Slow CH3NH3+ Diffusion in CH3NH3PbI3 Under Light Measured by

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Slow CHNH+ Diffusion in CHNHPbI Under Light Measured by Solid-State NMR and Tracer Diffusion Alessandro Senocrate, Igor Moudrakovski, Tolga Acartuerk, Rotraut Merkle, Gee Yeong Kim, Ulrich Starke, Michael Grätzel, and Joachim Maier J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06814 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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The Journal of Physical Chemistry

Slow CH3NH3+ Diffusion in CH3NH3PbI3 Under Light Measured by Solid-State NMR and Tracer Diffusion Alessandro Senocrate 1,2, Igor Moudrakovski1, Tolga Acartürk 1, Rotraut Merkle1, Gee Yeong Kim1, Ulrich Starke1, Michael Grätzel1,2 and Joachim Maier1*

[email protected] 1

Max Planck Institute for Solid State Research, Heisenbergstr. 1, 70569, Stuttgart, Germany Department of Chemistry and Chemical Engineering, École polytechnique fédérale de Lausanne, Lausanne CH-1015, Switzerland 2

Abstract We investigate methylammonium transport in methylammonium lead iodide under illumination and show this, as in the dark, to be measurable but negligible when compared with the major carriers. 1H and

13

C NMR spectra show constant linewidths as a function of temperature,

indicating absence of significant methylammonium (MA) diffusion.

13

C tracer exchange

experiments reveal two distinct diffusion processes, one attributed to bulk MA transport, 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. 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 neighbouring phases which are blocking for ion transfer.9,10 Such a situation is met in devices under operation, and the occurrence of these bulk polarisation 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 1 ACS Paragon Plus Environment

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affects the kinetic stability of such devices,15,16 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 methylammonium diffusion and presence of a large iodine transport due to 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,

methylammonium motion under light.

20–23

several studies

claim

substantial

In this contribution, we give for the first time direct

evidence of the absence of significant methylammonium diffusion under illumination, using tracer diffusion techniques and solid state NMR measurements. While a weak methylammonium transport is indeed observed, its contribution to the overall ion transport is very small and several orders of magnitude lower than that of the 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 methylammonium diffusion also under light in MAPI. Results & 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 (Fig. 1a). Upon annealing the samples at 333 K in the dark and under illumination, we can directly observe

13

C diffusion in the pure MAPI

sample by means of ToF-SIMS (Time of Flight Secondary Ions Mass Spectroscopy). As shown in Fig. 1, there is no difference in the diffusion profiles obtained by annealing with or without light, clearly showing that the methylammonium 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 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 2 ACS Paragon Plus Environment

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annealing step yields an increased

13

C 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 Fig. 1, we can adequately simulate our experimental profiles by ∗ considering two distinct diffusion processes with the same surface rate constant ( = 210-11 ∗ ∗ cm/s) but different diffusion coefficients (, ≥ 110-13 cm2/s and ,

= 310-17 cm2/s). The

fast diffusion coefficient is only a lower limit, as this process is distinctly surface controlled (Supporting Information Fig. S1), while 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 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 (910-15 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. In contrast, the fast diffusion coefficient obtained here would exceed the upper bound for bulk diffusion, even more so since it is only a lower limit (Supporting Information Fig. 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 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.

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Figure 1. (a) Schematics of the film tracer experiment performed with or without illumination. (b) Tracer 2 diffusion profiles in MAPI thin films, annealed at 333 K with and without 1 mW/cm 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 diffusion -11 ∗ processes with a single surface rate constant ( = 210 cm/s) but two different diffusion coefficients -13 2 -17 2 ∗ ∗ (, ≥ 110 cm /s and ,

= 310 cm /s). As a qualitative comparison, a lower limit for the -9 2 7 diffusion coefficient of iodine species was extracted at 410 cm /s at 378 K from conductivity data.

To confirm the above results, we performed 1H and

13

C solid-state NMR on MAPI thin films,

comparing dark and illuminated conditions. NMR has already been extensively used in hybrid halide perovskites to study dynamic processes.33–39 In our experiment, light is applied in-situ during the acquisition of a NMR spectrum on a thin film sample (Fig. 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 Fig. S2). As shown in Fig. 2b,c, illumination does not affect chemical shift nor signal shape for both 1H and

13

C at any

given temperature. Also, variation of the temperature under light does not influence 1H nor

13

C

signals, indicating 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

13

C linewidths, causing the

signals to greatly narrow (for detailed discussion see Supporting Information). Since 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 4 ACS Paragon Plus Environment

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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 Fig. S3).

1

Figure 2. (a) Schematics of the NMR experiment on thin film samples with in-situ illumination. (b) H and 13 13 15 (c) C stationary solid state NMR spectra acquired on C, N-enriched MAPI at different temperatures, 2 with or without in-situ llumination (10 mW/cm ). No significant changes are visible in the chemical shift nor in the signal shape upon changes in temperature or illumination (minor variations can be attributed to phasing issues in the in-house built flat coil). This similarity confirms the absence of light-induced temperature effects.

Conclusions In conclusion, our experiments quantify the methylammonium diffusion coefficient in MAPI under illumination. While a minor bulk MA transport is indeed discernible, its contribution to the overall ion transport -essentially carried by iodine anions- is 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



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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 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 thin film form.

Supporting Information Synthesis and experimental methods, detailed procedure for tracer diffusion profile analysis, simulated tracer profiles, NMR referencing, thin-film NMR system checks.

Acknowledgments This work has been performed within the framework of the Max Planck-EPFL Center for Molecular Nanoscience and Technology.

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(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 diffusion processes with a single surface rate constant (k*MA = 2·10-11 cm/s) but two different diffusion coefficients (D*(MA,1) ≥ 1·10-13 cm2/s and D*(MA,2) = 3·10-17 cm2/s). As a qualitative comparison, a lower limit for the diffusion coefficient of iodine species was extracted at 4·10-9 cm2/s at 378 K from conductivity data. 218x98mm (150 x 150 DPI)

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(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 llumination (10 mW/cm2). No significant changes are visible in the chemical shift nor in the signal shape upon changes in temperature or illumination (minor variations can be attributed to phasing issues in the in-house built flat coil). This similarity confirms the absence of light-induced temperature effects. 167x114mm (150 x 150 DPI)

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Slow CH3NH3+ Diffusion in CH3NH3PbI3 Under Light Measured by

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