Tao Zhu

Daming Zheng

Jiawen Liu

Laurent Coolen

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

PEAI-Based Interfacial Layer for High

Keywords: Perovskite solar cells, Methylammonium formamidinium lead iodide, (PEA)2PbI4, Electrical impedance spectroscopy, Interfacial passivation, Charge recombination reduction

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Introduction

Organic-inorganic hybrid perovskites (OIHPs) are among the most fascinating semiconductor materials due to their versatility, tunability, low temperature preparation and amazing optoelectronic properties. [START_REF] Saparov | Organic-Inorganic Perovskites: Structural Versatility for Functional Materials Design[END_REF] 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. [START_REF] Leblanc | Enhanced Stability and Band Gap Tuning of α-[HC(NH2)2]PbI3 Hybrid Perovskite by Large Cation Integration[END_REF] These compounds are promising for applications in many fields such as photodetectors, lasers, scintillation, LEDs and solar photovoltaics. [START_REF] Veldhuis | Perovskite Materials for Light-Emitting Diodes and Lasers[END_REF][START_REF] Kim | Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%[END_REF][START_REF] Lee | Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites[END_REF][START_REF] Liu | Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition[END_REF][START_REF] Burschka | Sequential Deposition as A Route to High-Performance Perovskite-Sensitized Solar Cells[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] Nie | High-Efficiency Solution-Processed Perovskite Solar Cells with Millimeter-Scale Grains[END_REF][START_REF] Zhang | Fast and Low Temperature Growth of Electron Transport Layers for Efficient Perovskite Solar Cells[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 Efficient 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][START_REF] Turren-Cruz | Methylammonium-Free, High-Performance, and Stable Perovskite Solar Cells on a Planar Architecture[END_REF] The latter is of special interest since the power conversion efficiency of solar cells based on OIHP has quickly increased, reaching a present certified record efficiency of 25.2%. 19 OIHP-based solar cells exhibit the highest performance among thin film technologies and their efficiency approaches that of the best silicon technology.

T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 3 Formamidinium lead tri-iodide (FAPbI3) perovskite is one of the best suited OIHP materials for perovskite solar cells (PSC) from the point of view of the bandgap for light harvesting of irradiation wavelengths ranging from the near-IR to the near-UV. This compound also allows the generation of a rather large photovoltage output. [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] Turren-Cruz | Methylammonium-Free, High-Performance, and Stable Perovskite Solar Cells on a Planar Architecture[END_REF] However, due to the large size of the FA cation, FAPbI3 is stable at room temperature in its non-perovskite yellow δ-phase which is poorly active in solar cells. [START_REF] Leijtens | Stability of Metal Halide Perovskite Solar Cells[END_REF] The stabilization of the photo-active black 3D perovskite α-phase needs the incorporation of extra monovalent cation such as cesium (Cs + ), rubidium (Rb + ), methylammonium (MA + )…. or of their mixture. Another important observation is that the most efficient OIHPs contain methylammonium (MA + ) as extra organic cation. [START_REF] Jeon | A Fluorene-Terminated Hole-Transporting Material for Highly Efficient and Stable Perovskite Solar Cells[END_REF][START_REF] Jiang | Surface Passivation of Perovskite Film for Efficient Solar Cells[END_REF][START_REF] Jung | Efficient, Stable and Scalable Perovskite Solar Cells Using Poly (3-Hexylthiophene)[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] Kim | Methylammonium Chloride Induces Intermediate Phase Stabilization for Efficient Perovskite Solar Cells[END_REF] High efficiency and high stability require dense, well-covering, uniform, pin-hole free perovskite layers as well as high phase purity.

The OIHP layer must be made of monolithic large crystal grains which go through the entire perovskite thin film. [START_REF] Nie | High-Efficiency Solution-Processed Perovskite Solar Cells with Millimeter-Scale Grains[END_REF] To increase the performances of PSCs, one must both optimize the OIHP layer and the interfaces with the selective contact layers. [START_REF] Zhu | Interfacial Engineering through Chloride-Functionalized Self-Assembled Monolayer for High Efficiency Perovskite Solar Cells[END_REF] Complex recipes have been developed for the preparation of high structural quality OIHP layers. Chloride anion additive is known, for a long time, to be beneficial to the formation of large crystal grains and has been used in a large extent in MAPbI3 perovskite layer synthesis. [START_REF] Edri | Elucidating the Charge Carrier Separation and Working Mechanism of CH3NH3PbI3x Clx Perovskite Solar Cells[END_REF][START_REF] Buin | Materials Processing Routes to Trap-Free Halide Perovskites[END_REF] On the other hand, only a few works have been developped chloride additives for the growth of mixed cation perovskite based on formamidinium and methylammonium. [START_REF] Jiang | Surface Passivation of Perovskite Film for Efficient Solar Cells[END_REF][START_REF] Kim | Methylammonium Chloride Induces Intermediate Phase Stabilization for Efficient Perovskite Solar Cells[END_REF][START_REF] Buin | Materials Processing Routes to Trap-Free Halide Perovskites[END_REF][START_REF] Mu | Quantitative Doping of Chlorine in Formamidinium Lead Trihalide (FAPbI3-xClx) for Planar Heterojunction Perovskite Solar Cells[END_REF][START_REF] Jeon | A Fluorene-Terminated Hole-Transporting Material for Highly Efficient and Stable Perovskite Solar Cells[END_REF] MACl solutions were used in several paper for the surface treatment of perovskite films by spin-coating. [START_REF] Xie | Vertical Recrystallization for Highly Efficient and Stable Formamidinium-Based Inverted-Structure Perovskite Solar Cells[END_REF][START_REF] Wu | Sequential Processing: Crystallization of Ultra smooth FA1-xMAxPbI3 Perovskite Layers for Highly Efficient and Stable Planar Solar Cells[END_REF][START_REF] Mateen | MACl-Induced Intermediate Engineering for High-Performance Mixed-Cation Perovskite Solar Cells[END_REF] Recently, MACl additive was employed to increase the solubility of GeI2 and allows the doping of the mixed cation perovskite layer with Ge. [START_REF] Kim | MACl-Assisted Ge Doping of Pb-Hybrid Perovskite: A Universal Route to Stabilize High Performance Perovskite Solar Cells[END_REF] Mu et al. used MACl combined with FAI in stoichiometric amounts to obatined mixed Cl and I halide perovskite compounds. [START_REF] Mu | Quantitative Doping of Chlorine in Formamidinium Lead Trihalide (FAPbI3-xClx) for Planar Heterojunction Perovskite Solar Cells[END_REF] Kim et al. were the first to employ MACl as an additive in the perovskite T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 4 precursor solution. They successfully produced an intermediate to the pure FAPbI3 α-phase without annealing. [START_REF] Kim | Methylammonium Chloride Induces Intermediate Phase Stabilization for Efficient Perovskite Solar Cells[END_REF] Cl ion was proved to stabilize α-phase FAPbI3 without annealing through enhancing the interactions of FA and I, and reducing the formation energy. The MA ion in the unannealed film caused a total volume shrinkage for the cubic α-phase FAPbI3 due to the small MA ion size and its large dipole moment, which also reduced the formation energy of α-phase FAPbI3.

However, the methylammonium content must be minimized to reduce the hydrophilicity of the material and its destabilization by degradation/ degazing (in CH3I and NH3). [START_REF] Turren-Cruz | Methylammonium-Free, High-Performance, and Stable Perovskite Solar Cells on a Planar Architecture[END_REF] For the interfaces, the perovskite/hole transporting layer (HTL) one is of utmost importance. The intercalation of an ad-hoc buffer layer enables the fast extraction of holes and their transfer to the HTL. 2-Phenylethylamonium iodide (PEAI) focused our attention since this compounds has been employed for the passivation of the OIHP grain boundaries and surfaces in several works. [START_REF] Lee | 2D Perovskite Stabilized Phase-Pure Formamidinium Perovskite Solar Cells[END_REF] You et al. found that PEAI itself can effectively passivate the perovskite defects and therefore improve the efficiency of the devices. [START_REF] Jiang | Surface Passivation of Perovskite Film for Efficient Solar Cells[END_REF] PEAI can also form a (PEA)2PbI4 2D-perovskite compound after reaction with I and Pb sources. [START_REF] Quan | Ligand-Stabilized Reduced-Dimensionality Perovskites[END_REF] We present PSCs which achieved very high power conversion efficiencies by the engineering of the OIHP precursor solution and interfaces. We used non-stoichiometric precursor solutions in which methylammonium chloride additive acted as a mediator to the growth of dense layers made of large crystal grains. By this technique, a small amount of MA cation was incorporated in FAPbI3, at the best for the α-phase stability. One of the interests of the presented recipe is that the solution has a rather low concentration of precursor. The technique is also reliable, robust and gives high efficiency with low result dispersion for a large range of additive concentration. The MACl best molar fraction is found at 48 mol.%. At lower concentration, the OIHP grain size was smaller and more grain boundaries were present. At higher concentration, more recombinations occurred which T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 5 mitigated the device fill factor (FF) and open circuit voltage (VOC). We also show that the PEAI treatment without thermal annealing leads notably to the spontaneous formation of (PEA)2PbI4 perovskite at the surface. It favors a fast transfer of the holes towards the HTL. A comprehensive investigation of the systems by impedance spectroscopy notably shows that recombinations are markedly reduced by using this buffer layer. Overall, the prepared cells were almost hysteresis free.

Their power conversion efficiency was greater than 22.1%. The power output of the produced devices was remarkably stable. These PSCs were proved resistant to electrical stresses, to light irradiation and to moisture.

Results and discussion

Perovskite layers description

We have investigated the preparation of OIHP layers from precursor solutions containing a stoichiometric amount of FAI and PbI2 and various molar fractions of methylammonium chloride additive, namely 40 mol.%, 48 mol.% and 55 mol.%. The related layers are noted MC40, MC48 and MC55, respectively. The precursor concentration was rather low at 1.2 M while a large amount of MACl was employed. The latter was crucial to govern the growth of the perovskite layers.

Scanning Electron Microscope (SEM) top-views in Figure S1(a-c) (Supporting Information) show that the presence of chloride gradually increased the size of the crystal grains. This anion reduces the density of nuclation seeds and allows the growth of larger grains. The maximum average grain size was close to 1 μm for the MC55 sample (Figure S1c, Supporting Information). SEM views of the layer cross-section (Figure S1d, Supporting Information) indicated a monolithic structure with the whole thickness of the perovskite layer occupied by single grains.

T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 6 XRD patterns revealed that the prepared layers were made of ultra-pure cubic α-phase 3Dperovskite. The peaks are indexed in Figure S2 (Supporting Information) and the layer is textured with the (100) family of plane parallel to the surface. No non-perovskite yellow δ-phase and PbI2 phase were detected by XRD. The layers were well-crystallized whatever the concentration of additive used. The diffraction peak intensity increased with the additive concentration due to the increase of perovskite grain size. The accurate compositions of the layers were determined by combining EDX and 1 H-NMR techniques. EDX showed that the I/Pb ratio was close to 3 for the three films (Table S1, Supporting Information). This technique also indicated that the Cl element was not present or present in a negligible quantity in the final layers. Liquid-state 1 H-NMR allowed us to finely titrate the FA and MA component ratio in the effective perovskite layers employed in the solar cells. The values are reported in Table S1 (Supporting Information). The interesting result is that x in FA1-xMAxPbI3 is very low and ranges between 0.061-0.064 (Table S1, Supporting Information). This parameter is almost irrespective of the concentration of MACl in the precursor solution. In summary, we can describe the mixed cation perovskite obtained with the various molar faction of MACl by the 3D perovskite formulae FA0.94MA0.06PbI3.

The absorbance spectra of the layers in Figure S3a (Supporting Information) exhibit an absorption edge at about 820 nm which is due to the direct optical transition in FA0.94MA0.06PbI3. The use of pure I as halide permits the extension of the absorption limit in the near-IR. We observed that increasing the MACl % favored the absorbance. It can be related to larger grains and likely to a slight increase of the perovskite layer thickness with MACl additive. Figure S3b (Supporting Information) displays the Tauc plots of the absorbance spectra. The linear fit intersection with the energy axis gives an optical bandgap energy (Eg) of 1.53 eV whatever the precursor solution composition. The fact that increasing the MACl concentration does not affect Eg is in good T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 7 agreement with a similar organic cation composition of the MC40, MC48 and MC55 layers. In Figure S3c (Supporting Information), the normalized photoluminescence (PL) spectra of the various layers overlap. The maximum of the emission was measured at about 811 nm. Time resolved photoluminescence (TRPL) decay curves measured from the layers top are presented in Figure S3d (Supporting Information). They were analyzed as described in the Annex 1 of the Supporting Information, using three-exponential decay function. The fit results are gathered in Table S2 (Supporting Information). The slower component had the best relative contribution (rc) to PL and we assigned it to the bimolecular recombination of the photogenerated charges in the bulk perovskite. The slow decay time is related to the structural quality of the perovskite layers.

The longer is this parameter, the higher is the structural quality of the material. From the fits, we measured τslow at 99 ns, 102 ns and 99 ns for MC40, MC48 and MC55 samples, respectively. The fact that this parameter is not longer for the MC55 layer with the bigger grains shows that the structural quality of the polycrystalline MC55 film was not significantly higher compared to the others. Nevertheless, all the prepared materials exhibited a rather long emission lifetime.

PEAI treatment

The solar cell study detailed below shows a marked improvement of the PSC performance after the perovskite surface treatment with a 10 mM solution of 2-Phenylethylammonium Iodide (PEAI) in isopropanol (2.49 mg/mL). This treatment was done after the preparation of the perovskite layer and before the HTL deposition. The solution of PEAI was spin-coated on top of the perovskite surface and, importantly, no post-deposition processing was implemented (especially no thermal annealing which is most often done after perovskite surface treatments). On SEM views (Figure 1), the PEAI treatment produced a change in the aspect of the perovskite grains surface, especially of the flat parts of the crystals suggesting a surface modification by a chemical reaction. We also observed the presence of an extra-phase with a haze aspect that accumulated in the deepest zones where most grain (multi-)boundaries are present. On the layer XRD patterns (Figure 2a), measured immediately after the PEAI treatment, an extra-peak appeared at an angle of 5.35° which is assigned to the (001) plane of the 2D perovskite with the (PEA)2PbI4 composition. [START_REF] Jiang | Surface Passivation of Perovskite Film for Efficient Solar Cells[END_REF] No PEAI diffraction peak at 4.7° was observed (Figure S3e, Supporting Information) therefore no crystallized PEAI was present on the surface at the concentration employed. [START_REF] Jiang | Surface Passivation of Perovskite Film for Efficient Solar Cells[END_REF] The effect of the PEAI solution concentration was followed and shown in Figure S3f (Supporting Information). The 2D peak appeared above 1mg/mL and was clearly present at 2.49 mg/mL, the concentration retained in this study. Its intensity increases with the concentration. We have noted that the presence of crystallized PEAI was detected only for the highest concentration (6 mg/mL). It was then mixed

with the 2D phase.

The presence of the 2D perovskite was further confirmed on the absorbance spectra (Figure 2b).

An absorption edge at 520 nm which corresponds to the near band edge transition in (PEA)2PbI4 is revealed in the differential curve (Figure 2b). On the TRPL curves (Figure 2c), we observed a more intense initial emission for the PEAI treated sample. We also observed a faster initial decay of PL for the treated sample. The latter trend was confirmed on the normalized TRPL curves shown in Figure S3h (Supporting Information). Table S2 (Supporting Information) report that the fast component was much faster for MC48-PEAI (0.47 ns) compared to the untreated MC48 sample compound compared to the 3D one. [START_REF] Quan | Ligand-Stabilized Reduced-Dimensionality Perovskites[END_REF] PEAI could remain present in an amorphous state in the deep zones of the perovskite layer. However this deposit looks poorly attached to the perovskite surface and is likely mainly removed upon the Spiro-OMeTAD deposition by spin-coating.

Perovskite solar cells performances

The solar cell structure is presented in Figure 3a. The FA0.94MA0.06PbI3 perovskite layer was contacted by selective contacts: TiO2 on one side and a spin-coated Spiro-OMeTAD layer on the other side. A gold contact was evaporated on the back of the cell. On the SEM cross-sectional view (Figure S1d, Supporting Information), the mp-TiO2 layer thickness was measured at about 130 nm. This layer was soaked with FA0.94MA0.06PbI3. The thickness of the capping pure perovskite layer was about 390 nm. The top Spiro-OMeTAD layer was about 150 nm thick. The performances of the PSCs were evaluated by measuring their J-V curves under one sun AM 1.5G illumination. The curves of cells prepared with various conditions and with and without PEAI are displayed in Figure 3b. Table 1 gathers the J-V curve parameters of the devices. The hysteresis between the reverse and the forward scans were small due to the low defect density in the perovskite of the hysteresis (Table 1). MACl additive molar fraction has also a significant effect on the cell electrical properties. In Table 1, Jsc is the highest for MC48-PEAI and MC55-PEAI and the best VOC is achieved by MC48-PEAI.

Overall, the best power output was reached for the MC48-PEAI devices. The improvement between MC40-PEAI and MC48-PEAI cells can be explained by larger grains and higher structural quality. The VOC parameter depends on the recombination rate, especially on the recombinations at and near the perovskite/selective contact interfaces. The lower efficiency of the MC55-PEAI cells compared to the MC48-PEAI ones is the effect of lower FF and Voc. We will show in Section 2.4 that these components are related to the internal resistances of the devices which are themselves related to the charge recombinations. We can conclude that MC55-PEAI are not the best devices due to a lower quality of the interfaces.

Overall, choosing MACl at 48mol.% appears as the best choice for the best performances. The observation made for the best cells were confirmed by the statistics of the J-V curve parameters distribution (Figure 3c). 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. We have also noted that the PEAI treatment especially boosts the JSC and the VOC of the cells by improving the perovskite/HTM interface, reducing the surface defects and favoring the fast charge transfer towards the selective contact. It also reduces the PCE dispersion. Table 1 shows that the best PCE was measured at 22.18%. This cell had a hysteresis index of only 3.7%. Importantly, this device was tracked at its maximum power output. It reached a stabilized efficiency of 22.08% after about 80s. This value is very close to the measurement on the reverse scan (Figure 3d).

The ability of the cells to collect light over a large wavelength range and to generate charge with a high efficiency was confirmed by measuring their EQE spectra. The effect of PEAI on the EQE spectrum is presented in Figure 3e. EQE was enhanced with the treatment over the whole visible range. It augmented the Jsc obtained by curve integration from 24.8 mA.cm -2 to 25.1 mA.cm -2 . It confirms that the treatment favors the charge generation and charge extraction at the T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 15 perovskite/HTL interface. Figure S3g (Supporting Information) shows the effect of the MACl molar fraction on the EQE spectra. By integrating the curves, JSC as high as 25.4 mA.cm -2 were calculated (24.9 mA.cm -2 , 25.1 mA.cm -2 and 25.4 mA.cm -2 for MC40-PEAI, MC48-PEAI and MC55-PEAI, respectively) and were in good agreement with the J-V curve data.

We also performed the analysis of the VOC as a function of the white light illumination power density. The curves with and without PEAI treatment are presented in Figure 4a. For the investigated cells, the VOC was superior over all the light intensity range after the PEAI treatment.

The VOC scaled logarithmically with the light power density (I) and followed the relationship:

qVOC= Eg + nIDkT ln(I/I0) (1)
with q the elementary charge, k the Boltzmann constant, T the absolute temperature and nID the ideality factor. The latter parameter is related to the main recombination phenomena occurring at the Voc. [START_REF] Tress | Interpretation and Evolution of Open-Circuit Voltage, Recombination, Ideality Factor and Subgap Defect States During Reversible Light-Soaking and Irreversible Degradation of Perovskite Solar Cells[END_REF] We found 1.40 < nID < 1.50 for matured cells (cells stored several days in N2 atmosphere) (Figure 4). A smaller slope was found after the PEAI treatment (Figure 4a). It is known that deviation of nID from 1 reflects the occurrence of defect-assisted recombination in the PSCs. nID values suggest that, at the open circuit, a Shockley-Read-Hall (RSH) recombination mechanism through perovskite intragap defects is the dominating recombination process at the Voc. We have observed that, like in the case of MAPbI3 perovskite, [START_REF] Tress | Interpretation and Evolution of Open-Circuit Voltage, Recombination, Ideality Factor and Subgap Defect States During Reversible Light-Soaking and Irreversible Degradation of Perovskite Solar Cells[END_REF]

Impedance spectroscopy study of the PSCs

To gain further insight into the cell functioning, the effect of the buffer layer and cell maturation, we have studied the solar cells by EIS. This characterization technique is performed in operando, is safe for the device and provides fruitful information on its functioning. [START_REF] Ulfa | Molecular Versus Polymeric Hole Transporting Materials for Perovskite Solar Cell Application[END_REF][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][START_REF] Pitarch-Tena | Impedance Spectroscopy Measurements in Perovskite Solar Cells. Device Stability During the Measurement and Noise Reduction[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] The PSCs have been investigated under white light irradiation with a power density of about 0.9 sun. The applied voltage (Vappl) was varied between the short circuit and the open circuit conditions. The Nyquist plots (imaginary part versus the real part of the complex impedance) of fresh cells polarized at various applied voltages and which underwent or not the PEAI treatment are presented in Figure 5a. Their shape was rather unusual for solar cells under illumination. [START_REF] Wang | Effects of Perovskite Monovalent Cation Composition on the High and Low Frequency Impedance Response of Efficient Solar Cells[END_REF] . i is√-1, and p is a number lower than 1. We have extracted the capacitances values from the ZCPE parameters by the approach described by Brug et al. [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] The formulae employed and the analysis protocol are more detailed in the Annex 2: Analysis of the impedance spectra of the Supporting Information. C2 versus Vappl of fresh cells is displayed as open symbols in Figure 5c. C2 is assigned to the intrinsic dielectric relaxation capacitance of the perovskite and to a depletion capacitance above 0.7V. We show below that its behavior is unchanged with the cell aging. Therefore, the bulk perovskite did not evolve during the first days of the PSC life. The resistance measured was quite large and, based on the general EEC, [START_REF] Ulfa | Molecular Versus Polymeric Hole Transporting Materials for Perovskite Solar Cell Application[END_REF] is assigned to the sum of two recombination resistances, R2 and R4. The large values result from the domination of the R4 parameter. The special low frequency behavior of the cell can be assigned to the interfacial reactivity and slow processes that occur at the interfaces. Perovskite/selective contacts Interfaces in fresh cells would not be stabilized either for the pristine or PEAI-treated cells. Further characterizations of the PSCs were carried out after their aging for a few days in a N2 filled glovebox (typically 6-7 days). This maturation period of the solar cells results in an increase of the overall efficiency of the PSC mainly due to the increase of the Voc and of the Jsc in a less extend as shown in Table S3 (Supporting Information). After this period of time, the Nyquist plots exhibited with the EEC presented in Figure 6d and also derived from the general EEC. Figure 6a presents the effect of applied voltage on the Nyquist impedance plot. When the voltage was increased, the two circle arcs were reduced in size due to a reduction of both R2 and R4 resistance as schematically displayed in the Annex 2 (Supporting Information). Figure 6b reports impedance spectra of a pristine and a PEAI-treated cells. The amplitude of the circles was increased by the treatment and therefore R2 and R4 were increased. Figure 6c shows that the amplitude of the circle arcs also varied with the MACl molar fraction employed to prepare FA1-xMAxPbI3. To quantify these changes, the spectra were fitted using the simplified EEC presented in Figure 6d. Due to the appearance of a second circle arc, we added a CPE capacitive element, noted CPE4, in parallel with R4 in the EEC of Figure 5b. C4 capacitance values were extracted from CPE4 as detailed in the annex 2 (Supporting Information). No inductive loop was found and we stopped the analysis at 0.1 Hz since at lower frequency, the signal could be rather noisy.

C2 is a light insensitive element. 17 Figure 5c and Figure 6e show that this parameter remained the same with a battery of changed parameters: the cell age, the PEAI treatment and the MACl additive fraction in the perovskite precursor solution. The curves were similar over the whole investigated applied voltage range. Below 0.5V, this capacitance remained almost constant. It is assigned to the intrinsic dielectric relaxation of the perovskite. [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] The composition of the perovskite of the various cells is about the same, consequently this parameter did not vary significantly in Figure 5c. Above 0.6 V, C2 increased due to a charge storage effect. At high Vappl, C2 is dominated by the capacitance due to the charge stored in a depleted region at the TiO2/perovskite layers. In the literature, the depleted region has been described as mainly extending either in the titania layer and/or in the T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 21 doped perovskite layer. [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][START_REF] Guerrero | Electrical Field Profile and Doping in Planar Lead Halide Perovskite Solar Cells[END_REF][START_REF] Jiang | Carrier Separation and Transport in Perovskite Solar Cells Studied by Nanometre-Scale Profiling of Electrical Potential[END_REF] The Mott-Schottky plot of C2 in Figure S4 (Supporting Information) results in a linear section. The linear fits extrapolate at the same potential for the various cells, between 1.08V-1.12V. It has been assigned to the built-in potential (Vbi) in the litterature. 47 The other electrical elements, R2, R4 and C4 were light sensitive and we will see that all of them are related to charge recombination phenomena. For R2, the PEAI treatment results in a significant increase of this parameter (Figure 7a). A consequence is that, above 0.7V, it shifts the resistance to higher potential. The same behavior was found for R4 which is also a recombination resistance.

Recombination resistances are inversely related to the recombination rate and they provide a direct quantitative view of the recombination processes. The higher is this resistance, the lower is the T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 22 recombination rate. The fact that the PEAI nanolayer shifts R2 and R4 curves toward higher voltage is favorable for the cell performances since it increases the cell VOC.

C4 is a light sensitive parameter which value dramatically increases with the light irradiation and can reach very high values. In our case, at high Vappl, C4 attained 1.10 -1 F.cm -2 . There is little doubt that C4 is also related to recombination of charges. Because charge recombinations in PSC are a phase delayed phenomenon, in addition to the real component, an imaginary component is also introduced. This imaginary component is analyzed here as a capacitance in the equivalent electrical circuits used to fit our data (Figures 6d). It is the main contribution of C4. This delayed recombination process is a particular 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. In Figures 7a-c, the three parameters R2, R4 and 1/C4 clearly appear as related since they exhibit a similar behavior. We can also note that their shape reproduces the J-V curve shape since the Rs+R2+R4 sum is the total resistance of the devices at the steady state.

These three parameters increased markedly with the PEAI treatment which passivates the interfacial defects, favors the hole transfer toward the HTL and lower the recombination phenomena. Therefore, higher JSC and VOC are achieved when the treatment is done (Figure 3).

Figure 7 shows some differences in the electrical behavior of cells prepared with various MACl additive fraction contents. The main one is the marked lowering of the R2 parameter for the MC55-PEAI cells. R4 and 1/C4 are also slightly lower. We can conclude that more recombinations occurred in these cells especially at high voltage. The surface of the MC55 film was more defective and the passivation was less effective for these cells. Consequently, the FF and VOC were the most impacted parameters which were significantly reduced at the high MACl molar fraction (Figure 3c). Because of the ion mobility in the perovskite and the dynamics of defect formation and repair T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 23 described, it is rather difficult to go further in the analysis and assigned a sound origin for R2 and R4. However in Figure 7, we remark that the behaviour of 1/C4 is similar to that of R2 which lead us to tentatively assign the later resistance to recombination occurring in the bulk, likely near the interface because this element is sensitive to the surface treatment. On the other hand, R4 would be an interfacial recombination resistance.

Operational stability of the devices

Developing highly stable PSCs that are resistant to moisture, light and electrical stresses is a great challenge for the future of this technology. We have performed several experiments which have demonstrated the high stability of the PSCs developed in this study. Impedance spectroscopy is a stressing operating mode for the cells. The operated devices are then submitted to a continuous light irradiation, to various applied voltages and to a superimposed ac stimulus. The full characterization of a solar cell by EIS lasts more than one hour. For each cell investigated, we controlled their stability by measuring their J-V curves before and after the EIS characterization. Results for some of them are gathered in Table S4 (Supporting Information). The performances of the pristine and of the PEAI-treated devices were slightly higher after EIS characterizations due to a better VOC. The prepared devices presented a high robustness and they met the stability requirement for reliable EIS characterizations as defined by one of us in Ref. [START_REF] Pitarch-Tena | Impedance Spectroscopy Measurements in Perovskite Solar Cells. Device Stability During the Measurement and Noise Reduction[END_REF] The stability was also followed for 5 hours under a continuous one sun AM 1.5G illumination and at a relative humidity (RH) of 45%. Figure 8a compares the pristine and the PEAI-treated unencapsulated MC48 cells with a MAPI unencapsulated cell. The latter was preferred to a pure FAPI cell due to the fact that the FAPI perovskite in its α-phase is unstable and quickly decomposes in its non-photoactive δ-phase. The power output slightly decreased and then increased for the pristine cell while it was highly stable and unchanged for the PEAI-treated one. On the other hand, the MAPI cell power output continuously decreased during the same test. The stability against moisture was tested by storing the unencapsulated cells in a chamber with a 90% RH and ambient light condition (Figure 8b). The power output was stable for 20h for the MC48-PEAI cell. At the same time, the MAPI cell was not stable, losing rapidly 60 % of its PCE within only 20h. The variations of the J-V curve parameters are shown in Figure S5 (Supporting Information).

Conclusions

We have developed the use of methyl ammonium chloride as an additive in rather low

Experimental Section

The full experimental details are given at the beginning of the Supporting Information. The fluorine-doped SnO2 (FTO) substrates (TEC 7 from Pilkington) were paterned, etched and cleaned as described in our previous Ref. [START_REF] Zhu | Interfacial Engineering through Chloride-Functionalized Self-Assembled Monolayer for High Efficiency Perovskite Solar Cells[END_REF] . The compact TiO2 layer, noted c-TiO2, was deposited by spray pyrolysis and the spin-coated mesoporous (mp-TiO2) were deposited as described in our previous works. [START_REF] Wang | Insights into the Hole Blocking Layer Effect on the Perovskite Solar Cell Performance and Impedance Response[END_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] T. Zhu, D. Zheng, J. The Spiro-OMeTAD (Borun New Material Technology) film and the gold back-contact were prepared as described in Ref. [START_REF] Zhu | Interfacial Engineering through Chloride-Functionalized Self-Assembled Monolayer for High Efficiency Perovskite Solar Cells[END_REF] Unless specified, the solar cells where matured by storing them several days in a N2 glovebox before to be characterized.

The J-V curves were measured as described in Ref. [START_REF] Zhu | Interfacial Engineering through Chloride-Functionalized Self-Assembled Monolayer for High Efficiency Perovskite Solar Cells[END_REF] using AM 1.5G illumination conditions (100 mW/cm 2 ) 49 calibrated with help of a reference silicon solar cell. [START_REF] Kusumawati | Effects of Graphene in Graphene/TiO2 Composite Films Applied to Solar Cell Photoelectrode[END_REF] The XRD patterns, the absorbance and photoluminescence spectra were measured as described in Ref. [START_REF] Zhu | Interfacial Engineering through Chloride-Functionalized Self-Assembled Monolayer for High Efficiency Perovskite Solar Cells[END_REF] and in the Supporting Information. The samples were observed with a high resolution Ultra 55 Zeiss FEG field-emission scanning electron microscope (FE-SEM) operated in the in-lens mode. The EDX spectra were measured with a Quantax system from Brucker operated at 15 kV. The time-resolved photoluminescence (TRPL) curves were measured as detailed in Ref. [START_REF] Ulfa | Charge Injection and Electrical Response in Low-Temperature SnO2-Based Efficient Perovskite Solar Cells[END_REF] and in the Supporting Information. The electrical impedance spectra between 1 MHz and 100 mHz, were measured at room temperature by a PGSTAT 12 equipment from Autolab. The AC signal was 20 mV. Voltages (Vappl) were applied between the short circuit and the open circuit. An halogen Schott lamp was employed to provide a light power density of ∼0.9 sun. The cell illuminated area was 0.16 cm 2 defined by a mask. All the measured cells had the same contact/illumination geometries.

The Z-view software from National Instrument was employed for the spectra analysis.

Fig. 1 .

 1 Fig. 1. SEM images of the MC48 perovskite layers surface, (a) before and (b) after the PEAI deposition. Yellow circles are regions where PEAI accumulates. The inset is a zoom view.

Fig. 2 .

 2 Fig. 2. (a) XRD patterns and (b) absorbance spectra of MC48 and MC48-PEAI layers. (c) TRPL curves and (d) steady PL spectra for pristine and PEAI-treated MC48 samples.

  , D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 10

( 2 .

 2 8 ns). We explain the initial stronger emission by the fact that the treatment passivates defects near the layer front surface (the irradiated one) and enhances the radiative recombination. However the faster decay is analyzed as an efficient quenching of the radiative recombination by the fast transfer of the hole toward the PEAI-based surface layer. Table S2 also compares τslow of the T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 11pristine and treated samples. We measured 100-110 ns for the former sample and about 117 ns after the surface treatment. The improvement was limted because this parameter is sensitive to the perovskite bulk properties, not the surface and near-surface ones. The effect of the treatment on the steady PL spectra of the layers is shown in Figure2d. After the treatment, the PL intensity was stronger due to the stronger emission observed on Figure2c. In summary, a (PEA)2PbI4 nanolayer is formed spontaneously on the FA0.94MA0.06PbI3 film surface, without any post-PEAI-deposition thermal annealing treatment. It is assigned to the dissolution of MA and FA cations at the surface by the isopropanol solvent employed and to the higher thermodynamic stability of the 2D

Fig. 3 .

 3 Fig. 3. (a) Device structure employed in the paper with the c-TiO2/mp-TiO2 bottom contact and PEAI-based buffer layer. (b) Typical J-V curves of PSCs prepared with various MACl% with and without the PEAI surface post-deposition treatment. (c) Statistical analysis of the PSC performances and J-V curve parameters. (d) Tracking curve and steady-state PCE of the best cell. (e) EQE spectra of MC48 solar cells with and without PEAI treatment.

  , D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 13 material and the large grains. We have reproducibly observed that the use of the PEAI treatment increased the cell efficiency. The beneficial effect was found on both the short circuit current, JSC,and on the open circuit voltage, VOC, parameters. The higher Jsc with the treatment is in agreement with the TRPL results detailed above. The PEAI-passivation surface nanolayer dramatically reduces the non-radiative recombination by passivating the perovskite surface defects. Moreover, the hole charge are quickly separated by transfer to the nanolayer and it reduces the charge recombination in the perovskite. This fast charge transfer is beneficial of the solar cell current generation. The VOC is increased significantly with the PEAI treatment due to surface defects passivation and reduction of the interfacial charge recombinations. This point will be confirmed in Section 2.4 by the electrical impedance spectroscopy (EIS) study. In average, the post-deposition treatment boosted the PCE by about 10%. Another beneficial effect of the treatment is the reduction

-37207 16 Fig. 4 .

 164 Fig. 4. Voc versus light intensity (power density) curves. (a) MC48 cells with and without PEAI treatment. (b) Effect of MACl%. The curve slopes provide the ideality factor, nID, values (numbers in bold).

Fig. 5 .

 5 Fig. 5. (a) Impedance spectra of MC48 and MC48-PEAI solar cells measured at various applied voltages. Green symbols: MC48; Red symbols: MC48-PEAI. (b) Equivalent electrical circuit used to fit the high frequency part of the spectra. (c) C2 parameter of fresh and matured cells. (d) R2+R4 parameter of fresh cells. (c) & (d) symbols: green diamonds = MC48 cell, red squares = MC48-PEAI cell; open symbols = fresh cells; full symbols = matured cells (stored several days in a N2filled glovebox).

  At high frequencies (f > 50-150 Hz), a circle arc was found. After an inductive transition at intermediate frequency, a special behavior was found that cannot be fitted with a classical electrical circuit. The imaginary part of the impedance was negative but was pinned, remaining almost constant over a T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 18 large frequency range, while at the same time, the real part of the spectra decreased as the frequency decreased. We have focused our analysis on the circle arc at f > 50-150 Hz. It was fitted with the equivalent electrical circuit (EEC) shown in Figure5b. We have employed in this paper the same notation as in the general EEC for the analysis of PSC impedance spectra presented in our previous work[START_REF] Ulfa | Molecular Versus Polymeric Hole Transporting Materials for Perovskite Solar Cell Application[END_REF] and shown in Figure S4a (Supporting Information). Rs is a series resistance measured at high frequencies which is due to the contacts and wires. The flattened circle arc was fitted by a constant phase element (CPE) in parallel with a resistance. The CPE impedance (ZCPE) varies as 𝑍 𝐶𝑃𝐸 = 1 𝑇(𝑖𝜔) 𝑝 in the complex plane. ω notes the angular frequency. It is related to the frequency as 𝑓 = 𝜔 2𝜋

Fig. 6 .

 6 Fig. 6. Nyquist plots of the EIS spectra. (a) Effect of the applied voltage on a 7 days old MC48-PEAI cell. (b) Effect of PEAI (MC48 and MC48-PEAI, Vappl=0.8V); (c) Effect of MACl (PEAItreated cells, Vappl=0.8V) and (d) Electrical equivalent circuit used to analyze the data. (e) Effect of MACl molar fraction on C2 parameter versus the applied voltage.

  , D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 20 two main relaxations as shown in Figure 6a-c. It suggests that during the maturation, reactions occurred that results in the stabilization of the low frequency signal. These spectra have been fitted

Fig. 7 .

 7 Fig. 7. (a-c) Effect of PEAI treatment on the impedance spectra parameter versus Vappl. (a) R2; (b) R4 and (c) 1/C4. Effect of MAI additive molar faction: (d) R2; (e) R4; and (f) 1/C4.

Fig. 8 .

 8 Fig. 8. (a) Tracking of MAPI, MC48 and MC48-PEAI solar cell normalized power outputs under continuous one sun AM1.5G illumination (unencapsulated devices, 45% RH). (b) Normalized PCE evolution of MC48-PEAI and MAPI unencapsulated cells upon storage in a chamber with a 90% RH.

26 A

 26 Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 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 32.4 mg, 38.9 mg and 44.6 mg (which represent 40 mol.%, 48 mol.% and 55 mol.% of precursor solution PbI2 content, respectively) of methylammonium chloride (MACl, Alfa aesar) in 800 μL DMF and 200 μL DMSO. The solutions was spin-coated as described in the Supporting Information to form films which were annealed at 153 °C for 13 min. These films and the corresponding solar cells are noted 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 perovskite layer. A one-step spin-coating program was employed: 2000 rpm/s acceleration, 3000rpm for 20s.

1 H

 1 -NMR spectra were recorded on a Bruker Avance 500 MHz Neo spectrometer. Chemical shifts were measured in ppm using the residual DMSO solvent signal as internal reference. Perovskite layers deposited on a sprayed c-TiO2 layer were dissolved in DMSO-d6 solvent (99.96 %D, T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 27 H2O<0.01%, Euriso-top) and the NMR spectra of the solutions were measured. These solutions were highly stable.

Supporting

  Information to this paper can be found online. SEM view of the layers and of a solar cell; XRD patterns of the FA0.94MA0.06PbI3 layers; perovskite layers titration; Absorbance and Tauc plot of the layers; PL spectra and TRPL curves of the layers; Effect of PEAI treatment on the perovskite layer XRD; EQE curves of the solar cells; TRPL curve analysis, decay times and their relative contribution; General equivalent electrical circuit of a perovskite solar cell under illumination; Mott-Schottky plots; Analysis of the impedance spectra; J-V curve parameters before and after EIS study of the PSCs; Stability measurement by J-V curve parameters tracking.

Table 1

 1 Photovoltaic J-V parameters of best FA0.94MA0.06PbI3 cells prepared with various MACl%, with and without the PEAI surface post-deposition treatment (under AM 1.5G conditions, 100 mW.cm -2 ).

	Cell	Scan direction	Voc /V	Jsc mA/cm 2 FF /%	PCE /%	HI a /%
	MC40-PEAI	Reverse 1.046 Forward 1.035	25.83 25.82	77.69 76.10	21.00 20.33	3.2
	MC48-PEAI	Reverse 1.060 Forward 1.052	25.94 25.93	80.62 78.26	22.18 21.35	3.7
	MC55-PEAI	Reverse 1.024 Forward 1.014	25.90 25.93	80.75 77.77	21.41 20.44	4.5
	MC48	Reverse 1.002 Forward 0.994	24.73 24.47	82.3 80.02	20.37 19.46	4.5
	a HI : Hysteresis index defined as [PCE(%)rev-PCE(%)for]/PCE(%)rev

  concentration mixed organic cation precursor solutions. MACl is shown to mediate the growth of the layers. The resulting films were uniform, compact, with no pinholes and made of remarkably large size perovskite crystal grains. By this technique, a small amount of MA cation was incorporated in FAPbI3, stabilizing the pure α-phase with FA0.94MA0.06PbI3 composition. The excess was removed by sublimation and degradation. The presented growth technique is reliable, T. Zhu, D. Zheng, J. Liu, L. Coolen, Th. Pauporté, ACS Appl. Mater. Interfaces 12 (2020) 37197-37207 25 robust and gives devices efficient over a large range of additive concentration. The best MACl molar fraction is found at 48mol.%. To further increase the PCE of the solar cells, a treatment with a PEAI solution has been implemented. This treatment, without any thermal annealing step, leads to the spontaneous formation of (PEA)2PbI4 perovskite at the surface. This buffer layer has been shown to favor a fast transfer of the holes towards the HTL and to reduce the recombinations at the perovskite/HTL interface. Overall, the optimized devices had a PCE higher than 22.1%. A comprehensive investigation of the systems by impedance spectroscopy has been carried out. The change in the impedance spectra within the first days of the device storage corresponds to a maturation step which involves reactions at the interface(s). The high frequency capacitance was irrespective of the PSC age and treatment was assigned at low Vappl to the intrinsic dielectric relaxation of the perovskite. The high frequency resistance R2 and R4 were analyzed as recombination resistances and C4 as a recombination capacitance. Furthermore, our results indicate that R2 and C4 are likely related to recombination centers localized near the interface whereas R4 would correspond to an interfacial recombination resistance. Overall, the variations of R2, R4 and 1/C4 explain the variations in FF and VOC of the PSCs with the MACl mol.% and the PEAI treatment. The power output of the produced devices was remarkably stable. These PSCs were proved resistant to electrical stresses, to light irradiation and to moisture.

TOC Graphic


Ludovic Favereau
email: ludovic.favereau@univ-rennes1.fr

Cassandre Quinton

Cyril Poriel

Thierry Roisnel

Denis Jacquemin

Jeanne Crassous

Persistent Organic Room-Temperature Phosphorescence in Cyclohexane-trans-1,2-Bisphthalimide Derivatives: The Dramatic Impact of Heterochiral vs. Homochiral interactions

Persistent metal-free Room-Temperature Phosphorescence (RTP) materials attract significant interest owing to the production of long-lived triplet excited states. Although several organic designs show RTP, the impact of intermolecular interactions on the triplet excitons stabilization and migrations remains hardly understood because obtaining different ordered intermolecular interactions while conserving identical molecular electronic properties is very challenging. We propose here a new strategy to circumvent this problem by taking advantage of the distinct molecular packing that can be found between enantiomer and racemic forms of a chiral molecule. Structural, photophysical and chiroptical investigations of chiral cyclohexane bisphthalimide derivatives showed that heterochiral and homochiral dimer interactions play a crucial role on the triplet excited state stabilization, resulting in higher RTP efficiency for enantiopure systems than for racemic one. This study paves the way to the use of molecular chirality to rationalize supramolecular properties arising from subtle intermolecular interactions.

TOC Graphic

 While phosphorescent metal-containing systems are well-known and rely on an efficient inter-system crossing (ISC) process induced by the large spin-obit couplings (SOCs) of metal, 12, 13 organic heavy-atom-free RTP materials have several advantages, such as lower cost and

toxicity, and more tuneable photophysical properties. [START_REF] Data | Recent Advancements in and the Future of Organic Emitters: TADF-and RTP-Active Multifunctional Organic Materials[END_REF][START_REF] Kenry; Chen | Enhancing the performance of pure organic room-temperature phosphorescent luminophores[END_REF] In organic systems, RTP can typically take place by improving ISC efficiency, which can be done either by increasing the SOCs using heavy halogen atoms such as bromine or iodide, or promoted by the presence of low-lying transitions following El-Sayed's rule. This can be achieved using heterocyles or carbonyl groups for instance. [START_REF] An | Stabilizing triplet excited states for ultralong organic phosphorescence[END_REF][START_REF] Hirata | Recent Advances in Materials with Room-Temperature Phosphorescence: Photophysics for Triplet Exciton Stabilization[END_REF][START_REF] Ma | Efficient and Long-Lived Room-Temperature Organic Phosphorescence: Theoretical Descriptors for Molecular Designs[END_REF] Besides these electronic factors, structural rigidity and ordered packing are also crucial parameters to minimize the radiationless deactivation of the ultra-long lived excited triplet states. [START_REF] Chen | Versatile Room-Temperature-Phosphorescent Materials Prepared from N-Substituted Naphthalimides: Emission Enhancement and Chemical Conjugation[END_REF][START_REF] Lucenti | Cyclic Triimidazole Derivatives: Intriguing Examples of Multiple Emissions and Ultralong Phosphorescence at Room Temperature[END_REF][START_REF] Lucenti | H-Aggregates Granting Crystallization-Induced Emissive Behavior and Ultralong Phosphorescence from a Pure Organic Molecule[END_REF][START_REF] Xie | How the Molecular Packing Affects the Room Temperature Phosphorescence in Pure Organic Compounds: Ingenious Molecular Design, Detailed Crystal Analysis, and Rational Theoretical Calculations[END_REF][START_REF] Bian | Simultaneously Enhancing Efficiency and Lifetime of Ultralong Organic Phosphorescence Materials by Molecular Self-Assembly[END_REF][START_REF] Gu | Prolonging the lifetime of ultralong organic phosphorescence through dihydrogen bonding[END_REF][START_REF] Huang | The influence of molecular conformation on the photophysics of organic room temperature phosphorescent luminophores[END_REF][START_REF] Mu | Mechano-induced persistent roomtemperature phosphorescence from purely organic molecules[END_REF][START_REF] Zhang | N-Alkylcarbazoles: homolog manipulating long-lived room-temperature phosphorescence[END_REF][START_REF] Wang | Mechanoluminescence or Room-Temperature Phosphorescence: Molecular Packing-Dependent Emission Response[END_REF] An effective understanding of the underlying photophysical process is needed to optimize the design of organic RTP molecular materials, and, more precisely, the impact of intermolecular interactions on the triplet exciton stabilization and migration needs to be understood. Such task is however made difficult, as obtaining different intermolecular interactions while keeping the same molecular electronic properties is often beyond reach, e.g., the addition of methylene substituent in a lateral alkyl chain may induce a completely different solid state packing. [START_REF] Huang | The influence of molecular conformation on the photophysics of organic room temperature phosphorescent luminophores[END_REF][START_REF] Zhang | N-Alkylcarbazoles: homolog manipulating long-lived room-temperature phosphorescence[END_REF] To circumvent this problem, chirality offers a unique possibility to investigate such parameters since enantiomer and racemic forms of a chiral molecule have identical chemical and physical properties in solution, [START_REF] Jacques | Enantiomers, Racemates, & Resolutions[END_REF] but may present distinct molecular packing as heterochiral and homochiral dimer interactions might differ, which in turn can yield different photophysical properties and/or electronic conduction. [START_REF] Josse | Enantiopure versus Racemic Naphthalimide End-Capped Helicenic Non-fullerene Electron Acceptors: Impact on Organic Photovoltaics Performance[END_REF][START_REF] Hatakeyama | Azaboradibenzo[6]helicene: Carrier Inversion Induced by Helical Homochirality[END_REF][START_REF] Réthoré | Chiral Molecular Metals: Syntheses, Structures, and Properties of the AsF6-Salts of Racemic (±)-, (R)-, and (S)-Tetrathiafulvalene-Oxazoline Derivatives[END_REF][START_REF] Pop | Chirality Driven Metallic versus Semiconducting Behavior in a Complete Series of Radical Cation Salts Based on Dimethyl-Ethylenedithio-Tetrathiafulvalene (DM-EDT-TTF)[END_REF][START_REF] Brandt | The added value of small-molecule chirality in technological applications[END_REF][START_REF] Yang | Emergent Properties of an Organic Semiconductor Driven by its Molecular Chirality[END_REF][START_REF] Rice | A computational exploration of the crystal energy and charge-carrier mobility landscapes of the chiral [6]helicene molecule[END_REF] Accordingly, organic chiral compounds with RTP property modulated by enantiopurity represents an exceptional opportunity for exploring the impact of intermolecular interactions on the efficiency of ultra-long RTP without altering the molecular electronic properties. Moreover, the possibility to obtain RT circularly polarized luminescence (CPL) with chiral organic emitters could bring additional advantages over unpolarized RTP, [START_REF] Chen | Long-Persistent Circularly Polarized Phosphorescence from Chiral Organic Ionic Crystals[END_REF][START_REF] Liang | Organic Room-Temperature Phosphorescence with Strong Circularly Polarized Luminescence Based on Paracyclophanes[END_REF][START_REF] Hirata | Circularly Polarized Persistent Room-Temperature Phosphorescence from Metal-Free Chiral Aromatics in Air[END_REF] notably in the design of efficient CP-OLEDs, in spintronic, optical information processing, as well as in bioimaging (ellipsometry-based tomography) and chiral sensing, where the state of light polarization (either left or right) may bring higher contrast and selectivity than for an non-polarized light.

During our work on chiral -conjugated molecules for chiroptoelectronic applications, [START_REF] Josse | Enantiopure versus Racemic Naphthalimide End-Capped Helicenic Non-fullerene Electron Acceptors: Impact on Organic Photovoltaics Performance[END_REF][START_REF] Bouvier | Helicenes Grafted with 1,1,4,4-Tetracyanobutadiene Moieties: pi-Helical Push-Pull Systems with Strong Electronic Circular Dichroism and Two-Photon Absorption[END_REF][START_REF] Dhbaibi | Exciton coupling in diketopyrrolopyrrole-helicene derivatives leads to red and near-infrared circularly polarized luminescence[END_REF] we serendipitously noticed that enantiopure cyclohexane-trans-1,2-bisphthalimide derivatives, (RR)-Cy(F2Pht)2, [START_REF] Li | Stable Enantiomers Displaying Thermally Activated Delayed Fluorescence: Efficient OLEDs with Circularly Polarized Electroluminescence[END_REF] display persistent RTP emission process once precipitated in the reaction medium. Irradiation of the obtained solid by UV light at  = 365 nm (benchtop UV lamp) affords light blue luminescence, which evolves in a yellow one, seen by the naked eye upon switching off the excitation light (Scheme 1). This behaviour prompted us to explore the possible impact of molecular enantiopurity on the RTP efficiency. Herein we report our results regarding the study of chiral organic persistent RTP molecular materials based on cyclohexane tetrafluorinated derivatives ((RR)-, (SS)-and (rac)-Cy(F2Pht)2, Figure 1). Crystals of Cy(F2Pht)2 display ultra-long yellow phosphorescence, which is dependent on both the enantiopurity of the molecular components and the temperature. While enantiopure and racemic crystals exhibit persistent phosphorescence at 77 K, only the former shows efficient RTP under ambient (air) conditions with a lifetime of 0. (RR)-, (SS)-and (rac)-Cy(F2Pht)2 were efficiently synthetized by mixing one equivalent of corresponding (RR)-, (SS)-or (rac)-cyclohexane-trans-1,2-diamine with excess of 5,6difluoroisobenzofuran-1,3-dione in acetic acid in 92% yield for the three compounds (Scheme 1). These chiral derivatives were fully characterized by NMR, chiral HPLC and mass spectrometry, [START_REF] Li | Stable Enantiomers Displaying Thermally Activated Delayed Fluorescence: Efficient OLEDs with Circularly Polarized Electroluminescence[END_REF] displaying characteristic signals of both phthalimide and cyclohexane units in the aromatic and aliphatic parts of the 1 H spectrum, respectively, with a C2 symmetry for the bisphthalimide moieties (see the SI for details). [START_REF] Gawronski | Excited States of the Phthalimide Chromophore and Their Exciton Couplings: A Tool for Stereochemical Assignments[END_REF][START_REF] Gawroński | Chirality of aromatic bis-imides from their circular dichroism spectra[END_REF] Crystals of both (RR)-and (rac)-Cy(F2Pht)2 have been obtained by slow diffusion of pentane vapours into chloroform solutions and their resulting X-ray structures show distinct features regarding the molecular geometry and the supramolecular arrangement: (RR)-and (rac)-Cy(F2Pht)2 crystallize in the C2221 and Pī space group, respectively (Figure 1 and table S1), with the expected equatorial conformation for the phthalimide substituents attached to the cyclohexane backbone. While the two phthalimide planes form an angle of ca. 56° in the racemic compound, the enantiopure (RR)-Cy(F2Pht)2

shows a more open structure with an angle of 71° (Figures 1a and1b). In addition, different N1-C1-C2-N2 torsion angles are found in (rac)-and (RR)-Cy(F2Pht)2: 49.3° and 60.6°, respectively, resulting in a less face-to-face phthalimides arrangement for the former. These significant differences may be attributed to symmetry reasons, which sometimes induce different packing between pure enantiomers and corresponding racemic mixture. [START_REF] Jacques | Enantiomers, Racemates, & Resolutions[END_REF] While (rac)and (RR)-Cy(F2Pht)2 form similar phthalimide dimers in a head-to-tail fashion, separated by 3.46 and 3.53 Å, respectively (Figures 1c and1d), expanding the packing to the supramolecular level reveals important differences between both crystals. Indeed, (RR)-Cy(F2Pht)2 dimers exhibit a continuous intermolecular chain along the x axis, maintained by identical - interactions involving each phthalimide unit (Figure 1f), whereas (rac)-Cy(F2Pht)2 dimers present - intermolecular interactions along the y axis with other dimer neighbours (blue colour, Figure 1e). In this additional dimer interaction, the phthalimide groups are separated by 3.45 Å and appear much less co-facially oriented than the previously described red one, affording distinct molecular arrangements for homo-and heterochiral dimer interactions.

Further single-crystal X-ray difraction analysis show that for the red dimers, the angle between transition dipoles and the interconnected axis is larger than the critical value of 54.7° ( = 71.5 and 77.6° for (rac)-and (RR)-Cy(F2Pht)2, respectively, Figure 1c,d), indicating the presence of H-aggregates (see the SI for details), while for the blue dimer, this angle is around 55.4° at the boarder between H-and J-aggregates states. [START_REF] An | Stabilizing triplet excited states for ultralong organic phosphorescence[END_REF][START_REF] Kasha | The exciton model in molecular spectroscopy[END_REF][START_REF] Eisfeld | The J-and H-bands of organic dye aggregates[END_REF] Fluorine atoms might have an indirect effect on the overserved solid-state arrnagement since in the crystal structure, one fluorine atom shows a short contact bond with an aromatic proton of a phthalimide fragment neighbors for (RR)-Cy(F2Pht)2 or with an alkyl proton of the cyclohexane core in the case of rac-Cy(F2Pht)2 (Figure 1), suggesting an effect on the molecular packing . The UV-vis spectrum of Cy(F2Pht)2 in dilute solution (10 -5 M) displays an intense absorption bands at ca. 230 nm ( = 5.5  10 4 M -1 cm -1 ) and smaller broader one at 285 nm ( = 2.0  10 3 M -1 cm -1 ), respectively assigned to -* and n-* transitions (Figure 2). [START_REF] Gawronski | Excited States of the Phthalimide Chromophore and Their Exciton Couplings: A Tool for Stereochemical Assignments[END_REF] At 298 K in dichloromethane solution, Cy(F2Pht)2 exhibits a very weak luminescence at ca. 310 nm ( < 0.01), presumably due to an efficient ISC process from the singlet to the triplet excited state of the molecule, as already reported for phthalimide derivatives. [START_REF] Wintgens | Spectroscopic properties of aromatic dicarboximides. Part1.-N-H and N-methyl-substituted naphthalimides[END_REF] Indeed, lowering the temperature to 77 K affords a residual fluorescence signal at 300 nm and mainly phosphorescence emission, with a maximum at 461 nm and a lifetime of ca. 1 s arising from -* triplet state, in line with the phosphorescence emission reported for phthalimide derivatives (Figures 2 andS9). To gain further details, theoretical calculations were performed on Cy(F2Pht)2 (see the SI). Upon light excitation, population of excited Cy(F2Pht)2 singlet states (Sn) occurs, followed by internal conversion and relaxation to the lowest excited singlet state (S1, Figure 2). From the latter, a residual fluorescence emission occurs experimentally recorded at 312 nm, which is well reproduced by the calculations (319 nm, Figure 2). This radiative deactivation process is in competition with the ISC process leading to Cy(F2Pht)2 triplet excited states. Theoretical calculations indicate the presence of eight triplet states close to S1 in energy, with moderate to rather strong spin orbit coupling with values up to 4.3 and 4.6 cm -1 for the S1T6 and S1T5 transitions. These SOC are of significant intensity for organic systems without heavy atom and highlight the potential of this molecular system to investigate phosphorescence emission, which was also well reproduced by theoretical calculation at 447 nm (Figure 2). For comparison purpose, mono phatalimide derivative CyF2Pht was also synthetized and its optical properties characterized. CyF2Pht displays almost the same UV-vis absorption and luminescence profiles than those of Cy(F2Pht)2, notably a similar low temperature phosphorescence emission at 460 nm with a lifetime of ca. 0.8 s. These results suggest a weak electronic interaction between the two phthalimide units in Cy(F2Pht)2, consistent with their arrangements (see Figure 1) and with its molar extinction coefficients twice as high as for CyF2Pht. As expected, the cyclohexane-trans-1,2-bisamido linker has a limited role of in term of photophysics due to its sp 3 core but remains crucial as it brings the property of chirality owing to the presence of the two enantiomers (RR and SS) for this conformation. 10 -5 (red), 10 -4 (green), 10 -3 (blue) and 10 -2 M (black) in dichloromethane solution at 298 K; right: Electronic Circular Dichroism spectra of (RR)-and (SS)-Cy(F2Pht)2 recorded at different concentration: 10 -5 (red), 10 -4 (green), 10 -3 (blue) and 10 -2 M (black) in dichloromethane solution at 298 K (due to rapid signal saturation upon increasing concentration, only the 230-450 nm region is displayed). Bottom: Energy-level diagrams and calculated ISC channels from the ground state S0 to the singlet excited states (Sn) and finally the triplet excited states (Tn) with the most significant corresponding spin-orbit coupling (SOC) constants.

Further insights have been obtained by recording the Electronic Circular Dichroism (ECD) of (RR)-and (SS)-Cy(F2Pht)2 (Figure S7), which exhibited the expected mirror-image spectra with positive intense bands at 230 and 242 nm ( ~ 35 M -1 cm -1 ), arising from two chiral excitonic couplings between the perpendicular in-plane electronic dipoles present in each phthalimide unit, [START_REF] Gawronski | Excited States of the Phthalimide Chromophore and Their Exciton Couplings: A Tool for Stereochemical Assignments[END_REF] and an additional very weak transition around 280 nm for (SS)-Cy(F2Pht)2 ( ~ -0.5 M -1 cm -1 ). Increasing solution concentrations up to 10 -2 M does not induce drastic changes in the UV-vis signature and mainly resulted in a small 15 nm red-shift of the band at 230 nm, a proportional intensity increase of the n-* transitions absorption at ca. 285 nm with a slight blue-shift of 3 nm, and the apparition of a weak band between 340 and 400 nm when the concentration attains 10 -3 -10 -2 M (Figure 2), suggesting the presence of intermolecular interactions. Likewise, ECD of concentrated solutions reveals the appearance of very weak mirror-image signals up to 350 nm, marked by two positive maxima at 315 and 330 nm for (SS)-Cy(F2Pht)2 (Figure 2), while the rest of the ECD spectrum becomes above the detection range of the detector. The obtained X-ray crystallographic structures and the slight blue-shift of the absorption spectra upon increasing concentration are consistent with a predominant contribution of H-aggregates between intermolecular phthalimide fragments, which have been identified as an important parameter for stabilization of triplet excitons and RTP in organic systems. [START_REF] Data | Recent Advancements in and the Future of Organic Emitters: TADF-and RTP-Active Multifunctional Organic Materials[END_REF][START_REF] Kenry; Chen | Enhancing the performance of pure organic room-temperature phosphorescent luminophores[END_REF][START_REF] Hirata | Recent Advances in Materials with Room-Temperature Phosphorescence: Photophysics for Triplet Exciton Stabilization[END_REF][START_REF] Lucenti | H-Aggregates Granting Crystallization-Induced Emissive Behavior and Ultralong Phosphorescence from a Pure Organic Molecule[END_REF] Similar experiments on monophtalimide model compound, CyF2Pht, gives identical optical signature for concentrated solution (Figures S13-S15), which confirms the involvement of intermolecular interactions. Such aggregation behaviour also modifies the luminescence properties of Cy(F2Pht)2 since the emission spectra display significant changes at high concentration including a decrease of the low emissive band around 310 nm (Figure 3a), concomitantly with the appearance of weakly luminescence around 420 nm, presumably arising from H-type dimers. Indeed, excitation of the sample at 360 nm, i.e., in the absorption band attributed to intermolecular interactions, results in similar fluorescence emission at 425 nm ( = 2.2 and 12 ns, Figure 3b). Moreover, excitation spectra at 425 nm clearly indicate that this emission band involved molecular and intermolecular optical transitions at 260-280 nm and between 340 and 400 nm, respectively (Figure S8). Overall, these photophysical characterizations in solution show the tendency of Cy(F2Pht)2 to form weakly fluorescent aggregates at high concentration, as expected for H-type molecular interactions, [START_REF] Lakowicz | Principles of Fluorescence Spectroscopy[END_REF] but do not afford any evidence of phosphorescence emission, nor of different signatures of the racemic and enantiopure supramolecular assemblies. The luminescence properties of the (RR)-, (SS)-, (rac)-Cy(F2Pht)2 crystals are displayed in Figure 4. At 298 K in air atmosphere, (rac)-Cy(F2Pht)2 crystals excited at 330 nm give a weak and broad emission spectrum with two hardly distinguishable and similarly intense bands at 435 and 530 nm. When excitation takes place at 350 nm, a similar luminescence profile is observed but with a slightly higher emission intensity for the 435 nm emission band. Such change becomes clearer when the crystals are excited at 370 nm, corresponding to the intermolecular interactions from the phthalimide aggregates, since the signal at 530 nm appears only as a shoulder of the main emission process at 435 nm. In comparison, while excitation of (RR)-Cy(F2Pht)2 crystals at 370 nm affords a similar emission spectrum with a maximum at 440 nm, the excitations at 330 and 350 nm result in different luminescence profiles. Indeed, upon excitation at 350 nm, (RR)-Cy(F2Pht)2 displays two well resolved emission bands at 440 and 525 nm, the latter being of higher intensity, which strikingly differ from (rac)-Cy(F2Pht)2.

Finally, the predominance of the emission process at 525 nm is even more pronounced when the enantiopure crystals are excited at 330 nm (I 530 /I 425 ratio of 14). Importantly, yellow RTP remains clearly present after switching off the excitation source at all these excitation wavelengths, suggesting that this process may be related to the emission band at 530 nm. Indeed, emission spectra of (RR)-Cy(F2Pht)2 crystals excited at 350 nm and recorded after 10, 30 and 50 ms of delay afford an identical vibronic responses with maxima at 496 and 532 nm.

To gain further insights, time-resolved luminescence measurements have been recorded at these two luminescence maxima. At 530 nm, a very long lifetime () of ~ 0.3 s was detected as the main decay component for the RTP process whereas at 435 nm, only a short nanosecond lifetime was recorded (Figure S9). Such measurements allowed us to assign the two observed emission bands at 435 and 530 nm to singlet (S1) and triplet (T1) emissions of the (RR)-Cy(F2Pht)2 aggregates, since the former is close to the fluorescence observed in concentrated solution (Figure 3) whereas the phosphorescence at 530 nm appears redshifted as compared to the molecular phosphorescence recorded in frozen solution of diluted Cy(F2Pht)2 at 77 K (Figure 2). We have also investigated the luminescence excitation spectra at 440 and 525 nm. The latter notably indicated that the electronic transitions responsible for the fluorescence and phosphorescence emissions involve different absorption bands, which may be assigned to intermolecular aggregates (350 -410 nm) and molecular absorption bands (350 nm), respectively (Figure 4 c), and suggest that the observed RTP process is more efficiently triggered when light excitations occurs on electronic transitions of the molecular unit (for exc < 350 nm). According to photophysical studies of phthalimides, [START_REF] Gawronski | Excited States of the Phthalimide Chromophore and Their Exciton Couplings: A Tool for Stereochemical Assignments[END_REF][START_REF] Wintgens | Spectroscopic properties of aromatic dicarboximides. Part1.-N-H and N-methyl-substituted naphthalimides[END_REF] the lower part of their absorption spectrum (280-340 nm) is dominated by n-* electronic transitions, which are associated to a high ISC rate between the singlet 1 (n-*) and lowest triplet 3 (,*) excited states in accordance with El-Sayed's rule, [START_REF] Lower | The Triplet State and Molecular Electronic Processes in Organic Molecules[END_REF] and as evidenced here by the recorded phosphorescence process in diluted phthalimide solution at 77 K. Such fast ISC process should occur in both racemic and enantiopure crystals of Cy(F2Pht)2 when exciting the molecular electronic transitions (exc 350 nm). However, when one excites the absorption band related to the intermolecular aggregates, that is, for exc > 350 nm, the ISC rate drops and fluorescence becomes the main radiative deactivation process. Interestingly, the racemic and enantiopure crystals did not emit with the same intensity, as can be seen in Figure 4. To ascertain such difference, a similar photophysical study was performed on (SS)-Cy(F2Pht)2 and afforded, as expected, similar results as (RR)-Cy(F2Pht)2 (see the SI for detail). Further, quantitative evidences of the impact of enantiopurity on the RTP efficiency were provided by luminescence quantum yield measurements recorded at different wavelengths. Both enantiopure crystals show close emission efficiencies with significant values of 2-3 and 3-6% when exciting either at 350 or 370 nm, respectively, while low quantum yields of 0.5 and 1% were measured for (rac)-Cy(F2Pht)2 at 350 or 370 nm respectively, unambiguously reflecting the more efficient RTP process in enantiopure crystal. This fact is also confirmed by naked eyes since the yellow RTP is less bright and shorter than the one observed from enantiopure crystals (see videos in the SI). Since the photophysical experiments were performed on crystals, the RTP process can be directly related to the presence of the illustrated intermolecular red dimers of phthalimide fragments in Figure 1. As mentioned above, these dimers show characteristics of H-type aggregates, which have been identified as a stabilization strategy for triplet excitons and an efficient approach to get RTP in organic systems. [START_REF] An | Stabilizing triplet excited states for ultralong organic phosphorescence[END_REF][START_REF] Lucenti | Cyclic Triimidazole Derivatives: Intriguing Examples of Multiple Emissions and Ultralong Phosphorescence at Room Temperature[END_REF][START_REF] Lucenti | H-Aggregates Granting Crystallization-Induced Emissive Behavior and Ultralong Phosphorescence from a Pure Organic Molecule[END_REF] Indeed, photophysical experiments performed on crystals of model compound CyF2Pht did not reveal any RTP process, which could be explained by the lack of "red" H-aggregates found in Cy(F2Pht)2 (Figure 5).

Accordingly, the higher RTP efficiency in enantiopure Cy(F2Pht)2 can be explained by the greater presence of such interactions in comparison with (rac)-Cy(F2Pht)2, highlighting the subtle and crucial role of the heterochiral and homochiral dimer interactions, ultimately leading to distinct molecular packing, and thus different RTP efficiency. Indeed, the presence of additional "blue" dimers for (rac)-Cy(F2Pht)2 result in less H-type aggregates and a less ordered molecular network, which presumably decreases the stabilization of triplet excitons, as for CyF2Pht. Such parameter was tentatively investigated by carrying out some photophysical experiments at 77 K to probe the impact of temperature on the stabilization of the phosphorescence arising from the supramolecular assembly (Figure S11-S12). Reducing the temperature clearly stabilizes and enhances the long emission process over the fluorescence one for both (rac)-Cy(F2Pht)2 and (RR)-Cy(F2Pht)2, which exhibited phosphorescence at 490 nm with a lifetime of 1.1 and 0.7 s., respectively. When light excitation is performed at 350 nm, only weak fluorescence is observed for (rac)-Cy(F2Pht)2 at 400 nm, indicating a critical role of the temperature on the deactivation of the triplet excited state for such derivative.

Unfortunately, no circularly polarized RTP was observed from enantiopure crystals despite several attempts, presumably due to the low value of polarized emission in these derivatives.

We have reported RTP process in chiral metal free tetrafluorinated cyclohexane-trans-1,2naphthalimide derivatives whose efficiency is modulated by the enantiopurity of the molecular component. This surprising aspect was assigned to a different organization of homo-and heterochiral dimers in the solid state, which has a crucial impact on the presence of H-type aggregates within the crystal network, as unambiguously ascertained by X-ray analysis. Such symmetry parameter results in an efficient RTP for enantiopure crystal of Cy(F2Pht)2 with a quantum yield of 3% and a lifetime of 0.3 s while, in contrast, (rac)-Cy(F2Pht)2 displays a quantum yield of 0.5% only under ambient (air) conditions. Photophysical and chiroptical measurements afforded further insights on the structural parameters that stabilized triplet excitons within these chiral molecular networks and notably the crucial impact of intermolecular H-aggregates and temperature. These findings clearly highlight an interesting feature of chiral molecular materials on the modulation of supramolecular material properties by the degree of enantiopurity. It is our hope that these results may help in designing more efficient chiral RTP materials in order to obtain circularly polarized phosphorescence in metal free compounds.

Scheme 1 .

 1 Scheme 1. Synthesis of (RR)-, (SS)-and (rac)-Cy(F2Pht)2 with their main photophysical characteristics and picture of the RTP process for (RR)-Cy(F2Pht)2, under UV light excitation at  = 365 nm (benchtop UV lamp) and after switched it off (time evolution).

Figure 1 .

 1 Figure 1. Comparison of the X-ray crystallographic structures of (rac)-Cy(F2Pht)2 (left, a) and (RR)-Cy(F2Pht)2 (right, b) at the molecular level with N1-C1-C2-N2 dihedral angle and angle between phthalimide plans; and at the supramolecular level with different views of intermolecular interactions between dimers, c) and d); and supramolecular packing e) and f).  angles represent the angle between the transition dipoles and interconnected axis of (rac)-Cy(F2Pht)2 and (RR)-Cy(F2Pht)2 with values of 71.5° and 77.6°, respectively for the red dimers, evidencing the presence of H aggregates, in comparison to the blue dimer ( = 55.4°, see SI for additional details).

Figure 2 .

 2 Figure 2. Top: left: UV-vis absorption spectrum of Cy(F2Pht)2 and CyF2Pht in dichloromethane solution (~10 -5 M, red and black, respectively) at 298 K together with their emission spectrum recorded in 2-MeTHF at 77 K (red and black dashed lines, respectively, exci = 280 nm) and the corresponding excitation spectrum of Cy(F2Pht)2 (red dotted line, exci = 460 nm); middle: UV-vis absorption spectra of Cy(F2Pht)2 recorded at different concentration:

Figure 3 .

 3 Figure 3. a) Luminescence spectra of Cy(F2Pht)2 recorded at different concentrations: 10 -6 (purple), 10 -5 (red), 10 -4 (green) and 10 -3 M (blue) in dichloromethane solution at 298 K (exci = 280 nm); b) luminescence spectra of Cy(F2Pht)2 recorded at different concentration: 10 -5 (red), 10 -4 (green), 10 -3 (blue) and 10 -2 M (black) in dichloromethane solution at 298 K (exci = 360 nm) with illustration of the presumably molecular aggregation in highly concentrated solutions.

Figure 4 .

 4 Figure 4. a) Luminescence spectra of (rac)-Cy(F2Pht)2 crystals (left, black lines) and (RR)-Cy(F2Pht)2 (right, red lines) recorded at different wavelength excitations: 330 (solid line), 350 (dashed line) and 370 nm (dotted line) at 298 K; b) top: pictures showing luminescence of (rac)-Cy(F2Pht)2 (left) and (RR)-Cy(F2Pht)2 (right) under UV light excitation with benchtop UV lamp and bottom: just after switched it off; c) steady-state (black solid line) and delayed emission spectra (red lines, exci = 280 nm), recorded at different times delays: 50 ms (solid line), 30 ms (dashed line) and 10 ms (dotted line) at 298 K; d) left: excitation of luminescence of (RR)-Cy(F2Pht) recorded at 440 (red) and 525 nm (black) and right: proposed mechanism of RTP generation with impact of excitation wavelengths.

Figure 5 .

 5 Figure 5. a) Luminescence quantum yields of (RR)-Cy(F2Pht)2 and (rac)-Cy(F2Pht)2 crystals recorded at 350 and 370 nm at 298 K; b) Luminescence spectra of CyF2Pht crystals recorded at different wavelength excitations: 310 (solid line), 350 (dashed line) and 370 nm (dotted line) at 298 K with X-ray crystallographic structures of intermolecular dimers.

Acknowledgments

We acknowledge the Ministère de l'Education Nationale, de la Recherche et de la Technologie, the Centre National de la Recherche Scientifique (CNRS). The PRISM core facility (Biogenouest©, UMS Biosit, Université de Rennes 1 -Campus de Villejean -35043 RENNES Cedex, FRANCE) is acknowledged for the ECD measurements. The authors thanked Dr. N.

Vanthuyne for the chiral HPLC analysis. Guillaume Juin is warmly thanked for his movie on the room temperature process.

Supporting Information

Additional detailed experimental information, synthesis, and graphs can be find in the Electronic Supporting Information.