This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.
Article http://pubs.acs.org/journal/acsodf
Orientation Control of Two-Dimensional Perovskites by Incorporating Carboxylic Acid Moieties Ryosuke Arai, Masahiro Yoshizawa-Fujita, Yuko Takeoka,* and Masahiro Rikukawa Faculty of Science and Engineering, Sophia University, 7-1 Kioi-cho, Chiyoda-ku, Tokyo 1028554, Japan S Supporting Information *
ABSTRACT: Two-dimensional perovskite compounds, (RNH3)2PbX4, have attracted much attention as quantum confinement materials. To achieve suitable orientation and exciton properties for optical applications, carboxy groups were introduced into the ammonium cations of two-dimensional perovskite compounds, which formed dimer structures based on the hydrogen bonding by the carboxy moieties. This structural organization allowed control of the layer orientation for favorable solar cells and thermal stability of the perovskites, while maintaining quantum confinement effects.
■
INTRODUCTION Organic−inorganic perovskite compounds have attracted significant attention since their initial application in photovoltaic devices by Miyasaka et al. in 2009.1 As research into perovskite solar cells (PSCs) has progressed over the last several years and new methods of fabricating perovskite layers and devices have been developed, the power conversion efficiency values of PSCs have increased from 3.8% to more than 20%.2−9 Organic−inorganic perovskite compounds are composed of metal ions, ammonium cations, and halogen anions, and they have the ability to be applied in the design of various analogues.10−14 Typically, three-dimensional (3D) perovskites with the general formula AMX3 (A = CH3NH3+, Cs+, CH(NH2)2+; M = Pb2+, Sn2+; X = Cl−, Br−, I−) have been studied as light harvesters in PSCs due to their outstanding properties, including high extinction coefficients, moderate band gaps, small exciton-binding energies, and long excitonand charge-diffusion lengths.15,16 However, PSCs based on 3D perosvkites have serious problems in terms of stability, especially in the presence of moisture, because 3D perovskites are ionic crystals. Therefore, improvements in the stability of PSCs are necessary. On the other hand, two-dimensional (2D) perovskites with the general formula A2PbX4 (A = RNH3+; X = Cl−, Br−, I−) are stable against moisture due to the presence of bulky and hydrophobic organic layers. Focusing on this issue, some groups have recently reported the application of 2D perovskite compounds and their analogues that have improved resistance to moisture.17−23 2D perovskite compounds not only show high moisture stability but also show exceptional optical properties. Because the bandgap of the organic layers is much larger than that of the [PbX6]4− octahedra layers (by at least 3 eV), the organic layers act as insulating barriers, while the inorganic layers behave as semiconductors. The interfaces between the inorganic and organic layers are intrinsically flat, © 2017 American Chemical Society
and it is thought that excitons are confined as a result and behave as though they are in an ideal 2D system.24,25 These stable excitons result in exceptional optical properties, such as a strong photoluminescence and high optical nonlinearity, and so organic−inorganic layered perovskites have the potential for application in a wide range of optical materials. As reported by Tsai et al.,22 the inorganic layers of 2D perovskites can act as semiconductors for PCEs when the 2D planes are positioned parallel to the substrate. In this study, to control the orientation of the 2D perovskites while retaining the quantum confinement structure, 2D perovskites were prepared using alkyl ammoniums having carboxy moieties (Figure 1).
Figure 1. Schematic view of a 2D perovskite containing carboxy groups. Received: April 7, 2017 Accepted: May 17, 2017 Published: May 25, 2017 2333
DOI: 10.1021/acsomega.7b00421 ACS Omega 2017, 2, 2333−2336
ACS Omega
■
Article
RESULTS AND DISCUSSION Hydroiodides, HOOC(CH2)n−1NH3I (n = 3, 4, or 7), were obtained by reacting equivalent amounts of the corresponding amines, HOOC(CH2)n−1NH2, with hydriodic acid. Perovskite solutions were prepared by reacting stoichiometric amounts of HOOC(CH2)n−1NH3I with PbI2 in N,N-dimethylformamide (DMF). Films were prepared on preheated glass substrates at 100 °C by spin-coating at 2000 rpm. For comparison, [CH3(CH2)n−1NH3]2PbI4 (n = 3, 4, or 7) spin-coated films were also fabricated by a similar procedure. From the elemental analysis data of microcrystalline, the compositions of [HOOC(CH2)n−1NH3]2PbI4 and [CH3(CH2)n−1NH3]2PbI4, with n values of 4 and 7, were confirmed. Figure 2 shows the out-of-plane X-ray diffraction (XRD) patterns generated by spin-coated films of [HOOC-
Figure 3. In-plane XRD patterns of films of (a) [HOOC(CH2)6NH3]2PbI4 and (b) [CH3(CH2)6NH3]2PbI4.
substrate in the [CH3(CH2)n−1NH3]2PbI4 (n = 7) spin-coated film. In contrast, in the case of the [HOOC(CH2)6NH3]2PbI4 film, a diffraction peak at 4.4° corresponding to the interlayer dspacing was observed in addition to a peak at 14.3°, ascribed to the [PbI6]4− octahedra. This peak indicates that a 2D perovskite layered structure was formed and that the crystalline growth was partly perpendicular to the film surface. This perpendicular crystalline growth is well suited to photovoltaic applications because it allows charges to migrate within the perovskite layers without disturbance. The [HOOC(CH2)6NH3]2PbI4 film also generated a ring of X-ray scattering 2D wide-angle X-ray scattering data, attributed to inorganic layers oriented randomly to the substrate. Differential scanning calorimetry (DSC) studies of these perovskites were undertaken to investigate their structural transitions. Figure 4 shows the DSC profiles obtained from
Figure 2. Out-of-plane XRD patterns of spin-coated films of (a) [HOOC(CH2)n−1NH3]2PbI4 and (b) [CH3(CH2)n−1NH3]2PbI4, with n = 3, 4, or 7.
(CH2)n−1NH3]2PbI4 and [CH3(CH2)n−1NH3]2PbI4, with n values of 3, 4, or 7. A series of diffractions corresponding to the interlayer spacing is clearly observed for each of the samples. On the basis of the diffraction peaks, the long cell dimensions along the c-axis were calculated to be 15.6 Å (n = 3), 12.1 Å (n = 4), and 18.7 Å (n = 7) for [HOOC(CH2)n−1NH3]2PbI4. Similarly, the d-spacing values for [CH3(CH2)n−1NH3]2PbI4 were found to be 12.5 Å (n = 3), 13.8 Å (n = 4), and 18.4 Å (n = 7), respectively. As for alkylammonium-based perovskites, the variation in the interlayer d-spacing as a function of n was linear and was also consistent with our previous data.11 In addition, for specimens containing carboxy groups, the d-spacings at n = 4 and 7 were similar to those in the alkyl group samples. This result suggests the formation of layered perovskite structures with the formula [HOOC(CH2)n−1NH3]2PbI4 (n = 4 or 7), as shown in Figure 1. However, the data for n = 3 deviate from the trend suggested by the d-spacing values for the n = 4 and 7 films (Figure S1). The relatively large d-spacing of the n = 3 material suggests that this specimen had a different structure from that of the 2D perovskites. The orientations of the 2D structures were investigated by grazing incident in-plane XRD, and Figure 3 presents the inplane XRD patterns of spin-coated films of [HOOC(CH 2 ) 6 NH 3 ] 2 PbI 4 and [CH 3 (CH 2 ) 6 NH 3 ] 2 PbI 4 . The [CH3(CH2)6NH3]2PbI4 film generated a diffraction peak attributed to the [PbI6]4− octahedra at 14.4° (d = 6.2 Å), even though the corresponding peak was not present in the out-of-plane XRD pattern. These results suggest that the 2D inorganic layers were oriented completely parallel to the
Figure 4. DSC profiles of (a) [HOOC(CH2)6NH3]2PbI4 and (b) [CH3(CH2)6NH3]2PbI4 crystals obtained during the second heating.
[HOOC(CH2)6NH3]2PbI4 and [CH3(CH2)6NH3]2PbI4 crystals prepared by precipitation, using DMF as the good solvent and dichloromethane as the poor solvent. Whereas the [CH3(CH2)6NH3]2PbI4 crystals showed phase transitions at −1.6, 12.3, and 35.9 °C, no transition peaks were generated by [HOOC(CH2)6NH3]2PbI4. Mercier26 has suggested that 2D perovskite compounds composed of ammonium cations with carboxy groups form dimer structures through hydrogen bonds associated with the carboxy groups. The results of the FT-IR measurements showed that the peaks of the C−O stretching vibration around 1720 cm−1 and the N−H deformation vibration around 1570 cm−1 were observed strongly for [HOOC(CH 2 ) 6 NH 3 ] 2 PbI 4 compared with HOOC(CH2)6NH3I. From these results, the hydrogen-bond interactions between the carboxylic acid moieties and ammonium 2334
DOI: 10.1021/acsomega.7b00421 ACS Omega 2017, 2, 2333−2336
ACS Omega
Article
measurements) and 10 wt % (for UV−vis absorption spectroscopy measurements). Similarly, the precursor solutions of [CH3(CH2)n−1NH3I]2PbI4 were obtained. Films were fabricated on preheated hydrophilic substrates by spin-coating at 2000 rpm using a Mikasa spin coater 1H-D7. The substrates were heated at ca. 100 °C during the spin-coating process to obtain high-quality films due to the high boiling point of DMF. Microcrystalline powders were obtained by pouring the precursor solutions into dichloromethane. After vacuum drying, orange (n = 4, 7) and yellow (n = 3) powders were obtained for [HOOC(CH2)n−1NH3I]2PbI4 and [CH3(CH2)n−1NH3I]2PbI4. Characterization. Optical absorption spectra of the spincoated films were recorded with a UV−vis−NIR spectrophotometer (UV-3100PC; Shimadzu) at room temperature. XRD patterns were obtained over a 2θ range of 1.5−35° with an Xray diffractometer (SmartLab; Rigaku) in conjunction with a Ni-filtered copper Kα target, operating at 45 kV and 200 mA. DSC was performed using DSC 7200 (Hitachi) under a nitrogen flow of 40 mL min−1.
ions hinder the motion of the organic amines between the inorganic layers and thus inhibit phase transition. Figure 5 presents the UV−vis absorption spectra of spin-coated films of [HOOC(CH 2 ) n−1 NH 3 ] 2 PbI 4 and
Figure 5. UV−vis absorption spectra of spin-coated films of (a) [HOOC(CH2)n−1NH3]2PbI4 and (b) [CH3(CH2)n−1NH3]2PbI4.
■
ASSOCIATED CONTENT
S Supporting Information *
[CH3(CH2)n−1NH3]2PbI4 (n = 3, 4, or 7). Exciton absorption peaks attributed to 2D quantum confinement structures were generated at approximately 500 nm by all [CH3(CH2)n−1NH3]2PbI4 films. The absorbance at this wavelength was increased upon increasing the chain length, n, suggesting the formation of stable excitons in the perovskites as the result of a suitable barrier layer thickness. In contrast, the [HOOC(CH2)2NH3]2PbI4 spin-coated film did not exhibit exciton absorption, presumably as a consequence of its different structure. The [HOOC(CH2)n−1NH3]2PbI4 (n = 4 or 7) films produced exciton absorption peaks at 505 and 502 nm, respectively. Therefore, the quantum confinement effects that are a feature of 2D perovskite compounds were expressed despite the introduction of carboxy groups.
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00421. Elemental analysis data of the compounds and other experimental data (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +81-3-3238-3451. Fax: +81-3-3238-3361. ORCID
Yuko Takeoka: 0000-0003-4958-3879 Notes
■
The authors declare no competing financial interest.
■
CONCLUSIONS Two-dimensional perovskite compounds incorporating ammonium cations with carboxy groups were fabricated and found to form dimer structures based on the hydrogen bonds of the carboxy moieties. Films of these compounds were randomly oriented with respect to the substrate. In addition, the thermal stabilities of these materials were increased due to inhibition of the phase transitions of organic layers upon introduction of carboxy groups. The length of the acid chains can be varied, and this has a direct impact on the exciton formation exhibiting quantum confinement effects.
ACKNOWLEDGMENTS This work was performed under the Advanced Low Carbon Technology Research and Development Program (ALCA) funded by Japan Science and Technology (JST).
■
REFERENCES
(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (2) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M. Sequential Deposition as a Route to High-performance Perovskite-sensitized Solar Cells. Nature 2013, 499, 316−320. (3) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-Performance Photovoltaic Perovskite Layers Fabricated Through Intramolecular Exchange. Science 2015, 348, 1234−1241. (4) Saliba, M.; Matsui, T.; Seo, J. Y.; Domanski, K.; Correa-Baena, J. P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A.; Grätzel, M. Cesium-containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency. Energy Environ. Sci. 2016, 9, 1989−1997. (5) Xiao, M.; Huang, F.; Huang, W.; Dkhissi, Y.; Zhu, Y.; Etheridge, J.; Gray-Weale, A.; Bach, U.; Cheng, Y.-B.; Spiccia, L. A Fast Deposition-Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells. Angew. Chem., Int. Ed. 2014, 53, 9898−9903.
■
EXPERIMENTAL SECTION Materials. Hydroiodic acid (HI, 57 wt % aqueous solution) and DMF (super dehydrated) were purchased from Wako Pure Chemical Industries, Ltd and used as received. Dichloromethane was purchased from Kanto Chemical Co., Inc. All other chemicals were obtained from Tokyo Chemical Industry Co., Ltd. Amine hydroiodides, HOOC(CH2)n−1NH3I and CH3(CH2)n−1NH3I, with n = 3, 4, and 7, were synthesized by neutralizing HOOC(CH2)n−1NH2 and CH3(CH2)n−1NH2 (n = 3, 4, and 7) with stoichiometric amounts of HI, respectively. Sample Preparation. HOOC(CH2)n−1NH3I and PbI2 were dissolved in DMF at 50 °C for 1 h to obtain precursor solutions for the preparation of [HOOC(CH2)n−1NH3I]2PbI4. The concentrations of these solutions were 40 wt % (for XRD 2335
DOI: 10.1021/acsomega.7b00421 ACS Omega 2017, 2, 2333−2336
ACS Omega
Article
(6) Kim, H. D.; Ohkita, H.; Benten, H.; Ito, S. Photovoltaic Performance of Perovskite Solar Cells with Different Grain Sizes. Adv. Mater. 2016, 28, 917−922. (7) Bi, D.; Tress, W.; Dar, M. I.; Gao, P.; Luo, J.; Renevier, C.; Schenk, K.; Abate, A.; Giordano, F.; Baena, J.-P. C.; Decoppet, J.-D.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M.; Hagfeldt, A. Efficient Luminescent Solar Cells based on Tailored Mixed-cation Perovskites. Sci. Adv. 2016, 2, No. e1501170. (8) Huang, F.; Dkhissi, Y.; Huang, W.; Xiao, M.; Benesperi, I.; Rubanov, S.; Zhu, Y.; Lin, X.; Jiang, L.; Zhou, Y.; Gray-Weale, A.; Etheridge, J.; McNeill, C. R.; Caruso, R. A.; Bach, U.; Spiccia, L.; Cheng, Y.-B. Gas-assisted Preparation of Lead Iodide Perovskite Films Consisting of a Monolayer of Single Crystalline Grains for High Efficiency Planar Solar Cells. Nano Energy 2014, 10, 10−18. (9) Chen, W.; Wu, Y.; Yue, Y.; Liu, J.; Zhang, W.; Yang, X.; Chen, H.; Bi, E.; Ashraful, I.; Grätzel, M.; Han, L. Efficient and Stable Large-area Perovskite Solar Cells with Inorganic Charge Extraction Layers. Science 2015, 350, 944−948. (10) Mitzi, D. B. Synthesis, Structure, and Properties of Organic− inorganic Perovskites and Related Materials. Prog. Inorg. Chem. 1999, 48, 1−121. (11) Takeoka, Y.; Asai, K.; Rikukawa, M.; Sanui, K. Systematic Studies on Chain Lengths, Halide Species, and Well Thicknesses for Lead Halide Layered Perovskite Thin Films. Bull. Chem. Soc. Jpn. 2006, 79, 1607−1613. (12) Takeoka, Y.; Kawahara, M.; Rikukawa, M. Construction of Novel Quantum-confined Structure with Lead Halide/Gemini Surfactant Hybrids. J. Phys. Chem. Solids 2012, 73, 1401−1403. (13) Era, M.; Morimoto, S.; Tsutsui, T.; Saito, S. Organic-inorganic Heterostructure Electroluminescent Device using a Layered Perovskite Semiconductor (C6H5C2H4NH3)2PbI4. Appl. Phys. Lett. 1994, 65, 676−678. (14) Takeoka, Y.; Asai, K.; Rikukawa, M.; Sanui, K. Hydrothermal Synthesis and Structure of Zero-dimensional Organic−inorganic Perovskites. Chem. Lett. 2005, 34, 602−603. (15) Baikie, T.; Fang, Y. N.; Kadro, J. M.; Schreyer, M.; Wei, F. X.; Mhaisalkar, S. G.; Gräetzel, M.; White, T. J. Synthesis and Crystal Chemistry of the Hybrid Perovskite (CH3NH3)PbI3 for Solid-state Sensitised Solar Cell Applications. J. Mater. Chem. A 2013, 1, 5628− 5641. (16) D’Innocenzo, V.; Grancini, G.; Alcocer, M. J. P.; Kandada, A. R. S.; Stranks, S. D.; Lee, M. M.; Lanzani, G.; Snaith, H. J.; Petrozza, A. Excitons versus Free Charges in Organo-lead Tri-halide Perovskites. Nat. Commun. 2014, 5, 3586−3591. (17) Smith, I. C.; Hoke, E. T.; Ibarra, D. S.; McGehee, M. D.; Karunadasa, H. I. A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability. Angew. Chem. 2014, 126, 11414− 11417. (18) Cao, D. H.; Stoumpos, C. C.; Farha, O. K.; Hupp, J. T.; Kanatzidis, M. G. 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications. J. Am. Chem. Soc. 2015, 137, 7843−7850. (19) Koh, T. M.; Shanmugam, V.; Schlipf, J.; Oesinghaus, L.; Buschbaum, P. M.; Ramakrishnan, N.; Swamy, V.; Mathews, N.; Boix, P. P.; Mhaisalkar, S. G. Nanostructuring Mixed-Dimensional Perovskites: A Route Toward Tunable, Efficient Photovoltaics. Adv. Mater. 2016, 28, 3653−3661. (20) Yao, K.; Wang, X.; Xu, Y. X.; Li, F.; Zhiu, L. Multilayered Perovskite Materials Based on Polymeric-Ammonium Cations for Stable Large-Area Solar Cell. Chem. Mater. 2016, 28, 3131−3138. (21) Hu, Y.; Schlipf, J.; Wussler, M.; Petrus, M. L.; Jaegermann, W.; Bein, T.; Buschbaum, P. M.; Docampo, P. Hybrid Perovskite/ Perovskite Heterojunction Solar Cells. ACS Nano 2016, 10, 5999− 6007. (22) Tsai, H.; Nie, W.; Blancon, J. C.; Stoumpos, C. C.; Asadpour, R.; Harutyunyan, B.; Neukirch, A. J.; Verduzco, R.; Crochet, J. J.; Tretiak, S.; Pedesseau, L.; Even, J.; Alam, M. A.; Gupta, G.; Lou, J.; Ajayan, P. M.; Bedzyk, M. J.; Kanatzidis, M. G.; Mohite, A. D.; et al.
High-efficiency Two-dimensional Ruddlesden-Popper Perovskite Solar Cells. Nature 2016, 536, 312−316. (23) Hamaguchi, R.; Yoshizawa-Fujita, M.; Miyasaka, T.; Kunugita, H.; Ema, K.; Takeoka, Y.; Rikukawa, M. Formamidine and Cesiumbased Quasi-two-dimensional Perovskites as Photovoltaic Absorbers. Chem. Commun. 2017, 53, 4366−4369. (24) Tanaka, K.; Takahashi, T.; Kondo, T.; Umebayashi, T.; Asai, K.; Ema, K. Image Charge Effect on Two-dimensional Excitons in an Inorganic-organic Quantum-well Crystal. Phys. Rev. B 2005, 71, 045312−045317. (25) Ema, K.; Inomata, M.; Kato, Y.; Kunugita, H.; Era, M. Nearly Perfect Triplet-Triplet Energy Transfer from Wannier Excitons to Naphthalene in Organic-Inorganic Hybrid Quantum-Well Materials. Phys. Rev. Lett. 2008, 100, 257401−257404. (26) Mercier, N. (HO2C(CH2)3NH3)2(CH3NH3)Pb2I7: A predicted Noncentrosymmetrical Structure Built up from Carboxylic Acid Supramolecular Synthons and Bilayer Perovskite Sheets. CrystEngComm 2005, 7, 429−432.
2336
DOI: 10.1021/acsomega.7b00421 ACS Omega 2017, 2, 2333−2336
