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Highly efficient perovskite solar cells based on mechanically durable molybdenum cathode
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Noble metal-free mesoscopic perovskite solar Cells with High efficiency and mechanical durability Inyoung Jeong, Hae Jin Kim, Byung-Seok Lee, Hae Jung Son, Jin Young Kim, Doh-Kwon Lee, Dae-Eun Kim, Jinwoo Lee, Min Jae Ko
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S2211-2855(15)00318-3 http://dx.doi.org/10.1016/j.nanoen.2015.07.025 NANOEN926
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Nano Energy
Received date: 8 July 2015 Accepted date: 28 July 2015 Cite this article as: Inyoung Jeong, Hae Jin Kim, Byung-Seok Lee, Hae Jung Son, Jin Young Kim, Doh-Kwon Lee, Dae-Eun Kim, Jinwoo Lee, Min Jae Ko, Noble metal-free mesoscopic perovskite solar Cells with High efficiency and mechanical durability, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.07.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Noble Metal-Free Mesoscopic Perovskite Solar Cells with High Efficiency and Mechanical Durability Inyoung Jeonga,b, Hae Jin Kimc, Byung-Seok Leea,d, Hae Jung Sona, Jin Young Kima,e, Doh-Kwon Leea,e, Dae-Eun Kimc, Jinwoo Leeb,*, Min Jae Koa,e,f,** a
Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea
b
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk 790-784, Republic of Korea
c
School of Mechanical Engineering, Yonsei University, Seoul 120-749, Republic of Korea
d Department of Nanomaterials Science and Engineering, University of Science and Technology (UST), Daejeon, Republic Korea e
Green School, Korea University, Seoul 136-701, Republic of Korea
f
KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Republic of Korea
*Corresponding author. Tel: +82-54-279-2395 **Corresponding author. Tel: +82-2-958-5518 E-mail address: [email protected] (J. Lee), [email protected] (M. J. Ko)
Abstract Noble metal-free mesoscopic perovskite solar cell based on low-cost molybdenum (Mo) cathode has been developed for the first time. By optimizing the thickness of Mo electrode and combination with high quality perovskite layer, the Mo cathode-based perovskite solar cell exhibited a best power conversion efficiency as high as 15.06% with invariance of current density-voltage curves at different scan rates and very small hysteresis according to scan direction. In addition to photovoltaic performances, importance of mechanical durability of a cathode material for perovskite solar cell was highlighted and results from nano-indentation and scratch test indicated that Mo electrode has superior resistance and restoration capability against external scratch or deformation compared to Au electrode. The Mo cathode-based perovskite solar cells have great potential for realizing low cost and high performance as well as mechanical durability.
Keyworkd: Organic-inorganic hybrid perovskite, photovoltaics, cathode, noble metal-free, Molybdenum
1
Introduction Recently, organic–inorganic hybrid perovskites have emerged as promising candidate materials in the field of photovoltaics. Research and development on solar cells based on perovskite-type compounds CH3NH3PbX3 (X = I-, Br-, Cl-, or mixed halides) have experienced unprecedented rapid progress in the past five years due to their unique characteristics, such as high light-absorption coefficient, long-range balanced carrier-diffusion length, high crystallinity, and excellent charge-carrier mobility.[1-13] In addition, perovskite solar cells can be prepared by inexpensive solution processing at low temperatures.[14, 15] From the first report on perovskite-based solar cells having the power conversion efficiency (PCE) of 3.8% by Kojima and co-workers in 2009, very recently, the record efficiency of 20.1% has been certified and it is anticipated that the PCE of perovskite solar cells will be further increased to above 25%, which is the level of state-of-the-art single-crystalline Si solar cells.[1, 16, 17] Therefore, perovskite solar cells satisfying low cost as well as high efficiency became a powerful rival to conventional Si and GaAs solar cells. However, most of state-of-the-art perovskite solar cells typically need expensive noble-metals (Ag or Au) as cathode materials, which raises the cost of perovskite solar cells and thus impedes their commercialization.[18] Therefore, the replacement of noble metals by inexpensive and earth-abundant materials for cathode is highly desirable to reduce the cost of perovskite solar cells. Many groups suggested various carbon nanomaterials as solution.[19-24] Li and coworkers applied laminated carbon nanotubes (CNTs) as the cathode of perovskite solar cells and the best CNT-based cells showed PCE of 9.9% and 6.87% with and without the hole-conductor material spiro-OMeTAD, respectively.[19] Mei et al. developed fully printable monolithic perovskite solar cells using carbon black/graphite composite counter electrode with certified PCE of 12.84% without a hole conductor.[22] However, the resulting open circuit voltage (Voc) and PCE of perovskite solar cells using carbonaceous electrodes are still lower than those of conventional devices equipped with Au electrode, which is mainly attributed to inefficient charge extraction. Meanwhile, replacing the noble metals (Au or Ag) by inexpensive metals with high work function can be a very simple way to reduce cost while retaining the photovoltaic performance. Jiang et al. used nickel as the cathode material of perovskite solar cells and thereby opened the possibility to replace commonly used noble metals (Ag or Au) with non-precious metals.[25] Molybdenum (Mo), one of the metals having a high work function combined with high electrical conductivity, might be a candidate material for low-cost and highly efficient perovskite solar cells. With respect to cost, the unit price of Mo is far lower than 0.1% of that of Au. In addition, Mo is chemically
2
more stable against reactions with halides and oxidation in air compared to Ag and Al, respectively.[26, 27] Motivated by the advantages of Mo, we developed highly efficient perovskite solar cells equipped with Mo cathode for the first time. We realized a PCE of 13.28%, which is comparable with that of Aucathode-based cells (14.07%) without discrepancy in the current density–voltage (J–V) curves measured at different scan rates, and only very small hysteresis is observed with respect to the scan direction. Eventually, the best-performing device exhibited the PCE of 15.06% and the performances of the Mo-cathode-based cells showed good reproducibility with a standard deviation of only 0.91% in efficiency. In addition to the high photovoltaic performance of the Mo-cathode-based devices, the durability of the Mo electrode must be sufficient to ensure high reliability and prolonged lifetime of the device. This is particularly important considering the fact that the outermost electrode layer is exposed to the external environment. In order to assess the durability of Mo, its mechanical and scratchresistance properties were compared to those of Au by using nanoindentation and wear testing methods, respectively. From the superior mechanical properties and wear resistance of Mo compared to those of Au, it was determined that Mo may serve as a durable electrode layer for perovskite solar cells.
Experimental Section Device Fabrication Patterned fluorine-doped tin oxide (FTO) substrates (TEC8, Pilkington) were cleaned by sonication in ethanol, acetone, and 2-propanol, followed by UV-ozone treatment for 20 min. The compact TiO2 layer was coated on the FTO glass by spin-coating the TiO2 precursor-containing titanium diisopropoxide bis(acetylacetonate) solution (75 wt% in isopropanol, Aldrich) mixed with 1-butanol at 2000 rpm for 40 s, followed by heating at 125 °C for 10 min. After heat treating at 500 °C for 30 min, lab-made TiO2 (20 nm) paste was diluted with anhydrous ethanol (2:7 weight ratio), spin-coated on the compact TiO2 layer and dried at 125 °C for 10 min, followed by annealing at 500 °C for 60 min. For the perovskite layer, PbI2 (500 mg, Sigma-Aldrich, 99%) dissolved in N,N-dimethylformamide (1 ml, DMF, SigmaAldrich, 99.8%) was spin-coated on top of the mesoporous TiO2 layer. The PbI2-coated electrode was sequentially spin-coated with methylammonium iodide (10 mg, CH3NH3I) solution in 2-propanol (1 ml, Sigma-Aldrich, 99.5%) and then heated to 80 °C for 20 min, resulting in the formation of dark brown perovskite film. We used hole transport material (HTM) composed of 2,2ƍ,7,7ƍ-tetrakis(N,N-p-
3
dimethoxyphenyl-amine)-9,9ƍ-spirobifluorene (56 mg, spiro-OMeTAD, Merck), 4-tert-butylpyridine (30 mg, Aldrich, 96%), and bis(trifluoromethane)sulfonimide lithium salt (5.8 mg, LiClO4, Aldrich, 99.95%) in anhydrous chlorobenzene (1 ml, Aldrich, 99.8%). After coating the HTM solution, Mo was sputtered on top of the spiro-OMeTAD layer using DC magnetron sputtering system under 4 mTorr of Ar. The thickness of the Mo electrodes was controlled by changing the sputtering time. For comparison, the Au electrode was deposited by thermal evaporation with a thickness of 80 nm. The metal electrodes were deposited using shadow masks to define the active area. The active area of each cell (0.1–0.15 cm2) was measured using an optical microscope.
Characterization The morphology and thickness of the films were examined by field-emission scanning electron microscopy (FE-SEM, FEI Inspect F). Elemental mapping was conducted using electron probe microanalysis (EPMA, JEOL JXA-8500F). The current density–voltage J–V characteristics were measured using a Keithley model 2400 source measurement unit and a solar simulator equipped with a 1000-W xenon lamp (Yamashida Denso, YSS-50S) and a KG-3 filter. A Si solar cell calibrated by the National Renewable Energy Laboratory (NREL) was used to adjust the light intensity to the AM 1.5G 1 -2
sun condition (100 mW cm ). During all J–V measurements, non-reflective black masks were attached to the devices to exclude diffused light scattering. The incident photon-to-current conversion efficiency (IPCE) was measured using a K3100 EQX spectral IPCE measurement system. The sheet resistances of the metal electrodes were measured through four-point probe measurements (LorestaGP MCP-T610, Mitsubishi Chemical Analytech). The mechanical properties of the thin films were measured by the nanoindentation method (Ultra-nanoindentation hardness tester, Anton-Paar). Nanoscratch tests were performed by using a nano-scratch tester (Nano-scratch tester, Anton-Paar) under a ramp loading that was continuously increased from 0.3 mN to 5 mN during the sliding. A diamond conical tip with 2 µm in radius was slid against Mo and Au electrodes which were deposited on each device. The Hertzian maximum contact pressure at 5 mN normal load for Mo and Au electrodes were calculated to be 5.6 GPa and 4.8 GPa, respectively. The scratch sliding stroke was set to be 1 mm with a sliding speed of 1 mm/min. All the mechanical characterization experiments were performed in Class 100 clean room under the environmental conditions of 25 °C and 30% RH.
4
Results and discussion
Figure 1. (a) Schematic and (b) SEM cross cross-sectional sectional image of the mesoscopic perovskite solar cell with the Mo cathode. (c) SEM top view of the thin Mo film covering covering the multilayer consisting of hole transport material (HTM)/perovskite/meso-TiO (HTM)/perovskite/meso 2. (d) Energy level diagram representing the device components.
Figure 1a depicts the schematic of the mesoscopic perovskite p te solar cell fabricated with Mo metal cathode. Each layer is well defined, as shown in the the scanning electron microscopy (SEM) cross crosssectional image of Figure 1b. To fabricate the 250-nm-thick 250 mesoporous TiO2 layer (meso (meso-TiO2), a labmade TiO2 paste diluted with ethanol (2:7 weight ratio) was spin-coated s onto to the TiO2 compact layer.[28] For the high-quality quality perovskite bilayer, we adopted the sequential sequential deposition method in which lead iodide (PbI2) is firstly coated and then methylammonium iodid iodide (CH3NH3I) is sequentially spin-coated onto the PbI2/meso-TiO TiO2 composite film. After post annealing at 80 °C, a bi bilayer structure
5
consisting of perovskite (CH3NH3PbI3)/meso-TiO2 composite and a 150-nm-thick CH3NH3PbI3 capping layer was formed. After that, the hole transport material (HTM) spiro-OMeTAD spiro OMeTAD was deposited on top of the perovskite capping layer, resulting in a uniform unifor and smooth multi-layer layer film, as shown in the cross crosssectional SEM image of Figure 1b. Finally, a thin M Mo film was sputtered by DC magnetron gnetron sputtering in form of a metal back contact to efficiently extract the holes from the spiro-OMeTAD. OMeTAD. The top top-view SEM image of Figure 1c confirmed that the sputtered thin thin Mo film densely covered the layers underneath (HTM/perovskite) and there was no pin-hole hole in the Mo layer. As depicted in Figure 1d, the high work function of Mo (- 4.6 eV) is favorable for hole extraction from spiro-OMeTAD, spiro OMeTAD, thus efficiently transporting holes to the external circuit.[29] circuit.
Figure 2. Current density–voltage voltage (J–V) ( characteristics of mesoscopic CH3NH3PbI3 perovskite solar cells prepared with Mo cathodes of different thickness. thickness. The thickness of Mo was controlled by changing the sputtering time and measured by cross-sectional cross SEM images.
6
Table 1. Photovoltaic parameters of perovskite solar cells equipped with Mo cathodes of different thickness. Thickness of Mo
Voc [V]
Jsc [mA cm-2]
FF [%]
PCE [%]
Rs a [Ω cm2]
RSH b [Ω square-1]
40 nm
0.95
20.16
59.55
11.43
9.43
14.0
80 nm
0.97
20.61
67.77
13.58
3.41
5.1
140 nm
0.87
20.68
57.03
10.25
10.04
2.2
210 nm
0.89
19.45
53.79
9.35
21.74
1.3
a
b
Rs: Series resistances calculated from the slope of the J–V curves at Voc; RSH: Sheet resistances of thin films measured using the four-point probe method.
We fabricated perovskite solar cells with a Mo cathode of different thickness by controlling the sputtering time and compared their photovoltaic performance. Figure 2 shows the J–V curves of mesoscopic CH3NH3PbI3 perovskite solar cells in dependence of the Mo-electrode thickness and the individual photovoltaic parameters are summarized in Table 1. When the Mo thickness increased from 40 nm to 80 nm, the short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) were enhanced. Especially the FF, which is directly related to the series resistance Rs of the device, was considerably increased because of the smaller sheet resistance RSH of the 80-nm-thick Mo electrode (5.1 ȍ/square) compared to that of the 40-nm-thick Mo electrode (14.0 ȍ/square). As summarized in Table 1, the four point probe sheet resistances of the Mo electrodes decreased with increasing thickness of the Mo layer. However, the photovoltaic performances of the devices with thicker Mo cathode (140 and 210 nm) were inferior to that of the cell having 80-nm-thick Mo. In particular, upon increasing the thickness of the Mo layer to more than 140 nm, the J–V characteristic showed s-shaped curves reflecting the poor FF. This may be attributed to the damage of the organic HTM layer caused by the bombardment by high-energy particles in the plasma during longer sputtering times, which has been similarly reported as plasma damage in the fabrication of organic light-emitting diodes.[30] As the sputtering time increased, we found that small fractions of Mo were observed under the HTM layer and in direct contact with the perovskite layer, as shown in the elemental mapping of Figure S1 obtained by electron probe microanalysis (EPMA). The direct contact between perovskite and Mo metal without a hole-selective contact (here, spiro-OMeTAD) is non-ohmic (metal-semiconductor interface) and this is likely to result in inefficient charge transportation and increase Rs.[31, 32] As summarized in Table 1, the larger Rs obtained for devices with thicker Mo electrode (140 nm and 210 nm) led to deleterious effects on device performances, especially FF. These trends have been observed in hole conductor-free heterojunction perovskite solar cells, and
7
recently, Habisreutinger et al. also reported that direct contact contact between perovskite and metal cathode results in poor Voc and FF.[33-35]
Figure 3. (a) Current density–voltage voltage (J–V) ( ) curves of perovskite solar cells prepared with Au and Mo cathode. (b) Hysteresis of the J–V V curves measured in forward (short circuit ĺ open circuit) and reverse (open circuit ĺ short circuit) scan direction and (c) J–V curves of Mo- cathode perovskite solar cells measured with different delay times for each 10 mV voltage step.
8
We chose the optimum thickness of 80 nm for the Mo cathode resulting in highly efficient perovskite solar cells and compared their photovoltaic characteristics with those of the perovskite solar cell equipped with an 80-nm-thick Au electrode. As shown in Figure 3a, the cell with the Mo cathode -2
exhibited high Jsc of 20.11 mA cm and Voc of 1.01 V, which are nearly the same values as those of the Au-cathode cell. Even though the Au cathode with higher electrical conductivity showed lower RSH than the Mo electrodes, the difference in FF of the devices was not significant (Table 2). Eventually, the Mo-cathode cell showed the PCE of 13.28%, which is comparable with that of the Au-cathode cell (14.07%). Meanwhile, perovskite solar cells suffer from anomalous hysteresis in their J–V characteristics, depending on the scan rate and direction, resulting in discrepancies and, usually, overestimation of their photovoltaic performance. To confirm the reliability and accuracy of our results, we measured the J–V curves of the Mo-cathode cell at different scan directions and rates. Very small hysteresis between forward and reverse scans resulted in the average PCE of 12.88% and no deviations in J–V could be observed for different scan rates, as shown in Figure 3b and c. The small hysteresis in dependence of the scan direction can be further reduced by controlling the crystallinity and morphology of the perovskite layer, as reported previously.[9, 36-38]
Table 2. Photovoltaic parameters of perovskite solar cells with either Au or Mo cathode. Cathode
Voc [V]
Jsc [mA cm-2]
FF [%]
PCE [%]
RSH [Ω square-1]
Au
1.01
20.05
69.36
14.07
0.8
Mo
1.01
20.11
65.39
13.28
5.1
9
Figure 4. (a) Current density–voltage voltage (J–V) ( curves of the best-performing performing cell based on Mo cathode under 100 mW cm
-2
AM 1.5G illumination (red circle) and in the dark (black (black square). (b) Incident
photon to current efficiency (IPCE) spectrum of the best-performing cell (black line)) and calculated current density (Jsc) by integrating the IPCE curve with an AM 1.5G spectrum (red line). (c) Transient photocurrent response of the Mo-cathode cathode cell during the repetitive turning on and o off of simulated sunlight. (d) Steady-state state photocurrent at the maximum power point (0.8 V) and stabilized PCE.
The J–V characteristics of the best--performing performing cell based on the Mo cathode are shown iin Figure 4a. The best cell showed the high PCE of 15.06% along with w Voc of 1.01 V, FF of 0.67, and remarkably -2
high Jsc of 22.13 mA cm . Since Voc is exceeding 1 V along with high Jsc, Mo can efficiently replace noble-metal metal electrodes (Au or Ag) and thereby reduce the cost of perovskite solar cell while simultaneously exhibiting high performance. performance Figure 4b shows the high and broad incident photon photon-tocurrent efficiency (IPCE) over the whole spec spectral tral range of visible light and the integrated Jsc calculated from the IPCE spectrum under AM 1.5G photon flux is in agreement with the Jsc measured in the J–V curve. To confirm how efficiently charges are extracted extracted and collected by Mo cathode of perovskit perovskite solar cells, we measured the transient photocurrent response during repetitive on/off cycles under
10
-2
simulated 1 sun illumination (100 mW cm ) without bias. Figure 4c shows that the photoresponse photorespo of the device equipped with Mo cathode is rapid and stable st over repeated light on/off cycles with steady -2
Jsc exceeding 20 mA cm , indicating that holes are efficiently extracted at at the Mo electrode/HTM interface. We also monitored the photocurrent of th the best-performing performing device under working conditions at the maximum mum power point (0.8 V) to corroborate the accuracy of the measured J–V J results. As shown in Figure 4d, even after continuous scanning for 10 min, the photocurrent is very stable and its -2
high value of 18.3 mA cm at 0.8 V resulted in the stabilized PCE of about 15%. This is consistent with the measured J–V curve shown in Figure 4a. To examine the reproducibility of the performance, we fabricated 34 devices based on Mo cathodes. The histograms istograms of the photovoltaic characteristics are shown in Figure S2 and the he average values with standard deviation of the 34 devices are summarized in Table S1. The histograms and graphs of of the normal distributions confirm that the performance of the Mo-cathode-based based cells show good reproducibility with standard deviation of only 0.91% in efficiency.
Figure 5. (a) Representative ative nanoindentation curves of Mo and Au electrodes deposited on perovskite solar cells.. (b) Hardness and elastic modulus of Mo and Au electrodes obtained from 10
11
nanoindentation tests. (c) Elasticity of Mo and Au electrodes. (d) Schematic of nano-scratch test and optical images of Au and Mo electrodes after the scratch test. As mentioned earlier, superior mechanical properties of the cathode layer that is exposed to the external environment is essential to prevent any damage or failure during operation and handling of the solar cell.[39] Furthermore, a cathode layer with high elasticity can prevent permanent deformation during bending or folding. In this study, the mechanical properties and scratch resistance of Mo and Au electrodes deposited on perovskite solar cells were characterized by nanoindentation and scratch test, respectively.[40] In the nanoindenation tests, Berkovich diamond tip with 100 nm in radius was used. From the nanoindentation loading and unloading curves of Mo and Au electrodes shown in Figure 5a, the hardness, elastic modulus, and elasticity of Mo and Au electrodes were obtained by using the Oliver-Pharr method.[41] It should be noted that the elasticity was calculated by dividing the area beneath the loading curve (total energy) by that of the unloading curve (elastic energy) which can be used to quantitatively assess the degree of elasticity.[42] As shown in Figure 5b, the hardness values of Mo and Au electrodes were measured to be 2.9 ± 0.5 GPa and 0.4 ± 0.1 GPa, respectively. As can be confirmed from the hardness value of Mo that is ~7 times higher than that of Au electrode, it can be stated that Mo electrode deposited on the device shows significantly higher resistance to the permanent deformation compared to Au electrode. The elastic modulus of Mo and Au electrodes were measured to be 24.8 ± 3.4 GPa and 19.6 ± 2.8 GPa, respectively. The elasticity of Mo and Au electrodes were calculated to be 93.8 ± 2.7% and 19.2 ± 2.7%, respectively, as shown in Figure 5c. Thus, it was found that Mo electrode has drastically higher restoration capability against deformation originating from bending or folding compared to Au. In order to investigate the scratch resistance of Mo and Au electrodes on devices, nano-scratch test were performed. Figure 5d shows the schematic of nano-scratch test with a ramp loading and optical images of Mo and Au electrodes after scratch test. The scratch images clearly show that Au was severely scratched and peeled off from the underlying HTM layer, while no evidence of surface damage was found for Mo electrode after scratch test. These results indicate that Mo can be used as a mechanically stable cathode material for devices that require structural flexibility.
12
Conclusions In summary, we demonstrated noble-metal-free highly efficient perovskite solar cells using sputtered Mo as the cathode material, thereby replacing the commonly used noble metals (Au and Ag). The Mo cathode with high work function and good electrical conductivity successfully replaced Au for perovskite solar cells, yielding comparable photovoltaic performances. Furthermore, due to the excellent mechanical properties of Mo, such as hardness and elasticity, compared with the Au electrode, the Mo electrode showed superior resistance and restoration capability against external scratch and deformation that can occur during the practical handling of these devices. By improving the quality of the perovskite films and our device-fabrication skills, we realized the best cell efficiency of as high as 15.06%, comparable to that of Au-based cells. This is the first report showing such a high efficiency in excess of 15% in mesoscopic perovskite solar cells based on noble-metal-free cathode. These results suggest that Mo is a promising candidate for replacing noble-metal electrodes in mesoscopic perovskite solar cells, thereby realizing low cost as well as high performance.
Acknowledgements The authors acknowledge funding support from the Global Frontier R&D Program on Center for Multiscale Energy System (2012M3A6A7054856) and 2015 University-Institute cooperation program funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea; This work was also supported by the KIST institutional programs (2V03780 and 2E25392). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2010-0018289).
Appendix. Supporting information Supplementary data associated with this article can be found in the online version at
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References [1]
A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050.
[2]
J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Park, N. G. Park, Nanoscale 3 (2011) 4088
[3]
M. Liu, M. B. Johnston, H. J. Snaith, Nature 501 (2013) 395.
[4]
J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Grätzel, Nature 499 (2013) 316.
[5]
H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. Humphry-Baker, J. H. Yum, J. E. Moser, M. Grätzel, N. G. Park, Sci. Rep. 2 (2012) 591.
[6]
S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T., Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 342 (2013) 341.
[7]
G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar, T. C. Sum, Science 342 (2013) 344.
[8]
M. Grätzel, Nat Mater, 13 (2014) 838.
[9]
N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu, S. I. Seok, Nat. Mater 13 (2014) 897.
[10]
N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Nature 517 (2015) 476.
[11]
N. G. Park, Mater. Today. 18, (2015) 65.
[12]
C. Wehrenfennig, G. E. Eperon, M. B. Johnston, H. J. Snaith, L. M. Herz, Adv. Mater. 26 (2014) 1584.
[13]
W.-J. Yin, T. Shi, Y. Yan, Adv. Mater. 26 (2014) 4653.
[14]
C.
Bi,
Y.
Yuan,
Y.
Fang,
J.
Huang,
Adv.
Energy
Mater.
(2014).
http://dx.doi.org/10.1002/aenm.201401616 in press. [15]
D. Liu, T. L. Kelly, Nat. Photonics 8 (2014) 133.
[16]
H.S. Jung, N. G. Park, Small, 11 (2015) 10.
[17]
M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Prog. Photovolt: Res. Appl. 23 (2015) 1.
[18]
G. Hodes, Science 342 (2013) 317.
[19]
Z. Li, S. A. Kulkarni, P. P. Boix, E. Shi, A. Cao, K. Fu, S. K. Batabyal, J. Zhang, Q. Xiong, L. H. Wong, N. Mathews, S. G. Mhaisalkar, ACS Nano, 8 (2014) 6797.
[20]
Z. Wei, H. Chen, K. Yan, S. Yang, Angew. Chem. Int. Ed. 53 (2014) 13239.
[21]
Z. Wei, K. Yan, H. Chen, Y. Yi, T. Zhang, X. Long, J. Li, L. Zhang, J. Wang, S. Yang, Energy Environ. Sci. 7 (2014) 3326.
[22]
A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, M. Xu, M. Hu, J. Chen, Y. Yang, M. Grätzel, H. Han, Science 345 (2014) 295.
[23]
K. Yan, Z. Wei, J. Li, H. Chen, Y. Yi, X. Zheng, X. Long, Z. Wang, J. Wang, J. Xu, S. Yang, Small, 11 (2015) 2269.
14
[24]
X. Wang, Z. Li, W. Xu, S. A. Kulkarni, S. K. Batabyal, S. Zhang, A. Cao, L. H. Wong, Nano Energy, 11 (2015) 728.
[25]
Q. Jiang, X. Sheng, B. Shi, X. Feng, T. Xu, J. Phys. Chem. C 118 (2014) 25878.
[26]
E. Della Gaspera, Y. Peng, Q. Hou, L. Spiccia, U. Bach, J.J. Jasieniak, Y.-B. Cheng, Nano Energy, 13 (2015) 249.
[27]
Y. Han, S. Meyer, Y. Dkhissi, K. Weber, J.M. Pringle, U. Bach, L. Spiccia, Y.-B. Cheng, J. Mater. Chem. A, 3 (2015), 8139
[28]
K. Yoo, J.-Y. Kim, J.A. Lee, J.S. Kim, D.-K. Lee, K. Kim, J.Y. Kim, B. Kim, H. Kim, W.M. Kim, J.H. Kim, M.J. Ko, ACS Nano, 9 (2015) 3760.
[29]
H.B. Michaelson, J. Appl. Phys. 48 (1977) 4729.
[30]
H.-K. Kim, K.-S. Lee, J.H. Kwon, Appl. Phys. Lett.88 (2006) 012103.
[31]
E. J. Juarez-Perez, M. Wuȕler, F. Fabregat-Santiago, K. Lakus-Wollny, E. Mankel, T. Mayer, W. Jaegermann, I. Mora-Sero, J. Phys. Chem. Lett. 5 (2014) 680
[32]
J.-J. Shi, W. Dong, Y.-Z. Xu, C.-H. Li, S.-T. Lv, L.-F. Zhu, J. Dong, Y.-H. Luo, D.-M. Li, Q.-B. Meng, Q. Chen, Chin. Phys. Lett, 30 (2013) 128402.
[33]
Y. Xu, J. Shi, S. Lv, L. Zhu, J. Dong, H. Wu, Y. Xiao, Y. Luo, S. Wang, D. Li, X. Li, Q. Meng, ACS Appl. Mater. Inter. 6 (2014) 5651.
[34]
W. A. Laban, L. Etgar, Energy Environ. Sci. 6 (2013) 3249.
[35]
S. N. Habisreutinger, T. Leijtens, G. E. Eperon, S. D. Stranks, R. J. Nicholas, H. J. Snaith, Nano Letters, 14 (2014) 5561.
[36]
Z. Xiao, Q. Dong, C. Bi, Y. Shao, Y. Yuan, J. Huang, Adv. Mater. 26 (2014) 6503.
[37]
G. Li, K.L. Ching, J.Y.L. Ho, M. Wong, H.-S. Kwok, Adv. Energy Mater. (2015). http://dx.doi.org/10.1002/aenm.201401775 in press.
[38]
F. X. Xie, D. Zhang, H. Su, X. Ren, K. S. Wong, M. Grätzel, W. C. H. Choy, ACS Nano, 9 (2014) 639.
[39]
H. J. Kim, D. E. Kim, Solar Energy, 86 (2012) 2049.
[40]
H. J. Kim, D. E. Kim, Tribol Lett, 49 (2013) 85.
[41]
W. C. Oliver, G. M. Pharr, J. Mater. Res. 7 (1992) 1564.
[42]
R. H. Kim, H. J. Kim, I. Bae, S. K. Hwang, D. B. Velusamy, S. M. Cho, K. Takaishi, T. Muto, D. Hashizume, M. Uchiyama, P. André, F. Mathevet, B. Heinrich, T. Aoyama, D. E. Kim, H. Lee, J.-C. Ribierre, C. Park, Nat Commun, 5 (2014) 3583.
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Vitae Inyoung Jeong received his B.S. degree (2011) and a M.S degree (2013) in the Department of Chemical Engineering at Pohang University of Science and Technology (POSTECH), Korea. He is currently urrently a Ph.D. course student in research group of Prof. Jinwoo Lee at the POSTECH. His current research focuses on synthesis and application cation of functional nanomaterials for Dye sensitized solar cell and Perovskite ovskite solar cell.
Hae-Jin Jin Kim received his B.S. degree (2009) in the Department o of Mechanical Engineering Engineering at Yonsei University, Korea. He is currentl currently a unified M.S/Ph.D. course student in the research group oup of Prof. Dae Dae-Eun Kim at Yonsei University. His current research focu focuses on the characterization of mechanical and tribological properties perties of thin film for enhanced durability of mechanical systems. systems
Byung-Seok Seok Lee granted a B. S. degree (2013) in Department of Physics at Chungbuk National University, Korea. He is studying for a M. S. degree under Prof. Doh-Kwon Doh Kwon Lee in Department of Nanomaterials Science and Engineering at University of Science and d Technology (UST), Korea. His research field is inorganic thin-film thin film solar cells by non non-vacuum process.
Dr. Hae Jung Son received her B.S degree in chemistry from Sungkyunkwan University (2000) and M.S. degree from the Korea Advanced Institute of Science and Technology (2002), Rep. of Korea Korea. After graduation, she worked for Samsung Advanced IInstitute of Technology as a research resea staff member until 2004. She received Ph.D. degree in chemistry from the University of Chicago in 2011 under the supervision of Prof. Luping Yu. Presently, she is a senior research scientist in Photo-electronic Photo Hybrids Research Center at Korea Institut Institute of Science and Technology, Technology Rep. of Korea.. Her research interest is the development of conjugated polymers for opto-electronic opto electronic applications. Dr. Jin Young Kim is a senior research scientist of Korea Institute o of Science and Technology (KIST), Korea. Before joining g KIST, he worked for National Renewable Energy Laboratory (NREL), USA A from 2007 to 2011 as a postdoctoral researcher and a scientist. Dr. Kim obtained his BS (2000), MS (2002), and PhD (2006) degrees from the he Department of Materials Science and Engineering of Seoul National University. Dr. Kim Kim’s research interest is mostly focused on the next generation eration solar cells and solar fuels generation.
16
Dr. Doh-Kwon Lee received his B.S. (1997), M.S. (1999), and Ph.D. degree (2005) in materials science and engineering from Seoul National University (Advisor: Prof. Han-Ill Yoo) in Korea. With his Ph.D. completed, he moved onto the Institute of Physical Chemistry, Justus-LiebigUniversity (JLU) Giessen in Germany, where he joined Prof. Janek’s research group as a senior scientist. Since 2009, Dr. Lee has worked as a senior/principal research scientist at Korea Institute of Science and Technology (KIST) in Korea. His current research interest is to develop solution-processed inorganic thin-film solar cells, organo-metal halide perovskite solar cells, and multi-junction solar cells. Prof. Dae-Eun Kim received his B.S. from Tufts University in 1984 and M.S. and Ph.D. from M.I.T. in 1986 and 1991, respectively. He was an Assistant Professor at the Ohio State University before joining the faculty of Yonsei University, where he is currently a Professor in the Department of Mechanical Engineering. He is also the Director of the Center for NanoWear, which is part of the Creative Research Initiative program sponsored by the National Research Foundation of Korea. His research interests include nano/bio-tribology, functional coatings, and micro-fabrication.
Prof. Jinwoo Lee obtained his B.S. (1998) and Ph.D. (2003) from the Department of Chemical and Biological Engineering of Seoul National University, Korea. After his postdoctoral research at Seoul National University (with Prof. Taeghwan Hyeon, 2003–2005) and Cornell University (with Prof. Ulrich Wiesner, 2005–2008), he joined the faculty of the Department of Chemical Engineering at Pohang University of Science and Technology (POSTECH) in June, 2008. His research field includes synthesis and applications of ordered functional mesoporous materials and shape controlled nanocrystals in energy conversion and storage devices.
Dr. Min Jae Ko is a principal research scientist at Korea Institute of Science and Technology (KIST) and an adjunct professor at Korea University. He obtained his B.S (1995) and M.S (1997) degrees from the Department of Fiber and Polymer Science, and Ph.D. (2001) from the Department of Materials Science and Engineering at Soul National University, Korea. He performed his postdoc work at MIT from 2001 to 2004. Then he moved to Samsung Electronics Co., as a senior research engineer in 2005. His research is focusing on the developments of materials and devices for the next generation flexible solar cells.
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Highlights z
Highly efficient and noble metal-free perovskite solar cell (PSC) were fabricated.
z
The Mo cathoded PSCs exhibit comparable performances to that of Au-based cells.
z
Best cell based on Mo cathode shows stabilized efficiency as high as 15%.
z
Nanoindentation and wear test show Mo can serve as a durable electrode for PSCs.
18
*Graphical Abstract
Noble metal-free mesoscopic perovskite solar Cells with High efficiency and mechanical durability Inyoung Jeong, Hae Jin Kim, Byung-Seok Lee, Hae Jung Son, Jin Young Kim, Doh-Kwon Lee, Dae-Eun Kim, Jinwoo Lee, Min Jae Ko
www.elsevier.com/nanoenergy
PII: DOI: Reference:
S2211-2855(15)00318-3 http://dx.doi.org/10.1016/j.nanoen.2015.07.025 NANOEN926
To appear in:
Nano Energy
Received date: 8 July 2015 Accepted date: 28 July 2015 Cite this article as: Inyoung Jeong, Hae Jin Kim, Byung-Seok Lee, Hae Jung Son, Jin Young Kim, Doh-Kwon Lee, Dae-Eun Kim, Jinwoo Lee, Min Jae Ko, Noble metal-free mesoscopic perovskite solar Cells with High efficiency and mechanical durability, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2015.07.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Noble Metal-Free Mesoscopic Perovskite Solar Cells with High Efficiency and Mechanical Durability Inyoung Jeonga,b, Hae Jin Kimc, Byung-Seok Leea,d, Hae Jung Sona, Jin Young Kima,e, Doh-Kwon Leea,e, Dae-Eun Kimc, Jinwoo Leeb,*, Min Jae Koa,e,f,** a
Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea
b
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Kyungbuk 790-784, Republic of Korea
c
School of Mechanical Engineering, Yonsei University, Seoul 120-749, Republic of Korea
d Department of Nanomaterials Science and Engineering, University of Science and Technology (UST), Daejeon, Republic Korea e
Green School, Korea University, Seoul 136-701, Republic of Korea
f
KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul 136-701, Republic of Korea
*Corresponding author. Tel: +82-54-279-2395 **Corresponding author. Tel: +82-2-958-5518 E-mail address: [email protected] (J. Lee), [email protected] (M. J. Ko)
Abstract Noble metal-free mesoscopic perovskite solar cell based on low-cost molybdenum (Mo) cathode has been developed for the first time. By optimizing the thickness of Mo electrode and combination with high quality perovskite layer, the Mo cathode-based perovskite solar cell exhibited a best power conversion efficiency as high as 15.06% with invariance of current density-voltage curves at different scan rates and very small hysteresis according to scan direction. In addition to photovoltaic performances, importance of mechanical durability of a cathode material for perovskite solar cell was highlighted and results from nano-indentation and scratch test indicated that Mo electrode has superior resistance and restoration capability against external scratch or deformation compared to Au electrode. The Mo cathode-based perovskite solar cells have great potential for realizing low cost and high performance as well as mechanical durability.
Keyworkd: Organic-inorganic hybrid perovskite, photovoltaics, cathode, noble metal-free, Molybdenum
1
Introduction Recently, organic–inorganic hybrid perovskites have emerged as promising candidate materials in the field of photovoltaics. Research and development on solar cells based on perovskite-type compounds CH3NH3PbX3 (X = I-, Br-, Cl-, or mixed halides) have experienced unprecedented rapid progress in the past five years due to their unique characteristics, such as high light-absorption coefficient, long-range balanced carrier-diffusion length, high crystallinity, and excellent charge-carrier mobility.[1-13] In addition, perovskite solar cells can be prepared by inexpensive solution processing at low temperatures.[14, 15] From the first report on perovskite-based solar cells having the power conversion efficiency (PCE) of 3.8% by Kojima and co-workers in 2009, very recently, the record efficiency of 20.1% has been certified and it is anticipated that the PCE of perovskite solar cells will be further increased to above 25%, which is the level of state-of-the-art single-crystalline Si solar cells.[1, 16, 17] Therefore, perovskite solar cells satisfying low cost as well as high efficiency became a powerful rival to conventional Si and GaAs solar cells. However, most of state-of-the-art perovskite solar cells typically need expensive noble-metals (Ag or Au) as cathode materials, which raises the cost of perovskite solar cells and thus impedes their commercialization.[18] Therefore, the replacement of noble metals by inexpensive and earth-abundant materials for cathode is highly desirable to reduce the cost of perovskite solar cells. Many groups suggested various carbon nanomaterials as solution.[19-24] Li and coworkers applied laminated carbon nanotubes (CNTs) as the cathode of perovskite solar cells and the best CNT-based cells showed PCE of 9.9% and 6.87% with and without the hole-conductor material spiro-OMeTAD, respectively.[19] Mei et al. developed fully printable monolithic perovskite solar cells using carbon black/graphite composite counter electrode with certified PCE of 12.84% without a hole conductor.[22] However, the resulting open circuit voltage (Voc) and PCE of perovskite solar cells using carbonaceous electrodes are still lower than those of conventional devices equipped with Au electrode, which is mainly attributed to inefficient charge extraction. Meanwhile, replacing the noble metals (Au or Ag) by inexpensive metals with high work function can be a very simple way to reduce cost while retaining the photovoltaic performance. Jiang et al. used nickel as the cathode material of perovskite solar cells and thereby opened the possibility to replace commonly used noble metals (Ag or Au) with non-precious metals.[25] Molybdenum (Mo), one of the metals having a high work function combined with high electrical conductivity, might be a candidate material for low-cost and highly efficient perovskite solar cells. With respect to cost, the unit price of Mo is far lower than 0.1% of that of Au. In addition, Mo is chemically
2
more stable against reactions with halides and oxidation in air compared to Ag and Al, respectively.[26, 27] Motivated by the advantages of Mo, we developed highly efficient perovskite solar cells equipped with Mo cathode for the first time. We realized a PCE of 13.28%, which is comparable with that of Aucathode-based cells (14.07%) without discrepancy in the current density–voltage (J–V) curves measured at different scan rates, and only very small hysteresis is observed with respect to the scan direction. Eventually, the best-performing device exhibited the PCE of 15.06% and the performances of the Mo-cathode-based cells showed good reproducibility with a standard deviation of only 0.91% in efficiency. In addition to the high photovoltaic performance of the Mo-cathode-based devices, the durability of the Mo electrode must be sufficient to ensure high reliability and prolonged lifetime of the device. This is particularly important considering the fact that the outermost electrode layer is exposed to the external environment. In order to assess the durability of Mo, its mechanical and scratchresistance properties were compared to those of Au by using nanoindentation and wear testing methods, respectively. From the superior mechanical properties and wear resistance of Mo compared to those of Au, it was determined that Mo may serve as a durable electrode layer for perovskite solar cells.
Experimental Section Device Fabrication Patterned fluorine-doped tin oxide (FTO) substrates (TEC8, Pilkington) were cleaned by sonication in ethanol, acetone, and 2-propanol, followed by UV-ozone treatment for 20 min. The compact TiO2 layer was coated on the FTO glass by spin-coating the TiO2 precursor-containing titanium diisopropoxide bis(acetylacetonate) solution (75 wt% in isopropanol, Aldrich) mixed with 1-butanol at 2000 rpm for 40 s, followed by heating at 125 °C for 10 min. After heat treating at 500 °C for 30 min, lab-made TiO2 (20 nm) paste was diluted with anhydrous ethanol (2:7 weight ratio), spin-coated on the compact TiO2 layer and dried at 125 °C for 10 min, followed by annealing at 500 °C for 60 min. For the perovskite layer, PbI2 (500 mg, Sigma-Aldrich, 99%) dissolved in N,N-dimethylformamide (1 ml, DMF, SigmaAldrich, 99.8%) was spin-coated on top of the mesoporous TiO2 layer. The PbI2-coated electrode was sequentially spin-coated with methylammonium iodide (10 mg, CH3NH3I) solution in 2-propanol (1 ml, Sigma-Aldrich, 99.5%) and then heated to 80 °C for 20 min, resulting in the formation of dark brown perovskite film. We used hole transport material (HTM) composed of 2,2ƍ,7,7ƍ-tetrakis(N,N-p-
3
dimethoxyphenyl-amine)-9,9ƍ-spirobifluorene (56 mg, spiro-OMeTAD, Merck), 4-tert-butylpyridine (30 mg, Aldrich, 96%), and bis(trifluoromethane)sulfonimide lithium salt (5.8 mg, LiClO4, Aldrich, 99.95%) in anhydrous chlorobenzene (1 ml, Aldrich, 99.8%). After coating the HTM solution, Mo was sputtered on top of the spiro-OMeTAD layer using DC magnetron sputtering system under 4 mTorr of Ar. The thickness of the Mo electrodes was controlled by changing the sputtering time. For comparison, the Au electrode was deposited by thermal evaporation with a thickness of 80 nm. The metal electrodes were deposited using shadow masks to define the active area. The active area of each cell (0.1–0.15 cm2) was measured using an optical microscope.
Characterization The morphology and thickness of the films were examined by field-emission scanning electron microscopy (FE-SEM, FEI Inspect F). Elemental mapping was conducted using electron probe microanalysis (EPMA, JEOL JXA-8500F). The current density–voltage J–V characteristics were measured using a Keithley model 2400 source measurement unit and a solar simulator equipped with a 1000-W xenon lamp (Yamashida Denso, YSS-50S) and a KG-3 filter. A Si solar cell calibrated by the National Renewable Energy Laboratory (NREL) was used to adjust the light intensity to the AM 1.5G 1 -2
sun condition (100 mW cm ). During all J–V measurements, non-reflective black masks were attached to the devices to exclude diffused light scattering. The incident photon-to-current conversion efficiency (IPCE) was measured using a K3100 EQX spectral IPCE measurement system. The sheet resistances of the metal electrodes were measured through four-point probe measurements (LorestaGP MCP-T610, Mitsubishi Chemical Analytech). The mechanical properties of the thin films were measured by the nanoindentation method (Ultra-nanoindentation hardness tester, Anton-Paar). Nanoscratch tests were performed by using a nano-scratch tester (Nano-scratch tester, Anton-Paar) under a ramp loading that was continuously increased from 0.3 mN to 5 mN during the sliding. A diamond conical tip with 2 µm in radius was slid against Mo and Au electrodes which were deposited on each device. The Hertzian maximum contact pressure at 5 mN normal load for Mo and Au electrodes were calculated to be 5.6 GPa and 4.8 GPa, respectively. The scratch sliding stroke was set to be 1 mm with a sliding speed of 1 mm/min. All the mechanical characterization experiments were performed in Class 100 clean room under the environmental conditions of 25 °C and 30% RH.
4
Results and discussion
Figure 1. (a) Schematic and (b) SEM cross cross-sectional sectional image of the mesoscopic perovskite solar cell with the Mo cathode. (c) SEM top view of the thin Mo film covering covering the multilayer consisting of hole transport material (HTM)/perovskite/meso-TiO (HTM)/perovskite/meso 2. (d) Energy level diagram representing the device components.
Figure 1a depicts the schematic of the mesoscopic perovskite p te solar cell fabricated with Mo metal cathode. Each layer is well defined, as shown in the the scanning electron microscopy (SEM) cross crosssectional image of Figure 1b. To fabricate the 250-nm-thick 250 mesoporous TiO2 layer (meso (meso-TiO2), a labmade TiO2 paste diluted with ethanol (2:7 weight ratio) was spin-coated s onto to the TiO2 compact layer.[28] For the high-quality quality perovskite bilayer, we adopted the sequential sequential deposition method in which lead iodide (PbI2) is firstly coated and then methylammonium iodid iodide (CH3NH3I) is sequentially spin-coated onto the PbI2/meso-TiO TiO2 composite film. After post annealing at 80 °C, a bi bilayer structure
5
consisting of perovskite (CH3NH3PbI3)/meso-TiO2 composite and a 150-nm-thick CH3NH3PbI3 capping layer was formed. After that, the hole transport material (HTM) spiro-OMeTAD spiro OMeTAD was deposited on top of the perovskite capping layer, resulting in a uniform unifor and smooth multi-layer layer film, as shown in the cross crosssectional SEM image of Figure 1b. Finally, a thin M Mo film was sputtered by DC magnetron gnetron sputtering in form of a metal back contact to efficiently extract the holes from the spiro-OMeTAD. OMeTAD. The top top-view SEM image of Figure 1c confirmed that the sputtered thin thin Mo film densely covered the layers underneath (HTM/perovskite) and there was no pin-hole hole in the Mo layer. As depicted in Figure 1d, the high work function of Mo (- 4.6 eV) is favorable for hole extraction from spiro-OMeTAD, spiro OMeTAD, thus efficiently transporting holes to the external circuit.[29] circuit.
Figure 2. Current density–voltage voltage (J–V) ( characteristics of mesoscopic CH3NH3PbI3 perovskite solar cells prepared with Mo cathodes of different thickness. thickness. The thickness of Mo was controlled by changing the sputtering time and measured by cross-sectional cross SEM images.
6
Table 1. Photovoltaic parameters of perovskite solar cells equipped with Mo cathodes of different thickness. Thickness of Mo
Voc [V]
Jsc [mA cm-2]
FF [%]
PCE [%]
Rs a [Ω cm2]
RSH b [Ω square-1]
40 nm
0.95
20.16
59.55
11.43
9.43
14.0
80 nm
0.97
20.61
67.77
13.58
3.41
5.1
140 nm
0.87
20.68
57.03
10.25
10.04
2.2
210 nm
0.89
19.45
53.79
9.35
21.74
1.3
a
b
Rs: Series resistances calculated from the slope of the J–V curves at Voc; RSH: Sheet resistances of thin films measured using the four-point probe method.
We fabricated perovskite solar cells with a Mo cathode of different thickness by controlling the sputtering time and compared their photovoltaic performance. Figure 2 shows the J–V curves of mesoscopic CH3NH3PbI3 perovskite solar cells in dependence of the Mo-electrode thickness and the individual photovoltaic parameters are summarized in Table 1. When the Mo thickness increased from 40 nm to 80 nm, the short circuit current density (Jsc), open circuit voltage (Voc), and fill factor (FF) were enhanced. Especially the FF, which is directly related to the series resistance Rs of the device, was considerably increased because of the smaller sheet resistance RSH of the 80-nm-thick Mo electrode (5.1 ȍ/square) compared to that of the 40-nm-thick Mo electrode (14.0 ȍ/square). As summarized in Table 1, the four point probe sheet resistances of the Mo electrodes decreased with increasing thickness of the Mo layer. However, the photovoltaic performances of the devices with thicker Mo cathode (140 and 210 nm) were inferior to that of the cell having 80-nm-thick Mo. In particular, upon increasing the thickness of the Mo layer to more than 140 nm, the J–V characteristic showed s-shaped curves reflecting the poor FF. This may be attributed to the damage of the organic HTM layer caused by the bombardment by high-energy particles in the plasma during longer sputtering times, which has been similarly reported as plasma damage in the fabrication of organic light-emitting diodes.[30] As the sputtering time increased, we found that small fractions of Mo were observed under the HTM layer and in direct contact with the perovskite layer, as shown in the elemental mapping of Figure S1 obtained by electron probe microanalysis (EPMA). The direct contact between perovskite and Mo metal without a hole-selective contact (here, spiro-OMeTAD) is non-ohmic (metal-semiconductor interface) and this is likely to result in inefficient charge transportation and increase Rs.[31, 32] As summarized in Table 1, the larger Rs obtained for devices with thicker Mo electrode (140 nm and 210 nm) led to deleterious effects on device performances, especially FF. These trends have been observed in hole conductor-free heterojunction perovskite solar cells, and
7
recently, Habisreutinger et al. also reported that direct contact contact between perovskite and metal cathode results in poor Voc and FF.[33-35]
Figure 3. (a) Current density–voltage voltage (J–V) ( ) curves of perovskite solar cells prepared with Au and Mo cathode. (b) Hysteresis of the J–V V curves measured in forward (short circuit ĺ open circuit) and reverse (open circuit ĺ short circuit) scan direction and (c) J–V curves of Mo- cathode perovskite solar cells measured with different delay times for each 10 mV voltage step.
8
We chose the optimum thickness of 80 nm for the Mo cathode resulting in highly efficient perovskite solar cells and compared their photovoltaic characteristics with those of the perovskite solar cell equipped with an 80-nm-thick Au electrode. As shown in Figure 3a, the cell with the Mo cathode -2
exhibited high Jsc of 20.11 mA cm and Voc of 1.01 V, which are nearly the same values as those of the Au-cathode cell. Even though the Au cathode with higher electrical conductivity showed lower RSH than the Mo electrodes, the difference in FF of the devices was not significant (Table 2). Eventually, the Mo-cathode cell showed the PCE of 13.28%, which is comparable with that of the Au-cathode cell (14.07%). Meanwhile, perovskite solar cells suffer from anomalous hysteresis in their J–V characteristics, depending on the scan rate and direction, resulting in discrepancies and, usually, overestimation of their photovoltaic performance. To confirm the reliability and accuracy of our results, we measured the J–V curves of the Mo-cathode cell at different scan directions and rates. Very small hysteresis between forward and reverse scans resulted in the average PCE of 12.88% and no deviations in J–V could be observed for different scan rates, as shown in Figure 3b and c. The small hysteresis in dependence of the scan direction can be further reduced by controlling the crystallinity and morphology of the perovskite layer, as reported previously.[9, 36-38]
Table 2. Photovoltaic parameters of perovskite solar cells with either Au or Mo cathode. Cathode
Voc [V]
Jsc [mA cm-2]
FF [%]
PCE [%]
RSH [Ω square-1]
Au
1.01
20.05
69.36
14.07
0.8
Mo
1.01
20.11
65.39
13.28
5.1
9
Figure 4. (a) Current density–voltage voltage (J–V) ( curves of the best-performing performing cell based on Mo cathode under 100 mW cm
-2
AM 1.5G illumination (red circle) and in the dark (black (black square). (b) Incident
photon to current efficiency (IPCE) spectrum of the best-performing cell (black line)) and calculated current density (Jsc) by integrating the IPCE curve with an AM 1.5G spectrum (red line). (c) Transient photocurrent response of the Mo-cathode cathode cell during the repetitive turning on and o off of simulated sunlight. (d) Steady-state state photocurrent at the maximum power point (0.8 V) and stabilized PCE.
The J–V characteristics of the best--performing performing cell based on the Mo cathode are shown iin Figure 4a. The best cell showed the high PCE of 15.06% along with w Voc of 1.01 V, FF of 0.67, and remarkably -2
high Jsc of 22.13 mA cm . Since Voc is exceeding 1 V along with high Jsc, Mo can efficiently replace noble-metal metal electrodes (Au or Ag) and thereby reduce the cost of perovskite solar cell while simultaneously exhibiting high performance. performance Figure 4b shows the high and broad incident photon photon-tocurrent efficiency (IPCE) over the whole spec spectral tral range of visible light and the integrated Jsc calculated from the IPCE spectrum under AM 1.5G photon flux is in agreement with the Jsc measured in the J–V curve. To confirm how efficiently charges are extracted extracted and collected by Mo cathode of perovskit perovskite solar cells, we measured the transient photocurrent response during repetitive on/off cycles under
10
-2
simulated 1 sun illumination (100 mW cm ) without bias. Figure 4c shows that the photoresponse photorespo of the device equipped with Mo cathode is rapid and stable st over repeated light on/off cycles with steady -2
Jsc exceeding 20 mA cm , indicating that holes are efficiently extracted at at the Mo electrode/HTM interface. We also monitored the photocurrent of th the best-performing performing device under working conditions at the maximum mum power point (0.8 V) to corroborate the accuracy of the measured J–V J results. As shown in Figure 4d, even after continuous scanning for 10 min, the photocurrent is very stable and its -2
high value of 18.3 mA cm at 0.8 V resulted in the stabilized PCE of about 15%. This is consistent with the measured J–V curve shown in Figure 4a. To examine the reproducibility of the performance, we fabricated 34 devices based on Mo cathodes. The histograms istograms of the photovoltaic characteristics are shown in Figure S2 and the he average values with standard deviation of the 34 devices are summarized in Table S1. The histograms and graphs of of the normal distributions confirm that the performance of the Mo-cathode-based based cells show good reproducibility with standard deviation of only 0.91% in efficiency.
Figure 5. (a) Representative ative nanoindentation curves of Mo and Au electrodes deposited on perovskite solar cells.. (b) Hardness and elastic modulus of Mo and Au electrodes obtained from 10
11
nanoindentation tests. (c) Elasticity of Mo and Au electrodes. (d) Schematic of nano-scratch test and optical images of Au and Mo electrodes after the scratch test. As mentioned earlier, superior mechanical properties of the cathode layer that is exposed to the external environment is essential to prevent any damage or failure during operation and handling of the solar cell.[39] Furthermore, a cathode layer with high elasticity can prevent permanent deformation during bending or folding. In this study, the mechanical properties and scratch resistance of Mo and Au electrodes deposited on perovskite solar cells were characterized by nanoindentation and scratch test, respectively.[40] In the nanoindenation tests, Berkovich diamond tip with 100 nm in radius was used. From the nanoindentation loading and unloading curves of Mo and Au electrodes shown in Figure 5a, the hardness, elastic modulus, and elasticity of Mo and Au electrodes were obtained by using the Oliver-Pharr method.[41] It should be noted that the elasticity was calculated by dividing the area beneath the loading curve (total energy) by that of the unloading curve (elastic energy) which can be used to quantitatively assess the degree of elasticity.[42] As shown in Figure 5b, the hardness values of Mo and Au electrodes were measured to be 2.9 ± 0.5 GPa and 0.4 ± 0.1 GPa, respectively. As can be confirmed from the hardness value of Mo that is ~7 times higher than that of Au electrode, it can be stated that Mo electrode deposited on the device shows significantly higher resistance to the permanent deformation compared to Au electrode. The elastic modulus of Mo and Au electrodes were measured to be 24.8 ± 3.4 GPa and 19.6 ± 2.8 GPa, respectively. The elasticity of Mo and Au electrodes were calculated to be 93.8 ± 2.7% and 19.2 ± 2.7%, respectively, as shown in Figure 5c. Thus, it was found that Mo electrode has drastically higher restoration capability against deformation originating from bending or folding compared to Au. In order to investigate the scratch resistance of Mo and Au electrodes on devices, nano-scratch test were performed. Figure 5d shows the schematic of nano-scratch test with a ramp loading and optical images of Mo and Au electrodes after scratch test. The scratch images clearly show that Au was severely scratched and peeled off from the underlying HTM layer, while no evidence of surface damage was found for Mo electrode after scratch test. These results indicate that Mo can be used as a mechanically stable cathode material for devices that require structural flexibility.
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Conclusions In summary, we demonstrated noble-metal-free highly efficient perovskite solar cells using sputtered Mo as the cathode material, thereby replacing the commonly used noble metals (Au and Ag). The Mo cathode with high work function and good electrical conductivity successfully replaced Au for perovskite solar cells, yielding comparable photovoltaic performances. Furthermore, due to the excellent mechanical properties of Mo, such as hardness and elasticity, compared with the Au electrode, the Mo electrode showed superior resistance and restoration capability against external scratch and deformation that can occur during the practical handling of these devices. By improving the quality of the perovskite films and our device-fabrication skills, we realized the best cell efficiency of as high as 15.06%, comparable to that of Au-based cells. This is the first report showing such a high efficiency in excess of 15% in mesoscopic perovskite solar cells based on noble-metal-free cathode. These results suggest that Mo is a promising candidate for replacing noble-metal electrodes in mesoscopic perovskite solar cells, thereby realizing low cost as well as high performance.
Acknowledgements The authors acknowledge funding support from the Global Frontier R&D Program on Center for Multiscale Energy System (2012M3A6A7054856) and 2015 University-Institute cooperation program funded by the National Research Foundation under the Ministry of Science, ICT & Future Planning, Korea; This work was also supported by the KIST institutional programs (2V03780 and 2E25392). This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2010-0018289).
Appendix. Supporting information Supplementary data associated with this article can be found in the online version at
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References [1]
A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050.
[2]
J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Park, N. G. Park, Nanoscale 3 (2011) 4088
[3]
M. Liu, M. B. Johnston, H. J. Snaith, Nature 501 (2013) 395.
[4]
J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Grätzel, Nature 499 (2013) 316.
[5]
H. S. Kim, C. R. Lee, J. H. Im, K. B. Lee, T. Moehl, A. Marchioro, S. J. Moon, R. Humphry-Baker, J. H. Yum, J. E. Moser, M. Grätzel, N. G. Park, Sci. Rep. 2 (2012) 591.
[6]
S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T., Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 342 (2013) 341.
[7]
G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar, T. C. Sum, Science 342 (2013) 344.
[8]
M. Grätzel, Nat Mater, 13 (2014) 838.
[9]
N. J. Jeon, J. H. Noh, Y. C. Kim, W. S. Yang, S. Ryu, S. I. Seok, Nat. Mater 13 (2014) 897.
[10]
N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Nature 517 (2015) 476.
[11]
N. G. Park, Mater. Today. 18, (2015) 65.
[12]
C. Wehrenfennig, G. E. Eperon, M. B. Johnston, H. J. Snaith, L. M. Herz, Adv. Mater. 26 (2014) 1584.
[13]
W.-J. Yin, T. Shi, Y. Yan, Adv. Mater. 26 (2014) 4653.
[14]
C.
Bi,
Y.
Yuan,
Y.
Fang,
J.
Huang,
Adv.
Energy
Mater.
(2014).
http://dx.doi.org/10.1002/aenm.201401616 in press. [15]
D. Liu, T. L. Kelly, Nat. Photonics 8 (2014) 133.
[16]
H.S. Jung, N. G. Park, Small, 11 (2015) 10.
[17]
M. A. Green, K. Emery, Y. Hishikawa, W. Warta, E.D. Dunlop, Prog. Photovolt: Res. Appl. 23 (2015) 1.
[18]
G. Hodes, Science 342 (2013) 317.
[19]
Z. Li, S. A. Kulkarni, P. P. Boix, E. Shi, A. Cao, K. Fu, S. K. Batabyal, J. Zhang, Q. Xiong, L. H. Wong, N. Mathews, S. G. Mhaisalkar, ACS Nano, 8 (2014) 6797.
[20]
Z. Wei, H. Chen, K. Yan, S. Yang, Angew. Chem. Int. Ed. 53 (2014) 13239.
[21]
Z. Wei, K. Yan, H. Chen, Y. Yi, T. Zhang, X. Long, J. Li, L. Zhang, J. Wang, S. Yang, Energy Environ. Sci. 7 (2014) 3326.
[22]
A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, M. Xu, M. Hu, J. Chen, Y. Yang, M. Grätzel, H. Han, Science 345 (2014) 295.
[23]
K. Yan, Z. Wei, J. Li, H. Chen, Y. Yi, X. Zheng, X. Long, Z. Wang, J. Wang, J. Xu, S. Yang, Small, 11 (2015) 2269.
14
[24]
X. Wang, Z. Li, W. Xu, S. A. Kulkarni, S. K. Batabyal, S. Zhang, A. Cao, L. H. Wong, Nano Energy, 11 (2015) 728.
[25]
Q. Jiang, X. Sheng, B. Shi, X. Feng, T. Xu, J. Phys. Chem. C 118 (2014) 25878.
[26]
E. Della Gaspera, Y. Peng, Q. Hou, L. Spiccia, U. Bach, J.J. Jasieniak, Y.-B. Cheng, Nano Energy, 13 (2015) 249.
[27]
Y. Han, S. Meyer, Y. Dkhissi, K. Weber, J.M. Pringle, U. Bach, L. Spiccia, Y.-B. Cheng, J. Mater. Chem. A, 3 (2015), 8139
[28]
K. Yoo, J.-Y. Kim, J.A. Lee, J.S. Kim, D.-K. Lee, K. Kim, J.Y. Kim, B. Kim, H. Kim, W.M. Kim, J.H. Kim, M.J. Ko, ACS Nano, 9 (2015) 3760.
[29]
H.B. Michaelson, J. Appl. Phys. 48 (1977) 4729.
[30]
H.-K. Kim, K.-S. Lee, J.H. Kwon, Appl. Phys. Lett.88 (2006) 012103.
[31]
E. J. Juarez-Perez, M. Wuȕler, F. Fabregat-Santiago, K. Lakus-Wollny, E. Mankel, T. Mayer, W. Jaegermann, I. Mora-Sero, J. Phys. Chem. Lett. 5 (2014) 680
[32]
J.-J. Shi, W. Dong, Y.-Z. Xu, C.-H. Li, S.-T. Lv, L.-F. Zhu, J. Dong, Y.-H. Luo, D.-M. Li, Q.-B. Meng, Q. Chen, Chin. Phys. Lett, 30 (2013) 128402.
[33]
Y. Xu, J. Shi, S. Lv, L. Zhu, J. Dong, H. Wu, Y. Xiao, Y. Luo, S. Wang, D. Li, X. Li, Q. Meng, ACS Appl. Mater. Inter. 6 (2014) 5651.
[34]
W. A. Laban, L. Etgar, Energy Environ. Sci. 6 (2013) 3249.
[35]
S. N. Habisreutinger, T. Leijtens, G. E. Eperon, S. D. Stranks, R. J. Nicholas, H. J. Snaith, Nano Letters, 14 (2014) 5561.
[36]
Z. Xiao, Q. Dong, C. Bi, Y. Shao, Y. Yuan, J. Huang, Adv. Mater. 26 (2014) 6503.
[37]
G. Li, K.L. Ching, J.Y.L. Ho, M. Wong, H.-S. Kwok, Adv. Energy Mater. (2015). http://dx.doi.org/10.1002/aenm.201401775 in press.
[38]
F. X. Xie, D. Zhang, H. Su, X. Ren, K. S. Wong, M. Grätzel, W. C. H. Choy, ACS Nano, 9 (2014) 639.
[39]
H. J. Kim, D. E. Kim, Solar Energy, 86 (2012) 2049.
[40]
H. J. Kim, D. E. Kim, Tribol Lett, 49 (2013) 85.
[41]
W. C. Oliver, G. M. Pharr, J. Mater. Res. 7 (1992) 1564.
[42]
R. H. Kim, H. J. Kim, I. Bae, S. K. Hwang, D. B. Velusamy, S. M. Cho, K. Takaishi, T. Muto, D. Hashizume, M. Uchiyama, P. André, F. Mathevet, B. Heinrich, T. Aoyama, D. E. Kim, H. Lee, J.-C. Ribierre, C. Park, Nat Commun, 5 (2014) 3583.
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Vitae Inyoung Jeong received his B.S. degree (2011) and a M.S degree (2013) in the Department of Chemical Engineering at Pohang University of Science and Technology (POSTECH), Korea. He is currently urrently a Ph.D. course student in research group of Prof. Jinwoo Lee at the POSTECH. His current research focuses on synthesis and application cation of functional nanomaterials for Dye sensitized solar cell and Perovskite ovskite solar cell.
Hae-Jin Jin Kim received his B.S. degree (2009) in the Department o of Mechanical Engineering Engineering at Yonsei University, Korea. He is currentl currently a unified M.S/Ph.D. course student in the research group oup of Prof. Dae Dae-Eun Kim at Yonsei University. His current research focu focuses on the characterization of mechanical and tribological properties perties of thin film for enhanced durability of mechanical systems. systems
Byung-Seok Seok Lee granted a B. S. degree (2013) in Department of Physics at Chungbuk National University, Korea. He is studying for a M. S. degree under Prof. Doh-Kwon Doh Kwon Lee in Department of Nanomaterials Science and Engineering at University of Science and d Technology (UST), Korea. His research field is inorganic thin-film thin film solar cells by non non-vacuum process.
Dr. Hae Jung Son received her B.S degree in chemistry from Sungkyunkwan University (2000) and M.S. degree from the Korea Advanced Institute of Science and Technology (2002), Rep. of Korea Korea. After graduation, she worked for Samsung Advanced IInstitute of Technology as a research resea staff member until 2004. She received Ph.D. degree in chemistry from the University of Chicago in 2011 under the supervision of Prof. Luping Yu. Presently, she is a senior research scientist in Photo-electronic Photo Hybrids Research Center at Korea Institut Institute of Science and Technology, Technology Rep. of Korea.. Her research interest is the development of conjugated polymers for opto-electronic opto electronic applications. Dr. Jin Young Kim is a senior research scientist of Korea Institute o of Science and Technology (KIST), Korea. Before joining g KIST, he worked for National Renewable Energy Laboratory (NREL), USA A from 2007 to 2011 as a postdoctoral researcher and a scientist. Dr. Kim obtained his BS (2000), MS (2002), and PhD (2006) degrees from the he Department of Materials Science and Engineering of Seoul National University. Dr. Kim Kim’s research interest is mostly focused on the next generation eration solar cells and solar fuels generation.
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Dr. Doh-Kwon Lee received his B.S. (1997), M.S. (1999), and Ph.D. degree (2005) in materials science and engineering from Seoul National University (Advisor: Prof. Han-Ill Yoo) in Korea. With his Ph.D. completed, he moved onto the Institute of Physical Chemistry, Justus-LiebigUniversity (JLU) Giessen in Germany, where he joined Prof. Janek’s research group as a senior scientist. Since 2009, Dr. Lee has worked as a senior/principal research scientist at Korea Institute of Science and Technology (KIST) in Korea. His current research interest is to develop solution-processed inorganic thin-film solar cells, organo-metal halide perovskite solar cells, and multi-junction solar cells. Prof. Dae-Eun Kim received his B.S. from Tufts University in 1984 and M.S. and Ph.D. from M.I.T. in 1986 and 1991, respectively. He was an Assistant Professor at the Ohio State University before joining the faculty of Yonsei University, where he is currently a Professor in the Department of Mechanical Engineering. He is also the Director of the Center for NanoWear, which is part of the Creative Research Initiative program sponsored by the National Research Foundation of Korea. His research interests include nano/bio-tribology, functional coatings, and micro-fabrication.
Prof. Jinwoo Lee obtained his B.S. (1998) and Ph.D. (2003) from the Department of Chemical and Biological Engineering of Seoul National University, Korea. After his postdoctoral research at Seoul National University (with Prof. Taeghwan Hyeon, 2003–2005) and Cornell University (with Prof. Ulrich Wiesner, 2005–2008), he joined the faculty of the Department of Chemical Engineering at Pohang University of Science and Technology (POSTECH) in June, 2008. His research field includes synthesis and applications of ordered functional mesoporous materials and shape controlled nanocrystals in energy conversion and storage devices.
Dr. Min Jae Ko is a principal research scientist at Korea Institute of Science and Technology (KIST) and an adjunct professor at Korea University. He obtained his B.S (1995) and M.S (1997) degrees from the Department of Fiber and Polymer Science, and Ph.D. (2001) from the Department of Materials Science and Engineering at Soul National University, Korea. He performed his postdoc work at MIT from 2001 to 2004. Then he moved to Samsung Electronics Co., as a senior research engineer in 2005. His research is focusing on the developments of materials and devices for the next generation flexible solar cells.
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Highlights z
Highly efficient and noble metal-free perovskite solar cell (PSC) were fabricated.
z
The Mo cathoded PSCs exhibit comparable performances to that of Au-based cells.
z
Best cell based on Mo cathode shows stabilized efficiency as high as 15%.
z
Nanoindentation and wear test show Mo can serve as a durable electrode for PSCs.
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*Graphical Abstract