Direct Experimental Evidence of Halide Ionic Migration under Bias in

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Direct Experimental Evidence of Halide Ionic Migration under Bias in CHNHPbI Cl Based Perovskite Solar Cells using GD-OES Analysis 3

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HEEJAE LEE, Sofia GAIASCHI, Patrick CHAPON, Arthur Marronnier, Heeryung LEE, JeanCharles Vanel, Denis Tondelier, Jean-Eric Boureé, Yvan Bonnassieux, and Bernard Geffroy ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00150 • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on April 2, 2017

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Contents Graphic 162x68mm (96 x 96 DPI)

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Figure 1. a) Detailed scheme of the perovskite planar solar cell architecture facing the plasma of GD-OES, b) Measured J-V curves of the best CH3NH3PbI3-xClx solar cell under 1 sun illumination scanned in the forward (blue line) and reverse (red line) directions, c) pictures of the perovskite solar cells before and after GD-OES measurement. Fig. 1a shows the inverted (or 366x182mm (150 x 150 DPI)


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Figure 2. GD-OES results of the reference PSCs device without applied voltage showing the relative atomic composition a) from Ag to ITO, and b) for PCBM, hybrid perovskite and PEDOT:PSS layers (zoom of shaded area of Fig. 2a). The GD-OES results performed o 307x129mm (150 x 150 DPI)


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Figure 3. GD-OES profile lines versus sputtering time for a) iodide, b) chloride, c) lead and d) nitrogen ions as a function of the amplitude of the applied bias. For better clarity, the profile line intensities are shifted. . For clarity, the intensity v 328x268mm (150 x 150 DPI)


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Figure 4. GD-OES profile lines of iodine species in perovskite films depending on a) negative and b) positive applied biases Fig. 4 focuses more specifical 486x165mm (150 x 150 DPI)


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Figure 5. Ionic migration characteristics in perovskite film. a) Rate of immobile I ions and average length of b) I- and c) Cl- ions depending on the applied bias. Fig. 5a reports the percentage 344x304mm (150 x 150 DPI)


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Figure. S 1 Schematic illustration of GD-OES measurement set-up. The GD plasma, generated in anode tube, sputters the surface of the analyzed sample (perovskite solar cell) layer by layer and excites the sputtered atoms. The photons emitted from excited atoms, are collected and analyzed to obtain the depthresolved elemental analysis, with nanometric depth resolution. In GD-OES a radio frequency (RF) plasma is used to obtain the elemental profile of conductive, insulating or hybrid materials. The GD plasma has a double role, firstly it sputters the surface of the sample and by optimizing the plasma conditions layer-bylayer erosion can be achieved and secondly it excites the sputtered atoms. More in details, by applying a RFpower to the sample, plasma is created in the tubular anode. The inner diameter of the anode defines the analyzed spot size, as the plasma is restricted to this area. When the discharge gas breaks down electrically, forming electrons and positively charged ions, the latter are accelerated towards the sample by the electric field and the ion bombardment of the surface of the sample causes the sputtering of the material to be analyzed. When entering the gas phase environment of the glow discharge, the sputtered atoms are excited by collisions with high-energy electrons, metastable argon atoms and ions. De-excitation of the excited species causes the emission of photons characteristic of the different elements composing the material, which are collected using an optical spectrometer. As the sample is continuously sputtered, the collected light reflects the temporal evolution of the sputtered species, therefore it is possible to obtain the depthresolved elemental analysis, with nanometric depth resolution. The GD-OES profile lines versu 161x128mm (150 x 150 DPI)


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Figure. S 2 GD-OES profile lines versus sputtering time for (a) iodide, (b) chloride, (c) lead and (d) nitrogen depending on the applied bias from -2.5V to +2.5 V. These are the original GD-OES results of Fig. 3 before shifting intensity. Fig. 3a and S2a display GD-OES 231x190mm (150 x 150 DPI)


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Figure. S 3 GD-OES profile lines of I species in perovskite films with (a) -1 V, (b) +1 V, (c) -2.5 V and (d) +2.5 V) and without applied bias. The dotted lines corresponds to the GD-OES profile line intensity without bias (red line) multiplied by (a) 42 %, (b) 56 %, (c) 38 %, and (d) 40 % respectively. When the negative bias is applied, the GD-OES profile lines of I species completely overlap with the dotted line in the range of 50 to 60 seconds (in the direction of PEDOT:PSS). When the positive bias is applied, they completely overlap in the range of 30 to 40 seconds (in the direction of PCBM). The overlapped ranges stay the same regardless of different values of bias. However, the percentage values of fixed ions vary while different values of bias are applied. This allows evaluating the percentage value of fixed ions in Fig. 5a. Based on this observation, we 287x188mm (150 x 150 DPI)


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Direct Experimental Evidence of Halide Ionic Migration under Bias in CH3NH3PbI3-xClx Based Perovskite Solar Cells using GD-OES Analysis Heejae Lee1, Sofia Gaiaschi2, Patrick Chapon2, Arthur Marronnier1, Heeryung Lee1, Jean-Charles Vanel1, Denis Tondelier1, Jean-Eric Bourée1, Yvan Bonnassieux1 and Bernard Geffroy*3,1 1. LPICM, Ecole polytechnique, Université Paris-Saclay, CNRS UMR7647, 91128, Palaiseau Cedex, France 2. Horiba Jobin Yvon S.A.S., 16-18 rue du canal, 91165, Longjumeau Cedex, France 3. LICSEN, NIMBE, CEA, Université Paris-Saclay, CEA Saclay, 91191, Gif-sur-Yvette Cedex, France * Corresponding author Phone number: +33 1 69 33 43 82 E-mail address: [email protected] ABSTRACT: In recent decades, the development of organic-inorganic hybrid perovskite solar cells (PSCs) has been increasing very quickly due to their high initial efficiency and low cost process. However, key points such as crystal growth mechanisms, current-voltage hysteresis and instability remain still unexplained or misunderstood. Among several possibilities, ionic migration in PSCs has been suggested to explain the hysteresis effect. However, direct experimental evidence of ionic migration under operation or measurement conditions of PSCs is still missing. This work shows directly the ionic migration of halogen components (I- and Cl-) of CH3NH3PbI3-xClx perovskite film under an applied bias using glow discharge optical emission spectrometry (GD-OES). Furthermore, no migration of lead and nitrogen ions is observed in the polarization timescale less than 2 minutes. The ratio of fixed to mobile iodide ions is deduced from the evolution of the GD-OES profile lines as a function of the applied bias. The average length of iodide and chloride ion migration is deduced from the experimental results.

Hybrid organic-inorganic perovskite materials (HOIPs) have recently emerged as an exciting topic of research in chemistry and materials science for their attractive optical-electrical properties. HOIPs have found potential applications in optoelectronics 1 ACS Paragon Plus Environment

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such as photovoltaics, photodetectors, light-emitting diodes and lasers.1-3 It has been discovered that these materials are able to exhibit remarkable performance in terms of photocurrent generation.4 Key features of these hybrid perovskites include direct bandgap, large exciton diffusion length and strong optical absorption.5-7 However, despite their processing advantages (low cost solution process) and outstanding solar to electrical energy conversion properties approaching the performance of standard inorganic materials such as silicon, HOIP materials suffer from several drawbacks. The stability of HOIP materials under operational conditions (bias, light, environment…) is one of the biggest challenges to be addressed before commercialization.8 HOIPs are mixed ionic-electronic conductors where ions as well as electron/hole carriers can migrate in the material under electric fields.9 It has been suggested that ionic migration in HOIPs could impact optoelectronic performance and affect device operation and long-term stability. The J-V hysteresis behavior in perovskite solar cells (PSCs) was first reported by Snaith et al.10 and Unger et al.11 in mesoporous structured perovskites and by Xiao et al.12 for planar heterojunction structures. Various mechanisms have been proposed to explain the origins of the J-V hysteresis such as filaments, giant dielectric constant, unbalance between hole and electron mobility, trapping or ferroelectricity effects and ionic migration effects.13-19 The general formula for HOIPs materials is ABX3 where A is an organic cation, B is a divalent metal and X is a halide anion. Methylammonium lead iodide (MAPbI3) materials have been extensively studied for photovoltaic applications. The possible mobile ions in MAPbI3 crystal include MA+ ions, Pb2+ ions, I- ions, vacancies and other defects such as hydrogen-related impurities (H+, H0 and H-).20 Considering the activation energy of ion migration,21-24 it is reasonable to expect that both the MA+ ions and I- ions are mobile in the MAPbI3 films, while the Pb2+ ions are almost immobile. However, whereas the migration of ions has been suggested by several groups,25-27 direct experimental evidence is still needed. In this work we show experimentally the migration of ions in hybrid perovskites CH3NH3PbI3-xClx based solar cells as a function of an applied bias using glow discharge optical emission spectrometry (GD-OES) (Fig 1a). GD-OES is a spectrochemical technique that allows direct determination of major and trace elements.28,29 The ratio of fixed ions versus mobile ions is deduced by applying electrical bias on the device. These results show directly that halogen ions (I- and Cl-) move through the device while lead and nitrogen ions are immobile. This migration of halogen ions influences the electrical characteristics of PSCs devices and may be responsible for the J-V hysteresis. Fig. 1a shows the inverted (or p-i-n) CH3NH3PbI3-xClx based planar structure PSCs devices used in this work (ITO = anode, Ag = cathode). The photovoltaic performance of a series of 10 samples is reported in table 1. Table 1 shows a good reproducibility of the devices and we can thus consider that all the samples used for GD-OES experiment have the same characteristics. The power conversion efficiency (PCE) under 1 sun equivalent illumination is 12.6 % for the best cell (11.6 % in average) with an active area of 0.28 cm2. Fig. 1b and table 1 represent the J-V characteristics of the best cell scanned in the forward and in the reverse directions. The hysteresis effect is small (less than 2.5 %) in our case. This is consistent with the results in the literature reporting that the p-i-n architecture does not show hysteresis while the n-i-p architecture shows significant hysteresis.27,30

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Table 1 Photovoltaic performance of perovskite solar cells used in this study. JSC VOC FF PCE (mA/cm2) (V) (%) (%) Average performance 0.92 (0.02) 64.2 (4) 11.7 (0.8) 19.9 (2) (10 samples) Best performance Forward scan 19.9 0.92 67.0 12.3 Reverse scan 20.2 0.93 67.0 12.6

Figure 1. a) Detailed scheme of the perovskite planar solar cell architecture facing the plasma of GD-OES, b) Measured J-V curves of the best CH3NH3PbI3-xClx solar cell under 1 sun illumination scanned in the forward (blue line) and reverse (red line) directions, c) pictures of the perovskite solar cells before and after GD-OES measurement.

The GD-OES results performed on the CH3NH3PbI3-xClx solar cell are reported in Fig. 2. It is worthy to note that the size of the GD-OES spot is 4 mm in diameter, therefore the analyzed area (Fig. 1a) is smaller than the active area of the devices (active area: 6 mm diameter). During a GD-OES analysis the sputtering starts from the surface of the sample (silver electrode) towards ITO layer. Thus, the GD-OES signals of interest composing the electron transport layer (ETL), the perovskite layer and the hole transport layer (HTL) are located between the Ag and In peaks (figure 2a). Fig. 2b corresponds to a zoom of Fig. 2a between 30 s and 60 s of sputtering time and it presents the elemental distribution of carbon (C), sulfur (S), chlorine (Cl), iodine (I), lead (Pb) and nitrogen (N), which are the main elements of 6,6-phenyl-C61-butyric acid methylester (PCBM) as ETL, hybrid perovskite and poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT:PSS) as HTL, respectively. It is worth noting that the C signal (red solid line) is composed of two peaks due to the PCBM (at sputtering time of 33 s) and PEDOT:PSS (at sputtering time of 47 s) respectively. Moreover, the sulfur (S) signal (one of the principle components of PEDOT:PSS) appears at the same 3 ACS Paragon Plus Environment

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sputtering time (between 45 and 55s) as the C signal. The signals of I, N, Pb, and Cl, which are the main elements of the perovskite film, can be observed at the same sputtering time between the two peaks of C. Note that the intensities of the GD-OES signal for each element (Fig. 2b) vary in large amplitude. However, these relative intensities do not correspond directly to the relative concentrations due to the independent sputtering and emission yields for each element.

Figure 2. GD-OES results of the reference PSCs device without applied voltage showing the relative atomic composition a) from Ag to ITO, and b) for PCBM, hybrid perovskite and PEDOT:PSS layers (zoom of shaded area of Fig. 2a).

The GD-OES profile lines versus sputtering time under different applied biases (2.5 V to +2.5 V vs. anode) are shown in Fig. 3 and S1 (see SI) for iodide, chloride, lead and nitrogen ions. For clarity, the intensity values in Fig. 3 are shifted vertically as compared to intensity values in Fig. S1 in order to highlight the ionic migration. In the GD-OES line profiles, the initial sputtering time (30 s) corresponds to the PCBMperovskite interface and the final sputtering time (60 s) to the PEDOT:PSS-perovskite interface. As shown in Fig. 3, two different behaviors are obtained: on one hand, the iodide and chloride ions (Fig. 3a and 3b respectively) move according to the sign of the voltage and on the other hand lead and nitrogen do not move at all in the polarization timescale less than 2 minutes (Fig. 3c and 3d respectively).


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Figure 3. GD-OES profile lines versus sputtering time for a) iodide, b) chloride, c) lead and d) nitrogen ions as a function of the amplitude of the applied bias. For better clarity, the profile line intensities are shifted.

Figure 4. GD-OES profile lines of iodine species in perovskite films depending on a) negative and b) positive applied biases.


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Fig. 3a and S2a display GD-OES profile lines of iodide ion versus sputtering time for different biases showing directly the iodide ion movements. For a negative bias, Iions move towards PCBM layer and for the positive bias I- ions move towards PEDOT:PSS layer. We can see that the iodide ion (negatively charged) moves in the inverse direction of the applied electric field as expected. This movement induces a screening of the external electric field applied to the solar cell. 13,18,24 Fig. 4 focuses more specifically on iodide ions and shows profile lines depending on applied bias. When the bias is negative (figure 4a), the mobile iodide ions move towards the silver cathode side (shorter sputtering time) and when the bias is positive (figure 4b), the mobile iodide ions are shifted towards the PEDOT: PSS side (longer sputtering time). These results confirm that the mobile iodide ions are negatively charged species. The red solid line (without applied bias) in fig. 4a and 4b corresponding to the profile line of I- in the perovskite film shows only one peak. However, we observe the appearance of two peaks when bias is applied on the device. We attribute these two peaks to fixed (sputtering time around 40 s) and mobile (sputtering time at 37 s for negative biases and 47 s for positive biases respectively) I species in the hybrid perovskite film. We have checked that the integrated signal for iodide ion is the same whatever is the applied bias. Based on this observation, we have calculated the percentage of fixed iodide ions by deconvolution of GD-OES signals for each bias with the unbiased signal (Fig. S3). Such deconvolution was achieved by performing the difference between the biased signals and the un-biased signal, for all the biases. If we consider that the un-biased signal corresponds to the sum of fixed and mobile ions, by performing the difference between each signal it is possible to determine the amount of ions that moved due to the applied bias. Indeed, as can be seen in Fig. S4 (see SI), this operation leads to the presence of two opposite peaks: a negative and a positive one. This can be explained by the loss or excess of mobile I ions, which move due to the applied bias. The average length of the ionic migration can be estimated as the distance between the negative and the positive peak derived from the difference between biased versus un-biased signals. Fig. 5a reports the percentage of fixed I- ions as a function of the applied bias using as explained in supporting information the statistical weight of fixed ions represented by the dotted lines (Fig. S3). As seen in Fig. 5a, the ratio of fixed to mobile I species decreases according to the applied bias following an exponential law. The fixed I species percentage saturates roughly at 35 % both in positive and negative applied biases. Consequently, 65% of iodide ions are able to migrate in the perovskite film under applied bias. Fig. 5b reported the average length of ion migration for iodine as a function of the applied bias. Independently to the sign of the applied bias, the average length value saturates around 110-130 nm (Fig. 5b). It can be deduced that the iodide ions migrate approximately 1/3 of the perovskite layer thickness.


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Figure 5. Ionic migration characteristics in perovskite film. a) Percentage of immobile I ions and average length of b) I- and c) Cl- ions depending on the applied bias.

Fig. 3b reports the shifted profile lines of chloride ions as a function of the applied bias. Without applying a voltage, the Cl concentration is higher at the PEDOT:PSS side than in the PCBM side as shown in Fig. 2b. The intensity value of Cl is significantly lower than the value of Pb and I. As seen in Fig. 3b and Fig. S2b, Cl shows similar migration behaviors as for iodine as a function of the applied bias. This effect is expected due to the fact that both ions have a negative charge. However, a one-peak pattern is observed for chlorine signal versus applied bias contrary to iodine signal. Since the peak moves according to the value of the applied bias, we consider that all the chloride ions are mobile. As shown in Fig. 5c, the average length values for chloride ions are 120 nm for -2.5 V and 20 nm for +2.5 V respectively. This behavior can be explain easily by the fact that the chloride ions are not distributed homogeneously within the perovskite layer but located deeper insight the perovskite layer (figure 2b). Average length continues to increase as a function of the applied bias without saturation effect contrary to iodine. This observation means that all the chloride ions are mobile in the halide perovskite film. This is not surprising since it is almost accepted that the chloride ion does not integrate within the MAPI structure and it is the reason why they are free to move and can migrate monotonically with the applied voltage. The distances 7 ACS Paragon Plus Environment

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found are not the same depending on the sign of the applied bias because of the initial unsymmetrical distribution of chloride ions without any bias (Fig. 2b). A CH3NH3+ rich environment is critical to slow down the growth of the crystal domains during annealing.31,32 The role of chlorine is to allow the removal of excess CH3NH3+ at a relatively low temperature. Moreover, Yu et al.31 reported that only a negligible amount of Cl atoms is present in the perovskite film after the annealing process. So it may be assumed that during annealing process there is formation of CH3NH3Cl gas,32 which escapes from the top of the surface of the perovskite layer. This represents an interpretation of the unsymmetrical chlorine distribution without any bias. GD-OES profile lines of Pb and N show no ionic migration under an applied bias in the polarization timescale less than 2 minutes (Fig. 3c, 3d, S2c, and S2d). These results are in accordance with the fact that the migration activation energies of Pb (2.31 eV) and MA (0.84 eV) ions vacancies are higher than the value of I ions vacancies (0.58 eV) as reported by Eames et al.23,33

Methods Solar cell fabrication and characterization. The ITO coated glass (from Xin Yan Tech.) is patterned by the wet etching process with zinc powder and HCl, cleaned with deionized H2O (DI water), acetone and isopropanol. The Cr and gold are deposited on ITO glass substrate for ohmic contact during J-V measurement. A PEDOT:PSS (50 nm thick) hole transport layer is deposited on ITO substrate via spin-coating process and heated at 120 °C for 20 minutes in N2 conditions. The perovskite solution composed of PbI2, PbCl2 and MAI with 1:1:4 molar ratio in N,N-dimethylformamide (DMF) is spun at 6000 r.p.m onto the PEDOT: PSS and heated at 80 °C for 2 hours in N2 conditions. The thickness of the perovskite film is 400 nm. A solution of 4 wt. % PCBM in chlorobenzene (CB) is then spun onto the perovskite as the hole blocking layer and the electron transport layer. The thickness of the PCBM layer is 50 nm. Finally, a 150 nm layer of Ag is vacuum-evaporated onto the PCBM as the cathode. The active area of the device is 0.28 cm2. J-V curves are recorded under illumination using a Keithley 2635 source-measure unit and a homemade acquisition program. The light source is an AM1.5 SolarCellTest 575 from ATLAS MTT, equipped with a metal halogen lamp of 100 mW.cm-2. GD-OES measurements. GD-OES analyses are performed using a GD Profiler 2 from HORIBA Jobin Yvon. The instrument is equipped with a RF-generator (at 13.56 MHz), a standard HORIBA Jobin Yvon glow discharge source with a cylindrical anode of 4 mm internal diameter and two optical spectrometers (a polychromator and a monochromator). The sample is mounted on an O-ring outside of the plasma chamber (Fig. 1a and S1) and acts as the negatively charged electrode. Samples are analyzed using the patented Ultra Fast Sputtering system, which consists in using 4% O2 mixed Ar as plasma gas34 to etch the organic-inorganic hybrid perovskite layer and organic layers (PEDOT:PSS and PCBM). The plasma is generated under these exact conditions: Ar-O pressure was set at 420 Pa and we used a 17 W applied power. A schematic set-up of the GD-OES experiment is shown in Fig. S1 (see SI). As the sample is continuously sputtered, the collected light reflects the temporal evolution of the sputtered species,35,36 therefore it is possible to obtain the depth-resolved elemental analysis, with nanometric depth resolution. Currently, GD-OES is already widely used to analyze various 8 ACS Paragon Plus Environment

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components of photovoltaic devices, such as metallic contacts, metal oxides, inorganic and organic materials.37-39 In summary, this GD-OES study has provided the direct experimental evidence of the halide (I and Cl) migration in the CH3NH3PbI3-xClx based perovskite films under applied bias. We verified that lead and MA ions are not migrating under the applied bias in the polarization timescale less than 2 minutes. We found that the ratio of fixed to mobile iodine saturates at 35 % and the average length of iodine migration is around 120 nm. All the chloride ions are mobile but they are not homogenously distributed in the hybrid perovskite layer without any bias: they are mostly located deeper inside the film closed to the PEDOT:PSS side. The migration behavior of each species observes through GD-OES is coherent with the activation energies reported so far in the literature.21-24 Based on GD-OES, this study gives a way for observing directly ionic movements in hybrid perovskite films. It makes a step forward in the quest of elucidating electrical phenomena usually observed in perovskite based solar cells like J-V hysteresis, external electric field screening or interfacial effects with electrodes.

Supporting Information The performance parameters of the PSCs; Schematic illustration of GD-OES measurement setup; Original GD-OES profile lines; GD-OES profile lines of I species; GD-OES intensity differences with and without applied voltage

Acknowledgements We would like to thank Université Paris-Saclay for the Ph.D. grant assigned to H. Lee.

References (1) Stoumpos, C. C.; Kanatzidis, M. G. The Renaissance of Halide Perovskite and TheirEvolution as Emerging Semiconductors. Acc. Chem. Res. 2015, 48, 2791–2802. (2) Docampo, P.; Bein, T. A Long-Term View on Perovskite Optoelectronics. Acc. Chem. Res. 2016, 49, 339−346. (3) 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. (4) Gao, P.; Graetzel, M. ; Nazeeruddin, M. K. Organohalide Lead Perovskites for Photovoltaic Applications. Energy Environ. Sci. 2014, 7, 2448-2463. (5) Graetzel, M. The light and shade of perovskite solar cells. Nat. Mater. 2014, 13, 838-842. (6) Green, M. A.; Ho-Baillie, A.; Snaith, H. J. The emergence of perovskite solar cells. Nat. Photonics 2014, 8, 506-514. (7) Liu, S.; Wang, L.; Lin, W.-C.; Sucharitakul, S.; Burda, C.; Gao, X. P. A. Imaging the Long Transport Lengths of Photo-generated carriers in Oriented Perovskite Films. Nano Lett. 2016, 16, 7925-7929. (8) Shahbazi, M.; Wang, H. Progress in research on the stability of organometal perovskite solar cells. Solar Energy 2016, 123, 74-87. 9 ACS Paragon Plus Environment

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Ionic transport in hybrid lead iodide perovskite solar cells. Nat. Commun. 2015, 6, 7497. (24) Yuan, Y.; Huang J., Ion migration in Organometal Trihalide Perovskite and Its Impact on Photovoltaic Efficiency and Stability. J. Acc. Chem. Res. 2016, 49, 286-293. (25) Yuan, Y.; Wang, Q; Shao, Y.; Lu, H ; Li, T ; Gruverman, Huang, J. Electric-FieldDriven Reversible Conversion Between Methylammonium Lead Triiodide Perovskite and Lead Iodide at Elevated Temperatures. Adv. Energy Mater. 2016, 6, 1501803. (26) Shao, Y.; Fang, Y.; Li, T.; Wang, Q.; Dong, Q.; Deng, Y.; Yuan, Y.; Wei, H.; Wang, M.; Gruverman, A.; Shield, J.; Huang, J. Grain boundary dominated ion migration in polycrystalline organic-inorganic halide perovskite films. Energy Environ. Sci. 2016, 9, 1752-1759. (27) Calado, P.; Telford, A. M.; Bryant, D.; Li, X.; Nelson, J.; O’Regan, B. C.; Barnes, P. R. F. Evidence for ion migration in hybrid perovskite solar cells with minimal hysteresis. Nat. Commun. 2016, 7, 13831. (28) Pisonero, J.; Fernández, B.; Pereiro, R.; Bordel, N.; Sanz-Medel, A.; Glowdischarge spectrometry for direct analysis of thin and untra-thin solid films. Trends Anal. Chem., 2006, 25, 11-18. (29) Sanchez, P.; Fernandez, B.; Menendez, A.; Pereiro, R.; Sanz-Medel, A.; Pulsed radiofrequency glow discharge optical emission spectrometry for the direct characterisation of photovoltaic thin film silicon solar cells. J. Anal. At. Spectrom. 2010, 25, 370-377. (30) Heo, J. H.; Han, H. J.; Kim, D.; Ahn, T. K.; Im, S. H. Hysteresis-less inverted CH3NH3PbI3 planar perovskite hybrid solar cells with 18.1% conversion efficiency. Energy Environ. Sci. 2015, 8, 1602-1608. (31) Dar, M. I.; Arora, N.; Gao, P.; Ahmad, S.; Graetzel, M.; Nazeeruddin, M. K. Investigation Regarding the role of chloride in organic-inorganic halide perovskite obtained from chloride containing precursors. Nano Lett. 2014, 14, 6991-6996. (32) Yu, H.; Wang, F.; Xie, F.; Li, W.; Chen, J.; Zhao, N. The role of chlorine in the formation process of CH3NH3PbI3-xClx perovskite. Adv. Funct. Mater. 2014, 24, 7102-7108. (33) Mosconi E. ; De Angelis F. Mobile Ions in Organohalide Perovskites: Interplay of Electronic Structure and Dynamics. ACS Energy Lett. 2016, 1, 182−188. (34) UFS European Pat. EP 2434275 A1. (35) King, B. V. Surface analysis methods in materials science, O’Connor, D. J., Sexton, B. A., Smart, R. St. C., Eds.; Springer: Verlag Berlin Heidelberg New York, 2003; pp 107-125. (36) Winchester, M. R.; Payling, R. Radio-frequency glow discharge spectrometry: A critical review. Spectrochimica. Acta Part B, 2004, 59, 607-666. (37) Ghanbari, N.; Waldmann, T.; Kasper, M.; Axmann, P.; Wohlfahrt-Mehrens, M. Inhomogeneous Degradation of Graphite Anodes in Li-Ion Cells : A Post Mortem Study Using Glow Discharge Optical Emission Spectroscopy (GD-OES). J. Phys. Chem. C 2016, 120, 22225-22234. (38) Mercier, D.; Bouttemy, M.; Vigneron, J.; Chapon, P.; Etcheberry, A. GD-OES and XPS coupling : A new way for the chemical profiling of photovoltaic absorbers. Appl. Surf. Sci. 2015, 347, 799-807. (39) Takahara, H.; Ishigami, R.; Kodama, K.; Kojyo, A.; Nakamura, T.; Oka, Y. Hydrogen analysis in diamond-like carbon by glow discharge optical emission spectroscopy. J. Anal. At. Spectrom. 2016, 31, 940-947. 11 ACS Paragon Plus Environment

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ACS Energy Letters


ACS Energy Letters

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Figure. S 4 GD-OES intensity differences with and without applied voltage for I (a) negative and (b) positive bias. The distance between the negative and the positive peaks indicates the average length of the ionic migration. Indeed, as can be seen in Fig. 276x107mm (150 x 150 DPI)


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Direct Experimental Evidence of Halide Ionic Migration under Bias in

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