Article Cite This: Chem. Mater. 2018, 30, 4590−4596
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Perovskite Single-Crystal Microarrays for Efficient Photovoltaic Devices Jiang Wu,† Fengjun Ye,† Wenqiang Yang,† Zhaojian Xu,† Deying Luo,† Rui Su,† Yifei Zhang,† Rui Zhu,*,†,‡,§ and Qihuang Gong†,‡,§ †
Chem. Mater. 2018.30:4590-4596. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/05/18. For personal use only.
State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, Department of Physics, Peking University, Beijing, 100871, China ‡ Collaborative Innovation Center of Quantum Matter, Beijing, 100871, China § Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi, 030006, China S Supporting Information *
ABSTRACT: Hybrid perovskite single crystals offer a great promise for optoelectronic devices, and patterning is broadly required in industrialized applications for functional purposes. However, established patterning techniques meet their limitations when it comes to hybrid perovskite single crystals with multilayered diode structures. In this work, an Ostwald ripening assisted photolithography (ORAP) patterning process, which employs wettability-assisted blade-coating and Ostwald ripening assisted crystallization, is developed for fabricating patterned perovskite single-crystal microarrays. Optoelectronic devices based on uniform perovskite single-crystal microarrays with a multilayered diode structure can be constructed through the ORAP process. To demonstrate the compatibility of the ORAP process with the manufacture of optoelectronic devices, patterned CH3NH3PbBr3 single-crystal microarray solar cells, which show enhanced performance than solar cells based on CH3NH3PbBr3 continuous single crystals reported, were fabricated. With the development of perovskite research, we are confident that the ORAP process opens a new avenue to fabricate optoelectronic devices based on perovskite microarrays.
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strated by Liu and co-workers.36 In addition, Feng and coworkers37 produced another method, which could fabricate perovskite arrays via splitting the precursor solution by a wettability-selected silicon template. However, the prepatterned seed layer, which is necessary for the growth of the crystal, limits the deposition of charge transport layers. Hence, patterning single-crystalline perovskite microplates on conventional charge transport layers is rarely achieved. Owing to this limitation, prototypical structure36 (substrate/perovskite) and photoconductor structure35 (electrode/perovskite/electrode), instead of the multilayered diode structure38 (electrode/charge transport layer/perovskite/charge transport layer/electrode), had been demonstrated in patterned single-crystalline perovskite microplates. In this work, we report an Ostwald ripening assisted photolithography (ORAP) process for patterning perovskite single-crystal microarrays. The ORAP process includes two steps: patterning the perovskite precursor solution into arrays through a wettability-assisted blade-coating process; then crystallizing the prepatterned solution arrays through an Ostwald ripening assisted crystallization strategy. In comparison to other patterning methods recently reported for
INTRODUCTION Benefiting from the excellent semiconducting properties previously reported,1−9 hybrid perovskite material is regarded as one of the most promising candidates for next-generation optoelectronic devices.10−16 At present, most of these optoelectronic devices comprising hybrid perovskite materials are based on polycrystalline thin-film structures, which normally introduces large amounts of defects at the grain boundaries.17 Better optoelectronic performance is expected with devices based on perovskite single crystals, as single crystals have no grain boundaries.18−31 As we know, compared to a continuous substance, materials arranged with some sort of regularity are required for many optoelectronic functional applications: for example, luminous pixel matrices in panel displays,32 photodetector microarrays in camera modules,33 and meandering conductive structures in integrated circuits.34 Thus, there is an increasing demand to develop patterning methods for hybrid perovskite materials, especially perovskite single crystals. In recent years, several patterning methods have been developed for the realization of hybrid perovskite crystalline microplates. For example, Wang and co-workers35 fabricated perovskite arrays through vapor-phase conversion on PbI2 seed arrays in prepatterned cells. Through a similar strategy, uniform perovskite arrays based on the boron-nitride film which was prepatterned by photolithography were demon© 2018 American Chemical Society
Received: March 4, 2018 Revised: June 10, 2018 Published: July 11, 2018 4590
DOI: 10.1021/acs.chemmater.8b00945 Chem. Mater. 2018, 30, 4590−4596
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Chemistry of Materials
Figure 1. Schematics of step 1 of the ORAP process: a wettability-assisted blade-coating process. (a) Preparing the photoresist template via photolithography. The photoresist was coated on the substrate with precoated poly-TPD layer. (b) Washing the sample with CB to duplicate the pattern of photoresist template on poly-TPD. (c) Washing the sample with DMF to remove the photoresist. (a−c) Reproduced with permission from ref 42. Copyright 2017 American Chemical Society. (d) On the hydrophobic surface of prepatterned poly-TPD, the CH3NH3PbBr3 perovskite solution was deposited by blade-coating method. (d1−d6) Illustrated the optical microscopic pictures with the movement (left to right) of the blade. Scale bar: 100 μm. was placed into a weighing bottle (50 mm × 30 mm) surrounded by 0.5 mL of isopropanol (IPA). After 10 min, one and only one larger crystal with several smaller crystals were deposited in each cell of the samples. Then the sample was placed into another weighing bottle surrounded by 0.1 mL of DMF. Through opening and closing the cover of the weighing bottle, uniform CH3NH3PbBr3 single-crystal microarrays were realized by Ostwald ripening. The sample was then placed on a hot plate (80 °C) for 1 min, and transferred into a N2filled glovebox, followed by washing with CB to remove the poly-TPD layer. The CH3(CH2)17SiCl3 blocking layer was then fabricated on the sample according to a reported procedure.41 To deposit the hole transporting layer, 28.5 μL of 4-tert-butylpyridine, 17.5 μL of acetonitrile, 9.1 mg of lithium bis(trifluoromethanesulfonyl)imide, and 80 mg of Spiro-OMeTAD were all dissolved in 1 mL of CB to create the Spiro-OMeTAD-based solution. The solution was then spin-coated on the patterned perovskite crystals at 2000 rpm for 30 s. Finally, we deposited a gold electrode (150 nm) through thermal evaporation to complete the device. See Figure S1 for the detailed fabrication process. Characterizations of Solar Cells. A Keithley 2400 sourcemeasure unit was employed to measure the current−voltage curves of the solar cells. The cells (active area: 8.63 mm2. The average perovskite coverage rate was counted under an optical microscope as 6.1%, so the actual active area turned out to be 0.53 mm2.) were illuminated by a 150 W class AAA solar simulator (XES-40S1, SANEI) at 100 mW·cm−2 with an AM 1.5G filter. Illumination intensity was calibrated by use of a standard monocrystalline silicon photodiode which was calibrated by the Newport TAC-PV lab. Other Characterizations. The scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) images were captured by a FEI Nova_NanoSEM 430 field-emission SEM. The height of the surface of patterned CH3NH3PbBr3 single-crystal microarrays was measured by a Dektak stylus profiler.
perovskite crystalline microplates, the ORAP process enables the realization of single-crystal microarrays with a multilayered diode device structure. Meanwhile, the ORAP process is compatible with the manufacturing process of most optoelectronic devices, owing to the elimination of the destructive processes on the perovskite single crystals such as etching, pressing, lifting-off, or transferring. Furthermore, patterned CH3NH3PbBr3 single-crystal microarray solar cells with a remarkable power conversion efficiency (PCE) of 7.84% were fabricated based on a multilayered diode structure, to demonstrate the compatibility and uniformity of the ORAP process.
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EXPERIMENTAL SECTION
Materials. The photoresist was purchased from Ruihong Tech. Poly(4-butylphenyl-diphenyl-amine) (poly-TPD) was bought from Xi’an Polymer Light Tech. MABr was synthesized by a previously reported method.39 OTS was purchased from TCI Company. The 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′,-spirobifluorene (Spiro-OMeTAD) was purchased from SunaTech. PbBr2, MA, hydrobromic acid, and acetic acid were bought from Sinopharm Group Company. All other liquid reagents were bought from Acros and used as received. Fabrication of Solar Cells Based on Patterned Perovskite Single-Crystal Microarrays. First, the FTO substrates were ultrasonically cleaned with detergent, deionized water, acetone, and isopropanol for 10 min. Then we fabricated the TiO2 electrontransporting layer on the precleaned FTO substrates according to a reported procedure.40 20 mg/mL poly-TPD in chlorobenzene (CB) was spin-coated on top of TiO2 coated FTO substrates at 3000 rpm to construct a hydrophobic film. A 1.7 μm thick photoresist layer with grid patterns was produced on the poly-TPD through photolithography. To duplicate the patterning on the poly-TPD layer, the sample was washed with CB. Then we washed the sample with N,Ndimethylformamide (DMF) to remove the photoresist template. To create the perovskite precursor solution, 5 mmol of MABr and 5 mmol of PbBr2 were dissolved in 1 mL of anhydrous dimethyl sulfoxide (DMSO), and stirred at 60 °C until dissolved completely. After that, the CH3NH3PbBr3 precursor solution in DMSO was patterned into the hydrophilic regions of the sample by blade-coating under a nonflowing atmosphere. The speed of the blade movement was 1.2 mm/min controlled by a stepping motor. Then the sample
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RESULTS AND DISCUSSION The fabrication of patterned single-crystalline CH3NH3PbBr3 microplate arrays using the ORAP process consisted of two steps. Step 1: CH3NH3PbBr3 precursor solution was patterned into arrays through a wettability-assisted blade-coating process (Figure 1). A 20 mg/mL poly-TPD solution (in CB) was first spin-coated on the top of a precleaned substrate. Then a photoresist template with microgrid patterns was obtained 4591
DOI: 10.1021/acs.chemmater.8b00945 Chem. Mater. 2018, 30, 4590−4596
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Chemistry of Materials
Figure 2. Schematics of step 2 of the ORAP process: an Ostwald ripening assisted crystallization process. (a) Placing the patterned CH3NH3PbBr3 precursor solution arrays into IPA atmosphere. (b) Crystallizing, then removing the IPA atmosphere. One and only one larger crystal with several smaller crystals were deposited in each cell. (c) Employing a moderate DMF atmosphere to make the smaller crystals dissolve totally but the larger one partially. (d) Removing the DMF atmosphere. The previous small crystals would now redeposit onto the larger crystal. (e) Removing the polyTPD by CB washing. (a1, b1−b3, c1, d1−d2, e1) Illustrated the optical microscopic pictures in the corresponding steps in chronological order. Scale bar: 100 μm.
through a conventional photolithographic approach.42,43 We washed the samples with CB, followed with DMF to duplicate the pattern of the photoresist template on poly-TPD, and then removed the photoresist template. After that, CH3NH3PbBr3 precursor solution (5 M, in DMSO) was patterned into the hydrophilic regions by blade-coating. The blade should move more slowly than most other reported cases44 as the solution needs some time to leave the hydrophobic area and flock into the hydrophilic regions. In the meantime, moving too slowly was also inapposite as the patterned arrays of the precursor solution should be kept intact as liquid for the next step, and moving too slowly would take too a long time so the solution began to crystallize. To overcome this issue, we employed a stepping motor to control the speed of blade movement, and operated the blade-coating in a nonflowing atmosphere to hinder the solution from crystallizing. Coming to step 2 of the ORAP process, the patterned CH3NH3PbBr3 precursor solution arrays were crystallized through an Ostwald ripening assisted crystallization process as we show in Figure 2. Initially, the patterned CH3NH3PbBr3 precursor solution arrays fabricated in Figure 1 were placed into an isopropanol (IPA) atmosphere. CH3NH3PbBr3 would crystallize into solid as IPA is a poor solvent of CH3NH3PbBr3. Through optimizing the concentration of IPA gas, one and only one larger crystal with several smaller crystals could be realized in each cell with long-range periodicity as shown in Figure 2b,b3. (See Video S2, Supporting Information.) Then
the IPA was removed and a DMF atmosphere was employed, which made the CH3NH3PbBr3 crystals deliquesce. The amount of DMF gas was limited and controlled to prevent the larger crystal from deliquescing totally. Subsequently, the DMF gas was removed and the sample started recrystallization. According to the theory of Ostwald ripening,45 smaller crystals would dissolve and redeposit onto the larger crystal. After 1−3 cycles of employment and retirement of the DMF atmosphere, single-crystalline CH3NH3PbBr3 microplate arrays were realized as displayed in Figure 2d. (See Video S3 in the Supporting Information for the first cycle, and Video S3 for the second cycle.) Finally, the unwanted poly-TPD could be washed away by CB. As a consequence, once the appropriate photolithographic templates were employed, controllable single-crystalline CH3NH3PbBr3 microplates with any longrange arrangements can be achieved by the use of the ORAP process. Figure 3 shows the SEM images of the patterned singlecrystalline CH3NH3PbBr3 microplates. Figure 3a−c displays SEM images of the CH3NH3PbBr3 microplate arrays at different magnifications. Figure 3a,b illustrates that the ORAP process could form perovskite microplate arrays with good conformity. Moreover, Figure 3c−e shows the ordered shape and uniform surface of the perovskite microplate units suitable for optoelectronic devices. Figure 4a shows the X-ray diffraction (XRD) pattern of a single-crystalline CH3NH3PbBr3 microplate obtained from the 4592
DOI: 10.1021/acs.chemmater.8b00945 Chem. Mater. 2018, 30, 4590−4596
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Figure 3. SEM images for the patterned single-crystalline CH3NH3PbBr3 microplates on FTO substrate fabricated through the ORAP process. (a− c) SEM images for CH3NH3PbBr3 microplate arrays at different magnifications. SEM images for the (d) edge and (e) center area of a CH3NH3PbBr3 crystal.
Figure 4. (a) X-ray diffraction (XRD) pattern for a single-crystalline CH3NH3PbBr3 microplate. (b) Normalized UV−vis absorption spectrum for the patterned CH3NH3PbBr3 single-crystal microarray. (c) Optical microscopic picture of the patterned CH3NH3PbBr3 single-crystal microarray. (d) Height profile characterization along the yellow line in (c).
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DOI: 10.1021/acs.chemmater.8b00945 Chem. Mater. 2018, 30, 4590−4596
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Chemistry of Materials
Figure 5. (a) Schematic of the solar cell based on patterned CH3NH3PbBr3 single-crystal microarray. (b) Cross-sectional diagram of the corresponding device. (c) EDS characterization and SEM image (inset) of silica glass/FTO/TiO2/patterned CH3NH3PbBr3 single-crystal/ CH3(CH2)17SiCl3 blocking layer. (d) Elemental analysis based on the EDS data. (e) Current density−voltage characterization of the devices with and w/o the blocking layer under 1 sun illumination. (f) Table of the device performance.
Figure 5e,f shows the current density versus voltage curves. Devices based on the structure of silica glass/FTO/TiO2/ patterned CH3NH3PbBr3 single-crystal/Spiro-OMeTAD/Au were also fabricated for comparison. The short-circuit current density (Jsc) was obtained based on the active area of the perovskite microplates, which was the device area multiplied by the average perovskite coverage rate (6.1%). Devices without the OTS blocking layer got a PCE of 0.35% (an open-circuit voltage (Voc) of 0.51 V, a Jsc of 1.62 mA·cm−2, and a fill factor (FF) of 0.42). For devices with the OTS layer, a significant PCE of 7.84% was achieved with a Voc of 1.04 V, a Jsc of 9.79 mA·cm−2, and a FF of 0.77, which is higher than those of devices based on continuous CH3NH3PbBr3 single crystal reported.27 There was a decline in Voc for the devices using patterned perovskite single-crystal arrays in comparison to devices based on continuous single crystals28 or films47 reported. This was probably caused by the potential shunting and the unequable diffusion lengths in the edge area of perovskite single crystals.
ORAP process. From the XRD analysis, it is noted that both 00l (l = 1, 2, 3, 4) reflections and 2θ positions are identical to the theoretical results as reported in other literature.46 This indicated that the microplate was formed in cubic phase and its surface belonged to the family of (001) planes. The absorption spectrum of the patterned CH3NH3PbBr3 single-crystal microarray is shown in Figure 4b. To evaluate the thickness and surface morphology of the patterned CH3NH3PbBr3 single-crystal microarray, Figure 4c,d shows its optical microscopic image and the surface height profiles. From Figure 4d, it can be observed that the microplates had a smooth surface, and the thicknesses of the microplates had a good conformity. The average thickness of the microplates turned out to be about 3 μm, which was controlled by technical specifications in the ORAP process. To confirm the compatibility of the ORAP process with the manufacture of optoelectronic devices, solar cells based on patterned perovskite single-crystal arrays were fabricated, wherein the device architectures are composed of glass/ FTO/TiO2/patterned CH3NH3PbBr3 single-crystal/CH3(CH2)17SiCl3 blocking layer/Spiro-OMeTAD/Au, as shown in Figure 5a,b. Through immersion, N-octadecyltrichlorosilane (CH3(CH2)17SiCl3; OTS) acted as a surface modifier to TiO2. Figure 5c,d shows the SEM and EDS characterizations of the silica glass/FTO/TiO2/patterned CH3NH3PbBr3 single-crystal/CH3(CH2)17SiCl3 blocking layer. Due to the condensation reaction between silane molecules and TiO2 surface,41 the alkyl chains are expected to assemble onto the uncovered TiO2 surface. Meanwhile, elemental analysis data based on the surface on the CH3NH3PbBr3 crystal contained no chlorine and silicon, suggesting that the OTS layer could block the shunting paths without impeding the transport of photocurrent.
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CONCLUSIONS In conclusion, we developed an Ostwald ripening assisted photolithography process, and this process was used to obtain the desired patterned perovskite single-crystal microarrays. The ORAP process solved the drawback that regular patterning methods cannot be applied to devices based on crystalline perovskite microplates with a multilayered diode structure. Patterned CH3NH3PbBr3 single-crystal microarray solar cells were fabricated, which showed superior performance than the devices based on CH3NH3PbBr3 continuous single crystals reported. With the development of perovskite research, we expect the ORAP process should be broadly applicable to other optoelectronic devices based on perovskite microarrays. 4594
DOI: 10.1021/acs.chemmater.8b00945 Chem. Mater. 2018, 30, 4590−4596
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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.chemmater.8b00945. Detailed device fabrication process and statistics of device performance (PDF) Crystallizing CH3NH3PbBr3 solution into solid (AVI) The first cycle for the Ostwald ripening of the singlecrystal microarray (AVI) The second cycle for the Ostwald ripening of the singlecrystal microarray (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Rui Zhu: 0000-0001-7631-3589 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.W. and F.Y. contributed equally to this work. This work was financially supported by the 973 Program of China (2015CB932203) and the National Natural Science Foundation of China (61722501, 91733301, 91433203, and 61377025). The authors thank Prof. Xinqiang Wang (Department of Physics, Peking University) for the calibration of solar simulator, and Dr. Rui Zhu (Electron Microscopy Laboratory of Peking University) for the SEM and EDS testing.
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DOI: 10.1021/acs.chemmater.8b00945 Chem. Mater. 2018, 30, 4590−4596
