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Letter 3
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Lead-Halide Perovskite Solar Cells by CHNHI-Dripping on PbI-CHNHI-DMSO Precursor Layer for Planar and Porous Structures using CuSCN Hole Transporting Material 2
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Seigo Ito, Soichiro Tanaka, and Hitoshi Nishino J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b00122 • Publication Date (Web): 18 Feb 2015 Downloaded from http://pubs.acs.org on February 20, 2015
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LeadLead-Halide Perovskite Solar Cells by CH3NH3I-Dripping on PbI2CH3NH3I-DMSO Precursor Layer for Plana Planar and Porous Structures Structures using CuSCN Hole Transporting Material Seigo Ito1*, Soichiro Tanaka1, and Hitoshi Nishino2
1
Department of Electric Engineering and Computer Science, School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hypogo 671-2280, Japan. 2
Energy Technology Laboratories, Osaka Gas Co., Ltd., 6-19-9 Konohana-Ku, Osaka 5540051, Japan.
Abstract In order to fabricate planar-structure CH3NH3PbI3 perovskite solar cells using CuSCN hole transporting material (HTM), the sequential fabrication scheme of the CH3NH3PbI3 layer has been improved. In the PbI2 layer fabricated by the spin-coating method, at first, small amounts of CH3NH3I (MAI) and DMSO were incorporated as the first-drip precursor layer on a flat TiO2 layer. On the first-drip precursor layers, a MAI solution was applied by either soaking (MAIsoaking method) or dripping using successive spin coating (MAI-dripping). The morphology and crystal transformations were observed by SEM and XRD, respectively. Using the normal sequential MAI-soaking method, we were unable to fabricate planar CH3NH3PbI3 perovskite solar cells with CuSCN HTM.
Using the MAI-dripping method, however, a significant
photovoltaic effect has been observed to be planar solar cells.
*Corresponding author’s email: [email protected]
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(TOC)
Keywords: Copper thiocyanate, flat TiO2, mesoporous titania, interdiffusion
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Recently, lead-based perovskites (CH3NH3PbX3 (X: Cl, Br, or I)) have emerged as a new, more cost-effective solar cell
1-4
. For the cost-effective fabrication of perovskite solar cells, CuSCN
has been applied as an inexpensive inorganic hole transporting material (HTM)
5-9
. Although
the perovskite solar cells with CuSCN can perform 12.4% conversions efficiency using porous TiO2 electron transporting layer 8, due to the short circuit caused by an interdiffusion between CH3NH3PbI3
and
CuSCN,
the
conversion
efficiency
of
planar-structured
was close to zero 7. Since the planar-structured CH3NH3PbI3 perovskite solar cells using organic-HTM (spiro OMe-TAD or P3TH) have gathered interest 1015
, it may be significant if planar-structured CH3NH3PbI3 solar cells using inorganic-HTM
(CuSCN) can be fabricated. Already, one step method for planar-structured CH3NH3PbI3 solar cells using CuSCN have been fabricated by one-step deposition with 6.4% conversion efficiency 9, in this paper, the new fabrication method for the planar-structured solar cells is disclosed with 7.2% conversion efficiency, which can be adapted to porous TiO2 electrode, as well. We have mixed two-types of CH3NH3PbI3 perovskite deposition methods: the two-drop spin-coating method
16, 17
and DMSO-incorporated method
18, 19
. The Park and Huang groups
founded the two-drop spin-coating method 16, 17, which is the sequential deposition 3 of PbI2 and CH3NH3I (methylannmonium iodide: MAI) during the one spin-coating procedure. Using the two-drop spin-coating method, the CH3NH3PbI3 perovskite crystal became larger, and the photovoltaic effect improved up to 17.0% conversion efficiency. The Seok and Han groups founded the DMSO-containing perovskite precursor
18, 19
, which can be changed to a smooth
CH3NH3PbI3-perovskite layer and high-efficiency CH3NH3PbI3 perovskite solar cells with 16.2% conversion efficiency. We have mixed these two methods by the sequential dripping deposition of MAI on a DMSO-MAI-containing perovskite precursor (named as “MAIdripping”), resulting in a less-pinhole perovskite layer which prohibits shunt contact between 3
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CH3NH3PbI3 and CuSCN. The precursor solution was modified to be the mixture of PbI2, MAI in a mixed solvent of DMF and DMSO (9/1, v/v). A small addition of MAI and DMSO can help the dissolution of PbI2 into the solvent (Fig. 1a [left]). In actuality, without MAI and DMSO, pure DMF is unable to easily dissolve PbI2 (Fig. 1a [right]). After proper purification, the PbI2 can be dissolved into solvents 20. Only a small amount of MAI in the precursor solution formed the CH3NH3PbI3-perovskite crystal (the ratio of MAI/PbI2 was only 0.24).
The
sequential deposition by MAI-dripping has been performed to fabricate good quality perovskite layers for the deposition of interdiffusing CuSCN HTM.
For comparison, a sequential
deposition of the precursor layer was performed by soaking it into MAI solution (named as “MAI-soaking”). The MAI-soaking deposition remained as the pinholes in the perovskite layer, resulting in the solar cells with quite low photovoltaic effect with CuSCN HTM. On the other hand, the MAI-dripping deposition formed less-pinhole perovskite layers, which can work for the planar-structured solar cells. This is the first report regarding the planar-structured sequential-deposited perovskite solar cells using CuSCN HTM.
F-doped SnO2 (FTO) glass (TEC-15, t = 2 mm, NSG-Pilkington) substrates were cleaned by ultrasonication with detergent water and rinsed with distilled water and ethanol, successively. The layers of TiO2, CH3NH3PbI3 and CuSCN were deposited in a normal atmosphere (without using a glove box). For the coating of dense (blocking) TiO2 layers on the cleaned FTO, a spray-pyrolysis solution was prepared with titanium di-isopropoxide bis (acetylacetonate) (TAA: 0.3 mL) in ethanol (4 mL, Kanto Chemical Co., Inc., Japan). The TAA solution was prepared by pouring 2-molar acetylacetone (Wako Pure Chemical Industries, Ltd., Japan) into 1-molar titanium isopropoxide (Kanto Chemical Co., Inc., Japan). The spray-pyrolysis solution (4.3 mL) was sprayed on the cleaned FTO on a hotplate at 450 °C. These dense TiO2 layers can be used as the flat‒TiO2 electrodes for the CH3NH3PbI3 perovskite solar cells.
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For the fabrication of nanocrystalline TiO2 (d = 36 nm) layers, at first, a TiO2 paste was prepared according to the literature 21. After the dilution of the TiO2 paste into the ethanol (TiO2 paste : ethanol = 1:3.5, v/v), the TiO2 colloidal solution was deposited on a dense TiO2 layer by the spin coating method (acceleration in 5 sec, and spun at 5000 rpm for 25 sec) and annealed at 500 ˚C for 30 min. The sizes of the FTO substrate and porous-TiO2 areas were 17 × 25 mm2 and 9 × 9 mm2, respectively. For perovskite-precursor deposition, PbI2 (1.3 M) and CH3NH3I (0.31 M) were dissolved into a mixed solution of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) (9:1, v/v) (Fig. 1a). For preparation of the CH3NH3PbI3 layer, the perovskite-precursor solution (50 µL) was applied on the TiO2 substrate and spread by the spin coating (Fig. 1b). For each PbI2 deposition by spin coating, the TiO2–coated substrates were heated beforehand using a hotplate at 70 °C. After deposition of the PbI2 solution, the substrates were spun at 6,500 rpm per 2 sec and remained spinning for 13 sec. For the MAI-dripping method, the speed was slowed to 4,000 rpm per 1 sec, and a 75 µL MAI solution (0.063 M in isopropanol) was dripped on the spinning substrates. The spinning was stopped 39 sec after the MAI-dripping. After the spin coating, the perovskite layer was heated on a hotplate at 100 ˚C for 15 min. The MAI-soaking process was performed for comparison (Fig. 1b). Just after spinning the precursor layer at 6,500 rpm for 13 sec, the substrates were removed from the spin coater as the “first-drop precursor layer.” The precursor layer soaked into a solution of CH3NH3I in 2propanol (0.063 M) for 20 s, and then rinsed into a 2-propanol solution and spun to dry at 4000 rpm for 8 sec (2 sec acceleration). Finally, the deposited CH3NH3PbI3 film was dried on a hotplate at 70 ˚C for 30 min. A CuSCN hole conductor was deposited on CH3NH3PbI3 layers by the doctor blading method at 65 ˚C (CuSCN solution was prepared by dissolving 6 mg of CuSCN (Kishida Chemical Co. Ltd.) into 1 mL of propyl sulfide)
5-8, 22
. Lastly, gold was evaporated on top to
form a gold layer with a thickness of ca. 50 nm as a back contact for all films. The crystal 5
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structures were characterized using X-ray diffraction (Miniflex II, Rigaku) with Cu Kα radiation. Photocurrent density‒voltage (J‒V) curves, with an area of 0.25 cm2, were measured for the sample by applying an external bias to the cell from the 0 V to 1 V (forward bias scan) and measuring the generated photocurrent with a DC voltage current source (6240, ADCMT Co. Ltd., Japan) under a solar simulator (AM1.5, 100 mW cm-2, calibrated by a standard photo detector [Bunkou Keiki Co. Ltd., Japan]) equipped with a 500 W Xe lamp (YSS-100A, Yamashita Denso Co. Ltd., Japan). We have confirmed the measurement accuracy using metal mask to regulate the light-irradiation area. In order consider the photovoltaic effects of solar cells, we have eliminate the results of revers bias voltage scan, which can be the overestimation of the results
23
. Therefore, every result in the manuscript is obtained only by forward bias
voltage scan, which should be believed in the scientific view point.
Figure 2 shows the surfaces and cross sections of deposited layers (first-drip precursor layer [PbI2-MAI-DMSO], MAI-soaking layer and MAI-dripping layer) without porous TiO2 layers. From the top view, the first-drip precursor layer of PbI2-MAI-DMSO was very smooth and flat. In the cross-section view, however, the existence of small particles in the layer can be confirmed, which would be the PbI2 precursor crystal (ca. 100 nm). By MAI-soaking method, perovskite nanoparticles with a 200-400 nm diameter were formed, which can be observed from the top view. From the cross-section view, the thickness of the perovskite layer was formed by only one to two of the nanoparticles. Hence, the layer looks to form small holes which can induce the short contact between compact TiO2 and CuSCN. Contrarily, by the MAI-dripping method (Fig. 2e), the perovskite crystal enlarged to ca. 1 µm, and smooth perovskite film covered the surface of planar TiO2 opened between the large perovskite crystals, which may prohibit short contact. Figure 3 shows XRD patterns of the TiO2, PbI2, first-drip layer, perovskite layers by MAI-soaking and MAI-dripping methods. Although an arbitrary unit (a.u.) has been utilized for 6
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the Y-axis in order to show all XRD patterns in Fig. 3, the relationship of each peak height was kept from each background. Therefore, we can compare each peak height in Fig. 3. The main XRD peaks of PbI2 were 12.9, 38.0 and 51.8 degrees. In contrast, the XRD peaks of the firstdrip precursor layer were 12.8, 38.8 and 52.3 degrees. The shift of XRD peaks from PbI2 to the first-drip precursor may be due to the intercalation of MAI and DMSO. In fact, because of the presence of MAI in the first-drip precursor, a small peak at 14.2 degrees, which presents the perovskite crystal, was observed. The intense peak of PbI2 in the first-drip precursor was converted by the application of MAI (MAI-dripping and MAI-soaking). Although the peak of perovskite at 14.3 degrees by MAI-dripping was higher than that by MAI-soaking, the remaining PbI2 peak by MAI-soaking was lower than that by MAI-dripping. Mainly, the other peaks showing perovskite crystals by the MAI-dripping method (20.2, 28.7, and 40.8 degrees) were more intense than those by the MAI-soaking method. Figure 4 shows the current-voltage curves of solar cells using the MAI-dripping or MAI-soaking methods. Each photovoltaic characteristic was summarized in Table 1 using three cells. The order of IPCE largeness (Figure S1) is compatible with the JSC in Table 1. Using the MAI-soaking method, the photocurrents were very close to zero without porous TiO2 layer, as published 7, resulting in low conversion efficiencies. This low photovoltaic effect is due to a short circuit, which was caused by the diffusion of CuSCN into the perovskite layer 7, 8. However, using the MAI-dripping method, the photocurrent density of the planar structure improved by up to 17 mA cm-2, due to the inhabitation of such a short circuit caused by the flat TiO2 layer. Hence, it was confirmed that the short circuit occurring using CuSCN on perovskite layers was caused by the pinholes in the perovskite layer, which can be eliminated by the fabrication of a pinhole-free perovskite layer. However, the fill factors of using a planar TiO2 layer were lower than those using a porous TiO2 layer. Therefore, the inhabitation of a short circuit can be performed much more effectively by a porous TiO2 layer than by a pinhole-free perovskite layer with the MAI7
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dripping method. Consequently, the best photovoltaic effect achieved by using the MAIdripping method on a porous–TiO2 layer for CuSCN HTM. As a comparison, experiments using the MAI-dripping method on first-drip precursor layers without MAI and/or DMSO have been performed (in Table 1). It can be noticed that, although the cell pattern number 3 performed low VOC of 0.25 V, other cells did not changed the
VOC with the difference of TiO2 structure (porous or planar), in spite of different first-drip precursor layers. The TiO2 structure varied specially FF. The low FF is due to the planar TiO2 structure. The additives of DMSO and MAI varied JSC. Without the two additives of DMSO and MAI at the same time, the JSC of planar cells can be quite low around 1.5 mA cm-2, resulting in the low conversion efficiency around 0.5%. The function of DMSO was reported to be the retardation of CH3NH3PbI3 rapid crystallization 18, 19. The function of MAI in PbI2 may be the enhancement of the CH3NH3PbI3 crystallization, because the small CH3NH3PbI3 peak at 14.2 degree can be observed on the XRD pattern of first-drop precursor in Figure 3. Moreover, the MAI can slow down the rapid recrystallization at the spin-coating of PbI2 layer, because MAI can help the dissolution of DMF solvent (as Figure 1a). These points may be the beneficial effects of MAI and DMSO in the first drip precursor. The porous TiO2 layer can partially prohibit the short circuit between TiO2 underlayer and CuSCN HTM, but it’s not perfect 7. Hence, the smooth perovskite layer due to MAI and DMSO in PbI2 layer can enhance the shunt resistance of cell, resulting in the high photovoltaic effects (Table 1). In conclusion, by using CuSCN hole transporting material, planar CH3NH3PbI3 perovskite solar cells have been fabricated at the first time by sequential deposition: MAIdripping method on PbI2-MAI-DMSO precursor layer (Fig. 1b). In order to obtain photocurrent densities as high as 17 mA cm-2, a smooth less-pinhole CH3NH3PbI3 perovskite layer should be fabricated. It was considered that such a smooth CH3NH3PbI3 perovskite layer would be able to block the short circuit between TiO2 and CuSCN. However, the fill factors of planar structure 8
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solar cells were not as high as those with porous TiO2 layers, due to the partial short circuit in the planar solar cells. Therefore, although a portion of the short circuit has been blocked using the smooth CH3NH3PbI3 perovskite layer, the prohibition of short circuits has not been perfected.
In order to improve the fill factor, new perovskite material to prohibit the
interdiffusion with CuSCN should be found. We hope that this finding can provide a future contribution for the industrial fabrication of low-cost planar perovskite solar cells using role-torole processing on flexible substrates.
Acknowledgements A part of this research was supported by Advanced Low Carbon Technology Research and Development Program, Japan Science and Technology Agency (ALCA-JST).
References (1) Kim, H-S.; Lee, C-R.; Im, J-H.; Lee, K-B.; Moehl, T.; Marchioro, A.; Moon, S-J.; HumphryBaker, R.: Yum, J-H.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338 643−647. (3) 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. (4) 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. (5) Ito, S.; Tanaka, S.; Vahlman, H.; Nishino, H.; Manabe, K.; Lund, P. Carbon-Double-Bond-Free Printed Solar Cells from TiO2/CH3NH3PbI3/CuSCN/Au: Structural Control and Photoaging Effects. ChemPhysChem, 2014, 15, 1194-2000. (6) Ito, S.; Tanaka, S.; Manabe, K.; Nishino, H. Effects of Surface Blocking Layer of Sb2S3 on Nanocrystalline TiO2 for CH3NH3PbI3 Perovskite Solar Cells. J. Phys. Chem. C, 2014, 118, 16995-17000. (7) Murugadoss, G.; Mizuta, G.; Tanaka, S.; Nishino, H.; Umeyama, T.; Imahori, H.; Ito, S. Double functions of porous TiO2 electrodes on CH3NH3PbI3 perovskite solar cells: Enhancement of perovskite crystal transformation and prohibition of short circuiting. APL Materials, 2014, 2, 081511. (8) Peng, Q.; 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. Nature Communications 2014, 5, 3834.
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(9) Chavhan, S.; Miguel, O.; Grande, H.-J.; Gonzalez-Pedro, V.; Sánchez, R. S.; Barea, E. M.; MoraSeró, I.; Tena-Zaera, R. Organo-metal halide perovskite-based solar cells with CuSCN as the inorganic hole selective contact. J. Mater. Chem. A, 2014, 2, 12754-12760. (10) Liu, D.; Kelly, T. L. Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nature Photonics, 2014, 8, 133−138. (11) Yella, A.; Heiniger, L.-P.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Nanocrystalline Rutile Electron Extraction Layer Enables Low-Temperature Solution Processed Perovskite Photovoltaics with 13.7% Efficiency. Nano Lett., 2014, 14, 2591–2596. (12) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395−398. (13) 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. (14) You, J.; Hong, Z.; Yang, Y.; Chen, Q.; Cai, M.; Song, T.-B.; Chen, C.-C.; Lu, S.; Liu, Y.; Zhou, H.; et al.; Low-Temperature Solution-Processed Perovskite Solar Cells with High Efficiency and Flexibility. ACS Nano 2014, 8, 1674-1680. (15) Kim, B. J.; Kim, D. H.; Lee, Y.-Y.; Shin, H.-W.; Han, G. S.; Hong, J. S.; Mahmood, K.; Ahn, T. K.; Joo, Y.-C.; Hong, K. S.; et al.; Highly efficient and bending durable perovskite solar cells: toward a wearable power source. Energy Environ. Sci., (2015) DOI: 10.1039/C4EE02441A. (16) 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. Nature Nanotecknology 2014, 9, 927932. (17) Xiao, Z.; Bi, C.; Shao, Y.; Dong, Q.; Wang, Q.; Yuan, Y.; Wang, C.; Gao, Y.; Huang, J. Efficient, high yield perovskite photovoltaic devices grown by interdiffusion of solutionprocessed precursor stacking layers. Energy Environ. Sci., 2014, 7, 2619-2623. (18) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells. Nature Materials, 2014, 13, 897-903. (19) Wu, Y.; Islam, A.; Yang, X.; Qin, C.; Liu, J.; Zhang, K.; Peng, W.; Han, L. Retarding the crystallization of PbI2 for highly reproducible planar-structured perovskite solar cells via sequential deposition. Energy Environ. Sci., 2014, 7, 2934-2938. (20) Wakamiya, A.; Endo, M.; Sasamori, T.; Tokitoh, N.; Ogomi, Y.; Hayase, S.; Murata, Y. Reproducible Fabrication of Efficient Perovskite-based Solar Cells: X-ray Crystallographic Studies on the Formation of CH3NH3PbI3 Layers. Chem. Lett., 2014, 43, 711-713. (21) Ito, S.; Zakeeruddin, S. M.; Comte, P.; Liska, P.; Kuang, D.; Grätzel, M. Bifacial dyesensitized solar cells based on an ionic liquid electrolyte. Nature Photonics 2008, 2, 693-696. (22) Tsujimoto, K.; Nguyen, D.-C.; Ito, S.; Nishino, H.; Matsuyoshi, H.; Konno, A.; Kumara, G. R. A.; Tennakone, K. TiO2 Surface Treatment Effects by Mg2+, Ba2+, and Al3+ on Sb2S3 Extremely Thin Absorber Solar Cellsnic liquid electrolyte. J. Phys. Chem. C., 2013, 116, 13465-13471. (23) Grätzel, M. The light and shade of perovskite solar cells. Nature Materials 2014, 13, 838-842 (commentary); Perovskite fever. Nature Materials 2014, 13, 837 (editorial).
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(Table)
Table 1. Average photovoltaic characteristics of perovskite solar cells using 3 cells. The numbers in the brackets show the results with the best-efficiency cells. Cell pattern number
TiO2 structure
PbI2 additives
1
MAI process Soaking
Porous 2
DMF +DMSO +MAI
3
Dripping Soaking
4 5 6 7
Planar
DMF DMF +DMSO DMF +MAI
Dripping
Jsc / mA cm-2
Voc / V
FF
Efficiency /%
17.73 (18.02) 18.27 (18.23) 0.29 (0.40) 17.41 (18.42) 1.51 (1.76) 1.53 (1.92) 1.65 (2.42)
0.86 (0.87) 0.96 (0.96) 0.25 (0.49) 0.94 (0.97) 0.92 (0.91) 0.92 (0.92) 0.93 (0.93)
0.63 (0.63) 0.66 (0.68) 0.34 (0.39) 0.36 (0.40) 0.38 (0.36) 0.31 (0.28) 0.37 (0.28)
9.57 (9.86) 11.49 (11.96) 0.03 (0.08) 6.00 (7.19) 0.52 (0.55) 0.43 (0.49) 0.53 (0.64)
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(Figures Figures) ures)
(a)
(b)
Figure 1. Fabrication information of perovskite solar cells; (a) Picture of PbI2 in DMF+MAI+DMSO (left) and DMF (right). (b) Fabrication procedures of CH3NH3PbI3 perovskite layer by the “MAI-dripping” and “MAI-soaking” processes. 12
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Figure 2. Surface (a, c, e) and cross sectional (b, d, f) views of SEM of PbI2-DMSOMAI precursor layer (a, b) and perovskite layers by the MAI-soaking (c, d) and MAIdripping (e, f) methods.
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Figure 3. X-ray diffraction patterns of TiO2, PbI2, first-drop precursor layer (PbI2+MAI+DMSO), and perovskite layers (by the MAI-soaking and MAI-dripping methods). The relationship of each peak height was kept from each background for the comparison of the peak height.
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Figure 4. Photocurrent density – voltage curves of CH3NH3PbI3 perovskite solar cells by the MAI-soaking and MAI-dripping methods on with/without porous TiO2 layers.
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Letter 3
3
Lead-Halide Perovskite Solar Cells by CHNHI-Dripping on PbI-CHNHI-DMSO Precursor Layer for Planar and Porous Structures using CuSCN Hole Transporting Material 2
3
3
Seigo Ito, Soichiro Tanaka, and Hitoshi Nishino J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b00122 • Publication Date (Web): 18 Feb 2015 Downloaded from http://pubs.acs.org on February 20, 2015
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LeadLead-Halide Perovskite Solar Cells by CH3NH3I-Dripping on PbI2CH3NH3I-DMSO Precursor Layer for Plana Planar and Porous Structures Structures using CuSCN Hole Transporting Material Seigo Ito1*, Soichiro Tanaka1, and Hitoshi Nishino2
1
Department of Electric Engineering and Computer Science, School of Engineering, University of Hyogo, 2167 Shosha, Himeji, Hypogo 671-2280, Japan. 2
Energy Technology Laboratories, Osaka Gas Co., Ltd., 6-19-9 Konohana-Ku, Osaka 5540051, Japan.
Abstract In order to fabricate planar-structure CH3NH3PbI3 perovskite solar cells using CuSCN hole transporting material (HTM), the sequential fabrication scheme of the CH3NH3PbI3 layer has been improved. In the PbI2 layer fabricated by the spin-coating method, at first, small amounts of CH3NH3I (MAI) and DMSO were incorporated as the first-drip precursor layer on a flat TiO2 layer. On the first-drip precursor layers, a MAI solution was applied by either soaking (MAIsoaking method) or dripping using successive spin coating (MAI-dripping). The morphology and crystal transformations were observed by SEM and XRD, respectively. Using the normal sequential MAI-soaking method, we were unable to fabricate planar CH3NH3PbI3 perovskite solar cells with CuSCN HTM.
Using the MAI-dripping method, however, a significant
photovoltaic effect has been observed to be planar solar cells.
*Corresponding author’s email: [email protected]
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(TOC)
Keywords: Copper thiocyanate, flat TiO2, mesoporous titania, interdiffusion
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Recently, lead-based perovskites (CH3NH3PbX3 (X: Cl, Br, or I)) have emerged as a new, more cost-effective solar cell
1-4
. For the cost-effective fabrication of perovskite solar cells, CuSCN
has been applied as an inexpensive inorganic hole transporting material (HTM)
5-9
. Although
the perovskite solar cells with CuSCN can perform 12.4% conversions efficiency using porous TiO2 electron transporting layer 8, due to the short circuit caused by an interdiffusion between CH3NH3PbI3
and
CuSCN,
the
conversion
efficiency
of
planar-structured
was close to zero 7. Since the planar-structured CH3NH3PbI3 perovskite solar cells using organic-HTM (spiro OMe-TAD or P3TH) have gathered interest 1015
, it may be significant if planar-structured CH3NH3PbI3 solar cells using inorganic-HTM
(CuSCN) can be fabricated. Already, one step method for planar-structured CH3NH3PbI3 solar cells using CuSCN have been fabricated by one-step deposition with 6.4% conversion efficiency 9, in this paper, the new fabrication method for the planar-structured solar cells is disclosed with 7.2% conversion efficiency, which can be adapted to porous TiO2 electrode, as well. We have mixed two-types of CH3NH3PbI3 perovskite deposition methods: the two-drop spin-coating method
16, 17
and DMSO-incorporated method
18, 19
. The Park and Huang groups
founded the two-drop spin-coating method 16, 17, which is the sequential deposition 3 of PbI2 and CH3NH3I (methylannmonium iodide: MAI) during the one spin-coating procedure. Using the two-drop spin-coating method, the CH3NH3PbI3 perovskite crystal became larger, and the photovoltaic effect improved up to 17.0% conversion efficiency. The Seok and Han groups founded the DMSO-containing perovskite precursor
18, 19
, which can be changed to a smooth
CH3NH3PbI3-perovskite layer and high-efficiency CH3NH3PbI3 perovskite solar cells with 16.2% conversion efficiency. We have mixed these two methods by the sequential dripping deposition of MAI on a DMSO-MAI-containing perovskite precursor (named as “MAIdripping”), resulting in a less-pinhole perovskite layer which prohibits shunt contact between 3
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CH3NH3PbI3 and CuSCN. The precursor solution was modified to be the mixture of PbI2, MAI in a mixed solvent of DMF and DMSO (9/1, v/v). A small addition of MAI and DMSO can help the dissolution of PbI2 into the solvent (Fig. 1a [left]). In actuality, without MAI and DMSO, pure DMF is unable to easily dissolve PbI2 (Fig. 1a [right]). After proper purification, the PbI2 can be dissolved into solvents 20. Only a small amount of MAI in the precursor solution formed the CH3NH3PbI3-perovskite crystal (the ratio of MAI/PbI2 was only 0.24).
The
sequential deposition by MAI-dripping has been performed to fabricate good quality perovskite layers for the deposition of interdiffusing CuSCN HTM.
For comparison, a sequential
deposition of the precursor layer was performed by soaking it into MAI solution (named as “MAI-soaking”). The MAI-soaking deposition remained as the pinholes in the perovskite layer, resulting in the solar cells with quite low photovoltaic effect with CuSCN HTM. On the other hand, the MAI-dripping deposition formed less-pinhole perovskite layers, which can work for the planar-structured solar cells. This is the first report regarding the planar-structured sequential-deposited perovskite solar cells using CuSCN HTM.
F-doped SnO2 (FTO) glass (TEC-15, t = 2 mm, NSG-Pilkington) substrates were cleaned by ultrasonication with detergent water and rinsed with distilled water and ethanol, successively. The layers of TiO2, CH3NH3PbI3 and CuSCN were deposited in a normal atmosphere (without using a glove box). For the coating of dense (blocking) TiO2 layers on the cleaned FTO, a spray-pyrolysis solution was prepared with titanium di-isopropoxide bis (acetylacetonate) (TAA: 0.3 mL) in ethanol (4 mL, Kanto Chemical Co., Inc., Japan). The TAA solution was prepared by pouring 2-molar acetylacetone (Wako Pure Chemical Industries, Ltd., Japan) into 1-molar titanium isopropoxide (Kanto Chemical Co., Inc., Japan). The spray-pyrolysis solution (4.3 mL) was sprayed on the cleaned FTO on a hotplate at 450 °C. These dense TiO2 layers can be used as the flat‒TiO2 electrodes for the CH3NH3PbI3 perovskite solar cells.
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For the fabrication of nanocrystalline TiO2 (d = 36 nm) layers, at first, a TiO2 paste was prepared according to the literature 21. After the dilution of the TiO2 paste into the ethanol (TiO2 paste : ethanol = 1:3.5, v/v), the TiO2 colloidal solution was deposited on a dense TiO2 layer by the spin coating method (acceleration in 5 sec, and spun at 5000 rpm for 25 sec) and annealed at 500 ˚C for 30 min. The sizes of the FTO substrate and porous-TiO2 areas were 17 × 25 mm2 and 9 × 9 mm2, respectively. For perovskite-precursor deposition, PbI2 (1.3 M) and CH3NH3I (0.31 M) were dissolved into a mixed solution of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) (9:1, v/v) (Fig. 1a). For preparation of the CH3NH3PbI3 layer, the perovskite-precursor solution (50 µL) was applied on the TiO2 substrate and spread by the spin coating (Fig. 1b). For each PbI2 deposition by spin coating, the TiO2–coated substrates were heated beforehand using a hotplate at 70 °C. After deposition of the PbI2 solution, the substrates were spun at 6,500 rpm per 2 sec and remained spinning for 13 sec. For the MAI-dripping method, the speed was slowed to 4,000 rpm per 1 sec, and a 75 µL MAI solution (0.063 M in isopropanol) was dripped on the spinning substrates. The spinning was stopped 39 sec after the MAI-dripping. After the spin coating, the perovskite layer was heated on a hotplate at 100 ˚C for 15 min. The MAI-soaking process was performed for comparison (Fig. 1b). Just after spinning the precursor layer at 6,500 rpm for 13 sec, the substrates were removed from the spin coater as the “first-drop precursor layer.” The precursor layer soaked into a solution of CH3NH3I in 2propanol (0.063 M) for 20 s, and then rinsed into a 2-propanol solution and spun to dry at 4000 rpm for 8 sec (2 sec acceleration). Finally, the deposited CH3NH3PbI3 film was dried on a hotplate at 70 ˚C for 30 min. A CuSCN hole conductor was deposited on CH3NH3PbI3 layers by the doctor blading method at 65 ˚C (CuSCN solution was prepared by dissolving 6 mg of CuSCN (Kishida Chemical Co. Ltd.) into 1 mL of propyl sulfide)
5-8, 22
. Lastly, gold was evaporated on top to
form a gold layer with a thickness of ca. 50 nm as a back contact for all films. The crystal 5
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structures were characterized using X-ray diffraction (Miniflex II, Rigaku) with Cu Kα radiation. Photocurrent density‒voltage (J‒V) curves, with an area of 0.25 cm2, were measured for the sample by applying an external bias to the cell from the 0 V to 1 V (forward bias scan) and measuring the generated photocurrent with a DC voltage current source (6240, ADCMT Co. Ltd., Japan) under a solar simulator (AM1.5, 100 mW cm-2, calibrated by a standard photo detector [Bunkou Keiki Co. Ltd., Japan]) equipped with a 500 W Xe lamp (YSS-100A, Yamashita Denso Co. Ltd., Japan). We have confirmed the measurement accuracy using metal mask to regulate the light-irradiation area. In order consider the photovoltaic effects of solar cells, we have eliminate the results of revers bias voltage scan, which can be the overestimation of the results
23
. Therefore, every result in the manuscript is obtained only by forward bias
voltage scan, which should be believed in the scientific view point.
Figure 2 shows the surfaces and cross sections of deposited layers (first-drip precursor layer [PbI2-MAI-DMSO], MAI-soaking layer and MAI-dripping layer) without porous TiO2 layers. From the top view, the first-drip precursor layer of PbI2-MAI-DMSO was very smooth and flat. In the cross-section view, however, the existence of small particles in the layer can be confirmed, which would be the PbI2 precursor crystal (ca. 100 nm). By MAI-soaking method, perovskite nanoparticles with a 200-400 nm diameter were formed, which can be observed from the top view. From the cross-section view, the thickness of the perovskite layer was formed by only one to two of the nanoparticles. Hence, the layer looks to form small holes which can induce the short contact between compact TiO2 and CuSCN. Contrarily, by the MAI-dripping method (Fig. 2e), the perovskite crystal enlarged to ca. 1 µm, and smooth perovskite film covered the surface of planar TiO2 opened between the large perovskite crystals, which may prohibit short contact. Figure 3 shows XRD patterns of the TiO2, PbI2, first-drip layer, perovskite layers by MAI-soaking and MAI-dripping methods. Although an arbitrary unit (a.u.) has been utilized for 6
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the Y-axis in order to show all XRD patterns in Fig. 3, the relationship of each peak height was kept from each background. Therefore, we can compare each peak height in Fig. 3. The main XRD peaks of PbI2 were 12.9, 38.0 and 51.8 degrees. In contrast, the XRD peaks of the firstdrip precursor layer were 12.8, 38.8 and 52.3 degrees. The shift of XRD peaks from PbI2 to the first-drip precursor may be due to the intercalation of MAI and DMSO. In fact, because of the presence of MAI in the first-drip precursor, a small peak at 14.2 degrees, which presents the perovskite crystal, was observed. The intense peak of PbI2 in the first-drip precursor was converted by the application of MAI (MAI-dripping and MAI-soaking). Although the peak of perovskite at 14.3 degrees by MAI-dripping was higher than that by MAI-soaking, the remaining PbI2 peak by MAI-soaking was lower than that by MAI-dripping. Mainly, the other peaks showing perovskite crystals by the MAI-dripping method (20.2, 28.7, and 40.8 degrees) were more intense than those by the MAI-soaking method. Figure 4 shows the current-voltage curves of solar cells using the MAI-dripping or MAI-soaking methods. Each photovoltaic characteristic was summarized in Table 1 using three cells. The order of IPCE largeness (Figure S1) is compatible with the JSC in Table 1. Using the MAI-soaking method, the photocurrents were very close to zero without porous TiO2 layer, as published 7, resulting in low conversion efficiencies. This low photovoltaic effect is due to a short circuit, which was caused by the diffusion of CuSCN into the perovskite layer 7, 8. However, using the MAI-dripping method, the photocurrent density of the planar structure improved by up to 17 mA cm-2, due to the inhabitation of such a short circuit caused by the flat TiO2 layer. Hence, it was confirmed that the short circuit occurring using CuSCN on perovskite layers was caused by the pinholes in the perovskite layer, which can be eliminated by the fabrication of a pinhole-free perovskite layer. However, the fill factors of using a planar TiO2 layer were lower than those using a porous TiO2 layer. Therefore, the inhabitation of a short circuit can be performed much more effectively by a porous TiO2 layer than by a pinhole-free perovskite layer with the MAI7
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dripping method. Consequently, the best photovoltaic effect achieved by using the MAIdripping method on a porous–TiO2 layer for CuSCN HTM. As a comparison, experiments using the MAI-dripping method on first-drip precursor layers without MAI and/or DMSO have been performed (in Table 1). It can be noticed that, although the cell pattern number 3 performed low VOC of 0.25 V, other cells did not changed the
VOC with the difference of TiO2 structure (porous or planar), in spite of different first-drip precursor layers. The TiO2 structure varied specially FF. The low FF is due to the planar TiO2 structure. The additives of DMSO and MAI varied JSC. Without the two additives of DMSO and MAI at the same time, the JSC of planar cells can be quite low around 1.5 mA cm-2, resulting in the low conversion efficiency around 0.5%. The function of DMSO was reported to be the retardation of CH3NH3PbI3 rapid crystallization 18, 19. The function of MAI in PbI2 may be the enhancement of the CH3NH3PbI3 crystallization, because the small CH3NH3PbI3 peak at 14.2 degree can be observed on the XRD pattern of first-drop precursor in Figure 3. Moreover, the MAI can slow down the rapid recrystallization at the spin-coating of PbI2 layer, because MAI can help the dissolution of DMF solvent (as Figure 1a). These points may be the beneficial effects of MAI and DMSO in the first drip precursor. The porous TiO2 layer can partially prohibit the short circuit between TiO2 underlayer and CuSCN HTM, but it’s not perfect 7. Hence, the smooth perovskite layer due to MAI and DMSO in PbI2 layer can enhance the shunt resistance of cell, resulting in the high photovoltaic effects (Table 1). In conclusion, by using CuSCN hole transporting material, planar CH3NH3PbI3 perovskite solar cells have been fabricated at the first time by sequential deposition: MAIdripping method on PbI2-MAI-DMSO precursor layer (Fig. 1b). In order to obtain photocurrent densities as high as 17 mA cm-2, a smooth less-pinhole CH3NH3PbI3 perovskite layer should be fabricated. It was considered that such a smooth CH3NH3PbI3 perovskite layer would be able to block the short circuit between TiO2 and CuSCN. However, the fill factors of planar structure 8
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solar cells were not as high as those with porous TiO2 layers, due to the partial short circuit in the planar solar cells. Therefore, although a portion of the short circuit has been blocked using the smooth CH3NH3PbI3 perovskite layer, the prohibition of short circuits has not been perfected.
In order to improve the fill factor, new perovskite material to prohibit the
interdiffusion with CuSCN should be found. We hope that this finding can provide a future contribution for the industrial fabrication of low-cost planar perovskite solar cells using role-torole processing on flexible substrates.
Acknowledgements A part of this research was supported by Advanced Low Carbon Technology Research and Development Program, Japan Science and Technology Agency (ALCA-JST).
References (1) Kim, H-S.; Lee, C-R.; Im, J-H.; Lee, K-B.; Moehl, T.; Marchioro, A.; Moon, S-J.; HumphryBaker, R.: Yum, J-H.; et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (2) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338 643−647. (3) 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. (4) 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. (5) Ito, S.; Tanaka, S.; Vahlman, H.; Nishino, H.; Manabe, K.; Lund, P. Carbon-Double-Bond-Free Printed Solar Cells from TiO2/CH3NH3PbI3/CuSCN/Au: Structural Control and Photoaging Effects. ChemPhysChem, 2014, 15, 1194-2000. (6) Ito, S.; Tanaka, S.; Manabe, K.; Nishino, H. Effects of Surface Blocking Layer of Sb2S3 on Nanocrystalline TiO2 for CH3NH3PbI3 Perovskite Solar Cells. J. Phys. Chem. C, 2014, 118, 16995-17000. (7) Murugadoss, G.; Mizuta, G.; Tanaka, S.; Nishino, H.; Umeyama, T.; Imahori, H.; Ito, S. Double functions of porous TiO2 electrodes on CH3NH3PbI3 perovskite solar cells: Enhancement of perovskite crystal transformation and prohibition of short circuiting. APL Materials, 2014, 2, 081511. (8) Peng, Q.; 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. Nature Communications 2014, 5, 3834.
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(9) Chavhan, S.; Miguel, O.; Grande, H.-J.; Gonzalez-Pedro, V.; Sánchez, R. S.; Barea, E. M.; MoraSeró, I.; Tena-Zaera, R. Organo-metal halide perovskite-based solar cells with CuSCN as the inorganic hole selective contact. J. Mater. Chem. A, 2014, 2, 12754-12760. (10) Liu, D.; Kelly, T. L. Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nature Photonics, 2014, 8, 133−138. (11) Yella, A.; Heiniger, L.-P.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Nanocrystalline Rutile Electron Extraction Layer Enables Low-Temperature Solution Processed Perovskite Photovoltaics with 13.7% Efficiency. Nano Lett., 2014, 14, 2591–2596. (12) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395−398. (13) 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. (14) You, J.; Hong, Z.; Yang, Y.; Chen, Q.; Cai, M.; Song, T.-B.; Chen, C.-C.; Lu, S.; Liu, Y.; Zhou, H.; et al.; Low-Temperature Solution-Processed Perovskite Solar Cells with High Efficiency and Flexibility. ACS Nano 2014, 8, 1674-1680. (15) Kim, B. J.; Kim, D. H.; Lee, Y.-Y.; Shin, H.-W.; Han, G. S.; Hong, J. S.; Mahmood, K.; Ahn, T. K.; Joo, Y.-C.; Hong, K. S.; et al.; Highly efficient and bending durable perovskite solar cells: toward a wearable power source. Energy Environ. Sci., (2015) DOI: 10.1039/C4EE02441A. (16) 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. Nature Nanotecknology 2014, 9, 927932. (17) Xiao, Z.; Bi, C.; Shao, Y.; Dong, Q.; Wang, Q.; Yuan, Y.; Wang, C.; Gao, Y.; Huang, J. Efficient, high yield perovskite photovoltaic devices grown by interdiffusion of solutionprocessed precursor stacking layers. Energy Environ. Sci., 2014, 7, 2619-2623. (18) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cells. Nature Materials, 2014, 13, 897-903. (19) Wu, Y.; Islam, A.; Yang, X.; Qin, C.; Liu, J.; Zhang, K.; Peng, W.; Han, L. Retarding the crystallization of PbI2 for highly reproducible planar-structured perovskite solar cells via sequential deposition. Energy Environ. Sci., 2014, 7, 2934-2938. (20) Wakamiya, A.; Endo, M.; Sasamori, T.; Tokitoh, N.; Ogomi, Y.; Hayase, S.; Murata, Y. Reproducible Fabrication of Efficient Perovskite-based Solar Cells: X-ray Crystallographic Studies on the Formation of CH3NH3PbI3 Layers. Chem. Lett., 2014, 43, 711-713. (21) Ito, S.; Zakeeruddin, S. M.; Comte, P.; Liska, P.; Kuang, D.; Grätzel, M. Bifacial dyesensitized solar cells based on an ionic liquid electrolyte. Nature Photonics 2008, 2, 693-696. (22) Tsujimoto, K.; Nguyen, D.-C.; Ito, S.; Nishino, H.; Matsuyoshi, H.; Konno, A.; Kumara, G. R. A.; Tennakone, K. TiO2 Surface Treatment Effects by Mg2+, Ba2+, and Al3+ on Sb2S3 Extremely Thin Absorber Solar Cellsnic liquid electrolyte. J. Phys. Chem. C., 2013, 116, 13465-13471. (23) Grätzel, M. The light and shade of perovskite solar cells. Nature Materials 2014, 13, 838-842 (commentary); Perovskite fever. Nature Materials 2014, 13, 837 (editorial).
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(Table)
Table 1. Average photovoltaic characteristics of perovskite solar cells using 3 cells. The numbers in the brackets show the results with the best-efficiency cells. Cell pattern number
TiO2 structure
PbI2 additives
1
MAI process Soaking
Porous 2
DMF +DMSO +MAI
3
Dripping Soaking
4 5 6 7
Planar
DMF DMF +DMSO DMF +MAI
Dripping
Jsc / mA cm-2
Voc / V
FF
Efficiency /%
17.73 (18.02) 18.27 (18.23) 0.29 (0.40) 17.41 (18.42) 1.51 (1.76) 1.53 (1.92) 1.65 (2.42)
0.86 (0.87) 0.96 (0.96) 0.25 (0.49) 0.94 (0.97) 0.92 (0.91) 0.92 (0.92) 0.93 (0.93)
0.63 (0.63) 0.66 (0.68) 0.34 (0.39) 0.36 (0.40) 0.38 (0.36) 0.31 (0.28) 0.37 (0.28)
9.57 (9.86) 11.49 (11.96) 0.03 (0.08) 6.00 (7.19) 0.52 (0.55) 0.43 (0.49) 0.53 (0.64)
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(Figures Figures) ures)
(a)
(b)
Figure 1. Fabrication information of perovskite solar cells; (a) Picture of PbI2 in DMF+MAI+DMSO (left) and DMF (right). (b) Fabrication procedures of CH3NH3PbI3 perovskite layer by the “MAI-dripping” and “MAI-soaking” processes. 12
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Figure 2. Surface (a, c, e) and cross sectional (b, d, f) views of SEM of PbI2-DMSOMAI precursor layer (a, b) and perovskite layers by the MAI-soaking (c, d) and MAIdripping (e, f) methods.
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Figure 3. X-ray diffraction patterns of TiO2, PbI2, first-drop precursor layer (PbI2+MAI+DMSO), and perovskite layers (by the MAI-soaking and MAI-dripping methods). The relationship of each peak height was kept from each background for the comparison of the peak height.
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Figure 4. Photocurrent density – voltage curves of CH3NH3PbI3 perovskite solar cells by the MAI-soaking and MAI-dripping methods on with/without porous TiO2 layers.
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