New Scalable Cold-Roll Pressing for Post-treatment of Perovskite

Article pubs.acs.org/JPCC

New Scalable Cold-Roll Pressing for Post-treatment of Perovskite Microstructure in Perovskite Solar Cells Bahram Abdollahi Nejand,† Saba Gharibzadeh,‡ Vahid Ahmadi,*,§ and Hamid Reza Shahverdi† †

Nanomaterials Group, Department of Materials Engineering, ‡Department of Physics, and §School of Electrical and Computer Engineering, Tarbiat Modares University, Tehran, Iran S Supporting Information *

ABSTRACT: We introduce a new cold-roll pressing technique to enhance microstructure of deposited pinholed perovskite layers in perovskite solar cells. This technique is proposed as an efficient post-treatment method for deposited perovskite layers that are mainly suffering from excessive number of pinholes. In this regard, we followed a set of sequential steps of partial dissolving the perovskite layer surface and a cold-roll pressing to compress and spread the semidissolved portion of the perovskite that is exposed to slight N,Ndimethylformamide (DMF) vapor. Because of applying no heating during the compressing process, the cells showed good stability against decomposition. Transformation of perovskite layer from columnar microstructures containing a large amount of uncoated TiO2 surface to a continuous and approximately pinhole-free structure resulted in an increase in the power conversion efficiency (PCE) of the cells from 8.16 to 13.24%, showing a 62% enhancement in the cell performance. For cell performance enhancement observed in this study, we can propose three considerable explanations: drop of the recombination regions, increasing the visible light absorbance, and the increase in contact surface between perovskite and TiO2 layer.

1. INTRODUCTION Because of their interesting photovoltaic properties, organometal halide perovskite solar cells have been an interest for research and industrial sections over the past five years. The number of works conducted on these cells indicates a considerable growth because of numerous advantages of these cells such as broad range of light absorption, covering the visible to near-infrared spectrum with high extinction coefficient (∼104 cm−1 at 550 nm) and long diffusion length around 1 μm.1−3 The certified power conversion efficiency (PCE) of the perovskite solar cells is 20.1% to date.4 A vast variety of methods has been reported for fabrication of perovskite layers including one-step5,6 and two-step7,8 chemical and physical sequential techniques.9,10 The perovskite deposition methods result in various morphologies and thicknesses that strongly affect the cell performance and parameters. It was declared that formation of pinhole-free perovskite layers considerably improves the cell parameters. The open circuit voltage and fill factor are among these parameters, which are induced by high recombination rate in the interface of electron and hole transport materials.10,11 Hence, many attempts such as solution engineering,12−15 vapor assisted solution process,11 two-source vapor deposition,10,16−18 solvent−solvent extraction,19 blow-drying process,20,21 and a solid state chemistry22 were undertaken to reach pinhole-free perovskite layers. As deposition of perovskite layers by vacuum technique involves high production cost, deposition by chemical routes attracts much attention because of its ease of fabrication and low-cost process. In this regard, the most common chemical deposition method for perovskite layers is spin coating. However, despite © XXXX American Chemical Society

producing thin uniform layers this method would not be a good scale-up candidate considering its limitations in large area deposition. Spray coating is one of the interesting and scalable methods in deposition of thin films because of its great potential for large area deposition in perovskite layers. Although attempts have taken to obtain uniform and pinhole-free perovskite layers by adjusting the concentration and substrate temperature in spray coating, the final product contains a considerably high number of pinholes that lead to high recombination in the interface of ETM and HTM layers.23,24 Rolling technique is among the promising method for spreading viscous liquid and paste, as it continuously forges and diminishes the thickness of sheets and layers by simultaneous rolling and pressing. Two kinds of hot- and cold-roll presses are used for various applications in sheet and film technology, by which different grain sizes can be reached. Because hot-roll pressing may affect the structure of sheets and layers, coldrolling technique can be a much promising method in the view of preserving the structure of layers against oxidation and same transformation states. In addition to the reported works in deposition of pinhole-free perovskite layers, a post-treatment method was reported where pinholed perovskite layer was constructed to pinhole-cured perovskite layer by hot pressureassisted method.25 In this method, slight smoothening of the perovskite layer, achieved by two-step deposition of PbI2 and Received: November 27, 2015 Revised: January 20, 2016

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DOI: 10.1021/acs.jpcc.5b11596 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Schematic device architectures and energy level diagrams of the FTO/TiO2/Columnar CH3NH3PbI3−xClx/spiro-OMeTAD/Au (a) and FTO/TiO2/Continuous CH3NH3PbI3−xClx/spiro-OMeTAD/Au (b).

and the final CH3NH3I was collected and dried at 60 °C in a vacuum oven for 24 h. Deposition of Perovskite Layer. Deposition of perovskite layer on thin TiO2 compact layers was done at room temperature and air atmosphere with 20% moisture content. For the perovskite layer, a 1:3 ratio of PbCl2/CH3NH3I with a concentration of 203 and 350 mg was mixed in 1 mL of N,Ndimethylformamide (DMF), respectively. The mixture solution was stirred at room temperature overnight and spray coated on the substrate at 180 °C. The spray coating was conducted at 2 mbar, 1 mL/min flow rate, 2 cm/sec spray head movement, 20 cm spray height from hot substrate, and moisture-free nitrogen gas as an inert carrier gas. To obtain 850 nm height perovskite columns, eight coating passes were carried out. Cold-Roll Pressing Modification of Perovskite Layer. Modification of noncontinuous perovskite layer achieved by spray coating was conducted by roll pressing at room temperature of 25 °C and low pressure of 0.2 MPa, while exposing it to slight DMF vapor in advance. In this technique, the noncontinuous perovskite layer, formed by spray coating at hot substrate, was exposed to DMF vapor at room temperature and then roll pressed by 0.2 MPa silicon-coated rollers to compress the melted portion of the perovskite. The DMF steam was produced by a slow transportation of DMF steam from 180 °C hot substrate with a 1.67 mL/min steam flow rate. This small amount of DMF seam is enough to slightly wet the perovskite surface for further cold-roll pressing. The cold-roll pressing was repeated 10 times to achieve a uniform perovskite layer. The rolling speed for first five steps of cold-roll pressing was around 0.5 rpm, while the second five steps were carried out with a rotation speed of 0.8 rpm. The roller diameter was 12 cm. Deposition of Spiro-OMeTAD/Au. Immediately after cooling the annealed perovskite layers to room temperature, the spiro-OMeTAD-based hole transporting layer (72 mg of spiro-OMeTAD, 17.5 μL lithium-bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg Li-TFSI in 1 mL acetonitrile) and 26.6 mg of 4-tertbutylpyridine all dissolved in 1 mL chlorobenzene) were deposited by spin-coating at 2000 rpm for 30 s. Putting the deposited spiro-OMeTAD in desiccator for 12 h, the thin 100

CH3NH3I, is not only a time-consuming process, but also requires high temperature and pressure.25 In the present study, we introduced a fast room-temperature low-pressure flattering method, which may attract much attention in the scale-up vision. Herein, we designed a new technique for converting the noncontinuous and pinholed perovskite layers to continuous pinhole-free perovskite layers through deposition of perovskite layer by spray coating followed by solvent vapor-assisted coldroll pressing. In this technique, despite the rough pinholed surface morphology of perovskite layers formed by spray coating, the engineered solvent vapor-assisted cold-roll pressing cures the structure to reach a pinhole-free and continuous perovskite layer. Because of the roll-to-roll mechanism and lowpressure and temperature processes, this technique might be favorable for fabrication of large area and massive flexible perovskite solar cells.

2. EXPERIMENTAL DETAILS Preparing Compact Layer. FTO-coated glass substrates were patterned by Zn powder and 2 M HCl solution etching. The patterned FTO substrates were cleaned by soap-deionized water solution, followed by ultrasonication at 50 °C with deionized water, ethanol, and isopropanol, and then subject to an O3/ultraviolet treatment for 20 min. After cleaning the substrates, a compact layer of TiO2 was created by spin coating of a 0.15 M titanium diisopropoxide bis(acetylacetonate) (75 wt % in isopropanol, Aldrich) in 1-butanol (Aldrich) solution at 2000 rpm for 60 s, followed by heating at 125 °C. After cooling down the coated layers to room temperature, a 0.3 M titanium diisopropoxide bis(acetylacetonate) solution in 1-butanol was spin-coated on the substrate and then heated at 125 °C for 30 min. The procedure was repeated twice to make a pinhole-free dense TiO2 film.26 Synthesis of CH3NH3I. The CH3NH3I was synthesized by stir-mixing 24 mL of CH3NH2 and 10 mL of HI in a 250 mL round-bottom flask at 0 °C for 2 h. The precipitate was collected using a rotary evaporator through careful removal of the solvents at 50 °C. The as-obtained product was redissolved in 100 mL of absolute ethanol and precipitated with addition of 300 mL of diethyl ether. This procedure was repeated thrice B


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Figure 2. Schematic of cold-roll pressing process containing three steps of spray coating (stages 1 and 2), vapor prewetting (stages 3 and 5), cold roll pressing (stages 4 and 6), and final compressed perovskite layer (stage 7).

structure approaches to the ideal p−i−n structure and alignment of perovskite solar cells. To study the introduced process for converting the columnar perovskite layer to continuous perovskite layer, prepatterned FTO substrates are coated by a thin compact layer of TiO2 to avoid direct connection between the hole transport material and FTO substrate. For the columnar perovskite layer, a respective 1:3 ratio of PbCl2/CH3NH3I in DMF was spraycoated on hot substrate layer to create a well-distributed columnar structure of perovskite (Figure 2, stages 1 and 2). The columnar structure of perovskite was exposed to DMF vapor to slightly wet the surface of the columnar perovskite layer (Figure 2, stage 3). This slightly wetted perovskite layer was immediately passed through a cold-roll pressing equipped by silicon-coated rollers to compress the softened portion of the columnar perovskite layer (Figure 2, stage 4). Immediately after passing the layer through the five rollers (Figure 2, stage 4), the exact same process was repeated to completely compress the perovskite layer to a uniform continuous perovskite layer (Figure 2, stages 5, 6, and 7). To investigate the microstructural transformation occurred through the cold-roll pressing, the microstructure of the perovskite was studied in every step and stage of the process. Figure 3 illustrates the SEM images of perovskite morphology in three steps of the process. As shown in Figures 3a,b and 4a, the perovskite layer deposited by spray coating on the hot substrate shows an island-like columnar structure of perovskite with an approximate grain size of 2 μm. The prewetted columnar perovskite structure was partially compressed and dried (Figures 3c,d and 4b) through the first step of cold-roll pressing, which involves five rollers. By repeating the prewetting and cold-roll pressing, the compressed structure of perovskite was transformed into a continuous and almost pinhole-free perovskite layer (Figures 3e,f and 4c). Figure 3g,h indicates higher magnification of continuous perovskite layer after two prewetting and cold-roll pressing stages. Studying the optical microscopy of transforming perovskite layer from columnar microstructure to continuous perovskite layer shows a regular spreading of perovskite columnar grains into bigger perovskite grains. This grain size can be attributed to compression of taller perovskite columns with smaller

nm gold contact was deposited on the spiro-OMeTAD by thin stainless steel shadow mask to create a 0.1 cm2 active area. Thin Film and Device Characterization. To study the microstructure and morphology of the deposited films, field emission scanning electron microscopy (FE-SEM, S4160 Hitachi Japan) and atomic force microscopy (AFM, Veeco PCResearch U.S.A.) were applied. The phase structure and crystal size of the films were also investigated by X-ray diffraction (XRD, Philips Expert- MPD). XRD analyses were performed at θ-2θ mode using Cu-Kα with wavelength of 1.5439 Å radiation, all at grazing incident angle of 2°. The optical characteristics of deposited films were analyzed by UV− vis spectroscopy using the wavelength range of 190−1000 nm. The photocurrent−voltage (I−V) characteristics of solar cells were measured under one sun (AM1.5G, 100 mW/cm2) illumination with a solar simulator (Sharif solar simulator). Steady-state PL measurements were acquired using a Varian Cary Eclipse fluorescence spectrometer.

3. RESULTS AND DISCUSSION Improving the structure and dealing with potential problems in deterioration of the photovoltaic parameters are two strongly undertaken efforts for achieving high performance solar cells. Reaching the electron and hole transport materials in the perovskite solar cell accelerates and increases the generated charges recombination. As a result, the PCE of the fabricated devices considerably declines by decreasing the fill factor and open circuit voltage. Therefore, achieving a continuous pinholefree perovskite layer would introduce an ideal structure for boosting the performance of the devices. Figure 1 depicts the energy level diagrams of FTO/TiO2/CH3NH3PbI3−xClx/spiroOMeTAD/Au device in two noncontinuous (Figure 1a) and continuous perovskite layers (Figure 1b). As shown in Figure 1, the generated electron and holes are separated in the interfaces of TiO2-perovskite and spiro-OMeTAD-perovskite layers, respectively. The respective transferred electrons and holes to the TiO2 and spiro-OMeTAD can be possibility recombined in the direct connection of TiO2 and spiro-OMeTAD (Figure 1a). On the other hand, by reconstructing the columnar perovskite layer to the continuous structure, the junction of the TiO2 and spiro-OMeTAD layers can be diminished and the fabricated C


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Figure 5. Cross-section FE-SEM images of as-deposited columnar perovskite layer by spray coating (a,b) and final continuous perovskite layer prepared by prewetting and cold-roll pressing (c,d).

grains (Figure 5a,b), the height of the perovskite columns decreases and thinner continuous perovskite layers are achieved (Figure 5c,d). The interface jointing and homogenization of the perovskite grains may come from a slight dissolving of the previous columnar perovskite followed by compressing by silicon coated rollers at room temperature (Figure 5c,d). The possible mechanism for the morphology transformation can be explained as follows. By reaching a small amount of the DMF solvent molecules to the surface of the perovskite, the surface of the columnar perovskite is partially dissolved into the solvent molecules, so that it flows aside the solid grains. Then, the coldroll pressing has sufficient time to spread the partial dissolved perovskite into free zone between the solid perovskite columnar grains. It is noteworthy that the hot roll pressing would solidify the dissolved perovskite just before compressing the softened perovskite. Thus, the cold-roll pressing provides sufficient time for complete compressing of the semidissolved perovskite. To validate mechanism involved in transformation of the perovskite columnar grains into smooth and continuous

Figure 3. FE-SEM images of as-deposited columnar perovskite layer by spray coating (a,b); compressed perovskite layer by first step of prewetting and cold-roll pressing (c,d); continuous perovskite layer prepared by second prewetting and cold-roll pressing (e,f); and higher magnification of continuous perovskite layer (g,h).

diameter into shorter perovskite columns with larger diameter (Figure 1S). As shown in Figure 1S, by two steps of prewetting and cold-roll pressing the perovskite columns are compressed and interfacially adhered to obtain a continuous pinhole-free perovskite layer. This process was investigated by AFM technique in the next part of this work. In addition, as shown in cross-section images in Figure 5, by compressing and interfacial adhering the columnar perovskite

Figure 4. A schematic diagram of deposited columnar perovskite layer by spray coating (a); compressed perovskite layer by first step of prewetting and cold-roll pressing (b); and continuous perovskite layer prepared by second prewetting and cold-roll pressing (c). D


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Figure 6. Topographic and phase images of as-deposited columnar perovskite layer by spray coating (a−c); compressed perovskite layer by first step of prewetting and cold-roll pressing (d−f); and continuous perovskite layer prepared by second prewetting and cold-roll pressing (g−i).

Figure 7. Linear (a,b,d,e,g,h) and area (c,f,i) topographic study of as-deposited columnar perovskite layer by spray coating (a−c); compressed perovskite layer by first step of prewetting and cold-roll pressing (d−f); and continuous perovskite layer prepared by second prewetting and cold-roll pressing (g−i).

perovskite layer was formed (Figure 6g,h). To investigate the existence of pinholes during the compressing process, we employed a phase mode of AFM, wherein the color changes in the image showed a phase alteration on the perovskite surface (Figure 6c,f,i). As shown in Figure 6c, the color change from brown to white indicates the transition of perovskite to substrate by which the pinholes are distinguishable. By transforming the columnar perovskite to compressed perovskite, the surface coverage by perovskite considerably was increased and a small amount of white color ascribed to the substrate was observed in the phase image (Figure 6f). As

perovskite layer, we studied the topographic results by tapping and phase modes of AFM (Figures 6 and 7). As shown in Figure 6a,b, the perovskite layer deposited by hot substrate spray coating shows an island-like columnar structure with approximate same heights. By exposing the columnar structure of perovskite to DMF solvent, followed by cold-roll pressing, the surface of the perovskite grains becomes rougher and compressed with more continuity in comparison with the previous columnar perovskite structure (Figure 6d,e). By further repeating the process, dissolution and compression of the perovskite grains proceeded and a continuous uniform E


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Figure 8. Grazing incident XRD pattern of columnar, compressed, and continuous perovskite double layers prepared by cold-roll pressing.

shown in Figure 6i, the final step of wetting and compressing diminishes and cures the pinholes, where no white region is distinguishable. The linear and planar analyses of AFM results show an obvious transformation of columnar structure to continuous structure of perovskite layer during the wetting and cold-roll pressing process (Figure 7). As presented in Figure 7a,b,d,e,g,h, by sequential prewetting and cold-roll pressing the approximate 2 μm columns are transformed into larger grains and continuous perovskite layer. The mean height resulted from area investigation of the perovskite layer by AFM mapping shows a drop in the height of perovskite grains at 100 μm2. As shown in Figure 7c,f, the mean height of columnar perovskite drops approximately from 850 to 520 nm by the first step of prewetting and cold-roll pressing. The further prewetting and cold-roll pressing reduces the mean height of perovskite to 150 nm, indicating the high deviation in smoothening of the perovskite layer (Figure 7i). It is not wise to conclude that the thickness of the perovskite reaches 150 nm because of removal and disappearance of the pinholes in the perovskite layer as the tip of the AFM apparently does not reach the substrate and just scans the top surface of the continuous perovskite layer. These results are in good agreement with the phase image mapping and SEM images (Figures 3, 5, and 6). The microstructural transformation of the perovskite layer from columnar to continuous morphology may affect the structure of perovskite. To ensure the phase and structure stability, we studied the XRD pattern of perovskite layers in three stages. Because decomposition of perovskite happens at high temperature and presence of moisture, as long as this process is conducted at room temperature, the only decomposition agent could be the moisture or unbalanced molar ratio of PbI2 and CH3NH3I during the partial dissolving and cold-roll pressing. As shown in Figure 8 and Figure S2, transformation of perovskite microstructure from columnar to continuous does not affect the crystallographic structure of the perovskite, implying the high stability in the crystal structure during the prewetting and cold-roll pressing. The selfcrystallization of perovskite after partial dissolution and

compression by cold-roll pressing probably could be attributed to the existence of undissolved crystalline perovskite nucleuses. The photovoltaic performance of the devices prepared by various microstructures of perovskite with same structure of the FTO/TiO2/Perovskite/spiro-OMeTAD/Au was studied in this work. As presented in Figure 9 and Table 1, the perovskite solar

Figure 9. J−V curves measured at AM1.5G solar illumination for different devices prepared by columnar, compressed, and continuous perovskite microstructures.

cell prepared by columnar microstructure does not show a good performance due to the large number of pinholes in the layer and the high thickness of perovskite columns. In comparison, by first step of the cold-roll pressing, fill factor of the cell considerably increased from 0.49 to 0.57 because of diminishing a large number of pinholes in this process. This reduction is reported as the main reason for the recombination in the perovskite solar cells and dropping the recombination possibility in lower thickness of perovskite layer. The increase in photocurrent from 18.31 to 19.19 mA/cm2 can be attributed F


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The Journal of Physical Chemistry C Table 1. Performance Parameters of Perovskite Solar Cells Prepared by Columnar, Compressed, and Continuous Perovskite Microstructures device FTO/TiO2/columnar perovskite/ spiro-OMeTAD/Au FTO/TiO2/ compressed perovskite/ spiro-OMeTAD/Au FTO/TiO2/ continuous perovskite/ spiro-OMeTAD/Au

Voc (V)

Jsc (mA/cm2)

FF

PCE (%)

0.91

18.31

0.49

8.16

0.92

19.19

0.57

10.06

1.05

20.68

0.61

13.24

to the high charge extraction induced by the increasing contact area between perovskite and TiO2 layer as well as the higher absorbance of the perovskite layer. Repeating the cold-roll pressing showed a more enhancement in perovskite solar cell performance from 10.06 to 13.24%. In this case, increasing the cell performance can be attributed to almost all cell parameters. Increasing the respective open circuit voltage and fill factor from 0.92 and 0.57 to 1.05 and 0.61 can be explained by disappearance of almost all pinholes in the perovskite layers. In addition to decreasing the pinholes in the perovskite layer by cold-roll pressing, the voltage and fill factor enhancement can be attributed to the drop in the perovskite thickness that causes high rate of carrier extraction and lower recombination possibility. In addition, inclining the short circuit density from 19.19 to 20.68 comes from the higher contact area and higher absorbance ability. The dark J−V study of the cells with various perovskite microstructures reveals the effect of microstructure transformation from columnar to continuous perovskite layer. As shown in the dark J−V study in Figure 9, by vanishing the pinholes and more TiO2 surface coverage by perovskite layer, the carrier recombination between TiO2 and spiro-OMeTAD considerably decreases. Therefore, reaching a continuous and pinhole-free perovskite layer would enhance the open circuit voltage and fill factors of the devices. The statistical data of fabricated 10 devices are presented in Figure S3. Optical properties of various perovskite microstructures detected during the cold-roll pressing were studied by UV− vis spectroscopy (Figure 10). As shown in Figure 10a, compressing the columnar perovskite causes higher absorbance of photons by perovskite, while continuing the process to reach continuous uniform perovskite shows highest absorbance ability of perovskite layer. As shown in Figure 10b, increasing the absorbance ability of perovskite by transformation of the columnar perovskite to uniform well-compressed perovskite can be explained by the higher absorbance ability of layer because of better coverage of the TiO2 by perovskite layer. The complete coverage of the TiO2 surface by perovskite, which shows a continuous and pinhole-free microstructure, strongly increases the absorbance ability of layer as compared to pinholed perovskite layers. In addition to the drop in recombination states in the continuous perovskite layers, this effect can enhance the cell performance by increasing the absorbance ability of the perovskite solar cells. Perovskite layer on an inert substrate shows a strong luminescence at 773 nm, depicting a narrow band gap of 1.5 eV. Exciting the perovskite layer by a 600 nm wavelength showed no luminescence of glass and TiO2 at the luminescence range of perovskite layer. To study the photoluminescence behavior of the double layers of perovskite and TiO2, perovskite layer was deposited on the glass/TiO2 layer based on the

Figure 10. Absorbance spectra of layers (a) and schematic light absorbance by columnar, compressed, and continuous microstructures of perovskite layers (b).

described method. The laser beam was entered from the opposite side of quencher layer. As shown in Figure 11a, TiO2 layers could quench the luminescence of perovskite at 773 nm by strong charge extracting ability. As shown in the figure, compressing the perovskite layer by cold-roll pressing obviously enhances the quenching ability of TiO2 layers probably because of increasing the contact area between perovskite and TiO2 layer. By further cold-roll pressing of the compressed perovskite, the strong charge extraction properties of the cell were observed when enhancing the contact area between perovskite and TiO2 layers (Figure 11b). As previously reported, increasing the contact surface area between perovskite and charge extraction materials strongly enhances the cell performance.7 Thus, the new approach of cold-roll pressing presented in this work can be an appropriate candidate in enhancement of the future cell performance. As widely reported, many perovskite solar cells suffer from hysteresis weakness induced from factors such as the scan rate, materials selection, and interface engineering between perovskite and HTM and ETM layers.27,28 In this regard, we studied the hysteresis behavior of one of best device after cold-roll pressing that almost does not contain any pinhole. To evaluate the hysterisis behavior of our devices, J−V curves were recorded every 10 mV at a scan rate of ∼0.5 V s−1 in two scan directions of forward bias to short circuit, and short circuit to forward bias. As shown in Figure 12, our device shows a hysteresis effect that may be induced by the previous reported reasons in the normal architectures of FTO/TiO2/perovskite/ spiro-OMeTAD/Au.27,29 As shown in the inset of Figure 12, the 13% PCE drop was seen in different scan direction with same scan rate.

4. CONCLUSION In summary, we introduced a successful technique of cold-roll pressing to enhance the deposited pinholed perovskite layers. We applied the principals of partial dissolution of the perovskite layer on the surface and capability of cold-roll pressing to G


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Figure 11. Photoluminescence study of single perovskite and coupled TiO2 layer with columnar, compressed, and continuous perovskite layers excited by 600 nm wavelength (a) and a schematic diagram of active area in charge extraction by columnar and continuous perovskite structure (b).

during the compressing process, the cells showed good stability against decomposition. Transformation of perovskite layer from columnar microstructure, which contains a large amount of uncoated surface, into a continuous and almost pinhole-free structure results in a 62% increase in the PCE of the cells. Declining the recombination regions, increasing the absorbance, and rising the contact surface between perovskite and TiO2 layer were proposed as a the most effective factors on the cell performance enhancement.

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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/acs.jpcc.5b11596. Optical microscopy of spray-coated and compressing perovskite microstructure, top and back side of prepared devices. (PDF)

Figure 12. J−V curves of prepared cold-roll pressing devices for the scan direction of forward bias to short circuit (FB-SC) and short circuit to forward bias (SC-FB).

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AUTHOR INFORMATION

Corresponding Author

spread and compress the semidissolved part of perovskite exposed by slight DMF vapor. Although no heating was applied

*E-mail: [email protected]. Tel/Fax: 0098 21 82883368 H


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The Journal of Physical Chemistry C Author Contributions

(13) Nie, W.; et al. High-Efficiency Solution-Processed Perovskite Solar Cells with Millimeter-Scale Grains. Science 2015, 347, 522−525. (14) Zhao, Y.; Zhu, K. Solution Chemistry Engineering toward HighEfficiency Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 4175− 4186. (15) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells. Science 2014, 345, 542−546. (16) Wang, S.; Ono, L. K.; Leyden, M. R.; Kato, Y.; Raga, S. R.; Lee, M. V.; Qi, Y. Smooth Perovskite Thin Films and Efficient Perovskite Solar Cells Prepared by the Hybrid Deposition Method. J. Mater. Chem. A 2015, 3, 14631−14641. (17) Docampo, P.; Ball, J. M.; Darwich, M.; Eperon, G. E.; Snaith, H. J. Efficient Organometal Trihalide Perovskite Planar-Heterojunction Solar Cells on Flexible Polymer Substrates. Nat. Commun. 2013, 4, 2761. (18) Jeon, Y.-J.; Lee, S.; Kang, R.; Kim, J.-E.; Yeo, J.-S.; Lee, S.-H.; Kim, S.-S.; Yun, J.-M.; Kim, D.-Y. Planar Heterojunction Perovskite Solar Cells with Superior Reproducibility. Sci. Rep. 2014, 4, 6953. (19) Zhou, Y.; Yang, M.; Wu, W.; Vasiliev, A. L.; Zhu, K.; Padture, N. P. Room-Temperature Crystallization of Hybrid-Perovskite Thin Films Via Solvent-Solvent Extraction for High-Performance Solar Cells. J. Mater. Chem. A 2015, 3, 8178−8184. (20) Zhang, M.; Yu, H.; Yun, J.-H.; Lyu, M.; Wang, Q.; Wang, L. Facile Preparation of Smooth Perovskite Films for Efficient Meso/ Planar Hybrid Structured Perovskite Solar Cells. Chem. Commun. 2015, 51, 10038−10041. (21) Hwang, K.; Jung, Y.-S.; Heo, Y.-J.; Scholes, F. H.; Watkins, S. E.; Subbiah, J.; Jones, D. J.; Kim, D.-Y.; Vak, D. Toward Large Scale Rollto-Roll Production of Fully Printed Perovskite Solar Cells. Adv. Mater. 2015, 27, 1241−1247. (22) Kumar, M. H.; Yantara, N.; Dharani, S.; Graetzel, M.; Mhaisalkar, S.; Boix, P. P.; Mathews, N. Flexible, Low-Temperature, Solution Processed Zno-Based Perovskite Solid State Solar Cells. Chem. Commun. 2013, 49, 11089−11091. (23) 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, 2944−2950. (24) Das, S.; Yang, B.; Gu, G.; Joshi, P. C.; Ivanov, I. N.; Rouleau, C. M.; Aytug, T.; Geohegan, D. B.; Xiao, K. High-Performance Flexible Perovskite Solar Cells by Using a Combination of Ultrasonic SprayCoating and Low Thermal Budget Photonic Curing. ACS Photonics 2015, 2, 680−686. (25) Xiao, J.; Yang, Y.; Xu, X.; Shi, J.; Zhu, L.; Lv, S.; Wu, H.; Luo, Y.; Li, D.; Meng, Q. Pressure-Assisted CH3NH3PbI3 Morphology Reconstruction to Improve the High Performance of Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 5289−5293. (26) Kim, H.-S.; et al. Lead Iodide Perovskite Sensitized All-SolidState Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (27) Heo, J. H.; Han, H. J.; Kim, D.; Ahn, T. K.; Im, S. H. HysteresisLess Inverted CH3NH3PbI3 Planar Perovskite Hybrid Solar Cells with 18.1% Power Conversion Efficiency. Energy Environ. Sci. 2015, 8, 1602−1608. (28) Kim, H.-S.; Park, N.-G. Parameters Affecting I−V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer. J. Phys. Chem. Lett. 2014, 5, 2927−2934. (29) Unger, E.; Hoke, E.; Bailie, C.; Nguyen, W.; Bowring, A.; Heumüller, T.; Christoforo, M.; McGehee, M. Hysteresis and Transient Behavior in Current−Voltage Measurements of HybridPerovskite Absorber Solar Cells. Energy Environ. Sci. 2014, 7, 3690− 3698.

The manuscript was written and approved by all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from Tarbiat Modares University and the equipment services from NOPL laboratory and thank Ms. A. Khazaeipour for her technical assistant at NOPL laboratory. The authors also acknowledge the Iran nanotechnology initiative council (INIC) for the partial support of this project.

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ABBREVIATIONS HTM, hole transport materials; ETM, electron transport materials; spiro-OMeTAD, 2,2′,7,7′-tetrakis (N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene; PCE, power conversion efficiency; Jsc, short-circuit current density; FF, fill factor; Voc, open-circuit voltages; PL, photoluminescence; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital

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REFERENCES

(1) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341− 344. (2) Im, J.-H.; Lee, C.-R.; Lee, J.-W.; Park, S.-W.; Park, N.-G. 6.5% Efficient Perovskite Quantum-Dot-Sensitized Solar Cell. Nanoscale 2011, 3, 4088−4093. (3) Minami, T.; Nishi, Y.; Miyata, T. Heterojunction Solar Cell with 6% Efficiency Based on an N-Type Aluminum−Gallium−Oxide Thin Film and P-Type Sodium-Doped Cu2O Sheet. Appl. Phys. Express 2015, 8, 022301. (4) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476−480. (5) Carnie, M. J.; Charbonneau, C.; Davies, M. L.; Troughton, J.; Watson, T. M.; Wojciechowski, K.; Snaith, H.; Worsley, D. A. A OneStep Low Temperature Processing Route for Organolead Halide Perovskite Solar Cells. Chem. Commun. 2013, 49, 7893−7895. (6) Lv, S.; et al. One-Step, Solution-Processed Formamidinium Lead Trihalide (FAPbI(3‑X)Clx) for Mesoscopic Perovskite-Polymer Solar Cells. Phys. Chem. Chem. Phys. 2014, 16, 19206−19211. (7) Im, J.-H.; Jang, I.-H.; Pellet, N.; Grätzel, M.; Park, N.-G. Growth of CH3NH3PbI3 Cuboids with Controlled Size for High-Efficiency Perovskite Solar Cells. Nat. Nanotechnol. 2014, 9, 927−932. (8) Qin, P.; Tanaka, S.; Ito, S.; Tetreault, N.; Manabe, K.; Nishino, H.; Nazeeruddin, M. K.; Grätzel, M. Inorganic Hole Conductor-Based Lead Halide Perovskite Solar Cells with 12.4% Conversion Efficiency. Nat. Commun. 2014, 5, 3834. (9) Chen, C.-W.; Kang, H.-W.; Hsiao, S.-Y.; Yang, P.-F.; Chiang, K.M.; Lin, H.-W. Efficient and Uniform Planar-Type Perovskite Solar Cells by Simple Sequential Vacuum Deposition. Adv. Mater. 2014, 26, 6647−6652. (10) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395−398. (11) Chen, Q.; Zhou, H.; Hong, Z.; Luo, S.; Duan, H.-S.; Wang, H.H.; Liu, Y.; Li, G.; Yang, Y. Planar Heterojunction Perovskite Solar Cells Via Vapor-Assisted Solution Process. J. Am. Chem. Soc. 2014, 136, 622−625. (12) Jin, Y.; Chumanov, G. Solution-Processed Planar Perovskite Solar Cell without a Hole Transport Layer. ACS Appl. Mater. Interfaces 2015, 7, 12015−12021. I


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