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Investigation of Ion Mediated Charge Transport in Methylammonium Lead Iodide Perovskite Joydeep Dhar, Sayantan Sil, Arka Dey, Dirtha Sanyal, and Partha Pratim Ray J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01047 • Publication Date (Web): 15 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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The Journal of Physical Chemistry

Investigation of Ion Mediated Charge Transport in Methylammonium Lead Iodide Perovskite Joydeep Dhar,† Sayantan Sil,† Arka Dey,† Dirtha Sanyal*,‡ and Partha Pratim Ray*,† †

‡

Department of Physics, Jadavpur University, Kolkata 700032, India.

Variable Energy Cyclotron Centre, 1/AF, Bidhannagar, Kolkata 700064, India. Corresponding Author’s E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT We have investigated the origin of ionic conductivity in methylammonium lead iodide (MAPbI3) by positron annihilation lifetime spectroscopy (PALS), supplemented by coincidence Doppler broadening spectroscopic (CDBS) techniques which reveal the presence of methylammonium (MA+) defects in the perovskite crystal lattice. Crystallinity and the defect concentration vary with the perovskite synthesis process which in turn governs the magnitude of ionic conductivity. Single crystalline perovskite contains lesser defects with equal probability of developing both, cationic and anionic (halide) vacancies; whereas polycrystalline perovskite sample developed through mechanical process carries mainly cationic i.e. MA+ vacancy in its crystal lattice as indicated by DC polarization experiment.


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INTRODUCTION In recent year, the methylammonium lead halide based perovskite materials have demonstrated a stupendous growth in photovoltaic (PV) performances as power conversion efficiency (PCE) exceeding 20% in a very short period of five years owing to their phenomenal material and electronic properties.1 But, the anomalous hysteresis observed in the photocurrent-voltage (J-V) measurements has complicated the performance analysis and opened up the scope of research in determining the origin of such intriguing behavior, critically observed for organo lead halide perovskite solar cell.2-4 Several studies have quite definitively established the intimate relation between ionic motions with the photocurrent hysteresis measured under the electric field.3,

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The ion migration phenomenon in the perovskite has been well explored in the literature.9-12 The ion diffusion has significant influence on their electrical properties. Very high static dielectric constant of ~70 having inverse relationship with frequency is an indicator of ionic character of hybrid organic-inorganic perovskite material.13 Similarly, giant permittivity (106) at low frequency of 0.1 Hz under one sun illumination originates from the ion transport.14 Furthermore, substantially lower field effect mobility measured in field effect transistor (FET) device than its Hall mobility is because of screening of the gate induced charge carrier at the dielectricsemiconductor interface by the mobile ions in the perovskite layer.15 The ion migration is also closely related to the device stability as reported by Bag et al.16 The importance of differentiating the electronic component of charge transport from its ionic part is well documented,17 but macroscopic origin of ionic conductivity is quite contradictory.12, 18-20

The migrating species in perovskite semiconductor is differently identified by various

research groups. The diffusion of either proton21 or methylammonium22 or halide ion11, 23 has been predicted with respective studies but without any coherency. Therefore, it has become


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imperative to determine the exact nature of vacancy site and establish the correlation between ionic conductivity with material properties. PALS, a well-known non-destructive nuclear solid state technique, has been widely used to identify defects and their chemical nature in different solids.24-25 The positron lifetime is inversely proportional to the electron density at the annihilation site. Hence by measuring the sub nano-second positron lifetime one can study the electron density distribution in the studied material.26-27 Since the electron density at the cation defect sites are lower compared to the electron density of the bulk of the material the lifetime of positrons annihilated at the defects is larger compared to the annihilation of positron in the bulk of the material. Besides, from CDB spectrum the change in the momentum of valence electron of the annihilating element can be determined. Thus, we can identify the nature of the defect site by measuring the orbital energy of the neighboring atom whose valence electron participates in the annihilation process. For the first time, we have used PALS along with CDBS to determine the relative defect concentration and have observed the presence of similar type of cationic defect in MAPbI3 irrespective of the material synthesis procedure. DC polarization measurements also corroborate our observation regarding the nature of the defect sites. RESULTS and DISCUSSION In this report, MAPbI3 was prepared in two different methods; synthesized from its basic precursors i.e. lead iodide (PbI2) and methylammonium iodide (MAI). In the first synthetic protocol, single crystal of PbI2 was converted into the MAPbI3 single crystal by reacting with MAI dissolved in isopropanol (IPA),28 whereas, the second synthesis method consists of mechanical grinding of two precursors to form MAPbI3,29 named as SC and Mech, respectively. According to the previous reports, following above mentioned synthesis procedures the


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synthesized two perovskite samples SC and Mech are single crystalline and polycrystalline in nature, respectively. Since, crystallinity of the material varies the density of the lattice defects which acts as ion transport pathway for ionic conductivity; we have chosen two samples with varied crystallinity to investigate the influence of defects concentration on the charge transport properties of CH3NH3PbI3 under dark condition. The single crystal to single crystal conversion procedure was recorded on video and has been provided in the supporting information. Material purity was confirmed from their powder X-ray diffraction (PXRD) pattern (Figure S1). Both of them exhibit almost identical diffraction pattern indicating they are identical crystallographically. The peak position suggests that both follow tetragonal crystal packing. Optical absorption spectra of two perovskite samples are shown in Figure 1. The optical bandgap increases for the sample Mech (1.55 eV) with respect to SC with band gap of 1.53 eV. We also notice a sharp peak at the absorption onset for SC corroborating its excitonic characteristic which is absent for the sample with mechanical grinding.30 The variation in the absorption property for the two samples with same chemical composition can be explained on the basis of their structural features. Field emission scanning electron microscopic (FESEM) images (Figure 2) unequivocally points out that the large differences in the particle size of the two perovskite samples is the origin for the changes observed in their optical properties. The size of the single crystal of MAPbI3 ranges from micron to millimeter (Figure 2a). On the contrary, for Mech, it is only few hundreds of nanometer (Figure 2b). Optical image of millimeter sized single crystal is shown in Figure 1. It is reported earlier that larger crystal with higher exciton binding energy imparts stability of the exciton as compared to smaller sized particle and such excitonic transition was recorded for larger crystallites, but only at low temperature.30-31 The presence of excitonic peak in sample SC indicates that the single crystallinity i.e. material without grain boundary


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having long range order is responsible for exhibiting the excitonic characteristics. The exciton binding energy is high for the single crystals as compared to the polycrystalline CH3NH3PbI3.30

Figure 1. (Left) UV-vis absorption onset for SC and Mech. (Right) Optical image of perovskite single crystal ranging over millimeter length-scale. Single crystal to single crystal transformation for the sample SC formation was confirmed from the transmission electron microscopic (TEM) images and selected area electron diffraction (SAED) pattern (Figure S2). The structural features as well as the SAED pattern completely change as PbI2 single crystal converts into the SC, showing hexagonal crystal packing for PbI2 and tetragonal for MAPbI3. But, polycrystalline nature of sample Mech is reflected from its SAED pattern (Figure S2).

Figure 2. FESEM images of SC (a) and Mech (b) samples. The micrometer sized rod-like single crystals were observed for SC, while aggregated nanoparticles were found for the sample Mech.


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For both the samples, significant variation in DC conductivity was observed when I-V characteristics were recorded within a potential window of ± 20 V using silver electrodes under dark conditions in their pellet form (Figure 3a). Cross-sectional FESEM images of the pellets prepared from two samples are shown in Figure S3 (a, b). The surface morphology and roughness of the pellets were studied by atomic force microscopy (AFM) (Figure S3 c, d). The surface of the pellet from sample SC was very smooth having some nanostructure and the roughness was below 10 nm. On the other hand, surface of the pellet from sample Mech was composed of smaller nanoparticles and the surface of the pellet was little rough with roughness ranges from 60-100 nm due to the presence of lot of interparticle edges. Sample Mech exhibited more than two order higher conductivity (3 ×10-6 S/cm) as compared to SC which has a conductivity of 5.5 ×10-9 S/cm. The operational stability of the samples during DC conductivity measurement was checked by performing repeated I-V measurement applying ± 20 V and ± 5 V alternately (Figure S4). We observed current increases marginally at ± 20 V scan and DC conductivity remained almost the same. The material stability was further assessed by collecting the optical microscopic images before and after the applying the voltage (Figure S5 a, b). Pellet did not show any sign of degradation. To see the actual colour changes that might have happened during degradation process we added few drops of water on the sample. The black coloured MAPbI3 immediately turned yellow (Figure S5 c). After sometime while water slowly evaporates we notice the black colour reappears due to the regeneration of MAPbI3 (Figure S5 d). The AC impedance measurement (Figure 3b) also followed the similar trend as observed in DC conductivity, i.e. SC had higher charge transfer resistance (RCT ~ 3.3 ×106 Ω) as compared to 9.6 ×104 Ω for Mech, calculated after fitting their Nyquist plots. For both the samples, a common


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feature noticed in impedance measurement is that the existence of Warburg element (WS) in the Nyquist plot at low AC frequency indicating ion diffusion to operate simultaneously along with electronic charge transport under the given electric field.16, 32-33 The electrical circuit elements are shown in the inset of Figure 3b. It was previously reported that the ionic component of the electrical conductivity can be successfully separated from its electronic counterpart by using ion blocking electrodes.34-35 Here, we have used graphite electrodes which block ion transfer to measure the electrical conductivity in AC and DC modes originating only from electronic character for both the perovskite materials. As expected, the WS vanishes and we observe only one semicircle in the Nyquist plot (Figure 3d) indicating ion transfer to the surface of the electrodes is prevented. As a consequence, the conductivity decreases and resistivity increases for both the samples (Figure 3c, d). The electrical conductivity reduces to 7.6×10-10 and 5.0×10-8 S/cm for SC and Mech, respectively from their initial conductivity value as mentioned previously. This can be clearly visualized from the Nyquist plots. The RCT value increases to 2.2 ×107 and 4.9 ×106 Ω for SC and Mech and thus, the change in DC conductivity exactly matches with the variation in frequency dependent resistance values. Substantial fall in electrical conductivity indicates that the ionic conductivity contributes significantly towards the total conductivity for both the perovskite samples, but at different magnitudes under dark condition. Using graphite electrodes, the decrease in electrical conductivity for SC and Mech was almost 85% and 99%, respectively.


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Figure 3. Dark current vs. voltage (I-V) (a, c) and Nyquist (b, d) plots for SC and Mech with silver (a, b) and graphite (c, d) electrodes. I-V plots for SC have been enlarged and shown in the inset of figure (a) and (c). The magnified part of Nyquist plot in figure (b) indicated by the arrow mark represents the sample Mech and is shown in the inset along with their common equivalent circuit. The fitted data points are shown in green over the respective Nyquist plots. Like Kim et al., we also observed particle size dependent capacitance for organo halide perovskite material (Figure S6, S7).36 But, capacitance and dielectric constant decreases for both the samples and the values become similar when silver electrodes were replaced with graphite (Figure S7). The observed large variation in frequency dependent capacitance and corresponding low frequency dielectric constant by changing silver electrodes with a pair of graphite electrodes


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indicates that the electrode polarization is operating at the silver-perovskite interfaces for both the samples.37 This is in general observed for ionic electrolytes38 and hence, proves the ionic nature of perovskite material. By varying the contact area and the thickness of the pellets the frequency dependent capacitance and dielectric constants were measured. From frequency dependent capacitance plot, the real part of dielectric constant was plotted against frequency in Figure S6. From the linear region the measured dielectric constants were 540 and 20 for Mech and SC samples arising due to the material polarization.32, 39 High frequency (107 Hz) dielectric constants of 52 and 18 for Mech and SC, respectively measured with silver electrodes reduced to 3.4 when measurement was performed with ion-blocking graphite electrodes. This suggests that material and electrode polarization significantly eliminated at higher frequency and with the use of graphite electrodes, respectively. The ionic transport for perovskites arises from the defects mediated hopping of ions under a potential gradient, as studied theoretically and experimentally.9, 11, 18, 22, 40-41 Following Kröger and Vink notation, there are three probable defect forming reactions maintaining mass and charge balances as shown below. The activation energy for Pb2+ migration is comparatively larger than the CH3NH3+ (MA+) or I- ion as estimated from different theoretical calculations.9, 11, 20

Hence, the probability of defect formations due to migration of MA+ and/or I- are the most

plausible one. nil

V ' MA + V' ' Pb + 3V I + MAPbI3

(1)

nil

V ' MA + V I + MAI

(2)

nil

V '' Pb + 2V I + PbI2

(3)


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Figure 4. DC polarization plots for SC (a, b) and Mech (c, d) at 20 V while silver electrode at negative (a, c) and positive (b, d) terminals. For clarity, data have been presented up to the individual saturation current. Here we utilize the Hebb-Wagner’s DC polarization techniques to identify the nature of defects present in the crystal lattice.42-43 For this purpose, an ion blocking graphite electrode was used in combination with a silver electrode to selectively tune the movement and transfer of charges of major ionic species onto the silver electrode depending on its electrical polarity. Recently, Troshin and co-workers demonstrated that in DC polarization experiment with MAPbI3 using two gold electrodes, initially current steadily increases after few minutes and then falls off exponentially with time when a constant bias of 100 V was applied for 2 h.44 They have shown that in such a scenario, electrochemical reactions take place over the two electrodes in which a part of MAPbI3 decomposes. Thus, ion migration leads to the rapid increase in current


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after short interval of few minutes which are required for ion movement and later maintains a steady current after decaying with time. This was also verified by us with two silver electrodes. We also had a similar observation with graphite/silver electrode combination; current sharply increases after a short delay of few minutes for both of our perovskite samples when the negative terminal of power source was connected to silver electrode (Figure 4 a, c). Therefore, it predicts the presence of cationic vacancy in the perovskite crystal lattice. From the energetic point of view, probability of monovalent MA+ migration is maximum in comparison with the diffusion of divalent Pb2+.9,

11, 20

For Mech, the rapid enhancement in DC current was of two orders in

magnitude from the initial value, whereas it was only of one order for SC. This is in line with our previous observations in I-V measurements where increase in conductivity by changing the electrode from graphite to silver for SC and Mech matches quite closely with the polarization results. After reversing the polarity of the silver electrode, that means by attaching the positive terminal of the power source we observed a similar polarization effect but only for SC sample (Figure 4 b). But, an exponential decay and then a stable current with time were noticed for sample Mech (Figure 4 d). Such observation can be explained as follows.40 When external bias was applied, instant electronic current was observed due to faster movement of electron and holes as compared to ions. After sometime, ions (here MA+) will start to flow towards graphite electrodes (according to the polarity shown in Figure 4d) and more and more ions accumulate at the perovskite-graphite interface. This results in the formation of ion induced electric field which acts opposite to the external electric field partially canceling the external bias and reducing the electronic current. Hence, we observe a regular decay and later a stable current with time after attaining the equilibrium. This result clearly suggests the presence of both MA+ and I- vacancies for SC, but principally only one type of vacancy defects i.e. MA+ in Mech sample. The iodide (I-


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) defect was seemed to be most mobile among all the ions due to low activation energy, high polarizability, short jump distances and large thermal displacements. In DC polarisation experiment we see the anion diffusion is faster and the peak appears in shorter time as compared to the peak due to cation diffusion supporting our assumption. Our observation was further corroborated by FESEM-EDX analysis (Figure S6, S7). For SC sample, the EDX measurement at multiple positions showed that atomic ratio of nitrogen, lead and iodine follows either ideal atomic ratio of perovskite of 1 : 1 : 3 or

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