Effect of Environmental Humidity in the Electrical Properties of Lead

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C: Energy Conversion and Storage; Energy and Charge Transport

Effect of Environmental Humidity in the Electrical Properties of Lead Halide Perovskites Alberto García-Fernández, Zahra Moradi, Juan Manuel Bermúdez-García, Manuel Sánchez-Andújar, Valero Alfonso Gimeno, Socorro Castro-García, Maria Antonia SeñarisRodriguez, Elena Mas-Marzá, Germà Garcia-Belmonte, and Francisco Fabregat-Santiago J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03915 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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The Journal of Physical Chemistry

Effect of Environmental Humidity in the Electrical Properties of Lead Halide Perovskites. Alberto García-Fernández1*, Zahra Moradi2,3, Juan Manuel Bermúdez-García1, Manuel Sánchez-Andújar1, Valero A. Gimeno3, Socorro Castro-García1, María Antonia SeñarísRodríguez1, Elena Mas-Marzá3, Germà Garcia-Belmonte3, Francisco FabregatSantiago3*

1 Faculty

of Science and Advanced Scientific Research Center (CICA), Universidade da Coruna, As Carballeiras, s/n - Campus de Elviña, 15071 A Coruna, Spain 2

Physics Department, Faculty of Science, University of Isfahan, Av. Hezar Jarib 81746-73441 Isfahan, Iran 3

Institute of Advanced Materials, Universitat Jaume I, Avda. V. Sos Baynat, s/n, 12006 Castelló, Spain

e-mail: [email protected], [email protected]

1 ACS Paragon Plus Environment

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Abstract In large quantities, water is detrimental to lead halide perovskite solar cells, mainly because of the decomposition of the perovskite layer. On the contrary, the presence of small quantities of water has been observed to play a key role in the crystallization of the perovskite and in the performance of the corresponding devices. However, it is still under debate the exact role of water during the operation of perovskite solar cells. In this paper, impedance spectroscopy is used to analyze the changes produced by environmental humidity in the electronic properties of methylammonium lead triiodide perovskite. Our results show that water absorbed from environmental humidity induces a huge increase in the capacitance of this material. This capacitance can reach values as large as the accumulation capacitance found in devices based in perovskite, which is the responsible of the characteristic large hysteresis observed between forward and reverse J-V curves. In parallel to this outstanding rise of the capacitance, water absorption produces a significant rise of the conductivity, in agreement with previous reports in the literature. Activation energy of 0.52 eV is found for electronic transport, a value in line with activation energy of ionic transport found in literature, what suggests ambipolar diffusion as the transport mechanism that links these two phenomena.


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1. Introduction The solar energy conversion efficiency for devices based on lead halides with perovskite structure has already reached performance values of 23.3%.1 These values surpass dye-sensitized, quantum dot, organic, amorphous silicon and other emerging photovoltaic technologies, thus making lead halide perovskites a very promising option for low-cost and high efficiency photovoltaic applications. Their unprecedented optoelectronic properties,2 which include high fluorescence yields and easy color tunability, make them also very interesting materials in applications for light-generation by electroluminescence (EL), such as light-emitting diodes and lasers. Despite the spectacular advances in solar cell efficiency, in lighting applications and in the understanding of the operation mechanisms of lead halide perovskites, there are still some questions which need to be unveiled, including the interaction of perovskite with environmental moisture. It is well-known that environmental humidity affects the commonly used methylammonium lead triiodide (MAPbI3) perovskite during its preparation (morphology, crystal quality, etc.) as well as once it is formed.3-7 Thus, when MAPbI3 is exposed to low humidity conditions at room temperature, water may easily penetrate into the perovskite along grain boundaries.8 Then new mono and di-hydrated phases, MAPbI3·H2O and MA4PbI6·2H2O, are formed, which can be dehydrated back to MAPbI3 by raising the temperature.8-11 Very interestingly, DFT calculations have pointed out that water adsorption into perovskite surface is heavily influenced by the orientation of the methylammonium cations close to the surface.12-13 Additionally, Rappe et al.14 have demonstrated that, depending on the methylammonium orientation in the structure, the water molecules can infiltrate into hollow sites of the surface and



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get trapped. Controlling dipole orientation via polling or interfacial engineering could thus enhance its moisture stability. With respect to electrical properties, films under humid environments have been described to increase their conductivity several orders of magnitude with respect to values obtained in dryer conditions.6,

11, 15

This increase in conductivity has been

attributed to the interfacial hydrated perovskite species formed,6 but may also have some contributions

from partial dissolution of ions in perovskite induced by water

absorption.11 On the other side, capacitive response of MAPbI3 upon water absorption remains still unclear. Even though different mechanisms have already been investigated in the literature, providing some hints on the origin of the capacitive response of the perovskite,16-18 some aspects are still unsettled mainly due to the complex morphology of perovskite solar cells and to the combination of its ionic and electronic properties. In this scenario, there are different mechanisms contributing to the capacitive response of these perovskites, which may be both intrinsic (from the bulk material: dipolar, ionic and electronic polarization effects) or extrinsic (interfacial and space charge polarization arising from the presence of a certain electronic and/or ionic conductivity, etc.).19-21 Former studies of the dielectric constant of bulk and single crystal of MAPbI3 showed a sharp change at T 162 K, associated with a phase transition from orthorhombic to tetragonal symmetry.22 Thus the value of the relative dielectric constant was found to be ~ 10 below 162 K, rising to about 20-30 at room temperature.16, 23 In contrast, devices based in MAPbI3 thin-films display very large capacitances at room temperature and low frequencies, which are amplified under illumination.24 These huge capacitances yield to the large hysteresis observed between forward and reverse J-V curves in many perovskite solar cell configurations.25-28


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The goal of this work is to elucidate the influence of the water molecules on the electric response of MAPbI3. For this purpose, we have prepared polycrystalline pellets and thin-films of MAPbI3 (see Figure 1) and exposed them to different environmental humidity conditions at room temperature. Our results show that after water absorption, MAPbI3 experiences compelling changes on its conductivity and capacitance. By the first time, we observe that the environmental moisture induces huge values of MAPbI3 capacitance, which are comparable to those initially attributed in the literature to colossal dielectric constants.16, 28-29 Recent studies have ascribed these extraordinarily high values of capacitance occurring in the perovskite to a charge accumulation process in the surface rather than a dielectric bulk effect.30 Additionally, our results suggest that the large dispersion found for electronic parameters such as mobility or carrier concentration may be related to the changes in conductivity associated to humidity absorption.

Figure 1: Configurations for electrical measurements: (a) polycrystalline pellet (b) coplanar flat thin-film.



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2. Experimental Synthesis. a) Synthesis of polycrystalline MAPbI3 All starting reagents, CH3NH2 solution (40 wt. % in H2O), PbI2 (99%) and HI solution (57 wt. % in H2O) were of analytical grade from Sigma-Aldrich and used without further treatment. Polycrystalline MAPbI3 powder was prepared at room temperature by means of a mechanosynthesis method. For this purpose, equimolar amounts of PbI2 and CH3NH3I were placed in an agate mortar and ground carefully for 15 minutes with a pestle until a visually homogenous black powder was obtained. This powder was used to obtain different pellets by coldpressing at 5 tons with an area of 1.33 cm2 and a thickness of 0.5 mm. Top and bottom surfaces were sputtered with gold to ensure a good electric contact as shown in Figure 1(a). The pellets were heated up until 150ºC along 3 hours to remove moisture (dry sample) and measured. After these measurements, the pellets were kept under environmental humidity of ~40% for 24h (wet sample), measured and dried again (re-dry) in the same conditions. b) Synthesis of MAPbI3 thin-films Substrate preparation. Fluorine doped tin oxide (FTO) coated glass substrates (25 x 25 mm2, Pilkington TEC15, ∼15/sq resistance) were scribed with a CO2 laser, producing a 90 m wide, 57.1 mm long line (see Figure S1 in Supporting Information section). The substrates were cleaned with soap (Extran® MA 02 neutral) and sonicated for 15 minutes in a solution of water and soap (Extran® MA 02 neutral). Then, the


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sheets were rinsed with MilliQ water and sonicated for 15 minutes in ethanol (95%). Finally, the substrates were rinsed with acetone and dried with compressed air. Perovskite deposition. A perovskite flat thin-film layer was deposited by spincoating 50 µL of a solution 1.35 M of CH3NH3I (Dyesol) and PbI2 (TCI) in a mixture of N,N-dimethylformamide (DMF) and dimethylsulfoxide (DMSO) 9:1 at 4000 rpm during 50s with an acceleration of 4000 rpm/s. During the spincoating procedure, diethyl ether was added to the substrate. Then, the film was heated at 100ºC for 3 min. The complete procedure of coplanar thin-film preparation is summarized in Figure S1 of Supporting Information (SI). A sketch of final configuration is plotted in Figure 1b. Scanning electron microscopic (SEM) pictures of the film (cross section and top view) are shown in Figure S2 of SI. To ease electrical contact, perovskite far from the scribed line was removed mechanically. Flat films kept under humidity conditions of 40% are called “wet” samples. To avoid film degradation, drying process was made keeping the films at 60ºC under dry conditions (less than 0.01% moisture) and labelled “dry”. After dry measurements some films were allow absorbing moisture from ambient with 40% relative humidity during at least 12 hours and labeled “wet after dry”. Wet thinfilms were measured in humid environment, dry ones in dry air. Thermal analysis. Thermogravimetric analyses (TGA) were carried out in a TGA-DTA Thermal Analysis SDT2960 equipment. For these experiments approximately 27 mg of perovskite polycrystalline powder were heated at a rate of 5 K/min from 300 K to 400 K using corundum crucibles under a flow of dry nitrogen.



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Powder X-ray diffraction. XRD patterns from the obtained powder and films were collected in a Siemens D-5000 diffractometer and a Bruker AXS-D4 Endeavor Advance X-ray diffractometer (respectively), using CuK radiation at room temperature. Scanning electron microscopy analysis.

The morphology and structural

properties of the films were analyzed using a JEOL 7001F scanning electron microscope with a film emission gun (SEMFEG). Dielectric properties and impedance spectroscopy (IS). The capacitance and impedance spectroscopy measurements of pelletized samples were performed as a function of frequency and temperature with a Solartron 1260A Impedance/Gain-Phase Analyzer. IS measurements were realized in the frequency range from 1 Hz up to 1 MHz using an amplitude of 0.5 V. Pelletized samples were fit into a parallel-plate sample holder placed in a Janis SVT200T cryostat cooled with liquid nitrogen. Temperature was fixed using a Lakeshore 332 controller, ranging from 100 K up to 350 K. These impedance spectroscopy measurements were carried out at 0V bias, in dark conditions in a nitrogen atmosphere, after several cycles of vacuum and nitrogen gas to ensure that the sample environment was free of water. The data were collected on heating direction and after a two minutes of stabilization at each temperature step. Coplanar thin-films were measured with an Autolab potentiostat PGSTAT204 equipped with a FRA32M impedance module, in the frequency range from 0.01 Hz up to 1 MHz using an AC signal amplitude of 50 mV. Wet samples were measured in environmental atmosphere. Dry samples were measured in a glove box under dry air with moisture level below 0.01%.


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3. Results and discussion 3.1

Basic characterization of materials

The obtained samples, both as polycrystalline powders and thin-films, were single phase materials as confirmed by XRD patterns obtained at room temperature (Figure S3 in Supporting Information, SI). We observed that XRD patterns of both samples prepared under humid atmosphere and after drying are similar. We did not observe the characteristic peaks of hydrated phase in none of the samples. Therefore, if formed, hydrated phase concentration is below the resolution limit of XRD. Note that in our case relative humidity is 40%. In other cases with larger humidity,31 the formation of different phases is observed by DRX. TGA analysis of dry and wet polycrystalline powders plot in Figure 2 shows that the wet sample displays a weight loss of about 0.9% at T ~(300-355) K. Wet sample was first dried for 3 hours at 150ºC and then kept at 40% humid atmosphere to ensure only water could provide an extra contribution to sample response. With this assumption in mind, we estimated a perovskite: water molar ratio of about 3:1 in the wet powder, while the dried sample is almost free of water molecules, which could be due to water remaining in dry sample after treatment described in experimental section or rehydration before TGA is done.



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Figure 2: Thermo-gravimetric analysis of wet and dry polycrystalline perovskite powder. This basic characterization confirms that the amount of water that can be absorbed by the perovskite, without any visible change in the crystalline structure by XRD, is surprisingly high. Furthermore, most of water contents is lost above de-hydration temperature. Therefore, data suggest that most of water absorbed by perovskite is incorporated, very likely in the channels of the structure, without forming a hydrate. 3.2

Electric properties of polycrystalline powders

Figure 3 shows the temperature dependence of the real part of the capacitance of dry and wet polycrystalline samples in the temperature range 100-350 K obtained from impedance spectroscopy. As it can be seen in this Figure, for T < 185 K the capacitive response is similar for both samples, meanwhile for T > 185 K, the samples showed a completely different behavior: while the capacitance of the dry sample remains nearly constant, the one from the wet sample raises up to two orders of magnitude to reach values in the order of nF·cm-2. In order to check the reversibility of this process, we heated the wet pellet at 150ºC for 3 hours to remove the absorbed water molecules (green line in Figure 3). Remarkably, the re-dried sample displays a similar capacitive response to the one 10 ACS Paragon Plus Environment

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obtained for the dry sample, demonstrating the reversibility of the water absorptiondesorption process in the MAPbI3 perovskite. These findings undoubtedly demonstrate that the exposition of MAPbI3 to environmental moisture induces a large increase of its capacitance values.

Figure 3: Capacitance of a MAPbI3 perovskite pellet at 10 Hz and different temperatures, after a complete cycle of drying and wetting with environmental moisture.

Another interesting feature is the sharp capacitance peak that is observed around 163 K in Figure 3, in good agreement with the crystalline phase transition of MAPbI3 from orthorhombic to tetragonal mentioned above.14,20,32 The presence of extrinsic factors, such as electronic and/or ionic conductivity, could result in the appearance of artifacts in the dielectric measurements.33-34 Therefore, we have made additional impedance analysis to deepen further into the capacitive behavior displayed by this compound.



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Figure 4: (a) Nyquist plot of wet (red dots) and dry (blue dots) pellets at 300 K. Dots represent experimental data and lines their fit to equivalent circuit in (b) that separates dielectric, electronic and ionic properties in 3 different branches.

Figure 4a shows Nyquist plots for the impedance spectra of polycrystalline dry and wet samples at 300 K (see more spectra at temperatures ranging 290 K < T < 350 K in Figure S4 of SI). In general, the wet samples present two arcs in the Nyquist plot, while for the dry ones only a single large semi-arc, slightly deformed, is detected. On the lack of a well established model to describe the impedance spectra of perovskite combining the effects of electronic and ionic behavior, we propose the equivalent circuit shown in Figure 4b. This circuit, that has been adapted to this specific configuration from previous works dealing with the analysis of perovskites,7,

35-38

consists of a resistance (RS), associated to wires and contacts, that is connected in series with the parallel combination of three branches. A first branch is associated to dielectric capacitance of the perovskite (Cd). The second one is mainly associated to electronic contribution to impedance spectra, which consist on a transport resistance (Rtr) connected in series with the parallel combination of a resistance associated with charge


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transfer from contact to perovskite (Rct) and the capacity associated to charge accumulation in this interface (Cint), that may be purely electronic17, 25 or associated to ionic accumulation in Helmholtz layer.16 Data obtained at different pellet thickness, (see Figures S4 and S5 of supporting info) showed that in general Rct is lower than Rtr, but it is not negligible and when Cint is large and Rtr small enough, two arcs appear as for wet samples and the two resistances may easily be decoupled. Finally, the third branch is associated to ionic movement in perovskite which may be modeled by an impedance element representing diffusion of ions blocked at the interface, which is given by

Z di  Rd

d j coth d j

(1)

with Rd the diffusion resistance,  d the ion diffusion frequency (which is related to the diffusion coefficient D   d L2d through the size of the diffusion layer, Ld) and j is the imaginary unit (  1 ). On the lack of more accurate models, the parallel combination of electronic and ionic transport agrees well with diffusion phenomena occurring in perovskite as proposed by several authors.7,

38-39

Equivalent circuit in Figure 4(b)

provides a better determination of electronic properties such conductivity and capacitance data (shown in Figures 5 and S5), than other alternative equivalent circuits as those shown in Figure S6 of supporting information. However accuracy of ionic transport data is limited and allows only qualitative analysis about ionic diffusion (see Figures S7 and S8 in supporting info). Therefore further work is needed to fully confirm the proposed equivalent circuit. Note that configuration of the samples only allows electronic charge (electrons or holes) crossing the complete sample, while ions are blocked at interfaces and so do equivalent circuits discussed here.



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Finally, when needed, a constant phase element (CPE) is used instead of Cint to obtain better fitting of the data. When this was the case (wet pellet at 290 K and 300 K), equivalent capacitance was calculated using

C int,eq 

( Rct Qint )1 / n Rct

(2)

as described in supporting information, with Qint the prefactor and n the index (with n 185 K, dominating the total capacitance of the pellet and yielding to the large values discussed above even at the highest measured temperatures. In the dry samples, Cint contribution is one order of magnitude smaller. According to the TGA analysis in Figure 2, if we assume that most of the water has been eliminated during the drying process, this capacitance would be associated to perovskite ionic species, like iodide, producing a double layer capacitance in the interface with gold. The increase of capacitance in wet samples is then attributable to ionic species originated by water. This increase, that shows a small peak centered at 290 K where hydration occurs, still remains at the temperatures were hydrated phase has disappeared, what suggest the main contribution to capacitance is the strong interaction between ionic species in the wet perovskite and the contacts. Note that for the dry sample, the relatively small value of Cint, combined with the 16 ACS Paragon Plus Environment

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large value attained by Rtr, makes the rise of the capacitance at 10 Hz and high temperatures in Bode plot of Figure 3 very small and below the real value obtained from the fit.

3.3

Electric properties of perovskite thin-films

Pellet configuration helps to understand the basic characteristics of bulk perovskite and the effect of humidity over them. However, despite this configuration is the standard for measuring dielectric and conductivity properties for many materials, it is far from the real situation in thin-film perovskite solar cells. To evaluate the effect of humidity in thin-film perovskites, we used a coplanar configuration as shown in Figure 1(b). This configuration allows the measurement of electrical characteristics of thin-film materials avoiding inconveniences such as the presence of pin holes in the film or material degradation due to the deposition of the top electrode. Several perovskite thin-films were prepared in coplanar configuration to analyze their transport and dielectric properties at room temperature (295 K) and 308 K. We have observed that all these samples presented the same behavior and good reproducibility under impedance spectroscopy measurements. Most representative results are plotted in Figures 5, 6 and S7. Concerning to transport properties, Figure 5a shows that the conductivity of the dry thin-films at room temperature, 9.2 10-9 S·cm-1, is similar to previous results reported in the literature for MAPbI3 perovskites.43 This value is larger than that of the dry pellets, 2.9 10-9 S·cm-1. The difference may be associated to incomplete water extraction in thin-films, due to the lower temperature used for drying these samples to avoid their 17 ACS Paragon Plus Environment


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degradation. For the perovskites with high moisture content we observe that the conductivity of the thin-film (1.4 10-6 S·cm-1) is greatly improved with respect to the dry case (see also Figure S11 in SI). As can be seen in Figure 5a, the conductivity of the wet film also exceeds the one of wet pellets. This difference may be attributed to the measuring conditions of the pellets, which are kept under dry nitrogen (to allow the temperature study) and therefore are expected to lose some of their initial moisture content. Finally wet films decrease their conductivity when rising the temperature from 295 to 308 K, following the same trend found for the pellets. The large variations found for the conductivity under water absorption may have two origins: (i) a change in mobility or (ii) a change in the number of free carriers (doping). The first option matches well with the large divergences in the values of mobility reported in the literature that ranges from 2.5 - 164 cm2·V-1·s-1. 43-45 Using these values of mobility, we can estimate the concentration of free carriers, N. For dry films of perovskite N would lie between 1·1010 and 2·108 cm-3 and for wet films between 2·1012 and 4·1010 cm-3. Pellets follow the same tendency with values approximately one order of magnitude smaller. Water absorption by perovskite thin-film also produces a rise in the capacitance that may be appreciated at temperatures higher than 185 K, see Figure 5b. The peak in the values of Cint wet may be again associated to hydrated phase, but the large difference between dry and wet samples found above 323 K (50ºC) indicates a strong effect of absorbed water in the interfacial capacitance, which is stronger than the one found for conductivity at these temperatures. To highlight the effect of water absorption on thin-film coplanar perovskite capacitance, Figure 6 shows the evolution of its bode plot under different wetting and drying cycles and compares it with the response of bare glass substrate measured in lab 18 ACS Paragon Plus Environment

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environment at relative humidity ~40%. As can be seen in this bode plot, at high frequencies the response is in all cases dominated by the glass substrate. This makes impossible an accurate determination of the dielectric response of perovskite in coplanar configuration (see Figure S12 in SI). At low frequencies, the capacitance depends strongly on the amount of absorbed water.

Figure 6: Real part of capacitance of coplanar thin-film perovskite electrodes after different wetting and drying treatments compared with glass substrate.

Therefore, black line in Figure 6 corresponds to the measurement of a fresh thin-film prepared under 40% relative humidity. The capacitance of the wet perovskite exceeds by more than 4 orders of magnitude the one of the glass substrate, reaching values as high as 1 mF·cm-2, a value very similar to those reported previously for perovskite solar cells.24 Then the sample was kept overnight in a glove box with continuous flux of dry air (HR < 0.01%) and the value of capacitance diminished around 1.5 orders of magnitude (red line). The sample was then put again in contact with environmental air at 40% humidity for 24 hours and the capacitance of the sample (green line) completely recovered, demonstrating the reversibility of the process. Finally, a second drying treatment was applied to the sample combining 16 hours under dry air and heating at 19 ACS Paragon Plus Environment


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60ºC for 6 hours (violet line). As can be seen in Figure 6, after this treatment the capacitance of the sample was reduced nearly to the same values of the glass substrate indicating that most of water was extracted from the film and showing that as in the case of the pellets, the remaining water still contributes to perovskite capacitance. Note that as for the case of conductivity, the increase in capacitance of wet pellets is much smaller than in the case of thin films what again may be attributed to the partial drying of pellets in the nitrogen atmosphere used for the temperature study. These results suggest that water absorbed by perovskite films during its preparation or storage at room conditions may have a relevant contribution to the huge capacitance found in these systems, which have been associated to interfacial effects.30 These huge capacitances are responsible of two important phenomena in perovskite devices: On one side, the characteristic hysteresis widely reported for some configurations of solar cells,16, 25, 28-29 on the other side, the large carrier accumulation at the interface induces larger recombination and thus a lower Voc.30, 37, 46 Therefore, reducing water content of the perovskite may contribute to reduce hysteresis and charge recombination associated to the presence of these large capacitances. On the other hand, the increase of conductivity indicates that the water absorbed by the perovskite is very convenient to improve the transport properties of the material. Therefore, to improve the solar cells performance, a trade-off is needed in order to optimize conductivity and capacitance contributions. In summary, our results suggest that upon water absorption a small amount contributes to a large increase in conductivity observable at room temperature, and very likely due to the formation of hydrated phases. However, most of the absorbed water does not form hydrates and may diffuse through the perovskite volume. The capacitance measured for wet samples attains the huge values observed in surface charge


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accumulation processes commonly observed in perovskite devices, showing that absorbed water has a key role in these phenomena. Thermal treatment at 60 ºC cancels the effect of the conductivity increase due to the hydrates, but some contribution from capacitance still remains.

4. Conclusions We have shown that environmental moisture plays an important role on the dielectric and transport properties of MAPbI3 perovskite thin-films and pellets, which are key parameters that determine the response of the perovskite solar cells. Both capacitance and conductivity are increased several orders of magnitude when the MAPbI3 perovskite is under environmental moisture conditions, with respect to the values reached for dry samples and measured in water free atmospheres. Specifically, moisture absorption induces a huge increase of the capacitance, yielding to values comparable to the ones commonly found on lead halide perovskite devices. This result suggests that absorbed water has an important role in charge accumulation phenomena observed in perovskite solar cells. We found that activation energy of electronic transport in dry perovskites obtained in this work is very similar to that of ionic transport reported by different authors and techniques, highlighting the strong interactions between these two processes. Our data also suggest that the large dispersion of mobility values found in the literature may be related to the modifications in conductivity associated to water absorption. Finally, we observed that water absorption from environment with up to 40% relative humidity may reach a perovskite to water molar ratio of 3:1 and the electrical phenomena associated to it showed a reversible response. From all this water, less than 10% contributes to form the hydrates that yield to a peak in conductivity, which is 21 ACS Paragon Plus Environment


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canceled above 60ºC. Strong contribution to capacitance due to water absorption was still observed at higher temperatures, suggesting specific interaction between ionic species in perovskite and contacts induced by water absorption.


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Acknowledgements Authors want to acknowledge Ministerio de Economía y Competitividad (MINECO) from Spain under project, MAT2013-47192-C3-1-R (NASCENT), EU-FEDER for financial support under the project ENE2014-56237-C4-4-R, Xunta de Galicia under the project GRC2014/042. J.M.B.-G. acknowledges Xunta de Galicia for a Postdoctoral Fellowship. Z.M was supported by Ministry of Science, Research and Technology of Iran, Z.M. thanks Prof. Hamid Reza Fallah for his worthwhile advices. E.M-M thanks the Ramón y Cajal Program from the Spanish government. Serveis Centrals d’Instrumentació Científica from UJI are acknowledged for SEM and DRX measurements.

Supporting Information Supporting Information Available: Scheme of thin-film synthesis process. SEM images of coplanar thin-film sample; XRD data of pellets and thin-films; Impedance spectra of dry and wet pellets at different temperatures; Alternative equivalent circuits for impedance analysis; Analysis of transport resistance and dielectric capacitance for pellets with different thickness; Arrhenius plot of conductivity; Dielectric constant of perovskite pellets at different temperatures and frequencies;

Conductivity and

capacitance data of thin-films obtained from impedance data at different voltages; Bode of Capacitance from thin-film samples after subtraction of glass capacitance; Procedure for the calculation of conductivity and permittivity of materials in pellets and co-planar configurations. This material is available free of charge via the Internet at http://pubs.acs.org



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References. 1. Research Cell Record Efﬁciency Chart by National Renewable Energy Laboratory https://www.nrel.gov/pv/assets/images/efficiency-chart-20180716.jpg (accessed August 2018). 2. 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. 3. Eperon, G. E.; Habisreutinger, S. N.; Leijtens, T.; Bruijnaers, B. J.; van Franeker, J. J.; deQuilettes, D. W.; Pathak, S.; Sutton, R. J.; Grancini, G.; Ginger, D. S.; et. al. The Importance of Moisture in Hybrid Lead Halide Perovskite Thin Film Fabrication. ACS Nano 2015, 9 , 9380-9393. 4. Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface Engineering of Highly Efficient Perovskite Solar Cells.. Science 2014, 345 , 542. 5. Gangishetty, M. K.; Scott, R. W. J.; Kelly, T. L. Effect of Relative Humidity on Crystal Growth, Device Performance and Hysteresis in Planar Heterojunction Perovskite Solar Cells. Nanoscale 2016, 8, 6300-6307. 6. Aranda, C.; Cristobal, C.; Shooshtari, L.; Li, C.; Huettner, S.; Guerrero, A. Formation Criteria of High Efficiency Perovskite Solar Cells Under Ambient Conditions Sustainable Energy Fuels 2017, 1, 540-547. 7. Kim, G. Y.; Senocrate, A.; Yang, T.-Y.; Gregori, G.; Grätzel, M.; Maier, J. Large Tunable Photoeffect on Ion Conduction in Halide Perovskites and Implications for Photodecomposition. Nat. Mater. 2018, 17, 445-449. 8. Leguy, A. M. A.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.; van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, et al. Reversible Hydration of CH3NH3PbI3 in Films, Single Crystals, and Solar Cells. Chem. Mater. 2015, 27, 3397-3407. 9. Christians, J. A.; Miranda Herrera, P. A.; Kamat, P. V. Transformation of the Excited State and Photovoltaic Efficiency of CH3NH3PbI3 Perovskite upon Controlled Exposure to Humidified Air. J. Am. Chem. Soc. 2015, 137, 1530-1538. 10. Halder, A.; Choudhury, D.; Ghosh, S.; Subbiah, A. S.; Sarkar, S. K. Exploring Thermochromic Behavior of Hydrated Hybrid Perovskites in Solar Cells. J. Phys. Chem. Lett. 2015, 6 , 3180-3184. 11. Müller, C.; Glaser, T.; Plogmeyer, M.; Sendner, M.; Döring, S.; Bakulin, A. A.; Brzuska, C.; Scheer, R.; Pshenichnikov, M. S.; Kowalsky, et al Water Infiltration in Methylammonium Lead Iodide Perovskite: Fast and Inconspicuous. Chem. Mater. 2015, 27, 7835-7841. 12. Mosconi, E.; Azpiroz, J. M.; De Angelis, F. Ab Initio Molecular Dynamics Simulations of Methylammonium Lead Iodide Perovskite Degradation by Water. Chem. Mater. 2015, 27, 4885-4892. 13. Zhang, L.; Sit, P. H. L. Ab Initio Study of Interaction of Water, Hydroxyl Radicals, and Hydroxide Ions with CH3NH3PbI3 and CH3NH3PbBr3 Surfaces. J. Phys. Chem. C 2015, 119, 22370-22378. 14. Koocher, N. Z.; Saldana-Greco, D.; Wang, F.; Liu, S.; Rappe, A. M. Polarization Dependence of Water Adsorption to CH3NH3PbI3 (001) Surfaces. J. Phys. Chem. Lett. 2015, 6, 4371-4378. 15. Sveinbjörnsson, K.; Aitola, K.; Zhang, X.; Pazoki, M.; Hagfeldt, A.; Boschloo, G.; Johansson, E. M. J. Probing Photocurrent Generation, Charge Transport, and


Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Recombination Mechanisms in Mesostructured Hybrid Perovskite through Photoconductivity Measurements. J. Phys. Chem. Lett. 2015, 6, 4259-4264. 16. Almora, O.; Zarazua, I.; Mas-Marza, E.; Mora-Sero, I.; Bisquert, J.; GarciaBelmonte, G. Capacitive Dark Currents, Hysteresis, and Electrode Polarization in Lead Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6, 1645-1652. 17. Almora, O.; Aranda, C.; Zarazua, I.; Guerrero, A.; Garcia-Belmonte, G. Noncapacitive Hysteresis in Perovskite Solar Cells at Room Temperature. ACS Energy Lett. 2016, 1, 209-215. 18. Cojocaru, L.; Uchida, S.; Sanehira, Y.; Nakazaki, J.; Kubo, T.; Segawa, H. Surface Treatment of the Compact TiO2 Layer for Efficient Planar Heterojunction Peroyskite Solar Cells. Chem. Lett. 2015, 44, 674-676. 19. Lunkenheimer, P.; Krohns, S.; Riegg, S.; Ebbinghaus, S. G.; Reller, A.; Loidl, A. Colossal Dielectric Constants in Transition-Metal Oxides. Eur. Phys. J. Spec. Top. 2009, 180, 61-89. 20. Sánchez-Andújar, M.; Yáñez-Vilar, S.; Pato-Doldán, B.; Gómez-Aguirre, C.; Castro-García, S.; Señarís-Rodríguez, M. A. Apparent Colossal Dielectric Constants in Nanoporous Metal Organic Frameworks. J. Phys. Chem. C 2012, 116, 13026-13032. 21. Walsh, A. Principles of Chemical Bonding and Band Gap Engineering in Hybrid Organic–Inorganic Halide Perovskites. J. Phys. Chem. C 2015, 119, 5755-5760. 22. Onoda-Yamamuro, N.; Matsuo, T.; Suga, H. Dielectric Study of CH3NH3PbX3 (X = Cl, Br, I). J. Phys. Chem. Solids 1992, 53, 935-939. 23. Samiee, M.; Konduri, S.; Ganapathy, B.; Kottokkaran, R.; Abbas, H. A.; Kitahara, A.; Joshi, P.; Zhang, L.; Noack, M.; Dalal, V. Defect Density and Dielectric Constant in Perovskite Solar Cells. Appl. Phys. Lett. 2014, 105, 153502. 24. Juarez-Perez, E. J.; Sanchez, R. S.; Badia, L.; Garcia-Belmonte, G.; Kang, Y. S. Photoinduced Giant Dielectric Constant in Lead Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 2390-2394. 25. Ravishankar, S.; Almora, O.; Echeverria-Arrondo, C.; Ghahremanirad, E.; Aranda, C.; Guerrero, A.; Fabregat-Santiago, F.; Zaban, A.; Garcia-Belmonte, G.; Bisquert, J. Surface Polarization Model for the Dynamic Hysteresis of Perovskite Solar Cells. J. Phys. Chem. Lett. 2017, 8, 915-921. 26. Tress, W.; Marinova, N.; Moehl, T.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Gratzel, 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. 27. Fabregat-Santiago, F.; Mora-Seró, I.; Garcia-Belmonte, G.; Bisquert, J. Cyclic Voltammetry Studies of Nanoporous Semiconductor Electrodes. Models and Application to Nanocrystalline TiO2 in Aqueous Electrolyte. J. Phys. Chem. B 2003, 107, 758-769. 28. Cojocaru, L.; Uchida, S.; Jayaweera, P. V. V.; Kaneko, S.; Nakazaki, J.; Kubo, T.; Segawa, H. Origin of the Hysteresis in I-V Curves for Planar Structure Perovskite Solar Cells Rationalized with a Surface Boundary-Induced Capacitance Model. Chem. Lett. 2015, 44, 1750-1752. 29. Kim, H.-S.; Jang, I.-H.; Ahn, N.; Choi, M.; Guerrero, A.; Bisquert, J.; Park, N.G. Control of I–V Hysteresis in CH3NH3PbI3 Perovskite Solar Cell. J. Phys. Chem. Lett. 2015, 6, 4633-4639. 30. Zarazua, I.; Bisquert, J.; Garcia-Belmonte, G. Light-Induced Space-Charge Accumulation Zone as Photovoltaic Mechanism in Perovskite Solar Cells. J. Phys. Chem. Lett. 2016, 7, 525-528.



Page 26 of 34

31. Salado, M.; Contreras-Bernal, L.; Caliò, L.; Todinova, A.; López-Santos, C.; Ahmad, S.; Borras, A.; Idígoras, J.; Anta, J. A. Impact of Moisture on EfficiencyDetermining Electronic Processes in Perovskite Solar Cells. J. Mat. Chem. A 2017, 5, 10917-10927. 32. Weller, M. T.; Weber, O. J.; Henry, P. F.; Di Pumpo, A. M.; Hansen, T. C. Complete Structure and Cation Orientation in the Perovskite Photovoltaic Methylammonium Lead Iodide Between 100 and 352 K. Chem. Commun. 2015, 51, 4180-4183. 33. Scott, J. F. Ferroelectrics go Bananas. J. Phys.: Condens. Matter 2008, 20, 021001. 34. Loidl, A.; Krohns, S.; Hemberger, J.; Lunkenheimer, P. Bananas Go Paraelectric. J. Phys.: Condens. Matter 2008, 20, 191001. 35. Bisquert, J.; Bertoluzzi, L.; Mora-Sero, I.; Garcia-Belmonte, G. Theory of Impedance and Capacitance Spectroscopy of Solar Cells with Dielectric Relaxation, Drift-Diffusion Transport, and Recombination. J. Phys. Chem. C 2014, 118, 1898318991. 36. Anaya, M.; Zhang, W.; Hames, B. C.; Li, Y.; Fabregat-Santiago, F.; Calvo, M. E.; Snaith, H. J.; Míguez, H.; Mora-Seró, I. Electron Injection and Scaffold Effects in Perovskite Solar Cells. J. Mat. Chem. C 2017, 5, 634-644. 37. Zarazua, I.; Han, G.; Boix, P. P.; Mhaisalkar, S.; Fabregat-Santiago, F.; MoraSero, I.; Bisquert, J.; Garcia-Belmonte, G. Surface Recombination and Collection Efficiency in Perovskite Solar Cells from Impedance Analysis. J. Phys. Chem. Lett. 2016, 7, 5105-5113. 38. Peng, W.; Aranda, C.; Bakr, O. M.; Garcia-Belmonte, G.; Bisquert, J.; Guerrero, A. Quantification of Ionic Diffusion in Lead Halide Perovskite Single Crystals. ACS Energy Lett. 2018, 3, 1477-1481. 39. Kerner, R. A.; Rand, B. P. Ionic–Electronic Ambipolar Transport in Metal Halide Perovskites: Can Electronic Conductivity Limit Ionic Diffusion? J. Phys. Chem. Lett. 2018, 9, 132-137. 40. Eames, C.; Frost, J. M.; Barnes, P. R. F.; O’Regan, B. C.; Walsh, A.; Islam, M. S. Ionic Transport in Hybrid Lead Iodide Perovskite Solar Cells. Nat. Comm. 2015, 6, 7497. 41. O'Kane, S. E. J.; Richardson, G.; Pockett, A.; Niemann, R. G.; Cave, J. M.; Sakai, N.; Eperon, G. E.; Snaith, H. J.; Foster, J. M.; et. al. Measurement and Modelling of Dark Current Decay Transients in Perovskite Solar Cells. J. Mat. Chem. C 2017, 5, 452-462. 42. Brivio, F.; Walker, A. B.; Walsh, A. Structural and Electronic Properties of Hybrid Perovskites for High-Efficiency Thin-Film Photovoltaics From First-Principles. APL Mat. 2013, 1, 042111. 43. Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K.; ; et.al. Low Trap-State Density and Long Carrier Diffusion in Organolead Trihalide Perovskite Single Crystals. Science 2015, 347, 519-522. 44. Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. ElectronHole Diffusion Lengths 175 μm in Solution-Grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967-970. 45. Brenner, T. M.; Egger, D. A.; Rappe, A. M.; Kronik, L.; Hodes, G.; Cahen, D. Are Mobilities in Hybrid Organic–Inorganic Halide Perovskites Actually “High”? J. Phys. Chem. Lett. 2015, 6, 4754-4757.


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46. Raga, S. R.; Barea, E. M.; Fabregat-Santiago, F. Analysis of the Origin of Open Circuit Voltage in Dye Solar Cells. J. Phys. Chem. Lett. 2012, 3, 1629-1634.



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TOC


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Figure 1: Configurations for electrical measurements: (a) polycrystalline pellet (b) coplanar thin-film. 207x76mm (96 x 96 DPI)

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Figure 2: Thermo-gravimetric analysis of wet and dry polycrystalline perovskite powder. 74x64mm (300 x 300 DPI)


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Figure 3: Capacitance of a MAPbI3 perovskite pellet at 10 Hz and different temperatures, after a complete cycle of drying and wetting with environmental moisture. 74x61mm (300 x 300 DPI)



Figure 4: (a) Nyquist plot of wet (red dots) and dry (blue dots) pellets at 300 K. Dots represent experimental data and lines their fit to equivalent circuit in (b) that separates dielectric, electronic and ionic properties in 3 different branches. 80x42mm (300 x 300 DPI)


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Figure 5: (a) Conductivity and (b) Capacitance of polycrystalline pellets and thin-films. Inset in Figure (a) shows Arrhenius plot of the conductivity of the dry polycrystalline sample. All data were obtained for wet and dried samples after fitting impedance spectra to model in Figure 4b. 74x33mm (300 x 300 DPI)



Figure 6: Real part of capacitance of coplanar thin-film perovskite electrodes after different wetting and drying treatments compared with glass substrate. 97x74mm (150 x 150 DPI)


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Effect of Environmental Humidity in the Electrical Properties of Lead

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