D Zhu

M.-N Zheng

Th Rager

Sol Pauporté

Rrl

Tao Zhu

Daming Zheng

Marie-Noelle Rager

Thierry Pauporté
email: thierry.pauporte@chimieparistech.psl.eu

Prof. Th Mr T Pauporté

Mr D Zhu

Zheng

Dr Marie-Noelle Rager

M.-N Rager

The Stabilization of Formamidinium Lead Tri-Iodide Perovskite through a Methylammonium-Based additive for High Efficiency Solar Cells

Keywords: Perovskite solar cell, Stabilization by methylammonium, NMR, High efficiency, Organic cations composition

Nowadays, complex chemistry and precursor solution compositions are developed to stabilize hybrid perovskite films and boost the efficiency of perovskite solar cells (PSCs). In this context, determining the actual composition of these layers, especially in organic cation, and understanding the chemistry behind is challenging. In the present paper, we consider the introduction of methylammonium (MA + ) in formamidinium lead iodide (FAPbI3) 3D perovskite to stabilize the α-phase which quantity must be minimized to reduce the material hydrophilicity and its possible destabilization by degassing. The key effects of methylammonium chloride (MACl) additive on the growth of FA1-xMAxPbI3 perovskite layers are studied. Liquid-NMR has been used to analyze the photovoltaic layers. NMR peaks and their origin have been identified. We have then reliably measured MA and FA content in films actually used in PSCs and prepared over a large additive molar concentration ratio. We have quantified x at 0.06±0.01 for the pure films which corresponds to the best entropic compound stabilization. It results in perovskite solar cells (PSCs) with a stabilized power conversion

efficiency as high as 22.06 %. These PSCs are shown highly stable under solar irradiation and high moisture.

Organic-inorganic hybrid perovskites (OIHPs) are among the most fascinating semiconductor materials due to their versatility, tunability, low temperature preparation and amazing optoelectronic properties. [1] Their direct transition bandgap can be tuned by playing on their composition, they exhibit long charge carrier lifetimes and they can be prepared at low cost starting from precursor solutions which are processed at low temperature. OIHPs have emerged as one of the most promising family of materials for photovoltaic solar cell application. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] The record power conversion efficiency of perovskite solar cells (PSCs) has reached a certified record efficiency of 25.2%. [16] PSCs have the highest performance for the thin film solar cell technologies and they are closely approaching the efficiency of the best silicon solar cells.

Formamidinium lead tri-iodide (FAPbI3) perovskite is among the best suited OIHP material for the application from the point of view of the bandgap for light harvesting over a vast wavelength range (from the near-IR to the near-UV) and for generating a rather large photovoltage output. [6,9] However, FAPbI3 is stable at room temperature in its δ-phase, a photoinactive compound in solar cells. The stabilization of the photo-active black perovskite α-phase requires the incorporation of an extra monovalent cation. [6,17] To the best of our knowledge and as summarized in Table S1 (Supporting Information), PSCs reaching PCE>22% reported in the literature contain methylammonium (MA + ) as extra-organic cation. [10][11][12][13][14][15] However, the presence of MA + presents disadvantages as this organic compound is hydrophilic and its excess can be degassed out of the layer. [9] It is therefore of utmost importance to incorporate enough MA + for stabilization and a minimum of this cation for the long-term stability. The other important points is that, for a high efficiency and a high stability, the layer must be made of large grains which go through the entire perovskite thin film. [18,19] To increase the performances of PSCs, complex recipes have been developed. Remarkable results have been achieved by employing additives and precursor solutions which composition is far from the actual composition of the final perovskite layer. For instance chloride is a wellknown additive which favor the formation of large grains and has been used in a large extent in MAPbI3 perovskite layers. [20,21] Only a small number of papers have employed chloride additives for the growth of perovskites mixing formamidinium and methylammonium cations. [11,15,[22][23][24] MACl was used by several authors for processing the perovskite layers upon spin-coating to increase their quality. [23,24] In another paper, MACl was combined with FAI to force the stoichiometric introduction of Cl in perovskite compounds and obtained mixed halide anion materials. [22] Kim et al. showed that MACl successfully induces an intermediate to the pure FAPbI3 α-phase without annealing. [15] However, a minimum of methylammonium must be incorporated in the perovskite to minimize its hydrophobicity and its destabilization. [9] We describe the accurate quantification of organic cations in perovskite thin films employed in high efficiency solar cells that have been produced from non-stoichiometric precursor solutions. The technique is based on liquid-NMR. It is applied to FA1-xMAxPbI3 layers, well-crystallized in their α-phase. The optimized films are free of δ-phase and of PbI2.

We show that MACl must be added in a large excess in the precursor solution to favor the layer crystallinity. We demonstrate that the actual methylammonium molar content in the films is low (x~0.06) and does not vary with the initial additive content between 40 mol% and 55 mol%.

It is explained by the fact that the best amount of MA for the thermodynamic stabilization of FA1-xMAxPbI3 is incorporated while all the excess is eliminated by the annealing treatment at 153°C. The obtained cells have a high efficiency and exhibit a high stability.

The perovskite layers were prepared as detailed in the Supporting Information. Table S1 (Supporting Information) shows that we employed a low concentration of precursor at 1.2M of PbI2 compared to the other recipes yielding very high efficiencies PSCs. The morphology of perovskite films with 40 mol%, 48 mol% and 55 mol% molar fractions of MACl additive (the films and cells are denoted MC40, MC48 and MC55, respectively, in the following) are presented in Figure 1a-c. The layers were uniform, compact, contained no pinholes and were made of remarkably large size perovskite grains. The various MACl fractions employed gave a similar morphology. The statistical analysis of the grain size, done with the Image J software, is presented in Figure 1d. The average values were 604 nm, 775 nm and 982 nm for MC40, MC48 and MC55 films, respectively. The grain size gradually increased with the MACl molar fraction used. The cross-sectional views of the layer in the solar cells confirmed the large size of the grains (Figure 1e). Remarkably, they also showed monolithic large grains, their thickness being the perovskite layer thickness. We therefore conclude that, in the solar cells, each grain is contacted by the electron transporting layers on one side (TiO2) and by the hole transporting layer on the other side (Spiro-OMeTAD) (Figure 1e).

The phase and the purity of the produced compounds were checked by X-ray diffraction (XRD). The patterns shown in Figure 2a and 2b correspond to the ultra-pure cubic α-phase perovskite. We have noted that high concentation of MACl, with an optimum at 48 mol.% was required to get the high purity α-phase, while MC30 and MC20 films contained both PbI2 and δ-phase impurities (See Figure S1, Supporting Information). The reflection peaks have been indexed accordingly. The fact that the strongest diffraction peaks belong to the (h00) family of plane shows a strong texturation of the layer with a preferential orientation along the [100] crystallographic direction. The XRD peak intensity followed the additive fraction in relation with the grain enlargement with increasing the chloride concentration. Another interesting information extracted from the XRD patterns is that the peak positions are almost unchanged with the fraction of MACl between 40% and 55% and that these parameters are very close to those of the α-FAPbI3 compound (Figure S1, Supporting Information). On the other hand, the XRD peaks of the FA1-xMAxPbI3 are significantly shifted compared to the MAPbI3 compound (Figure S1, Supporting Information). Replacing FA by MA is known to produce a shrinkage of the crystal lattice and a reduction of the lattice parameter. 15 It suggests that, in the present case, x in FA1-xMAxPbI3 does not change significantly with the initial amount of MACl in the precursor solution.

Energy dispersive x-ray spectrometry (EDX) allowed us to determine the layers I/Pb atomic ratio. The average values were 2.96, 3.15 and 3.06 for MC40, MC48 and MC55 samples, respectively. The theoretical ratio is 3 and the presence of PbI2 would reduce this parameter.

Within the uncertainty of the technique, the compounds were close to the stoichiometry without a significant amount of PbI2 present. Also, EDX analysis revealed that the final layers, after annealing, were almost free of chloride element, regardless of the MACl fraction in the precursor solution. In hybrid 3D perovskite, the integration of iodide is thermodynamically much more favorable than that of chloride. In the precursor solution employed, I is added in a stoichiometric ratio and no Cl need to be integrated in the structure for its stabilization. [22] Therefore, almost all chloride ions provided were eliminated during the annealing step. For the charge compensation, cations must be eliminated with Cl -. A small amount of MA is integrated in the structure to stabilize the black α-phase of perovskite while the organic cation excess is also eliminated upon the layer annealing.

To accurately determine how much MA is incorporated in the films and then to answer the critical issue of the actual organic cation composition of the prepared layers, a NMR study was conducted. To the best of our knowledge, in the works investigating perovskite composition by NMR, the analyzed compounds are precipitated powders, [7,[25][26][27] crystals, [7,26,28] or films produced by drop-casting [25] or by spin-coating combined with vacuum evaporation. [29] By using them, one can get a quite large amount of material that permits solid-state-and/or liquid-NMR investigations. However, the drawback of these techniques is that they deviate from the ones employed for very high efficiency PSC fabrication. In our case we have investigated the films actually employed in the perovskite solar cells. They have been directly dissolved in DMSO-d6 solvent (see the Experimental Section). The liquid-state 1 H-NMR and 13 C-NMR spectra of the MC48 layer and their correlation figure are disclosed in Figure S2a, S2b and S2c (Supporting Information), respectively. In the NMR spectra, we observed more peaks than expected from our MAI and FAI reference spectra (Figure 3a and 3b, respectively). The 1 H NMR spectrum showed a singlet at δ 7.84 ppm, which was correlated with the 13 C NMR peak at δ 157.4 ppm, and a singlet at δ 2.37 ppm correlated with the 13 C NMR peak at δ 24.7 ppm. [25] Based on our references (Figure 3a and3b), these peaks were attributed to the CH of the formamidinium and the CH3 of methylammonium, respectively. [27] We also observed a small 1 H peak at δ 7.94 ppm, correlated with the 13 C peak at δ 155.4 ppm and a 1 H peak at δ 2.81 ppm correlated with a 13 C peak at δ 28.0 ppm. A very small 1 H peak was also detected at δ 2.96 ppm. These three features are absent from the FAI and MAI NMR spectra (Figure 3) and it suggests that they result from reaction products between FA and MA. MAI in solution undergoes deprotonation and methylamine and HI are formed in equilibrium with MAI. Upon the writing of this paper, an article by Wang et al. [27] appeared which shows that the nucleophilic N atom in MA can react with the electrophilic imine group of FAI. It results in the formation of N-methyl FAI (CH3-NH2-CH=NH•HI) by condensation. The 1 H-NMR peak at δ 7.94 ppm fit with the CH in N-methyl FAI and that at δ 2.81 ppm with the CH3 in this compound. The very small peak at δ 2.96 ppm could be assigned to CH3 in N,N'-dimethyl FAI (CH3-NH2-CH=N-CH3•HI) formed by a second condensation reaction. We conclude that, following the layer dissolution in DMSO solution, N-methyl FAI is formed by the partial reaction between FAI and MA and that this compound is detected on the NMR spectra. A small amount of N,N'-dimethyl FAI is also formed. The spectra were similar for the MC40, MC48 and MC55 layers (Figure S2, Supporting Information). The 1 H-NMR peaks of formamidnium, methylammonium, N-methyl FAI and N,N'-dimethyl FAI were present with about the same ratio.

The main advantage of the liquid-state 1 H-NMR technique is its high sensitivity. As a consequence, by integrating the spectrum peaks, we have been able to quantify the relative amount of FA and MA incorporated in the perovskite layers actually used in the solar cells.

The DMSO peak was retained as an internal reference. The integration value of MAI peak was corrected of the 13 C satellite of DMSO. The CH3 of N-methyl FAI peak was corrected of the second N-methyl FAI peak which appeared at the same chemical shift. We have also supposed that the two CH3 of N,N'-dimethyl FAI had the same integration. The results gave x values in FA1-xMAxPbI3 equal to 0.064, 0.064 and 0.061 for MC40, MC48 and MC55 layers, respectively (x was slightly less for the MC30 (0.060) and MC20 (0.040) films (Table S2, Supporting Information) but they were not phase pure). Within the uncertainty of the technique, the remarkable point is that this parameter is almost irrespective of the MACl mol% present in the precursor solution which was varied in a large extent and is determined at 0.06±0.01. We can conclude that this value corresponds to the optimum for which the prepared black α-phase perovskite is the most stable. The excess of MACl is removed upon the annealing at 153°C by decomposition/sublimation. [9] For the charge and chemistry balance, a small amount of FA must be also eliminated likely by similar processes. The incorporation of a small amount of MA for the stabilization of FAPI is in agreement with recent computational investigation which calculated in FA1-xMAxPbI3 a minimum of the Gibbs free energy for x <0.2, while this parameter continuously increases at higher x. [30] The optical properties of the layers were characterized by measuring their absorbance curves. The spectra of FA1-xMAxPbI3 layers are compared to those of MAPbI3 and FAPbI3 in Figure S3 (Supporting Information). They exhibit a direct optical transition at about 820 nm.

We observed that FAPbI3 and FA1-xMAxPbI3 layers had about the same absorbance in the 550 nm-800 nm range. In Figure 2c. the linear fit intersection of the Tauc plot with the energy axis gives an optical bandgap energy (Eg) of 1.537 eV, regardless of the additive fraction. It confirms the same stoichiometry for the perovskite layers prepared. This value was very close to our Eg value of FAPbI3, 1.528 eV, and far apart the Eg of MAPbI3 (1.597 eV). These results are in line with the NMR study. This low Eg is a positive property for the light harvesting in the near-IR.

We have also measured the photoluminescence (PL) spectra of the layers. The maximum emission wavelength was the same for the prepared mixed cation perovskites and their normalized spectra overlapped (Figure 2d). In the same figure, we have reported the FAPbI3 and MAPbI3 spectra. FA1-xMAxPbI3 layers had a maximum emission wavelength (811 nm) very close to that of FAPbI3 (814 nm). Using the approximation of a linear variation and the emission maximum of MAPbI3, we found x at 7.5±2.5%, which is in agreement with the NMR study.

Perovskite solar cells were prepared by completing the layers stacking by spin-coating a Spiro-OMeTAD layer and by evaporating a gold back-contact. An SEM view of the device structure is presented in Figure 1e. Typical J-V curves are displayed in Figure 4a. Table 1 summarize the J-V curve parameters of devices among the best. We focus our discussion on the highest concentrations of MACl since the performances of MC20 and MC30 cells were rather lower (Table S3a, Supporting Information). Table 1 shows that the cells were almost hysteresis free. The very low hysteresis can be related to the very large grains and low defect density in the structure. JSC was the highest for MC48 and MC55 cells and the VOC was the highest for the MC48 ones. Overall, the best power output was achieved for MC48 devices.

These trends can be explained by the larger grains, higher structural quality and less defects in the case of the MC48 perovskite layer. The VOC depends on the recombination rate in the grains bulk, at the grains boundaries and at the perovskite interfaces with the selective contact. Overall, choosing MACl at 48% molar fraction appears as the best compromise for the best performances.

The observation made for the best cells were confirmed by the statistics of the J-V curve parameters distribution (Figure 4b). We noted the high JSC measured with the presented perovskite. The high charge generation efficiency over a large wavelength range was confirmed by EQE measurements (Figure 4c). The large grains, high crystallinity and high absorbance over a large visible-near infrared range of the produced perovskite layers gave rise to high performances. Figure 5a shows the reverse and forward scans for the best cell with a 22.15% PCE measured on the reverse scan. The hysteresis index of this cell was only 4.0%. By tracking, the steady power output was reached after about 80s and measured at 22.06% which is very close to the PCE measured on the reverse scan (Figure 5b).

We have also noted that the prepared cells had a remarkable stability. The photostability was followed for 5 hours under a continuous one sun AM 1.5G illumination at ambient temperature and 45% of relative humidity (RH). Figure S4a (Supporting Information) compares unencapsulated MC48, MAPI and FAPI cells. The latter had a very low efficiency (Table S3b, Supporting Information) and were not stable when stored under ambient conditions. Their PCEs degraded quickly upon tracking at the maximum power point under one sun (Figure S4a). On the contrary, the power output was highly stable and unchanged in the case of the FA0.94MA0.06PbI3 cell of the present study. In the same figure, we also include the MAPI cell power output which continuously decreased during the test. The stability against moisture was tested by storing the unencapsulated cells in a chamber with a 90% RH and ambient light conditions (Figure S4b, Supporting Information). The MC48 power output was stable for 20h. The MAPI cell lost 60 % of its initial PCE within only 20h. The variations of the J-V curve parameters are shown in Figure S4b (Supporting Information). This high stability is explained by the optimal incorporation of a minimum of MA for the α-phase stabilization and large grains.

In summary, we have described the reproducible synthesis of mixed organic cation halide perovskite films using methyl ammonium chloride as additive in a large amount and a rather low-concentration precursor solution compared to the other high efficiency PSC works. We have shown that the optimum amount of this additive is 48 mol%. The perovskite compound had a low bandgap, very large grains and was optimized to be free of parasitic phases.

Importantly, we have identified by liquid NMR the organic compounds formed after the dissolution of the layers in DMSO and shown the presence of condensation products between MA and FA. On this basis, we have been able to determine the actual methylammonium fraction in the FA1-xMAxPbI3 perovskite photovoltaic films. We have shown that the MA amount was very low at x~0.06±0.01. This small amount is analyzed as the best for the entropic stabilization of the FA1-xMAxPbI3 compound. It resulted in a high PCE up to 22.15% and an excellent stability of the prepared solar cells.

Experimental Section

The full experimental details are provided in the Supporting Information. In brief, the fluorinedoped SnO2 (FTO) substrates were cleaned and the compact TiO2 hole blocking layer (c-TiO2) and the mesoporous TiO2 (mp-TiO2) layer were deposited as described in our Ref. [17] . We prepared a mixed cation precursor solution with a 1.2M concentration by mixing 206 mg of formamidinium iodide (FAI, greatcell), 553 mg of PbI2 (TCI), and 16.2 mg, 24.3 mg, 32.4 mg, 38.9 mg and 44.6 mg of methylammonium chloride (Alfa Aesar) in 800 μL DMF and 200 μL DMSO. They corresponded to 20 mol%, 30mol%, 40 mol%, 48 mol% and 55 mol% of FAI, respectively. The solutions were spin-coated on top of the FTO/c-TiO2/mp-TiO2 substrates. 100 µL of chlorobenzene was dropped upon spinning to produce the layers. For the FAPbI3 films, a precursor solution with a 1.2 M concentration was prepared by mixing 103.18 mg of FAI and 276.7 mg of PbI2 in 100 μL DMSO and 400 μL DMF. The spin coating procedure was the same.

All the layers were annealed at 153°C, 13 min for the FA1-xMAxPbI3 films and for 18 min for the FAPbI3 films. Spiro-OMeTAD (Borun New Material Technology) was employed as the HTM and prepared as in Ref. [17] . The solar cells characterizations, the XRD patterns, the absorbance and photoluminescence spectra measurements as well as the Tauc plot analyses of the films were performed as described in our previous work. [17] A Quantax system from Bruker operated at 15 kV was employed for the EDX spectra measurements.

For NMR ( 1 H and 13 C) the chemical shifts, in ppm, are referred to the residual DMSO solvent signal. Perovskite layers, deposited on a sprayed c-TiO2 layer, were dissolved in DMSO-d6 solvent (99.96 %D, H2O<0.01%, Euriso-top) and the NMR spectra of the solutions were measured by a Bruker Avance 500 MHz Neo spectrometer.

Methylamonium iodide: 1 H NMR (DMSO, 500 MHz): δ = 2.37 (s, CH3). 13

Supporting Information

The Supporting Information is available online. Table of review of perovskite solution compositions for very high efficiency PCS with their references; Experimental Section; XRD patterns of α-FAPbI3, MAPbI3, MC20, MC30, MC40, MC48 and MC55 thin films; 1 H NMR spectra of MC48, MC40 and MC55, 13 C NMR spectrum and c 1 H & 13 C NMR spectrum correlation diagram of MC48; Table of x; Absorbance spectra of α-FAPbI3, MAPbI3, MC20, MC30, MC40, MC48 and MC55 thin films; Table of J-V curves parameters of MC20, MC30, MC48 and FAPI cells; Stability trackings.

Table of contents entry

MACl additive is employed to synthesize FA1-xMAxPbI3 films. The best molar fraction of this additive is determined. The MA content in the thin films actually used in solar cells is determined at x= 0.06. This amount is thermodynamically the best for the stabilization of this highly efficient perovskite. The perovskite solar cell achieves a stabilized power conversion efficiency as high as 22.06%.

Keywords

Supporting Information

The

Experimental Section

Preparation of Substrate, compact-TiO2 and mesoporous TiO2 Layers: Fluorine-doped SnO2 glovebox before to be characterized.

Characterization Methods: The J-V curves were recorded by a Keithley 2410 digital sourcemeter, using a 0.1 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 (100 mW/cm 2 ).

The illuminated surface was delimited by a black mask with an aperture diameter of 3 mm. The power density was calibrated at 100 mW.cm -2 by the use of a reference silicon solar cell.

The structure of the organolead perovskite film was characterized by a PANanalytical X-Pert high-resolution X-ray diffractometer (XRD) operated at 40 kV and 45 mA and using the CuKα radiation with λ= 1.5406 Å. The film specular absorbance was measured by a Cary 5000 UV-Vis-NIR spectrophotometer. A glass/FTO/c-TiO2/mp-TiO2 sample was used as the baseline.

The photoluminescence spectra were measured by a Cary Eclipse fluorescence spectrophotometer. The sample morphologies were examined with a high resolution Ultra 55

Zeiss FEG field-emission scanning electron microscope (FE-SEM) in the in-lens mode. The EDX spectra were measured with a Quantax system from Bruker operated at 15 kV.

NMR ( 1 H and 13 C) spectra were recorded on a Bruker Avance 500 MHz Neo spectrometer.

Chemical shifts are given in ppm using the residual DMSO solvent signal as an internal reference. Perovskite layers deposited on a sprayed c-TiO2 layer were dissolved in DMSO-d6 solvent (99.96 %D, H2O<0.01%, Euriso-top) and the NMR spectra of solutions were measured.

Methylamonium iodide: 1 H NMR (DMSO, 500 MHz): δ = 2.37 (s, CH3). 13 C NMR (DMSO, 125 MHz): δ = 24.7.

T

  . Zhu, D. Zheng, M.-N. Rager, Th. Pauporté, Sol. RRL 2020, 2000348.

  C NMR (DMSO, 125 MHz): δ = 24.7. Formamidinium iodide : 1 H NMR (DMSO, 500 MHz): δ = 7.84 (s, CH). 13 C NMR (DMSO, 125 MHz): δ = 157.4. N-methyl formamidinium iodide : 1 H NMR (DMSO, 500 MHz): δ = 7.94 (s, CH), 2.81 (s, CH3). 13 C NMR (DMSO, 125 MHz): δ = 155.4 (CH), 28.0 (CH3). N, N'-dimethyl formamidinium iodide: 1 H NMR (DMSO, 500 MHz): δ = 2.96 (s, CH3NH2CH=NCH3), 2.81 (s, CH3NH2CH=NCH3).

Figure 1 .

 1 Figure 1. (a-c) Typical SEM images of the perovskite layers surface with various MACl additive mol%. (a-c) Share the same scale bar. (d) Statistical analysis of the effect of MACl additive fraction on the grain size. (e) Cross-sectional SEM view of a MC48 perovskite solar cell.

Figure 2 .

 2 Figure 2. (a) XRD patterns of the perovskite layers. (b) is a zoom view of (a). # symbol marks a FTO substrate peak. (c) Absorbance Tauc plots and (d) normalized photoluminescence spectra of α-FAPbI3, MAPbI3, MC40, MC48 and MC55 layers.

Figure 3 .

 3 Figure 3. 1 H-NMR spectra of (a) MAI, (b) FAI and (c) MC48 layer with indexation. 1. NH=CHNH3I; 2. CH3NH2CH=NHI; 3. CH3NH3I; 4. CH3NH2CH=NHI + CH3N=CHNH2CH3I; 5. CH3N=CHNH2CH3I (N,N'-dimethyl FAI).

Figure 4 .

 4 Figure 4. (a) J-V curves of MC40, MC48 and MC55. solar cells (reverse scan). (b) Statistical analysis of the J-V parameters. (c) EQE spectra and integration curve of JSC.

Figure 5 .

 5 Figure 5. (a) J-V forward and reverse curves of the best FA1-xMAxPbI3 cell. (b) Tracking of the PCE.

:

  Perovskite solar cell, Stabilization by methylammonium, NMR, High efficiency, Organic cations composition Tao Zhu, a Daming Zheng, a Marie-Noelle Rager, b Thierry Pauporté* a The Stabilization of Formamidinium Lead Tri-Iodide Perovskite through a Methylammonium-Based additive for High Efficiency Solar Cells ToC figure T. Zhu, D. Zheng, M.-N. Rager, Th. Pauporté, Sol. RRL 2020, 2000348. Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018.

(

  FTO) substrates (TEC 7 from Pilkington) were etched pattern by zinc powder and 10% HCl solution prior to be cleaned with soap and water. The substrates were subsequently immersed for 20 min in a concentrated 2.2 M NaOH in ethanol/water (10:1 v/v %) and then rinsed with deionized water in an ultrasonic bath for 15 min. The substrates were subsequently heated at 500°C for 15 min. The compact TiO2 hole blocking layer, noted c-TiO2, was prepared by aerosol spray pyrolysis. A TiO2 nanoparticle solution employed for the preparation of the mesoporous layer, noted mp-TiO2, was prepared in advance and stirred for at least 12h. The TiO2 NR30-D paste (from Greatcell) was diluted in ethanol with a 1:7 w/w ratio. 45 μL of the solution was dropped on the compact TiO2 layer and spin-coated at 2000 rpm for 15 s. The layer was then dried on a hotplate at 70 °C for 5-10 min and finally heated at 500 °C under an air flux for 30 min, cooled down to 200 °C and removed from the hotplate.Preparation of Perovskite Layer:A mixed cation precursor solution with a 1.2M concentration was prepared by mixing 206 mg of formamidinium iodide (FAI, greatcell), 553 mg of PbI2 (TCI), and 16.2 mg, 24.3 mg, 32.4 mg, 38.9 mg and 44.6 mg (20 mol%, 30mol%, 40 mol%, 48 mol% and 55 mol%, respectively) of methylammonium chloride (MACl, Alfa aesar) in 800 μL DMF and 200 μL DMSO. The solution was stirred for a minimum of 2-4h at room temperature in a N2 glovebox. 45 μL of this solution was placed on top of the substrates. A two-step spincoating program was employed: first spinning at 1000 rpm for 10 s and then at 6000 rpm for 30 s. 100 µL of chlorobenzene was dripped 20 s after the starting of the spinning routine. The films were then annealed at 153 °C for 13 min. These films and corresponding solar cells are noted MC20, MC30, MC40, MC48 and MC55. The PEAI post-deposition treatment consisted in dropping 60 μL of a 10 mM 2-Phenylethylamine Hydroiodide (PEAI) solution (2.49 mg in 1 mL of isopropanol) onto the cold perovskite film. A one-step spin-coating program was employed: 2000 rpm/s acceleration, 3000rpm for 20s. For the FAPbI3 films, the precursor solution had a 1.2 M concentration. 103.18 mg of FAI and 276.7 mg of PbI2 were mixed in 100 μL DMSO and 400 μL DMF. The spin coating procedure was the same and the annealing time at 153°C was 18 min.HTM layer and Gold Contact:The hole transporting material (HTM) solution was prepared by dissolving 78 mg of Spiro-OMeTAD (Borun New Material Technology) in 1 mL of chlorobenzene. Then, 17.9 µL of bis(trifluoromethylsulfonyl)imide lithium salt solution (Li-TFSI) (Sigma Aldrich) solution (517 mg in 1 mL ACN), 30.4 µL of TBP (tert-butylpyridine) (Sigma Aldrich) and 14 µL of tris(2-1H-pyrazol-1-yl)-4-tert-butylpyridine)-cobalt(III) tris (bis(trifluoromethylsulfonyl)imide) (Dyesol, FK209) (376 mg in 1 mL acetonitrile) were added to this solution. 40 µL of the HTM solution was spin-coated at 4000 rpm for 30 s. Finally, the device was completed by thermally evaporating a 70-80 nm thick gold back contact on the Spiro-OMeTAD layer. The solar cells where matured by storing them several days in a N2

Formamidinium iodide : 1 H

 1 NMR (DMSO, 500 MHz): δ = 7.84 (s, CH).13 C NMR (DMSO, 125 MHz): δ = 157.4.N-methyl formamidinium iodide: 1 H NMR (DMSO, 500 MHz): δ = 7.94 (s, CH), 2.81 (s, CH3). 13 C NMR (DMSO, 125 MHz): δ = 155.4 (CH), 28.0 (CH3). N, N'-dimethyl formamidinium iodide: 1 H NMR (DMSO, 500 MHz): δ = 2.96 (s, CH3NH2CH=NCH3), 2.81 (s, CH3NH2CH=NCH3).

Figure S1 .Figure S2a: 1 H

 S11 Figure S1. XRD patterns of α-FAPbI3, MAPbI3, MC20, MC30, MC40, MC48 and MC55 thin films prepared by spin-coating. # symbol marks a FTO substrate peak. (b) is a zoom view of (a) with a longer counting time.

Figure S2c: 1 H

 1 Figure S2c: 1 H-13 C HSQC NMR spectrum of MC48 film. : Formamidinium; ◼ : Methylamonium, •: N-methyl formamidinium.

Figure S2d: 1 H

 1 Figure S2d: 1 H NMR spectrum of the MC40 film. : Formamidinium; ◼ : Methylamonium, •: N-methyl formamidinium; : N, N'-dimethyl formamidinium.

Figure S2e: 1 H

 1 Figure S2e: 1 H NMR spectrum of the MC55 film. : Formamidinium; ◼ : Methylamonium, •: N-methyl formamidinium; : N, N'-dimethyl formamidinium.

Table 1 .

 1 Photovoltaic J-V parameters of FA0.94MA0.06PbI3 cells among the best, prepared with various MACl%, (under AM 1.5G conditions, 100 mW/cm 2 ).

	Cell	Scan Direction Voc/ V	Jsc / mA.cm -2 FF/ %	PCE/ %	HI a /%
	MC40	Reverse Forward	1.043 1.035	25.79 25.82	78.21 76.11	21.05 20.34	3.3
	MC48	Reverse Forward	1.066 1.048	25.95 25.94	80.08 78.16	22.15 21.25	4.0
	MC55	Reverse Forward	1.026 1.012	25.90 25.92	80.63 77.83	21.42 20.42	4.5
	[a] Hysteresis index defined as [PCE(%)rev-PCE(%)for]/PCE(%)rev

Stabilization of Formamidinium Lead Tri-Iodide Perovskite through a Methylammonium-Based additive for High Efficiency Solar Cells

	Tao Zhu, a Daming Zheng, a Marie-Noelle Rager, b Thierry Pauporté* a

Table S1 :

 S1 Review of perovskite structure and precursor solution composition of PSCs with PCE > 22% published in the literature.

	Perovskite structure	[Pb 2+ ]	[FAI]	[MAI]	[MACl]	MA/ MA+FA	MACl/ MAI+FAI	PCE %/ year	Ref.
	(FAPbI 3 ) 0.95 (MAPbBr 3 ) 0.05	1.48M 1.41M	0.07M	0.49M	4.7mol%	33mol%	23.2% 2018 23.3% 2019	[1] [2]
	(FAPbI 3 ) 0.92 (MAPbI 3 ) 0.08 (FAPbI 3 ) 0.87 (MAPbBr 3 ) 0.13	1.5M	0.523M 0.523M	0.04M 0.08M(MABr)	0.133M 0.133M	7.1mol% 13.3mol%	24mol% 22mol%	23.32% 2019	[3]
	FA 1-x MA x PbI 3	1.5M	0.523M	0.04M	0.133M	7.1mol%	24mol%	23.0% 2020 [4]
	(FAPbI 3 ) 0.92 (MAPbBr 3 ) 0.08	1.53M 1.4M	0.11M	0.5M	7.3mol%	33mol%	23.4% 2019 [5]
	FA 1-x MA x PbI 3	1.8M	1.8M	0	0.72M	0	40mol%	24.02% 2019	[6]
	FA 0.94 MA 0.06 PbI 3	1.2M	1.2M	0	0.58M	0	48mol%	22.18%	this work
	K 0.03 Cs 0.05 FA 0.83 MA 0.17 Pb 0.97 Ge 0.0 3 (I 0.9 Br 0.1 ) 3	1.25M 1M	0.2M (MABr) 0.5M	17mol%	42mol%	22.7% 2020	[7]
	Cs 0.05 FA 0.85 MA 0.10 Pb(I 0.97 Br 0.03 ) 3 excess 0.06M PbI 2	1.61M 1.31M	0.15M	0.08M CsI	10mol%	5mol%Cs 23.5% 2020 [8]
	Cs 0.06 FA 0.9 MA 0.04 PbI 3 (excess 0.1M PbI 2 )	1.6M	1.38M	0.06M	0.06M CsI	4mol%+	6mol%Cs 22.2% 2019 [9]

Table S2 :

 S2 x in FA1-xMAxPbI3 films determined by 1 H-NMR.

		MC20	MC30	MC40	MC48	MC55
	MACl mol.%	20	30	40	48	55
	x	0.040	0.060	0.064	0.064	0.061

Acknowledgements T.Z. and D.Z. thank the CSC-Paristech and CSC programs for Ph.D scholarship. The ANR agency and the Ile de France Region are acknowledged for financial support via the Moreless project ANR-18-CE05-0026 and the SESAME equipment project, respectively. The NSFC (21703070), the Science and Technology Program of Guangzhou (201804010176 ) and the Guangdong Basic and Applied Basic Research Foundation (2020A1515010378) are acknowledged for funding.

Received: ((will be filled in by the editorial staff))

Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))

Table S3. J-V curves parameters of (a) MC20, MC30 and MC48 cells, without PEAI surface treatment; (b)


Daming Zheng

Tao Zhu

Thierry Pauporté
email: thierry.pauporte@chimieparistech.psl.eu

Using Monovalent-to

Keywords: Perovskite solar cells, Impedance spectroscopy, Mono-and Multi-cations, Recombination, Ionic mobility

Introduction

Organic-inorganic hybrid perovskites (OIHP) mix both organic and inorganic components and have the ABX3 general formula, with A a monovalent cation, B a divalent metal cation and X a halide anion. They have emerged recently as a versatile and exciting class of semiconductor materials for applications in opto-electronics. [START_REF] Kim | Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%[END_REF] These compounds combine the key properties of traditional inorganic semiconductors such as gallium arsenide (GaAs) and silicon (Si), including high charge carrier mobility and device efficiency, with the advantageous properties of organic semiconductors such as preparation simplicity, low-temperature processing, lowcost and property tunability. Impressive progresses have been made recently on the application of OIHPs in various fields such as lasers, [START_REF] Leyden | Distributed Feedback Lasers and Light-Emitting Diodes Using 1-Naphthylmethylamnonium Low-Dimensional Perovskite[END_REF] LEDs, [START_REF] Wei | An Overview on Enhancing the Stability of Lead Halide Perovskite Quantum Dots and Their Applications in Phosphor-Converted LEDs[END_REF] photodetectors, [START_REF] Miao | Recent Progress on Highly Sensitive Perovskite Photodetectors[END_REF] scintillators [START_REF] Mykhaylyk | Bright and Fast Scintillation of Organolead Perovskite MAPbBr3 at Low Temperatures[END_REF] and photovoltaic solar cells. [START_REF] Kim | Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%[END_REF][START_REF] Zhu | Actual Organic Cations Composition Determination in Perovskite Thin Films. Application to Formamidinium Lead Iodide Stabilization for High Efficiency Solar Cell[END_REF][START_REF] Zhang | Electrochemical Design of Nanostructured ZnO Charge Carrier Layers for Efficient Solid-State Perovskite-Sensitized Solar Cells[END_REF][START_REF] Zhang | Fast and Low Temperature Growth of Electron Transport Layers for Efficient Perovskite Solar Cells[END_REF][START_REF] Yoo | An Interface Stabilized Perovskite Solar Cell with High Stabilized Efficiency and Low Voltage Loss[END_REF][START_REF] Liu | Ultrahydrophobic 3D/2D Fluoroarene Bilayer-Based Water-Resistant Perovskite Solar Cells with Efficiencies Exceeding 22%[END_REF][START_REF] Pitarch-Tena | Impedance Spectroscopy Measurements in Perovskite Solar Cells. Device Stability During the Measurement and Noise Reduction[END_REF][START_REF] Nie | Solar cells. High-Efficiency Solution-Processed Perovskite Solar Cells with Millimeter-Scale Grains[END_REF][START_REF] Zhang | Effects of Oxide Contact Layer on the Preparation and Properties of CH3NH3PbI3 for Perovskite Solar Cell Application[END_REF][START_REF] Zhang | One-Dimensional Self-Standing TiO2 Nanotube Array Layers Designed for Perovskite Solar Cell Applications[END_REF][START_REF] Tu | Mixed-Cation Perovskite Solar Cells in Space[END_REF][START_REF] Yang | Iodide Management in Formamidinium-Lead-Halide-Based Perovskite Layers for Eefficient Solar Cells[END_REF][START_REF] Leblanc | Lead-and Iodide-Deficient (CH3NH3)PbI3 (d-MAPI): The Bridge Between 2D and 3D Hybrid Perovskites[END_REF][START_REF] Wang | Insights into the Hole Blocking Layer Effect on the Perovskite Solar Cell Performance and Impedance Response[END_REF][START_REF] Wang | Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells[END_REF] This material family exhibits excellent optoelectronic properties, an adjustable bandgap, long charge carrier diffusion length, [START_REF] Stranks | Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber[END_REF] low-cost and low-temperature solution processibility. [START_REF] Kim | Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%[END_REF][START_REF] Stranks | Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber[END_REF] OIHPs thin films can be printed on flexible substrates for lightweight devices. [START_REF] Leung | A Self-Powered and Flexible Organometallic Halide Perovskite Photodetector with Very High Detectivity[END_REF] The certified present record efficiency for a PSC is 25.5%. 22 The structural unit of OIHP compounds contains an anionic corner-sharing octahedral network stabilized by cations. The stability of the 3D perovskite phase is limited by geometry and oxidation state restrictions, quantified by a tolerance factor (tf). The well-known Goldsmith tolerance factor has been refined recently by Bartel et ) (1) where nA is the oxidation state of A (+1), ri is the ionic radius of ion i (2.70 Å for methyl ammonium (MA), 2.79 Å for formamidinium (FA), 1.81 Å for Cs and 1.19 Å for Pb 2+ ), rA > rB by definition. tf < 4.18 indicates that the perovskite structure is allowed. MA and FA are the two main organic monovalent cations that can be employed as organic A. However, due to the large size of FA, increasing the complexity of the perovskite was proved necessary to increase the PSC stability and performances. [START_REF] Matsui | Cesium-Containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency[END_REF][START_REF] Saliba | Incorporation of Rubidium Cations into Perovskite Solar Cells Improves Photovoltaic Performance[END_REF][START_REF] Turren-Cruz | Methylammonium-Free, High-Performance, and Stable Perovskite Solar Cells on a Planar Architecture[END_REF] Above the pioneering popular monovalent mono-cation MAPbI3 compound, a huge research effort has been directed towards the development of double-cation FA1-xMAxPbX3 and triple-cation M a y(FA1-x MAx)PbX3 (with M a an alkali metal) perovskites to improve the device performances and stability. In the latter case, Cs is the most popular elemental monovalent cation incorporated. However, the full entropic stabilization of this triple cation perovskite also requires the addition of Br in complement to I for X. [START_REF] Matsui | Cesium-Containing Triple Cation Perovskite Solar Cells: Improved Stability, Reproducibility and High Efficiency[END_REF] Alternative strategies of stabilization by using additives have been developed lately [START_REF] Zhu | Actual Organic Cations Composition Determination in Perovskite Thin Films. Application to Formamidinium Lead Iodide Stabilization for High Efficiency Solar Cell[END_REF][START_REF] Kim | Methylammonium Chloride Induces Intermediate Phase Stabilization for Efficient Perovskite Solar Cells[END_REF] and, to get stable double cation perovskite, we have developed recently the use of MACl as an additive in the FA1-xMAxPbI3 precursor solution. [START_REF] Zhu | Actual Organic Cations Composition Determination in Perovskite Thin Films. Application to Formamidinium Lead Iodide Stabilization for High Efficiency Solar Cell[END_REF] This compound has allowed the direct deposition of the perovskite phase upon the spin-coating and then the growth of well-crystalized FA1-xMAxPbI3 layers upon the annealing post-deposition step.

The purpose of the present paper is to investigate the electrical response, especially the electrical impedance spectra, of high efficiency PSCs. The perovskite composition has been varied, especially the number of monovalent cation employed to stabilized the perovskite photoactive phase. Based on this compositional study and the literature data, we propose a better understanding of the main features found in their impedance spectra and provide an answer to the question: what are the expected impedance spectra for the best PSCs? More precisely, we compare the following perovskites: MAPbI3, FA0.94MA0.06PbI3 and Cs0.08FA0.80MA0.12Pb(I0.88Br0.12)3. They have been the subject of a thorough optimization work in the group for the last years. [START_REF] Zhu | Actual Organic Cations Composition Determination in Perovskite Thin Films. Application to Formamidinium Lead Iodide Stabilization for High Efficiency Solar Cell[END_REF][START_REF] Zhu | Interfacial Engineering through Chloride-Functionalized Self-Assembled Monolayer for High Efficiency Perovskite Solar Cells[END_REF] The best performances were achieved for the second one at 22.2 %. The basic electrical characterization of a PSC just consists in measuring their J-V curves in the reverse and forward scan directions and in quantifying the hysteresis amplitude.

However, it provides poor information about the dynamic processes occurring in the cells. In the present work, these measurements have been completed by an investigation of the effect of light intensity on the steady-state Voc and by impedance spectroscopy measurements. The former provides information and quantifications about the recombination mechanism. The latter gives an in-depth and complete view of the physical phenomena occurring in PSCs in operation.

With the help of complementary measurements, the spectral features have been assigned to physical phenomena. The various relaxations have been decomposed and quantified using electrical elements. Their variation has been analyzed with the applied voltage and the chemical nature of the perovskite. The present comparative study has allowed us to clarify the assignment of some electrical elements and to confirm others. Moreover, we have attached a special attention to the stability of the systems under irradiation and electrical stresses since these measurements provide a good quantification of the PSC stability. 11

Experimental

The Fluorine-doped SnO2 (FTO) substrates (TEC 7 from Pilkington) were etched, patterned and a compact TiO2 layer (c-TiO2) was deposited by spray pyrolysis as detailed in our Ref. [START_REF] Wang | Insights into the Hole Blocking Layer Effect on the Perovskite Solar Cell Performance and Impedance Response[END_REF] .

A mesoporous TiO2 layer (mp-TiO2) was subsequently prepared by spin-coating as described in our Ref. [START_REF] Zhu | Interfacial Engineering through Chloride-Functionalized Self-Assembled Monolayer for High Efficiency Perovskite Solar Cells[END_REF][START_REF] Ulfa | Oxide Hole Blocking Selective Contacts in Perovskite Solar Cells[END_REF] . for 2h before use. The spin-coating program was 1000 rpm for 10s and 6000 rpm for 30 s. 100 µL of chlorobenzene was dripped 30s after the starting of the spinning routine. The layers were finally annealed on a hotplate at 105 °C for 60 min. The best performances were achieved after the device preparation. These layers are denoted MAPI throughout the paper.

MAPI layers preparation

CsFAMA layers preparation:

A precursor solution corresponding to a Cs0.08FA0.80MA0.12Pb(I0.88Br0.12)3 perovskite layer composition was prepared. First, 179 mg of formamidinium iodide (FAI), 17.4 mg of methylammonium bromide (MABr), 27.0 mg of CsI, 548 mg of PbI2 and 57.1 mg of PbBr2 were mixed in 220 μL DMSO and 780 μL DMF. The solution was stirred for a minimum of 3-4 h at room temperature in a N2 filled glovebox before use. [START_REF] Ulfa | Oxide Hole Blocking Selective Contacts in Perovskite Solar Cells[END_REF][START_REF] Nazeeruddin | Inexpensive Hole Transporting Materials Derived from Tröger's Base Afford Efficient and Stable Perovskite Solar Cells[END_REF][START_REF] Xia | Retarding Thermal Degradation in Hybrid Perovskites by Ionic Liquid Additives[END_REF] The Spiro-OMeTAD hole transporting layer (HTL) was spin-coated on the top of the perovskite layers and a 70-80 nm thick gold back contact was thermally evaporated on the HTL as detailed in Ref. [START_REF] Ulfa | Oxide Hole Blocking Selective Contacts in Perovskite Solar Cells[END_REF] (see section A of the Supporting Information).

The sample morphologies were examined with a high resolution Ultra 55 Zeiss FEG fieldemission scanning electron microscope (FE-SEM) in the in-lens mode. The time-resolved photoluminescence (TRPL) measurements were performed through a microscope lens (numerical aperture 0.7). The perovskites layers were spin-coated onto a glass/FTO/c-TiO2/mp-TiO2 substrate. The top of the OIHP layers was excited by a 470 nm diode laser (Picoquant)

and the emission was filtered by a 488-nm longpass filter. It was analyzed for time-resolved photoluminescence decay, by a PerkinElmer SPCM avalanche photodiode combined with a Picoharp acquisition card (500 ps characteristic time of the total system response function) used with the laser in a pulsed mode at a 10 nW excitation power (pulse duration 70 ps).

The J-V curves were recorded by a Keithley 2410 digital sourcemeter, using a 0.1 V.s -1 voltage scan rate. [START_REF] Bahers | Effect of Solvent and Additives on the Open Circuit Voltage of ZnO Based Dye-Sensitized Solar Cells: A Combined Theoretical and Experimental Study[END_REF] The solar cells were illuminated with a solar simulator (Abet Technology Sun 2000) equipped with a xenon lamp, which light was filtered to mimic AM 1.5G conditions (100 mW/cm 2 ). [START_REF] Bahers | Effect of Solvent and Additives on the Open Circuit Voltage of ZnO Based Dye-Sensitized Solar Cells: A Combined Theoretical and Experimental Study[END_REF] The illuminated surface was delimited by a black mask with an aperture diameter of 3 mm. The power density was calibrated at 100 mW.cm -2 by the use of a reference silicon solar cell. 33 The electrical impedance spectra of the PSCs were measured in air, at room temperature and ambient humidity (~45% < RH < ~60%). A PGSTAT 12 apparatus from Autolab was employed. The frequency ranged between 1 MHz and 20 mHz, the ac signal amplitude was 20 mV. All the measured solar cells were unencapsulated and had the same contact geometries. The impedance spectra were measured at room temperature, over a large applied voltage (Vappl) range, under ∼90 mW/cm 2 light power density irradiation supplied by a halogen Schott lamp equipped with an optical fiber light guide. The cell illuminated area was delimited by a 0.16 cm 2 mask. The full recording of the spectra and characterization of a cell lasted about 1h. The spectra were analyzed using the Z-view software from National Instrument.

Results and discussion

Perovskites characterizations.

The morphology of the three different perovskite films as revealed by SEM is disclosed in Figure 1. Top views show their poly-crystallinity and that they are made of merged crystal grains. The crystal grain size analysis and distribution were performed using the Image J software (Figure 1d). In the MAPI layer, they had an average size of 410 nm (Figure 1a). The layers were well-covering. The CsFAMA layers were made of smaller crystal grains with an average size of 160 nm (Figure 1b). They were compact, uniform with no pinholes. The bright grains observed on the surface have been assigned to PbI2 by EDX analysis. The FAMA layers were nicely compact, uniform and made of remarkably large crystal grains. Their average size was measured at 1020 nm (Figure 1c). The formation of large grains is due to the use of a chloride additive (MACl) in the precursor solution. Chloride is known to favor the growth of well-crystallized layers. [33][START_REF] Rong | Synergy of Ammonium Chloride and Moisture on Perovskite Crystallization for Efficient Printable Mesoscopic Solar Cells[END_REF][START_REF] Dai | Scalable Fabrication of Efficient Perovskite Solar Modules on Flexible Glass Substrates[END_REF][START_REF] Aldibaja | Effect of Different Lead Precursors on Perovskite Solar Cell Performance and Stability[END_REF][START_REF] Xie | Vertical Recrystallization for Highly Efficient and Stable Formamidinium-Based Inverted-Structure Perovskite Solar Cells[END_REF] In summary, using mixed cation precursor produces more compact layers while the presence of a chloride additive mediates the growth of the perovskite and produces large grains. Further information on the layers morphology was gained from SEM cross-sectional images of the perovskite films (Figure 2). They confirmed that the CsFAMA layer had the smallest grains (Figure 2b). The images of the FAMA layer in Figure 2c shows remarkably monolithic large crystal grains. The interesting point is that they extend through the entire perovskite layer thickness and that each large crystal is contacted on one side by TiO2 and, on the other side, by Spiro-OMeTAD. This ideal morphology contrasts with the CsFAMA film one where many grain boundaries are present in the layer bulk. Therefore, under light irradiation, the charges photogenerated in CsFAMA layers can encounter grain boundaries, be trapped and recombined there, before to be collected. The layers thicknesses were evaluated from the SEM views and found to be 360-380 nm, 440-460 nm and 410-430 nm for MAPI, CsFAMA and FAMA, respectively.

Detailed structural, compositional and optical characterizations of the layers are provided in the Supporting Information (Section B). Briefly, XRD patterns revealed that the three samples were well-crystallized (Figure S1

Perovskite solar cells performances

The architecture of the perovskite solar cells is presented in Figure 3a. The J-V curves are shown in Figure 3b and Figure S3 (Supporting Information) and the J-V curve parameters of the best cells are gathered in Table 1. The maximum power conversion efficiency (PCE) of MAPI cell achieved 18.93%, stabilized at 18.65%. It was 20.36%, stabilized at 20.03% for CsFAMA cells. We have demonstrated elsewhere that the intercalation of a 4-Chlorobenzoic acid self-assembled monolayer at the TiO2/CsFAMA interface boosts the performance to 21.35%. [START_REF] Ulfa | Oxide Hole Blocking Selective Contacts in Perovskite Solar Cells[END_REF] MAPI and CsFAMA compounds have the same Eg, however, CsFAMA cells present a higher Jsc and Voc. The former is mainly due to a higher absorbance (Figure S2a We also noted that the HI parameter is similar for the MAPI and the CsFAMA cells with values of ca 6-7% for the best cells. On average, it was 11-14% for MAPI and 6-8 % for CsFAMA cells. This parameter was significantly better in the case of the FAMA cells with reproducible and remarkably low values at only 3-4%. The ability of the PSCs to collect light and to generate charge with a high efficiency was studied by measuring their EQE spectra (Figure 4b). MAPI cells presented the lowest performances, especially at high wavelengths, for which the irradiation penetration depth is the longest. The integration of the spectra gave a Jsc of 21.1 mA.cm -2 . For the CsFAMA cell, the charge generation edge was similar but the EQE was higher compared to MAPI. The integration gave a Jsc of 22.2 mA.cm -2 . Using the FAMA perovskite produced two major effects: firstly, it

shifted the charge generation edge toward higher wavelengths at about 820 nm, in good agreement with the absorbance spectra of Figure S2 (Supporting Information); secondly, a high efficiency of 90% or above was achieved in the full visible and near-IR range. The integration of the spectrum provided a calculated Jsc of 25.5 mA.cm -2 in good agreement with the J-V measurements. Further information on the charge collection was extracted from TRPL experiments. In Figure 4c, at short times zoomed in the inset, CsFAMA and FAMA had a similar behavior. They presented a fast decay with characteristic times at 2.0 ns and 0.5 ns, respectively (Table S2, Supporting Information). The amplitude of the decay was markedly higher for FAMA compared to CsFAMA. The fast component is assigned to the non-radiative recombinations of the charges that are injected into TiO2. [START_REF] Yoo | Post-Treatment of Perovskite Film with Phenylalkylammonium Iodide for Hysteresis-Less Perovskite Solar Cells[END_REF] Our results show that the charge injection is faster and more efficient for FAMA compared to CsFAMA. For MAPI perovskite, the first decay component was much slower at 27 ns. It is the main relative contribution to the decay (75%) (Table S2, Supporting Information). It suggests that in the case of MAPI, the electron injection towards TiO2 is slower and more difficult in agreement with the lower EQE found (Figure 4b). We measured the cells Voc under various continuous light power densities (I). A white light source was employed and the curves are disclosed in Figure 5. CsFAMA cells presented a

superior Voc over all the investigated light intensity range. The VOC scaled logarithmically with I and followed the relationship:

qVOC = Eg + nIDkT ln(I/I0) (2)
with q the elementary charge, k the Boltzmann constant, T the absolute temperature and nID the ideality factor. The latter parameter was determined from the curve fits. It is related to the main recombination phenomena occurring at the Voc. [START_REF] Tress | Interpretation and Evolution of Open-Circuit Voltage, Recombination, Ideality Factor, Subgap Defect States During Reversible Light-Soaking and Irreversible Degradation of Perovskite Solar Cells[END_REF] In classical semiconductor theory, a value of 1 for this parameter indicates a band-to-band recombination. [START_REF] Tress | Interpretation and Evolution of Open-Circuit Voltage, Recombination, Ideality Factor, Subgap Defect States During Reversible Light-Soaking and Irreversible Degradation of Perovskite Solar Cells[END_REF][START_REF] Contreras-Bernal | Origin and Whereabout of Recombination in Perovskite Solar Cells[END_REF] Deviation of nID from 1 to 2 reflects the occurrence of trap-assisted Shockley-Read-Hall (SRH) recombination through perovskite intragap defects. [START_REF] Tress | Interpretation and Evolution of Open-Circuit Voltage, Recombination, Ideality Factor, Subgap Defect States During Reversible Light-Soaking and Irreversible Degradation of Perovskite Solar Cells[END_REF] The higher is nID, the higher is the SRH recombination processes.

nID was the lowest for FAMA at ~1.4 which had a superior quality compared to CsFAMA (nID ~1.5), and especially compared to MAPI (nID ~2.0) (Figure 5). SRH recombination increases from FAMA, CsFAMA and MAPI. However, we will see below that, in PSCs, interfacial recombination always occurs and must be taken into account. Figure 5 illustrates that for an optimized functioning over a large light power density, the nID parameter must be the lower.

Impedance spectroscopy

The electrical response of the cells has been in-depth investigated by electrical impedance spectroscopy (EIS), a technique which provides in operando characterizations and is safe for the devices. [START_REF] Zhang | Fast and Low Temperature Growth of Electron Transport Layers for Efficient Perovskite Solar Cells[END_REF][START_REF] Pitarch-Tena | Impedance Spectroscopy Measurements in Perovskite Solar Cells. Device Stability During the Measurement and Noise Reduction[END_REF][START_REF] Wang | Insights into the Hole Blocking Layer Effect on the Perovskite Solar Cell Performance and Impedance Response[END_REF][START_REF] Wang | Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells[END_REF] By sweeping the frequency over a wide range, the various electrical phenomena occurring in the PSCs at various time scales can be discriminated. To have a comprehensive overview of the electrical response and well-understand the J-V curves, we have measured spectra under illumination with an applied voltage (Vappl) varied from the short circuit to the open circuit. This technique provides a deep understanding of the devices functioning. Figure 6 shows the effect of Vappl and perovskite on the spectra Nyquist plots (imaginary versus real part plots). We can note that, for all these high-efficiency cells, no inductive loop at intermediate frequencies was observed. It confirms our previous observation [START_REF] Wang | Insights into the Hole Blocking Layer Effect on the Perovskite Solar Cell Performance and Impedance Response[END_REF] that this loop is an electric response due to parasitic reactions and is therefore the signature of not optimized devices. [START_REF] Wang | Insights into the Hole Blocking Layer Effect on the Perovskite Solar Cell Performance and Impedance Response[END_REF] They have been analyzed by using equivalent electrical circuits (EEC). The most general one, established in our previous work [START_REF] Ulfa | Molecular versus Polymeric Hole Transporting Materials for Perovskite Solar Cell Application[END_REF] takes into account the possible presence of an inductive loop [START_REF] Guerrero | Properties of Contact and Bulk Impedance in Hybrid Lead Halide Perovskite Solar Cells Including Inductive Loop Elements[END_REF] and is presented in Figure 7a. In the present work, we have kept the same notations. For each potential, the electrical elements have been extracted by fitting the spectra.

For CsFAMA and FAMA, two circle arcs were observed on the spectra (Figure 6b and Figure 6c). They have been analyzed by using the type III EEC presented in Figure 7b which contains three resistances and two constant phase elements (CPE). Rs is the series resistance due to the contacts and external wires electrical contributions. It is determined by extrapolating the real part of the spectra at high frequencies to the X-axis. Capacitances have been extracted from the CPE elements using the Brug's protocole [START_REF] Brug | The Analysis of Electrode Impedance Complicated by the Presence of a Constant Phase Element[END_REF] as explained in our previous work. [START_REF] Wang | Insights into the Hole Blocking Layer Effect on the Perovskite Solar Cell Performance and Impedance Response[END_REF][START_REF] Wang | Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells[END_REF][START_REF] Ulfa | Molecular versus Polymeric Hole Transporting Materials for Perovskite Solar Cell Application[END_REF][START_REF] Ulfa | Charge Injection and Electrical Response in Low-Temperature SnO2-Based Efficient Perovskite Solar Cells[END_REF][START_REF] Ulfa | Impact of Organic Hole Transporting Material and Doping on the Electrical Response of Perovskite Solar Cells[END_REF][START_REF] Zhu | Electrical Response of High Efficiency and Stable Solar Cells Based on MACl Mediated Grown FA0.94MA0.06PbI3 Perovskite[END_REF] and detailed in section D of the Supporting Information. CPE2 and CPE4 have given C2 and C4, respectively. The type IV EEC shown in Figure 7c was employed for fitting the MAPI spectra (Figure 6a). For these cells, a shoulder was found at intermediate frequencies for most of the Vappl which led us to employ an EEC with three relaxations, a R3//CPE3 element being added. The C2 element is a light insensitive parameter [START_REF] Wang | Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells[END_REF] which values were well-reproducible for each perovskite. Figure 8 displays the variation of C2 with the applied voltage (Vappl) for the three investigated perovskites. C2 was almost constant below 0.6V while, above this value, it increased and formed a S-shaped curve. In the case of FAMA cells, a fast C2 increase was observed at large Vappl. Below 0.6V, C2 is assigned to the bulk dielectric relaxation of the perovskite material. It is related to the relative permittivity of the perovskite, εr, according to:

C2=εrε0ρ/d (3)
with d the perovskite layer thickness, ρ the layer roughness factor and ε0 the vacuum permittivity (8.85×10 -12 F.m -1 ). The C2 average values for MAPI is 1.6 10 -7 F.cm -2 with d measured on SEM cross-sectional views at circa 370 nm (Figure 2a). Assuming ρ=1, εr is calculated at ~66 for MAPI. This value is in good agreement with our previous study on a MAPI layer prepared using another protocole. [START_REF] Wang | Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells[END_REF] The same calculation gives εr ~41 (C2~0.77 10 -7 F.cm - 2 ) for CsFAMA and εr ~41 (C2~0.89 10 -7 F.cm -2 ) for MAPI. These values must be taken with care and are overestimated since they neglect any roughness of the layer. is found to vary with the perovskite. τslow is measured at 59 ns, 93 ns and 117 ns for MAPI, CsFAMA and FAMA, respectively (Table S2, Supporting Information). τslow is classically assigned to bulk recombinations (either radiative [START_REF] Lee | 2D Perovskite Stabilized Phase-Pure Formamidinium Perovskite Solar Cells[END_REF][START_REF] Zhu | Tailored Amphiphilic Molecular Mitigators for Stable Perovskite Solar with 23.5% Efficiency[END_REF] or non-radiative [START_REF] Zhu | Tailored Amphiphilic Molecular Mitigators for Stable Perovskite Solar with 23.5% Efficiency[END_REF][START_REF] Bi | Efficient Luminescent Solar Cells Based on Tailored Mixed-Cation Perovskites[END_REF] ) of the photogenerated charges in the perovskite. It scales with the crystal quality and it mitigates with the density of recombination sites in the bulk and at the grain boundaries. We find in Figure 9b an exponential relationship between R2 and τslow .

R2=1/k2= A exp(τslow) (

A is a constant and the fit is excellent (R 2 =0.999). This relationship between R2 and τslow has been assessed by completing the series with a FAMA cell that did not undergo the PEAI treatment (FAMA* dot in Figure 9b). It allows us to assign the R2 component to the recombination occurring in the perovskite layer, especially at the grain boundaries.

Recombination resistance R2 is inversely related to the recombination rate (k2) and it provides a direct quantitative view of the recombination rate. The higher is this resistance, the lower is the recombination rate. R4 is also a recombination resistance but originates from recombinations occurring at or near the interfaces hereafter called interfacial recombinations. In Figure 10a, for Vappl< 0.8V, MAPI

and CsFAMA have about the same R4 value while FAMA has the highest R4. This cell presents the best interfaces. Above 0.9V, R4 of CsFAMA is shifted to higher potential compared to MAPI and FAMA. This is in agreement with the higher Voc for CsFAMA cells. The fact of shifting R2 and R4 curves toward higher voltage is favorable for the cell performances since it increases the cell VOC.

In Table S3 (Supporting Information) we report typical values of R2 and R4 at the Voc for the three kinds of cells. The R2/R4 ratio quantifies the relative interfacial versus bulk recombination rates (k4/k2). We observed that in all cases the interfacial recombination rate is dominant. It increases in the order FAMA (3.0), CsFAMA (4.3) and MAPI (7.3). Extracting the resistance values at the Voc, therefore, provides a finer view of the recombination phenomena than steady-state Voc measurements. We have measured R2 at various light intensities and reported their value as a function of the Voc. An exponential fit was found and an ideality factor, noted n'ID, has been then determined. [START_REF] Contreras-Bernal | Origin and Whereabout of Recombination in Perovskite Solar Cells[END_REF][START_REF] Correa-Baena | Change from Bulk to Surface Recombination Mechanism between Pristine and Cycled Perovskite Solar Cells[END_REF] The ideality factors obtained from EIS and open-circuit potential measurements (nID, see section 3.2) are disclosed in Table S4 (Supporting Information). Similar values are found from both techniques and they change in a similar manner with the perovskite employed. It confirms R2 as a bulk recombination resistance. C4 is a light-sensitive parameter which value dramatically increases with the light irradiation and can reach very high values. [START_REF] Wang | Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells[END_REF] In our case, C4 attained more than 1.10 -1 F.cm -2 at high Vappl.

Jacobs et al. [START_REF] Jacobs | The Two Faces of Capacitance: New Interpretations for Electrical Impedance Measurements of Perovskite Solar Cells and their Relation to Hysteresis[END_REF] have shown that such high values cannot be explained by a charge accumulation layer and cannot be the result of photogenerated charges separated across an interface. There is little doubt that C4 is related to the recombination of charges. Because charge recombination in PSC is a phase delayed phenomenon, in addition to the real component, an imaginary component is also introduced. This imaginary component is analyzed in our work as a capacitance in the equivalent electrical circuits used to fit our data (Figure 7). It is the main contribution of C4. This delayed recombination process is a special behavior occurring in PSCs due to the ionic conductivity of the perovskite. The movements of ions introduce defects that act as recombination centers and affect the dynamics of the charges. To better follow the effect of Vappl and perovskite material, we have chosen to plot the 1/C4 parameter in Figure 10b. This parameter can be directly compared with the recombination resistances. As expected, it dramatically increases from MAPI to CsFAMA and FAMA. Recombinations are suppressed in the latter device. We can also remark that the plot of 1/C4 is similar to that of R2 (Figure 9a).

The low-frequency capacitance C4 modulates the electrical response of the cells under voltage scan at rather low rate in the J-V curve. It produces a current which depends on the scan direction, rate and the value of C4. The higher C4, the higher the current difference between the forward and reverse scans. A consequence is that C4 is related to the J-V curve hysteresis. It increases from FAMA, CsFAMA and MAPI which exhibit HI at ~3%, ~8% and ~14%, respectively (see Figure 10b). We confirm here that C4 is closely related to the hysteresis index: the higher is C4, the higher is the hysteresis amplitude. [START_REF] Wang | Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells[END_REF][START_REF] Ulfa | Molecular versus Polymeric Hole Transporting Materials for Perovskite Solar Cell Application[END_REF] In the case of the MAPI cells, we have introduced an intermediate frequency relaxation to well-fit the spectra. The variations of the R3 and C3 parameters with Vappl are presented in and C3 between 0.0V and 0.6V. It reminds the behavior of C2 and may be related to bulk properties rather than interfacial ones. However, with the actual knowledge on the EIS response of PSCs, it is difficult to clearly state which physical process occurring in the cells is at the origin of these parameters.

The conclusion of the EIS investigation is that for PSC of very high efficiency, Rs must be minimized; R2, R4 and 1/C4 must be the highest as possible to get high Jsc and FF. Moreover, for a high FF, the drop of these light-sensitive components must be as abrupt as possible and for a high Voc, the drop must be voltage-shifted at most.

Electrical stability of the devices

The future of the perovskite technology will depend on the ability of obtaining stable devices. Their robustness against electrical stresses and light irradiation are key parameters that must be carefully investigated. We performed a first test consisting in measuring the J-V curves of the cells, before and after their EIS investigation. [START_REF] Pitarch-Tena | Impedance Spectroscopy Measurements in Perovskite Solar Cells. Device Stability During the Measurement and Noise Reduction[END_REF] EIS measurements, in which PSCs are submitted to a continuous light irradiation, to the application of many voltages and to a superimposed ac stimulus during about one hour, are especially stressing for the PSCs. Table 2 reports the J-V curves result before and after the EIS characterization, for the different perovskites. The performances of the MAPI cells measured on the reverse scan slightly increased upon the measurements. The PCE of the forward scan was decreased due to the FF parameter mitigation. The CsFAMA cells were the most affected by the EIS measurements.

Their efficiency was measured lower after the EIS characterization. FAMA devices were remarkably stable since they exhibited a slightly higher PCE after EIS characterizations due to the increase of the VOC. Their HI parameter remained low and almost unchanged. final PCE was the best for FAMA. We have also found that MAPI was decomposed upon aging in PbI2 with the elimination of MAI (MAI can be degraded into volatile CH3I and NH3 molecules). [START_REF] Turren-Cruz | Methylammonium-Free, High-Performance, and Stable Perovskite Solar Cells on a Planar Architecture[END_REF] On the other hand, CsFAMA and FAMA were degraded in the photo-inactive one-dimensional (1D) δ-phase that may be regenerated in their photoactive α-phase, for instance by heating (Figure S5, Supporting Information).

Scheme1. Main information extracted from the electrical impedance spectroscopy data and their correlations with other measurements. Light sensitive element.

Conclusions

In conclusion, we have fully investigated the electrical response of solar cells prepared with perovskite ranging from the mono-cation to the tri-cation composition. The best results have been found with the double MA/FA lead iodide compound which was prepared using MACl additive in the precursor solution. This strategy has been found the most accurate to achieve high efficiency and stability. It is more efficient than to complicate the perovskite composition.

FAMA presented the best morphology, crystallinity and optical properties for the application.

The ideality factor decreased from MAPI, CsFAMA and FAMA. For the later SRH recombination with the less intensity was found. The cells have been thoroughly investigated by impedance spectroscopy over a wide applied potential range. We have proposed two EECs based on our general EEC for analyzing the electrical response of the three kinds of solar cells.

For all these optimized devices, with high efficiency for their category, no inductive loop at intermediate-frequency due to parasitic reaction was found. The main information extracted from the electrical impedance spectroscopy data and their correlations with other measurements are summarized in Scheme 1. We have extracted the dielectric permittivity of the three perovskites from the high-frequency capacitance C2 and measured them at 66, 41 and 41 for MAPI, CsFAMA and FAMA, respectively. We have found a fair relationship between the highfrequency resistance R2 and the slow decay time τslow of the OIHP TRPL. This has led us to assign R2 to recombination occurring in the perovskite bulk and grain boundaries. The lowfrequency resistance R4 has been assigned to the recombination at and near the interfaces. The low-frequency capacitance C4 which modulates the electrical response of the cells under voltage scan at rather low rate has been assigned to a phase delayed recombination phenomenon, the delay being a consequence of the ionic mobility and defect formation. For these three recombination related electrical elements, the best values have been found for our highly efficient FAMA cell which is almost hysteresis-free. Finally, under electrical and light irradiation stresses, the investigated FAMA cell was highly stable while MAPI was more stable than CsFAMA. Finally, the strategy of mediating the growth of FA1-xMAxPbI3 perovskite double cation by MACl additive is found the best to suppress recombinations, suppress ion mobility, reach high efficiency with low hysteresis and stabilize the perovskite compound.

: A 1 . 45 M 5 mg

 1455 MAPI precursor solution was prepared by mixing 668.PbI2 and 230.5 mg MAI in 1ml DMSO. The solution was stirred and kept warm at 100°C

Figure 1 .

 1 Figure 1. SEM top-views of the perovkite layers investigated. (a) MAPI, (b) CsFAMA and (c) FAMA. (d) Grain size statistical analysis of the perovskite layers.

Figure 2 .

 2 Figure 2. SEM cross-sectional views of the perovskite layers. (a) MAPI, (b) CsFAMA and (c) FAMA.

  , Supporting Information) and absence of pinholes. Defects in the MAPI layer can result in recombination pathways between holes and electrons that lower the FF and Voc parameters. The better Voc of CsFAMA cells is assigned to less recombinations occurring in the bulk and at the interfaces. The FAMA cells exhibited the highest efficiency. The PCE could reach a remarkable 22.18 % value and the stabilized PCE achieved 22.08%. Jsc was boosted due to a wider wavelength range for the light-harvesting (Figure S2, Supporting Information) and to the high structural quality and big grains. These cells also presented the best FF. The lower bandgap is at the origin of the lower Voc of the FAMA cells compared to the CsFAMA ones. All the trends obtained for the best cells were confirmed by the statistical analysis disclosed in Figure 4a.

Figure 3 .

 3 Figure 3. (a) Exploded schematic view of the perovskite solar cells. (b) Reverse J-V curves of MAPI, CsFAMA and FAMA cells.

Figure 4 .

 4 Figure 4. (a) Statistical analysis of the PSCs performances and J-V curves parameters. (b) EQE spectra of the three perovskites with the integrated Jsc curves. (c) TRPL curves. The inset is a zoom view at short times.

Figure 5 .

 5 Figure 5. Variation of Voc as a function of the light power density. The curve slopes provide the ideality factor values (nID, numbers in bold).

Figure 6 .

 6 Figure 6. Impedance spectra of the investigated PSCs under illumination. Effect of the applied voltage for (a) MAPI, (b) CsFAMA and (c) FAMA. (d) Comparison of the EIS at 0.7V for the three cells.

Figure 7 .

 7 Figure 7. (a) General equivalent electrical circuit. (b,c) EECs employed for fitting (b) the FAMA and CsFAMA spectra and (c) the MAPI spectra.

Figure 8 .

 8 Figure 8. C2 parameter versus the applied voltage. Comparison of the three perovskites.

Figure 9 .

 9 Figure 9. (a) R2 parameters as a function of Vappl for the three perovskites. (b) R2 (measured at 0.3V) versus τslow of TRPL (FAMA* is a FAMA cell without PEAI treatment). Data recorded under illumination.

Figure 10 .

 10 Figure 10. (a) R4 and (b) 1/C4 parameters versus Vappl for the three perovskites. (c) Variation of R3 and C3 of MAPI cells with Vappl.

Figure 10c .

 10c Figure 10c. They were batch dependent but we reproducibly observed rather flat curves for R3

20 . 73

 2073 We also compared the stability of the unencapsulated solar cells under a one sun AM 1.5G light irradiation and at a relative humidity (RH) of 45%, tracking them at their maximum power points (MPP). The curves are disclosed in Figure11a. During 200 min, the MAPI cell presented the fastest decrease. The fact that the MAPI cell was less stable under the solar simulator compared to the EIS measurement could be due to the UV light component in the former irradiation spectrum. The CsFAMA cell PCE decreased more slowly at the beginning but the two curves crossed at 200 min. After 300 min, the MAPI cell had lost 37% of its initial PCE and the CsFAMA cell 42%. The FAMA cell was remarkably stable and no PCE loss was observed. Figures11b-dare the J-V curves measured before and after the tracking experiments. MAPI cell presented a lower Voc and a large hysteresis after the stability test. CsFAMA cell exhibited a decrease of the Jsc and an enlargement of the hysteresis. We noted a remarkable stability of the FAMA cell under the test since the J-V curves were unchanged after the long irradiation and tracking period.

Figure 11 .

 11 Figure 11. (a) Tracking of MAPI, CsFAMA and FAMA solar cells normalized power outputs under continuous one sun AM1.5G illumination (unencapsulated devices 45% RH). (b-d) J-V curves were measured before and after the tracking experiment under 1sun AM1.5G illumination. (b) MAPI; (c) CsFAMA and (d) FAMA cells.

  al.[START_REF] Bartel | New Tolerance Factor to Predict the Stability of Perovskite Oxides and Halides[END_REF] :

	𝑡 𝑓 =	𝑟 𝑥 𝑟 𝐵	-𝑛 𝐴 (𝑛 𝐴 -	⁄ 𝑟 𝐴 𝑟 𝐵 ln( 𝑟 𝐴 𝑟 𝐵 ⁄ )

Table 1 .

 1 Typical photovoltaic J-V parameters of cells among the best.

	Sample	Scan direction	Voc /V	Jsc mA/cm	2	FF /%	PCE /%	Stabilized PCE a /%	HI b /%
		Reverse	1.063	23.08		77.43	18.93
	MAPI							18.65	6.5
		Forward	1.059	22.51		73.99	17.70
		Reverse	1.110	23.13		79.27	20.36
	CsFAMA							20.03	6.1
		Forward	1.101	23.13		75.00	19.10
		Reverse	1.060	25.94		80.62	22.18
	FAMA							22.08	3.7
		Forward	1.052	25.93		78.26	21.35

a Measured by tracking at the maximum power point.

b HI : Hysteresis index defined as: [PCE(%)rev-PCE(%)for]/PCE(%)rev

Table 2 .

 2 J-V curves parameters of PSCs with various perovskites before and after their investigation by EIS.

	Sample			Voc /V	Jsc	/	FF / %	PCE / %	HI /%
					mA.cm -2
	MAPI	Before After	Reverse Forward Reverse Forward	1.045 1.026 1.057 1.033	22.19 22.18 23.19 23.13		78.42 70.19 75.87 65.41	18.18 15.97 18.60 15.63	12 16
	CsFAMA	Before After	Reverse Forward Reverse Forward	1.077 1.052 1.049 1.027	21.81 21.81 21.64 21.66		77.63 73.63 74.18 69.23	18.24 16.88 16.83 15.40	7 8
	FAMA	Before After	Reverse Forward Reverse Forward	1.035 1.031 1.044 1.045	25.65 25.69 25.71 25.71		79.62 77.81 79.36 77.17	21.13 20.61 21.29	2 3

Acknowledgements

Mr J. Liu and Dr L. Coolen (INSP, Sorbonne University, France) are acknowledged for TRPL measurements. T.Z. thanks the Chinese Scholarship Council and D.Z thanks the Chinese Scholarship Council-Paristech programme for Ph.D scholarships (grant numbers 201706340053 and 201806310126, respectively). The ANR agency is acknowledged for financial support (Moreless project ANR-18-CE05-0026).

TOC

Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/ XXXXXXXXXXXXXXXXXXXXXXXXXX Complementary experimental details; XRD patterns of the perovskite layers; EDX analysis; Optical characterizations of the three perovskite layers; Fitting of the TRPL curves; TRPL curves parameters; J-V forward and reverse curves of the three perovskites solar cells; Analysis of the impedance spectra; R2 and R4 measured at the Voc; Comparison of the ideality factors determined from EIS and Voc measurements; Aging tests at high moisture.