On the True Composition of Mixed-Cation Perovskite Films - ACS

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On the True Composition of Mixed-Cation Perovskite Films Carina Pareja-Rivera, Ana Lucía Solis-Cambero, Mireille Natalie Sánchez-Torres, Enrique Lima, and Diego Solis-Ibarra ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01577 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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ACS Energy Letters

On the True Composition of Mixed-Cation Perovskite Films Carina Pareja-Rivera, Ana L. Solís-Cambero, Mireille Sánchez-Torres, Enrique Lima and Diego Solis-Ibarra* Laboratorio de Fisicoquímica y Reactividad de Superficies (LaFReS), Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, CU, Coyoacán, 04510, Ciudad de México, México AUTHOR INFORMATION Corresponding Author *[email protected]

Abstract: Herein we show that mixed-cation perovskites films made by spin-coating do not necessarily have the same stoichiometry as the precursor solution they are made from and propose a simple method to quantify the true composition of such films.

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Lead based perovskites, with general formula APbI3 (A = CH3NH3+, CH(NH2)2+, or Cs) have disrupted the field of optoelectronics and have shown remarkable performance in solar cells with efficiencies of up to 23.3%.1 While early devices relied on materials with one A-site cation (most notably MAPbI3; MA = CH3NH3+), the advantages of mixing A-site cations quickly became apparent in terms of device performance and stability.2,3 In fact, almost of all today’s high efficiency (>20%) devices are made from a combination of two, three, or even four A-site cations.3,4 The most common film fabrication technique for perovskite-based solar cells is by spin-coating. This method is convenient and affordable, but is also highly sensitive to deposition conditions such as temperature, spin velocity, and casting solvent(s).5 In fact, it has been shown that the composition of mixed-halide perovskites can change significantly depending the synthesis method.6-8 To date, most studies assume that the stoichiometry of the starting precursor solution is equal to the composition of the final film, but little to no attention has been paid to actually determining the composition of such films. Herein, we show that this assumption can be


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ACS Energy Letters

erroneous and propose a simple method to determine the true composition of double and triplecation films made by spin-coating. Determination of the true composition of mixed-cation perovskite films is challenging for several reasons. First, technologically relevant films are a few hundred nanometers thick, thus yielding a few milligrams of material at best, depending on film area. Second, relevant cations include formamidinium (FA) and methylammonium (MA), which limits the ability of common techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX) or inductively coupled plasma (ICP), all of which are insensitive to light atoms (such as C, H or N). Third, the effects of cation-mixing on the properties of the materials (i.e., absorption or photoluminescence) are not always linear,9 and it is therefore challenging to estimate the relative concentration of cations by simple extrapolation and even more so when three or more cations are present. Given these challenges, we employed quantitative nuclear magnetic resonance (qNMR) to determine the actual composition of the films. Solution-state 1H qNMR is a widely accessible and affordable technique that can also be very sensitive and precise (precision and accuracy are both approximately 1%).10 To quantify the amount of A-site cation in perovskite samples (Scheme 1), we prepared films using standard spin-coating procedures for two double and three triple-cation perovskites (see Supporting Information (SI)).11 We then scraped off several films made by the exact same procedure, carefully weighing the resulting powders and dissolving them in a known volume of deuterated DMSO with a known amount of an internal standard. Finally, an 1H NMR spectrum was measured, and the signals from the non-labile carbon-bound protons of MA and FA were integrated and normalized using the signal from the internal standard. As the integrated peak of


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each signal is directly proportional to the number of protons, these results allowed us to determine the absolute mass of FA and MA, and by inference, the mass of Cs (Figure 1).

Scheme 1. Schematic representation of our method to quantify cations in mixed-cation perovskites by qNMR.


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ACS Energy Letters

Figure 1. Average concentration and MA:FA:Cs ratios of perovskites films made from solutions with different stoichiometries. The filled boxes show average concentration values with error while the diamond markers show the precursor solution (or nominal) concentration. The composition determined for MA:FA perovskites showed decent agreement with the precursor solution stoichiometry (Figure 1 and Table S2) in accordance with previous results.12 The films that contained cesium however, showed significant discrepancies between the stoichiometries of the precursor solutions and that of the obtained films, with MA concentrations up to 24% smaller than that of the solution concentration and FA and Cs concentrations up to 33 and 220% higher, respectively, than their nominal compositions (Figure 1 and Table S2). The lower than expected MA concentration across all triple cation films can presumably be explained by the lower solubility and higher volatility of FA and Cs cations with respect to MA. We note that the compositions obtained from our method are an average value of the composition of the films obtained from the specific method used, and do not consider local inhomogeneities and assume that our films are phase pure, both of which are not always true as previously demonstrated using NMR methods by Vela6 and Emsley7 (for further discussion on the assumptions and limitations of our technique please refer to the SI).


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To verify the validity of our results, we performed EDX to determine approximate Cs:Pb ratios in our Cs-containing films. EDX results (Figure S5) are consistent with the results obtained by our qNMR protocol: the films consistently showed higher than expected Cs/Pb ratios. The discrepancy between the solution and film compositions is remarkable and indicate that the precursor solution and films obtained from them do not necessarily possess the same stoichiometry. While the quantitative results obtained in this study only pertain to the particular film preparation conditions and the solution compositions used herein, our method is a simple option to enable others to precisely determine the stoichiometry of films fabricated under the specific conditions and compositions that best suit their work, which is important because film compositions will likely be extremely sensitive to the specific deposition conditions. Significantly, this method is not limited to films made by spin-coating of to the cations used in this study, nor is it even limited to three dimensional (3D) perovskites. This method is applicable to numerous types of perovskite materials and compositions. In particular, two dimensional (2D) perovskites,13 2D/3D perovskites,14 and even passivated films15 could be studied using this procedure, since it can be easily adapted to detect virtually any other organic molecule incorporated in perovskite films and complemented, if necessary, with other methods. In conclusion, we have developed a simple, yet precise, method to determine the formula of double and triple-cation perovskite films. Knowing the true composition of films used in the state-of-the-art perovskite solar cells is of fundamental importance for understanding and improving these materials, and we hope this method will enable such studies in the near future.


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ACS Energy Letters

ASSOCIATED CONTENT Supporting Information. Detailed experimental procedures (PDF) is available free of charge on the ACS Publications website.

AUTHOR INFORMATION [email protected]; http://www.iim.unam.mx/diegosolis/; @PerovskitesMX Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Profs. A. Walsh and B. Rodríguez-Molina for helpful discussions and Dr. C. LealCervantes and Dr. A. Jaffe for critical reading of the manuscript. The authors thank funding from PAPIIT IA202418 and CONACYT projects FC-2015-2/829 and 220436. We also acknowledge the support of G. Cedillo, A. Tejeda, J. Romero, C. Ramos, E. Morales, A. Vivas, A. Pompa, C. González, and M. Canseco.

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Philippe, B.; Saliba, M.; Correa-Baena, J.-P.; Cappel, U. B.; Turren-Cruz, S.-H.; Grätzel, M.; Hagfeldt, A.; Rensmo, H. Chemical Distribution of Multiple Cation (Rb+, Cs+, MA+, and FA+) Perovskite Materials by Photoelectron Spectroscopy. Chem. Mater. 2017, 29 (8), 3589–3596. McMeekin, D. P.; Wang, Z.; Rehman, W.; Pulvirenti, F.; Patel, J. B.; Noel, N. K.; Johnston, M. B.; Marder, S. R.; Herz, L. M.; Snaith, H. J. Crystallization Kinetics and Morphology Control of Formamidinium–Cesium Mixed‐Cation Lead Mixed‐Halide Perovskite via Tunability of the Colloidal Precursor Solution. Adv. Mater. 2017, 29 (29), 1607039. Rosales, B. A.; Hanrahan, M. P.; Boote, B. W.; Rossini, A. J.; Smith, E. A.; Vela, J. Lead Halide Perovskites: Challenges and Opportunities in Advanced Synthesis and Spectroscopy. ACS Energy Lett. 2017, 2 (4), 906–914. Kubicki, D. J.; Prochowicz, D.; Hofstetter, A.; Zakeeruddin, S. M.; Grätzel, M.; Emsley, L. Phase Segregation in Cs-, Rb- and K-Doped Mixed-Cation (MA) X(FA) 1– xPbI 3Hybrid Perovskites From Solid-State NMR. J. Am. Chem. Soc. 2017, 139 (40), 14173– 14180. Yoon, S. J.; Stamplecoskie, K. G.; Kamat, P. V. How Lead Halide Complex Chemistry Dictates the Composition of Mixed Halide Perovskites. J. Phys. Chem. Lett. 2016, 7 (7), 1368–1373. Stoumpos, C. C.; Kanatzidis, M. G. The Renaissance of Halide Perovskites and Their Evolution as Emerging Semiconductors. Acc. Chem. Res. 2015, 48 (10), 2791–2802. Rizzo, V.; Pinciroli, V. Quantitative NMR in Synthetic and Combinatorial Chemistry. J. Pharm. and Bio. Anal. 2005, 38 (5), 851–857. Li, L.; Wang, F.; Wu, X.; Yu, H.; Zhou, S.; Zhao, N. Carrier-Activated Polarization in Organometal Halide Perovskites. J. Phys. Chem. C 2016, 120 (5), 2536–2541. Van Gompel, W. T. M.; Herckens, R.; Reekmans, G.; Ruttens, B.; D’Haen, J.; Adriaensens, P.; Lutsen, L.; Vanderzande, D. Degradation of the Formamidinium Cation and the Quantification of the Formamidinium–Methylammonium Ratio in Lead Iodide Hybrid Perovskites by Nuclear Magnetic Resonance Spectroscopy. J. Phys. Chem. C 2018, 122 (8), 4117–4124. Smith, I. C.; Hoke, E. T.; Solis-Ibarra, D.; McGehee, M. D.; Karunadasa, H. I. A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability. Angew. Chem. Int. Ed. 2014, 53 (42), 11232–11235. Grancini, G.; n-Carmona, C. R. A.; Zimmermann, I.; Mosconi, E.; Lee, X.; Martineau, D.; Narbey, S.; Oswald, F.; De Angelis, F.; Graetzel, M.; Nazeeruddin, M. K. One-Year Stable Perovskite Solar Cells by 2D/3D Interface Engineering. Nature Commun. 2017, 8, 1–8. deQuilettes, D. W.; Koch, S.; Burke, S.; Paranji, R. K.; Shropshire, A. J.; Ziffer, M. E.; Ginger, D. S. Photoluminescence Lifetimes Exceeding 8 µs and Quantum Yields Exceeding 30% in Hybrid Perovskite Thin Films by Ligand Passivation. ACS Energy Lett. 2016, 1 (2), 438–444.


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ACS Energy Letters

Scheme 1. Schematic representation of our method to quantify cations in mixed-cation perovskites by qNMR. 139x285mm (300 x 300 DPI)



Figure 1. Average concentration and MA:FA:Cs ratios of perovskites films made from solutions with different stoichiometries. The filled boxes show average concentration values with error while the diamond markers show the precursor solution (or nominal) concentration. 811x289mm (300 x 300 DPI)


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On the True Composition of Mixed-Cation Perovskite Films - ACS

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