Precise Morphology Control and Continuous Fabrication of Perovskite

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Precise Morphology Control and Continuous Fabrication of Perovskite Solar Cells Using Droplet-Controllable Electrospray Coating System Seung Chan Hong,‡,§,† Gunhee Lee,‡,§,† Kyungyeon Ha,§,† Jungjin Yoon,‡,§ Namyoung Ahn,‡,§ Woohyung Cho,§ Mincheol Park,‡,§ and Mansoo Choi*,‡,§ ‡

Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, Republic of Korea Global Frontier Center for Multiscale Energy Systems, Seoul National University, Seoul 151-744, Republic of Korea

§

S Supporting Information *

ABSTRACT: Herein, we developed a novel electrospray coating system for continuous fabrication of perovskite solar cells with high performance. Our system can systemically control the size of CH3NH3PbI3 precursor droplets by modulating the applied electrical potential, shown to be a crucial factor for the formation of perovskite films. As a result, we have obtained pinhole-free and large grain-sized perovskite solar cells, yielding the best PCE of 13.27% with little photocurrent hysteresis. Furthermore, the average PCE through the continuous coating process was 11.56 ± 0.52%. Our system demonstrates not only the high reproducibility but also a new way to commercialize high-quality perovskite solar cells. KEYWORDS: perovskite solar cell, electrospray, continuous, droplet size, controllable reproducible and uniform fabrication of high-quality thin films through simple setup and process. With this advantage, the electrospray coating process has been widely utilized to fabricate light absorbing layers of organic photovoltaics,14 charge transport layers15 and transparent conducting layers16 of photovoltaics. Moreover, this low-cost system has the capability of controlling the droplet conditions, and can further be extended to the continuous and scalable fabrication technique by combining with roll-to-roll and multinozzle processing17 for the potential use in mass production. This electrospray system, however, has never been attempted for continuous fabrication of perovskite solar cells. In this study, we developed a continuous and dropletcontrollable electrospray coating system for fabricating perovskite solar cells. With a simple setup, this system can consistently provide stable electrospray conditions and a uniform coating process, which leads to the fabrication of pinhole-free and large-grain-sized perovskite films. Furthermore, the perovskite precursor droplets can be precisely and easily produced in the desired size by adjusting the applied electrical potential. We have also investigated the effect of the droplet size on morphology and crystallinity of the perovskite films as well as device performance. This holds significance of

H

ybrid organic−inorganic perovskites as light absorber materials in solar cells have recently received considerable attention due to their outstanding properties such as high absorption coefficient,1 long-range carrier lifetime,2 and appropriate bandgap.3 Only in a few years, the power conversion efficiency (PCE) of perovskite solar cell has been remarkably improved from 3.8 to 22.1%, 4−6 thereby strengthening its position as the next-generation solar cell. Expectedly, the demand for research on commercializing these high-performance solar cells has increased with the foremost priority lying on the development of continuous and scalable fabrication techniques. To date, this demand has been explored by spray-coating based fabrication techniques such as ultrasonic spray coating,7,8 hand spray coating9,10 and electric-fieldassisted spray coating.11,12 These methods, however, have technical issues to overcome such as excessive complexity and challenging controllability in the system. (Table S1) In addition, the need of fundamental studies on droplets generated by the spray has been emphasized in previous research because droplet conditions play an important role in determining the quality of thin films and device performance.13 Unfortunately, this demand has not been dealt with or fulfilled, because of the droplet controllability problem of existing spraycoating methods. The electrospray coating system could be an alternative method to the conventional spin-coating and spray-coating methods for commercializing the perovskite solar cells. It can consistently generate monodisperse droplets, which allows © 2017 American Chemical Society

Received: November 24, 2016 Accepted: February 20, 2017 Published: February 20, 2017 7879

DOI: 10.1021/acsami.6b15095 ACS Appl. Mater. Interfaces 2017, 9, 7879−7884

Letter

ACS Applied Materials & Interfaces

Figure 1. Schematic illustrations of (a) a new electrospray system and (b) the formation process of perovskite films through the electrospray system. The inset photographs show the electro-spraying of perovskite solutions and fabricated perovskite film through the system. The setup consists of a nozzle, a syringe pump, a high-voltage power supply, a hot plate, and a visualization system (CCD camera).

our study, as there has been no experimental analysis of perovskite precursor droplet size, despite the fact that it is one of the crucial factors to determine the device performance. Through systematic optimizations, we have successfully fabricated inverted planar perovskite solar cells that show the best PCE of 13.27% with little photocurrent hysteresis. Furthermore, the reproducible feature of the electrospray coating technique has been confirmed by fabricating the perovskite solar cells with the average PCE of 11.71 ± 0.81%. The continuous process system of the electrospray has also been verified by consecutively producing perovskite solar cells with the average PCE of 11.56% with a small standard deviation of 0.52%. We have simply configured an electrospray system of which schematic diagram is presented in Figure 1a. The setup consists of a nozzle, a programmable fluid pumping system, a power supply and a hot-plate. Contrary to the conventional electrospray systems, we have optimized the inner diameter of the nozzle to solve issues of instability and uncontrollability of electrospray that arise from high conductivity of perovskite solutions. The formation of a stable cone jet in electrospray system is determined by the balance between two characteristic times: the dielectric relaxation time (τe), which indicates the response time of the fluid when voltage is applied, and the characteristic time of Taylor cone formation (τh).18,19#tab; τe =

εε0 Rη , τh = K γ

(MAPbI3) perovskite solution is precisely controlled by using a digital syringe pump (LEGATO 100, KdScientific) which supplies the fluid to the silica nozzle through a polytetrafluoroethylene (PTFE) tube. In addition to this basic setup, we have installed a CCD camera (SCC-B2331, Samsung Electronics) and lighting equipment between the nozzle and the hot-plate to closely observe the electrospraying of perovskite solutions in real-time. The formation process of a perovskite thin film by using the electrospray system is presented in Figure 1b. First of all, poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) was spin-coated on an ITO/Glass substrate that was placed on the hot plate with a temperature of 65 °C. Highly unipolar-charged, microsized droplets were generated on the tip of the Taylor cone when the high electrical potential was applied to the nozzle.20,21 The generated droplets were dispersed due to the Coulomb repulsive force and densely deposited without any rebound effect with the help of the electrical gradient between the nozzle and the substrate.14,22 The perovskite precursor droplets deposited on the substrate were immediately heated and vaporized. As a result of the solvent vaporization, the perovskite crystals were simultaneously formed in the droplets. At the initial stage of the perovskite layer coating, the perovskite crystals were sparsely formed on the substrate due to the insufficient amount of the droplets. However, the coverage and thickness of the perovskite film were gradually increased through the continuous electrospray. Finally, the substrate was fully covered with the perovskite film after 2 min. After it was coated with the perovskite layer, the substrate was annealed under 100 °C for 5 min. The temperature of the substrate during the electrospray coating plays an important role in fabricating a uniform and dense perovskite film because the temperature determines the rates of solvent vaporization and perovskite crystallization in the droplets. In the case of a low-temperature substrate (45 °C), lots of pinholes were generated in the perovskite film due to the coffee ring effect induced by slow solvent vaporization as can be seen in Figure S1a. On the other hand, when the temperature of the substrate was as high as 85 °C, there was little time to be sufficiently wetted due to fast vaporization, which resulted in the formation of particle-shaped perovskite crystals and low uniformity of the perovskite film (Figure S1c). Therefore, we chose the temperature of 65 °C to vaporize the solvent in an appropriate rate, which produced a highly dense and uniform perovskite film shown in Figure S1b.

(1)

where ε is the relative permittivity, K is the electrical conductivity, η is the viscosity, γ is the surface tension of the solvent, ε0 is the permittivity of free space, and R is the inner radius of the nozzle. When electrical potential is applied, if the τe is much shorter than the τh, the fluid tends to be ejected before the Taylor cone is formed on the nozzle tip, which consequently induces unstable and uncontrollable electrospray. Therefore, for high conductivity fluids such as perovskite precursor solution, τh should be shortened by reducing the inner diameter of the nozzle to form a stable Taylor cone.18,19 Thus, we have used a microsized silica nozzle with the inner diameter of 15 μm (Nano NC, Korea) which is not commonly used in the conventional methods. The nozzle is connected to a metal holder to which high voltage is applied by a DC highpower supplier (Nano NC, Korea). To generate a stable electrical gradient between the nozzle and the substrate, we have used an electrically grounded hot-plate which was placed 60 mm under the nozzle. The flow rate of CH3NH3PbI3 7880

DOI: 10.1021/acsami.6b15095 ACS Appl. Mater. Interfaces 2017, 9, 7879−7884

Letter

ACS Applied Materials & Interfaces

Figure 2. (a) Correlation between the electrical potential applied to the nozzle and the size of generated droplets. Grayscale images show the Taylor cone and generated droplets depending on the applied electrical potential. (b−g) SEM images of the perovskite films depending on three different applied electrical potentials of (b−d) 10.5, 11.5, and 12.5 kV and (e−g) their magnified images.

On the basis of the above results, we analyzed the relationship between the droplet size and the morphology of perovskite films. Figure S2 shows the growth mechanism of perovskite films with respect to the droplet size. If it is too large, solvent vaporization and perovskite crystallization proceed slowly, which eventually results in the coffee ring effect. That is, uneven perovskite films with pinholes and insufficiently grown grains are formed. In contrast, if it is too small, the solution is rapidly vaporized before the perovskite is sufficiently crystallized. This leads to porous and highly rough films with many pinholes and small grains. Through the combined focused ion beam/scanning electron microscopy (FIB/SEM) system, we confirmed the morphologies of the perovskite films that had been fabricated with the different droplet sizes controlled by adjusting the potentials (10.5, 11.5, and 12.5 kV). The three different films have a similar thickness of 350 nm (see Figure S3), although there was a striking contrast in their morphologies as shown in Figure 2b−g. These results reveal that the perovskite film fabricated at 11.5 kV (Figure 2c, f) has the best quality in terms of surface coverage, uniformity and crystallinity. This means that the droplets generated at 11.5 kV are most appropriate for producing uniform, pinhole-free, and large grain size perovskite films

To identify the effect of the droplet size on the morphology of perovskite films, we first investigated the droplet features that affect the evaporation rate. The droplet size can be controlled by adjusting the electrical potential applied to the nozzle. Figure 2a shows the droplet diameters and Taylor cone images of electrospray as a function of applied voltage (see the detailed method in the experimental section.) The initiating value of the applied electrical potential for the cone-jet mode was 9.5 kV, generating droplets with over 9 μm in diameter at the tip of the Taylor cone. As the applied electrical potential increased, the cone became thinner and sharper by the reinforced electrical force along the downward direction. As a result of the increase in the applied electrical potential from 9.5 to 11.5 kV, the average droplet sizes were gradually declined from 9.3 to 4.5 μm in diameter, which is in accordance with the previous report.23,24 However, when the value of the applied electrical potential was over 12.5 kV, more than two Taylor cones (multijet mode) were formed by strong electrical force which can lead to jet instability.24−26 Therefore, the droplet size decreased from 3.1 μm in diameter at 12.5 kV to 2.3 μm in diameter at 13.5 kV. From these results, we have demonstrated that the droplet size could be controlled from 9.3 to 2.3 μm by adjusting the electrical potential. 7881

DOI: 10.1021/acsami.6b15095 ACS Appl. Mater. Interfaces 2017, 9, 7879−7884

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ACS Applied Materials & Interfaces

Figure 3. (a) Schematic illustration of an inverted planar MAPbI3 perovskite solar cell fabricated by the electrospray coating system and a crosssectional SEM Image of perovskite solar cells fabricated under the condition of 11.5 kV. (b) J−V curves of best performing perovskite solar cells in each cases (10.5, 11.5, and 12.5 kV). (c) PCE histogram of 25 cells fabricated under the applied electrical potential of 11.5 kV.

Table 1. Photovoltaic Parameters of the Perovskite Solar Cells Depending on the Different Applied Electrical Potentialsa

a

electrical potential (kV)

VOC (V)

JSC (mA/cm2)

FF

PCE (%)

best PCE (%)

10.5 11.5 12.5

0.95 ± 0.01 0.97 ± 0.01 0.88 ± 0.05

14.96 ± 1.06 16.87 ± 0.92 14.57 ± 1.48

66.94 ± 1.42 71.82 ± 2.83 60.28 ± 2.33

9.53 ± 0.64 11.71 ± 0.81 7.72 ± 0.62

10.86 13.27 9.05

The average values were calculated from 25 devices for each case.

(Figure 3). Especially in the 11.5 kV device, the highest PCE of 13.3% was achieved with negligible photocurrent hysteresis, Jsc of 18.55 mA/cm2, Voc of 0.96 V, and FF of 0.75 (Figure S7a). We also measured the stabilized current density at maximum power point and external quantum efficiency (EQE) spectra as can be seen in Figure S7b. The stabilized power conversion efficiency is 13.0% and the integrated Jsc is 15.8 mA/cm2 (Figure S8), which are well consistent with measured results from J−V curves. Moreover, the average PCE for the 11.5 kV devices were 11.71 ± 0.81% with a small standard deviation, which is indicative of reliable reproducibility of our electrospray system (Figure 3d). We additionally performed electrochemical impedance spectroscopy (EIS) measurements as shown in Figures S9 and S10 in order to deeply understand why all the parameters increased in the 11.5 kV devices based on the equivalent circuit analysis. The results indicate that the 11.5 kV devices have pinhole-free and large grain-sized perovskite films, which results in the decreased recombination loss. To prove the feasibility of continuous and reproducible features of our electrospray system, we have demonstrated continuous fabrication of perovskite solar cells through stopand-go as well as nonstop movements. All experiments have been performed by using the droplets generated at the potential of 11.5 kV in order to produce the highest quality perovskite films as aforementioned. In the case of the stop-and-go movement, substrates of 2.5 cm × 2.5 cm were sequentially coated by perovskite films as seen in Movie S1. In the case of the nonstop movement, perovskite films were continuously coated on substrates of 2.5 cm × 10 cm with a moving speed of

because of their well-adjusted balance between evaporation and crystallization rates. On the other hand, in the case of 10.5 kV (Figure 2b, e), perovskite films with pinholes and small grains were formed due to their droplets that were too large. In 12.5 kV condition (Figure 2d, g), the droplets were too small to crystallize perovskite sufficiently, resulting in porous and rough perovskite films with particle-shaped perovskite crystals. Furthermore, X-ray diffraction (XRD) patterns of the perovskite films obtained under three different applied voltages were measured to compare their crystallinity (Figure S4). The peaks of these XRD patterns correspond to those of spin-coated MAPbI3 films shown in Figure S5, which means that the pure MAPbI3 films were formed through our electrospray system. We also prepared inverted planar perovskite solar cells with the structure of ITO/PEDOT:PSS/MAPbI3 perovskite/fullerene (C60)/bathocuproine (BCP)/LiF/Al (Figure 3a). Current density−voltage (J−V) curves of three types of devices made with different applied electrical potentials were measured under one-sun illumination as shown in Figure 3b. The detailed photovoltaic properties of devices are shown in Table 1, and PCE histograms of all devices are shown in Figure S6. As a result of measurement, the 11.5 kV devices (a device fabricated by applying X kV of electrical potential in the electrospray system is referred to as the X kV device) show the highest average PCE, short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF) as compared with the 10.5 kV and 12.5 kV devices. It is in accordance with the case of perovskite film; the quality of the perovskite film (Figure 2) determines the performance of the perovskite solar cells 7882

DOI: 10.1021/acsami.6b15095 ACS Appl. Mater. Interfaces 2017, 9, 7879−7884

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ACS Applied Materials & Interfaces

Figure 4. (a) Photograph of the continuous electrospray coating system and perovskite films being produced by the system. (b) Photograph of fabricated perovskite films with six different areas marked. (c) J−V curves of perovskite solar cells in each marked areas.

1.25 cm/min (see Movie S2). Figure 4 and Figure S11 show the coating process, the locations and the device performance of manufactured cells for the both cases. The average PCEs were 11.56 ± 0.52% and 11.06 ± 0.55% with small standard deviations for the stop-and-go and the nonstop cases, respectively (see the detailed photovoltaic parameters in Table S2 and Figure S11). Moreover, the device performance of the both cases is comparable with that of the individually fabricated devices in Table 1, which indicates the possibility of realization of mass production when using our electrospray system. Thus, our electrospray systems can be considered as a promising manufacturing process. as it can continuously produce high-quality perovskite films with high reproducibility. We have developed a continuous electrospray coating system with a simple setup for rapid and scalable fabrication of perovskite solar cells. The droplet size can be controlled by adjusting the applied electrical potential in our system, which leads to the dramatic quality improvement of the perovskite films. We have analyzed the correlation between the applied electrical potential and the droplet size, and obtained pinholefree and uniform perovskite films with large grains, which are supported by SEM and XRD analysis. As a result of optimization, the best PCE of 13.27% has been achieved with no photocurrent hysteresis while the average PCE of 25 cells has been 11.71% with a small standard deviation of 0.81%. The EIS measurement clearly explains the enhancement of the device performance in terms of carrier recombination loss. Furthermore, we have demonstrated the continuous coating system, which results in the average PCE of 11.56% with a small standard deviation. Our electrospray system is a low-cost continuous technique for highly efficient perovskite solar cells through precise droplet-size management because of its reproducibility and scalability.

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EQE spectra and the integrated Jsc, steady-state photocurrent density and PCE, Nyquist plots of the fabricated devices; continuous fabrication of perovskite solar cells through nonstop movements (PDF) Movie S1, continuous fabrication of perovskite solar cells using stop-and-go and movement (AVI) Movie S2, continuous fabrication of perovskite solar cells using nonstop movement (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Mansoo Choi: 0000-0002-3009-375X Author Contributions †

S.C.H., G.L., and K.H. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the Global Frontier R&D Program of the Center for Multiscale Energy Systems funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea (2012M3A6A7054855). The XRD patterns were measured at Research Institute of Advanced Materials in Seoul National University.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15095. Experimental details; strengths and weaknesses of existing spray-coating methods; schematic description of the growth mechanism of perovskite films; photovoltaic parameters, SEM images, cross-sectional SEM images, XRD patterns, histograms of PCE, J−V curves, 7883

DOI: 10.1021/acsami.6b15095 ACS Appl. Mater. Interfaces 2017, 9, 7879−7884

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ACS Applied Materials & Interfaces of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead(II) Iodide. J. Am. Chem. Soc. 2015, 137, 8696−8699. (6) NREL Solar Cell Efficiency Chart 2016. http://www.nrel.gov/ ncpv/ (accessed November, 2016). (7) Barrows, A. T.; Pearson, A. J.; Kwak, C. K.; Dunbar, A. D. F.; Buckley, A. R.; Lidzey, D. G. Efficient Planar Heterojunction MixedHalide Perovskite Solar Cells Deposited via Spray-Deposition. Energy Environ. Sci. 2014, 7, 2944. (8) Huang, H.; Shi, J.; Zhu, L.; Li, D.; Luo, Y.; Meng, Q. Two-Step Ultrasonic Spray Deposition of CH3NH3PbI3 for Efficient and LargeArea Perovskite Solar Cell. Nano Energy 2016, 27, 352−358. (9) Gamliel, S.; Dymshits, A.; Aharon, S.; Terkieltaub, E.; Etgar, L. Micrometer Sized Perovskite Crystals in Planar Hole Conductor Free Solar Cells. J. Phys. Chem. C 2015, 119, 19722−19728. (10) Ramesh, M.; Boopathi, K. M.; Huang, T. Y.; Huang, Y. C.; Tsao, C. S.; Chu, C. W. Using an Airbrush Pen for Layer-by-Layer Growth of Continuous Perovskite Thin Films for Hybrid Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 2359−2366. (11) Chandrasekhar, P. S.; Kumar, N.; Swami, S. K.; Dutta, V.; Komarala, V. K. Fabrication of Perovskite Films Using an Electrostatic Assisted Spray Technique: The Effect of the Electric Field on Morphology, Crystallinity and Solar Cell Performance. Nanoscale 2016, 8, 6792−6800. (12) Ishihara, H.; Sarang, S.; Chen, Y.-C.; Lin, O.; Phummirat, P.; Thung, L.; Hernandez, J.; Ghosh, S.; Tung, V. Nature Inspiring Processing Route Toward High Throughput Production of Perovskite Photovoltaics. J. Mater. Chem. A 2016, 4, 6989−6997. (13) Chandrasekhar, P.; Kumar, N.; Swami, S. K.; Dutta, V.; Komarala, V. K. Fabrication of Perovskite Films Using an Electrostatic Assisted Spray Technique: The Effect of the Electric Field on Morphology, Crystallinity and Solar Cell Performance. Nanoscale 2016, 8, 6792−6800. (14) Park, S. E.; Kim, S.; Kim, K.; Joe, H. E.; Jung, B.; Kim, E.; Kim, W.; Min, B. K.; Hwang, J. Fabrication of Ordered Bulk Heterojunction Organic Photovoltaic Cells Using Nanopatterning and Electrohydrodynamic Spray Deposition Methods. Nanoscale 2012, 4, 7773− 7779. (15) Kim, M. C.; Kim, B. J.; Yoon, J.; Lee, J. W.; Suh, D.; Park, N. G.; Choi, M.; Jung, H. S. Electro-Spray Deposition of a Mesoporous TiO2 Charge Collection Layer: Toward Large Scale and Continuous Production of High Efficiency Perovskite Solar Cells. Nanoscale 2015, 7, 20725−20733. (16) Kim, T.; Canlier, A.; Kim, G. H.; Choi, J.; Park, M.; Han, S. M. Electrostatic Spray Deposition of Highly Transparent Silver Nanowire Electrode on Flexible Substrate. ACS Appl. Mater. Interfaces 2013, 5, 788−794. (17) Khan, M. K. I.; Mujawar, L. H.; Schutyser, M. A.; Schroën, K.; Boom, R. Deposition of Thin Lipid Films Prepared by Electrospraying. Food Bioprocess Technol. 2013, 6, 3047−3055. (18) Fernández de La Mora, J The Fluid Dynamics of Taylor Cones. Annu. Rev. Fluid Mech. 2007, 39, 217−243. (19) Obata, D.; Tasaka, H.; Katsuki, S.; Akiyama, H. Formation of Liquid Cone Jet Dependent on Rise Time of Driving Voltage. J. Electrost. 2015, 76, 274−277. (20) Zeleny, J. Instability of Electrified Liquid Surfaces. Phys. Rev. 1917, 10, 1−6. (21) Taylor, G. Disintegration of Water Drops in an Electric Field. Proc. R. Soc. London, Ser. A 1964, 280, 383−397. (22) Kang, J. S.; Park, I.; Jung, J. H.; Kim, S. S. Free-Surface Electrospray Technique Using a Multi-Hole Array. J. Aerosol Sci. 2013, 55, 25−30. (23) Park, I. Experimental Study on Water Electrospray: Characteristics of Jet Instability and Droplet Generation. PhD thesis, Korea Advanced Institute of Science and Technology, Daejeon, South Korea, June, 2015. (24) Lee, A.; Jin, H.; Dang, H. W.; Choi, K. H.; Ahn, K. H. Optimization of Experimental Parameters to Determine the Jetting Regimes in Electrohydrodynamic Printing. Langmuir 2013, 29, 13630−13639.

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DOI: 10.1021/acsami.6b15095 ACS Appl. Mater. Interfaces 2017, 9, 7879−7884