Article Cite This: J. Phys. Chem. C 2018, 122, 21803−21806
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Slow CH3NH3+ Diffusion in CH3NH3PbI3 under Light Measured by Solid-State NMR and Tracer Diffusion Alessandro Senocrate,†,‡ Igor Moudrakovski,† Tolga Acartürk,† Rotraut Merkle,† Gee Yeong Kim,† Ulrich Starke,† Michael Grätzel,†,‡ and Joachim Maier*,† †
Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569 Stuttgart, Germany Department of Chemistry and Chemical Engineering, É cole Polytechnique Fédérale de Lausanne, Lausanne CH-1015, Switzerland
‡
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S Supporting Information *
ABSTRACT: We investigate methylammonium (MA) transport in MA lead iodide under illumination and show this, as in the dark, to be measurable but negligible when compared with the major carriers. 1H and 13C nuclear magnetic resonance (NMR) spectra show constant linewidths as a function of temperature, indicating the absence of significant MA diffusion. 13C tracer-exchange experiments reveal two distinct diffusion processes, one attributed to bulk MA transport and the other most probably due to higher dimensional defects. The former process has a diffusion coefficient that is consistent with the upper limit extracted from NMR measurements. Derived bulk conductivities for MA cations are orders of magnitude below the experimental ionic conductivity, corroborating the picture of pure iodine transport under illumination, as it was previously experimentally shown only for the dark situation.
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this contribution, we give for the first time direct evidence of the absence of significant MA diffusion under illumination, using tracer diffusion techniques and solid-state nuclear magnetic resonance (NMR) measurements. Whereas a weak MA transport is indeed observed, its contribution to the overall ion transport is very small and several orders of magnitude lower than that of iodine.4,7,11,19 Notwithstanding the important implications of these results for the study of halide perovskites, the NMR measurements themselves deserve attention as, to the best of our knowledge, no solid-state measurements in thin-film samples with in situ illumination have ever been performed. In this respect, we recognize that a few reports have dealt with coupling light and NMR measurements for studying photochemistry in liquids.24,25 Through the use of these powerful techniques, we unambiguously rule out any significant MA diffusion also under light in MAPI.
INTRODUCTION Hybrid organic−inorganic halide perovskites are semiconducting materials heavily researched for their application in photovoltaic and optoelectronic devices.1,2 Notably, in these compounds, charge transport is not purely of electronic nature but contains a significant ionic contribution.3−8 Any investigation concerned with the material’s photoelectrochemical and electrical properties must take this aspect into account. In particular, one must reckon with the formation of stoichiometric gradients seizing the entire bulk of the material, whenever this is charged by a current while sandwiched between neighboring phases, which are blocking the ion transfer.9,10 Such a situation is met in devices under operation, and the occurrence of these bulk polarization phenomena in halide perovskites was experimentally observed.4,7,11 Ion redistribution is also expected to be involved in all interfacial processes that involve the perovskite layer occurring in solar cell devices, both with and without a current load.12−14 Moreover, ion migration strongly affects the kinetic stability of such devices15,16 and is directly relevant for the degradation kinetics of the materials. It is therefore of great importance to understand the ion conduction processes taking place in these compounds. In this respect, our group has extensively studied the nature of ion conduction in methylammonium lead iodide (MAPI), the archetypal hybrid halide perovskite. These studies, focused on equilibrium conditions (in the dark), show absence of any significant MA diffusion and presence of a large iodine transport carried by vacancies.4,7,11 This is also in agreement with several computational studies.17,18 As far as the situation under illumination is concerned, we unambiguously showed that light can induce a large enhancement of ion conduction in MAPI, which was attributed to an increased concentration of iodine vacancies.19 Nevertheless, several studies claim substantial MA motion under light.20−23 In © 2018 American Chemical Society
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RESULTS AND DISCUSSION Let us start by considering the tracer diffusion experiments, obtained by mechanically contacting two thin-film samples, one of which is 13C,15N-enriched (Figure 1a). Upon annealing the samples at 333 K in the dark and under illumination, we can directly observe 13C diffusion in the pure MAPI sample by means of time-of-flight secondary ion mass spectroscopy. As shown in Figure 1, there is no difference in the diffusion profiles obtained by annealing with or without light, clearly showing that the MA transport is not altered by illumination. As we reported elsewhere, ion conduction in MAPI is instead greatly enhanced by light, and we attributed this effect to an augmented iodide Received: July 16, 2018 Revised: August 22, 2018 Published: August 31, 2018 21803
DOI: 10.1021/acs.jpcc.8b06814 J. Phys. Chem. C 2018, 122, 21803−21806
Article
The Journal of Physical Chemistry C
× 10−11 cm/s) but different diffusion coefficients (D*MA,1 ≥ 1 × 10−13 cm2/s and D*MA,2 = 3 × 10−17 cm2/s). The fast diffusion coefficient is only a lower limit, as this process is distinctly surface-controlled (Supporting Information Figure S1), whereas this is not the case for the sluggish process. Even though we have no direct evidence on the nature of these two processes, we recognize that this situation could be met if a comparatively fast diffusion through grain boundaries (or other extended defects) would happen alongside a slower bulk diffusion (providing the distance between the grain boundaries is sufficiently high). Note that such an occurrence is rather common in oxide perovskites, where A- or B-site cations have been often reported to have faster grain boundary diffusivities with respect to the bulk.27−31 Interestingly, the sluggish diffusion coefficient found here (attributable to bulk diffusion) is consistent with our previously reported upper limit for bulk MA diffusion (9 × 10−15 cm2/s at 333 K)7 and orders of magnitude lower than what was found for iodine transport.7 In addition, the absence of a second diffusion process in tracer experiments carried out on bulk pellet samples,7 where the grain size is distinctly larger, supports the above hypothesis. By contrast, the fast diffusion coefficient obtained here would exceed the upper bound for bulk diffusion, even more so because it is only a lower limit (Supporting Information Figure S1). Note that the comparison between diffusion coefficients in pellets and thin films can only be done on a qualitative level, as these values are affected by both defect concentrations and microstructural features. Nonetheless, we recognize that such minor MA transport could still be of relevance for the degradation kinetics of halide perovskites and for cation-exchange reactions in nanocrystals.32 Concerning the latter, it is not a contradiction (it is, instead, rather expected) that the time constant of the bulk tracer diffusion given here is larger than the value reported for cation exchange, as this second process represents a chemical diffusion. To confirm the above results, we performed 1H and 13C solidstate NMR spectroscopy on MAPI thin films, comparing dark and illuminated conditions. NMR spectroscopy has already been extensively used in hybrid halide perovskites to study dynamic
Figure 1. (a) Schematics of the film tracer experiment performed with or without illumination. (b) Tracer diffusion profiles in MAPI thin films, annealed at 333 K with and without 1 mW/cm2 illumination. The similarity of the two profiles confirms the absence of light-induced temperature effects. No degradation was observed in any of the used conditions. The simulated curve is obtained by considering two * = 2 × 10−11 diffusion processes with a single surface rate constant (kMA cm/s) but two different diffusion coefficients (D*MA,1 ≥ 1 × 10−13 cm2/s * = 3 × 10−17 cm2/s). As a qualitative comparison, a lower limit and DMA,2 for the diffusion coefficient of iodine species was extracted at 4 × 10−9 cm2/s at 378 K from the conductivity data.7
conductivity due to an increased iodine vacancy concentration.19 This interpretation is now corroborated by the present experimental results showing that, both in the dark7 and under illumination, MA diffusion in MAPI is detectable but very small. Owing to the short diffusion distance present in thin films (300 nm thickness), the annealing step yields an increased 13C concentration in the entire sample. We note that, however, a single diffusion process taking place in a finite medium cannot properly describe our experimental data.26 On the other hand, the flat tail visible in the profile indicates the overlapping of two different diffusion processes. This occurrence is not surprising, especially considering that a film-to-film mechanical contact (as the one used for the isotope treatment) can easily be inhomogeneous. As shown in Figure 1, we can adequately simulate our experimental profiles by considering two distinct diffusion processes with the same surface rate constant (kMA * =2
Figure 2. (a) Schematics of the NMR experiment on thin-film samples with in situ illumination. (b) 1H and (c) 13C stationary solid-state NMR spectra acquired on 13C,15N-enriched MAPI at different temperatures, with or without in situ illumination (10 mW/cm2). No significant changes are visible in the chemical shift or in the signal shape upon changes in the temperature or illumination (minor variations can be attributed to phasing issues in the inhouse built flat coil). This similarity confirms the absence of light-induced temperature effects. 21804
DOI: 10.1021/acs.jpcc.8b06814 J. Phys. Chem. C 2018, 122, 21803−21806
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The Journal of Physical Chemistry C processes.33−39 In our experiment, light is applied in situ during the acquisition of a NMR spectrum on a thin-film sample (Figure 2a). As we are not aware of any other example where NMR measurements were performed under such conditions, we double-checked the successful in situ illumination (Supporting Information Figure S2). As shown in Figure 2b,c, illumination does not affect the chemical shift or the signal shape for both 1H and 13C at any given temperature. Also, variation of the temperature under light does not influence the 1H or 13C signals, indicating the absence of translational motion in the temperature range under concern. This aspect is discussed in detail for the dark situation elsewhere.7 Here, it suffices to state that the presence of rapid translational motion (diffusion) involving MA cations would result in the averaging of the intermolecular interactions composing the 1H or 13C linewidths, causing the signals to greatly narrow (for detailed discussion, see Supporting Information). As we see no signs of this process taking place in the temperature range probed (253−333 K), we conclude that MA cations are not involved in any significant long-range diffusion process under illumination. This situation is analogous to dark conditions.7 As expected, no significant changes are visible between the NMR spectra of MAPI thin films and bulk powders (Supporting Information Figure S3).
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been performed within the framework of the Max Planck-EPFL Center for Molecular Nanoscience and Technology.
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(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) 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. (3) Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. Giant Switchable Photovoltaic Effect in Organometal Trihalide Perovskite Devices. Nat. Mater. 2015, 14, 193− 198. (4) Yang, T.-Y.; Gregori, G.; Pellet, N.; Grätzel, M.; Maier, J. The Significance of Ion Conduction in a Hybrid Organic-Inorganic LeadIodide-Based Perovskite Photosensitizer. Angew. Chem., Int. Ed. 2015, 54, 7905−7910. (5) Calado, P.; Telford, A. M.; Bryant, D.; Li, X.; Nelson, J.; O’Regan, B. C.; Barnes, P. R. F. Evidence for Ion Migration in Hybrid Perovskite Solar Cells with Minimal Hysteresis. Nat. Commun. 2016, 7, 13831. (6) Li, C.; Tscheuschner, S.; Paulus, F.; Hopkinson, P. E.; Kießling, J.; Köhler, A.; Vaynzof, Y.; Huettner, S. Iodine Migration and Its Effect on Hysteresis in Perovskite Solar Cells. Adv. Mater. 2016, 28, 2446−2454. (7) Senocrate, A.; Moudrakovski, I.; Kim, G. Y.; Yang, T.-Y.; Gregori, G.; Grätzel, M.; Maier, J. The Nature of Ion Conduction in Methylammonium Lead Iodide: A Multimethod Approach. Angew. Chem., Int. Ed. 2017, 56, 7755−7759. (8) Yang, D.; Ming, W.; Shi, H.; Zhang, L.; Du, M.-H. Fast Diffusion of Native Defects and Impurities in Perovskite Solar Cell Material CH3NH3PbI3. Chem. Mater. 2016, 28, 4349−4357. (9) Wagner, C. Galvanische Zellen Mit Festen Elektrolyten Mit Gemischter Stromleitung. Z. Elektrochem. 1956, 60, 4−7. (10) Yokota, I. On the Theory of Mixed Conduction with Special Reference to Conduction in Silver Sulfide Group Semiconductors. J. Phys. Soc. Jpn. 1961, 16, 2213−2223. (11) Senocrate, A.; Yang, T.-Y.; Gregori, G.; Kim, G. Y.; Grätzel, M.; Maier, J. Charge Carrier Chemistry in Methylammonium Lead Iodide. Solid State Ionics 2018, 321, 69−74. (12) Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. Understanding the rate-dependent J-V hysteresis, slow time component, and aging in CH3NH3PbI3 perovskite solar cells: the role of a compensated electric field. Energy Environ. Sci. 2015, 8, 995−1004. (13) Jamnik, J.; Maier, J. Transport across Boundary Layers in Ionic Crystals Part I: General Formalism and Conception. Ber. Bunsen-Ges. 1997, 101, 23−40. (14) Levine, I.; Nayak, P. K.; Wang, J. T.-W.; Sakai, N.; Van Reenen, S.; Brenner, T. M.; Mukhopadhyay, S.; Snaith, H. J.; Hodes, G.; Cahen, D. Interface-Dependent Ion Migration/Accumulation Controls Hysteresis in MAPbI3 Solar Cells. J. Phys. Chem. C 2016, 120, 16399− 16411. (15) Cheng, Y.; Li, H.-W.; Qing, J.; Yang, Q.-D.; Guan, Z.; Liu, C.; Cheung, S. H.; So, S. K.; Lee, C.-S.; Tsang, S.-W. The Detrimental Effect of Excess Mobile Ions in Planar CH3NH3PbI3 Perovskite Solar Cells. J. Mater. Chem. A 2016, 4, 12748−12755. (16) Carrillo, J.; Guerrero, A.; Rahimnejad, S.; Almora, O.; Zarazua, I.; Mas-Marza, E.; Bisquert, J.; Garcia-Belmonte, G. Ionic Reactivity at Contacts and Aging of Methylammonium Lead Triiodide Perovskite Solar Cells. Adv. Energy Mater. 2016, 6, 1502246.
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CONCLUSIONS In conclusion, our experiments quantify the MA diffusion coefficient in MAPI under illumination. Whereas a minor bulk MA transport is indeed discernible, its contribution to the overall ion transportessentially carried by iodine anionsis negligible. This conclusion, previously drawn only for equilibrium conditions (in the dark), is now extended to the situation under illumination. These observations are consistent with recent experimental reports indicating that it is possible to fabricate perovskite homojunctions by tuning MAI composition in MAPI.40 Nevertheless, we note that such MA transport, albeit much more sluggish than iodine conduction,7 could still be of importance for the degradation kinetics or cation-exchange reactions in halide perovskites.32 From a methodological point of view, we showed that the application of tracer studies and NMR measurements as a function of illumination is feasible and offers a powerful tool to analyze the coupling of light and transport in different materials, even in a thin-film form.
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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.jpcc.8b06814.
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REFERENCES
Synthesis and experimental methods, detailed procedure for tracer diffusion profile analysis, simulated tracer profiles, NMR referencing, and thin-film NMR system checks (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Alessandro Senocrate: 0000-0002-0952-0948 Rotraut Merkle: 0000-0003-3811-8963 Michael Grätzel: 0000-0002-0068-0195 Joachim Maier: 0000-0003-2274-6068 21805
DOI: 10.1021/acs.jpcc.8b06814 J. Phys. Chem. C 2018, 122, 21803−21806
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DOI: 10.1021/acs.jpcc.8b06814 J. Phys. Chem. C 2018, 122, 21803−21806
