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The Effect of Impurities on the Impedance Spectroscopy Response of CHNHPbI Perovskite Solar Cells 3

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Sonia R. Raga, and Yabing Qi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11584 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 4, 2016

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The Journal of Physical Chemistry

The Effect of Impurities on the Impedance Spectroscopy Response of CH3NH3PbI3 Perovskite Solar Cells Sonia R. Raga, Yabing Qi*. Energy Materials and Surface Sciences Unit (EMSS), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Kunigami-gun, Okinawa 904-0495, Japan. Corresponding Author

* E-mail: [email protected] , Tel.: +81 989668435

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ABSTRACT

The fabrication of CH3NH3PbI3 perovskite by solution processes is known to induce impurities in the film, which is usually correlated to the low photovoltage or fill factor of the solar cells. The nature of such defects is not always easy to identify, and is described in general as grain boundaries containing dangling bonds, vacancies, interstitials, or impurities. In this work, perovskite films were fabricated with a very large and known concentration of impurities (PbI2, Pbx(O/OH)y) and further characterized using chronoamperometry and impedance spectroscopy (IS). The IS data was carefully analyzed and compared with existing literature, and a new interpretation is proposed according to the results. The IS features at high frequency and close to open circuit were attributed to the photogenerated charge recombination. Despite large impurity levels (>30 mol. %), minor differences were observed in regard to charge recombination, which minimally affected the photovoltage.


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INTRODUCTION Methylammonium lead halide (MAPbI3) perovskites have been in the spotlight in recent years due to the remarkably high solar cell efficiencies achieved with low-cost solution-processed films.1-2 The unique properties of MAPbI3, such as diffusion lengths up to 3 mm for single crystal,3 a large dielectric constant,4 a switchable photovoltaic effect,5 and high tolerance to crystal defects 6 have attracted the attention of the scientific community. Despite a huge research effort on metalorganic perovskite materials and devices, several topics remain controversial, including the impact of impurities on solar cell performance. Several reports concluded that defects and grain boundaries in perovskite films are the main cause of performance losses or hysteresis.7-10 As a result, much effort has been devoted to increase grain sizes to minimize grain boundaries to reduce charge recombination.11-16 On the contrary, some other studies claimed that grain boundaries in perovskite films are beneficial for charge dissociation and photocurrent collection.17-19 Several types of defects in perovskite lattices, such as ion vacancies, interstitials, and antisites, have been reported to be mainly shallow energy levels with a minimal effect on charge recombination.6,

19

Such vacancy/interstitial defects are closely correlated with ion

migration, one of the proposed leading causes of hysteresis in solar cell j-V curves.9, 20-22 To date, there is no unified understanding of the effects of defects on photovoltaic performance of perovskite solar cells. Moreover, the nature of such defects in perovskite films is usually not experimentally identified and usually other impurities in the film originated by the fabrication process are not taken into account. Other impurities account as oxides, remaining solvent or unreacted precursors that may be originated by the solution-processed techniques under air conditions. Our focus is to study the effects of known impurity contents in the film in a systematic manner, for that we prepared perovskite films containing various concentrations of


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PbI2 and/or Pbx(O/OH)y up to 33 mol.% and compared with high quality pure MAPbI3 films.23 Solar cells fabricated with the various-quality perovskites have been characterized using impedance spectroscopy (IS) and carefully monitored by j-V curves before, during and after the IS scans. The first aim of this work is to analyze IS on these solar cells in a comprehensive way, focussing on the suitability of previously reported models for the fitting of our data. We correlated the IS results not only with the shape of a j-V curve measured with the solar simulator but also with the current dynamics that affect hysteresis and the steady state output. Perovskitebased solar cells degrade relatively rapidly (~1h). It is therefore necessary to cross check in realtime impedance spectra with the j and V values obtained during IS (steady state) and not only with the j-V measured initially with the solar simulator. EXPERIMENTAL SECTION Glass coated with fluorine-doped SnO2 (FTO, 15 Ohm/sq.) was first etched using Zn and HCl and cleaned by sequential sonication in a soapy bath, Milli-Q water and isopropanol. The FTO was covered with a 70 nm compact layer of TiO2 by the spray pyrolysis method using a precursor solution of titanium diisopropoxide bis(acetylacetonate) solution 75% in isopropanol (Aldrich). A 150 nm-thick TiO2 mesostructured (mp-TiO2) layer was deposited by spin-coating a diluted TiO2 paste (Dyesol, 90-T) and then annealed at 450ºC for 30 min. The TiO2 substrates were cleaned with plasma and UV-O3 just before PbI2 deposition. PbI2 was dissolved in dimethylformamide (DMF, 460 mg mL−1) and kept stirring at 70°C until completely dissolved. The PbI2 solution was spin coated on the substrates, previously heated at 70°C, at 4000 rpm for 30 s and then dried on the hot plate at 70°C for 5 minutes in the N2 glovebox. The substrates were taken out to the air for perovskite formation by subsequent MA and HI exposures, similar to our previous work and detailed as follows.23


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MA step cell. Methylamine gas exposure step: The PbI2 substrate was placed in a closed container at room temperature, ~ 200 µL of methylamine (MA) solution in ethanol (33% wt., Aldrich) was placed in the same closed container with the PbI2 substrate. The PbI2 film turns transparent within 3 s because of the excess CH3NH2. The transparent PbI2 substrate is immediately removed and placed on a hot-plate at 70ºC for 5 min, the film turns dark brown immediately. The reaction happening during MA step is as follows: CH3NH2 + PbI2 + air CH3NH3PbI3 + Pb- oxides/ hydroxides Where at least 1/3 of the film content is in the form of Pb-subproducts. HI step cell. Hydroiodic acid gas exposure step: The perovskite films prepared by MA step were subsequently exposed to HI to convert lead oxide to PbI2. The substrates were placed on a hot plate at 70ºC and covered by a glass petri dish of 5 cm in diameter. The HI exposure was performed by dropping some HI (57%, TCI co.) ~50 µL around the petri dish covering the samples. The HI gas filled the space inside the petri dish, reacting with the film during 2 minutes. During this process HI mostly reacts with the Pb-oxides forming PbI2 again, as can be seen in XRD, Figure 1. Seq. Sequential MA gas and HI gas exposures. The MA step and HI step were sequentially performed on the sample four times in order to minimize the content of impurities. Except for the first MA step, where the film turns transparent, further exposures were done with the substrate on the hot plate at 70ºC, and MA gas was introduced during 4 minutes but the film does not turn transparent. Sim. Simultaneous HI gas and MA gas exposures: For the simultaneous HI and MA exposures, the substrates were placed on the hot plate at 70ºC and covered by a glass petri dish with 5 cm in


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diameter, with a small gap of ~0.5 mm between the hot plate surface and petri dish to let the air flow inside the petri dish. For the simultaneous HI and MA exposures, HI ~100 µL and MA solution ~400 µL were placed inside the petri dish and next to the samples. In this reaction, a white-dense homogeneous smoke was formed induced by the air flow below the Petri dish. We identified the deposited powder as methylammonium iodide by XRD. The substrates were left for 10 min and the surface was blown with N2 gas. The film obtained by this process was much darker than others. Solar cell fabrication: The hole transport layer was deposited on the perovskite film by spin coating at 2000 rpm a chlorobenzene solution containing 59 mM 2,2′,7,7′-tetrakis(N,N′-di-pmethoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), 172 mM tert-butylpiridine and 32 mM of Li salt pre-dissolved in acetonitrile. The spiro-OMeTAD layer was exposed to ambient air for 3h before loading in the metal evaporation chamber. The top contact was fabricated by thermal evaporation of gold, with a final thickness of 80 nm. The devices were not sealed. A total of 6 devices were fabricated and tested for each fabrication method. Characterization: The film crystallinity was analyzed by X-ray diffraction (XRD, D8 Bruker) with diffraction angle resolution of 0.01°. All samples were characterized and stored in the air, in a dark cabinet, the measured relative humidity and temperature of the laboratory were 50% and 25 °C, respectively. Current-voltage (j-V) characteristics of solar cells were measured under 1sun illumination (AM 1.5G, 100 mW⋅cm−2) using solar simulator (Newport Oriel Sol 1A) and a Keithley 2400 source meter. The scan was performed from 1.2 V to 0 V at 0.1 V/s. All measurements were performed without a mask in order to compare with the impedance data. The Impedance Spectroscopy measurements were taken with an Autolab PGSTAT204 equipped with a frequency response analyzer module FRA32M. The AC modulation was 10 mV, the


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frequencies ranged from 1 MHz to 0.1 Hz and DC voltages from -0.2 V to 1.2 V at a step of 0.1 V. The DC current, DC voltage and time signals were recorded at each frequency point during the impedance for further analysis of the data, for the plotting of in situ j-V curves (Figure 3b) the values of current were extracted from the point measured at 100 Hz. The light source for all IS measurements was a cool white LED, calibrated to give the same photocurrent on a Si solar cell than the measured under 1 sun AM1.5. Note that the spectra of the white LED is different from the AM 1.5. The IS data was analyzed using licensed Z-View software (Scribner Associates Ltd.). RESULTS AND DISCUSSION For this work, five different types of solar cells with different perovskite quality were fabricated. First, a PbI2 layer was deposited by spin-coating onto a FTO substrate with TiO2 compact and mesoporous layers. A solar cell labelled as “PbI2 cell” was fabricated only with this PbI2 layer. The PbI2 layer was treated with methylamine gas (MA) in a chamber in ambient air and converted to a film containing a mixture of perovskite (XRD in Figure 1a) with a large quantity (33 mol.%) of Pb compounds.23 Pb can form dozens of oxide/ hydroxide compounds with different stoichiometry,24-25 thus for the sake of simplicity the Pb compounds in our samples will be mentioned hereafter in the text as Pbx(O/OH)y. The absence of additional peaks in the XRD result for sample “MA cell” (Figure 1a) suggests that Pbx(O/OH)y mainly exists in the amorphous state. A solar cell labelled as “MA cell” was fabricated using the PbI2 layer exposed to only MA. For the third device, which is labelled as “HI cell”, PbI2 exposed to MA was subsequently exposed to HI gas in a chamber in ambient air. When exposed to HI, Pbx(O/OH)y re-converts to crystalline PbI2 showing the intense PbI2-related XRD peak at 12.6° (Figure 1a).23 For the fourth sample labelled as “Seq.”, the PbI2 exposed first to MA was then exposed sequentially twice to


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HI gas and MA gas. These exposures increase the content of stoichiometric perovskite by reconverting the Pbx(O/OH)y and PbI2, as it can be inferred from the strong XRD peak at 14.1° corresponding to perovskite and the significantly decreased PbI2-related XRD peak at 12.6° observed for Seq. sample (Figure 1a). Finally, for the last device labelled as “Sim.”, the PbI2 film exposed to MA gas was then exposed simultaneously to HI and MA gases in a chamber in ambient air. During simultaneous exposure to both gases we expect stoichiometric conversion of PbI2 to MAPbI3. XRD in Figure 1a shows a strong perovskite XRD peak at 14.1° and tiny peaks at 9.8° and 19.7° corresponding to MAI formed as a by-product of the reaction between MA and HI. The pictures and j-V curves corresponding to these five different types of solar cell devices are shown in Figure 1b-c. The photovoltaic parameters extracted from the j-V curves and a summary of the film composition are shown in Table 1.

Figure 1. a) XRD plot for the perovskite films with different impurity content, see details in Table 1. The inset shows the XRD peaks appearing between 5° and 25°, the MAI diffraction peak is marked with * and an unknown peak at 8.9° is marked as #. b) Pictures of the solar cell devices containing different perovskite films taken at the end of all measurements. Note that the area non-covered by the Au contact bleached, but under the Au the film is still dark. c) j-V curves


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for the best device fabricated using each perovskite film. The solid line corresponds to reverse scan and the dashed line corresponds to forward scan. Film impurity content PbI2

Voc (V)

Jsc (mA cm-2)

FF (%)

PCE (%)

0.701

3.5

60

1.5

MA cell

33% Pbx(O/OH)y

1.020

15.1

47

7.2

HI cell

33% PbI2

1.096

15.5

57

9.6

Seq.

10% PbI2

1.023

17.6

59

10.7

Sim.

MAI, PbI2 traces

1.061

20.2

61

13.0

Table 1. Summary of the approximate film composition and the photovoltaic parameters obtained from the reverse scan of the j-V curve. Impurity content has a minimal effect in Voc, but a large effect on Jsc and FF. Despite the differences in preparing these five types of solar cells, the open circuit voltage (Voc) values for all our devices range from 1.020 to 1.096 V, which is close to Voc of the reported highest performance MAPbI3 perovskite solar cells.26 This is a rather surprising observation, i.e. a device containing more than 30 mol. % amorphous Pbx(O/OH)y or crystalline PbI2 exhibits no obvious Voc loss as compared with the best performing MAPbI3 solar cell reported. In solar cells, photogenerated electrons and holes are effectively separated to the selective contacts. The high charge densities created at the interfaces splits the quasi-Fermi level, determining the Voc of the device, see Figure 2a. Two main variables can reduce the charge density in the device resulting in a Voc decrease (i) the photocurrent; jsc relates to Voc through the expression

ܸ௢௖ =

௞் ௤

ln

௝ೞ೎

(1)

௝బ


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where T is temperature, k is Boltzmann constant, q the electron charge and j0 the dark current, and (ii) the electron-hole recombination; higher recombination will reduce the charge density. For thin film solar cells, the non-radiative charge recombination is often assisted by localized states in the band gap. The film defects such as grain boundaries or impurities have found to be the main cause of charge recombination in Si or thin film solar cells, and extrapolated to explain the perovskite device performance.27 On the other hand, theoretical studies claim that lattice defects such as ion vacancies or interstitials create shallow energy trap states that have a minimal effect on the recombination.6,

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the surprisingly small differences in Voc observed on the

different samples in this work seem to be consistent with such a claim, given that the impurities may induce structural defects at its interface with perovskite. Specifically, one of the impurities in this work Pbx(O/OH)y has a large band gap and is thus acting as an inert material embedded into perovskite matrix. In the literature, band gap energies in the range [1.35 to 4 eV] can be found for PbxOy, and [3.28 to 4.51 eV] for Pbx(OH)y.

24-25

The larger band gap corresponds to

lower oxidation state (Pb2+), the most stable state of Pb. Our fabrication processes are carried out at low temperatures, thus we believe the as formed Pbx(O/OH)y will exist in the form of low Pb oxidation states, coinciding with the large band gaps. Other possibility is that the electron conduction through perovskite to the selective contact interface is faster than the trapping into the gap states. In the case of the other impurity defect in this work PbI2, several studies have shown that it can serve as a passivation layer

28-29

For PbI2 cell, the Voc obtained is low (0.7 V)

compared with the large band gap (2.3 eV) of the material. In this case, low carrier diffusion length of PbI2 may explain the large recombination of the charges across the ~200 nm-film, before reaching the contacts.30 We also measured our devices 8 days after the IS characterization, stored in ambient air in dark to check their stability, see Figure S1. The devices exhibited similar


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performance, except for the large efficiency improvement of MA cell from 7.2% to 10.5% and the large efficiency decrease of HI cell from 9.6% to 6.2%. The performance increase of MA cell is likely a result of reorganization of the crystalline structure. Similar performance increase of perovskite solar cells is often observed in the first few hours of solar cell measurements. Evolution of the crystalline phase of perovskite was reported by Park et al. for films stored in Ar atmosphere for several days.31

Figure 2: a) Theoretical scheme of the energy levels for a solar cell with structure TiO2/MAPbI3/spiro-OMeTAD, the energy level of pure PbI2 is also represented overlapping the MAPbI3. The quasi-Fermi level splitting is marked in red dashed lines. Voc close to 1.1 V is obtained, only 100 mV below the theoretical energy difference between TiO2 conduction band and spiro-OMeTAD valence band. b) Plot of the real part of the dielectric constant (ε’) vs. frequency for the impedance taken at 0.5V. c) Nyquist plot taken at 0.5V for all samples, the point marked as void circle represents 100 kHz, void triangle 1 kHz and green circle 1 Hz.


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Despite the rather small Voc variations, substantial differences in FF and photocurrent were observed for the five types of devices. To understand the underlying processes causing the different performance in these solar cells, IS measurements were performed on the same fresh devices (Figure 1) which remain stable after 8 days (Figure S1). The IS scan was performed with an amplitude of 10 mV, in a DC voltage range from -0.2 V to +1.2 V under constant white LED illumination. More details can be found in the experimental section. We will first describe our observations from the Nyquist and dielectric constant (ε) -frequency plots. This preliminary visual analysis is important to consider which equivalent circuit may be the best for the fitting. Two arcs can be observed in the Nyquist plot for all samples for applied voltages below the Voc (Figures 2c and S2). The low frequency arc is largely deformed. A deformed arc may be due to the combination of two arcs with similar time constants, or because it is originated from a nonideal capacitor. A constant phase element (CPE) is commonly employed in the equivalent circuits for the fitting in order to compensate for non-homogeneous systems.32 The impedance of a CPE follows the equation:

ܼொ =

ଵ

(2)

௒బ ሺ௝ఠሻ೙

Where Y0 is a constant with units of F cm−2 sn−1 and n is a factor in the range between 0 and 1. However, equation (2) does not give any direct physical interpretation except for the following cases; (i) for n= 1 ZQ= 1/wC, (ii) for n= 0 ZQ = R and (iii) for n= 0.5 ZQ = Warburg. For n values lower than but close to 1, a widely employed equation to calculate capacitance (C) is:

‫=ܥ‬

ሺோ∗௒బ ሻభ/೙

(3)

ோ

Where R is the resistance in parallel to the CPE. Although equation (3) is not the only existing way to calculate the capacitance, it is the most widely equation used in the literature for the majority of the cases for n > 0.7.33


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In our devices, samples MA cell, HI cell and Seq., have low values of n, below 0.8 for all the samples. In the case of the PbI2 cell, three features are observed in the Nyquist plot. The second arc of the PbI2 sample appears at higher characteristic frequencies (~650 Hz) than the second arc of the other four perovskite devices indicating a different electric process. A third feature appears at extremely low frequencies similar to a largely distorted arc, almost horizontal line in the Nyquist plot, see Figure 2c. At the lowest frequencies, the PbI2 cells exhibit large values of the dielectric constant (ε), see Figure 2b, only one order of magnitude lower than the giant ε of perovskite. Sanchez et al. proposed that the low-frequency feature is originated by slow phenomena such as interfacial electronic or ionic charge accumulation in the perovskite film.34 Interestingly, for the PbI2 sample and the different perovskite solar cells, at voltages greater than Voc (where the current flows in a reverse direction), the low frequency arc converts to an inductive loop in the Nyquist plot, see Figure S2b-d. Similar inductive loops are often found for perovskite materials, a recent study attributed the inductive loop and j-V curve hysteresis of SrTiO3 perovskite to the bias induced ion motion.35 Because of the many similarities between the PbI2 cell and the other four types of perovskite cells in IS spectra, we believe that they all have a similar origin. All five types of cells share the Pb and I in common, we anticipate that the low frequency features may be induced by iodine interaction (accumulation/adsorption) at the interfaces.36 Based on the above-mentioned considerations, the Nyquist plot was adjusted to a simple equivalent circuit of two series-connected resistor-CPE elements in parallel (see Figure S3b). The resistance and capacitance obtained from the fitting of the high frequency (HF) arc (1 MHz 1 kHz) were labelled as RHF and CHF, respectively. From the fitting of the low frequency (LF) arc (1 kHz - 0.1 Hz), we obtained the low-frequency resistance (RLF) and the CPE parameters Y0 and


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n. For the PbI2 device, three RC in series were used for the fitting; for PbI2 the LF feature is only seen at a few intermediate voltages. As explained above, because of the low values obtained for n in the fitting of the LF region, we were unable to calculate real capacitances.37 On the contrary, we employed the raw values of n as an approach to explain the differences among the different samples. Note that the parameters n and Y0 do not have any defined physical meaning yet, thus theoretical modelling needs to be developed for a quantitative analysis of the LF region. In order to accurately correlate the impedance results with the real performance of the device, it is necessary to obtain the steady-state j-V curve. During the course of this study, we realized that during the time that IS scan takes place (usually 1h), every sample evolves in a different way (Figure S4). From the j-V curves in Figure S4 we observed variations in FF, photocurrent and photovoltage for all samples, including the PbI2 sample. In the literature, IS is commonly employed to understand the electronic mechanisms in the device. IS results are often correlated to the Voc and FF values of the solar cell j-V curve that is measured with a calibrated solar simulator. When measuring the j-V curve of a perovskite solar cell, it is necessary to consider two main effects; (i) the hysteresis: j-V curves for perovskite solar cells may have different shapes depending on the scan rate and direction. For sufficiently long dwell times the steadystate j-V can be obtained.38-39 Figure S5 shows an example of the photocurrent decay obtained during the first scan at Vapp = -0.2 V. (ii) The aging: perovskite is a structure that can evolve over time due to processes such as degradation, ion migration, etc. Applying continuous AC perturbations can accelerate these processes. In other words, the slow measurement conditions for IS are not always equivalent to the “fast” j-V scans that is conventionally used to determine solar cell performance parameters, e.g. Voc, Jsc, and FF. For a proper and accurate comparison of the IS data with the real performance of the devices, we extracted the DC photocurrent from the


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IS data and constructed the steady-state j-V curves (Figure 3b). We can observe from Figure S5a that the steady-state for the samples containing impurities is largely decreased as a function of time, whereas the pure PbI2 sample or the high-quality perovskite (Sim.) shows little decay in photocurrent. Our findings suggest that the transient currents can be modulated by the presence of a large quantity of impurities. We will focus first on the features at high frequencies (HF). In the literature, the HF arc is mainly associated with the charge transfer resistance at the interface with the selective contacts (TiO2 or spiro-OMeTAD) or the transport resistance through the spiro-OMeTAD layer.8, 40-42 The HF arcs for all perovskite and PbI2 sample have a characteristic frequency around 70 kHz (see Figure S2), i.e. processes occurring in a time scale of ~14 µs. The fitted values of RHF (Figure 3a) show nearly constant values close to 16-19 Ω cm2 for all samples except PbI2 in the range of voltages of 0 V to 0.7 V (close to the maximum power point). After 0.7 V (0.6 V for PbI2), RHF decreases gradually by ~10 times to values as low as 2-7 Ω cm2. The two regimes of RHF suggest that there are two different electrical processes occurring at similar time scales. A method to check the possible contribution of RHF to the series resistance of the solar cells is to correct the voltage drop on the j-V curves as VF = Vapp + i*R, where VF is the corrected voltage, Vapp is the applied voltage, i the photocurrent and R the sum of all series resistances in our device (Rs and RHF). In ideal devices with zero series resistance and no shunt paths, the FF will take values close to but lower than 1. In our case, the voltage correction leads to FF greater than 1 (Figure S6), especially for the perovskites with less impurities. This indicates that the origin of RHF is an electrical process occurring not in series but in parallel to the injection of photogenerated carriers in our device. When two resistances are connected in parallel in an equivalent circuit, the smaller resistance will determine the diameter of the corresponding arc in the Nyquist plot. At voltages


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above 0.7 V (0.6 for PbI2) RHF starts decreasing, coinciding with the onset in photocurrent on the j-V curves (see Figure 3a-b) where charge recombination is dominating over charge extraction. In addition, the values of RHF close to the open circuit are clearly larger for the HI cell corroborated by a higher steady-state Voc of the sample (Figure 3b), which indicates that RHF is possibly originated from the charge recombination. In the case of the Sim. cell, with much larger jsc than the other three perovskite cells, an increased Voc should be expected according to Equation 1. However, it has the same Voc than MA cell and Seq. cell, correlating with a slightly smaller RHF (higher recombination) close to Voc values for Sim cell. The same effect is observed for MA cell measured after 8 days exposed to air, jsc increased largely while retaining same Voc, and smaller RHF, see Figure S7. For applied voltages n > 0.75 and, as far as we know, no references are found using n< 0.7.37 Other works also found depressed arcs at the LF region and attributed it to different phenomena. For example, Bag et al. adjusted the LF arc to a Warburg element, as it appeared as a linear region at the frequency range of 4 kHz- 100 Hz.45 They obtained a diffusion coefficient of 3.6·10-12 cm2s-1 and attributed the ion diffusion to the alkyl ammonium counter-ion.45 The Warburg element is equivalent to a CPE where n= 0.5, representing the impedance of semi-infinite diffusion to the contact interface. In our case, see Figure 4, most of the points lay below 0.8 and are different than 0.5, therefore cannot be fitted as a Warburg neither a capacitor. In addition, our proposed origin of charge recombination for the high-frequency element is incompatible with a seriesconnected ionic diffusion. Some other groups fitted the depressed arc in the LF region with a Havriliak- Negami element instead of a simple CPE.46-47 In their work, the LF feature was associated to photoinduced carrier hopping and drift occurring sequentially and assisted by deep traps.47 We found that there is a clear trend for the parameter n depending on the perovskite film


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quality; for the samples containing larger quantity of impurities the parameter is largely decreased. There is also dependence for the n value on the applied voltage; for all samples the LF arc becomes more deformed when increasing the applied voltage, before converting to the inductive feature close to Voc (Figure S2).

Figure 4. parameter n obtained from the fitting of the CPE, according to Equation 2. For the PbI2 sample, the parameter n corresponding to features at medium and low frequencies are shown. For n= 1 the CPE equals to pure capacitor. For 1> n >0.75 (green background on the plot) the capacitance can be obtained through Equation 2. For n < 0.75 the CPE cannot be longer fitted to a capacitor. It is worth pointing out that the current experiment was done with the same selective contacts in all samples, thus we attributed the observed differences to the quality of perovskite films. However, it is also possible that the hole or electron selective contacts may also influence the way ions accumulate/interact at the interface and thus modify the shape of the Nyquist plots.


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CONCLUSIONS To summarize, we fabricated and characterized various perovskite solar cells containing known large amounts of PbI2 and Pbx(O/OH)y as a byproduct of the fabrication method, and compared to solar cells with high quality perovskite and pure PbI2. It was found that such impurities in perovskite films have minimal effect on the recombination, mainly reflected on the high Voc for all perovskite samples. Instead, the impurity content in the film was found to influence the shape of the low-frequency feature, inducing more depressed arcs in the Nyquist for the larger contents in impurities. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] , Tel.: +81 989668435 Notes The authors declare no competing financial interests. ASSOCIATED CONTENT Supporting Information. j-V curves of the solar cells after 8 days of storage in the air, Nyquist plots for all samples at different applied voltages, equivalent circuit and simulated fitting curves, j-V curves measured before and after the IS scans, evolution of the photocurrent during the measurement of IS, j-V curves simulated by removing the effect of the series resistances in the device, j-V and RHF for MA cell fresh and after 8 days, CHF-Vapp for all the devices, values of resistance and capacitance for the low frequency arc fitted by constraining n > 0.7 and with a free value of n.


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ACKNOWLEDGMENT This work was supported by funding from the Energy Materials and Surface Sciences Unit of the Okinawa Institute of Science and Technology Graduate University, the OIST R&D Cluster Research Program and JSPS KAKENHI Grant Number 15K17925. We thank Dr. Emilio Juarez Perez for fruitful discussion. We thank Steven D. Aird, the Technical Editor at Okinawa Institute of Science and Technology Graduate University for valuable suggestions in revising the manuscript. REFERENCES 1. Saliba, M.; Matsui, T.; Seo, J.-Y.; Domanski, K.; Correa-Baena, J.-P.; Nazeeruddin, M. K.; Zakeeruddin, S. M.; Tress, W.; Abate, A.; Hagfeldt, A., et al. Cesium-Containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency. Energy Environ. Sci. 2016, 9, 1989-1997. 2. Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. HighPerformance Photovoltaic Perovskite Layers Fabricated Through Intramolecular Exchange. Science 2015, 348, 1234-1237. 3. Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-Hole Diffusion Lengths > 175 µm in Solution-Grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967-970. 4. Juarez-Perez, E. J.; Sanchez, R. S.; Badia, L.; Garcia-Belmonte, G.; Kang, Y. S.; MoraSero, I.; Bisquert, J. Photoinduced Giant Dielectric Constant in Lead Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 2390-2394. 5. Xiao, Z.; Yuan, Y.; Shao, Y.; Wang, Q.; Dong, Q.; Bi, C.; Sharma, P.; Gruverman, A.; Huang, J. Giant Switchable Photovoltaic Effect in Organometal Trihalide Perovskite Devices. Nat. Mater. 2015, 14, 193-198. 6. Yin, W.-J.; Shi, T.; Yan, Y. Unusual Defect Physics in CH3NH3PbI3 Perovskite Solar Cell Absorber. Appl. Phys. Lett. 2014, 104, 063903. 7. Cui, P.; Fu, P.; Wei, D.; Li, M.; Song, D.; Yue, X.; Li, Y.; Zhang, Z.; Li, Y.; Mbengue, J. M. Reduced Surface Defects of Organometallic Perovskite by Thermal Annealing for Highly Efficient Perovskite Solar Cells. RSC Adv. 2015, 5, 75622-75629. 8. Bai, Y.; Chen, H.; Xiao, S.; Xue, Q.; Zhang, T.; Zhu, Z.; Li, Q.; Hu, C.; Yang, Y.; Hu, Z., et al. Effects of a Molecular Monolayer Modification of NiO Nanocrystal Layer Surfaces on Perovskite Crystallization and Interface Contact toward Faster Hole Extraction and Higher Photovoltaic Performance. Adv. Funct. Mater. 2016, 26, 2950-2958. 9. 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.


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