Elucidating the Failure Mechanisms of Perovskite Solar Cells in Humid

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Elucidating the Failure Mechanisms of Perovskite Solar Cells in Humid Environments Using In Situ Grazing-Incidence Wide Angle X-ray Scattering Kyle M. Fransishyn, Soumya Kundu, and Timothy L Kelly ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01300 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 16, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Energy Letters

Elucidating the Failure Mechanisms of Perovskite Solar Cells in Humid Environments Using In Situ Grazing-Incidence Wide Angle X-ray Scattering Kyle M. Fransishyn, Soumya Kundu, and Timothy L. Kelly* Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

The main downfall of perovskite solar cells is that they degrade rapidly when exposed to heat, light, and moisture. Although previous work has elucidated the decomposition pathways of lead halide perovskites, there has been little direct insight into how perovskite decomposition affects device performance. Therefore, in order to better understand this correlation, we performed in situ

grazing-incidence

wide

angle

x-ray

scattering

(GIWAXS)

measurements

on

methylammonium lead iodide solar cells. We show that the formation of hydrate phases is not the most important device degradation pathway; rather, as water penetrates the cell, the mobility of iodide ions increases, leading to corrosion of the metallic top contact. Furthermore, the work reveals a temperature-dependence to the perovskite decomposition pathway, with higher temperatures suppressing the formation of both intermediates and byproducts. The work suggests that the rigorous exclusion of moisture and the design of corrosion-resistant electrodes may help produce longer-lived perovskite solar cells.

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Solar cells based on lead halide perovskites are one of the most promising new photovoltaic technologies, with the record cell efficiency (> 22%) now competing with that of market-leading technologies like multi-crystalline silicon and cadmium telluride.1 The main drawback of lead halide perovskites, however, is their instability with respect to environmental stimuli such as heat, light, oxygen, and moisture.2-4 Exposure to these factors can lead to rapid perovskite decomposition and cell failure. Although our understanding of perovskite decomposition pathways is improving, the exact relationship between perovskite decomposition and cell failure is still unclear. One of the earliest noted instabilities of lead halide perovskites (such as the parent member of the family, CH3NH3PbI3) is their sensitivity toward both liquid and vapor-phase water. While exposure to liquid water leads to the immediate and irreversible decomposition of CH3NH3PbI3, the reaction with water vapor is more complex; in high relative humidity (RH) environments, CH3NH3PbI3 reacts to form a series of crystalline hydrate phases,5,6 before finally degrading into PbI2. The formation of these hydrate phases in CH3NH3PbI3 thin films was reported in separate studies by Christian et al.,7 Yang et al.,8 and Leguy et al.;9 collectively, the results suggest that CH3NH3PbI3 first reversibly forms a monohydrate phase (CH3NH3PbI3·H2O), before decomposing irreversibly to PbI2 and a dihydrate phase ((CH3NH3)4PbI6·2H2O).9 The reversible nature of the monohydrate formation made in situ grazing-incidence wide angle x-ray scattering (GIWAXS) methods critical to the elucidation of the decomposition mechanism.8 This type of in situ characterization has been invaluable in improving our understanding of the chemical processes that occur in lead halide perovskite thin films; in situ x-ray scattering methods10 have yielded important information on structural changes occurring in the perovskite as it forms, crystallizes, or degrades, with early examples focusing on probing perovskite


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formation and crystallization mechanisms.11-15 Yet although these methods have yielded important insights into the chemistry of perovskite films, the impact of these chemical changes on device performance is only poorly understood. A perovskite solar cell is substantially more complex than a thin film, with the addition of electrodes, electron- and hole-transport layers, and all of their associated interfaces. As a result, it is not clear whether perovskite decomposition is actually what limits device lifetime, and exactly what impact perovskite decomposition has on device efficiency. Several research groups have therefore begun to explore the potential of operando GIWAXS measurements16,17 – where diffraction patterns are acquired on complete devices during operation, and diffraction data correlated with cell performance. As an excellent example of this approach, the effect of the tetragonal-to-cubic phase transition on CH3NH3PbI3based solar cells was studied using operando GIWAXS.16 It was found that there was no discontinuity in cell performance as the devices were heated through the phase transition at 6065 °C, indicating that these cells have a high degree of tolerance to structural changes occurring within the perovskite layer. More recent work has even probed the photovoltaic performance of perovskite solar cells during the perovskite formation and crystallization process by depositing precursor solutions onto pre-fabricated interdigitated electrodes.17 Moderately high open-circuit voltage (Voc) was observed even before the appearance of crystalline perovskite phases, again highlighting the robust nature of this class of photovoltaic materials. Motivated by a desire to understand the impact of hydration on the efficiency of CH3NH3PbI3based solar cells, we carried out an in situ GIWAXS study where we exposed devices to a high humidity environment (RH ≈ 80%). Measurements were carried out at the Hard X-ray MicroAnalysis (HXMA) beamline of the Canadian Light Source, taking advantage of the high xray flux of the synchrotron and the fast response of a CCD area detector to simultaneously track


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changes in cell performance and perovskite structure in real time. The experiments reveal that the absorption of moisture by the perovskite layer increases the mobility of iodide ions, which subsequently migrate to the metal top electrode, leading to corrosion and a rapid loss of cell performance. Even when corrosion-resistant electrodes are used, this ion migration leads to a screening of the built-in potential and a loss of carrier collection efficiency. The experiments also show that the moisture-induced decomposition mechanism of CH3NH3PbI3 is different for cells under full illumination as opposed to the dark, suggesting the possibility of diurnal variation in the degradation rate of field-deployed modules.

Figure 1. (a) Diagram of the humidity-control apparatus and in situ sample chamber. (b) Photograph of a perovskite cell in the sample holder, showing the electrical leads and bottom-up source of illumination. (c) Device architecture of the perovskite solar cells studied in this work. The devices in this study were prepared by depositing the CH3NH3PbI3 absorber layer onto a ZnO electron-transport layer using a two-step deposition method.18,19 Our preliminary work used 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD) as the hole-transport layer, in keeping with previous cell designs from our group;19 however, the


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devices were found to degrade extremely rapidly in humid environments. Both we and others have ascribed this rapid cell failure to the high levels of lithium dopants used in SpiroOMeTAD-based devices.20,21 We therefore switched to a poly(3-hexylthiophene) (P3HT) holetransport layer, and our initial experiments were performed on devices with an ITO/ZnO/CH3NH3PbI3/P3HT/Ag device structure. Although the power conversion efficiencies (PCEs) of the P3HT-based devices (Table S1, Figure S1) were lower than those of SpiroOMeTAD-based cells, the cells were much more robust, making them more suitable for the in situ GIWAXS experiments. The in situ studies were performed in a gas-tight chamber (Figure 1 and Figure S2) equipped with electrical test leads, a flange-mounted RH sensor, and a glass window to allow for cell illumination. An x-ray flight tube and Kapton window allowed for simultaneous acquisition of GIWAXS data. The humidity was controlled by flushing the chamber with a constant flow (3 SLPM) of nitrogen with the desired humidity; the RH was controlled by a system of water bubblers and mass flow controllers (Figure 1a). The cells were kept under constant illumination, and I-V curves acquired at 1 minute intervals; since the illumination was not an AM1.5G source and the cells were not apertured during measurement, we report only currents (not current densities), and only normalized changes in PCE. Device parameters and J-V curves for representative cells prior to transport to the synchrotron (properly apertured and tested with a Class AAA AM1.5G solar simulator) are shown in Table S1 and Figure S1. GIWAXS patterns were acquired at 3 minute intervals using fast shutters and a 3 s acquisition time; the less frequent sampling and short exposure time was done to minimize beam damage on the sample. We first studied the degradation of a device in a dry (RH = 0-5%) environment, where the chamber was continuously purged with dry nitrogen gas. The initial I-V curve revealed a fully


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functional device (Figure S4a), while the initial GIWAXS pattern (Figure S4b) showed the tetragonal phase of CH3NH3PbI3, as identified by the diagnostic (211) peak at 1.67 Å−1 (this peak is absent in the high temperature cubic phase). I-V curves (Figure S4c) and GIWAXS patterns (Figure S4d) were acquired for > 1.5 h, and the correlated data are shown in Figure S4e. The perovskite film is almost entirely unchanged over the course of the experiment, as evidenced by the lack of change in either the intensity or the full-width-at-half-maximum (FWHM) (Figure S5) of the (110) scattering peak (q = 1.01 Å–1). Additionally, no new phases or decomposition products are observed in the GIWAXS pattern (Figure S4d). This indicates that beam damage is likely to be minimal over the course of these experiments, and that the perovskite films are relatively stable under illumination in a nitrogen environment. There is a slow loss in PCE after ca. 15 min of testing, primarily driven by a drop in Voc, after which the PCE stabilizes somewhat. There are several possible explanations for this “burn-in” period, including the thermallyinduced degradation of either the perovskite22-24 (a noted problem on ZnO substrates25,26) or the hole-transport layer.27 The formation of trap states at the band edges would be expected to narrow the effective band gap and/or increase the recombination velocity, reducing Voc. However, regardless of the exact cause of this small efficiency loss, both the perovskite absorber layer and the device performance were relatively stable over the course of this control experiment. Using a fresh device (whose initial GIWAXS pattern and I-V curve are shown in Figure S7), we then repeated the experiment, only this time in a high humidity environment (RH ≈ 80%). The I-V curves, GIWAXS patterns, normalized device parameters and perovskite (110) peak intensity are all plotted in Figure 2. Clearly, the presence of water vapor accelerates the degradation of the perovskite; there is a ca. 15% drop in the intensity of the (110) scattering


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peak, although again, no new phases or decomposition products are observed (Figure 2b). This may suggest a gradual amorphization of the perovskite layer, accelerated by the presence of water vapor; however, there is no increase in the FWHM of the (110) reflection (Figure S8), and the peak actually narrows slightly (∆FWHM = –0.001 Å–1) over the course of the experiment. This is consistent with previous reports of humidity-induced recrystallization and annealing of perovskite films.28,29 Adsorption of water at the grain boundaries29 may result in grain ripening and an increase in the coherent scattering length (decrease in FWHM), despite a decrease in the overall amount of crystalline material due to the loss of small grains (decrease in intensity). It should be emphasized that all of these changes are quite modest, and any structural changes in the perovskite layer are relatively minor. In contrast, the PCE of the cell drops precipitously during the first 15 minutes of the experiment; after only 30 minutes, the device is effectively non-functional. This rapid loss in performance is primarily due to a loss of fill factor (FF) and open-circuit voltage; these parameters decrease sharply in the first 30 minutes, while the shortcircuit current (Isc) drops more gradually over the course of the entire experiment.


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Figure 2. (a) I-V curves acquired at 1 min intervals (t = 0 h, purple; t = 1.6 h, red) and (b) azimuthally-integrated GIWAXS patterns for a ITO/ZnO/CH3NH3PbI3/P3HT/Ag cell after exposure to a humid (RH ≈ 80%) nitrogen environment. (c) Normalized intensity of the (110) scattering peak and electrical device parameters over the course of the experiment. From the x-ray scattering and I-V data in Figure 2, it is clearly evident that the failure of the perovskite cell is not strongly correlated with the observed perovskite decomposition; however, given the results of the control experiment in dry nitrogen (Figure S4), it is clearly a moisturedriven phenomenon. One explanation for this behavior is electrode corrosion driven by moistureinduced iodide migration. Both Kato et al.30 and Kundu et al.31 have previously observed iodide migration in perovskite solar cells, leading to corrosion of the silver top electrode. The formation


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of a thin layer of AgI at the P3HT/Ag interface would be expected to impede charge transport across the interface and lead to a high surface recombination velocity; this is entirely consistent with the observed drop in both fill factor (the series resistance increases and the shunt resistance decreases) and open-circuit voltage. The fact that this is only observed in the presence of humidity suggests that moisture is first absorbed into the perovskite layer. This helps solvate ions within the film, possibly leading to a more disordered structure in which ion migration is easier. Once these mobile iodide ions reach the silver top contact, electrode corrosion and cell failure is inevitable. In order to test this hypothesis, we replaced the silver contact with a more oxidatively robust gold electrode. Device parameters and calibrated J-V curves for representative cells prior to transport to the synchrotron are shown in Table S2 and Figure S9. Again, our first experiment was to test a representative device in a dry (RH = 0-5%) environment (Figure S11). For this particular cell, we observed a slow loss of intensity in the (110) scattering signal over the 6.5 h experiment (35% drop in intensity). Again, no new phases or byproducts were observed by GIWAXS, and the FWHM of the scattering peak decreased slightly (∆FWHM = –0.002 Å–1) (Figure S12). Whatever the cause of the observed loss in scattering intensity (residual moisture, photochemical changes, thermal degradation, or x-ray beam damage), it has only a moderate effect on device performance; the loss in fill factor and short-circuit current appear to be correlated to the degradation of the perovskite, while the open-circuit voltage decays more slowly over the course of the experiment. This is consistent with previous reports on the robust electronic nature of lead halide perovskites.32 Lattice fluctuations and disorder (whether dynamic or static) lead to reduced scattering lifetimes, but low trap state densities and shallow trap state


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energies33,34 can nonetheless lead to long charge carrier lifetimes, even in imperfect or degraded materials. We then repeated the experiment in a humid (RH ≈ 85%) environment. The initial I-V curve and GIWAXS pattern of the cell are shown in Figure S14; the remaining data are shown in Figure 3. The results are a stark contrast to those observed with the silver electrode (Figure 2). The intensity of the (110) reflection actually increases over the course of the experiment, and the FWHM decreases markedly (∆FWHM = –0.007 Å–1) (Figure S15), indicating a substantial increase in the coherent scattering length. Clearly, recrystallization and annealing of the perovskite can occur in the presence of moisture.28,29 This annealing is also consistent with our hypothesis of moisture-induced ion migration, as it is much more pronounced at higher RH. The electrical data also suggest a substantially reduced level of electrode corrosion; the Voc decreases rapidly in the first 30 minutes, but the magnitude of the Voc loss is small, and the Voc deteriorates much more slowly afterward. The lack of corrosion observed with the gold electrode supports our earlier conclusion that iodide migration was responsible for the failure of the silver-based devices. In this case, cell failure appears to proceed by a very different mechanism. The initial I-V curve of the cell (Figure S14a) is entirely normal in appearance; however, as the device is exposed to the humid environment, an abnormal “bump” develops near the maximum power point (ca. 0.7 V) (Figure 3a), where the current is substantially higher than at Isc. This feature is known to occur in cells that display substantial hysteresis in their I-V curves.35 It results from charge carrier collection efficiencies that vary as a function of applied voltage, a result of slow ion migration within the perovskite layer. This clearly indicates that the degree of hysteresis increases over the course of the experiment. Since the degree of hysteresis is closely linked to the mobility of ions within the perovskite layer,35,36 the data fully supports our hypothesis that ions are becoming


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more mobile as the perovskite layer absorbs moisture from the high humidity environment. Further evidence of the effect of humidity on ion migration comes from a comparison of the RH and the fill factor over the course of the experiment (Figure S16). The net result of the abnormal I-V curve is an artificially inflated fill factor; from Figure S16, it can be observed that the degree of inflation in the fill factor is tightly correlated with the humidity measured inside the chamber. The net result of this humidity-induced ion migration is a reduction in the charge carrier collection efficiency and a rapid drop in the photocurrent, which leads to a loss in cell performance. These results are entirely consistent with previous work on moisture-uptake in perovskite thin films, which show moisture penetrates into the body of crystalline grains at RH > 70%.29 Although the cell failure mechanism is different than in the case of the silver top electrode, the root cause – moisture-induced ion migration in the perovskite layer – is entirely the same.


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Figure 3. (a) I-V curves acquired at 1 min intervals (t = 0 h, purple; t = 6 h, red) and (b) azimuthally-integrated GIWAXS patterns for a FTO/ZnO/CH3NH3PbI3/P3HT/Au cell after exposure to a humid (RH ≈ 85%) nitrogen environment. (c) Normalized intensity of the (110) scattering peak and electrical device parameters over the course of the experiment. One question that remained unanswered throughout these experiments was the lack of both hydrate phases and decomposition products in the GIWAXS patterns. Previous in situ GIWAXS studies have clearly shown the formation of the monohydrate phase, followed by the appearance of crystalline lead iodide, on the timescale of minutes to hours;8 no such intermediates or byproducts were observed in any of the in situ experiments. We therefore exposed another fresh ITO/ZnO/CH3NH3PbI3/P3HT/Ag device to a RH ≈ 80% atmosphere, but this time we did not


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illuminate it or apply any bias (Figure 4). Under these conditions, we observed the expected formation of the monohydrate phase at q = 0.61 Å−1, followed by the appearance of PbI2 at q = 0.94 Å−1 shortly thereafter, with a concomitant loss in intensity of the CH3NH3PbI3-based scattering peaks. We then switched on the halogen light source used for the preceding in situ experiments. We observed a near-instantaneous loss of both the monohydrate and PbI2 peaks, along with a regeneration of the perovskite (110) signal. Clearly, there is a process (either photochemical or thermal) by which the humidity-induced decomposition can be reversed. Although several previous studies have noted the reversibility of monohydrate formation,7-9,29 in this case the partial pressure of water vapor in the atmosphere remains unchanged, yet the monohydrate reverts back to the perovskite structure. We then switched off the light and left the device to decompose further in the dark; as expected, the monohydrate peak at q = 0.61 Å−1 reappeared, along with additional PbI2. We then illuminated the device with an LED-based light source of roughly the same intensity. Crucially, the LED source lacks the intense near-infrared output of the halogen-based source (Figure S17), limiting sample heating and allowing photochemical and thermal effects to be probed separately (the halogen lamp increased the sample temperature to ≈ 40 °C, whereas no temperature increase was observed with the LED source). The LED source had no appreciable impact on the GIWAXS pattern, and the film continued to decompose under illumination. The results show that photothermal annealing of the perovskite layer effectively suppresses the formation of hydrate phases and crystalline decomposition products, suggesting a possible diurnal variation in the degradation pathway of field-deployed perovskite solar cells.


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Figure 4. GIWAXS pattern of an ITO/ZnO/CH3NH3PbI3/P3HT/Ag device exposed to a nitrogen atmosphere with RH ≈ 80%. The device was left in the dark and illuminated for brief periods with either a halogen or LED light source, as noted in the figure. While illuminated, the x-ray exposures were limited to 0.5 s and the scattering intensities were corrected to compensate. These results provide important insight into the degradation mechanisms of perovskite solar cells, and help explain how changes to the perovskite absorber layer affect device performance. Despite the moisture-sensitivity of the CH3NH3PbI3 absorber, it is not decomposition of the perovskite layer that leads to cell failure in humid environments. Rather, moisture ingress leads to an increase in the ion mobility within the film; this in turn leads to either electrode corrosion (for silver electrodes) or a loss of photocurrent due to a screening of the built-in potential (for gold electrodes). Additionally, the experiments reveal a temperature dependence of the perovskite decomposition pathway. At room temperature, the CH3NH3PbI3 layer decomposes by the monohydrate-to-PbI2 pathway previously described in the literature;7-9 however, when illuminated by a near-infrared-emitting source, the amount of crystalline material in the perovskite layer decreases more slowly, without the appearance of further decomposition


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products. The results show the importance of environmentally-controlled and in situ experiments in elucidating the failure mechanisms of perovskite solar cells.

ASSOCIATED CONTENT Supporting Information. Experimental details, calibrated device performance parameters and J-V curves, additional I-V curves and GIWAXS patterns, output spectra for the halogen and LED light sources. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The Natural Sciences and Engineering Research Council of Canada (NSERC, RGPIN-201703732) and the University of Saskatchewan are acknowledged for financial support. T.L.K. is a Canada Research Chair in Photovoltaics. The research was undertaken, in part, thanks to funding from the Canada Research Chair program. Research described in this paper was performed at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, NSERC, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research. Technical support from HXMA beamline scientist Dr. Chang-Yong Kim is


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gratefully acknowledged. Dr. Riccardo Comin, Ms. Xiwen Gong and Prof. Edward Sargent are acknowledged for helpful discussions regarding the design of the in situ chamber and the likely degree of perovskite beam damage.

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