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Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells Pengjiu Wang, Maria Ulfa, and Thierry Pauporte J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11010 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells Pengjiu Wang, Maria Ulfa, Thierry Pauporté* Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie Paris (IRCP), 11 rue P. et M. Curie, F-75005 Paris, France *Author for correspondence. E-mail: [email protected]

Abstract The partial replacement of methylammonium by formamidinium and cesium in organolead trihalide materials is of great importance to improve the performance and stability of photovoltaic solar cells. However, the effect of multiple cations on the cell functioning and their electrical characteristics remains to be clarified. By using the impedance spectroscopy technique, we have investigated the electrical response to a small ac perturbation applied to solar cells implementing hybrid perovskites with various compositions, polarized over a large potential range. The solar cell preparation protocols have been optimized to reach power conversion efficiencies higher than 17%. The impedance response has been investigated both under light and in the dark to discriminate the light sensitive parameters. The spectra have been carefully analyzed using an ad hoc equivalent circuit and the data have been discussed in the light of the existing literature. The spectra showed no intermediate frequency inductive loop due to the absence of multistep charge transfer involving surface states. Large inductive loop is found the signature of poorly functioning solar cells. Except the high frequency capacitance which is the bulk response of perovskite, the other parameters are influence by interface and contact phenomena, ionic conductivity and charge accumulations. The scaling of the low frequency capacitance with the hysteresis amplitude is clearly stated by our comprehensive study. Moreover, no diffusion impedance due to the diffusion of ionic species is observed. However, ion mobility results in a strong effect on recombinations and has a strong influence on the low frequency impedance response of the system.

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1. INTRODUCTION In recent years, halide perovskites (HP) have appeared as one of the most promising family of materials for the development of high-performance optoelectronic devices1-4 and have revolutionized the field of photovoltaic solar cells.5-15 The record power conversion efficiency (PCE) of perovskite solar cells (PSCs) has increased rapidly and achieves a current certified world record of 22.7%.16 Within a few years, solar materials based on 3D lead halide perovskites have attracted an extraordinary attention and given rise to an abundant literature. These materials are direct bandgap semiconductors with opto-electronic properties that can be tuned and adjusted by playing on their composition. They adopt the ABX3 general chemical formula, where A is a monovalent cation (classically Cs+, Rb+, CH3NH3+ (noted MA+) or HC(NH2)2+ (FA+)), B is a bivalent metal cation such as Pb2+, Sn2+ or Ge2+, and X is a halide anion (typically Cl−, Br−, I− or their mixtures).17 Various techniques have been developed for the preparation of perovskite layers such as spin-coating, dipcoating, 2-step interdiffusion, chemical vapor deposition, ink-jet printing and thermal evaporation.18 Spin-coating is the most popular technique and appears as an easily implementable growth technique, performed using precursor solutions, at low temperature and ambient pressure. Recent reports have shown the great interest of using a mixture of monovalent cations in HPs to get more stable and more efficient PSCs.19-22 Higher resistance to moisture is due to the use of Cs+ and FA+ cations which are less hydrophilic than MA+. These perovskites are also more thermally stable. Also mixing I- and Br- improves the material robustness. MAxFA1-xPb(I1-xBrx)3 solar cells have achieved high efficiency.21-22 These materials show less halide segregation. Moreover, it has been shown that using a small amount of Cs+ is beneficial by avoiding the formation of the inert nonperovskite hexagonal γ-phase (yellow phase). In most papers dedicated to PSCs, the cells are mainly electrically characterized by their J-V curves. These curves show a dynamic effect with current density values measured under the reverse scan direction being higher than those measured upon the forward scan.23 This hysteresis effect is linked to a capacitive electrical behavior of the cells. It is generally admitted that this effect is related to the accumulation of mobile ions (and ion vacancies) at the interfaces near the contacts due to ion migration under an electric field. It alters the band structure at the interface and modulates the barrier heights for electron/hole collection. It is strongly influenced by the interfaces between the selective contacts and the perovskite layer.23 Using a mesoporous oxide layer on top of the compact (hole blocking) layer (BL) reduces the hysteresis amplitude. However deeper electrical characterizations of the cells are required to better understand their functioning. In the impedance spectroscopy (IS) technique, the device electrical response is linearized by superimposing a small ac perturbation to the dc polarization which can be varied over a large range. Impedance spectroscopy has proved to be a useful tool to investigate different solar cells, especially dye-sensitized solar cells.24-27 In this case, 2 ACS Paragon Plus Environment

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charge accumulation occurs in the thick mesoporous oxide layer giving rise to a chemical capacitance. Moreover, there is a limitation of electron transport in the mesoporous oxide phase which is observed on the impedance spectra and which can be fully analyzed.24 Important parameters such as the electron diffusion coefficient and the electronic conductivity of the oxide layer can be extracted at various applied voltages (Vappl) and then over a large density of states range. IS also emerges as a powerful technique for the investigation of PSCs.28-44 By this technique the phenomena of charges (ions and electrons) transfer and accumulation can be thoroughly investigated. It is now recognized that hybrid perovskite solar cells contain different kinds of mobile charge carriers, ionic and electronic, that are important to consider to explain the cell electrical response.45 Ionic defects can easily drift under internal electrical field, and they accumulate at the interfaces.46,47 Due to these interface modifications which intervene with the ion transport, the optimum situation in which every single feature in the impedance spectra is produced by a single physical process is not observed and perovskite solar cells present complex spectra in which a single feature is affected by different processes. In a previous paper, we have investigated the effect of BLs on the IS response of PSCs and the BL preparation procedure has been optimized.28 In this work, the two-step perovskite preparation technique (first deposition of a PbI2 film which is subsequently converted into CH3NH3PbI3 by reaction with CH3NH3I) was employed for the absorber preparation. We investigate here the effect of the perovskite composition and especially its monovalent cation content on the cell performances and impedance spectra. All the HP layers have been produced by a one-step technique using the dripping by an antisolvent upon the spinning to produce a precursor film which is subsequently annealed to get the perovskite layer. In the literature, most of the comprehensive IS studies of PSCs have been performed on devices with a rather low efficiency.29,30,38,48,49 In the present work, the cell preparation protocols have been optimized and the study has been carried out typically on cells with a PCE higher than 17%. Moreover the study has been conducted both under light shining and in the dark to discriminate the light sensitive parameters.

2. EXPERIMENTAL SECTIONS The F-doped tin oxide coated glass (TEC7, Pilkington) was cut, and etched patterned using HCl 10% and Zn powder. They were then cleaned by using a concentrated 2.5 mol.L-1 NaOH ethanolic solution, rinsed with water, clean with a detergent, rinsed with MilliQ water and dried with compressed air. The substrates were then annealed 30 min at 500°C. The TiO2 blocking layer was prepared by an aerosol spray pyrolysis technique as described elsewhere.28 A precursor solution was prepared by mixing 0.6 mL of titanium isopropoxide (TTIP), 0.4 mL of acetyl acetone in 7 mL of isopropanol. The substrate was placed on a hotplate at 455°C for 20 min prior to start the spraying.



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The deposited layer was then annealed at 455°C for 40 min before to be let to cool down. The mesoporous TiO2 layer was prepared by diluting the NR30-D paste from Dyesol with ethanol (1:8 mass ratio). 40 µL of the solution was deposited on the blocking layer and spin-coated 20s at 5000 rpm. The layer was then dried on a hotplate at 100°C for 10 min and subsequently annealed at 500°C for 30 min. The CH3NH3PbI3 (MAPI) precursor solution was prepared by mixing 553 mg of PbI2 and 190 mg of MAI (Dyesol) in 1 mL of DMSO solvent. The solution was heated at 70°C until full dissolution. 50 µL of this solution was deposited on the substrate, and spun at 6000 rpm for 35s. The layer was dripped with 100 µL chlorobenzene after 25s. The perovskite layer was finally annealed at 105°C for 1h on a hotplate. The samples were then removed from the hotplate and let to cool down to room temperature during 10 min. The cells prepared with this perovskite layer are denoted MAPI in the following. For the FA0.87MA0.13Pb(I0.87Br0.17)3 (FAMA) preparation the precursor solution contained 1M FAI, 0.2M MABr, 1.1M PbI2 and 0.22 M PbBr2 dissolved in a 4:1 volume ratio mixture of DMF and DMSO. In the case of CsxFAMA, a 1.5 M CsI predissolved solution was mixed into the perovskite precursor solution. 33 µL and 66 µL were added to 1 mL of the FAMA precursor solution to get a x of 0.05 and 0.1, respectively. The corresponding cells are denoted Cs5FAMA and Cs10FAMA, respectively. 45µl of the perovskite precursor solutions was spin-coated on FTO/ TiO2 (BL) /mesoTiO2 at 1000 rpm for 20 s (acceleration 200 rpm/s) and then at 6000 rpm for 30 s (acceleration 3000 rpm/s). During the second step, after 20s, 100 µl Chlorobenzene was dripped on the sample. The perovskite layer was finally annealed at 100 ºC for 1 h. A solution was prepared by dissolving 72 mg of Spiro-OMeTAD in 1 mL chlorobenzene. Then, 17.5µl of bis(trifluoromethylsulfonyl)imide lithium salt solution (LiTFSI) solution (520 mg in 1 mL ACN), 28 µL of TBP (tert-butylpyridine) and 6 µL of tris(2-1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III)-tris(bis(trifluoromethylsulfonyl)imide) (300 mg in 1 mL ACN) were added to this solution. 35 µL of the HTM solution was spin-coated at 4000 rpm for 20 s. Finally, the device was completed by thermally evaporating a 70-80 nm thick gold back contact on the Spiro-OMeTAD layer. The solar cell surface area delimited by the back contact was about 0.24 cm2. A schematic of the solar cell configuration is shown in Figure 1a. The J-V curves were recorded by a Keithley 2410 digital sourcemeter, using a 0.10 V.s-1 voltage scan rate. The solar cells were illuminated with a solar simulator (Abet Technology Sun 2000) filtered to mimic AM 1.5G conditions.50 The illuminated surface was delimited by a black mask with an aperture diameter of 3 mm. The power density was calibrated to 100 mW.cm-2 by the use of a reference silicon solar cell.51 The impedance spectra were measured at 4 ACS Paragon Plus Environment

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room temperature, between 600 kHz and 30 mHz, using a PGSTAT 30 apparatus from Autolab. All the measured cells had the same contact geometries.28 The AC signal was 20 mV. All the impedance spectra were measured at room temperature, over a large Vappl range, both in the dark and under a ~1 sun light power provided by a halogen Schott lamp. These spectra were analyzed using the Z-view software from National Instrument.

3. RESULTS AND DISCUSSION The present work was carried out to unveil the role of the cations in the perovskite performances and impedance responses. All the cells were prepared by the one-step technique but using various cations (MA+, FA+, Cs+) and compositions. In every case, the precipitation of the precursor layer was triggered by benzene anti-solvent dripping upon the spin-coating. The deposited precursor film was subsequently annealed to produce the perovskite layers. Figure 1b-e shows top views on the investigated perovskites. The layers were uniform in morphology, well-capping the TiO2 mesoporous layer and were made of large grains. We observed that the grains of the layers containing formamidinium cation (FA+) were smaller than the MAPI grains and that cesium cation also decreased the perovskite gain size. A large literature on impedance spectroscopy study of PSC has deals with cells with rather low efficiency.29,30,38,48,49 We have investigated in a previous work28 the IS response of high efficiency perovskite solar cells with MAPI prepared by the 2-step technique. The extensive study by IS of high efficiency PSC with various cation composition and prepared by the spincoating/dripping technique remains to be done. In the present case, the cell preparation protocols have been optimized and the study has been carried out typically on cells with a PCE higher than 17%. Typical J-V curves are shown in Figure 1f. In Table 1 are gathered the best cell J-V curves parameters and the average values obtained for the forward and backward scan directions. From these data it is concluded that using the FA cation allows the increase of Jsc. Also we have found that the hysteresis could be small for the MAPI cells, but variations from one batch to another were observed and difficult to control. We obtained higher efficiency using multiple cations HPs.



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(a)

(f)

Figure 1. (a) Exploded schematic view of the perovskite solar cells. (b-e) SEM top views of the hybrid perovskite layers: (b) MAPI, (c) FAMA, (d) Cs5FAMA and (e) Cs10FAMA. (f) Typical reverse scan J-V curves of the perovskite solar cells prepared with perovskites of various compositions.

Table 1. Effect of cation on the Perovskite solar cell J-V curves parameters (AM1.5G filtered 100 mW.cm-2 illumination). Perovskite

MAPI

Besta Avgb

FAMA

Best Avg

Cs5FAMA

Best Avg

Cs10FAMA

Best Avg

Scan direction Reverse Forward Reverse Forward Reverse Forward Reverse Forward Reverse Forward Reverse Forward Reverse Forward Reverse Forward

Voc (V)

Jsc (mA.cm-2)

FF (%)

PCE (%)

1.01 1.00 1.01 (0.02) 0.987 (0.02) 1.02 1.01 1.01 (0.008) 0.989(0.0136) 1.02 1.01 1.01(0.008) 1.01(0.004) 1.00 0.91 0.99(0.012) 0.96(0.031)

22.15 22.72 20.83 (0.63) 21.02 (1.06) 24.05 23.24 23.34(0.665) 23.27(2.60) 22.59 23.00 21.99(0.544) 22.62(0.58) 24.10 24.14 23.15(0.581) 23.35(0.42)

78.75 63.02 76.5 (2.70) 59.3 (2.91) 76.1 55.02 75.93(1.33) 50.72(2.60) 73.88 62.58 74.45(2.32) 60.58(2.11) 77.16 55.20 76.91(1.22) 59.31(3.78)

17.68 14.45 16.10 (1.00) 12.42 (1.22) 18.67 12.92 17.93(0.49) 11.55(0.823) 17.06 14.67 16.51(0.33) 13.90(0.50) 18.66 12.18 17.45(0.77) 13.40(1.23) 6

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a

Best cell; b averaged values. The values in bracket are the standard deviations.

On the basis of the J-V curves alone, it is difficult to extract clear conclusions about the influence of the various cations on the functioning of the PSCs. To gain a deeper insight into this understanding, we have investigated the solar cell electrical response to a small ac perturbation applied to the devices polarized at various potentials. The devices have been investigated both under light and in the dark. Impedance spectroscopy (IS) utilizes small amplitude perturbations in order to linearize the system response in the frequency domain. Linearization is obviously convenient when one wants to model a system in terms of linear circuit elements (resistors, capacitors and inductors). The decoupling of physical processes with different characteristic times is then made possible. IS spectra interpretation requires electrical models and determination of the physical origin of the observed electrical behavior. IS spectra of the various cells, measured under light at two different Vappl, are shown in Figure 2a-d as Nyquist plots (imaginary versus real part of the impedance). They are characterized by both a high frequency and a low frequency loops. The frequency transition between them lies at 260-500 Hz. An intermediate arc of circle could appear as a shoulder of the low frequency loop for some cells and applied voltages (Figure S1, Supporting Information). It shows the occurrence of a middle frequency relaxation. Also some tail was found at the lowest frequency points of the first loop for some other spectra (Figure 2). It should be noted that no inductive loop was observed at intermediate frequencies whereas we have reported the occurrence of this inductive loop on IS spectra of cells prepared by a two-step technique with various blocking layers in Ref.28. We have noted there an increase in this loop amplitude when the performance of the cell decreased and found it to be the signature of poorly working cells. The inductive behavior has been ascribed to multistep electron injection process occurring at the interface, the electrons would not be injected directly but through surface states.52 The present study shows that no such phenomenon would occurs for perovskite prepared by a one-step technique on TiO2, mixing or not cations.



(a)

(b)

(c)

(d)

(e)

(g)

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(f)

(h)


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Figure 2. Nyquist plots of impedance spectra of PSCs prepared with various hybrid perovskites. (a-d) Measured under light at 0.2V (a,b) and 0.6V (c,d) applied voltages. (e,f) Measured in the dark at 0.2V (e) and 0.6V (f) applied voltages. (g) Three relaxations and (h) two relaxations equivalent electrical circuits.

The general equivalent electrical circuit (EEC) used to fit the spectra is shown in Figure 2g. Rs is a series resistance measured by extrapolating the high frequency loop to the x-axis. It is mainly due to gold and FTO contact electrodes and external wires. The high frequency and low frequency arcs of circle were fitted by the R1//CPE1 and R3//CPE3 electrical circuits, respectively. Due to the complexity and non-general occurrence of the middle frequency relaxation, the R2//CPE2 circuit has been neglected in most cases and will not be discussed in detail. The arcs of circle in the spectra were not strictly semicircles and showed some dispersion. The fit required the use of constant phase elements (CPE) to model the capacitive behavior as detailed in our previous work.28 The CPE impedance is defined by Z CPE =

Q ω where ω is the angular frequency related to the frequency as f = , j is the p 2π ( jω)

square root of -1 and p is a number lower than 1. The equivalent capacitances have been then calculated from these CPEs using the Brug’s protocol described in Ref.[53]. The cells have also been characterized in the dark. Typical spectra for the various perovskite are shown in Figure 2e and 2f. Two relaxations are found and, compared to the spectra measured under light, the high and low frequency resistances were larger. The frequency transition between the two loops was in the 5-20 Hz range for MAPI and FAMA and 200-600 Hz for the perovskite with Cs cation. Also we could note that for all the perovskites, an inductive loop was measured at very low frequency at Vappl higher than 0.7V as shown in Figure S2 (Supporting Information). The spectra were fitted by the EEC of Figure 2h. For the fit, the low frequency inductive part, if any, was discarded. Using the EEC of Figure 2g,h, the various electrical parameters have been determined over a large frequency range and the effects of the perovskite composition on these parameters have been studied. The high frequency capacitance C1 was extracted from the CPE1 parameters. The high frequency arc of circle was close to a semicircle and the p parameter ranged between 0.9 and 1 (see Figure S3, Supporting Information). C1 values as a function of Vappl are displayed in Figure 4. Figure 4a shows that for various solar cell batches the C1 values were the same and that light had no influence on this parameter (Figure 3a). Figures 3 also shows that for the various perovskites, C1 remains practically constant between 0 and 0.6V and slightly increases at higher Vappl. All these observations show that C1 is mainly related to the intrinsic dielectric relaxation of the bulk perovskites.28,38 It corresponds to the geometrical capacitance of the perovskite layers. Figure 3b shows that, below 0.6V, C1 ranges between 1.2-1.8 10-8 F (0.3-0.45 10-8 F.cm2) for formamidinium containing perovskites and 5-6 10-8 F (1.2-1.5 10-8 F.cm2) for MAPI. This capacitance is expressed by C=εrε0S/d, with ε0 the vacuum permittivity 9 ACS Paragon Plus Environment


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(8.85×10−12 F.m-1), εr the relative permittivity, S the cell surface area and d the la perovskite layer thickness. Because the organolead perovskite layer is not perfectly smooth (Figure 1), one must take into account the roughness factor, ρ, S=ρSgeo, with Sgeo the geometric surface area. εr is reported for MAPI, however, various values can be found in the literature: 6.5,54 22,31 23,31 24.1,55,56 31.9,54 3731 and 70.54 C1 can be estimated taking the following values: S=0.24 cm2, ρ=1.2 to 2, εr=30 and d=300 to 500 nm. One finds then 1.5 10-8 F < C1 < 4.210-8 F, which is in good agreement with the obtained experimental data. The higher capacitances for MAPI cell suggest a higher εr for the methylammonium based perovskite compared to the formamidinium ones. The constancy of this parameter with Vappl up to 0.6V shows that no significant charge accumulation occurs at the contact in these PSCs whatever the perovskite used and that, unlike DSSCs, no chemical capacitance is found 24,26,57

The slight C1 increase above 0.6V could be due to the charge carrier accumulation in TiO2

above this Vappl.38

(a)

(b)

Figure 3. (a) Effects of batch and of light shining on the C1 parameter of MAPI cells under various applied potentials (Under light: full symbols; in the dark: cross symbol). (b) Effect of bulk perovskite composition on C1 measured at various applied voltages. Figure 4a compares for various perovskites the high frequency resistance R1 determined from the fitting of the impedance spectra measured over a large Vappl. This parameter varies in a large extent with the HP used as the absorber. MAPI (both prepared by a one-step or two step techniques28) PSCs show higher resistance than those with an absorber containing the Cs+ cation. A decrease of this parameter with the Cs+ concentration is found. We have shown in a previous work that this resistance scale with the TiO2 blocking layer thickness and crystallinity.28 Results from our group also demonstrate that this resistance has a contribution from the HTM (see Figure S4a, Supporting Information).


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(a)

(b)

Figure 4. (a) Variation of R1 with Vappl for various perovskite (MAPI(2) is a cell prepared with a twostep perovskite); (b) The HF relaxation time for various perovskites as a function of the applied voltage. The high frequency resistance has been also determined in the dark and much higher values have been found in this case (Figure S4b, Supporting Information). Light shining has a major effect on R1 parameter which is reduced in a large extend by photogenerated charges. This is in agreement with the work by Pockett et al.44 who found that R1 scales with light intensity at the Voc. These authors have described this resistance as a recombination resistance. However, R1 is a complex element that gathers a large variety of contributions. Changing the perovskite has a significant influence on this parameter by affecting the intrinsic resistances of the absorber layer and the interfaces which, in turn, affect the recombinations. Figure 4b shows the high frequency relaxation time defined as τHF=R1C1 of the cells with various perovskite absorbers. τHF is in the 10 µs range and is reduced by adding FA+ and Cs+ cations in the perovskite. The interest of analyzing cells without inductive loop in the middle frequency range is that the low frequency parameters R3 and C3 can be correctly estimated without the need of measurements at very low frequencies.28 The low frequency capacitance, C3, of cells with various perovskites has been extracted from the fit of the impedance spectra. Figure 5a shows that this parameter increases with Vappl and that values are of the same order of magnitude for the various perovskite. C3 increases dramatically by 2-3 orders of magnitude under the light shining. This surprising effect has been welldescribed in the literature.32 It has been assigned to the displacement of the ions, their accumulation at the interface and charge compensation under light shining. PSCs are known to present J-V curves with a hysteresis between the forward and the backward scan direction. This hysteresis effect correspond to a capacitive behavior and can be further studied by IS measurements. In the case of MAPI PSC, we have selected several solar cells, which preparation covered a long period of time, with various hystereses. The amplitude of the hysteresis has been quantified using an hysteresis index14 defined as:



HI(%) =

( PCE ) REV − ( PCE ) FOR ( PCE ) AVG

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(1)

with (PCE)REV, PCE)FOR and (PCE)AVG the power conversion efficiencies determined on the reverse scan, forward scan and by averaging both, respectively. Clearly, in Figure 5b, the C3 parameter scale with HI(%). The same behavior has been found in the case of the multiple cation perovskite cells. In Figure S5b (Supporting Information), C3 for two cells with extreme HI, namely 12% and 46% (the former and the latter cells having an efficiency of 17.1% and 15.4%, respectively) are displayed and show a difference of about one order of magnitude and confirm the strong correlation between C3 and the hysteresis amplitude (HI).

(a)

(b)

Figure 5. (a) Effect of perovskite and light shining on the low frequency C3 parameter. (b) Low frequency C3 parameter of MAPI cells for hysteresis indexes (HI) of 11%, 22% and 30%.

The low frequency impedance arc of circle was more depleted that the high frequency one showing a more imperfect capacitance behavior (Figure 2). The variation of the p parameter of the CPE3 element (Figure 2g,h) with the perovskite and Vappl is shown in Figure S6 (Supporting Information). Values of about 0.7 are measured for all the cells and p has a tendency to decrease with the applied voltage. We can note that its value tends to 0.5 at the Voc. This has led some authors to analyze the low frequency loop of the IS measured at the Voc as a Warburg impedance due to the diffusion of alkyl ammonium counter-ion and to calculate their diffusion coefficient from the impedance spectra.43 However, our results show that the low frequency loop cannot be analyzed as a diffusion impedance since the p-parameter is much higher than 0.5 over a large Vappl range. It has been proposed by Zarazua et al.45 that local doping at the interface is controlled by ionic defects, so that the capacitance needs charge compensation by ion for the accumulation capacitance to take place. This is a self-doping mechanism that obviously depends on the details of contact materials and surface morphology. 12 ACS Paragon Plus Environment

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The low frequency resistance, R3, is presented in Figure 6a for the various perovskites under light and in Figure S7 (Supporting Information) in the dark. Figure S7 (Supporting Information) shows that this resistance decreases in a large extent with light shining and that dark R3 tends to light R3 at high Vappl. This decrease was also found for instance Pockett et al. for measurements at the Voc for various light intensity.44 This resistance can be considered as an estimation of the recombination resistance. The lower R3 for Cs10FAMA is in agreement with the lower Voc for this perovskite-based device. The resistance decreases with the applied voltage and then more recombinations occur by increasing this parameter. τLF=R3C3 is the relaxation time calculated at low frequency. Figure 6b shows that it lies in the several seconds range. It shows the occurrence of a slow process which is the ionic relaxation in the perovskite near the interfaces. The slow ionic movements and redistribution govern the recombination phenomena occurring in the cells.

(a)

(b)

Figure 6. Effect of the perovskite monovalent cation composition on the (a) low frequency resistance R3 and (b) and low frequency relaxation time, τLF, of the PSCs measured under light at various voltages.

4. CONCLUSION We have optimized the protocol for the preparation of perovskites with various monovalent cations to reach power conversion efficiencies higher than 17% and up to 18.6%. The cell electrical properties have been studied and the devices have been fully investigated by impedance spectroscopy. The IS response have been investigated both under light and in the dark to discriminate the light sensitive parameters. The spectra have been carefully analyzed using an ad hoc equivalent circuit and the data has been discussed in the light of the existing literature. We have found that, far from the optimum situation in which every single feature in the impedance pattern is produced by a single physical process, perovskite solar cells present complex spectra in which a single feature is affected by 13 ACS Paragon Plus Environment


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different processes. For these cells, with perovskite prepared by the dripping technique, the spectra showed no inductive loop in the intermediate frequency range. The inductive loop is assigned to multistep charge transfer that could involve surface states and that is detrimental for the solar cell functioning. Our studies show that a large inductive loop is the signature of poorly functioning solar cells. The high frequency capacitance is the bulk response of perovskite and is lower in the presence of FA+ and Cs+. The other electrical parameters are influenced by interface phenomena, ionic conductivity and charge accumulations. The high frequency resistance combines many contributions (selective contact resistances, interfacial phenomena, intrinsic resistance of the perovskite). Our comprehensive study has also clearly demonstrated the scaling of the low frequency capacitance with the hysteresis amplitude. Moreover no diffusion impedance due to the diffusion of ionic species has been observed. However, ion mobility results in a strong effect on recombinations and has a strong influence on the low frequency impedance response of the system.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc. Nyquist plots of IS spectra of MAPI cells; Nyquist plot of an impedance spectrum of a FAMA cell in the dark; Variation of the CPE1 p parameter with the applied voltage; Effect of the HTM and of dark on the R1 parameter; C3 parameter in the dark; Effect of hysteresis on C3 for Cs5FAMA cells; CPE3-p parameter; variation of the R3 parameter in the dark as a function of the applied potential.

ACKNOWLEDGEMENTS P.W. thanks the China Scholarship Council (CSC) for scholarship funding. M.U. acknowledges the Indonesia Endowment Fund for Education (LPDP) scholarship for funding.

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