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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Effect of Halide Ion Migration on the Electrical Properties of Methylammonium Lead Tri-Iodide Perovskite Solar Cells Heejae Lee,† Sofia Gaiaschi,‡ Patrick Chapon,‡ Denis Tondelier,† Jean-Eric Boureé ,† Yvan Bonnassieux,† Vincent Derycke,§ and Bernard Geffroy*,§,† †

LPICM, CNRS, Ecole Polytechnique, Institut Polytechnique de Paris, route de Saclay, 91128 Palaiseau, France Horiba Jobin Yvon S.A.S., 16-18 rue du canal, 91165 Longjumeau Cedex, France § LICSEN, NIMBE, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France Downloaded via UNIV OF SOUTHERN INDIANA on July 24, 2019 at 11:21:21 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Ionic migration in halide perovskite materials is now well recognized to affect the electrical properties of perovskite solar cells. Native point defects such as vacancies are considered as responsible for migration of ions. In order to interpret the unusual behavior of current−voltage curves in methylammonium lead tri-iodide perovskite (MAPI) solar cells, we combined two independent experiments: temperaturedependent conductivity measurements (213−363 K) under dark and glow discharge optical emission spectroscopy (GDOES) under electrical bias conditions. From the film conductivity measurements, we observed two different conduction regimes: an ionic conduction regime at temperatures higher than 263 K, associated with an activation energy of 0.25 eV, and an electronic conduction regime below 263 K. At room temperature, both conductivities are around 10−7 S/cm. From GD-OES, we observed directly a reversible migration of iodide ions under positive or negative bias (minute time scale) at room temperature and deduced an iodide ion diffusion coefficient of 1.3 × 10−12 cm2·s−1 and a mobility of 5 × 10−11 cm2·V−1·s−1.

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CsPbBr3 and CH3NH3PbBr3,8 indicate that lead−halide perovskite materials (both hybrid and inorganic) are subject to several types of disorders with different nature. In particular, lead halide perovskites are recognized as mixed conductors possessing both electronic and ionic conductivity, as was noted by many authors.9−13 Moreover, because structural fluctuations in perovskites are decorrelated from point defects,8 we will henceforth make a difference between dynamical disorder with typically femtosecond time scale and quasi-static disorder with second time scale. Numerous studies have been devoted to the high sensitivity of halide perovskites to moisture, ultraviolet light, or thermal stress.9,14−21 If we take as an example the most studied methylammonium lead tri-iodide (MAPI), this compound is prone to degradation by humidity due to the hygroscopic nature of the alkylammonium cation present in the unit cell.15,22 Moisture or oxygen ingress corresponds to extrinsic instability which can be greatly limited using encapsulation with a thin layer23 or more recently using the engineering of multidimensional junctions (2D/3D perovskite interface).24 On the contrary, intrinsic instability refering to both thermal instability and instability under light illumination is much more

INTRODUCTION Organic−inorganic metal halide perovskites have rapidly emerged as a new class of materials with remarkable optoelectronic properties, notably as solar cells with power conversion efficiency (PCE) reaching 23% under AM1.5 conditions, so approaching the efficiency of the sophisticated monocrystalline silicon solar cells. However, contrary to the silicon cells, which account for 95% of the global market, the hybrid perovskite cells are poorly stable under operating conditions, which is the key issue for a future commercialization. First let us remind that the physico-chemistry of the hybrid perovskites is complex and far from being understood. These low-cost materials behave like semiconductors with intriguing charge transport properties. On the basis of several experiments,1−3 apparent contradictory results have been observed like high lifetimes and long charge carrier diffusion lengths4 simultaneously with moderate mobilities5 and low-temperature preparation simultaneously with low defect density. All these facts and the observation that CsPbBr3 solar cells are as efficient as CH3NH3PbBr3 cells6 lead to approach the outstanding properties of these hybrid perovskites by assuming that the inorganic part dominates the optoelectronic behavior. The early observation of ionic conduction in the perovskitetype halides CsPbCl3 and CsPbBr3,7 together with the recent observation of large anharmonic thermal fluctuations in both © XXXX American Chemical Society

Received: May 16, 2019 Revised: June 28, 2019 Published: July 3, 2019 A

DOI: 10.1021/acs.jpcc.9b04662 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Perovskite solar cell p-i-n architecture. (a) Layer stack on glass substrate. (b) Photo of the final device. (c) Energetic diagram of the solar cell.

difficult to control.21 MAPI is unstable at temperatures higher than 85 °C18 and is thermally decomposed at 200 °C.25 The formation energy of 0.11 eV calculated in the tetragonal phase below 327 K26 suggests a “soft matter” nature,18 compatible with the large anharmonic thermal fluctuations already mentioned.8 In the present paper, we show that current−voltage hysteresis observed at room temperature in MAPI since the early days27−29 can be partly attributed to ion migration. We have based our conclusion on two independent experiments: on one side temperature-dependent current−voltage characteristics of solar cells under dark and on the other side direct observation of ion migration in solar cell under bias by glow discharge optical emission spectroscopy (GD-OES) measurements at room temperature.30 The idea of working in dark conditions comes from the fact that we can get rid of the photogenerated carriers (electrons and holes) which would hide the contribution from mobile ions in the material. We explain how the halide ionic migration influences the electrical characteristics of halide perovskite solar cells (PSCs) and may be responsible for the J−V hysteresis under dark.

atmosphere in order to form the CH3NH3PbI3 (MAPI) layer. The thickness of the perovskite thin film is around 400 nm. A solution of 4 wt % PCBM in chlorobenzene is then spun onto the perovskite layer in N2 atmosphere. The thickness of the PCBM layer is around 50 nm. Finally, a 100 nm layer of Ag is deposited on top of PCBM layer as the cathode by using a thermal-vacuum-evaporation process. The active area of the device is 0.28 cm2. A picture of the final device is shown in Figure 1b. The solar cells were not encapsulated because the J−V measurement system is located in a glovebox. The thickness of the different layers was measured with a Veeco Dektak 150 profilometer. Solar Cell Characterization. The current−voltage (J−V) characteristics of perovskite solar cells are measured in N2filled glovebox using a Keithley 2635 source-meter unit both in dark and under illumination conditions. The light source is an AM1.5 solar simulator (SCP575PV SolarCellTest 575 from ATLAS MTT) with a metal halogen lamp calibrated with a photodiode at 100 mW/cm2. Conductivity Measurements. The MAPI film was deposited on glass substrate using the same synthesis process described before. Aluminum electrodes were then deposited on the MAPI film in order to measure the in-plane conductivity versus temperature (temperature range 213−363 K) by placing the sample under vacuum in a N2-cooled cryostat with a temperature controller. The electrical measurements were performed with a Keithley 2635 source meter. J−V measurements of solar cells as a function of temperature were also performed in the same conditions. GD-OES Measurements. The glow discharge optical emission spectroscopy (GD-OES) analyses were performed using a HORIBA Jobin Yvon GD-Profiler 2. The instrument consists of a low-pressure plasma source (glow discharge) and a fast-optical detection (Figure S1 in SI). GD-OES relies on the controlled erosion of the material by a plasma and the simultaneous optical detection of the elemental species excited by the plasma. GD-OES provides fast elemental chemical analysis of the material as a function of the etching depth. A RF-generator (at 13.56 MHz) and a standard HORIBA Jobin Yvon glow discharge source with a cylindrical anode of 4 mm internal diameter are used for generating the plasma. The solar cell is mounted on an O-ring at one side of the plasma chamber and used as the cathode. The plasma conditions used for the analysis of the solar cells structure are 420 Pa and 17 W applied power. For etching the silver electrode, the organic− inorganic hybrid perovskite layer and organic layers (PCBM and PEDOT:PSS), 4% oxygen mixed with argon is used as plasma gas. As the sample is continuously sputtered, the collected light reflects the temporal evolution of the sputtered species, therefore it is possible to obtain the depth-resolved

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EXPERIMENTAL SECTION Solar Cell Fabrication. The perovskite solar cell structure consisting of an inverted planar p-i-n configuration is the following: Glass/ITO/PEDOT:PSS/halide perovskite/ PCBM/Ag (Figure 1a). The photogenerated electrons are collected by the silver electrode through the fullerene derivative, 6,6′-phenyl-C61-butyric acid methyl ester (PCBM) layer acting as an electron transport layer (ETL) and the holes are collected by the ITO electrode through the PEDOT:PSS thin film working as a hole transport layer (HTL). The ITO coated on top of glass is first patterned by wet etching process using zinc powder and HCl solution. The ITO-coated glass is purchased from Xin Yan Tech. Deionized water (DI water), acetone, and isopropanol are used for cleaning the patterned ITO right after the wet etching process. Cr and gold are successively deposited on ITO and glass substrate for getting ohmic contacts for the J−V measurements (Figure 1b). The PEDOT:PSS layer is deposited on ITO substrate (50 nm thickness) by a spin-coating process and annealed at 120 °C for 20 min in N2. The perovskite precursor solution is prepared by mixing PbI2, PbCl2, and MAI with 1:1:4 molar ratio in N,Ndimethylformamide (DMF). Recently Stone et al. have shown that addition of PbCl2 in the precursor solution improves the optoelectronic properties of the perovskite film by slowing down the conversion of precursor into perovskite.31 Sixty microliters of the solution is spun at 6000 r.p.m on top of the PEDOT:PSS layer and annealed at 80 °C during 2 h in N2 B


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sharply and this could be correlated with the tetragonal-cubic phase transition occurring around 340 K in MAPI material. Temperature-Dependent Conductivity of Perovskite Films. As mentioned earlier, halide perovskites are known to exhibit ionic conduction due to anion migrations.7 In order to investigate this point further, it is essential to measure the electrical properties as a function of the temperature. It is wellknown that in ionic crystals, the ionic conduction depends essentially on the motion of vacancies, the conductivity increasing exponentially with temperature. In Figure 4, the conductivity of the perovskite film measured in the dark is plotted versus 1/T in the Nernst−Einstein formalism.32 Two conduction regimes are clearly observed: ionic conduction regime at high temperature and electronic conduction regime at low temperature. At high temperature, the conductivity of the film is dominated by ionic transport because the electronic conduction is weak in dark (low carrier density). However, the ionic conduction decreases by decreasing the temperature and become negligible at low temperature where the electronic conduction is dominant. The change from ionic to electronic conduction is found at 263 K. This temperature value is consistent with previous results reported in literature for 3D MAPI film.33 Remarkably, this temperature corresponds to the onset of voltage shift discussed previously. The ion activation energy (Ea) can be derived from the temperature-dependent electrical conductivity (σ) using the Nernst−Einstein relation

elemental analysis with nanometric depth resolution. However, the GD-OES technique is destructive and a crater is made in the sample after analysis. Furthermore, the GD-OES measurements can be performed only at room temperature and under dark because of technical limitations.

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RESULTS AND DISCUSSIONS Solar Cell Performance. Table 1 shows the photovoltaic performance of the solar cells. The average power conversion Table 1. Photovoltaic Performance of the Perovskite Solar Cell Used in This Study

average performance

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

20 (±2)

0.92 (±0.02)

64 (±4)

11.8 (±0.8)

efficiencies (PCE) is 11.8% under 1 sun illumination. As shown in Table 1 and Figure 2b, the reproducibility of the solar cell fabrication process is quite good with a margin error below 7%. This is notably due to the good crystallization of the perovskite film as shown in Figure 2a with average grain size exceeding 500 nm. Reproducibility is critical in the context of this study, because the GD-OES technique is destructive and needs a new sample per measurement. V0 Shift Characterization. Figure 3a shows the room temperature J−V characteristics of the solar cell under illumination and in the dark for two different starting biases (2.5 and 1 V respectively). Under illumination, the J−V curve is not affected by the starting voltage. Conversely, in the dark a voltage shift (recorded when the current is zero) is observed when the starting voltage is 2.5 V (black dot-solid line). In dark conditions, the voltage is expected to be at zero when the current is zero as observed when the initial voltage is 1 V (red dot-solid line). In Figure 3b, the voltage shift is plotted as a function of the initial applied voltage at room temperature. A shift is observed only for positive biases higher than 1.5 V. In Figure 3c, the voltage shift is reported as a function of the temperature in forward (−1 to 1.5 V) and in reverse (1.5 V to −1 V) scan directions. Figure 3c is obtained from the experimental J−V curves measured in dark from 213 to 360 K with a scanning rate of 125 mV/s (Figures S2 and S3 in SI). The voltage shift is observed only in the reverse scan direction (starting with positive bias) consistent with the result of Figure 3b. The voltage shift starts to increase when the temperature is above 263 K. Above 340 K, the voltage shift increases more

ij −E yz σ ·Τ = σ0 expjjj a zzz j kBT z k {

(1)

where kB is the Boltzmann constant, σ0 is a constant, and T is the absolute temperature. The activation energy of ionic conductivity is a quantitative characterization of the rate of ion migration and is sensitive to the crystallinity of the MAPI perovskite, which depends on the fabrication process. From the Arrhenius plot in Figure 4, the activation energy Ea of the ions is 0.253 eV in the ionic conduction regime (high temperature). This value is comparable to 0.19 eV reported by Eames et al.12 The activation energy of electrons (holes) deduced from the conduction regime at lower temperature is 0.112 eV. This value is comparable to the 0.09 eV reported by Zhao et al.21 It is likely that, similar to migration of Br− ions in CH3NH3PbBr3 and based on nudged elastic band density functional theory (DFT) simulations, the I− migration benefits from CH3NH3+ alignment under the electric bias.34

Figure 2. (a) SEM image of the perovskite film after synthesis. (b) Distribution of PCE values for a series of perovskite solar cells. C


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Figure 3. (a) J−V characteristics of the perovskite solar cells in dark and under 1 sun illumination as a function of the initial applied voltage. (b) Voltage shift (for J = 0 mA/cm2) as a function of the applied initial voltage. (c) Voltage shift as a function of the temperature for forward (−1 V to 1.5 V) and reverse (1.5 V to −1 V) scan directions.

to the silver cathode side and the longer sputtering time to the ITO anode side. The value of the sputtering time is proportional to the position of the ions in the perovskite layer. Either 1.5 V (Figure 5a) or −1.5 V (Figure 5b) was applied in the PSCs devices during 30 s before etching in order to induce halide ions displacements as we have shown previously.30 The red curves in Figure 5a,b show one broad peak centered around 38 s of sputtering time without applying the bias (GD-OES measurement done at 0 min). However, after applying the bias during 30 s (GD-OES measurement done at 1 min.), a second peak shifted at higher sputtering time for 1.5 V bias and at lower sputtering time for −1.5 V bias is observed. These changes of the profile lines are attributed to iodide ion migration due to the applied bias.30 These second peaks begin to shrink after discontinuance of voltage supply and disappear after 3 min when the device was positively biased and in 4 min when the device was negatively biased. GD-OES results show the reversibility of iodide ionic migration, that is, when the applied voltage is suppressed, the iodide ions return to their initial position within the perovskite layer. The same behavior is also observed for the chloride ions (blue curves in Figures 5a and 5b) in the perovskite film. The initial peak is at 50s of sputtering time before applying the voltage. However, we observe the movement of the peaks when a bias is applied to the device. The shifted peaks are at 54 and 42 s of sputtering time under positive bias (+1.5 V) and negative bias (−1.5 V), respectively. We attribute these peak movements to the chloride ion migration due to the applied bias. These shifted peaks get back to their initial position (sputtering time at 50 s) after 1 min, which is shorter than for iodide ions. We note that the chloride ion signal is much lower than the iodide ion signal suggesting a lower concentration of chloride ions in the MAPI film. Contrary to iodide ions, for chloride ions we observe only one peak moving as a function of

Figure 4. Conductivity of the MAPI film as a function of the temperature.

The ionic (σi) and electronic (σe) conductivity of the halide perovskite film can be deduced from the results in Figure 4. At 300 K in the dark, σi and σe are in the same range: 1.5 × 10−7 and 10−7 S/cm, respectively. Reversibility of Halide Ion Migration. As shown above, a characteristic temperature of around 263 K is observed for V0 shift as well as for the transition between the ionic and electronic conduction regimes. This correlation between two distinct measurements strongly suggests that ion movements within the MAPI material is at the origin of the observations. In order to investigate further ionic migration in our halide perovskite solar cells, we performed GD-OES measurements at room temperature. Unfortunately, GD-OES cannot be performed as a function of temperature for technical reasons. Figure 5 shows GD-OES profile lines of iodide and chloride ions versus sputtering time for different measurement times (from 0 to 4 min). Considering the plasma etching direction (from silver to ITO), the shorter sputtering time corresponds D


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Figure 5. Chronological record of iodide and chloride ions distribution in MAPI layer using a GD-OES system. (a) Positive (+1.5 V) or (b) negative (−1.5 V) bias is applied during 30 s in order to generate the ionic migration. The red and blue profile lines correspond to iodide and chloride ions, respectively.

used in J−V measurement, the slow recovering shown here could explain why the initial applied voltage is critical for dark J−V performance. As shown in our previous article30 we can estimate the average distance of iodide ion displacement by considering the difference between the sputtering time for the peak before and after applying the bias, respectively. The average displacements for iodide ions are around 100 and 70 nm for an applied bias of 1.5 V and −1.5 V, respectively. We also reported that the iodide ion motion is weaker for the negative polarization than for the positive one.30 Taking into account the imposed bias voltage, the evolution of the I− distribution (Figure 5) and the perovskite layer thickness, an estimation of the diffusion coefficient of D ∼ 10−12 cm2·s−1 and of the mobility of 5 × 10−11 cm2·V−1·s−1 can be deduced for the iodide ions at room temperature. These values should be compared with the bromide mobility of 3 × 10−10 cm2·V−1·s−1

the applied bias. This observation means that all the chloride ions are mobile in the halide perovskite film. This is not surprising since it is largely accepted that the chloride ions do not integrate within the MAPI structure and it is the reason why they are all free to move and can migrate monotonically with the applied voltage. This behavior can be explained by the fact that the chloride ions are not distributed homogeneously within the perovskite layer but located deeper inside the perovskite layer. It is indeed reported that chlorine is released from the surface of the solar cell after formation of the perovskite film.31 The reversibility of ionic migration discussed above is consistent with previous articles reporting that the recovery time of the short circuit current (JSC) due to the ionic migration is around 100 s for both directions of applied bias. Given the short voltage scanning time (around 40 s) usually E


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The Journal of Physical Chemistry C deduced from nanoprobe X-ray fluorescence measurements in CH3NH3PbBr3 crystal under bias.34 As the migration of the halide ions is rather low, the built-in electric field inside the perovskite layer evolves with time in a minute time scale. So, the concentration of electrons and holes inside the perovskite layer varies also with time, which explains the observed changes of the J−V characteristics with the scanning rate. The built-in field is not totally canceled when changing the direction of biasing, which is at the origin of the voltage shift observed for reverse scan.

(3) Brenner, T. M.; Egger, D. A.; Kronik, L.; Hodes, G.; Cahen, D. Hybrid Organic-Inorganic Perovskites − Low-Cost Semiconductors with Intriguing Charge Transport Properties. Nat. Rev. Mater. 2016, 1, 15007. (4) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; Leijtens, T.; Hertz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342, 341− 344. (5) Brandt, R. E.; Stevanovic, V.; Ginley, D. S.; Buonassisi, T. Identifying Defect-Tolerant Semiconductors with High MinorityCarrier Lifetimes: Beyond Hybrid Lead Halide Perovskites. MRS Commun. 2015, 5, 265−275. (6) Kulbak, M.; Cahen, D.; Hodes, G. How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr3 Cells. J. Phys. Chem. Lett. 2015, 6, 2452−2456. (7) Mizusaki, J.; Arai, K.; Fueki, K. Ionic Conduction of the Perovskite-type Halides. Solid State Ionics 1983, 11, 203−211. (8) Yaffe, O.; Guo, Y.; Tan, L. Z.; Egger, D. A.; Hull, T.; Stoumpos, C. C.; Zheng, F.; Heinz, T. F.; Kronik, L.; Kanatzidis, M. G.; et al. Local Polar Fluctuations in Lead Halide Perovskite Crystals. Phys. Rev. Lett. 2017, 118, 136001. (9) Leijtens, T.; Eperon, G. E.; Noel, N. K.; Habisreutinger, S. N.; Petrozza, A.; Snaith, H. J. Stability of Metal Halide Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1500963. (10) Azpiroz, J. M.; Mosconi, E.; Bisquert, J.; De Angelis, F. Defect Migration in Methylammonium Lead Iodide and its Role in Perovskite Solar Cell Operation. Energy Environ. Sci. 2015, 8, 2118−2127. (11) Yang, T. Y.; Gregori, G.; Pellet, N.; Grätzel, M.; Maier, J. The Significance of Ion Conduction in a Hybrid Organic-Inorganic LeadIodide-Based Perovskite Photosensitizer. Angew. Chem., Int. Ed. 2015, 54, 7905−7910. (12) 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. Commun. 2015, 6, 7497. (13) Meloni, S.; Moehl, T.; Tress, W.; Franckevičius, M.; Saliba, M.; Lee, Y. H.; Gao, P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Rothlisberger, U.; et al. Ionic Polarization-Induced Current-Voltage Hysteresis in CH3NH3PbX3 Perovskite Solar Cells. Nat. Commun. 2016, 7, 10334. (14) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable InorganicOrganic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764−1769. (15) Leijtens, T.; Bush, K.; Cheacharoen, R.; Beal, R.; Bowring, A.; McGehee, M. D. Towards Enabling Stable Lead Halide Perovskite Solar Cells; Interplay between Structural, Environmental, and Thermal Stability. J. Mater. Chem. A 2017, 5, 11483−11500. (16) Leguy, A.; Hu, Y.; Campoy-Quiles, M.; Alonso, M. I.; Weber, O. J.; Azarhoosh, P.; Van Schilfgaarde, M.; Weller, M. T.; Bein, T.; Nelson, J.; et al. The Reversible Hydration of CH3NH3PbI3 in Films, Single Crystals and Solar Cells. Chem. Mater. 2015, 27, 3397−3407. (17) Hoke, E. T.; Slotcavage, D. J.; Dohner, E. R.; Bowring, A. R.; Karunadasa, H. I.; McGehee, M. D. Reversible Photo-Induced Trap Formation in Mixed-Halide Hybrid Perovskites for Photovoltaics. Chem. Sci. 2015, 6, 613−617. (18) Conings, B.; Drijkoningen, J.; Gauquelin, N.; Babayigit, A.; D’Haen, J.; D’Olieslaeger, L.; Ethirajan, A.; Verbeeck, J.; Manca, J.; Mosconi, E.; et al. Intrinsic Thermal Instability of Methylammonium Lead Trihalide Perovskite. Adv. Energy Mater. 2015, 5, 1500477. (19) Sutton, R. J.; Eperon, G. E.; Miranda, L.; Parrott, E. S.; Kamino, B. A.; Patel, J. B.; Hörantner, M. T.; Johnston, M. B.; Haghighirad, A. A.; Moore, D. T.; et al. Bandgap-Tunable Cesium Lead Halide Perovskite with High Thermal Stability for Efficient Solar Cells. Adv. Energy Mater. 2016, 6, 1502458. (20) Bush, K. A.; Bailie, C. D.; Chen, Y.; Bowring, A. R.; Wang, W.; Ma, W.; Leijtens, T.; Moghadam, F.; McGehee, M. D. Thermal and Environmental Stability of Semi-Transparent Perovskite Solar Cells

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CONCLUSIONS In summary, we demonstrated the effect of iodide migration in methylammonium lead tri-iodide perovskite solar cells on the current−voltage characteristics under dark. A shift of the J−V characteristics is observed for reverse scan direction above 263 K, which is found to be the frozen temperature for iodide ion migration. Activation energy of 0.25 V for iodide ions migration was derived from Arrhenius plot for crystalline MAPI films associated with a conductivity of around 1.5 × 10−7 S/cm at room temperature under dark. Using GD-OES measurements, it is shown that the iodide ion motion is induced by the electric field across the perovskite solar cell and they return to their initial position few minutes after removing the bias. On the basis of these results, a mobility of 5 × 10−11 cm2·V−1·s−1 and a diffusion coefficient of ∼10−12 cm2·s−1 have been deduced for iodide ion migration at room temperature.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b04662. Glow discharge optical emission spectroscopy (GDOES) working principle; dark J−V characteristics of MAPI solar cells as a function of temperature and scanning direction (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone number: +33 1 69 33 43 82. E-mail address: bernard. geff[email protected]. ORCID

Vincent Derycke: 0000-0002-3272-9694 Bernard Geffroy: 0000-0001-8654-6140 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Université Paris-Saclay for the Ph.D. grant assigned to H. Lee. B.G. acknowledge the French Research Agency for financial support (Persil project, ANR-16CE05-0019).

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REFERENCES

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