Humidity-Induced Grain Boundaries in MAPbI3

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Humidity-Induced Grain Boundaries in MAPbI Perovskite Films Dan Li, Simon A. Bretschneider, Victor W. Bergmann, Ilka M. Hermes, Julian Mars, Alexander Klasen , Hao Lu, Wolfgang Tremel, Markus Mezger, Hans-Jürgen Butt, Stefan A. L. Weber, and Rüdiger Berger J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00335 • Publication Date (Web): 04 Mar 2016 Downloaded from http://pubs.acs.org on March 4, 2016

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The Journal of Physical Chemistry

Humidity-Induced Grain Boundaries in MAPbI3 Perovskite Films Dan Li †, Simon A. Bretschneider †, Victor W. Bergmann †, Ilka M. Hermes †, Julian Mars †, Alexander Klasen ‡, Hao Lu †, Wolfgang Tremel ‡, Markus Mezger †, Hans-Jürgen Butt †, Stefan A. L. Weber †, Rüdiger Berger †,∗ †

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

‡

Institute of Inorganic Chemistry and Analytical Chemistry, Johannes Gutenberg University

Mainz, Duesbergweg 10-14, 55128 Mainz, Germany

AUTHOR INFORMATION Corresponding Author * Tel: +49-6131-379114, Email: [email protected] Present Address: Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

Markus Mezger and Stefan A. L. Weber are also working in Institute of Physics, Johannes Gutenberg University Mainz, Staudingerweg 7, 55128 Mainz, Germany; Alexander Klasen is now working in Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany 1 ACS Paragon Plus Environment


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ABSTRACT Methylammonium halide perovskites (MAPbI3) are very sensitive to humid environments. We performed in-situ scanning force microscope and in-situ X-ray diffraction measurements on MAPbI3 films to track changes in the film morphology and crystal structure upon repeated exposure to a high relative humidity environment (80%). We found that the appearance of monohydrate (MAPbI3⋅H2O) Bragg reflections coincided with the appearance of additional grain boundaries. Prolonging the exposure time to humidity induced more grain boundaries and steps in the MAPbI3 films and the peak intensities of the monohydrate MAPbI3⋅H2O increased. The monohydrate was not stable under dry atmosphere and could be reversed to MAPbI3. However, the humidity-induced grain boundaries persisted. The presence of these additional grain boundaries was most likely the reason for an increase in hysteresis in JV behavior upon humidity exposure. Morphological changes were not observed for exposure to humidity ≤ 50% for a duration of 144 h.

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Methylammonium metal halide perovskite materials have emerged in 2009 as promising absorber materials for solar cells.1 Recently, the power conversion efficiency (PCE) has increased to efficiencies > 20%.2, 3 Among various hybrid metal halide perovskite materials,4-7 methylammonium lead halide perovskite CH3NH3PbX3 (MAPbX3, X = I, Br or Cl) is the most investigated material due to its broad absorption range and long charge carrier diffusion lengths (> 1 µm).8-10 Key requirements for a high PCE are perovskite films with a smooth surface, large grain size and full surface coverage.11-16 On one hand, the fabrication of MAPbI3-based solar cells were usually recommended to be carried out in a controlled environment with a humidity level of < 1%.17 On the other hand, in 2014, Zhou et al. and You et al. reported on methylammonium lead halide perovskite solar cells with efficiencies > 15% that were prepared in a controlled humid environment of ≈ 30%.18, 19 The humid environment has been proven to have positive impact on the formation of MAPbI3 perovskite films.20-22 However, the perovskite films are not stable upon longer exposure to humidity,23-26 thus the stability of perovskite solar cells still hampers their commercialization. Therefore, it is crucial to have a better understanding of morphology degradation pathways under detrimental environments (e.g. humidity, light etc.). Humidity can induce decomposition of dark brown MAPbI3 crystals into yellow PbI2, resulting in a declining device performance.24, 26-29 Niu et al. suggested that MAPbI3 films decomposed into methylammonium iodide (CH3NH3I, MAI) solution and PbI2 solid in the presence of H2O.27, 30 Christians et al.26 and Yang et al.24 proposed the formation of a pale yellow hydrated intermediate phase (MA)4PbI6⋅2H2O in a high humid environment. After flushing with dry gas or storing in vacuum, the pale yellow (MA)4PbI6⋅2H2O film rapidly dehydrated and regenerated the characteristic dark brown color of a MAPbI3 film. This phenomenon was accompanied by a partial recovery of perovskite absorbance which indicated a partial reversibility of (MA)4PbI6⋅2H2O hydrate phase.24, 26 Recently, Leguy et 3 ACS Paragon Plus Environment


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al.23 introduced a degradation model of MAPbI3 upon exposure of the compound to humidity. First, a monohydrate MAPbI3⋅H2O at an early humidity induced-degradation stage was observed, which was fully reversible upon subsequent drying in gas. Second, a dihydrate (MA)4PbI6⋅2H2O formed upon longer exposure time. Leguy et al. found that the hydration process was isotropic and homogeneous throughout the film.23 Based on time-resolved ellipsometry, X-ray diffraction (XRD) and JV measurements, Leguy et al. proposed a microscopic degradation mechanism. The hydration of the MAPbI3 led to a separation of grains which resulted in electrical isolation and increased charge recombination at the grain interface. Interestingly, the JV behavior recovered almost to its original state upon subsequent drying of the film. However, Leguy et al. noticed an increase in the magnitude of the JVhysteresis behavior that might be associated with perovskite crystal size. In this context, Kim and Park reported that a decrease in crystal size led to an increasing hysteresis.31 Thus it is still an open question to which extend the temporary exposure to humidity leads to irreversible changes in the film. In this work, we performed in-situ scanning force microscope (in-situ SFM) and in-situ XRD measurements to track changes in the film morphology and the crystal structure. The aim is to refine the current picture of humidity-induced degradation of inverted planar MAPbI3 perovskite solar cells. To track humidity-induced morphology changes, in-situ SFM experiments were performed in the center of MAPbI3 perovskite films on top of PEDOT:PSS coated ITO substrates. The MAPbI3 perovskite films were prepared as reported by de Quilettes et al.32 (see Supporting Information). This preparation procedure is often used for a one-step preparation of planar type MAPbI3 perovskite solar cells leading to PCEs up to 17.1%.19 At first, we varied the exposure time and probed time-resolved morphology evolution under a controlled humidity level of 80% (Figure 1). Before humidity exposure, the MAPbI3 film contained crystallites up to a diameter of 4 µm, exhibiting a smooth topography profile with a 4 ACS Paragon Plus Environment

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peak-to-peak roughness below 3.0 nm after subtracting the background slope (Figure 1a). After 4 hours of exposure to a humidity of 80% the topography in the MAPbI3 film changed (black profile line in Figure 1b). Additional steps with a height up to 5.0 nm appeared (Figure 1b). With prolonging exposure time, more steps were introduced and groves with depths of ≈ 1.5 nm appeared in the film (Figure 1c). We attribute these groves to grain boundaries which were generated in MAPbI3 perovskite crystallites upon exposure to humidity (Figure 1b and 1c). Upon exposing the MAPbI3 film to dry N2 gas, the topography was conserved and the grain boundaries did not disappear (Figure 1e). Thus the humidity-induced morphology degradation was irreversible in terms of the surface microstructure. Upon further elongating the exposure time to 144 hours (6 days) more new grain boundaries formed and the depth of grain boundaries increased (Figure 1f). An analysis of the grain sizes revealed that only grains > 500 nm are subject to additional grain boundaries upon exposure to humidity. Furthermore, the change in morphology was reflected in a change in the root-mean-square roughness (rms) of the perovskite film, which increased from 60 - 65 nm to 80 nm in the measured area of around 155 µm2 for the longest exposure time investigated here (Supporting Information Figure S1).



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Figure 1. SFM images and profiles of the MAPbI3 films in a humidity level of 80% at different exposure times (a) initial morphology, after (b) 4 hours, (c) 6 hours, (d) 8 hours, (e) flushing with dry N2 overnight (13 hours) and (f) 80% humidity for 144 hours. The SFM profiles were extracted from the same area along the black profile lines indicated in each SFM image. SFM experiments were performed without external illumination. The SFM setup is operated in the dark at constant temperature of 20 ºC with a laser power less than 1.0 mW at a wavelength of 670 nm to read out the cantilever deflection. The laser focus was positioned slightly away from the apex of the cantilever to avoid illumination of the sample.


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To correlate the appearance of grain boundaries with the formation of additional phases, we performed time-resolved in-situ XRD measurements on a freshly prepared sample that was not exposed to humidity before. The XRD data for a pristine MAPbI3 film showed Bragg reflections at angles of 14.38º, 20.27º, 23.80º, 24.77º and 28.72º (Figure 2a at 0h). They were attributed to the (110), (200), (211), (202) and (220) crystal planes of the tetragonal perovskite structure.26, 33 The weak Bragg reflection at 12.90º indicated the presence of PbI2 crystallites in the fresh prepared MAPbI3 film.26, 34 The PbI2 is likely caused by thermal annealing at 100 ºC during film preparation.35 We then flushed the experimental chamber with 80% humid N2 gas. In-situ XRD measurements in reflection geometry were made every 15 minutes. After exposure to 80% humidity for 5.5 hours and 6.5 hours respectively, additional Bragg reflections at 2θ = 10.75º and 8.76º were observed (Figure 2a). They corresponded to the (1 01) and (100) Bragg reflections of monoclinic MAPbI3⋅H2O with space group P21/m, indicating the formation of monohydrate.23 For visualization of the transformation process, we plotted the X-ray diffraction pattern versus time in a contour plot (Figure 2b).

Figure 2. Time-resolved in-situ XRD of a MAPbI3 film during first hydration-dehydration round and the zoom-in image at 2θ angle from 8.0º to 13.5 º (a) and the corresponding contour plot showing changes in XRD peak intensity during the first hydration-dehydration cycle (b). 7 ACS Paragon Plus Environment


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Interestingly, the appearance of the (100) and (101) diffraction peaks coincided in time with the appearance of grain boundaries observed by in-situ SFM (Figure 1c). Upon further exposure of the film to a humid environment, the peak intensities of the monohydrate phase increased (Figure 2a at 7 h, 8 h, and 9 h). The hydration was stopped after 9 h by flushing the experimental chamber with dry N2 gas. The XRD data revealed that monohydrate MAPbI3⋅H2O still existed after 10 min drying, but disappeared after 25 min (Figure 2a). From these results, we can conclude that the formation of the monohydrated phase at ≈ 5.5 h is linked to the appearance of steps and grain boundaries observed by in-situ SFM. At the initial stage (0 - 5.5 h) water molecules diffused into MAPbI3 structure but did not form a significant portion of the hydrated phase. When a critical uptake of water was reached, the hydrated methylammonium lead iodide phase (MAPbI3⋅H2O) formed. The transformation of MAPbI3 perovskite to the monohydrate phase MAPbI3⋅H2O led to a 6% volume expansion, which was based on the relative lattice parameters of MAPbI3 (247 Å3 per formula unit) and MAPbI3⋅H2O (263 Å3 per formula unit).23, 36, 37 Thus, stress inside the film built up and was released by creating additional steps and grain boundaries. Directly after the first exposure to humidity and subsequent drying in N2 gas, a second hydration-dehydration experiment was performed. Now the Bragg reflection at 2θ angle of 10.75° indicating (100) reflection of monohydrate MAPbI3⋅H2O was first detected after ≈ 7 h. The Bragg reflection at 8.76° ( 1 01) of monohydrate MAPbI3⋅H2O appeared after ≈ 9 h (Figure 3a and 3c). At times > 9 h the peak intensity of monohydrate gradually increased with prolonging exposure time, while the peak intensity of the MAPbI3 perovskite at (110) Bragg reflection only slightly decreased (Figure 3a and 3c). Thus, a significant fraction of MAPbI3 crystals was not directly affected by humidity. It is worth noticing that the formation of monohydrate was a non-linear process (Figure 3c). Finally, after flushing the experimental


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chamber with dry N2 for 4 min, the (100) and ( 1 01) Bragg reflections of monohydrate MAPbI3⋅H2O remained. However, they disappeared before the third measurement, starting after 20 min dehydration (Figure S2). Thus, similar to the first humidity exposure, the dehydration process was much faster than the hydration.



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Figure 3. Contour plots of time-resolved peak intensity of XRD data at second (a) and third (b) hydration-dehydration cycles, (c) Integrated intensities of the Bragg reflections as a function of exposure time (2θ = 14.38º perovskite (110); 10.75º monohydrate (101); 8.76º monohydrate (100)). The monohydrate Bragg reflections disappeared before the third measurement after 30 min was performed. In the third, long term exposure cycle, the Bragg reflection intensity of monohydrate decreased after flushing with dry N2 for 15 min.

A third long-term exposure of the sample to a humidity of 80% for 65 h was carried out (Figure 3b and S3). The XRD measurement indicated the presence of MAPbI3⋅H2O after 5 h of exposure to humidity. The peak intensities of the (100) and ( 1 01) reflections in monohydrate increased with prolonging exposure time. Simultaneously the peak intensity of (110) reflection in MAPbI3 perovskite structure decreased (Figure 3c). Even close to the end of the experiment after 65 h, no saturation of the peak intensities was observed. The cell was then flushed with dry N2 gas. Even after this long-term humidity exposure, the monohydrate phase MAPbI3⋅H2O disappeared. However, the Bragg reflection corresponding to the MAPbI3 perovskite structure clearly did not return to its initial intensity value (Figure 3c). A possible explanation is that only a fraction of the hydrated phase was converted back into the MAPbI3 perovskite structure. Additionally, a new Bragg reflection at 2θ = 12.45º appeared during the dehydration process. This peak indicates the formation of crystalline PbI2 at the late degradation stage (Figure S3).26 To study the effect of very long exposure (144 hours, i.e. 6 days) of MAPbI3 films to 80% humidity, we performed ex-situ XRD measurements on a powder diffractometer in Bragg-Brentano geometry. In contrast to the in-situ experiments on the time-scale up to 65 h, these films showed the formation of a dihydrate phase (MA)4PbI6⋅2H2O (Figure S4). 10 ACS Paragon Plus Environment

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Notably, the Bragg reflections of the MAPbI3⋅H2O appeared ≈ 1.5 h later in the 2nd hydration-dehydration cycle compared to the first and third one. One possibility for a delay in the formation of a hydrated phase could be the formation of a thin protective skin layer that formed on top of MAPbI3 film after the first hydration-dehydration process. Such a skin could prevent the penetration of water molecules in the second hydration cycle.38 In order to probe the possible presence of a skin layer we performed depth profiling X-ray photoelectron spectroscopy (XPS) on both reference and 10 h humidity-exposed MAPbI3 films (Figure S5). The depth profiling XPS revealed almost identical atomic ratios of I/Pb and N/Pb for both reference and humidity-exposed MAPbI3 films within the error of the measurement. Therefore we excluded the formation of a skin layer on top of MAPbI3 film during exposure to humidity. Finally, we were interested whether lower humidity levels can lead to the formation of new grain boundaries, too. Therefore, we exposed freshly prepared MAPbI3 films to lower humidity atmosphere of 0%, 20% and 50% and characterized the perovskite morphology by in-situ SFM. Before humidity exposure, all the MAPbI3 perovskite films contained large crystallites (Figure S 6a, S6c, S6e). We found no changes in morphology at low humidity environments ≤ 20% (Figure S6a - 6d) even after an exposure for 144 hours. This observation is in agreement with UV-visible absorption measurements reported by Yang et al.24 At a humidity of 50% holes formed in the perovskite film after 24 hours (Figure S6e and S6f). The size and the number of holes did not change for longer exposure time up to 144 h. This observation is also in agreement with the report from Matsumoto et al. who found that the optical properties of the MAPbI3 perovskite film did not change upon exposure to a relative humidity of 40% in the dark.39 Our ex-situ UV-visible spectra for a film stored at 50% humidity stored film still showed comparable absorption to initial pristine MAPbI3 film over the visible to near-IR spectrum range (Figure S7). Additionally, the XRD data revealed no 11 ACS Paragon Plus Environment


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changes of Bragg reflections (Figure S4). In summary, SFM, UV-visible spectra and X-ray diffraction experiments did not indicate degradation in the macrostructure and crystal structure of MAPbI3 perovskite film after exposure for 144 hours to humidity up to 50%.24, 26 Based on all findings described above, we propose a degradation model, which is based on the model outlined previously by Leguy et al.23 First water molecules diffused into MAPbI3 films upon exposure to humidity most probably along already existing grain boundaries (Figure 4a). In the beginning this neither had an effect on the MAPbI3 crystal structure nor on the surface morphology. The monohydrate MAPbI3⋅H2O could only form after a prolonged exposure time to an environment of humidity above 50%. Once the monohydrated phase formed, additional steps and grain boundaries appeared on the MAPbI3 films, which induced a rougher surface (Figure 4b, the hydrated phase is sketched in a light blue color). At later stages, more and more grain boundaries were observed by in-situ SFM for continuing exposure to humidity (Figure 1d and Figure 4c). Accordingly, the intensity of MAPbI3⋅H2O monohydrate Bragg reflections increased. Under a dry atmosphere the monohydrate MAPbI3⋅H2O converted back to MAPbI3 again. However, the grain boundaries persisted. After longer humidity exposure for 65 h, PbI2 was detected during the following dehydration process under dry atmosphere (Figure 4d and Figure S4b). Furthermore, after dehydration the intensity of MAPbI3 at (110) reflection did not return to its initial intensity value. This observation might be explained by a partially irreversible degradation of MAPbI3 films that led to the formation of PbI2.


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Figure 4. Proposed degradation model of MAPbI3 perovskite films prepared on top of a PEDOT:PSS coated ITO substrate32 at microscopic scale under humidity exposure (80%) and dry N2 gas flushing. Grain boundaries in the MAPbI3 phase are indicated with black lines. The square on the right side of the scheme shows the experimental atmospheres (80% humidity and dry N2) in our measurements.

Our measurements confirmed the formation of monohydrate MAPbI3⋅H2O under high humidity environment (80%), which was reversible under dry N2 flushing. The results were consisted with the reports on reversibility between perovskite phase and hydrated phase during humidity-induced degradation process.23,24 However, our measurements did not exhibit that degradation primarily started from or along the grain boundaries as proposed by Leguy et al.

23

In particular, we found irreversible morphology features such as grain boundaries and

steps which persisted after drying the MAPbI3 film overnight. The presence of grain boundaries could limit the charge transport40 and act as recombination centers.15,

41-43



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Quilettes et al. claimed the grain boundaries exhibited faster non-radiative decay which decreases the device performance.32 Therefore, we assumed the presence of these grain boundaries was most likely the reason for an increase in hysteresis upon humidity exposure that was measured by Leguy et al.23 This interpretation is consistent with the finding that a decrease in crystal size leads to an increasing hysteresis.31 Interestingly we noticed that morphological changes could not be observed for exposure to humidity ≤ 50% for a duration of 144 h. Thus passivation layers which are able to keep the humidity in devices below this value could be very beneficial for device performance. ASSOCIATED CONTENT SUPPORTING INFORMATION AVAILABLE Detailed sample preparation procedure and experimental methodology, in-situ SFM images, in-situ and ex-situ XRD spectra, UV-visible absorption spectra, XPS spectra of perovskite films. AUTHOR INFORMATION Corresponding Author ∗Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors would like to thank Dr. T. Weidner for the XPS experiment, and M. Steiert for the ex-situ X-ray diffraction measurements. D. Li thanks China Scholarship Council (CSC) for financial support. R. Berger, Victor W. Bergmann, and Stefan A. L. Weber thank the 14 ACS Paragon Plus Environment

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International Research Training Group 1404 ‘Self-organized Materials for Optoelectronics’ (DFG) for support.

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TOC GRAPHICS


Humidity-Induced Grain Boundaries in MAPbI3

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