Efficient Planar-Heterojunction Perovskite Solar Cells Fabricated by

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Efficient Planar-Heterojunction Perovskite Solar Cells Fabricated by High-Throughput Sheath-Gas-Assisted Electrospray Sunghoon Han, Hyungchae Kim, Seojun Lee, and Changsoon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18643 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 6, 2018

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ACS Applied Materials & Interfaces

Efficient Planar-Heterojunction Perovskite Solar Cells Fabricated by High-Throughput Sheath-GasAssisted Electrospray †

†

Sunghoon Han, Hyungchae Kim, Seojun Lee,

†,§

and Changsoon Kim

*,†

†Graduate School of Convergence Science and Technology, and Inter-University Semiconductor Research Center, Seoul National University, Seoul 08826, Republic of Korea. KEYWORDS: electrospray, perovskite solar cells, sheath gas, high throughput deposition, electrostatic deposition

ABSTRACT

When a perovskite precursor solution is electrosprayed using the conventional method where the nebulization of the solution is primarily governed by electrostatics, its high electrical conductivity tends to cause electrospray instabilities and thus makes high quality perovskite films very difficult to obtain. Here, we report high-throughput fabrication of efficient perovskite solar cells (PSCs) whose CH3NH3PbI3-xClx films are deposited using a sheath-gas-assisted electrospray system. Our system, based on strong pneumatic nebulization as well as high-voltage electrostatic

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charging of droplets, enables very stable, high-flow electrospray of small charged droplets even for the highly conductive perovskite precursor solution. Consequently, with control of the drying rate of the droplets deposited on substrates by adjusting the substrate temperature during deposition, crystalline, void-free CH3NH3PbI3-xClx films with near 100% surface coverage and high thickness uniformity is obtained. Inverted planar-heterojunction PSCs employing these films have a maximum power-conversion efficiency of 14.2% with a small standard deviation of 0.9%, comparable to that of the spin-coated device.

INTRODUCTION Organometal halide perovskites, such as CH3NH3PbX3,1,2 HC(NH2)2PbX3,3-5 and CH3NH3PbI3xXx

6

(X = Cl, Br, I), are emerging as the most promising photovoltaic material among materials

for the third-generation solar cells, including organic small molecules,7, 8 polymers,9 and nanocrystal quantum dots.10 The promising properties of the perovskites, including broad and strong optical absorption, ambipolar charge transport, long carrier diffusion length, and solution processability, have resulted in a great success in their application to solar cells. To date, power conversion efficiencies (PCEs) exceeding 20% have been achieved in perovskite solar cells (PSCs),1-5 comparable to those of copper indium gallium selenide (CIGS) and cadmium telluride (CdTe) based thin film cells.11 However, since the fabrication of high-efficiency PSCs is still based on laboratory-scale spin coating, the development of a cost-effective, high-throughput process is required for their commercialization. Recently, various fabrication methods such as spray pyrolysis,12 ultrasonic spray coating,13 doctor blade,14,15 inkjet printing,16 and chemical vapor deposition17 have been reported, but the resulting PCEs are much lower than those of the devices fabricated by spin-coating.


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Electrospray, which has been applied to the deposition of photoactive and charge transport layers of organic light emitting diodes18,19 and solar cells20,21 that require precise control of film thickness and surface uniformity, is one of the most promising techniques for high-throughput fabrication of PSCs. In order to form a high-quality perovskite film using the conventional electrospray, a perovskite precursor solution must be sprayed in the cone-jet mode. The cone-jet mode, in which monodisperse droplets are sprayed in appropriate ranges of applied voltages (VA) and solution flow rates (Qsol), allows constant electrostatic deposition of uniform and reproducible thin films. When a liquid being electrosprayed is a leaky dielectric, a strong electric shear stress directed along the surface of a Taylor cone is established, accelerating charges near the cone surface toward the apex where a stable jet is ejected. In the case of a perovskite precursor solution, however, due to its high electrical conductivity (σ), the charge relaxation time in the solution becomes smaller than the characteristic time required to form a Taylor cone, leading to a significant reduction of the electric shear stress. As a result, the cone-jet mode is very difficult to achieve, and the solution is electrosprayed in one of the unstable modes, such as the pulsating, vibrating-jet, and multi-jet modes.22 This phenomenon has been observed for various high-σ liquids and has been explained theoretically.23-25 Recently, by decreasing the Taylor-cone characteristic time using a spray nozzle whose diameter is only 15 µm, S. C. Hong et al. have overcome this limitation to demonstrate PSCs with PCEs as high as 13.27%.26 However, with the physical dimension of the nozzle reduced, the size of the Taylor cone also decreases, which decreases the maximum flow rate that the cone-jet mode can allow and thus limits the deposition throughput. Here, we demonstrate that efficient PSCs can be fabricated using an electrospray process, with a throughput that is several tens of times higher than that achieved using the conventional electrospray. This was enabled by using sheath-gas-assisted electrospray—a technique that has pre-


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viously been applied to electrospray ionization mass spectrometry,27 atomization of highly viscous and/or conductive liquids,28-30 and generation of uniform nanoparticles,31,32 where a strong aerodynamic force generated by the flow of a sheath gas having a high dielectric strength, such as N2, CO2, and SF6, pneumatically nebulizes the liquid, thus allowing for stable electrospray of high-σ liquids at high VA. Exploiting this unique feature, we show that a highly conductive precursor solution for CH3NH3PbI3-xClx (MAPbI3-xClx) is successfully nebulized in a stable jet mode at high Qsol of 50 µL/min, ~60 times higher than that achieved using the conventional electrospray.26 At the same time, owing to the sheath gas, the stable jet mode is maintained at high VA of 20 kV, leading to charged droplets with an average diameter as small as 5.4 µm. Films of MAPbI3-xClx with almost complete surface coverage and high thickness uniformity have been obtained via control of the solvent evaporation rate on substrates by adjusting the substrate temperature during deposition. The maximum PCE of inverted planar-heterojunction PSCs employing these films is 14.2%, which is, to the best of our knowledge, the highest among such PSCs fabricated by solution processes other than spin coating, suggesting that sheath-gas-assisted electrospray may accelerate the commercialization of PSCs. RESULTS AND DISCUSSION The concept and apparatus of the sheath-gas-assisted electrospray system are schematically shown in Figure 1a. The main components of the system are a coaxial dual nozzle where a high electrical bias, between 0 and 20 kV, is applied, and a grounded hot plate positioned 17 cm away from the nozzle. The coaxial dual nozzle is comprised of a center nozzle and an annular outer nozzle arranged coaxially with the center nozzle. The center nozzle, with the outer and inner diameters of 0.23 and 0.10 mm respectively, is for injection of the perovskite precursor solution composed of CH3NH3I (MAI)33 and PbCl2 in N, N-dimethylformamide (DMF) at a molar ratio


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of 3:1, while the outer nozzle, with the outer and inner diameters of 0.63 and 0.33 mm respectively, is for flowing a N2 sheath gas to nebulize the precursor solution with high Qsol while enabling stable electrosprays at high VA. In sheath-gas-assisted electrospray, contact between the precursor solution and sheath gas occurs at the outlet of the coaxial dual nozzle. The velocity of the sheath gas ejected from the outer nozzle is much greater than that of the solution ejected from the center nozzle, so that a high pneumatic shear force is generated at the gas–liquid interface during the contact, pneumatically nebulizing the liquid. Also, the flow of N2, with its dielectric strength larger than that of air, isolates the center nozzle from the surrounding air, allowing for the application of high VA while preventing a corona discharge.24 This is a very important feature for obtaining high quality MAPbI3-xClx film. With VA, charges are induced on the surface of the precursor solution ejected at the nozzle. The associated electrostatic free energy, which increases with VA, partly offsets the intrinsic molecular forces giving rise to surface tension, making it easier for the sheath gas to tear the solution into finer droplets.34 Moreover, as the charge-to-surfacearea ratio of these droplets increases during solvent vaporization, which is accelerated by the dry ambient of the sheath gas, further disintegration of the droplets occurs via Coulomb fission.27,28 The droplets deposited on the substrate are agglomerated into a film and its drying rate is controlled by adjusting the substrate temperature, which determines the surface coverage, thickness uniformity, and crystallite size of the resulting MAPbI3-xClx film. Figure 1b show the images capturing the evolution, with increasing VA, of the pattern of the spray ejected from the tip of a single nozzle used in the conventional electrospray, compared with those of sheath-gas-assisted electrospray using the coaxial dual nozzle shown in Figure 1c.


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Figure 1. (a) Schematic of the sheath-gas-assisted electrospray system composed of a coaxial dual nozzle and a grounded hot plate. (b, c) Images capturing the evolution of spray patterns with increasing VA, for (b) the conventional electrospray using a single nozzle and (c) sheath-gasassisted electrospray employing a coaxial dual nozzle. For both sets of experiments, Qsol = 5 µL/min. The conventional electrospray was performed using a single nozzle with the inner and outer diameters of 0.10 and 0.23 mm, respectively, with Qsol = 5 µL/min. Without electrical bias, the dripping mode22 is observed, where a series of large discrete drops falls from the nozzle. At the


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onset of the falling, the volume of the drops, having been increased by continuous feeding of the solution, reaches a critical value at which the gravitational force on the drop overcomes the surface tension. When VA is small (2, 3, and 4 kV), the electric force operating in parallel to the gravitational force decreases the critical volume (Figure S1), thereby increasing the dripping rate at given Qsol. VA larger than or equal to 5 kV causes the electrospray instabilities, phenomena commonly observed in electrospray of liquids with high σ. Firstly, the pulsating mode22 is formed, where a long jet is ejected from the elongated meniscus at the outlet of the nozzle with irregular intervals (VA = 5 kV). At VA = 8 kV, multiple jets consisting of many large droplets are ejected from the hemispherical meniscus, with an increased pulsating rate. As VA increases, the pulsating mode is developed into the vibrating-jet mode,22 where divergent multiple jets are ejected from the flat meniscus in various directions (VA = 12 kV). With a further increase in VA, a cloud of small droplets begins to form, with increased divergence of multiple jets and an emergence of sparking (VA = 15 kV). The impossibility in establishing the cone-jet mode is due to the fact that the electric shear stress accelerating the charges near the surface of the cone towards its apex required for the cone-jet mode to occur is significantly reduced due to a short charge relaxation time caused by high σ (1.3 × 10-2 S/cm) of the perovskite precursor solution.23,25 When the electrospray experiment was performed using only the solvent (DMF)—that is, without the precursor ions—whose surface tension (~37.1 mN/m) is very similar to that of the precursor solution (Figure S2) but whose σ (2.5 × 10-6 S/cm) is much lower, electrospray in the cone-jet mode was established at VA = 8 kV (Figure S3), confirming that the lack of the cone-jet mode in the conventional electrospray of the perovskite precursor solution is indeed attributed to its high σ. In contrast, in sheath-gas-assisted electrospray, a single stable jet mode is established in all cases (without bias, and VA = 5, 8, 13, and 20 kV) with the same Qsol (5 µL/min) as in the conventional


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electrospray, owing to the N2 sheath gas flowing through the outer nozzle at a flow rate (QN2) of 0.45 L/min. This is because, in this case, the atomization of the precursor solution at the nozzle tip does not entirely rely on the electrical force as in the conventional electrospray, but is enabled by the strong pneumatic shear force generated by the flow of the sheath gas. Furthermore, a large dielectric strength of the N2 sheath gas allows for a single stable jet without a corona discharge even at VA = 20 kV, at which unstable multiple jets are formed in the conventional electrospray (Figure S3). Figure 2a is the image of the spray pattern established by sheath-gas-assisted electrospray performed at VA = 20 kV, and QN2 = 0.5 L/min, and significantly increased Qsol of 50 µL/min, showing that the stable spray mode is maintained even at very high Qsol, ~60 times higher than that achieved with a single nozzle by S. C. Hong et al.26 In general, establishing the cone-jet mode with high Qsol is very difficult in the case of the conventional electrospray for the following reason. When Qsol is increased beyond a certain value, the concomitant increase in hydrodynamic force acting on the liquid near the nozzle tip forces out the liquid before the ions move to the surface of the meniscus to form a Taylor cone, resulting in a thick, continuous jet. In the demonstration by S. C. Hong et al., Qsol (= 0.8 µL/min) is likely to have been further limited by their strategy for overcoming the difficulty in electrospray of a high-σ solution—that is, the reduction of the physical dimension of the nozzle, whose inner diameter in their case was 15 µm, to decrease the characteristic time related to the formation of the Taylor cone below the fast charge relaxation time.26 In contrast, the pneumatically-assisted atomization in our case allows for the stable spray mode at Qsol = 50 µL/min, which, at a nozzle-to-substrate distance of 17 cm, translates to a deposition rate of 5 nm/s over an effective area of 4 cm2, indicating that the sheath-gas-assisted electrospray is much more suitable for high-throughput fabrication of PSCs than the convention-


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al electrospray.

Figure 2. (a) Photograph of the single stable jet mode established at Qsol = 50 µL/min using sheath-gas-assisted electrospray performed with VA = 20 kV and QN2 = 0.5 L/min. (b) The size distribution of precursor droplets depending on VA: without bias (black squares), 5 kV (red circles), 10 kV (blue triangles), and 20 kV (green diamonds), measured 10 cm away from the nozzle. To verify the combined effect of electrostatic charging and pneumatic nebulization on the droplet size reaching the substrate region, the size distribution of droplets sprayed with Qsol = 50


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µL/min, QN2 = 0.5 L/min, and varying VA (without bias, 5 kV, 10 kV, and 20 kV) was measured using an instrument based on laser diffraction (Spraytec). For the unbiased case, the average diameter (Davg) of droplets measured 10 cm below the nozzle is 10 µm (black squares, Figure 2b). This represents a typical case of the pneumatic atomization, in which a strong pneumatic shear force between the solution and the sheath gas streams disintegrates the solution into droplets. These droplets experience rapid vaporization due to the dry ambient provided by the sheath gas, resulting in smaller droplets impinging on substrates. When VA is applied, Davg, which, in all three cases, is smaller than that for the unbiased case, decreases with increasing VA, confirming the benefit of increasing the free energy of nebulized droplets by charging.34 Specifically, at VA = 5 kV (red circles), 10 kV (blue triangles), and 20 kV (green diamonds), the values of Davg are 8.6 µm, 6.2 µm, and 5.4 µm, respectively. We note that the case of VA = 20 kV not only has the smallest Davg = 5.4 µm, which is approximately 50% of the unbiased case, but also has a negligible fraction of droplets that are too large (> 10 µm), which is crucial to obtain high-quality MAPbI3-xClx films, as discussed below. The scanning electron microscope (SEM) images in Figure 3 show the morphological change of the MAPbI3-xClx films deposited by sheath-gas-assisted electrospray with varying substrate temperature (TS). All films were deposited at Qsol = 50 µL/min, QN2 = 0.5 L/min, and VA = 20 kV for 2 min on 30-nm-thick poly (3, 4-ethylenedioxythiophene):poly (styrenesulfonic acid) (PEDOT:PSS) layers coated on glass substrates pre-coated with indium tin oxide (ITO). Next, the samples were annealed at 100 °C under a nitrogen ambient, and were then examined using an SEM. Top views are shown in Figure 3a, with high-magnification images shown in the insets. For cross-sectional views shown in Figure 3b, additional layers required for PSCs were deposited so that the samples comprise: glass / 185 nm ITO / 30 nm PEDOT:PSS / 600 nm MAPbI3-xClx /


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40 nm [6,6]-Phenyl-C61-butyric acid methyl ester (PC60BM) / 3 nm polyethyleneimine ethoxylate (PEIE) / 100 nm Ag. The films deposited at TS = 40 °C have surface coverage (Θ) estimated to be 86%, with small voids in their interior (Figure S4 and Figure 3a). Θ increases with TS, and reaches almost 100% at TS = 60 °C, where the films are void-free, as shown in Figure 3b. We note that the values of Θ were slightly overestimated, as they were estimated from the SEM images shown in Figure 3a with a field of view of 150 µm × 90 µm where uncovered regions with sub-micron sizes were not resolved. When TS = 70 °C, the size of the perovskite crystals is decreased, and the number of voids is increased. The observed morphological change can be explained in terms of change in evaporation rate of the solvent on the substrate. When TS is low (≤ 50 °C), since the solvent evaporation rate on the substrate is much smaller than the solvent feeding rate provided by droplets reaching the substrate, the film remains in a wet state during and after the electrospray deposition. Ions in this precursor wet film are sufficiently mobile so that the film grows into relatively large crystals upon annealing, resulting in many uncovered regions. At high TS, on the other hand, droplets on the substrate rapidly dry before having a chance to coalesce with droplets that subsequently impinge on the neighboring region, thereby decreasing the crystal size and generating voids. The solvent evaporation rate at TS = 60 °C is such that the resulting film has a degree of dryness that is appropriate for yielding a void-free film with near 100% coverage. Meanwhile, MAPbI3-xClx films sprayed without electrical bias applied to the nozzle, whose SEM images are shown in Figures S5 and S6, have the following features that are inferior to those obtained with VA = 20 kV. First, due to the lack of an electric field attracting charged droplets towards the substrate, the deposition yield is much lower than the biased case; under the same condition (Qsol = 50 µL/min and QN2 = 0.5 L/min), the film thickness of the unbiased case is, on average, smaller by a factor of five than that of the biased case. Second, circular


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patterns, which are due to large droplets reaching the substrate dissolving the pre-deposited film, are observed throughout the substrates when TS = 50, 60, and 70 °C. This result indicates that both pneumatic atomization and charging of droplets by electrical bias are crucial for stable, high-throughput electrospray deposition of MAPbI3-xClx films suitable for high-efficiency PSCs.

Figure 3. (a) Top SEM images of MAPbI3-xClx films deposited by sheath-gas-assisted electrospray performed at Qsol = 50 µL/min, QN2 = 0.5 L/min, and VA = 20 kV with different TS. The insets are high magnification images of the main images. The scale bars in the insets are 3 µm. (b) Cross-sectional SEM images of solar cells employing MAPbI3-xClx films such as those shown in (a).

To investigate the effect of TS on the crystallinity of the MAPbI3-xClx films deposited by sheath-gas-assisted electrospray, the x-ray diffraction (XRD) patterns were obtained for 600-nmthick MAPbI3-xClx films deposited on glass / ITO / PEDOT:PSS whose temperatures during deposition were maintained at TS = 40, 50, 60, and 70 °C. As can be seen from the indexed XRD patterns in Figure 4a, the diffraction peaks at 14.1°, 28.4°, 31.8°, 40.4°, and 43.8° in all films


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correspond to (110), (220), (310), (224), and (330) planes of MAPbI3-xClx crystals in the tetragonal phase, respectively.35 The strong diffraction peaks corresponding to (110) and (220) planes indicate that the perovskite films deposited by sheath-gas-assisted electrospray are highly crystalline. The small unidentified peak at 13.5° is possibly due to an intermediate phase such as PbCl2–MAI–DMF complex or (CH3NH3)4PbI6·2H2O hydrate compound.36 The average size (L) of crystalline domains estimated from the full width at half-maximum of the (110) peak using the Scherrer equation37 decreases with TS (L = 290, 230, 190, and 130 nm when TS = 40, 50, 60, and 70 °C, respectively), in agreement with the trend shown in the SEM images in Figure 3b. To investigate the optical properties of these MAPbI3-xClx films, the transmittance (T) spectra were measured. Samples have layer structures identical to those used in the XRD measurement, and the values of 1−T are plotted in Figure 4b. All films exhibit strong absorption over the visible region that sharply decreases beyond a wavelength of 750 nm, consistent with MAPbI3-xClx films deposited by spin-coating.6 The values of 1−T are almost independent of wavelength in a shortwavelength region (< 500 nm), meaning that the MAPbI3-xClx films are optically thick in this region and that the differences in 1−T among the four samples are primarily attributed to the differences in Θ. In a wavelength region > 550 nm, the amount of increase in 1−T with TS is larger than what can be accounted for by the increase in Θ. In this wavelength region, where the MAPbI3-xClx films are not optically thick, optical absorption is likely to be increased by light scattering due to surface roughness. In fact, in the SEM images in the insets of Figure 3a, the surface roughness on length scales comparable to the wavelength appear to increase with TS.


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Figure 4. Characterizations of electrospray-deposited MAPbI3-xClx films. (a) indexed XRD patterns of the perovskite films deposited at TS = 40, 50, 60, and 70 °C. (b) Spectra of (1−transmittance) of the films. Figure 5a shows the current density vs voltage (J–V) characteristics, measured under 1 sun AM 1.5G illumination, of inverted planar-heterojunction PSCs comprising: glass / 185 nm ITO / 30 nm PEDOT:PSS / 600 nm MAPbI3-xClx / 40 nm PC60BM / 3 nm PEIE / 100 nm Ag, where the MAPbI3-xClx layers were deposited by sheath-gas-assisted electrospray. The PC60BM and PEIE layers were deposited by spin-coating, and the top Ag contacts were formed by thermal evaporation in vacuum. The photovoltaic parameters such as short-circuit current density (JSC), open-


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circuit voltage (VOC), fill factor (FF), and PCE are summarized in Table 1, and are also plotted, together with Θ, as functions of TS in Figure S7. The results clearly show that the dependence of all parameters on TS is similar to that of Θ. In the devices with TS = 40 and 50 °C, where Θ = 86 and 94%, respectively, direct electrical contact between the PEDOT:PSS and PC60BM layers inevitably occur in uncovered regions such as those shown in the SEM images (Figures 3a and S4), thereby limiting all device parameters. Furthermore, since the uncovered regions are unevenly distributed throughout the substrates, standard deviations of the parameters are relatively larger for these devices. In contrast, the solar cell with TS = 60 °C having a void-free MAPbI3-xClx film with almost complete coverage (Θ = 99%) and uniform thickness has the highest performance, comparable to the spin-coated device: PCE = 14.2% with its standard deviation of only 0.9%, JSC = 22.4 mA/cm2, VOC = 0.92 V, and FF = 0.69. To the best of our knowledge, this is the highest PCE among MAPbI3-xClx inverted planar-heterojunction PSCs fabricated by solution processes other than spin coating. Despite its highest absorption among the four types of devices and Θ = 95%, the device with TS = 70 °C has a PCE similar to that of the device with TS = 50 °C, which is likely due to the internal voids present in the former device limiting the collection of photogenerated charge carriers. The external quantum efficiency (EQE) spectra measured under shortcircuit conditions are shown in Figure 5b. The variation of EQE with TS is found to agree well with the general trend of Θ, with the shapes of the EQE spectra being almost identical. This indicates that Θ is likely the main factor determining JSC. In order to evaluate the stability of the solar cell with TS = 60 °C, we measured its J at a forward bias of 0.71 V, at which the device delivers the maximum power, under continuous 1 sun AM 1.5G illumination. Under an ambient laboratory condition with 30% relative humidity and without encapsulation, the initial value of J (=18.4 mA/cm2), corresponding to a PCE of 13.1%, was maintained for 2 hr with negligible var-


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iation, as shown in Figure 5c.

Figure 5. (a) Current density vs voltage (J–V) characteristics of PSCs measured in reverse scans, whose MAPbI3-xClx films are deposited by sheath-gas-assisted electrospray at different TS. (b) External quantum efficiency (EQE) spectra of the PSCs measured under short-circuit conditions.


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(c) J measured at a maximum power point of 0.71 V under continuous 1 sun illumination as a function of time and the corresponding PCE for the device with TS = 60 °C. The acquisition rate was 0.5 point/s.

Table 1. Photovoltaic parameters of electrospray-fabricated and spin-coated PSCs. Substrate temperature

Surface coverage

JSC

VOC

(mA/cm2)

(V)

PCE (max.)

PCEa

(%)

(%)

FF

(°C)

(%)

40

86

18.5

0.85

0.57

9.0

7.1 ± 2.2

50

94

20.8

0.91

0.65

12.3

11.0 ± 1.3

60

99

22.4

0.92

0.69

14.2

13.3 ± 0.9

70

95

21.5

0.90

0.64

12.3

10.9 ± 1.2

22.2

0.94

0.75

15.6

14.8 ± 0.7

Spin-coating

a Averaged over 25 devices; the values following ‘±’ are the standard deviations.

CONCLUSION Highly crystalline, void-free MAPbI3-xClx films with very high surface coverage and thickness uniformity were obtained by sheath-gas-assisted electrospray, and inverted planar-heterojunction PSCs employing these films exhibited a maximum PCE of 14.2%. Owing to the flow of a N2 sheath gas through the annular outer nozzle of the coaxial dual nozzle, stable electrospray of a highly conductive perovskite precursor solution was achieved even at both high Qsol = 50 µL/min and VA = 20 kV, which was impossible to attain with the conventional electrospray. Furthermore, due to pneumatic nebulization and a dry ambient provided by the sheath gas, and charging of droplets by electrical bias, the droplets were readily disintegrated into a cloud of droplets with


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Davg = 5.4 µm at high Qsol = 50 µL/min. The control of solvent evaporation rate on substrates during deposition was found to be crucial for obtaining high quality MAPbI3-xClx films, and the maximum PCE of 14.2% was obtained for a device deposited at TS = 60 °C, which is comparable to that of devices fabricated by spin-coating. In addition, PSCs fabricated by sheath-gas-assisted electrospray possess high operational stability, with the performance maintained at least for 2 hr under continuous 1 sun illumination in ambient without encapsulation. For sheath-gas-assisted electrospray to become a commercially viable fabrication method for PSCs, there are more demonstrations that need to be made in the future. An important near-term goal is the fabrication of module-sized PSCs, while, in a longer term, the electron- and hole-transport layers as well need to be deposited by electrospray.

EXPERIMENTAL SECTION Materials and Synthesis. All reagents and solvents were purchased and used without purification. Methylammonium iodide (CH3NH3I, MAI) was synthesized following a method reported elsewhere.33 Briefly, MAI was prepared by reacting 24 mL methylamine (33 wt% in absolute ethanol, Sigma-Aldrich) with 10 mL hydroiodic acid (57 wt% in water, Sigma-Aldrich) in an ice bath at 0 °C for 2 hr with stirring in N2 atmosphere. The resulting solution was evaporated at 60 °C for 1 hr to precipitate MAI, which was washed three times with 300 mL diethyl ether (anhydrous, ≥ 99%, Sigma-Aldrich) and then dried in vacuum. MAPbI3-xClx perovskite precursor solution was prepared by mixing MAI with lead (II) chloride (PbCl2, 99%, Sigma-Aldrich) at 10 wt% (3:1 molar ratio) in N, N-dimethylformamide (DMF, anhydrous, 99.8%, Sigma-Aldrich). [6,6]-Phenyl-C61-butyric

acid

methyl

ester

(PC60BM)

and

poly

(3,

4-


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ethylenedioxythiophene):poly (styrenesulfonic acid) (PEDOT:PSS, Clevios P VP AI 4083) were purchased from Nano-C, Inc. and Heraeus, respectively. Polyethyleneimine (PEIE) (80% ethoxylated, MW = 70,000 g mol-1, 35–40 wt% in H2O), methanol (ACS reagent, 99.8%), ethyl alcohol (200 proof, anhydrous, ≥ 99.5%), and chlorobenzene (ACS reagent, 99.8%) were purchased from Sigma-Aldrich. Sheath-gas-assisted electrospray. A high-voltage power supply (PS375/+20 kV, Stanford Research Systems) was used to apply a positive bias between 0 and 20 kV to a coaxial dual nozzle (NNC-DN-2332, NanoNC) consisting of a center nozzle and an annular outer nozzle. The MAPbI3-xClx precursor solution was injected into the center nozzle at a flow rate of 50 µL min-1 using a syringe pump (Legato 100, KD Scientific). At the same time, a sheath gas (N2) was injected into the annular outer nozzle at a flow rate of 0.5 L min-1, measured by a gas flowmeter (RMA-22-SSV, Dwyer). Substrate temperatures were maintained at 40, 50, 60, or 70 °C during deposition by a grounded hot plate positioned 17 cm away from the coaxial dual nozzle. Before electrospray depositions, a chamber (90 cm × 70 cm × 70 cm) enclosing the electrospray setup was filled with dry nitrogen until 30% relative humidity was reached. Device Fabrication. ITO-coated glass substrates (15 Ω/sq., 25 mm by 25 mm) were sequentially cleaned with detergent, deionized water, acetone, and isopropyl alcohol, followed by baking for 20 min at 150 °C in a vacuum oven prior to film depositions. PEDOT:PSS (filtered with a 0.45 µm PVDF filter) was spin coated at 5000 rpm for 45 s on UV ozone-treated substrates, and then baked for 30 min at 150 °C on a hot plate. The MAPbI3-xClx precursor solution (filtered with a 0.20 µm PTFE filter) was deposited by sheath-gas-assisted electrospray for 2 min on glass / ITO / PEDOT:PSS to obtain MAPbI3-xClx films, after which the samples were transferred into a N2 glove box and annealed for 1 hr at 100 °C. PCBM (2 wt% in chlorobenzene) and PEIE (0.58


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vol% in methanol) solutions were sequentially spin coated on the MAPbI3-xClx films at 1500 rpm for 45 s and 5000 rpm for 45 s, respectively. Finally, top Ag contacts, 2 mm in diameter, were formed by thermal evaporation in vacuum (~10-7 Torr) through a shadow mask with circular openings. Characterization. The J–V characteristics were measured using a source meter (2400, Keithley) and a solar simulator (PEC-L01, Peccell Technologies) calibrated with a standard silicon solar cell (BS-520BK, Bunkoukeiki). The top and cross-sectional images of the perovskite films were obtained using a field-emission scanning electron microscope (S-4800, Hitachi). The EQE spectra were measured with a broadband, laser-driven light source (EQ-99, Energetiq), a lock-in amplifier (SR830, Stanford Research Systems), and a current preamplifier (SR570, Stanford Research Systems). The X-ray diffraction patterns of the MAPbI3-xClx films were measured using an X-ray diffractometer (D8 Advance, Bruker) and the optical measurements of the perovskite films were carried out using a UV-visible spectrophotometer (Lambda 35, Perkin-Elmer). The images of the spray patterns were obtained using a digital microscope (Smart G-scope, Genie Technologies). The size distribution of precursor droplets was measured using an instrument based on laser diffraction (Spraytec, Malvern).

ASSOCIATED CONTENT Supporting Information Additional data and results (spray pattern, contact angle, and SEM images, photovoltaic parameters of perovskite solar cells) (PDF) AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected] ORCID Changsoon Kim: 0000-0002-3749-2235 Present Addresses §DRAM Device and PI Technology Group, R&D Division, SK Hynix Inc., Gyeonggi 17336, Republic of Korea. Author Contributions H.K. conceived the main idea of application of sheath-gas-assisted electrospray; S.H. designed the experiments; S.H. and S.L. carried out initial experiments to optimize the electrospray conditions; S.H. performed all other experiments; S.H., H.K., S.L., and C.K. analyzed the data; S.H. and C.K. wrote the manuscript; C.K. supervised the project. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Global Frontier R&D Program on the Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Science and ICT, Korea. The authors thank Yoonseok Ka and Jongcheon Lee for help in the EQE measurements. The authors also thank Dr. Gyumin Kim for his help with device fabrication, and Keetea Kim and Chanhyuk Ji for their help in the synthesis of CH3NH3I.


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Efficient Planar-Heterojunction Perovskite Solar Cells Fabricated by

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