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The Role of Water in the Reversible Optoelectronic Degradation in Hybrid Perovskites at Low Pressure Genevieve Noelle Hall, Michael Elias Stuckelberger, Tara Nietzold, Jessi Hartman, Ji-Sang Park, Jérémie Werner, Bjoern Niesen, Marvin L. Cummings, Volker Rose, Christophe Ballif, Maria K. Y. Chan, David P Fenning, and Mariana I. Bertoni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06402 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on October 31, 2017

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The Journal of Physical Chemistry

The Role of Water in the Reversible Optoelectronic Degradation in Hybrid Perovskites at Low Pressure Genevieve N. Hall,†,‡ Michael Stuckelberger,† Tara Nietzold,† Jessi Hartman,¶ Ji-Sang Park,¶ J´ er´ emie Werner,§ Bjoern Niesen,§ Marvin L. Cummings,¶,k Volker Rose,¶,k Christophe Ballif,§ Maria K. Chan,¶,⊥ David P. Fenning,# and Mariana I. Bertoni∗,† School of Electrical, Computer, and Energy Engineering, Arizona State University, Tempe, AZ 85287, United States, School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, United States, Center for Nanoscale Materials, Argonne National Laboratory, ´ IL 60439, United States, Institute of Microengineering, Ecole Polytechnique F´ ed´ erale de Lausanne, CH-2000 Neuchˆ atel, Switzerland, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, United States, Computation Institute, University of Chicago, Chicago, IL 60637, United States, and Department of Nanoengineering, University of California San Diego, La Jolla, CA 92093, United States E-mail: [email protected]

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To whom correspondence should be addressed ASU ‡ School of Molecular Sciences, Arizona State University, Tempe, AZ 85287, United States ¶ ANL § EPFL k APS ⊥ UC # UCSD †

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Abstract There is no doubt about the potential offered by the low-cost chemistry and high efficiency associated with hybrid organic-inorganic perovskite solar cells. However, the service lifetimes of these devices have to be increased from months to years to capitalize on their potential. The archetypal hybrid perovskite for solar cells, methylammonium lead iodide (MAPI), readily degrades in ambient atmosphere under standard operating conditions. Understanding the origin and effects of this degradation can pave the way to better engineer the solar devices and the perovskite material itself. Herein we present the effects of atmospheric pressure on the electrical performance of MAPI solar cells. Solar cell parameters, especially open circuit voltage, are significantly affected by the total ambient pressure and present an unexpected reversible behavior upon pressure cycling. We complement photoluminescence studies as a function of ambient atmosphere and temperature with first-principles DFT calculations. The results suggest that the reversible intercalation of water in the MAPI unit cell is a necessary component underlying this behavior.

Introduction Perovskite solar cells (PSCs) are poised to facilitate lower electricity prices than other photovoltaic technologies because of their low energetic production costs, 1 accessible and abundant material constituents, 2,3 efficiencies on par with those of multicrystalline silicon and cadmium telluride cells 4 and potential for integration in tandem configurations. 5 However, the short service lifetime of PSCs on the order of months, for best in class, 6–8 hampers the broad adoption of this technology. Synthesizing a long lasting perovskite absorber requires in-depth understanding of the various chemical factors that affect degradation and the governing mechanisms that drive it. To date we know that ambient atmosphere impacts the degradation rate of PSCs made with the archetypal perovskite absorber, methylammonium lead iodide (CH3 NH3 PbI3 or 2


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MAPI). Water has been proposed by multiple authors to interfere with hydrogen bonds formed between halide ions and hydrogen atoms in the CH3 NH3 + group. 9,10 Oxygen has been reported to affect the electronic properties of MAPI 11 as well, and, under high laser power, bond to the lead ions. 12 Heat, illumination, and applied bias voltage can also affect PSC performance. 6,13–15 Furthermore, many PSC structures contain spiro-MeOTAD as the hole transport layer (HTL), the electrical properties of which are also known to vary with atmospheric exposure 16 and illumination. 17 It is well established that long-term exposure to humidity (hours if high humidity, weeks if low) degrades PSC performance. Whether water vapor improves or degrades the optoelectronic properties of MAPI appears dependent on the amount and duration of the exposure. 18 Recent work showed that water vapor enters the CH3 NH3 PbI3 crystal structure at room temperature under relative humidities of 10%, transforming it into a monohydrate (CH3 NH3 PbI3 ·H2 O) and then a dihydrate (CH3 NH3 PbI3 ·2H2 O). 9,11 Modeling indicates that hydration leads to the formation of PbI2 as complexing water and/or hydroxyl species loosen the lattice binding of methylamine, which evaporates. 10,19 Lead iodide (PbI2 ) is frequently observed in degraded cells. On the other hand, some groups report that humidity exposure during or shortly after MAPI film fabrication enhances cell performance. For example, Petrus et al. exposed perovskite layers to 75% relative humidity for a variety of time periods on the order of hours. 20 On completing device fabrication with hole transport layer and gold contact, they found that long exposure to a humid atmosphere consistently improved both cell efficiency and open circuit voltage. The authors suggest that pseudo-solvation of MAPI constituents in the monohydrate allows small crystals to reorganize into larger ones. 20 Zhou et al. performed similar exposure tests and showed that the open circuit voltage (VOC ) of cells exposed to water during fabrication was substantially higher than that of unexposed control cells. 21 This VOC enhancement was only slightly reduced after they attempted to dry the MAPI films under vacuum. 21 The PSC research community increasingly acknowledges that humidity affects methylammonium lead iodide films and cells on a time scale of seconds to minutes. 11,18,22 Detailed atmospheric

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studies are not only relevant to the long-term deployment of PSCs, but also to the accurate interpretation of laboratory results. Here, perovskite solar cell performance is found to change upon exposure to vacuum, a common experimental environment. On a time scale of seconds, VOC hysteretically decreased with decreasing ambient pressure (down to 10−5 Torr) and increased with increasing pressure (up to 760 Torr). To the best of our knowledge, this is the first time this effect has been reported. Herein we describe the effect of low operating pressure on solar cell performance and propose that exposure to vacuum effectively dehydrates the material, reducing the unit cell volume and concomitantly the band gap and VOC of functioning solar cells. This hypothesis was evaluated by experimentally studying the band gap changes under different humidity levels and corroborated by first-principles density functional theory (DFT) calculations.

Experimental Perovskite solar cells were fabricated with the following layer stack: glass/ITO/c-TiO2 /mTiO2 /CH3 NH3 PbI3 /spiro-MeOTAD/Au previously reported by Werner et al.. 23 The perovskite absorber was prepared under a nitrogen atmosphere from a 1.2 M solution of PbI2 :methyl ammonium iodide (1:1). To investigate the effect of total pressure on PSC performance, a PSC was placed inside a vacuum chamber and illuminated by a Schott KL 2500 halogen lamp (intensity equivalent to 0.01 suns, as calculated from PSC current) through a window port. The PSC was electrically connected with a feedthrough to a CH-Instruments potentiostat in two-terminal operation. The VOC was monitored over time as the pressure in the chamber was decreased from atmospheric pressure (760 Torr, ∼30% relative humidity, ∼20◦ C) to ∼10−5 Torr and then increased back to 760 Torr. Following standard pumping down protocols, a roughing and turbo pump were used sequentially. Venting was performed through a needle valve close to the sample loading port. Pressure - voltage - time data were continually recorded and full J-V curves (0.1 V/s, scanned from high to low voltage) were

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obtained at atmospheric pressure, at the lowest pressure achieved, and once the chamber returned to atmospheric pressure. To identify effects contributing to the changes seen upon pressure cycling, further experiments evaluated the effect of oxygen or humidity pressure, light exposure, and temperature on voltage. First, to study the effect of oxygen or humidity on PSC performance, a cell was placed in a Linkam HFS600E probe stage inside a Newport J-V tester equipped with a solar simulator (AM 1.5g Class A). Nitrogen gas (99.999%) was flowed over the PSC and J-V curves (light and dark, 10 V/s, scanned in both directions using a Keithley 2440 sourcemeter in four terminal operation) were collected every 30 seconds for 30 minutes. The PSC was left in the dark except when collecting light J-V curves. Second, to investigate the effect of increasing light exposure, the PSC was stabilized under nitrogen in the Linkam stage for 1.5 hours and then exposed to light for various lengths of time (1 s to 21 min.). Light and dark J-V curves were collected (10 V/s, scanned in both directions) after each light exposure. Finally, the effect of temperature change on performance was evaluated by photoluminescence (PL). A PSC was placed inside the temperature-controlled Linkam stage and the atmosphere inside the stage was controlled to be either dry or humid nitrogen gas. A dry nitrogen atmosphere was achieved by flowing 99.999% nitrogen gas through a Drierite column. Humid nitrogen was obtained by bubbling the nitrogen gas through water before entering the stage. PL measurements were performed with a 532 nm laser in a Renishaw Confocal Invia spectrometer. Laser power (initially 40 W/cm2 ) was decreased with decreasing temperature to avoid saturation of the silicon detector since the intensity of the PL signal increased with decreasing temperature.

We performed first-principles density functional theory (DFT) calculations to investigate the effects of water incorporation into MAPI on its physical properties. We used the generalized gradient approximation (GGA) 24 and projector-augmented wave (PAW) potentials, 25 as implemented in the Vienna Ab-initio Simulation Package (VASP) code. 26 The wave func-

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tions were expanded in plane waves up to a cutoff of 500 eV. The internal atomic positions were fully relaxed until the residual forces on the atoms were smaller than 0.03 eV/Å, and a 3 x 3 x 3 k-point grid was used for Brillouin zone integration. The Pb d -states were treated explicitly as valence states. Since we employed a pseudo-cubic cell, the cubic root of the unit cell volume was calculated as the lattice constant. We also examined three different Van der Waals corrections, namely Tkatchenko-Scheffler (TS), 27 DFT-D2, 28 and optPBE. 29 To find the stable positions of a water molecule in the unit cell, we obtained the energy surface of an H2 O molecule in (100) plane containing I atoms and a plane containing N atoms. These DFT calculations involve the insertion of water molecule(s) into the interstitial space(s) in bulk MAPI unit- and super-cells.

Results and Discussion Figure 1 shows the cyclic changes in open circuit voltage with pressure. As pressure was cycled between atmospheric and 10−5 Torr we observed a decrease in the VOC of the PSC with decreasing pressure (Fig. 1(a), blue markers) and an increase in VOC with increasing pressure (Fig. 1(a), orange markers). The same behavior was observed for multiple samples and throughout multiple pumpdowns and vents. The observation that the effect of pressure is reversible and repeatable suggests that the mechanism causing the VOC change is not permanently altering the PSC structure. The step-like features of Figure 1 (a) between 7600.1 Torr and 0.1-10−5 Torr during pumpdown and vent, respectively, are correlated with the pressure-time relationship shown in Fig. 1(b). The irregularities originate from the different pumping rates of the roughing and turbo pumps, indicating some impact of time-dependent kinetics. Figure 1(c) presents the J-V curves collected before and after pumpdown and after vent. The VOC clearly decreases at low pressure and returns to its original value at ambient pressure. All J-V curves presented here were fit to a double-diode model with the first ideality factor (n1 ) kept equal to 1. Note that a one-diode model could not adequately

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describe all J-V curves. With decreasing pressure, the series resistance (Rs ), the second ideality factor (n2 ), and the dark currents for the first and second diodes (J01 and J02 ) increase, which is consistent with the changes in VOC (for the full set of solar cell parameters see Table S1). Concomitant with the VOC changes, the fill factor (FF ) is lower at low pressure (∼ 71%) and higher at high pressure (∼ 74%). Although we find that increasing duration of light exposure produces a small decrease in VOC the step-like features following the pump rates suggest that the major dependency is with pressure, Fig. 1(b). The VOC increase between the end of pumpdown and the beginning of the vent seen in Fig. 1(a), on the other hand, can be attributed to VOC recovery under dark conditions between measurements. The pressure-related increase in Rs and decrease in FF with decreasing pressure can be associated with an effect on the absorber itself or changes at the interfaces of the device. To deconvolute the effect of total pressure change from the effect of partial pressure change of the various gases in the air, we flowed nitrogen over the cell for 30 minutes and monitored the J-V characteristics. During this study we kept the total pressure constant while increasing the partial pressure of nitrogen and decreasing the partial pressure of all other constituent gases (O2 , H2 O, etc.). Figure 2 presents the resulting VOC decrease with increasing duration of nitrogen exposure. Note that the magnitude of this change - roughly 20 mV - is the same as that of the change produced by varying pressure. Based on these observations we can conclude that the partial pressures of specific gases affect PSC performance, and that total pressure is not the sole cause of the VOC change. To assess any effect on the VOC by heating due to illumination of the cell, we performed photoluminescence (PL) measurements at various temperatures under 1 atm of nitrogen. We found that the PL peak position presents a blue shift with increasing temperature. The slope is positive: +242 ± 2 µeV/K for T > 175K (Fig. S1), which is in good agreement with recent reports. 30–32 Rather than decreasing with increasing temperature as in most inorganic semiconductors, 33 the MAPI band gap increases with increasing temperature. This means that if the cell under vacuum is heated by illumination without the benefit of convective

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Figure 1: (a) Open circuit voltage changes versus pressure during pumpdown, decreasing pressure, (blue markers) and vent, increasing pressure, (orange markers). Solid markers depict respective time stamps for reference. (b) Solid lines depict pressure as a function of time and dashed lines show VOC vs. time for the same pumpdown (blue) and vent (orange) from (a). (c) Solar cell current-voltage response before and after pumpdown and vent.

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Figure 2: Open circuit voltage versus time, collected from full J-V curves acquired every 30 seconds over 30 minutes under nitrogen atmosphere at ambient pressure (760 Torr). cooling, an increase in band gap and hence, by first approximation, in VOC would be expected with decreasing pressure. In contrast, we observe a VOC decrease at low pressure (Fig. 1). Furthermore, on the basis of the heat capacity of the cell and the light intensity in the pressure experiment, we estimate that the cell could at most heat ∼3 K. Thus, temperature effects cannot explain the VOC decrease seen under vacuum. We hypothesize that the pressure-dependence of VOC is related to water deintercalation from partially hydrated MAPI. The hypothesis that variation in the water content of the MAPI alters the band gap to produce the observed VOC variations (Fig. 1) is supported by PL data as a function of relative humidity (Fig. 3). In Fig. 3 we present the PL peak position of a PSC held at 10◦ C while being exposed to a nitrogen gas stream cycled from dry to humid to dry conditions twice. The temperature of the nitrogen was adjusted and bubbled through water before entering the sample stage, enabling us to alter the total water content of the gas stream. When exposed to gaseous H2 O after 10 min. in dry N2 , there is a sudden decrease in band gap after which the band gap of the MAPI seems to increase slightly as a function of exposure time. This increase is partially reversible upon drying (20-30 min., Fig. 3). When water is allowed to condense on the PSC from a supersaturated N2 stream (30-40 min.), we observe a dramatic increase in band gap that is also partially 9



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reversible (40-50 min.). We attribute the delay in the band gap recovery (five to eight minutes after the atmosphere is changed back to dry nitrogen, minutes 45-48 in Fig. 3) to evaporation of condensed water from the surface of the cell. The rapid decrease in band gap likely indicates deintercalation of water from the MAPI. The sudden recovery of the band gap serves as evidence that the presence of water reversibly increases the band gap of MAPI. Similar sensitivity of PL to environmental gases and a considerable reduction of PL intensity under vacuum on MAPI single crystals was reported by Fang et al., 34 who drew a correlation between ambient gas molecules and surface recombination velocity. Here, we see a shift in the band gap between a dry N2 environment and a water-saturated ambient of 100 meV, on the order of the VOC decrease seen under vacuum. The mechanism may be dominated by the kinetics of a diffusion-like process, where the magnitude of the external forces (e.g., water concentration, pressure differential) drive the speed at which water exchanges through the thin film and device layers.

Figure 3: Photoluminescence peak wavelength of PSC on 10 ◦ C stage under flow of dry nitrogen. Data were collected at one minute intervals. Intensity values are provided in Fig. S2. We performed first-principle density functional theory (DFT) calculations to determine the effect of water on band gap and therefore VOC . We first performed sampling to find the 10


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parameter. The net effect is therefore a slight decrease in lattice parameter and hence band gap and VOC . To further evaluate this hypothesis, we performed DFT calculations of the thermodynamic favorability for MAPI to incorporate or release water molecules. To examine the stability of the water molecules in MAPI, we calculated the binding energy of water molecules as follows: (Etot [(MAPI)x (H2 O)y - Etot [(MAPI)x ] - Etot [(H2 O)y ])/y where x and y are the number of MAPI formula units and H2 O molecules in the supercell, respectively, and Etot is the total energy of the supercell. As shown in Fig. 5(c), our DFT calculations indicate that water molecules are stable in MAPI, regardless of whether any van der Waals correction is used. The configurational entropy of water in bulk MAPI is expected to be smaller than that in gas phase, so the Gibbs free energies of water incorporation should be of smaller magnitudes than the binding energies. During pumpdown, the free energy of a water molecule in gas phase decreases because of the dependence of free energy (G) on pressure (p), ∆G(p) = kB T [ln(p/p0 )]. For p and p0 of 760 vs. 10−5 Torr, the change in free energy is ∼0.5 eV, which is comparable to the binding energies. Thus we expect that, thermodynamically, water should be removed at least partially from MAPI during pumpdown (when pressure decreases) and reincorporated during vent (when pressure returns to atmospheric), which results in a reversible change of the band gap. The hole transport layer of these cells, spiro-MeOTAD, may further contribute to their atmospheric sensitivity: water and oxygen interact directly with spiro-MeOTAD on a molecular level. Ono et al. established that oxygen and water molecules are incorporated into spiro-MeOTAD upon exposure to air. 36 Recent work indicates that exposure to water permanently enhances the conductivity of spiro-MeOTAD, while exposure to oxygen reversibly enhances it. 16 However, the mere increase of the conductivity of the hole transport layer does not directly predict an improved open circuit voltage, in fact, the recombination at the interface plays a crucial role. 34 Pressure-dependent experiments with hole-transport layer free MAPI devices showed a reversible shift in photovoltaic properties (not shown), similar to those shown here for the spiro-MeOTAD containing devices.

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The data presented here indicate a significant effect of vacuum conditions on VOC , which we interpret based on PL and DFT studies to be a result of changing water content in the MAPI that alters the band gap. Awareness of the changing material and functional properties of perovskite devices during characterization under vacuum conditions is critical for assessing characterization results, e.g. band edge determinations by ultraviolet or Xray photoelectron spectroscopy or chemical analysis by electron dispersive spectroscopy. An important point to acknowledge is that the short circuit current density of our devices under study decreases slowly over time, regardless of pressure change. This seems to indicate that multiple degradation processes - both permanent and reversible - are at work.

Conclusions We observe that the VOC of perovskite solar cells decreases with decreasing pressure and shows a reversible behavior upon increasing pressure. A change in total pressure alone or heating of the cell under vacuum cannot explain the pressure dependence of VOC observed. Photoluminescence measurements in varying relative humidity indicate a stark blue shift when water is present, indicating a change in the material that leads to a larger band gap. DFT calculations provide evidence that partial water hydration of the film is associated with an increase in band gap due to increased Pb-I bond distance. The calculations also demonstrate that water release at high vacuum is thermodynamically favorable. Our observations suggest that the solar cell parameter changes are due to changes in water partial pressure in the ambient. Given this sensitivity and reversible optoelectronic response, PSC-like devices could act as air pressure or humidity sensors. Previous work on the effect of vacuum on PSCs reported that vacuum irreversibly degraded MAPI to PbI2 . We do not observe such irreversible degradation in several hours of experiment under medium to high vacuum. The effects seen here make clear the need to carefully report and assess the effects of ambient conditions during PSC characterization for fundamental material study not just when analyzing device characteristics. This work emphasizes the extreme atmospheric sensitivity of 14


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MAPI-based PSCs and the elasticity of the unit cell to exchange water with the environment.

Acknowledgement Use of the Center for Nanoscale Materials and the Advanced Photon Source, both Office of Science user facilities, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This material is based upon work supported in part by the National Science Foundation (NSF) and the Department of Energy (DOE) under NSF CA No. EEC-1041895. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect those of NSF or DOE. The project comprising this work is evaluated by the Swiss National Science Foundation and funded by Nano-Tera.ch with Swiss Confederation financing and by the Swiss Federal Office of Energy, under Grant SI/50107201. G.N. Hall was supported by an IGERT-SUN NSF fellowship (Award 1144616). D. P. Fenning acknowledges the support of UCSD start up funds.

Supporting Information Available The following files are available free of charge. • Table S1: JV curve parameters from double diode fit, referenced in Figure 1c • Figure S1: Change in photoluminescence peak position with temperature. • Figure S2: Photoluminescence peak intensity of PSC under the conditions described in Fig. 3 This material is available free of charge via the Internet at http://pubs.acs.org/.

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