Yuze Li

Jia-Jiu Ye

Asma Medjahed

Dmitry Aldakov

Stéphanie Pouget

Elisabeth Djurado

Lin Xu
email: linxu@jlu.edu.cn

Peter Reiss
email: peter.reiss@cea.fr

High Fill Factor and Reduced Hysteresis Perovskite Solar Cells Using Small Molecule-Engineered Nickel Oxide as Hole Transport Layer

Keywords: NiO, hole transport layer, perovskite solar cells, fill factor, 4-dimethylaminopyridine

Nickel oxide (NiOx) is an emerging hole transport layer (HTL) material in halide perovskite solar cells (PSCs), combining high hole mobility, transparency, and stability. Current limitations of the device performance are mainly related to the inefficient hole extraction caused by contact problems between NiOx and the perovskite layer. Based on its expected strong interaction with both the NiOx surface and the perovskite layer, we selected 4-dimethylaminopyridine (DMAP) as a molecular passivation agent for the HTL. Photoelectron spectroscopy and photophysical studies demonstrate that DMAP passivation creates a more favorable band alignment at the NiOx/perovskite interface. This leads to decreased carrier recombination near the interface and to enhanced hole transfer. In addition, X-ray diffraction reveals reduced strain, improved crystalline quality, and a redistribution of excess PbI2 in perovskite layers grown on DMAP-passivated NiOx. As a consequence, PSCs with the DMAP-modified HTL exhibit a strongly increased fill factor and power conversion efficiency with values close to 80% and 18%, respectively. Moreover, they show negligible hysteresis and enhanced environmental stability compared to devices with untreated HTLs.

Introduction

Organic-inorganic lead halide perovskite solar cells (PSCs) constitute an emerging family of photovoltaic devices, which benefit from the highly advantageous features of the perovskite active material such as efficient light absorption, [START_REF] De Wolf | Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance[END_REF] long charge carriers diffusion length, [START_REF] Adhyaksa | Carrier Diffusion Lengths in Hybrid Perovskites: Processing, Composition, Aging, and Surface Passivation Effects[END_REF] and low fabrication cost. With a power conversion efficiency (PCE) of more than 25% (25.7% certified) achieved in only 12 years of development, they are one of the most promising upcoming photovoltaic technologies. [START_REF] Kim | Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%[END_REF][START_REF] Jeong | Stable Perovskite Solar Cells with Efficiency Exceeding 24.8% and 0.3-V Voltage Loss[END_REF] The ambipolar character of halide perovskite materials enables a variety of device designs, encompassing mesoscopic and planar devices with n-i-p or p-i-n architectures. The more widely studied n-i-p structures, in which metal oxides (TiO2, SnO2, ZnO, etc.) and spiro-OMeTAD (N 2 ,N 2 ,N 2′ ,N 2′ ,N 7 ,N [START_REF] Van Reenen | Modeling Anomalous Hysteresis in Perovskite Solar Cells[END_REF] ,N 7′ ,N 7′ -octakis(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine) are commonly used as electron transport layer (ETL) and hole transport layer (HTL), respectively, [START_REF] Kim | Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%[END_REF] are prone to hysteresis effects when measuring the J/V curves, making it difficult to properly assess the PCE. [START_REF] Tavakoli | Surface Engineering of TiO2 ETL for Highly Efficient and Hysteresis-Less Planar Perovskite Solar Cell (21.4%) with Enhanced Open-Circuit Voltage and Stability[END_REF] This phenomenon is commonly attributed to unbalanced charge transport and accumulation of carriers near the interfaces due to a large defect density near the perovskite surface, [START_REF] Kim | Mechanism of Carrier Accumulation in Perovskite Thin-Absorber Solar Cells[END_REF] interstitial excess ions screening effects, [START_REF] Van Reenen | Modeling Anomalous Hysteresis in Perovskite Solar Cells[END_REF] or ferroelectric properties. [START_REF] Stoumpos | Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and near-Infrared Photoluminescent Properties[END_REF] The p-i-n architecture is an appealing alternative as it provides negligible hysteresis effects and the possibility of simpler and low-temperature fabrication processes, making it also highly suitable for use in tandem solar cells. [START_REF] Huang | PSS Composite Layer: A Route to Suppressed Hysteresis and Enhanced Open-Circuit Voltage in a Planar Perovskite Solar Cell[END_REF] Among inverted structure PSCs, poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) are commonly used HTLs, [START_REF] Bi | Non-Wetting Surface-Driven High-Aspect-Ratio Crystalline Grain Growth for Efficient Hybrid Perovskite Solar Cells[END_REF][START_REF] Luo | Enhanced Photovoltage for Inverted Planar Heterojunction Perovskite Solar Cells[END_REF][START_REF] Salim | Perovskite-Based Solar Cells: Impact of Morphology and Device Architecture on Device Performance[END_REF] with a highest certified PCE of 24.3%. [START_REF] Li | Organometallic-Functionalized Interfaces for Highly Efficient Inverted Perovskite Solar Cells[END_REF] However, electrode corrosion caused by the acidity of PEDOT:PSS causes reduced operational stability, and the interest in PTAA for use in scalable PSCs is limited due to its high cost. Nickel oxide (NiOx) has recently sparked a significant interest as a potential candidate for inorganic HTLs. NiOx possesses an appropriate bandgap (around 3.5 eV) as well as a deeplying valence band maximum (VBM: 5.0-5.2eV), [START_REF] Wang | P-Type Mesoscopic Nickel Oxide/Organometallic Perovskite Heterojunction Solar Cells[END_REF][START_REF] Chan | Enhanced Hole Injections in Organic Light-Emitting Devices by Depositing Nickel Oxide on Indium Tin Oxide Anode[END_REF] both beneficial for a suitable band alignment with lead halide perovskites. Moreover, the inexpensive and facile preparation processes of NiOx-based HTLs enable a straightforward scale-up for industrial production. Several approaches for preparing NiOx HTLs have been reported, such as solution-based processes, [START_REF] Manders | Solution-Processed Nickel Oxide Hole Transport Layers in High Effi Ciency Polymer Photovoltaic Cells[END_REF] pulsed laser deposition, [START_REF] Park | Efficient CH3NH3PbI3 Perovskite Solar Cells Employing Nanostructured P-Type NiO Electrode Formed by a Pulsed Laser Deposition[END_REF] and the sol-gel technique. [START_REF] Kim | High-Performance and Environmentally Stable Planar Heterojunction Perovskite Solar Cells Based on a Solution-Processed Copper-Doped Nickel Oxide Hole-Transporting Layer[END_REF] In parallel, different types of dopants have been explored to improve the conductivity of NiOx, such as Cu, Mg and Li. [START_REF] Li | Overcoming the Limitations of Sputtered Nickel Oxide for High-Efficiency and Large-Area Perovskite Solar Cells[END_REF] As an example, Li and coworkers developed Mg-doped NiOx HTLs of enhanced conductivity leading to a maximum PCE of 18.2% for small-area devices and 15.3% for 1 cm 2 devices. [START_REF] Ru | High Electron Affinity Enables Fast Hole Extraction for Efficient Flexible Inverted Perovskite Solar Cells[END_REF] To further improve the hole-extraction capability of the NiOx layer, Ru et al. synthesized molecular TCNQ (7,7,8,8-tetracyanoquinodimethane) derivatives of controlled electron affinity to dope the precursor solution of NiOx nanoparticles. The resulting HTLs showed a marked conductivity improvement and a decreased energy gap, yielding a promising PCE of PSCs (22.13%). [START_REF] Bai | Effects of a Molecular Monolayer Modification of NiO Nanocrystal Layer Surfaces on Perovskite Crystallization and Interface Contact toward Faster Hole Extraction and Higher Photovoltaic Performance[END_REF] Although the PCEs of PSCs based on NiOx HTLs have been largely improved, there is still a gap when compared with the performance of solar cells in n-i-p configuration, with the major limitations being the fill factor (FF) and short-circuit current density (Jsc). The lower crystalline quality of the perovskite grown on NiOx and the poor contact, caused by surface defects of NiOx, have been identified as the main limiting factors. [START_REF] Xu | Inverted Perovskite Solar Cells Employing Doped NiO Hole Transport Layers: A Review[END_REF] Surface modification is an effective way to passivate the NiOx layer, improving the interfacial contact and adjusting the energy levels, which can lead to a better hole extraction and thereby enhance the FF and Jsc. [START_REF] Li | Overcoming the Limitations of Sputtered Nickel Oxide for High-Efficiency and Large-Area Perovskite Solar Cells[END_REF] In contrast to the large choice of passivation layers reported for n-i-p PSCs, much fewer works concern NiOx in p-i-n configuration. As typical examples diethanolamine (DEA) and ferrocenedicarboxylic acid (FDA) have been proposed, interacting via the amine or the carboxylic acid function with the NiOx surface, respectively. [START_REF] Xu | Inverted Perovskite Solar Cells Employing Doped NiO Hole Transport Layers: A Review[END_REF][START_REF] Zhang | Efficient and Ultraviolet Durable Planar Perovskite Solar Cells: Via a Ferrocenecarboxylic Acid Modified Nickel Oxide Hole Transport Layer[END_REF] In another example, by bridging the NiOx nanocrystalline film with the perovskite layer via a self-assembled monolayer consisting of two types of molecules comprising carbazole and phosphonic acid groups, interfacial recombination was mitigated and hole extraction improved, resulting in an efficiency of 24.7% for flexible all-perovskite tandem solar cells. [START_REF] Li | Flexible all-perovskite tandem solar cells approaching 25% efficiency with molecule-bridged hole-selective contact[END_REF] Further examples of NiOX functionalization have been summarized in recent reviews. [START_REF] Huang | Recent Advances in Nanostructured Inorganic Hole-Transporting Materials for Perovskite Solar Cells[END_REF][START_REF] Zhu | Progress of solution-processed metal oxides as charge transport layers towards efficient and stable perovskite solar cells and modules[END_REF] In this work, we explore 4-dimethylaminopyridine (DMAP), a bifunctional molecule assuring strong interaction with both the HTL and the perovskite layer, as the passivation agent for sol-gel processed NiOx HTLs. To enhance the conductivity of NiOx, 5 mol% of Cu dopant is used, [START_REF] Zhao | Optical and Electrochemical Properties of Cu-Doped NiO Films Prepared by Electrochemical Deposition[END_REF] and the resulting HTL is therefore referred to as Cu:NiOx. DMAP preferentially binds to undercoordinated Ni cations at the HTL surface via its pyridine nitrogen, due to its high nucleophilicity. [START_REF] Barlow | Electrochemical Evaluation of 4-(Dimethylamino)Pyridine Adsorption on Polycrystalline Gold[END_REF] Upon deposition of the perovskite layer on DMAP-passivated Cu:NiOx, the remaining dimethylamino moiety can efficiently interact with lead ions. Therefore, DMAP can passivate defects at the HTL-perovskite interface and act at the same time as a nucleation center for the perovskite layer. Our results demonstrate that the treatment of the HTL with DMAP not only improves the perovskite crystalline quality but also leads to a more favorable energy level alignment at the HTL/perovskite interface, facilitating hole extraction. As a consequence, planar p-i-n PSCs with DMAP-passivated NiOx present significantly improved performances compared to identical devices without DMAP, reaching FF and PCE values of almost 80% and 18%, respectively, and showing strongly reduced hysteresis. Moreover, enhanced environmental stability of the solar cells is observed when exposing them to ambient air with 50% humidity.

Results and Discussion

In the DMAP molecule, electron donation from the dimethylamino group into the aromatic ring and resonance stabilization lead to high basicity of the pyridine nitrogen. Therefore, as illustrated in Fig. 1A, DMAP is expected to coordinate with the NiOx film surface via its pyridine nitrogen. The dimethylamino group, on the other hand, is available for coordinating Pb ions of the perovskite layer via the electron lone pair of its nitrogen atom. To probe this interaction, DMAP and PbI2 were mixed and annealed at 150°C for 5 min. A new compound formed, as indicated by the color change from yellow (PbI2) to white (cf. Fig. S1). Fig. 1B presents the Fourier-transform infrared (FTIR) spectra of DMAP and the new complex. Because of the coordination of the dimethylamino group to the Pb center, the spectrum of DMAP-PbI2 shows a clear change in the signals related to the symmetric and asymmetric stretching vibrations of the C-H bonds of the methyl groups in the range of 3000-2800 cm -1 (indicated by the dashed line). Furthermore, the amine C-N bond stretching vibration (ν(C-N)) shifts from 1373 cm -1 (DMAP) to 1381 cm -1 (DMAP-PbI2). [START_REF] Sundaraganesan | FT-Raman Spectra and Ab Initio HF and DFT Calculations of 4-N,N′-Dimethylamino Pyridine[END_REF] To further check the chemical state of the N atom in the complex, X-ray photoelectron spectroscopy (XPS) was performed (Fig. S2). XPS cannot distinguish between the pyridine and amine nitrogen atoms in DMAP. Nonetheless, the N1s spectrum displays a shift to lower energy (-0.16 eV) after coordination with PbI2 signaling a weak reduction of the nitrogen atom due to the interaction between PbI2 and DMAP.

In the one-step perovskite deposition process from solutions comprising methylammonium iodide (MAI) and PbI2, the perovskite precursor solution contains iodoplumbate intermediates (PbIn 2-n , n = 2-6) and lead-solvent complexes. [START_REF] Hamill | Influence of Solvent Coordination on Hybrid Organic-Inorganic Perovskite Formation[END_REF][START_REF] Rahimnejad | Coordination Chemistry Dictates the Structural Defects in Lead Halide Perovskites[END_REF] The presence and concentration of these compounds subsequently determine the defect density and morphology of the fully formed perovskite active layer. [START_REF] Stewart | Molecular Origins of Defects in Organohalide Perovskites and Their Influence on Charge Carrier Dynamics[END_REF] We expect that DMAP favors the assembly of iodoplumbates on the NiOx surface, leading to nucleation centers for the perovskite layer. The interaction between the DMAP passivation layer and the Cu:NiOx HTL was also probed by XPS measurements. On the survey spectrum (Fig. S3), the appearance of the N 1s peak at ca. 400 eV after the passivation unambiguously demonstrates the presence of DMAP on the Cu:NiOx surface. High-resolution Ni 2p spectra recorded for both passivated and pristine NiOx films have a broad multipeak nature due to the complex shake-up losses typical for this transition metal, shown in Fig. 1D and Fig. S4 a/b. The spectra of both the DMAP-passivated and the non-passivated HTL can be fitted using the same set of peaks without any contributions from Ni 0 and Ni 3+ . [START_REF] Biesinger | X-Ray Photoelectron Spectroscopic Chemical State Quantification of Mixed Nickel Metal , Oxide and Hydroxide Systems[END_REF] The main Ni 2p3/2 component is situated at 853.55 eV for NiOx and 853.43 eV for NiOx /DMAP. As discussed before, the nucleophilic pyridine nitrogen present in DMAP coordinates to Ni 2+ [START_REF] Hercules | Electron Spectroscopy for Chemical Analysis[END_REF][START_REF] Grim | X-Ray Photoelectron Spectroscopy of Some Nickel Dithiolate Complexes[END_REF] donating electrons and thus inducing the observed 0.12 eV shift of the Ni 2p3/2 peak to lower binding energies for the passivated sample. This electron-donating character of DMAP [START_REF] Garnier | Structure and Reactivity in Neutral Organic Electron Donors Derived from 4-Dimethylaminopyridine[END_REF] should also lower the work function (WF) of NiOx. [START_REF] Lindell | Characterization of the Interface Dipole at the Paraphenylenediamine-Nickel Interface: A Joint Theoretical and Experimental Study[END_REF] Concomitantly, the decrease of free carrier (hole) concentration can lead to a reduction of the conductivity of the passivated NiOx layer. [START_REF] Qiu | Enhanced Physical Properties of Pulsed Laser Deposited NiO Films via Annealing and Lithium Doping for Improving Perovskite Solar Cell Efficiency[END_REF] To probe the effects of DMAP passivation on the energy level positions of the HTL, ultraviolet photoelectron spectroscopy (UPS) was performed (cf. Fig. 1E-F; full spectra: Fig. S5). After DMAP treatment, it can be seen that the WF shifts from -4.71 [START_REF] Lee | Cu-Doped NiOx as an Effective Hole-Selective Layer for a High-Performance Sb2Se3 Photocathode for Photoelectrochemical Water Splitting[END_REF] to -4.51 eV. The changed WF of DMAP modified Cu:NiOx results in a larger built-in potential Vbi and a wider depletion region. This built-in field can improve the charge separation, charge transport, and the collection of photogenerated carriers, as well as suppress the back transfer of holes from the HTL to the perovskite layer. [START_REF] Yang | Interface Engineering in Planar Perovskite Solar Cells: Energy Level Alignment, Perovskite Morphology Control and High Performance Achievement[END_REF] Besides the shift of WF, the valence band maximum (VBM) of the HTL with DMAP passivation is lowered from -5.0 eV [START_REF] Lee | Cu-Doped NiOx as an Effective Hole-Selective Layer for a High-Performance Sb2Se3 Photocathode for Photoelectrochemical Water Splitting[END_REF] to -5.26 eV (cf. After deposition of the CsFAMA triple-cation perovskite, the effect of the presence of DMAP on the crystalline quality of this layer was studied with X-ray diffraction. Figure 2A displays the diffractograms obtained for perovskite films grown on bare and on DMAP-passivated Cu:NiOx HTLs.

In both cases, the peak positions are coherent with the diffraction pattern expected for the cubic -phase (space group Pm3 ̅ m) of the triple-cation perovskite with low methylammonium content, adopting essentially the crystal structure of FAPbI3. [START_REF] Zheng | Manipulation of Facet Orientation in Hybrid Perovskite Polycrystalline Films by Cation Cascade[END_REF] However, a slight shift of the diffraction peaks to larger angles is observed in the case of the DMAP-passivated sample (Fig. 2B), corresponding to a decrease of the lattice parameter from 6.276 ± 0.005 Å to 6.260 ± 0.005 Å. A peak at 44.1° associated with the polycrystalline FTO substrate (ICDD PDF4 #04-008-8130) is visible in both cases, indicating that the X-rays penetrated the entire perovskite layer. Moreover, a peak is present at 14.69° corresponding to the (001) Bragg reflection of hexagonal PbI2 (ICDD PDF4 #01-079-0803). The relative intensities of the peaks are similar for the two perovskite patterns. Here the ratio between the perovskite phase (022) and (002) peak intensities is taken as an example. For both the perovskite layer grown on the bare Cu:NiOx HTL and the DMAP-passivated HTL the ratio is close to 0.06. Therefore, it can be concluded that the use of DMAP does not change the preferential orientation within the perovskite layer. [START_REF] Bouchard | Direct Evidence of Chlorine-Induced Preferential Crystalline Orientation in Methylammonium Lead Iodide Perovskites Grown on TiO2[END_REF] For a more detailed comparison, the perovskite film microstructure has to be considered, as it can directly affect the performance of the PSC. [START_REF] Zou | Reduced Defects of MAPbI 3 Thin Films Treated by FAI for High-Performance Planar Perovskite Solar Cells[END_REF][START_REF] Medjahed | Unraveling the Formation Mechanism and Ferroelastic Behavior of MAPbI3 Perovskite Thin Films Prepared in the Presence of Chloride[END_REF] In the present case, it mainly concerns the perpendicular crystallite size and strain (distortion). The configuration used for the XRD measurements was optimized to favor a high diffracted intensity. The counterpart is a non-negligible contribution of the instrumental resolution to the broadening of the Bragg peaks. While a quantitative determination of this contribution is difficult to achieve in a reliable way, qualitative information can be obtained when analyzing the evolution of the full width at half maximum (FWHM) of the peaks as a function of the scattering angle 2. From crosssectional SEM images, the perpendicular crystallite size appears to be larger than 100 nm (shown in Fig. S7). Therefore, the peak broadening stems mainly from the presence of distortion in the perovskite lattice. This hypothesis is confirmed by the analysis of the 2 dependencies of the FWHM values of the (00l) diffraction peaks, as displayed in Fig. 2C. If the contribution of the crystallite size were the dominant factor for the observed line broadening, the FWHM values should exhibit a lower increase with 2(cf. Supplementary Note 1). As an example, 100-nm sized crystallites would induce a peak broadening of 0.10° and 0.12° at 2 = 30° and 70°, respectively. Hence, for both types of perovskite layers, the peak broadening mainly arises from the presence of strain. However, there is a distinct difference in the slope of the broadening with increasing 2 which is much lower in the case of the sample containing the DMAP-passivated HTL. It can therefore be concluded that DMAP improves the crystalline quality of the perovskite layer by significantly reducing strain.

Since the thicknesses of the perovskite layers based on the two different substrates are nearly the same (around 650 nm, cf. Fig. S7), we are able to investigate whether the differences in the perovskite crystallization affect the light-harvesting properties. The comparison of the UV-vis absorption spectra (Fig. 2D) shows a slight enhancement of the light absorption in the whole spectral range when DMAP passivation was used. In addition, steady-state photoluminescence (PL) measurements were performed to study the hole extraction efficiency. While both samples exhibit an emission peak centered at 760 nm, the PL intensity of the two stacks differs markedly (Fig. 2E). At a first glance, this difference is unexpected when considering that at least 90% of the light should be absorbed in the first 200 nm of the perovskite film and thus should not be directly affected by the interface modification. [START_REF] De Wolf | Organometallic Halide Perovskites: Sharp Optical Absorption Edge and Its Relation to Photovoltaic Performance[END_REF] Moreover, the DMAP-passivated HTL leads to a perovskite layer of better crystalline quality, which should in principle lead to lower quenching due to a decreased trap density. However, the modified HTL has also an improved interface with the perovskite layer. As a consequence, the observed significant PL quenching behavior indicates that the perovskite film grown on Cu:NiOx/DMAP enables faster charge transfer kinetics and higher hole extraction ability than that grown on the non-passivated HTL. These features effectively reduce the radiative carrier recombination in the PSC active layer and increase the FF of the device.

To learn more about the underlying charge carrier recombination mechanisms, the relationship between the Voc of the PSCs and the incident light intensity was examined. If the recombination of free electrons and holes in the active layer dominates, the slope of Voc vs. light intensity is larger than kT/q, and Voc strongly depends on the incident light intensity (where q is the electron charge, k is the Boltzmann constant, and T is the Kelvin temperature). [START_REF] Bouchard | Direct Evidence of Chlorine-Induced Preferential Crystalline Orientation in Methylammonium Lead Iodide Perovskites Grown on TiO2[END_REF] As visible in Fig. 2F, the slope of the stack without the DMAP passivation layer is 2.1 kT/q, whereas that of the stack containing the DMAP layer is significantly lower (1.19 kT/q). This result provides strong evidence that the interfacial barrier for hole extraction is considerably reduced when the DMAP passivation of the HTL is applied. These findings further support the assumption that carrier recombination is largely suppressed by the passivation layer, which can be attributed to a combination of factors including reduced perovskite crystal strain/better perovskite crystalline quality, as well as improved hole extraction due to the modified HTL/perovskite interface. Top-view scanning electron microscopy (SEM) images (Figure 3A) demonstrate that without DMAP passivation a few holes as well as a large number of small dots are visible on the perovskite surface. According to the literature, [START_REF] Jiang | Planar-Structure Perovskite Solar Cells with Efficiency beyond 21%[END_REF] the latter can be attributed to excess PbI2, which appears brighter than the CsFAMA perovskite in the dark-field images due to its higher electron density. In presence of the DMAP passivation layer, the amount of excess PbI2 on the surface and the number of pinholes reduce significantly (Fig. 3C). The surface roughness of the perovskite layers grown on the two types of substrates was investigated with AFM, as illustrated in Figure 3B/D. The lateral sizes of the crystallites are around 400 nm on both types of HTL, but perovskite films grown on bare Cu:NiOx exhibit a higher roughness of 17.31.6 nm RMS, compared to those on NiOx/DMAP (12.40.8 nm RMS). Concluding, SEM and AFM analyses demonstrate that the DMAP passivation layer not only induces a favorable perovskite morphology, i.e., a dense film of low surface roughness, but also results in the reduction of the amount of excess PbI2 on the perovskite layer surface. Due to the high-lying conduction band edge of PbI2 (3.4eV), [START_REF] Cho | Highly Efficient Perovskite Solar Cells with a Compositionally Engineered Perovskite/Hole Transporting Material Interface[END_REF] its large excess at the perovskite surface generates an energy barrier for charge extraction [START_REF] Bi | Unraveling the Effect of PbI2 Concentration on Charge Recombination Kinetics in Perovskite Solar Cells[END_REF] and decreases the device stability under illumination. [START_REF] Schoonman | Organic-Inorganic Lead Halide Perovskite Solar Cell Materials: A Possible Stability Problem[END_REF] Accordingly, it has been reported that only a small amount of unreacted PbI2 near the perovskite surface is desirable for decreasing recombination processes at the perovskite/charge transport layer interface. [START_REF] Liu | Is Excess PbI2 Beneficial for Perovskite Solar Cell Performance?[END_REF][START_REF] Merdasa | Impact of Excess Lead Iodide on the Recombination Kinetics in Metal Halide Perovskites[END_REF] To gain further insight how DMAP affects the PbI2 distribution within the perovskite layer, grazing incidence X-ray diffraction (GIXRD) was performed. Changing the incident angle ω of the X-ray beam allows for the analysis of the perovskite phase with different penetration depths in the layer. The probed depth h can be determined using the following formula, where D is the penetration depth calculated from the mass absorption coefficient and the density of the material (cf. perovskite contributions with the incident angle show distinct differences. To quantify this evolution, the ratio of the areas of the PbI2 (001) and -phase CsFAMA (001) peaks was calculated and its evolution as a function of the probed depth is depicted in Figure 4C. It should be noted that the values at 650 nm (entire film thickness) are from the standard θ-2θ WAXS measurement, which accounts for the complete amount of the PbI2 and perovskite phases in the layer. At this point, the ratio of the peak areas is very similar for the samples with and without DMAP passivation showing that the total amounts of PbI2 and perovskite in the active layer are essentially the same. Also, for both samples the amount of PbI2 is maximum in the upper part of the layer, demonstrating that the growth of the perovskite phase is bottomup, with at least part of the excess lead iodide remaining near the surface. However, the higher ratio value for the non-passivated sample clearly indicates that the amount of untransformed PbI2 close to the perovskite surface is significantly higher, which was also observed in SEM (Fig. 3A). This surfacesensitive XRD technique thus proves that the DMAP passivation layer influences the distribution of PbI2 throughout the active layer, and promotes the formation of the perovskite phase near the surface. In contrast, as illustrated in Figure 4D, for samples without DMAP passivation excess PbI2 precipitates from perovskite crystals and enriches the surface of the perovskite layer in the process of annealing.

After the introduction of the passivation layer, the coordination of DMAP molecules with PbI2 is supposed to slow down the conversion of PbI2 to the perovskite at the initial deposition process. Indeed, the nature of the first perovskite plane at the interface with the substrate has been shown to govern the texture and strain of the active layer. [START_REF] Medjahed | Microstructure of Methylammonium Lead Iodide Perovskite Thin Films: A Comprehensive Study of the Strain and Texture[END_REF] A relatively stable, ultrathin PbI2 layer can ensure the uniform growth of the perovskite layer in the following annealing step and at the same time reduce the enrichment of PbI2 on the surface of the active layer, which has been shown to decrease the probability of charge recombination at the interface between the perovskite and top charge transport layer (ETL). [START_REF] Wolff | Nonradiative Recombination in Perovskite Solar Cells: The Role of Interfaces[END_REF] Based on the above analyses, the DMAP-modified Cu:NiOx layer is expected to be beneficial for enhancing the solar cell device performance, due to a better interfacial contact and improved crystalline quality of the perovskite layer. Optimization of the thicknesses of the different layers in our devices using the stack FTO/ Cu:NiOx/DMAP/CsFAMA/PCBM/BCP/Ag resulted in 40~60 nm for the HTL, around 650 nm for the CsFAMA perovskite, 70 nm for [60]PCBM, 4 nm for BCP and 200 nm for the Ag counter-electrode. The J-V curves of the best performing devices are shown in Figure 5A and their J-V parameters are summarized in Table 1. For devices with the passivation layer, the PCE reached values up to 17.90%, i.e., more than 2% higher than the best devices without DMAP (15.73% PCE).

Figures 5B/C

show the statistical distribution of the PCE and FF measured on 34 devices (comparison of Voc and Jsc cf. Figure S9). Figure 5C demonstrates that the devices based on the DMAP-modified Cu:NiOx layer systematically present a higher FF, which is the principal parameter improved by the passivation even though statistically a small increase of the Jsc is also observed. For the cells based on bare Cu:NiOx layers, the average FF lies around 0.63, whereas for devices with the DMAP/ Cu:NiOx HTL it accounts for 0.75 (highest value: 0.78). These results confirm the advantages of the DMAP passivation discussed before, namely enhanced hole extraction, improved perovskite crystalline quality, and slightly enhanced light-harvesting of the devices. Furthermore, Figure 5D shows that the J-V hysteresis almost disappeared for the cells containing DMAP, with the hysteresis index decreasing from around 6% to 0.9%. The smaller hysteresis is a further indicator of the better charge extraction induced by the more favorable energy level alignment and less nonradiative recombination at the interface between the perovskite and the HTL. Finally, we analyzed the stability of both types of devices under ambient air (50% humidity, 25°C) in the dark, which can give a first indication of the environmental robustness of the obtained solar cells.

Voc (V) Jsc (mA/
As visible in Figure 5E, the DMAP-modified device maintained 87% of its initial PCE after 72h storage, against 77% for the cell without DMAP modification. This difference stems essentially from the FF (cf. Fig. 5F), which is strongly decreasing with storage time under ambient air when utilizing bare Cu:NiOx HTLs. We hypothesize that the higher crystalline quality and lower strain of the perovskite layer formed on the passivation layer accounts for the observed enhanced stability. Furthermore, as demonstrated above, the presence of the DMAP passivation layer alters the distribution of lead iodide inside the perovskite crystal, resulting in fewer pinholes and defects, which directly affects the FF. Over time, these defects may extend in the cells without DMAP, negatively impacting their performance. [START_REF] Sherkar | Recombination in Perovskite Solar Cells: Significance of Grain Boundaries, Interface Traps, and Defect Ions[END_REF] Yet another explanation could be related to the removal of impurity ions at the NiOX surface due to the DMAP passivation, an effect which has been shown to improve the performance of PSCs when treating the HTL with ionic liquids. [START_REF] Wang | Critical Role of Removing Impurities in Nickel Oxide on High-Efficiency and Long-Term Stability of Inverted Perovskite Solar Cells[END_REF] We underline that the presented stability test aims at the comparison of the behavior of perovskite solar cells fabricated under identical conditions with the only difference being the DMAP passivation layer. In a more applied perspective, the operational stability of the PSCs could be further strongly enhanced by employing appropriate encapsulation techniques.

Conclusions

We demonstrated the beneficial effects of the bifunctional molecule DMAP, used to passivate Furthermore, the cells with DMAP passivation exhibited strongly reduced hysteresis and improved stability in a high humidity environment. This study contributes to the emerging field of interface engineering of perovskite solar cells by correlating the manifold changes induced by the functionalization of the Cu:NiOx HTL with the device behavior. In particular, our results demonstrate both the opportunities and challenges related to the functionalization of the substrate used for the growth of the perovskite layer. The quality of the latter is strongly dependent on the precise preparation conditions but the underlying substrate also plays an important role. [START_REF] Dunlap-Shohl | Synthetic Approaches for Halide Perovskite Thin Films[END_REF] When novel molecules for the functionalization of the charge transport layers are used, their simultaneous influence on the perovskite nucleation and growth and the interfacial electronic properties are generally difficult to predict, and only very few examples exhibit a synergistic beneficial behavior. Based on the results obtained on Cu:NiOx HTLs, we expect that DMAP can also be successfully used for the functionalization of other interfaces within perovskite solar cells, in particular those involving metal oxide charge transport layers.

Furthermore, the proposed approach could be combined with the latest advances in p-i-n PSCs achieved through the engineering of the perovskite/ETL interface via functionalization with organometallic compounds, which led to record efficiency and stability. [START_REF] Li | Organometallic-Functionalized Interfaces for Highly Efficient Inverted Perovskite Solar Cells[END_REF] 4. Experimental section ). The mole ratio of metal ions: MEA was maintained at 1:1 in solution.

The dissolution took place while stirring in a sealed glass vial under air at 40° C for 1 h. The solution appeared homogeneous and deep green after approximately 40 min.

Device Fabrication:

FTO-coated glass substrates were sequentially cleaned in acetone, detergent solution, deionized water, and isopropyl alcohol baths for 30 min each. Then, the substrates were exposed to UV-ozone for 45 min to remove residual organic pollutants. The Cu:NiOx precursor was deposited by spin-casting onto the FTO substrates (70 μL solution, acceleration 3000 rpm/s, speed 3000 rpm, duration 30 s). Unless otherwise stated, the Cu:NiOx precursor films were heated to 275 °C for 45 min in air, then the temperature was raised to 500°C for 60 min in air. For surface modification, a DMAP solution in IPA with the concentration of 20 mg/mL, was spin-coated (70 μL solution using the same spinning parameters) onto the as-prepared Cu:NiOx films. Then the substrates were annealed on a hotplate at 100°C for 5 min inside an Ar-filled glovebox.

For the deposition of the perovskite films, we followed the procedure reported in our previous work. [START_REF] Ye | Doped Bilayer Tin(IV) Oxide Electron Transport Layer for High Open-Circuit Voltage Planar Perovskite Solar Cells with Reduced Hysteresis[END_REF] Briefly, a precursor solution was prepared by mixing PbI2 AFM images were acquired with a Bruker Dimension Icon atomic force microscope. Photoluminescence spectra were recorded using a FLUOROLOG-3 spectrophotometer (Jobin-Yvon). The UPS measurements for all samples were performed on an M-XPS spectrometer from Omicron Nanotechnology equipped with a monochromated Al Kα source (hν=1486.6 eV) and an ultraviolet He lamp (He I=21.2 eV). Photoelectrons were collected under normal geometry by a hemispherical analyzer,

  Fig. S6), which is beneficial for hole extraction. The energy level scheme of the perovskite solar cell is shown in Figure S6. Summarizing, DMAP has not only the role of an anchoring layer for the perovskite on the Cu:NiOx HTL but also induces a more favorable energy level alignment at the HTL-perovskite interface, which is expected to improve hole extraction.

Figure 1 :

 1 Figure 1: A) Perovskite solar cell architecture used in this work; inset: schematic view of the interaction of DMAP with the HTL and the perovskite. B) FTIR spectra of DMAP and a DMAP-PbI2 mixture heated to 150 °C for 5 min. C) Enlarged view of the FTIR spectra. D) High-resolution XPS spectra of Ni 2p. E/F) UPS spectra of the Cu:NiOx HTL with DMAP passivation layer.

Figure 2 :

 2 Figure 2: Comparison of the stacks FTO/Cu:NiOx/DMAP/CsFAMA and FTO/Cu:NiOx/CsFAMA. A/B) XRD patterns: A) overview, B) enlarged view highlighting the shift of the diffraction peaks to larger angles in the DMAP-containing stack. C) Linewidth (FWHM) of the (00l) perovskite diffraction peaks as a function of 2. D) UV-vis absorption spectra. E) Steady-state PL spectra (excited at 400 nm from the perovskite side). F) Device VOC vs. incident light intensity.

Figure 3 :

 3 Figure 3: A/C) Top-view SEM and AFM images of the CsFAMA perovskite layer grown on the bare Cu:NiOx HTL, excess PbI2 and pinholes are marked with red circles. B/D) Top-view SEM and AFM images of the perovskite grown on DMAP-passivated Cu:NiOx.

Fig

  scattering angle 2θ were performed at different fixed incident angles  whose choice determines the depth probed in the sample. Figure4Bpresents the data obtained for  values varying from 0.3° to 1.0°, which correspond to probed thicknesses ranging between approximately 40 nm and 200 nm (the full XRD patterns obtained with the different incident angles are shown in FigureS8). For all depths probed, the diffraction peaks characteristic of the cubic perovskite phase are present as well as the (001) Bragg peak of PbI2, while the contribution of FTO is missing as expected. Importantly, for both types of samples (with/without DMAP), the variation of the relative intensities of the PbI2 and

Figure 4 :

 4 Figure 4: A) Scheme representing the principle of the GIXRD measurements used to probe different depths h of the perovskite layer by varying the incident angle ω (D: penetration depth, PVK: perovskite). B) XRD patterns obtained for different incident angles. C) Relative intensities of the PbI2 and -phase CsFAMA (001) peaks of the samples with/without DMAP passivation of the Cu:NiOx HTL. D) Schematic illustration of the DMAP-induced differences in the perovskite layer composition near the HTL and near the surface.

Figure 5 :

 5 Figure 5: Comparison of the solar cell performances and stability of devices with or without DMAP passivation of the Cu:NiOx HTL. A) J-V curves of the best performing solar cells. B/C) Statistical distribution of the PCE and FF values obtained from 34 devices of each type. D) J-V curves obtained by reverse and forward scans. E/F) Relative evolution of PCE and FF with the storage time of the nonencapsulated solar cells in the ambient atmosphere (50% humidity).

  Cu:NiOx HTLs, on the performance and stability of planar p-i-n perovskite solar cells. DMAP enables the cross-linking between the Cu:NiOx HTL and the triple-cation (CsFAMA) perovskite via the coordination of the pyridine nitrogen on the metal sites of the Cu:NiOx HTL and interaction of the remaining dimethylamino function with Pb ions. The use of DMAP results in an improved perovskite crystalline quality with reduced strain, a better perovskite film morphology without pinholes, and a more favorable energy level alignment for hole extraction. These improvements combined with a modified PbI2 distribution throughout the perovskite layer enhance the charge carrier extraction and transport and reduce undesired charge recombination processes at the interfaces, which in turn results in better solar cell performances. The main factor of improvement was the fill factor, which led to an average increase of the PCE by more than 2.2% compared to devices without DMAP up to values close to 18%.

( 1 .

 1 45 M), FAI (1.15 M), PbBr2 (0.21 M), MABr (0.21 M), and MACl (0.45 M) in 1 mL of a mixed solvent of DMF/DMSO (4:1 v/v). Then, 17.5 μL of CsI solution (2 M in DMSO) was added to form the triple cation (Cs/MA/FA) perovskite. Next, the obtained solution was deposited onto the HTL substrates by two consecutive spin-coating steps, at 1000 and 4000 rpm for 10 and 25 s, respectively. During the second spin-coating step, 150 μL of chlorobenzene was dropped onto the substrate after 15 s. Then, the substrate was immediately moved to a hotplate and annealed at 100°C for 50 min. After cooling down to room temperature, 60 μL of a 20×10 -3 M [60]PCBM solution in CB was spin-coated onto the perovskite surface at a spinning rate of 1500 rpm for 45 s followed by spin-coating of a BCP solution (1mg/mL in IPA solution, 4000 rpm, 30 s). To complete the device, 200 nm thick Ag electrodes were deposited on top by thermal evaporation.The electrode area is 0.909 cm[START_REF] Adhyaksa | Carrier Diffusion Lengths in Hybrid Perovskites: Processing, Composition, Aging, and Surface Passivation Effects[END_REF] 4.4. Characterization:A Zeiss Ultra 55 high-resolution scanning electron microscope was used to examine the morphology of the films and acquire cross-sectional images of the entire device. A Panalytical Empyrean diffractometer with a cobalt anode (Co K1 = 1.7890, Co K2 = 1.7929), a Göbel mirror, and a 1D Pixcel detector were used to perform XRD analyses. The UV-vis absorption spectra were measured using a Shimadzu UV-1800 spectrometer. Current-voltage measurements of the devices were performed with a Keithley 2400 Source meter under AM 1.5G illumination with a xenon-lamp-based calibrated solar simulator (Newport, AAA) with 100 mW/cm² intensity unless stated otherwise. The cells were measured with a mask (0.09 cm 2 ) and the scan rate was 20 mV•s -1 for both forward and reverse scans.

cm 2 )

				Average	Hysteresis
			FF (%)	PCE (%)
				PCE (%)	index (%)
	w/o DMAP
	1.13	20.80	68.93	16.20
	descending
				15.73	6.04
	w/o DMAP
	1.10	20.55	67.46	15.25
	ascending
	w DMAP
	1.10	20.94	78.46	17.98
	descending
				17.90	0.90
	w DMAP
	1.09	20.91	78.22	17.82
	ascending

Table 1 :

 1 Device parameters and hysteresis of the best performing PSCs using DMAP-passivated and non-passivated Cu:NiOx HTLs. The hysteresis index is calculated as follows:

	𝐻𝑦𝑠𝑡𝑒𝑟𝑒𝑠𝑖𝑠 𝑖𝑛𝑑𝑒𝑥 =	𝑃𝐶𝐸 𝑑𝑒𝑠𝑐𝑒𝑛𝑑𝑖𝑛𝑔 -𝑃𝐶𝐸 𝑎𝑠𝑐𝑒𝑛𝑑𝑖𝑛𝑔 𝑃𝐶𝐸 𝑎𝑣𝑒𝑟𝑎𝑔𝑒	* 100%

with an angular acceptance of 8°. XPS analyses were carried out with a Versa Probe II spectrometer (ULVAC-PHI) equipped with a monochromated Al Ka source (h = 1486.6 eV). The core-level peaks were recorded with a constant pass energy of 23.3 eV. The XPS spectra were fitted with CasaXPS software using Shirley background and a combination of Gaussian (70%) and Lorentzian (30%) distributions. Binding energies were referenced with respect to adventitious carbon (C 1s BE = 284.8 eV).

Acknowledgment

The authors acknowledge the French Research Agency ANR for financial support (Grant PERSIL ANR-16-CE05-0019-02), and the LABEX Lanef (ANR-10-LABX-51-01) for its support with mutualized instrumentation. Part of this work was realized on the SyMMES' Hybrid-En facility, and Etienne Omnès is thanked for experimental support. The PhD thesis of Yuze Li is funded by UGA IDEX (project IRS C-Super), which is gratefully acknowledged. # Present address: Key Laboratory of Photovoltaic and Energy Conservation Materials, Institute of Solid-State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China

Supporting Information

XPS and UPS spectra and data, energy level scheme and cross-sectional SEM images of the devices, full-range GIXRD patterns, statistical distribution of the solar cell parameters, photographs of DMAP and PbI2 powders and their mixture after heating to 150°C.


Julie Ripoll

Fati Chen

Céline Mandier

Eric Rivals

Joint transcriptome and translatome analysis: a reproducible pipeline

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Joint transcriptome and translatome analysis: a reproducible pipeline

Julie RIPOLL 1 , Fati CHEN 2 , Céline MANDIER 1 and Eric RIVALS RNA sequencing (RNA-seq) is often used to elucidate the regulation of gene expression, as it provides an in-depth view of transcribed RNAs and a relative quantification of each gene or transcript. However, several studies have found that variations in RNA transcript levels do not necessarily correlate well with protein levels [START_REF] Battle | Impact of regulatory variation from rna to protein[END_REF], suggesting that translation also plays a key role. To study the role of translation in regulating gene expression, the translatome can be studied by selecting ribosome-bound RNAs, releasing ribosomes, and then sequencing the selected population of RNAs as in RNA-seq. This approach, called polysome sequencing (POL-seq), is a census assay that provides relative expression levels of genes/transcripts during translation. Using the workflow manager Snakemake [START_REF] Köster | Snakemake--a scalable bioinformatics workflow engine[END_REF] and Conda [START_REF]Anaconda Documentation[END_REF] environments, we developed a bioinformatics pipeline to jointly analyze these transcriptome and translatome fractions.

To promote reusability of individual steps, we propose a lightweight wrapper system to facilitate interoperability on different hardware without requiring an Internet connection. Moreover, it preserves the command inspection in Snakemake's dry-run mode. This pipeline is divided into two parts for primary and secondary analysis. The primary analysis part consists of 4 steps: i) quality control, ii) cleaning of the sequencing reads, iii) mapping of the reads to the reference genome, and iv) counting of the mapped reads per gene. The secondary analysis part encapsulates all statistical analyses to estimate the differential expression of genes and then perform functional enrichment analyses (i.e. gene ontology and pathway databases).

For the statistical part, all samples are normalized together to avoid any comparison problems between fractions. We then combine the comparisons of each fraction, visualized by a log-log plot, and filter the data according to their transcriptional and translational expression levels. This allows us to classify differentially expressed mRNAs into eight categories. These categories represent mRNAs that are regulated by transcription only, by translation only, or by both transcription and translation. In the case of conjoint regulations, we determine whether these regulations have combined or opposing effects.

We will illustrate this pipeline with results from a publication with our collaborators [START_REF] Gabriel Therizols | Alteration of ribosome function upon 5-fluorouracil treatment favors cancer cell drugtolerance[END_REF].

Acknowledgements

This project was supported by the Ligue Contre le Cancer and by the Institut National du Cancer (INCa, FluoRib grant -PLBIO18-131). The development of pipeline for the community is supported by ANR-11-INBS-0013236.