Communication pubs.acs.org/cm
Chlorine in PbCl2‑Derived Hybrid-Perovskite Solar Absorbers Vanessa L. Pool,†,⊥ Aryeh Gold-Parker,†,‡,⊥ Michael D. McGehee,§ and Michael F. Toney*,† †
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States Department of Chemistry, Stanford University, 333 Campus Drive, Stanford, California 94305, United States § Department of Materials Science and Engineering, Stanford University, 496 Lomita Mall, Stanford, California 94305, United States ‡
S Supporting Information *
M
Of the two explanations of chlorine’s role in improving performance (electronic and morphological), only the morphological explanation is consistent with reports that no chlorine remains in the fully converted films16,17 and this has been a major argument against the electronic mechanism.10 With a number of groups reporting the presence of Cl in the final films, the role that Cl plays in improving device performance remains an open question. While there is ample evidence showing that Cl plays a role in the morphology of Cl derived perovskites, the Cl remaining in the film may also play an electronic role. In addition, despite previous work quantifying Cl, its chemical state in the film is unknown. In this study, we have characterized PbCl2 -derived CH3NH3PbI3−xClx with X-ray absorption near edge structure (XANES) in order to verify the amount of Cl remaining in the film after annealing and to determine the chemical state of the Cl with a goal of understanding the role Cl plays in the final hybrid-perovskite film. X-ray absorption measurements have previously been reported on mesoporous18 and planar19 films; however, the data from these studies are too noisy to perform meaningful XANES analysis. Our high-quality data allow for quantification of Cl concentration and analysis of its chemical state. These results suggest that an electronic role for Cl cannot be ruled out. Perovskite films were made from a 1:3 molar mixture of PbCl2 and MAI in DMF (MA, methylammonium =CH3NH3+), spin-coated onto a substrate. Two types of substrates were used: 100 nm-thick silicon nitride windows (SiN) for simultaneous florescence and transmission XANES, and fluorine-doped tin oxide with compact titanium dioxide (FTO/TiO2) for fluorescence-only XANES. More specifics on film preparation are provided in the SI. The films prepared on SiN were annealed ex situ in dry air (3 ppm of H2O) at 95 °C for different times: 0 min (“unannealed”), 15, 60, 90, and 120 min (“fully annealed”). These ex situ annealed films were used to quantify the amount of Cl and to get accurate XANES for intermediate annealing times. To verify XANES for films grown on the (nonstandard) SiN, films were also spin-coated onto FTO/TiO2, and then annealed in situ in a helium atmosphere during the XANES experiment. All films annealed less than 120 min were transported to the experimental chamber without exposure to ambient air. Cl Kedge XANES measurements were performed on beamline 4-3
ethylammonium lead trihalide perovskite solar cells (CH3NH3PbI3−xClx) are a promising photovoltaic absorber, demonstrating efficiencies approaching those of crystalline silicon.1 In the first report of using methylammonium lead triiodide perovskite as an absorber for solar cells, the perovskite was deposited in a single step from a precursor solution of CH3NH3I and PbI2.2 Interest in hybrid-perovskite grew in 2012 with coincident reports of devices with efficiencies of 9.7%3 and 10.9%.4 Numerous studies have since reported benefits from inclusion of chlorine in the fabrication of triiodide perovskite cells. Liu, Snaith et al. reported a record planar device (η = 15.4%) in which CH3NH3I and PbCl2 were vapor-deposited onto a substrate before annealing.5 Soon after, Stranks, Snaith et al. reported measurements of carrier diffusion lengths greater than 1 μm in PbCl2-derived films, compared to 100 nm for chlorine-free CH3NH3PbI3.6 Commonly, PbCl2-derived perovskites are represented as CH3NH3PbI3−xClx, where x represents the small amount of chlorine that may remain in these films. Colella et al. correlate XRD measurements with DFT calculations and report a maximum concentration of x = 0.12 that is possible in the triiodide structure.7 There are two distinct explanations as to how the presence of Cl in the synthesis improves performance. The first (“electronic”) is that the chloride ions have an electronic effect in the films, possibly by doping the perovskite, passivating interfaces and/or grain boundaries, or causing band bending at the TiO2 interface.8 The second hypothesis (“morphological”) is that the presence of chloride influences film formation and leads to larger crystalline domains, but Cl does not necessarily remain in the film. A number of recent papers have offered morphological explanations for the improvements observed in triiodide perovskites processed with Cl.9−11 Some report chlorine concentrations below the detection limit of energy dispersive X-ray spectrometry (EDX) and X-ray photoelectron spectroscopy (XPS), and there is one report of Cl just above the EDX detection limit.12 Other research groups have observed chlorine in these films using other analytic methods. In 2014, we reported detectable chlorine in PbCl2-derived films, measured using X-ray fluorescence (XRF), even though we were unable to detect Cl using XPS or EDX.13 Supporting this finding, chlorine concentrations in CH3NH3PbI3−xClx thin films were recently quantified in two separate studies: Cojocaru et al. report x = 0.06 using ion chromatography;14 Li et al. report a range from x = 0.17 ± 0.04 using potentiometric titration.15 © 2015 American Chemical Society
Received: September 15, 2015 Revised: October 23, 2015 Published: October 27, 2015 7240
DOI: 10.1021/acs.chemmater.5b03581 Chem. Mater. 2015, 27, 7240−7243
Communication
Chemistry of Materials at the Stanford Synchrotron Radiation Lightsource (SSRL). The XANES data analysis was preformed in Athena.20 For further details on the setup and analysis, see the SI. XANES is useful in identifying the chemical state of the absorbing element and hence its chemical species. Appropriate standards are needed to compare with the measured spectra, and for this study we used PbCl2, MACl, and MAPbCl3. PbCl2 is a precursor, MACl is a known side-product,13 and MAPbCl3 is a possible intermediate.21 MAPbCl3 also allows investigation of the hypothesis that Cl incorporates into the perovskite. Figure 1 shows the Cl K-edge XANES for the three standards and the ex situ anneal sequence for the films. The unannealed
Figure 2. Measured Cl K-edge XANES spectra shown with solid lines (120 min, PbCl2 and MACl). Dashed red line shows the best linear combination fit of PbCl2 and MACl to the perovskite spectrum.
Cl K-edge spectral features are sensitive to the local environment of Cl and so indicates the chemical species.22,23 MAPbCl3 was measured as a proxy for Cl substituting for I in small concentrations; however, it is not an accurate standard as is discussed in the SI. The resemblance of the fully annealed perovskite spectrum to the MAPbCl3 spectrum can at most be taken as an indication that Cl is in a similar local environment. Although this may indicate that the Cl is substituting for I, it could also indicate Cl at the grain boundaries, interfaces, or amorphous or poorly crystalline MAPbCl3-like regions. Determining the exact Cl location within the film is beyond the scope of this paper and requires XANES modeling, as discussed in the SI. To quantify the amount of Cl present in the perovskite films, the Cl K-edge and Pb M3-edge were measured in a continuous scan and the edge steps compared. The ratio in step heights of the Cl- and Pb- edges is directly proportional to the ratio of concentrations of these two elements. To determine the proportionality, the same XANES measurements were performed on a PbCl2 standard film. This method is discussed in greater detail in the SI. Figure 3 shows the concentration of Cl for films annealed from 0 to 120 min. These transmission data are quantitative, while the fluorescence measurements are
Figure 1. Cl K-edge XANES for MACl, PbCl2, MAPBCl3, and films annealed for 0−120 min taken with fluorescence yield.
film closely resembles MACl, but has an additional shoulder at 2821.5 eV. With increased annealing time, the shoulder at 2821.5 eV increases in prominence and lowers in energy as the film becomes more fully converted and the main peak appears to broaden and shift to higher energy. The trends in the XANES spectra seen in the ex situ films on SiN windows were also seen in the in situ samples grown on FTO. This progression of spectral shape reflects the chemical changes taking place while the perovskite forms. By understanding this progression, we can understand the reaction pathways taking place during conversion. Further analysis was performed to rule out the possibility that Cl remaining in the fully converted perovskite film may be the result of unconverted precursor materials. If this were the case, then the experimental spectrum of the fully annealed perovskite could be reproduced with a linear combination fit (LCF) of the PbCl2 and MACl reference spectra. As shown in Figure 2, the agreement between this LCF and the experimental spectrum is poor. This excludes the possibility that the Cl in the fully converted film is solely unconverted MACl and PbCl2 precursors, an important conclusion from this work. As seen in Figure 1, there is resemblance between the fully annealed perovskite spectrum and the MAPbCl3 spectrum. The
Figure 3. Number of Cl atoms per Pb atom in films annealed at 95 °C for different times. Blue dots are calculated from transmission data of the ex situ samples; black diamonds are calculated from florescence data of the in situ samples. Square shows PbCl2 assumed to be 2Cl:1Pb. 7241
DOI: 10.1021/acs.chemmater.5b03581 Chem. Mater. 2015, 27, 7240−7243
Communication
Chemistry of Materials Notes
scaled to match the initial transmission data. Although the fluorescence data are not strictly quantitative, they do validate the transmission data and show that the substrate does not alter the results. The amount of Cl in the films was seen to decrease throughout the annealing process as would be expected. This observation is consistent with our previous report that MACl sublimes from the film throughout the anneal.13 Concentrations for intermediate annealing times are reported in Table S1 in the SI. The unannealed film was expected to have 2 Cl atoms per formula unit, as PbCl2 is the only source of Pb and Cl. The unannealed film is seen to have 1.7 Cl atoms per formula unit indicating that some Cl leaves the film through room temperature degassing or during the spin-casting process. The film annealed for 120 min was found to have x = 0.05 ± 0.03 Cl atoms per formula unit (i.e., per Pb atom). This is consistent with independent reports by Cojocaru et al. and Li et al. mentioned earlier.14,15 Our results, using a distinct technique, provide compelling evidence for chlorine in fully annealed PbCl2-derived perovskite films at this non-negligible concentration. In summary, by measuring the Cl K-edge XANES of perovskite films, we have verified that there is Cl remaining in the fully converted film in a form other than MACl and PbCl2. A series of films made from the single-step solutionprocess method were annealed in dry air at 95 °C for times varying from unannealed to fully annealed (120 min). We find that x = 0.05 ± 0.03 Cl atoms per formula unit remain in the fully annealed films. This is a non-negligible amount, allowing for the possibility of Cl having an electronic effect (though this data is not evidence of such an effect). The XANES for the unannealed film suggests that, upon spin-coating, Cl in the film is predominantly MACl. The Cl XANES for the fully annealed film rules out the possibility that the remaining Cl is exclusively unconverted MACl and PbCl2. This suggests that Cl may be incorporating into the film in a way that influences the electronic structure either directly (e.g., substitution or interstitial addition) or indirectly (e.g., accumulating at grain boundaries or other poorly crystalline regions). A better understanding of the role of chlorine in perovskite solar cells will help us continue to improve their performance.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1147470 (AG) and the Bay Area Photovoltaic Consortium (VLP). Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC National Accelerator Laboratory and an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Stanford University. We thank Erik Nelson and Matthew Latimer for their assistance on beamline 4-3, and Doug Van Campen and Valery Borzenets for design and fabrication of the in situ annealing setup. We thank Chris Tassone, Kevin Stone, and Andrea Bowring for helpful discussions.
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ABBREVIATIONS X-ray absorption near edge structure (XANES); linear combination fit (LCF); methylammonium, CH3NH3 (MA); energy dispersive X-ray spectrometry (EDX); X-ray photoelectron spectroscopy (XPS); X-ray fluorescence (XRF)
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b03581. Details about sample preparation, XANES experimental setup, data analysis, and a discussion of the MAPbCl3 standard (PDF).
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REFERENCES
(1) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. Il. Compositional engineering of perovskite materials for highperformance solar cells. Nature 2015, 517 (7535), 476−480. (2) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131 (17), 6050−6051. (3) Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E.; et al. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2012, 2, 591. (4) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 2012, 338 (6107), 643−647. (5) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501 (7467), 395−398. (6) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 2013, 342 (6156), 341−344. (7) Colella, S.; Mosconi, E.; Fedeli, P.; Listorti, A.; Gazza, F.; Orlandi, F.; Ferro, P.; Besagni, T.; Rizzo, A.; Calestani, G.; et al. MAPbI3−xClx Mixed Halide Perovskite for Hybrid Solar Cells: The Role of Chloride as Dopant on the Transport and Structural Properties. Chem. Mater. 2013, 25 (22), 4613−4618. (8) Colella, S.; Mosconi, E.; Pellegrino, G.; Alberti, A.; Guerra, V. L. P.; Masi, S.; Listorti, A.; Rizzo, A.; Condorelli, G. G.; De Angelis, F.; et al. The Elusive Presence of Chloride in Mixed Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 3532−3538. (9) Tidhar, Y.; Edri, E.; Weissman, H.; Zohar, D.; Hodes, G.; Cahen, D.; Rybtchinski, B.; Kirmayer, S. Crystallization of methyl ammonium lead halide perovskites: implications for photovoltaic applications. J. Am. Chem. Soc. 2014, 136 (38), 13249−13256. (10) Yu, H.; Wang, F.; Xie, F.; Li, W.; Chen, J.; Zhao, N. The Role of Chlorine in the Formation Process of “CH3NH3PbI3−xClx” Perovskite. Adv. Funct. Mater. 2014, 24 (45), 7102−7108. (11) Williams, S. T.; Zuo, F.; Chueh, C.; Liao, C.; Liang, P.; Jen, A. K.-Y. Role of Chloride in the Morphological Evolution of Organo-
AUTHOR INFORMATION
Corresponding Author
*M. F. Toney. E-mail:
[email protected]. Author Contributions ⊥
These authors contributed equally.
Funding
This work was supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1147470 (AG) and the Bay Area Photovoltaic Consortium (VLP). 7242
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Chemistry of Materials Lead Halide Perovskite Thin Films. ACS Nano 2014, 8 (10), 10640− 10654. (12) de Quilettes, D. W.; Vorpahl, S. M.; Stranks, S. D.; Nagaoka, H.; Eperon, G. E.; Ziffer, M. E.; Snaith, H. J.; Ginger, D. S. Impact of microstructure on local carrier lifetime in perovskite solar cells. Science (Washington, DC, U. S.) 2015, 348 (6235), 683−686. (13) Unger, E. L.; Bowring, A. R.; Tassone, C. J.; Pool, V. L.; Goldparker, A.; Cheacharoen, R.; Stone, K. H.; Hoke, E. T.; Toney, M. F.; Mcgehee, M. D. Chloride in Lead Chloride-Derived Organo-Metal Halides for Perovskite-Absorber Solar Cells. Chem. Mater. 2014, 26, 7158. (14) Cojocaru, L.; Uchida, S.; Jena, A. K.; Miyasaka, T.; Nakazaki, J.; Kubo, T.; Segawa, H. Determination of Chloride Content in Planar CH3NH3PbI3−xClx Solar Cells by Chemical Analysis. Chem. Lett. 2015, 44, 21−24. (15) Li, Y.; Sun, W.; Yan, W.; Ye, S.; Peng, H.; Liu, Z.; Bian, Z.; Huang, C. High-Performance Planar Solar Cells Based On CH3NH3PbI3−xClx Perovskites with Determined Chlorine Mole Fraction. Adv. Funct. Mater. 2015, 25, 4867−4873. (16) Grancini, G.; Marras, S.; Prato, M.; Giannini, C.; Quarti, C.; De Angelis, F.; De Bastiani, M.; Eperon, G. E.; Snaith, H. J.; Manna, L.; et al. The Impact of the Crystallization Processes on the Structural and Optical Properties of Hybrid Perovskite Films for Photovoltaics. J. Phys. Chem. Lett. 2014, 5 (21), 3836−3842. (17) Zhang, W.; Saliba, M.; Moore, D. T.; Pathak, S. K.; Hörantner, M. T.; Stergiopoulos, T.; Stranks, S. D.; Eperon, G. E.; AlexanderWebber, J. a; Abate, A.; et al. Ultrasmooth organic−inorganic perovskite thin-film formation and crystallization for efficient planar heterojunction solar cells. Nat. Commun. 2015, 6, 6142. (18) Sabba, D.; Dewi, H. A.; Prabhakar, R. R.; Baikie, T.; Chen, S.; Du, Y.; Mathews, N.; Boix, P. P.; Mhaisalkar, S. G. Incorporation of Cl in sequentially deposited lead halide perovskite films for highly efficient mesoporous solar cells. Nanoscale 2014, 6, 13854−13860. (19) Starr, D. E.; Sadoughi, G.; Handick, E.; Wilks, R. G.; Alsmeier, J. H.; Köhler, L.; Gorgoi, M.; Snaith, H. J.; Bär, M. Direct observation of an inhomogeneous chlorine distribution in CH 3 NH 3 PbI 3−x Cl x layers: surface depletion and interface enrichment. Energy Environ. Sci. 2015, 8 (5), 1609−1615. (20) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: Data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12 (4), 537−541. (21) Williams, S. T.; Chueh, C.-C.; Jen, A. K.-Y. Navigating OrganoLead Halide Perovskite Phase Space via Nucleation Kinetics toward a Deeper Understanding of Perovskite Phase Transformations and Structure-Property Relationships. Small 2015, 11 (26), 3088−3096. (22) Zhu, F.; Takaoka, M.; Shiota, K.; Oshita, K.; Kitajima, Y. Chloride chemical form in various types of fly ash. Environ. Sci. Technol. 2008, 42 (11), 3932−3937. (23) Bodeur, S.; Maréchal, J. L.; Reynaud, C.; Bazin, D.; Nenner, I. Chlorine K shell photoabsorption spectra of gas phase HCl and Cl2 molecules. Z. Phys. D: At., Mol. Clusters 1990, 17 (4), 291−298.
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