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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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DOI: 10.1039/C3CC49458F
Cite this: DOI: 10.1039/c0xx00000x
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Hong Zhang,a Yantao Shi,*a Feng Yan,b Liang Wang,a Kai Wang,a Yujin Xing,a Qingshun Dong,a and Tingli Ma*a,c 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x The ionic liquid N-butyl-N’-(4-pyridylheptyl)imidazolium bis(trifluoromethane)sulfonimide (BuPyIm-TFSI) was used as a dual-functional additive to improve the electrical properties of the hole-transporting material (HTM) for perovskite solar cells. BuPyIm-TFSI improved the conductivity of HTM and reduced dark current of the solar cell simultaneously, thereby greatly increasing the power conversion efficiency. As a typical 3rd generation photovoltaic cell, mesoscopic solar cells have been studied intensively since 1991.1 However, no breakthroughs are available for conventional dye-sensitized solar cells (DSCs), regardless of the many so-called strategies that have been tested experimentally or theoretically. In 2012, this stagnant situation was successfully overcome by using organolead halide perovskites as sensitizer in all-solid-state mesoscopic solar cells.2 Up to now, power conversion efficiencies (PCEs) of higher than 15% have been reported by the groups of H. J. Snaith3 and M. Grätzel,4 either based on a 3D porous framework or a 2D planar structure. Undoubtedly, perovskite solar cells open up a new and promising avenue for future thin-film solar cell development.5 Perovskite solar cells generally comprise a conductive substrate, a compact TiO2 layer, an organolead halide perovskite light-absorption layer (single or combined with a porous scaffold), a hole-transporting layer (HTM), and a metal cathode (Ag or Au). As of this writing, studies have mainly focused on designing scaffold layers,6 optimizing the perovskite layer,3, 4, 7 exploring alternative structures,8 and probing into inherent mechanisms.9 As one of the key components in perovskite solar cells, HTM separates photo-excited electron-hole pairs and transports the holes to the external circuit. Thus photovoltaic performance of perovskite solar cells is highly dependent on the properties of HTM in which 2,2′,7,7′-tetrakis(N,N-di-p--methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD) is currently widely used as the matrix for hole transportation.2-7 However, spiroMeOTAD alone in HTM is insufficient for obtaining high PCEs because of low conductivity and other problems. Adding different kinds of functional additives to spiro-MeOTAD can further improve the properties of HTM and ensure the high efficiency perovskite solar cells. For example, lithium10 or cobalt salts4 are frequently used for p-doping to increase the hole concentration. Despite the high volatility and unpleasant smell, TBP (4-tertbutylpyridine) is usually added into HTM to suppress chargerecombination, as in traditional DSCs.11 For the first time, we use a stable ionic liquid N-butyl-N’-(4-pyridylheptyl)imidazolium This journal is © The Royal Society of Chemistry [year]
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bis(trifluoromethane) sulfonimide (BuPyIm-TFSI) as a dualfunctional additive to simultaneously improve the electrical property of HTM and suppress charge combination in perovskite solar cells. BuPyIm-TFSI improved the conductivity of HTM and reduced the dark current of the solar cell.The PCE greatly increased from 3.83% (with no additive in HTM) to 7.91%, and these rates are comparable to those in conventional HTM containing lithium salt and TBP. Moreover, using such dualfunctional additive can effectively simplify the components of HTM and help reduce the production cost.
Fig. 1. Molecular structures of the BuPyIm-TFSI and spiro-OMeTAD
Molecular structures of BuPyIm-TFSI and spiro-MeOTAD are shown in Fig. 1, the molecular structure of BuPyIm-TFSI was characterized by NMR spectra in Supporting Information. As illustrated in Fig. 1, BuPyIm-TFSI is an ionic liquid and its cation comprises imidazolyl, pyridyl and alkyl chains that are used to either link or modify the two heteroatom rings. Unlike ordinary organic additives, ionic liquids are nearly non-volatile and usually have excellent stability. As shown in Fig. 2a, subsequent thermal gravimetric analysis indicated that the decomposition temperature of BuPyIm-TFSI was higher than 300°C. Undoubtedly, such satisfactory thermal stability will be meaningful to the long-term durability of perovskite solar cells. More importantly, the elaborate structural design enables dual function of the BuPyIm-TFSI. On one hand, based on the mechanism proposed by H. J. Snaith, free proton can be released from imidazolyl for p-doping and thus increase the conductivity of HTM by partial oxidation of spiro-MeOTAD.12 On the other hand, pyridyl can function in the same way as TBP to suppress the charge recombination that occurs at the TiO2/perovskite [journal], [year], [vol], 00–00 | 1
ChemComm Accepted Manuscript
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Dual Functional Additive for HTM Layer in Perovskite Solar Cells
ChemComm
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DOI: 10.1039/C3CC49458F
HTM
Compositions
VOC (V)
JSC (mA cm-2)
FF
PCE (%)
HTM1
Pristine spiro
0.63±0.03
12.26±0.20
0.50±0.08
3.83 ±0.20
HTM2
Spiro+BuPyIm-TFSI
0.87±0.04
16.26±0.30
0.56±0.03
7.91 ±0.30
HTM3
Spiro+Li+TBP
0.91±0.05
15.56±0.50
0.57±0.05
8.16±0.25
XRD spectroscopy (Fig. S1). We also verified that the perovskite was uniformly distributed throughout the mesoporous TiO2 films by performing cross-sectional scanning electron microscopy with elemental mapping via energy dispersive X-ray analysis (Fig. S2)
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Fig. 3. (a) Cross-sectional SEM image of a complete photovoltaic device based on HTM2. (b) SEM image of the top view of perovskite layer.
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Fig. 2. (a) Thermal gravimetric analysis curves of BuPyIm-TFSI; (b) J-V curves of CE/HTM/CE devices for different HTM.
interface. In total, three HTM samples we prepared and designated as HTM1, HTM2 and HTM3, respectively. HTM1 contained only spiro-MeOTAD, whereas HTM2 contained BuPyIm-TFSI and spiro-MeOTAD. For comparison, we prepared HTM3 using spiro-MeOTAD, lithium salt (LiTFSI) and TBP based on published works.4 Conductivities of these three HTMs were characterized using linear sweep voltammetry by sandwiching HTM between two platinum electrodes, as reported previously.13 As shown in Fig. 2b, BuPyIm-TFSI improved the conductive property of HTM2 compared with HTM1 and HTM3. Based on a previously reported theory, an ionic liquid such as BuPyIm-TFSI is considered as Brϕnsted acid and can release free protons to facilitate the oxidization of spiro-MeOTAD, which increase the hole concentration and the subsequent conductivity of HTM.12 We fabricated our perovskite solar cells using a previously described method, in which one sequential deposition route was used to well control the morphology of CH3NH3PbI3.4 The cross section and the top view of the perovskite solar cells are shown in Fig. 3. In addition to the necessary layer of dense TiO2, we also added one mesoporous layer of anatase TiO2 (approximately 300 nm thick) to support the CH3NH3PbI3, which was prepared by two deposition steps using PbI2 and CH3NH3I solutions consecutively. The formation of CH3NH3PbI3 was confirmed by
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Photovoltaic performance of the perovskite solar cells based on HTM1, HTM2, and HTM3 was measured under AM 1.5, 100 mW/cm2 light illumination. J-V curves of these three perovskite solar cells are illustrated in Fig. 4 and the detailed parameters have been summarized in Table 1. All of the above mentioned measurements were repeated more than three times. HTM2-based perovskite solar cells showed a much better performance than HTM1-based ones that contained only spiro-MeOTAD. The opencircuit voltage (Voc) and short-circuit current density (Jsc) of HTM1-based perovskite solar cells were ca. 0.63 V and 12.26 mA cm-2, respectively. The Voc and Jsc values were greatly enhanced to ca. 0.87 V and 16.26 mA cm-2, respectively, in HTM2-based perovskite solar cells with BuPyIm-TFSI. The fill factor, which reflects the inherent resistance and the degree of chare recombination, also increased from ca. 0.50 to 0.56. HTM2-based perovskite solar cells achieved a PCE of 7.91% because of the improved parameters, and this PCE value was much higher than that of HTM1-based cells (3.83%).This result was comparable to that obtained for HTM3-based cells containing LiTFSI and TBP, as shown in Table 1.
Fig. 4. J-V curves of the peroviskite solar cells employing different HTM.
2 | Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]
ChemComm Accepted Manuscript
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Table 1 Photovoltaic parameters of the peroviskite solar cells under 100 mWcm-2 light illumination.
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DOI: 10.1039/C3CC49458F
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Fig. 5. IPCEs of the peroviskite solar cells employing different HTM
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Incident photon-to-current conversion efficiencies (IPCEs) of our perovskite solar cells were characterized and shown in Fig. 5. For this type of solar cells, the spectrum rang, within which sunlight can be effectively converted into electricity, is very wide. The spectrum ranged from 300 nm to higher than 750 nm, with the maximum at ca. 520 nm. Compared with HTM1-based solar cells, HTM2- and HTM3-based perovskite solar cells demonstrated higher IPCE within this broad range, especially from 350 nm to 750 nm, where more than 50% of the photons can be successfully converted into electricity. This result was in agreement with J-V measurements. In summary, the validity of ionic liquid BuPyIm-TFSI is reported for the first time as a stable and dual-functional additive to enhance HTM conductivity and suppress charge combination in perovskite solar cells simultaneously. Various characterizations were carried out and some relevant mechanisms were determined to support our conclusions. BuPyIm-TFSI in HTM improved photovoltaic parameters, such as photocurrent, photovoltage, and fill factor. The PCE was enhanced remarkably. Using such dual functional additive can effectively simplify the components of HTM and help reduce production cost. This work is supported by the National Natural Science Foundation of China (Grant No. 51273032), Doctoral Found of Ministry of Education of China (Grant No. 2110041110003), International Science & Technology Cooperation Program of Chi na (Grant No. 2013DFA51000),Fundamental Research Funds for the Central Universities (Grant No. DUT12RC(3)57) and Open Project Program of the State Key Laboratory of Physical Chemical of Solid Surfaces, Xiamen University (Grant No. 201210). This research was also supported by the State Key Laboratory of Fine Chemicals of China. We also thank Prof. Yi Xiao and Mr. Youdi Zhang for helping evaporate sliver electrode.
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State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology (DUT), 2 Linggong Rd., 116024, Dalian, P. R. China. Fax: +86-411-84986230; Tel: +86-411-84986230; E-mail: [email protected]; [email protected] b Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, P. R. China c Graduate School of Life Science and Systems Engineering,Kyushu Institute of Technolog, 2-4 Hibikino, Wakamatsu, Kitakyushu, Fukuoka, 808-0196, Japan † Electronic Supplementary Information (ESI) available: [Experimental details, fabrication and characterization of devices].See DOI: 10.1039/b000000x/ 1 (a) O’Regan, M. Grätzel, Nature 1991, 353, 24; (b) A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, 6595; (c) W. Chen, Y. Qiu, S. Yang, Phys. Chem. Chem. Phys., 2012, 14, 10872; (d) H. Zhang, Y. Shi, L. Wang, C. Wang, H. Zhou, W. Guo and T. Ma, Chem. Commun., 2013, 49, 9003; (e) D. Xu, X. Chen, L. Wang, L. Qiu, H. Zhang and F. Yan, Electrochim. Acta, 2013,106, 181. 2 (a) 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 and N.-G. Park, Sci. Rep. , 2012, 2, 1; (b) M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science, 2012, 338 , 643; 3 M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, 395. 4 J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin and M. Grätzel, Nature, 2013,499, 316 . 5 (a) N.-G. Park, J. Phys. Chem. Lett., 2013, 4, 2423; (b) B. Cai, Y. Xing, Z. Yang, W.-H. Zhang and J. Qiu, Energy Environ. Sci., 2013, 6, 1480; (c) H. Chen, X. Pan, W. Liu, M. Cai, D. Kou, Z. Huo, X. X. Fang and S. Dai, Chem. Commun., 2013, 49, 7277. 6 (a) J. M. Ball, M. M. Lee, A. Hey and H. Snaith, Energy Environ.Sci.,2013, 6, 1739; (b) J. Qiu, Y. Qiu, K. Yan, M. Zhong, C. Mu, H. Yan and S. Yang, Nanoscale, 2013, 5, 3245; (c) D. Bi, L. Häggman, G. Boschloo, L. Yang, E. M. Johansson and A. Hagfeldt, RSC Adv., 2013, 3, 18762; (d) M. J. Carnie, C. Charbonnaeu, M. L. Davies, J. Troughton, T. M. Watson, K. Wojciechowski, H. Snaith and D. A. Worsley, Chem. Commun., 2013,49, 7893. 7 (a) J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal and S. I. Seok, Nano lett., 2013, 13, 1764; (b) G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely and H. J. Snaith, Adv. Funct. Mater., 2013, DOI: 10.1002/adfm.201302090. 8 (a) W. A. Laban and L. Etgar, Energy Environ. Sci. , 2013, 6, 3249; (b) W. Li, J. Li, L. Wang, G. Niu, R. Gao and Y. Qiu, J. Mater. Chem. A, 2013, 1, 11735; (c) Z. Ku, Y. Rong, M. Xu, T. Liu and H. Han, Sci. Rep., 2013, DOI: 10.1038/srep03132; (d) J. You, Z. Hong, Y. Yang, Q. Chen, M. Cai, T.-B. Song, C.-C. Chen, S. Lu, Y. Liu, H. Zhou and Y. Yang, ACS Nano, 2014. 8,1674. 9 (a) H.-S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F. FabregatSantiago, E. J. Juarez-Perez, N.-G. Park and J. Bisquert, Nature commun., 2013, DOI:10.1038/ncomms3242; (b) G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar and T. C. Sum, Science, 2013, 342, 344. 10 A. Abate, T. Leijtens, S. Pathak, J. Teuscher, R. Avolio, M. E. Errico, J. Kirkpatrik, J. M. Ball, P. Docampo, I. McPherson and H. J. Snaith, Phys. Chem. Chem. Phys., 2013,15, 257. 11 W.H. Howie, J.E. Harris, J.R. Jennings, L.M. Peter, Sol. Energy Mater. Sol. Cells, 2007, 91, 424. 12 A. Abate, D. J. Hollman, J. Teuscher, S. Pathak, R. Avolio, G. D’Errico, G. Vitiello, S. Fantacci and H. J. Snaith, J. Am. Chem. Soc. , 2013, 135, 13538. 13 M. Xu, Y. Rong, Z. Ku, A. Mei, X. Li and H. Han, J. Phys. Chem. C, 2013, 117, 22492.
Notes and references This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3
ChemComm Accepted Manuscript
Published on 25 March 2014. Downloaded by National Dong Hwa University Library on 26/03/2014 13:24:07.
5
To better understand the inherent mechanism underlying the improvements in HTM2-based perovskite solar cells, we first provide insight into the process of charge recombination by characterizing the dark current shown in Fig. 4. An HTM2-based solar cell exhibited a much lower dark current density than an HTM1-based solar cell, indicating that BuPyIm-TFSI can suppress charge recombination. Similar to the pronounced effect of TBP on the interfaces of either perovsikte/TiO2 in perovskite solar cells or dye/TiO2 in traditional solid state DSCs, BuPyImTFSI also form a barrier layer that retards charge recombination between electrons in TiO2 and holes in the HTM.
ChemComm
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The ionic liquid N-butyl-N’-(4-pyridylheptyl) imidazolium bis(trifluoromethane) sulfonimide (BuPyIm-TFSI) was used as an additive to improve HTM conductivity and reduce dark current of the solar cell simultaneously, thereby greatly increasing the power conversion efficiency.
ChemComm Accepted Manuscript
Published on 25 March 2014. Downloaded by National Dong Hwa University Library on 26/03/2014 13:24:07.
DOI: 10.1039/C3CC49458F
ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: H. Zhang, Y. shi, F. Yan, L. Wang, K. Wang, Y. Xing, Q. Dong and T. L. ma, Chem. Commun., 2014, DOI: 10.1039/C3CC49458F.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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DOI: 10.1039/C3CC49458F
Cite this: DOI: 10.1039/c0xx00000x
CMMUNICATION
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Hong Zhang,a Yantao Shi,*a Feng Yan,b Liang Wang,a Kai Wang,a Yujin Xing,a Qingshun Dong,a and Tingli Ma*a,c 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x The ionic liquid N-butyl-N’-(4-pyridylheptyl)imidazolium bis(trifluoromethane)sulfonimide (BuPyIm-TFSI) was used as a dual-functional additive to improve the electrical properties of the hole-transporting material (HTM) for perovskite solar cells. BuPyIm-TFSI improved the conductivity of HTM and reduced dark current of the solar cell simultaneously, thereby greatly increasing the power conversion efficiency. As a typical 3rd generation photovoltaic cell, mesoscopic solar cells have been studied intensively since 1991.1 However, no breakthroughs are available for conventional dye-sensitized solar cells (DSCs), regardless of the many so-called strategies that have been tested experimentally or theoretically. In 2012, this stagnant situation was successfully overcome by using organolead halide perovskites as sensitizer in all-solid-state mesoscopic solar cells.2 Up to now, power conversion efficiencies (PCEs) of higher than 15% have been reported by the groups of H. J. Snaith3 and M. Grätzel,4 either based on a 3D porous framework or a 2D planar structure. Undoubtedly, perovskite solar cells open up a new and promising avenue for future thin-film solar cell development.5 Perovskite solar cells generally comprise a conductive substrate, a compact TiO2 layer, an organolead halide perovskite light-absorption layer (single or combined with a porous scaffold), a hole-transporting layer (HTM), and a metal cathode (Ag or Au). As of this writing, studies have mainly focused on designing scaffold layers,6 optimizing the perovskite layer,3, 4, 7 exploring alternative structures,8 and probing into inherent mechanisms.9 As one of the key components in perovskite solar cells, HTM separates photo-excited electron-hole pairs and transports the holes to the external circuit. Thus photovoltaic performance of perovskite solar cells is highly dependent on the properties of HTM in which 2,2′,7,7′-tetrakis(N,N-di-p--methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD) is currently widely used as the matrix for hole transportation.2-7 However, spiroMeOTAD alone in HTM is insufficient for obtaining high PCEs because of low conductivity and other problems. Adding different kinds of functional additives to spiro-MeOTAD can further improve the properties of HTM and ensure the high efficiency perovskite solar cells. For example, lithium10 or cobalt salts4 are frequently used for p-doping to increase the hole concentration. Despite the high volatility and unpleasant smell, TBP (4-tertbutylpyridine) is usually added into HTM to suppress chargerecombination, as in traditional DSCs.11 For the first time, we use a stable ionic liquid N-butyl-N’-(4-pyridylheptyl)imidazolium This journal is © The Royal Society of Chemistry [year]
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bis(trifluoromethane) sulfonimide (BuPyIm-TFSI) as a dualfunctional additive to simultaneously improve the electrical property of HTM and suppress charge combination in perovskite solar cells. BuPyIm-TFSI improved the conductivity of HTM and reduced the dark current of the solar cell.The PCE greatly increased from 3.83% (with no additive in HTM) to 7.91%, and these rates are comparable to those in conventional HTM containing lithium salt and TBP. Moreover, using such dualfunctional additive can effectively simplify the components of HTM and help reduce the production cost.
Fig. 1. Molecular structures of the BuPyIm-TFSI and spiro-OMeTAD
Molecular structures of BuPyIm-TFSI and spiro-MeOTAD are shown in Fig. 1, the molecular structure of BuPyIm-TFSI was characterized by NMR spectra in Supporting Information. As illustrated in Fig. 1, BuPyIm-TFSI is an ionic liquid and its cation comprises imidazolyl, pyridyl and alkyl chains that are used to either link or modify the two heteroatom rings. Unlike ordinary organic additives, ionic liquids are nearly non-volatile and usually have excellent stability. As shown in Fig. 2a, subsequent thermal gravimetric analysis indicated that the decomposition temperature of BuPyIm-TFSI was higher than 300°C. Undoubtedly, such satisfactory thermal stability will be meaningful to the long-term durability of perovskite solar cells. More importantly, the elaborate structural design enables dual function of the BuPyIm-TFSI. On one hand, based on the mechanism proposed by H. J. Snaith, free proton can be released from imidazolyl for p-doping and thus increase the conductivity of HTM by partial oxidation of spiro-MeOTAD.12 On the other hand, pyridyl can function in the same way as TBP to suppress the charge recombination that occurs at the TiO2/perovskite [journal], [year], [vol], 00–00 | 1
ChemComm Accepted Manuscript
Published on 25 March 2014. Downloaded by National Dong Hwa University Library on 26/03/2014 13:24:07.
Dual Functional Additive for HTM Layer in Perovskite Solar Cells
ChemComm
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DOI: 10.1039/C3CC49458F
HTM
Compositions
VOC (V)
JSC (mA cm-2)
FF
PCE (%)
HTM1
Pristine spiro
0.63±0.03
12.26±0.20
0.50±0.08
3.83 ±0.20
HTM2
Spiro+BuPyIm-TFSI
0.87±0.04
16.26±0.30
0.56±0.03
7.91 ±0.30
HTM3
Spiro+Li+TBP
0.91±0.05
15.56±0.50
0.57±0.05
8.16±0.25
XRD spectroscopy (Fig. S1). We also verified that the perovskite was uniformly distributed throughout the mesoporous TiO2 films by performing cross-sectional scanning electron microscopy with elemental mapping via energy dispersive X-ray analysis (Fig. S2)
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Fig. 3. (a) Cross-sectional SEM image of a complete photovoltaic device based on HTM2. (b) SEM image of the top view of perovskite layer.
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Fig. 2. (a) Thermal gravimetric analysis curves of BuPyIm-TFSI; (b) J-V curves of CE/HTM/CE devices for different HTM.
interface. In total, three HTM samples we prepared and designated as HTM1, HTM2 and HTM3, respectively. HTM1 contained only spiro-MeOTAD, whereas HTM2 contained BuPyIm-TFSI and spiro-MeOTAD. For comparison, we prepared HTM3 using spiro-MeOTAD, lithium salt (LiTFSI) and TBP based on published works.4 Conductivities of these three HTMs were characterized using linear sweep voltammetry by sandwiching HTM between two platinum electrodes, as reported previously.13 As shown in Fig. 2b, BuPyIm-TFSI improved the conductive property of HTM2 compared with HTM1 and HTM3. Based on a previously reported theory, an ionic liquid such as BuPyIm-TFSI is considered as Brϕnsted acid and can release free protons to facilitate the oxidization of spiro-MeOTAD, which increase the hole concentration and the subsequent conductivity of HTM.12 We fabricated our perovskite solar cells using a previously described method, in which one sequential deposition route was used to well control the morphology of CH3NH3PbI3.4 The cross section and the top view of the perovskite solar cells are shown in Fig. 3. In addition to the necessary layer of dense TiO2, we also added one mesoporous layer of anatase TiO2 (approximately 300 nm thick) to support the CH3NH3PbI3, which was prepared by two deposition steps using PbI2 and CH3NH3I solutions consecutively. The formation of CH3NH3PbI3 was confirmed by
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Photovoltaic performance of the perovskite solar cells based on HTM1, HTM2, and HTM3 was measured under AM 1.5, 100 mW/cm2 light illumination. J-V curves of these three perovskite solar cells are illustrated in Fig. 4 and the detailed parameters have been summarized in Table 1. All of the above mentioned measurements were repeated more than three times. HTM2-based perovskite solar cells showed a much better performance than HTM1-based ones that contained only spiro-MeOTAD. The opencircuit voltage (Voc) and short-circuit current density (Jsc) of HTM1-based perovskite solar cells were ca. 0.63 V and 12.26 mA cm-2, respectively. The Voc and Jsc values were greatly enhanced to ca. 0.87 V and 16.26 mA cm-2, respectively, in HTM2-based perovskite solar cells with BuPyIm-TFSI. The fill factor, which reflects the inherent resistance and the degree of chare recombination, also increased from ca. 0.50 to 0.56. HTM2-based perovskite solar cells achieved a PCE of 7.91% because of the improved parameters, and this PCE value was much higher than that of HTM1-based cells (3.83%).This result was comparable to that obtained for HTM3-based cells containing LiTFSI and TBP, as shown in Table 1.
Fig. 4. J-V curves of the peroviskite solar cells employing different HTM.
2 | Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]
ChemComm Accepted Manuscript
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Table 1 Photovoltaic parameters of the peroviskite solar cells under 100 mWcm-2 light illumination.
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DOI: 10.1039/C3CC49458F
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Fig. 5. IPCEs of the peroviskite solar cells employing different HTM
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Incident photon-to-current conversion efficiencies (IPCEs) of our perovskite solar cells were characterized and shown in Fig. 5. For this type of solar cells, the spectrum rang, within which sunlight can be effectively converted into electricity, is very wide. The spectrum ranged from 300 nm to higher than 750 nm, with the maximum at ca. 520 nm. Compared with HTM1-based solar cells, HTM2- and HTM3-based perovskite solar cells demonstrated higher IPCE within this broad range, especially from 350 nm to 750 nm, where more than 50% of the photons can be successfully converted into electricity. This result was in agreement with J-V measurements. In summary, the validity of ionic liquid BuPyIm-TFSI is reported for the first time as a stable and dual-functional additive to enhance HTM conductivity and suppress charge combination in perovskite solar cells simultaneously. Various characterizations were carried out and some relevant mechanisms were determined to support our conclusions. BuPyIm-TFSI in HTM improved photovoltaic parameters, such as photocurrent, photovoltage, and fill factor. The PCE was enhanced remarkably. Using such dual functional additive can effectively simplify the components of HTM and help reduce production cost. This work is supported by the National Natural Science Foundation of China (Grant No. 51273032), Doctoral Found of Ministry of Education of China (Grant No. 2110041110003), International Science & Technology Cooperation Program of Chi na (Grant No. 2013DFA51000),Fundamental Research Funds for the Central Universities (Grant No. DUT12RC(3)57) and Open Project Program of the State Key Laboratory of Physical Chemical of Solid Surfaces, Xiamen University (Grant No. 201210). This research was also supported by the State Key Laboratory of Fine Chemicals of China. We also thank Prof. Yi Xiao and Mr. Youdi Zhang for helping evaporate sliver electrode.
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State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology (DUT), 2 Linggong Rd., 116024, Dalian, P. R. China. Fax: +86-411-84986230; Tel: +86-411-84986230; E-mail: [email protected]; [email protected] b Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, P. R. China c Graduate School of Life Science and Systems Engineering,Kyushu Institute of Technolog, 2-4 Hibikino, Wakamatsu, Kitakyushu, Fukuoka, 808-0196, Japan † Electronic Supplementary Information (ESI) available: [Experimental details, fabrication and characterization of devices].See DOI: 10.1039/b000000x/ 1 (a) O’Regan, M. Grätzel, Nature 1991, 353, 24; (b) A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, 6595; (c) W. Chen, Y. Qiu, S. Yang, Phys. Chem. Chem. Phys., 2012, 14, 10872; (d) H. Zhang, Y. Shi, L. Wang, C. Wang, H. Zhou, W. Guo and T. Ma, Chem. Commun., 2013, 49, 9003; (e) D. Xu, X. Chen, L. Wang, L. Qiu, H. Zhang and F. Yan, Electrochim. Acta, 2013,106, 181. 2 (a) 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 and N.-G. Park, Sci. Rep. , 2012, 2, 1; (b) M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science, 2012, 338 , 643; 3 M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, 395. 4 J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin and M. Grätzel, Nature, 2013,499, 316 . 5 (a) N.-G. Park, J. Phys. Chem. Lett., 2013, 4, 2423; (b) B. Cai, Y. Xing, Z. Yang, W.-H. Zhang and J. Qiu, Energy Environ. Sci., 2013, 6, 1480; (c) H. Chen, X. Pan, W. Liu, M. Cai, D. Kou, Z. Huo, X. X. Fang and S. Dai, Chem. Commun., 2013, 49, 7277. 6 (a) J. M. Ball, M. M. Lee, A. Hey and H. Snaith, Energy Environ.Sci.,2013, 6, 1739; (b) J. Qiu, Y. Qiu, K. Yan, M. Zhong, C. Mu, H. Yan and S. Yang, Nanoscale, 2013, 5, 3245; (c) D. Bi, L. Häggman, G. Boschloo, L. Yang, E. M. Johansson and A. Hagfeldt, RSC Adv., 2013, 3, 18762; (d) M. J. Carnie, C. Charbonnaeu, M. L. Davies, J. Troughton, T. M. Watson, K. Wojciechowski, H. Snaith and D. A. Worsley, Chem. Commun., 2013,49, 7893. 7 (a) J. H. Noh, S. H. Im, J. H. Heo, T. N. Mandal and S. I. Seok, Nano lett., 2013, 13, 1764; (b) G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely and H. J. Snaith, Adv. Funct. Mater., 2013, DOI: 10.1002/adfm.201302090. 8 (a) W. A. Laban and L. Etgar, Energy Environ. Sci. , 2013, 6, 3249; (b) W. Li, J. Li, L. Wang, G. Niu, R. Gao and Y. Qiu, J. Mater. Chem. A, 2013, 1, 11735; (c) Z. Ku, Y. Rong, M. Xu, T. Liu and H. Han, Sci. Rep., 2013, DOI: 10.1038/srep03132; (d) J. You, Z. Hong, Y. Yang, Q. Chen, M. Cai, T.-B. Song, C.-C. Chen, S. Lu, Y. Liu, H. Zhou and Y. Yang, ACS Nano, 2014. 8,1674. 9 (a) H.-S. Kim, I. Mora-Sero, V. Gonzalez-Pedro, F. FabregatSantiago, E. J. Juarez-Perez, N.-G. Park and J. Bisquert, Nature commun., 2013, DOI:10.1038/ncomms3242; (b) G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar and T. C. Sum, Science, 2013, 342, 344. 10 A. Abate, T. Leijtens, S. Pathak, J. Teuscher, R. Avolio, M. E. Errico, J. Kirkpatrik, J. M. Ball, P. Docampo, I. McPherson and H. J. Snaith, Phys. Chem. Chem. Phys., 2013,15, 257. 11 W.H. Howie, J.E. Harris, J.R. Jennings, L.M. Peter, Sol. Energy Mater. Sol. Cells, 2007, 91, 424. 12 A. Abate, D. J. Hollman, J. Teuscher, S. Pathak, R. Avolio, G. D’Errico, G. Vitiello, S. Fantacci and H. J. Snaith, J. Am. Chem. Soc. , 2013, 135, 13538. 13 M. Xu, Y. Rong, Z. Ku, A. Mei, X. Li and H. Han, J. Phys. Chem. C, 2013, 117, 22492.
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ChemComm Accepted Manuscript
Published on 25 March 2014. Downloaded by National Dong Hwa University Library on 26/03/2014 13:24:07.
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To better understand the inherent mechanism underlying the improvements in HTM2-based perovskite solar cells, we first provide insight into the process of charge recombination by characterizing the dark current shown in Fig. 4. An HTM2-based solar cell exhibited a much lower dark current density than an HTM1-based solar cell, indicating that BuPyIm-TFSI can suppress charge recombination. Similar to the pronounced effect of TBP on the interfaces of either perovsikte/TiO2 in perovskite solar cells or dye/TiO2 in traditional solid state DSCs, BuPyImTFSI also form a barrier layer that retards charge recombination between electrons in TiO2 and holes in the HTM.
ChemComm
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The ionic liquid N-butyl-N’-(4-pyridylheptyl) imidazolium bis(trifluoromethane) sulfonimide (BuPyIm-TFSI) was used as an additive to improve HTM conductivity and reduce dark current of the solar cell simultaneously, thereby greatly increasing the power conversion efficiency.
ChemComm Accepted Manuscript
Published on 25 March 2014. Downloaded by National Dong Hwa University Library on 26/03/2014 13:24:07.
DOI: 10.1039/C3CC49458F
