting) used in the laboratory do not comply with high-throughput and large-scale production. Gains in the scalability and stability of large active sized perovskite photovoltaic materials have lagged behind gains in the energy-conversion efficiency. For example, spin-coating is by far the most common film-formation method for obtaining perovskite thin layers by wet chemistry but does not scale-up well across a larger area (≥1 cm2), and too much material may be wasted during the process. Scaling up the size of perovskite solar cells without significant efficiency losses is a remaining challenge to be resolved.10-14 Multi-method coaters are available to evaluate various methods during scale-up. It was also found that the morphology of crystalline perovskite film has significant implications for manufacturing.15-18 Large number of results regarding perovskite growth have been achieved using scalable solution deposition methods, such as

blade-coating,19-20

slot-die

coating,21

spray-coating,22-29

inkjet-printing,30

and

electrodeposition.31 Therefore, the control of the crystal growth over a large area is a critical issue for scaling-up the perovskite absorber layer.32-38 Spray-coating denotes the formation of a thin film via micron-sized droplets sprayed towards the substrate.39 The quality of the final film is not only highly dependent of the match in polarity between the ink used and the substrate but also the processing conditions, such as the working distance, coating speed, and number of spray passes. The droplet generation in spray-coating can


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be achieved via ultrasonic vibration with a directed carrier gas40-45 or assisted by an electric field.4647

Recently, our group demonstrated the ultrasonic spray deposition of a small-area perovskite

layer through a one-step deposition route44 and a sequential process involving two deposition steps,45 in which a perovskite film with a high crystallinity and surface coverage were obtained. To extend this scalable processing technique to large-area perovskite films while maintaining the good photovoltaic performance of the corresponding cell, the ultrasonic spray deposition must be optimized by comparing different precursor recipes, tuning the deposition parameters, and optimizing the perovskite layer thickness. There are at least three important concepts in our ultrasonic spray-coating process described in this paper. First, widely used CH3NH3PbI3 perovskite film with an active area of >1 cm2 is developed only via a one-step solution method, and no other modification or treatments, e.g., additive in the precursor solution, anti-solvent dripping, postsolvent vapor annealing are applied in the manufacture compared with established perovskite photovoltaic technologies. A highly reproducible large-area perovskite device is produced here. Second, the perovskite crystals with large crystallites are spray-deposited and crystallized via postthermal annealing for a certain time, most importantly, under ambient conditions. Third, the issues of the scale-up production via high-throughput ultrasonic spray deposition and the long-term device stability are addressed.

The establishment of reliable scale-up processes for the fabrication of perovskite layers is a crucial step towards the commercialization of this technology. Herein, we introduce an alternative processing approach for the preparation of a large-area perovskite photoactive layer involving scalable spray deposition of CH3NH3PbI3. Using an automated ultrasonic sprayer, the effects of both the formulation of the perovskite precursor inks and the number of spray passes on the film properties and single-junction device performance obtained with each system were studied, while


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all the other coating parameters can be accurately reproduced. The best photovoltaic performance for a 1 cm2 active area device with good environmental stability was achieved with a PCE of 12.30% for the optimized sample C2 (15% precursor concentration and 2 spray passes). The same preparation condition was used to scale the areas up to 2 and 3 cm2 with PCEs of 10.18% and 7.01%, respectively. These results provide the possible routes for solving the critical issues in the large-area processing and scaling of perovskite photovoltaic technology.

EXPERIMENTAL SECTION

Materials. All the materials and solvents were purchased and used as received, including PbI2 (99.99%,

Alfa-Aesar),

CH3NH3I

(99.9985%,

Dyesol),

poly(3,4-

ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS, CLEVIOS™ P VP AI 4083), C60 (>99.5%, Solenne BV), 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP, >99%, Lumtec), dimethylsulfoxide (DMSO, >99.9%, Sigma-Aldrich) and γ-butyrolactone (GBL, >99.9%, SigmaAldrich). Device Fabrication. The pre-patterned ITO substrates (2.5 × 2.5 cm2) were ultrasonically cleaned with diluted detergent, deionized water, acetone and 2-propanol successively, and blowdried in nitrogen, followed by oxygen plasma treatment for 5 min. A 30 nm-thick layer of PEDOT:PSS was deposited by spin-coating at 5000 rpm for 30 s and annealed at 140 oC for 20 min. A total concentration of 9, 12, 15, and 18% of CH3NH3PbI3 precursor solution was prepared by dissolving PbI2 and CH3NH3I at a molar ratio of 1:1 in anhydrous DMSO:GBL co-solvents (1:1 volume ratio), which represents sample A, B, C and D, respectively. Then, the photoactive perovskite layer were spray-coated onto ITO/PEDOT:PSS substrate through a commercial


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ultrasonic nozzle (Sono-Tek; with a power of 2 W and frequency of 80 kHz). The atmosphere during spray deposition was controlled at the relative humidity (RH) 30-45% of and room temperature. The customized spray head driving system features programmable 3-axes coordinated motion via servomotors. The distance between nozzle and substrate in Z direction is fixed at 3.5 cm. The perovskite solution was atomized by the nitrogen gas at constant pressure of 0.01 MPa with a flow rate of ~5 ml hr-1 controlled by syringe pump. The spray pattern width is approximately 1.5 cm. The nozzle walks along the Y direction with a moving speed of 0.5 cm s-1 and single/multiple spray passes (including 1, 2, 3 and 4 passes), where the multiple reciprocating spray passes contain the several consecutive single pass back and forth in a desired area at a deposition temperature of 80 oC to achieve the required thickness. The time period between each spray passes is 10 s. Thereafter, the substrate was put onto a hotplate for 30 min at 120°C. The devices were finalized by thermal evaporation of C60 (20 nm), BCP (7 nm) and Ag (100 nm) at a chamber pressure of 10-6 torr. Characterization. The UV-Vis absorption spectra were recorded with a JASCO V-670 UVVis spectrophotometer. Steady state photoluminescent (PL) spectra were measured using an optical microscope based detection system (UniRAM, Protrustech) with a 500 nm excitation laser. The thin film morphology was measured using the optical microscope (OM, Leica 2700M) scanning electron microscope (SEM, JEOL JSM-7600F). X-ray diffraction (XRD) analysis was performed on Bruker D8A X-ray diffractometer with a Cu Kα radiation. Atomic force microscope (AFM) was performed using Seiko SPA400 in tapping mode. Current density-voltage (J-V) curves of all solar cells under illumination were measured by applying an external voltage bias to the cell and recording the photocurrent with Keithley 2400 programmable source meter. A light source integrated with a Xe lamp with an illumination power


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of 100 mW cm-2 simulated AM1.5G was used as solar simulator (provided by Enli Technology Co. Ltd.), calibrated by a Si reference cell. The external quantum efficiency (EQE) data were obtained by a spectral response/quantum efficiency measurement system (Enli Technology Co. Ltd.). All the measurements of the solar cells were performed under ambient atmosphere (RH 3045%) at room temperature without encapsulation. A black mask with the aperture area of 1×1 cm2, 1×2 cm2 and 1.25×2.4 cm2 (all the them are smaller than the defined electrode area of solar cells) was applied on the top of cell during the measurement.

RESULTS AND DISCUSSION

An inverted planar photovoltaic device with the structure of ITO/PEDOT:PSS/ CH3NH3PbI3/C60/BCP/Ag was used in this study. The perovskite thin film was produced via ultrasonic spray-coating from an equimolar PbI2 and CH3NH3I precursor solution in DMSO:GBL (1:1 in volume), as depicted in the schematic of the precise spray pathway (Figure 1), which is mechanically automated by pre-programmed sequences. The nozzle atomizes the precursor solution into a continuous spray of micrometer-sized droplets that land on the substrate and form a wet layer, followed by thermal annealing to form the perovskite film. The device performance was tuned by increasing the wet layer thickness and related morphologies, controlled by the number of passes of the spray-coating (single or multiple passes) and the precursor concentrations, whereas the other spray parameters, including the nozzle height, moving speed, atomization pressure, substrate temperature, and solution flow rate, were kept the same. The CH3NH3PbI3 film was grown onto a prepared substrate via a one-step method, and the combined effects of the number of spray passes (1-, 2-, 3-, and 4-pass coatings) and the precursor concentration (samples


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subjected to 9, 12, 15, and 18% were labeled as A, B, C, and D, respectively) were quantitatively analyzed for a total of 16 samples (the abbreviated name of each device is presented in Table S1). For example, C2 represents the device with coating of two passes from a 15% precursor solution. Finally, multiple 100-nm-thick Ag electrodes with an area of 1.1 × 1.1 cm2 were evaporated, and a precise aperture mask with a real active size of 1 × 1 cm2 was positioned directly on the active side of the cell. Table S2 shows all the photovoltaic parameters of the 16 samples, including the open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and power-conversion efficiency (PCE), measured under the simulated AM 1.5G 100-mW cm-2 illumination. A statistical study on the performance of more than 15 device cells for each sample was conducted. The PCE of our champion cell (device C2) was optimized to be 12.30% with a Voc of 0.83 V, Jsc of 21.9 mA cm-2, and FF of 67.6%. The cross-sectional scanning electron microscopy (SEM) image shown in Figure 1 demonstrates that the CH3NH3PbI3 photo-absorber layer in device C2 was pinhole-free with a grain size of hundreds of nanometers. The processing variables, including the concentration of the precursor solution and the number of back-and-forth sweep passes, involved in the one-step processing of the perovskite film is discussed as follows with respect to improving the film morphology, crystallization, and device performance.


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Figure 1. (a) Schematic illustration of the ultrasonic spray-coating process and post-annealing used in this study. (b) SEM cross section image of spray-coated perovskite solar cell (sample C2) on ITO glass substrate.

At a low solution flow rate and nozzle scan speed, a compact and continuous large-area perovskite film can be deposited via our automatic ultrasonic spray-coating system, and the perovskite film thickness and quality can be fundamentally regulated according to the precursor concentration and number of coating passes. First, the concentration of the precursor solution was varied from 9, to 12, 15, and 18% and spray-coated on top of the ITO/PEDOT:PSS substrate with a fixed two-coating pass, yielding devices A2, B2, C2, and D2, respectively. The thickness of the perovskite layer is generally determined by the amount of droplets dispensed per unit area. As expected, the film thickness increases from 193 ± 12 to 385 ± 25 nm as the precursor concentration increases (from device A2 to D2; Figure S1), suggesting that a wide range of controlled film


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thicknesses was obtained through the ultrasonic spray-coating technique. The film turns dark brown at a higher precursor concentration (Figure S2) which is probably because of increased precursor interaction upon annealing. To deeply understand the variation in the optical properties, the optical absorption spectra of the perovskite layer prepared with different precursor concentrations are shown in Figure 2. All the A2, B2, C2, and D2 perovskite films exhibit broad light absorption and an identical absorption shape in the UV-vis light region and an absorption onset at 785 nm, which agrees with the bandgap (1.55 eV) of the CH3NH3PbI3 film, similar to previously reported data for this material.48 Moreover, the absorption increases almost linearly with respect to the precursor concentration, which clearly demonstrates the improved lightharvesting capability.

2.5

Absorbance (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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D2 C2 B2 A2

2.0 1.5 1.0 0.5 0.0 500

550

600

650

700

750

800

Wavelength (nm) Figure 2. Absorption spectrum of ultrasonic spray-coated CH3NH3PbI3 film produced by sample A2, B2, C2 and D2.


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X-ray diffraction (XRD) patterns of the CH3NH3PbI3 film prepared from different precursor concentrations are presented in Figure 3. The XRD patterns of all the samples show major diffraction peaks at 2θ = 20.4° and 41.0°, corresponding to the preferential (112) and (224) orientations of CH3NH3PbI3 crystals, respectively, and weak diffraction peaks at 2θ = 14.5° and 28.9°, corresponding to the preferential (110) and (220) orientations, respectively. The increased intensities and reduced full width at half maximum (FWHM) of the (112) reflection peak for sample C2 with the major part of the precursor converted into CH3NH3PbI3 crystals (Figure S3) indicate the strong dependence of the film crystallinity and quality on the concentration of the precursor solution. Both the increased crystallinity and the large crystallites size, as evidenced by the narrow XRD peak of sample C2, are strongly related to the device performance. The appearance of a peak at 2θ = 12.5° for samples A2 and B2 indicates the presence of the PbI2 inside the perovskite film. At lower precursor concentrations (9% and 12%), the solvent evaporates quickly during the deposition. Having no sufficient time to the formation of CH3NH3PbI3 crystal, the PbI2 is left after the release of the organic species/solvent, which can be monitored according to the light-grey color of these two films. In particular, both major PbI2 and minor perovskite phases coexist for sample A1 (at the lowest precursor concentration and 1 spray pass), as shown in Figure S4, indicating that the observed XRD pattern is likely related to unreacted PbI2 and the release of CH3NH3I. Besides, owing to the discontinuous and thin perovskite layer, and the top C60 layer may directly contact the bottom PEDOT:PSS layer, reducing the charge transfer and extraction to the counter electrode. On the other hand, sample D2—with a higher precursor concentration—also has a very small peak, which is attributed to the lattice plane of PbI2. This may be caused by the incomplete reaction of PbI2 with CH3NH3I, and a higher loading of precursor


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materials with a slight residual of PbI2 phase necessitates a higher post-annealing temperature for overcoming the activation energy and achieving a higher conversion rate.

Figure 3. XRD pattern of ultrasonic spray-coated CH3NH3PbI3 film produced by sample A2, B2, C2 and D2.

The morphological changes of perovskite films formed with different precursor concentrations can be directly observed by top-view images. As shown in Figure 4, the uniform flower-like perovskite crystallites in all four films comprise different sizes of grains (from hundreds of nanometer to micrometer); the size in perovskite crystallites increases as the precursor concentration (film thickness) increases. When the precursor solution concentration is increased, perovskite crystals with a prolonged growth period may promote the continuous growth and high crystallization of the perovskite film. When the concentration is increased up to 18% (for sample D2; Figure 4d), the film morphology only exhibits a slight difference—with a similar large


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crystallites size—from that prepared from the 15% solution (for sample C2; Figure 4c). Large perovskite domain is generally beneficial for the light absorption and charge transport in the optoelectronic film.49-51 To clarify the effect of the precursor concentration on the quality of the resulting film, steady-state photoluminescence (PL) measurements were performed, as shown in Figure 5, indicating the similar trends. The strong PL peak of the sample C2 perovskite film is centered at 767 nm, with the narrowest FWHM of 41 nm. When defects or trapping sites are present inside the perovskite film, the photogenerated carriers become trapped at these sites and recombine without photon emission.52 Therefore, the extent of the PL quenching indicates higher non-radiative recombination due to the activated traps and higher photon scattering. It apparently explains the enhanced film quality of sample C2 due to its less PL quenching.

Figure 4. Surface SEM image (top view) of the ultrasonic spray-coated CH3NH3PbI3 film fabricated through sample (a) A2, (b) B2, (c) C2 and (d) D2.


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Figure 5. PL spectra of the ultrasonic spray-coated CH3NH3PbI3 film fabricated through A2, B2, C2 and D2 condition.

The current density–voltage (J–V) curves of the same batch of devices with different precursor concentrations are shown in Figure 6, and the corresponding photovoltaic parameters are summarized and compared in Figure 7 and Table S2. It is clearly shown that the photovoltaic performance, such as Jsc and FF, can be significantly improved by increasing the spraying precursor concentration to 15% with a suitable perovskite film thickness of ~350 nm and good film quality. Large perovskite crystallites sizes with a reduced overall grain boundary and less defects due to the reduced PbI2 at the boundary may explain the highest PCE of sample C2. However, the introduction of precursors in excess degrades the performance of the corresponding device slightly (especially in Jsc) because of the limited exciton diffusion and small amount of PbI2 impurities in the thickest perovskite film (sample D2), although the crystallites size of the


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perovskite film is large and the light absorption is greatly elevated. Sample C2, fabricated by using the moderate concentration, exhibits the best performance, with significantly improved device performance parameters (Voc of 0.83 V, Jsc of 21.94 mA cm-2, FF of 67.6%, and overall PCE of 12.30%) at the active area of 1 cm2. The trend of the precursor concentration-dependent device performance is similar for the 1-, 3-, and 4-pass coatings. The PCE increases consistently with an increasing precursor concentration, reaching its maximum at the concentration of 15%. Hence, the single- and multiple-pass deposition of the perovskite precursor coating with the same concentration (15%) is studied later to explore the effect of the number of spray passes on the photovoltaic characteristics of the resulting perovskite film.

0 -2

Current Density (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-5

D2 C2 B2 A2

-10 -15 -20 -25 0.0

0.2

0.4

0.6

0.8

1.0

Voltage (V) Figure 6. J-V curve for the best performing perovskite solar cell devices based on the A2, B2, C2 and D2 film (varied by concentrations of precursor solution).


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Figure 7. Dependence of the best device parameters on the perovskite solar cells with various precursor concentrations and numbers of spray passes.

As described in the following, the perovskite precursor solution was coated under four different conditions, varying the ultrasonic spray-coating passes of 1, 2, 3, and 4. The film thickness increases linearly with respect to the number of passes, from 162 ± 7 nm to 490 ± 19 nm (Figure S5). A precise control in film thickness is possible via successive spray depositions under an identical process. To obtain more insight into the effect of the number of coating passes on the quality of the resulting perovskite film, the corresponding SEM images, absorption spectra, and


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XRD spectra were recorded as a function of the number of passes. Figure 8 presents a top-section SEM image of the perovskite film deposited from the 15% precursor solution after post-annealing at 120 ℃, which exhibits similar crystalline perovskite features to clusters of flower-like crystallites with a domain size in the range of tens to hundreds of micrometers. The crystal structures from different passes are quite similar, but the average crystallites size of the CH3NH3PbI3 film increases significantly with increasing spray passes, accompanied by a reduced grain boundary. It can be probably explained that the coating process allows the underlying film is re-dissolved when the surface is re-coated and then the crystals merge into larger domain. Such film morphologies may yield effective photogenerated charge transport in the active layer and at the charge-extraction interfaces. Figure S6 shows the XRD patterns of CH3NH3PbI3 films with different numbers of spray passes, revealing that the diffraction peaks (112) and (224) of the sample C2 film is stronger than the other three films. The high intensities of the (112) peaks suggest that the crystal domains were highly oriented along the (112) plane via the spray-coating process. The decrease in the FWHM of the (112) peak for the sample C2 film (Figure S7) suggests an increased crystal coherence length, which accords with the increased grain size in the SEM image (Figure 8). These findings indicate an enhanced crystallinity with less structural defects and/or grain boundaries for the sample C2 film, which is due to the suitable balance among the precursor concentration, deposited temperature, and number of spray passes. Notably, the weak peak at 12.5° indicates the existence of a small amount of PbI2 for the sample C1, C3, and C4 films. Sample A1 and B1 show inadequate perovskite crystallization relative to sample C1 under the same single-spray pass condition. A similar situation arises when sample C4—fabricated with a high precursor concentration and large number of spray passes—exhibit less unreacted PbI2 than


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samples A4 and B4, indicating that the effect of the precursor concentration is more important for the formation of a high-quality perovskite film than the number of spray passes.

Figure 8. Surface SEM image (top view) of the ultrasonic spray-coated CH3NH3PbI3 film fabricated through sample (a) C1, (b) C2, (c) C3 and (d) C4.

The UV-vis absorption and PL emission spectra are also measured, as shown in Figures S8 and S9, respectively. The thickness of the CH3NH3PbI3 layer for devices produced from different numbers of spray passes can be directly correlated with the intensity of optical absorption. With increasing spray passes, a redshift and enhanced absorption across the whole spectrum were observed, which are attributed to the increased optical density of the thicker perovskite film with a large crystallites size. To further investigate the emission properties of these coated perovskite films, all the studied samples were prepared on the plasma-cleaned glass substrate to exclude the influence of charge transfer between the hole-transport and active layer. The PL emission peak


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located at 767 nm originating from CH3NH3PbI3 can be clearly observed. The C2 film exhibits the highest PL intensity and the narrowest peak width (Figure S9), owing to the optimized film quality with less defect traps of the film. These results, together with the SEM image and XRD pattern, clearly confirm the restricted charge-recombination channel in the C2 film processed with the 15% precursor solution and two spray passes, because of the reduced rain boundary and lower defect density.53-54 Parallel to the perovskite film characterization discussed above, a series of devices employing the perovskite film with single and multiple spray passes were examined. The J–V curves of representative devices are shown in Figure 9a, and the resultant photovoltaic parameters are summarized in Figure 7 and Table S2. The results show that the optimal number of spray passes for modifying the perovskite film is two when the film is applied in solar cells, as evidenced by the high PCE of 12.30% for sample C2. When the single coating pass is applied during the film formation (especially in the case of a lower precursor concentration (samples A1 and B1)), the performance is poorer due to the discontinuous and thin perovskite layer and top C60 layer may be directly contacted with bottom PEDOT:PSS layer, resulting in reducing charge transfer and extraction to the courter electrode. For example, sample C1 has a lower average PCE of 8.26 ± 0.44% (maximum of 9.01%). On the other hand, the thicker perovskite film and incomplete perovskite conversion for samples C3 and C4 (increase in the number of spray passes) may hinder the charge transport in the film and increase the charge trapping even though the desired morphology with large perovskite crystallites is obtained. The decreased device performance contributes to the decline of Jsc and FF. The best-testing sample C2 devices exhibits a PCE of 12.30 (12.20)% with a Voc of 0.83 (0.86) V, a Jsc of 21.94 (20.87) mA cm-2, and an FF of 67.6 (67.4)%, measured under a forward (reverse) voltage scan (Figure S10). Hysteresis-less is detected regardless of the scan direction, suggesting that the continuous ultrasonic spray deposition


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can contribute to the formation of a perovskite film with improved morphology and enhanced crystallinity. As indicated by EQE spectrum shown in Figure 9b, the C2 devices exhibit EQE values above 80% from 430 to 740 nm. The integrated EQE spectrum yields a Jsc of 20.02 mA cm-2, which agrees with the value measured from J–V scanning.

Figure 9. (a) J-V curve for the best performing perovskite solar cell devices based on the C1, C2, C3, and C4 film (varied by precursor coating passes). (b) EQE curves of perovskite solar cells produced by C2 film with the integrated Jsc over the EQE spectrum.

The stability of the perovskite solar cell based on C2 film was investigated by storing the unencapsulated devices in a 40% relative humidity environment. As the data shown in Figure 10a, we traced the device performance as a function of the storage time and found that the PCE of the sample C2 device retains ~80% of the original value after 60 days and ~65% after 105 days of storage, which is much better than the spin-coated CH3NH3PbI3 film. The good device stability is attributed to the large crystallites with reduction in the grain boundaries of the hydrophobic perovskite layer prepared by the ultrasonic spray-coating method, which protects perovskite film against moisture (or oxygen) penetration. The robustness against moisture, which stems from the


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hydrophobic nature, can be evaluated according to the contact angle between the CH3NH3PbI3 film and water. The contact angle of the ultrasonic spray-coating C2 film is ~76°, whereas that for the perovskite thin film subjected to spin-coating is ~64° (Figure 10b). The less hydrophilic properties of spray-coated perovskite film as compared to spin-coated one is probably due to its rougher surface from the root-mean-square roughness (Rrms) of 47.1 and 8.5 nm, respectively, as measured by AFM image (Figure S11). Besides, another possible reason is the contraction of the unit cells since the compact and stable spray-coated perovskite crystals are preferentially oriented along the (112) and (224) directions (Figure 3). Relatively, the major peaks of spin-coated perovskite film are from (110) and (220) facets (Figure S12). The position of the main peaks from spin-coated to spray-coated perovskite film shifts toward higher diffraction angle, which indicates the decreasing trend in the lattice constant.55-56 The introduction of these close-packed perovskite crystals from spray-coating technique can potentially overcome the water vulnerability of perovskite. Therefore, the device degradation can be mitigated through the optimized spraying procedure demonstrated in this study.


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Figure 10. (a) Normalized PCE profiles of C2 perovskite solar cells over time during storage at 30-45% RH environment without additional encapsulation. (b) Water contact angle test for perovskite film through ultrasonic spray-coating and spin-coating process.

Finally, high-throughput ultrasonic spray-coating was used to scale-up the coating of the perovskite layer to a larger area, whereby a perovskite film with high crystallinity and surface coverage were using the same processing condition in a single step under an ambient atmosphere as that for preparing C2 film. Scale-up of the active layers from 1 to 2 and 3 cm2 were performed to elucidate the relationship between the active area and the device performance. Photographs of the corresponding devices arranged by increasing area are presented in Figure 11a, while Figure 11b shows the J–V curves for perovskite solar cells with these three cell areas. The PCE of the solar cells decreases continuously, reaching 10.18% and 7.01% when the active area is increased to 2 and 3 cm2, respectively. The Voc is independent of the cell area. However, the Jsc and FF significantly decline, decreasing to 20.3 and 17.3 mA cm-2 and to 60.5% and 49.5%, respectively, when the active area is increased to 2 and 3 cm2. Importantly, a progressive increase in series resistance (Rs) shunt was accompanied by a progressive decrease in resistance (Rsh) upon increasing the active cell area (Rs/Rsh exhibit values of 4.8/1802.0, 7.0/1580.5, and 9.4/555.2 Ω cm2 for active areas of 1, 2, and 3 cm2, respectively), which is also consistent with the reduction in Jsc and FF.57 Here, the slight inhomogeneities of the active layer (probably at the edge of the film) may cause local variations in the photoinduced current as well as a reduced FF owing to the current leakage of the device in a very large size of active area. These results suggest that stable and high-performance centimeter-sized perovskite solar cells can be manufactured in air using the scalable ultrasonic spray-coating method. Further optimization of perovskite solar cells via


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elemental-composition engineering (with mixed cations and halide anions) of the perovskite absorber and interface engineering (hole/electron-transport layer) of the cell using our ultrasonic spray-coating strategy is in progress.

Figure 11. (a) Photograph of 1, 2, and 3 cm2 perovskite solar cells device. (b) J-V curve for the perovskite solar cell devices derived from C2 film with a 1, 2, and 3 cm2 active area.

CONCLUSION We demonstrated a scalable ultrasonic spray-coating technique for fabricating large-area perovskite thin films with large crystallites, reduced grain boundaries, and high crystallinity under ambient conditions. By subjecting the widely used photo-absorbing material CH3NH3PbI3 to a


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simple one-step solution process, a device exhibits PCE of 12.30% with an aperture area of 1 cm2 was achieved by manipulating the precursor concentration and the number of spray passes. A device based on the C2 film retained ~80% of its initial PCE after 60 d sitting in an ambient atmosphere. The spray-coating technique, without other treatments (especially anti-solvent dripping), is high-throughput and reproducible way, as evidenced by C2-based cells, which exhibited an average PCE of 11.43 ± 0.43% for 15 cells. The PCE reaches 10.18% and 7.01% when the active area is scaled to 2 and 3 cm2, respectively. The proposed ultrasonic spray-coating technique significantly promotes low-cost and mass production of large-area perovskite film, requiring no strictly controlled atmospheres.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Summary of sample name; summary of the photovoltaic parameters; perovskite film thickness; photograph, the intensity and FWHM of the (112) diffraction peak, XRD pattern, absorption spectrum, PL spectra and AFM image of the ultrasonic spray-coated CH3NH3PbI3 film; J-V curve in forward and reverse scan of ultrasonic spray-coated perovskite solar cells (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]


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ACKNOWLEDGMENT The authors gratefully acknowledge the funding from the Ministry of Science and Technology (MOST) of Taiwan. We also thanks for Advanced Laboratory of Accommodation and Research for Organic Photovoltaics (MOST, Taiwan) to perform the photovoltaic parameter measurements.

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Scalable Ultrasonic Spray Processing Technique for Manufacturing

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