Solvent-Assisted Gel Printing for Micropatterning Thin Organic

Aug 29, 2016 - The color change observed after the thermal treatment clearly suggests ... S–O stretch band of the residual DMSO in an as-cast precur...
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Solvent-Assisted Gel Printing for Micropatterning Thin Organic−Inorganic Hybrid Perovskite Films Beomjin Jeong,† Ihn Hwang,† Sung Hwan Cho,† Eui Hyuk Kim,† Soonyoung Cha,‡ Jinseong Lee,† Han Sol Kang,† Suk Man Cho,† Hyunyong Choi,‡ and Cheolmin Park*,† †

Department of Materials Science and Engineering and ‡School of Electrical and Electronic Engineering, Yonsei University, Yonsei-ro 50, Seodaemun-gu, Seoul, 03722 Republic of Korea S Supporting Information *

ABSTRACT: While tremendous efforts have been made for developing thin perovskite films suitable for a variety of potential photoelectric applications such as solar cells, field-effect transistors, and photodetectors, only a few works focus on the micropatterning of a perovskite film which is one of the most critical issues for large area and uniform microarrays of perovskite-based devices. Here we demonstrate a simple but robust method of micropatterning a thin perovskite film with controlled crystalline structure which guarantees to preserve its intrinsic photoelectric properties. A variety of micropatterns of a perovskite film are fabricated by either microimprinting or transfer-printing a thin spin-coated precursor film in soft-gel state with a topographically prepatterned elastomeric poly(dimethylsiloxane) (PDMS) mold, followed by thermal treatment for complete conversion of the precursor film to a perovskite one. The key materials development of our solventassisted gel printing is to prepare a thin precursor film with a high-boiling temperature solvent, dimethyl sulfoxide. The residual solvent in the precursor gel film makes the film moldable upon microprinting with a patterned PDMS mold, leading to various perovskite micropatterns in resolution of a few micrometers over a large area. Our nondestructive micropatterning process does not harm the intrinsic photoelectric properties of a perovskite film, which allows for realizing arrays of parallel-type photodetectors containing micropatterns of a perovskite film with reliable photoconduction performance. The facile transfer of a micropatterned soft-gel precursor film on other substrates including mechanically flexible plastics can further broaden its applications to flexible photoelectric systems. KEYWORDS: micropatterns, organic−inorganic hybrid perovskite films, solvent-assisted gel printing, controlled crystallization, imprinting, transfer printing, photodetector, dimethyl sulfoxide the properties of a perovskite film are extremely sensitive to various conditions during fabrication steps such as humidity and annealing temperature,18−20 it is important to find a way for micropatterning perovskites with dense and uniform crystalline structures and thus not harming their photoelectrical properties when fabricated in large area arrays of the devices. Although conventional photolithography, followed by dry and wet etching processes, easily accessible in current semiconducting industry, is most common and powerful for micropatterning, the patterning based on photolithography requires the additional design of materials and processes not to damage the intrinsic properties of a perovskite film, in particular during rather harsh develop and etching processes. Non-

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he recent development of solution-processed organic− inorganic hybrid perovskites has drawn great interest in a variety of cost-effective but high-performance photoelectric devices such as solar cells, photodetectors, field-effect transistors, nonvolatile memories, and light-emitting diodes.1−8 Their extraordinary materials properties are responsible for the realization of the high-performing devices including highabsorption coefficient, low-exciton binding energy, excellent charge carrier mobility, extremely long carrier diffusion length, narrow emission band, composition-dependent tunable band gap and so on. While numerous previous works have focused on fabricating thin perovskite films with controlled crystalline structures which play a prime role for improving device performance,9−14 only a few works have addressed the development of large area and uniform microarrays of individual perovskite devices, which primarily requires micropatterned perovskite films.15−17 In particular, considering that © 2016 American Chemical Society

Received: August 14, 2016 Accepted: August 29, 2016 Published: August 29, 2016 9026

DOI: 10.1021/acsnano.6b05478 ACS Nano 2016, 10, 9026−9035

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Figure 1. (a) Schematic illustration of the SAGP procedure. Micropatterning begins with the formation of a perovskite precursor film in gel state via spin-coating of a solution in high-boiling point solvent, DMSO (I). A transparent film in gel state is developed (II), and subsequently a topographically patterned PDMS mold is placed on the precursor film with an appropriate pressure (III), giving rise to a molded precursor film (IV). The following thermal treatment at 80 °C for 15 min (V) successfully results in a micropatterned perovskite film over large area (VI). (b) Photographs (scale bar is 1 cm), (c) FT-IR transmittance spectra, and (d) HR-XRD patterns of the as-spun MAPbBr3 precursor film in soft-gel state and the film after thermally annealed one at 80 °C for 15 min.

prepared by spin-coating.28−30 To make these techniques applicable for the micropatterning of a perovskite film, however, a thin precursor film should be sufficiently moldable with minimum perovskite crystals before thermal annealing of the film at the precursor-to-perovskite conversion temperature. In order to provide sufficient fluidity of a thin precursor film at room temperature for efficient pattern formation, we employed a high-boiling point solvent, dimethyl sulfoxide (DMSO), instead of commonly used dimethylformamide (DMF) to the fabrication of a thin precursor film. With this simple solvent design, here we present a nondestructive micropatterning process, solvent-assisted gel printing (SAGP), to fabricate a variety of micropatterns of perovskites with their controlled crystal structures. Our method is based on a modified microimprinting of a thin spin-coated precursor film in soft-gel state with a topographically prepatterned elastomeric poly(dimethylsiloxane) (PDMS) mold, followed by thermal treatment at 80 °C as schematically illustrated in Figure 1. The residual solvent in a thin precursor film makes the film readily moldable upon imprinting process with a topographically patterned PDMS mold. Subsequent thermal annealing not only eliminates the solvent but also helps complete conversion to a perovskite film, giving rise to a variety of micropatterns of the perovskite film replicating the topographic PDMS patterns with the controlled crystalline structures over large area. Our SAGP is also suitable for micropatterning other halide-based perovskite films such as CH 3 NH 3 PbBr 3 (MAPbBr 3 ) and CH3NH3PbI3 (MAPbI3). We have observed that the intrinsic photoelectric properties of a perovskite film are preserved after

destructive micropatterning techniques can be alternatives, including inkjet printing21 and direct laser writing,22 microcontact printing,23,24 micromolding in capillaries,25−27 and microimprinting28−30 and so on, but still only a few can be applicable for fabrication of micropatterns of thin and uniform perovskite films without complicated additional processes such as etching and lift-off. For example, the selective growth of perovskite crystals was made on the hydrophilic regions of a prepatterned silicon substrate developed by high-cost photolithographic patterning processes.15−17 Spin-coating a perovskite precursor solution commonly used for photoelectric devices is one of the most common processes for fabricating a large area, uniform film, but extremely rapid solvent evaporation during the process significantly affects the growth dynamics of the film involving strong ionic interactions between the metal cations and halogen anions.8−14 The resulting crystalline film is largely varied in the crystalline orientation and grain shape and size, depending not only on the processing conditions but also on the lead halides.8−14,18−20 The annealing process has been frequently applied for complete conversion from precursor state to final perovskite crystals. A micropatterning method which can address the aforementioned issues of film uniformity as well as controlled microstructure should be, therefore, on demand for reliable arrays of perovskite photoelectric devices. We envisioned that nondestructive microprinting techniques such as imprinting and transfer printing may be most suitable for micropatterning a perovskite film because they involve thermally induced molding and/or selective detachment of a thin resist polymer film frequently 9027

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Figure 2. Bright-field OM images of micropatterned MAPbBr3 films by SAGP with periodic lines (a), squares arrayed with p4mm symmetry (b), and hexagons with p6mm symmetry (c). (d) A FE-SEM image of the MAPbBr3 circles arrayed with p6mm symmetry in tilted view. A TMAFM image of a circle in height contrast and its cross-sectional profile are represented in the inset of (d). Bright-field OM images of micropatterned MAPbI3 films printed with periodic lines (e) and MAPbBr3 line patterns printed on a PET substrate (f) and a photograph (the inset of (f)). (g) A FE-SEM image and (h) a TM-AFM image in height contrast of MAPbBr3 line-patterns of 400 nm in width. Threedimensional topographic view of the TM-AFM image is also shown in (h). A photograph in the inset of (a) and (g) exhibits the light interference pattern confirming large area, homogeneous pattern formation on a substrate by SAGP. The scale bar in (a−f) is 30 μm.

demonstrating the effectiveness of our SAGP. A MAPbBr3 perovskite precursor film in gel state was first spin-coated on a hydrophilic SiO2 substrate treated with UV-oxygen plasma as shown in the steps I and II of Figure 1a. The spin-coating was done at room temperature (∼20 °C) with a solution of CH3NH3Br and PbBr2 dissolved in a high-boiling point solvent, DMSO (boiling point = 189 °C). The evaporation of DMSO during spin-coating was limited, leaving some of residual solvent in a thin precursor film after spin-coating. The residual solvent rarely evaporated at room temperature gives rise to a film in moldable gel state suitable for subsequent microimprinting processes. The film in gel state was readily deformed when a topographically prepatterned PDMS was in conformal contact on the film, followed by the gentle compression with an appropriate pressure as represented in steps III and IV of Figure 1a. A micropatterned precursor film replicated from the

SAGP such as photon absorbance and photoluminescence (PL). In particular, arrays of parallel-type photodetectors with one-dimensional line patterns of a perovskite film are readily fabricated, and the individual device exhibits highly reliable characteristic photoconduction behavior. Furthermore, we demonstrate that a soft-gel perovskite precursor film is selectively detached with a topographically patterned PDMS mold and transferred to a desired substrate which can diversify its applications.

RESULTS AND DISCUSSION A micropattern of a thin perovskite film is obtained by imprinting a spin-coated precursor film in soft-gel state with a topographically prepatterned elastomeric PDMS mold, as schematically illustrated in Figure 1a. A representative CH3NH3PbBr3 (MAPbBr3) perovskite was selected for 9028

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ACS Nano PDMS mold was subject to thermal annealing at 80 °C which facilitated the crystallization of perovskite in the precursor film (step V of Figure 1a). The PDMS mold was readily removed after complete crystallization of the perovskite film, giving rise to a micropatterned perovskite film with high degree of crystallization as illustrated in step VI of Figure 1a. We examined the physical properties of a gel-type precursor film spin-coated with DMSO. The precursor film in gel state was almost optically transparent, and its surface was a little sticky as shown in Figure 1b, which implies that certain amount of DMSO still exists in the film after spin-coating. The results also indicate that the perovskite crystallization arising from the reaction between MABr and PbBr2 rarely occurred due to the presence of the residual DMSO. On the other hand, the film turned yellowish when thermally annealed at 80 °C for 15 min, as shown in the bottom image of Figure 1b. The color change observed after the thermal treatment clearly suggests that the conversion to MAPbBr3 perovskite crystals was successfully made by complete evaporation of the residual DMSO. It should be noted that when a commonly used dimethylformamide (DMF, bp = 153 °C) was employed as a solvent, the crystallization instantly occurred even at room temperature right after spin-coating, giving rise to a yellowish perovskite film (see Supporting Information, Figure S1). The solid-type yellowish film was, however, hardly suitable for micropattering due to a lack of the fluidity of the film upon molding process as shown later. MAPbBr3 films both in gel state and in solid state after thermal treatment were further analyzed by FT-IR transmittance measurement, as shown in Figure 1c. Apparently, the characteristic transmittance peak is observed at 1014 cm−1 with very high intensity arising from S−O stretch band of the residual DMSO in an as-cast precursor gel film.31 The peak almost disappeared after thermal annealing, indicative of complete removal of DMSO. In addition, the peaks at 1583 and 1479 cm−1 were observed which arose from N−H bend of a methylammonium component of MAPbBr3. The sulfoxide oxygen atom in a DMSO molecule strongly coordinates with a Pb2+ ion, giving rise to a stable complex when the lead halides are dissolved in the DMSO.32 A Pb−O bond of 2.386 Å in length is formed with DMSO solvent, while the longer Pb−O bond of 2.431 Å in length is formed with DMF, which indicates that DMSO is more strongly coordinative with Pb2+ ions than DMF.32−34 The strong interaction in turn delays the evaporation of DMSO and effectively hinders the reaction between MABr and PbBr2 during spin-coating, making the perovskite crystallization significantly retarded. The slow crystallization was confirmed by high-resolution X-ray diffraction (HR-XRD) measurement, as shown in Figure 1d. The XRD pattern of an as-spun film exhibits very weak intensity of the characteristic peak at 15.0° which corresponds to (100) planes of MAPbBr3 perovskite, evidencing that the crystallization rarely occurred with DMSO at room temperature. On the other hand, highly crystalline perovskites were formed with the complete evaporation of DMSO after thermal treatment, as confirmed with the highly intense peak at 15.0°. The extremely slow crystallization of a precursor film at room temperature with DMSO rendered the film to remain in the gel state which allowed us to fabricate diverse micropatterns of the films as shown next. Our SAGP successfully developed a variety of micropatterns of organic−inorganic hybrid perovskite films with different geometries and dimensions over a large area, as shown in Figure 2. Representatively, micropatterns of MAPbBr3 films

were fabricated, including periodic lines, squares and hexagons as shown in Figure 2a−c, respectively. In particular, the optimization of patterning processes was made with a PDMS mold of periodic lines with a width, height, and periodicity of 5, 1.5, and 10 μm, respectively. First, the role of solvent was examined by employing the mixture of cosolvents of DMF and DMSO. As described previously, a film spin-coated with DMF was rarely suitable for micropatterning due to the lack of fluidity of the film arising from perovskite crystals developed during spin-coating. Interestingly, when 10 vol % of DMSO was added in solvent, a spin-coated film became softened, making the film partially moldable even though the resulting micropatterns were very defective (see Supporting Information, Figure S2). A film was further softened with the amount of DMSO in cosolvent, and thus the defects of the patterned lines decreased with DMSO. Finally, highly uniform, well-defined line pattern was obtained over a large area when a pure DMSO was used (Figure 2a), and the results clearly indicate that the high-boiling point DMSO solvent plays one of the most critical roles for making a precursor film moldable in gel state due to its extremely slow evaporation rate during spin-coating. We also examined the effect of various factors on micropattern formation of a perovskite film including the concentration of a precursor solution, imprinting pressure, time, and temperature (see Supporting Information, Figure S3). Unfilled patterns were developed with a precursor solution whose concentration was 25 kPa was sufficient to successfully mold a precursor film in gel state, giving rise to discrete microlines of a perovskite film (see Supporting Information, Figure S3b). For our thermal treatment process for the conversion of a precursor film to micropatterned perovskite one, we found that the treatment at 80 °C for 15 min was optimal for uniform pattern formation. Periodic lines were very defective when fabricated with insufficient imprinting time (30, 120, 300 s at 80 °C) due to the residual DMSO (see Supporting Information, Figure S3c). In addition, high-temperature annealing above 100 °C deteriorated the quality of micropatterns due to too fast evaporation of DMSO, and in consequence, lines were not completely long with many broken parts. With the optimized SAGP conditions, we were able to develop a large area micropattern of a thin MAPbBr3 perovskite film over 1.5 × 1.5 cm2 as shown in the inset of Figure 2a (also see Supporting Information, Figure S4). Periodic squares and hexagons of thin MAPbBr3 perovskite films arrayed into a p4mm and p6mm symmetry, respectively, were successfully fabricated by our SAGP as shown in Figure 2b,c, respectively. A scanning electron microscope (SEM) image in Figure 2d also shows that the perovskite circles are very uniform in height (∼530 nm), as exhibited in a TM-AFM image in height contrast and cross-section profile in the inset of Figure 2d. The perovskite micropatterns are readily controlled in size. Periodic lines, squares, and hexagons with three different sizes in width of 2, 5, 10 μm were produced as well (see Supporting Information, Figure S5). Our SAGP process was also applicable for another perovskite material, CH3NH3PbI3 (MAPbI3) in which uniform arrays of MAPbI3 lines with width and periodicity of 5 and 10 μm, respectively, 9029

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Figure 3. A HR-XRD pattern (a), a FE-SEM image of the surface morphology (b), and a magnified FE-SEM image (c) of periodic lines of MAPbBr3 film prepared by SAGP. A HR-XRD pattern (d), a FE-SEM image of the surface morphology (e), and a magnified FE-SEM image (f) of periodic lines of MAPbI3 film prepared by SAGP. A FE-SEM cross-section image of a domain of the MAPbBr3 (g) and MAPbI3 (h) line patterns. The scale bar is 10 μm in (b) and (e) and 1 μm in (c) and (f−h).

were demonstrated in Figure 2e. It should be noted that optimal annealing temperature for complete crystallization of a perovskite film depends upon halides of perovskites. In our patterning processes, however, we found that both MAPbBr3 and MAPbI3 were successfully converted from precursor films with DMSO to highly crystalline perovskite films upon thermal annealing at 80 °C as confirmed in Supporting Information, Figure S3. Furthermore, our SAGP which requires the relatively low-temperature annealing at 80 °C allows for micropatterning a perovskite film on a mechanically flexible plastic substrate unless the substrate is damaged by DMSO. A periodic line pattern of MAPbBr3 was successfully developed on a polyethylene terephthalate (PET) substrate, as shown in Figure 2f. In addition, perovskite patterns were successfully developed on other technically useful substrates such as indium tin oxide (ITO), fluorine doped tin oxide (FTO), Au, and Al2O3 films, making our process suitable for various photoelectronic applications (see Supporting Information, Figure S6) A micropatterned perovskite film should be controlled in thickness to broaden its applicability. We employed a set of PDMS molds with 4 different pattern heights to our SAGP and MAPbBr3 micropatterns with various thicknesses were successfully fabricated with the heights of approximately 372, 650, 1400, and 2300 nm, as confirmed by TM-AFM (see Supporting Information, Figure S7). Furthermore, SAGP was suitable for micropatterning a perovskite film with submicron scale. Periodic lines of 400 and 150 nm in width and height, respectively, were successfully developed over large area, as shown in Figure 2g,h. A uniform height was obtained with the 400 nm line pattern, as shown in Figure 2h. It should be also noted that the micropatterns developed by SAGP have little variation in height as confirmed by both TM-AFM in height contrast and cross-sectional SEM results (see Supporting Information, Figure S7)

It is important to examine the molecular and microstructure of patterned perovskite domains for further utilization of the micropattern for high-performance photoelectric applications such as PL and photodetection devices. The crystalline structure of MAPbBr3 line-patterns shown in Figure 2a was investigated by HR-XRD, as shown in Figure 3a. The diffraction pattern of a line-patterned MAPbBr3 perovskite film by SAGP exhibits the characteristic peaks at 2-θ of 15.0° and 30.1° which correspond to (100) and (200) planes of a cubic P/m3̅m phase of MAPbBr3, respectively.8,35 The diffraction pattern of a micropatterned MAPbBr3 film is identical with that of a nonpatterned, flat film, and the results clearly show that the crystal structure of MAPbBr3 was rarely affected by SAGP process and the patterned film with high crystallinity can be also suitable for various photoelectronic applications as shown later (see Supporting Information, Figure S8). The averaged crystal size of the patterned MAPbBr3 lines was approximately 42.8 nm when calculated by Scherrer equation, which suggests that the individual line is polycrystalline and consists of aggregation of nanometer sized crystallites (see Supporting Information, Table S1). The averaged crystal size of a nonpatterned, flat was approximately 42.2 nm almost same as that of the patterned film. It is worth noting that any unreacted residue of PbBr2 was rarely observed after SAGP process since the characteristic diffraction peaks of PbBr2 crystals at 18.8° and 37.6° were not detected.35 The microstructure of individual MAPbBr3 perovskite lines was examined by SEM, and the results are shown in Figure 3b,c. Individual lines are very smooth on surface without physical voids or mechanical cracks. Both crystalline and microstructure of MAPbI3 line patterns (the same one in Figure 2e) were also investigated by HR-XRD and SEM, respectively, and the results are shown in Figure 3d−f. The diffraction pattern of SAGP processed MAPbI3 lines exhibits the characteristic peaks at 14.1°, 24.5°, and 28.3° that correspond 9030

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Figure 4. (a) Absorbance and (b) PL spectra and (c) fluorescence OM images of periodic lines of MAPbBr3 and MAPbI3 films prepared by SAGP. (d) The spectrally resolved photocurrent spectrum of periodic lines of a MAPbI3 film of a two terminal parallel-type photodetector at voltage bias of 5 V. (e) The current−voltage (I−V) characteristics with and without the illumination of 532 nm laser and (f) the temporal photoresponse behavior of Au/MAPbI3 line pattern/Au photodetectors.

to (110), (202), and (220) planes of a tetragonal I4/mcm phase, respectively. Again, the diffraction pattern was the same as that of a nonpatterned, flat MAPbI3 film (see Supporting Information, Figure S8).35 In addition, the absence of a peak at 12.6° corresponding to (100) plane of PbI2 crystals confirms that there remained no unreacted PbI2 residues. The polycrystalline individual lines were developed by SAGP with the averaged crystals of 30.9 nm in size which was very similar to those of nonpatterned, flat film (32.1 nm). The surface morphology evidenced by FE-SEM in Figure 3e,f also exhibits uniform and smooth MAPbI3 patterns without physical defects and holes by our SAGP. The cross-sectional views of MAPbBr3 and MAPbI3 lines in Figure 3g,h, respectively, show that both lines have trapezoid cross-section with the height of approximately 1.1 μm. We speculate that preferential contraction of perovskite lines during crystallization occurred at the top regions of the lines upon thermal annealing. Both crystalline and microstructural results indicate that our SAGP offers a platform for developing a variety of micropatterned films with high-quality perovskite crystalline structures. The SAGP capable of fabricating micropatterned perovskite films with high crystallinity and structural integrity can make these patterned films suitable for various photoelectronic devices. For this purpose, we first investigated the optical absorbance and PL properties of SAGP processed MAPbBr3 and MAPbI3 films as shown in Figure 4a,b, respectively. The absorbance spectra of the patterned MAPbBr3 and MAPbI3 lines in the width of 5 μm exhibit the characteristic absorbance shoulders at 523 and 778 nm, respectively, which are identical with those of the unpatterned films (see Supporting Information, Figure S9). The sharp PL peaks at 521 nm and at 776 nm were observed for MAPbBr3 and MAPbI3 linepatterns, respectively, as shown in Figure 4b. Again, the PL properties of the patterned perovskite films are the same as those of nonpatterned, flat films (see Supporting Information, Figure S9). The fluorescence microscope images shown in Figure 4c also exhibit highly uniform green and red line patterns arising from the characteristic PL of MAPbBr3 and red MAPbI3, respectively.

Based on the optical absorbance and PL properties of SAGP processed perovskite films, we fabricated a two terminal, parallel-type photodetection device containing periodically patterned perovskite films as shown in Figure 4d−f. Periodic MAPbI3 lines of 5 and 10 μm in width and periodicity, respectively, were first formed by our SAGP process. MAPbI3 perovskite was selected because MAPbI3 has been extensively exploited as a photodetecting perovskite due to its promising properties such as broadband absorption, high detectivity, and fast response time.36 The arrays of Au electrodes with a channel length of 50 μm were deposited across the MAPbI3 line pattern by thermal evaporation, as schematically shown in the inset image in Figure 4d. First of all, we employed a tunable laser to examine the photocurrent properties as a function of input wavelength, and the results shown in Figure 4d clearly display that significant photocurrent was developed in the broadband wavelength covering the whole visible regime ranging from 450 to 800 nm when 5 V bias was applied to our Au/MAPbI3/Au device. In order to further examine the photodetection performance of our device with the patterned perovskite film, we employed a laser with the wavelength and power of 532 nm and 10 mW cm−2, respectively, and estimated the photocurrent as a function of bias voltage and the results are shown in Figure 4e. The pseudo-ohmic photocurrent behavior was observed with the significant enhancement of the photocurrent value under the laser excitation. The ohmic characteristics of a thin perovskite film in photocurrent were attributed to the small band energy difference between conduction band of a perovskite film and work function of Au as demonstrated in the previous work.37 The device also showed excellent photocurrent switching properties with a photocurrent value of the scale of a few nanoamperes at low voltage bias of ±5 V, as shown in Figure 4f. The multiple photocurrent switching reliably occurred with the bias voltage at 3 V. The switching time we measured at our facility was approximately 100 ms, which was the limitation of our facility. Different from the optical absorbance and PL properties in which nonpatterned perovskite films showed almost identical performance with those micropatterned by SAGP, photodetection performance of 9031

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Figure 5. (a) Schematic illustration of the procedure of solvent-assisted gel transfer printing. A precursor film in gel state is selectively detached and transferred to a topographically patterned PDMS mold. The precursor film on the PDMS mold is readily transferred to a desired substrate when combined with thermal annealing at 80 °C for 15 min, giving rise to a micropatterned perovskite film on the substrate. (b and c) Bright-field OM images of periodic circles of 5 μm in diameter arrayed with p4mm symmetry (b) and hexagons of 20 μm in side length with p6mm symmetry (c) of thin MAPbBr3 films transferred onto a Si substrate. The insets of (b) and (c) show magnified images. A bright-field OM image of a PDMS mold with a line pattern (d) before imprinting and (e) after 10 times imprinting events. The photographs of the PDMS molds in large scale are provided in the inset of (d) and (e).

structure of a perovskite film is under way as a function of pattern geometries as well as dimensions. Our SAGP was also combined with conventional detachment and transfer printing, and the resulting gel-assisted transfer printing provides more freedom for choosing substrates, as schematically illustrated in Figure 5a. A precursor film in gel state was selectively detached when a topologically patterned PDMS mold was in conformal contact on the film, followed by the removal of the mold. The surface of the prepatterned PDMS mold was treated by O2 plasma for facile detachment and transfer of the precursor film containing polar residual

a nonpatterned film was hardly observed in the two terminal parallel-type device architecture. We observed that the spincoating, followed by thermal treatment of a thin perovskite film resulted in perovskite crystals not continuously connected with each other, making it difficult to form photoconduction channels between the two electrodes (see Supporting Information, Figure S10). The results again support the usefulness of our SAGP in which the crystallization of a precursor film in the confined geometry was significantly helpful for developing compact and dense crystalline structure of a perovskite film. Detailed investigation of crystalline 9032

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films. In particular, the two terminal, parallel-type photodetector arrays containing micropatterned MAPbI3 films exhibited the characteristic photocurrent in broad band visible range with the reliable multiple photocurrent switching properties. Our SAGP was further extended when combined with a conventional detachment and transfer printing technique in which micropatterned perovskite films were successfully transferred on a desired substrate. Our results clearly suggest that gel-based soft printing technique developed in this work can open up an emerging research area for accelerating the commercialization of perovskite films.

DMSO. It is essential that adhesion of a precursor film on the surface of a PDMS mold should be controlled, depending upon printing techniques. While weak adhesion is required in solvent-assisted gel imprinting, strong adhesion of a precursor film on the surface of a PDMS mold should be guaranteed for transfer printing. For the purpose, a PDMS mold was treated with oxygen plasma, and the surface energy of the PDMS mold was controlled as a function of the treatment time (see Supporting Information, Figure S11). The successful detachment of a precursor film to a PDMS mold was achieved when the mold was treated for the time longer than 10 s. The work of adhesion is one of the most important parameters for successful pattern transfer, and it has been conveniently controlled with oxygen plasma, as reported in the previous works.38−40 The pattern transfer from PDMS surface to a target substrate was accomplished with the thermal treatment at 80 °C for 15 min which again facilitated the conversion of the precursor to crystalline perovskite film, as shown in Figure 5a. Representatively, a MAPbBr3 precursor film in gel state selectively detached by a PDMS mold was successfully transferred onto a Si substrate, giving rise to circular perovskite microdomains of 5 μm in diameter arrayed into p4mm symmetry, as shown in Figure 5b. Our gel-assisted transfer printing also developed the micropattern of hexagons with each side of 20 μm arrayed into p6mm symmetry, as shown in Figure 5c. To demonstrate that PDMS molds were reused for the costeffectiveness of our process, we examined the suitability of a PDMS mold for repeated micropatterning events. As shown in Figure 5, a PDMS mold was able to develop a micropatterned film even after 10 patterning events without any additional cleaning process of the used mold. After a few tens of imprinting processes, however, some residuals of the perovskite were observed on the PDMS mold. A surface clean PDMS mold was readily achieved by removing the residuals with an adhesive Scotch tape. The cleaned mold could be reused for many times (see Supporting Information, Figure S12). The facile fabrication of micropatterned perovskite films based on our SAGP, in particular, with the photoelectronic properties very comparable with those of thin and flat perovskite ones clearly suggests that our patterning technique can be diversely utilized for developing arrays of various organic−inorganic hybrid perovskite-based devices such as photodetectors,4,5 nonvolatile memories,7 and light-emitting diodes.8

EXPERIMENTAL SECTION Materials Preparation. Methylammonium halides such as CH3NH3Br and CH3NH3I were synthesized as reported in previous works.8,20 In brief, methylamine (40% in methanol, purchased from Tokyo Chemical Industry) was reacted with HI or HBr having the mixing portion of methylamine:HI of 9:10 and methylamine:HBr of 6:10 in volume by vigorous stirring of the solutions in an ice bath for 2 h. The precipitates were extracted by heating at 60 °C for 1 h with a rotary evaporator. After this, the precipitates were dissolved in ethanol and recrystallized by diethyl ether. Finally the recrystallized powder was washed three times by diethyl ether for 30 min each and dried in a vacuum oven for 24 h. All the chemical solvents used in this work and lead halide PbBr2 and PbI2 (99.999%, trace metal basis) were purchased from Sigma-Aldrich. A variety of elastomeric poly(dimethylsiloxane) (PDMS) molds were fabricated by curing PDMS with a curing agent having 10:1 weight ratio on prepatterned Si masters. The topologically patterned PDMS molds were fabricated with a variety of periodic lines, hexagons, and squares. The size of a domain of each pattern was determined from the prepatterned Si masters ranging from 2 to 10 μm. A Si master was also employed with the periodic lines of 400 nm in width. A variety of substrates were prepared for SAGP, including poly(ethylene terephthalate) (PET), ITO, FTO, Au, and Al2O3. PET and FTO substrates were purchased from Aldrich. ITO substrates were purchased from Freemteck, Inc., South Korea. A thin layer of Au was deposited on a Si substrate by thermal evaporation, and Al2O3 was deposited on a Si substrate by atomic layer deposition with 50 nm thickness, respectively. Solvent-Assisted Gel Printing (SAGP). Imprinting. A micropatterned perovskite film was fabricated on the SiO2 substrate. First, A SiO2 substrate was cleaned in acetone and 2-propanol and treated with UV-oxygen plasma for 15 min to make the substrate hydrophilic. After then, a perovskite precursor solution (MAX and PbX2 with 1:1 molar ratio with 30 wt %, X = Br, I) dissolved in dimethyl sulfoxide (DMSO) was spin-coated on the SiO2 substrate at 3000 rpm for 30 s at room temperature (20 °C) in a N2-filled glovebox. After spin-coating the DMSO-based perovskite precursor solution, a film in soft-gel state was produced due to the residual DMSO in the film arising from its extremely slow evaporation rate at 20 °C. Microimprinting was subsequently performed by gently compressing a topographically prepatterned PDMS mold in conformal contact on the perovskite precursor gel film with an appropriate pressure. Then, the molded gel film with the PDMS mold was slowly heated to 80 °C and thermally annealed for 15 min to facilitate the crystallization of the perovskite as well as the complete evaporation of DMSO. For comparison, we prepared nonpatterned, flat perovskite films. Spin-coated precursor films were subsequently annealed at 80 °C for 15 min in N2-filled glovebox. Detachment and Transfer Printing. A precursor film in gel state, as described previously, was selectively detached and transferred on the surface of a topographically prepatterned PDMS mold. The micropatterned precursor film in gel state on the PDMS mold was further transferred on a desired substrate when combined with the thermal treatment at 80 °C for 15 min, giving rise to a micropatterned perovskite film on the substrate. Soft contact pressure of 1 kPa was used for efficient pattern transfer. For efficient pattern transfer, a

CONCLUSIONS We demonstrated that the DMSO with high-boiling temperature remaining in a thin spin-coated perovskite precursor film was extremely useful for fabricating a variety of micropatterns of the perovskite film. The residual DMSO not only hampered the perovskite crystallization at room temperature but also made a precursor film remain in gel state which allowed for imprinting the film with a topographically patterned PDMS mold. The combined thermal treatment upon imprinting gave rise to welldefined micropatterned perovskite films with high crystallinity as well as structural rigidity. With optimized patterning conditions including the concentration of a precursor solution, imprinting pressure, temperature, and time, two representative organic−inorganic hybrid perovskite films of MAPbBr3 and MAPbI3 were successfully micropatterned with various pattern geometries and dimensions by our SAGP. We revealed that SAGP did not harm both photoabsorbance and PL properties of the perovskite films and the properties of micropatterned films were almost identical with those of thin, flat perovskite 9033

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ACS Nano PDMS mold was treated with oxygen plasma (Femto Science, Model: Cute) with 100 W at 10−3 Torr, 40 sccm for appropriate time. Fabrication of a Two Terminal, Parallel-Type Photodetector Array. Micropatterned MAPbI3 film fabricated by SAGP was used as a photodetecting layer. Periodic lines of MAPbI3 with the width and periodicity of 5 and 10 μm, respectively, were prepared by SAGP on a 280 nm-thick SiO2 substrate. Arrays of the two terminal Au electrodes with the terminal gap of 50 μm were thermally evaporated at a 10−6 Torr and a rate of 1 Å s−1 with a shadow mask vertically set on MAPbI3 line pattern. The Au electrodes were 100 nm in thickness. Characterization and Measurement. Fourier transform infrared (FT-IR) spectra were recorded under the attenuated total reflectance (ATR) mode (Spectrum 100, PerkinElmer). A bright-field optical microscope was used to visualize the perovskite films (Olympus BX 51M). Surface morphology was analyzed by using field emission scanning electron microscopy (FE-SEM, LEO 1550 VP). X-ray diffraction patterns were collected at a rate of 3° min−1 using Rigaku SmartLab HR-XRD equipment. Optical absorbance spectra were acquired by using UV−vis-NIR spectrometer (lambda 750, PerkinElmer). PL spectra were obtained using LS 55 fluorescence spectrometer (PerkinElmer). A photocurrent of a two terminal, parallel-type photodetector was measured by using a semiconductor system (4200-SCS, Keithley Instruments Inc.) with a home-built monochromatic laser with 532 nm wavelength. For the spectrally resolved photocurrent measurement, we used supercontinuum laser source which provided broadband radiation from visible to infrared range (460−2000 nm wavelength). After passing through a monochromator, the wavelength-selected laser source was focused onto biased samples. The generated photocurrent was measured by current preamplifier and lock-in amplifier with optically chopping frequency of 60 Hz.

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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/acsnano.6b05478. Experimental details and data (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by a grant from the National Research Foundation of Korea (NRF) funded by Korean government (MEST) (no. NRF-2014R1A2A1A01005046, NRF-2016M3A7B4910530), and Global Ph.D. Fellowship Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2013H1A2A1033524). This work is also based upon work supported by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under Industrial Technology Innovation Program (no. 10063274). REFERENCES (1) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, 316−319. (2) He, M.; Pang, X.; Liu, X.; Jiang, B.; He, Y.; Snaith, H.; Lin, Z. Monodisperse Dual-Functional Upconversion Nanoparticles Enabled Near-Infrared Organolead Halide Perovskite Solar Cells. Angew. Chem. 2016, 128, 4352−4356. 9034

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