Chan Myae

Myae Soe

Constantinos C Stoumpos

Mikaël Kepenekian

Boubacar Traoré

Hsinhan Tsai

Wanyi Nie

Binghao Wang

Claudine Katan

Ram Seshadri

Aditya D Mohite

# Mika݁ሷ

Kepenekian

Jacky Even

Tobin J Marks

Mercouri G Kanatzidis
email: m-kanatzidis@northwestern.edu

New Type of 2D Perovskites with Alternating Cations in the Interlayer Space

Keywords: Layered compounds, perovskite photovoltaics, solvent-engineering, hot-casting, crystallinity

INTRODUCTION

Two-dimensional (2D) layered halide perovskites are highly promising candidates for optoelectronic applications, and this has sparked new investigations of these materials from the synthetic, physicochemical and applications point of view. [START_REF] Stoumpos | High Members of the 2D Ruddlesden-Popper Halide Perovskites: Synthesis, Optical Properties, and Solar Cells of (CH 3 (CH 2 )[END_REF][2][START_REF] Blancon | Extremely efficient internal exciton dissociation through edge states in layered 2D perovskites[END_REF][START_REF] Cao | 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications[END_REF][START_REF] Mao | Role of Organic Counterion in Lead-and Tin-based Two-dimensional Semiconducting Iodide Perovskites and Application in Planar Solar Cells[END_REF][START_REF] Stoumpos | Ruddlesden-Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors[END_REF][START_REF] Mao | White-light Emission and Structural Distortion in New Corrugated 2D Lead Bromide Perovskites[END_REF][START_REF] Smith | A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability[END_REF] Unlike the more extensively studied three-dimensional (3D) AMX 3 perovskites (A + = Cs, CH 3 NH 3 (MA) or HC(NH 2 ) 2 (FA))

where the possibility of a 3D stable crystal structure is limited by the size and shape of the A + cation [START_REF] Kieslich | Solid-state principles applied to organicinorganic perovskites: new tricks for an old dog[END_REF][START_REF] Stoumpos | The Renaissance of Halide Perovskites and Their Evolution as Emerging Semiconductors[END_REF] the lower-dimensional perovskite semiconductors offer a much higher degree of structural versatility and permit better control and fine-tuning of the optoelectronic properties. [START_REF] Stoumpos | Structure-Band Gap Relationships in Hexagonal Polytypes and Low-Dimensional Structures of Hybrid Tin Iodide Perovskites[END_REF] Conceptually, upon "cutting" the 3D parent compound along a specific crystallographic plane, infinite nanosheets with a slab thickness of atomic or molecular size can be obtained, which are separated by organic cations acting as spacers and in this context, they can be considered as natural quantum wells (QWs). These ultrathin negatively charged perovskite nanosheets are held together by intercalating organic cations, which balance the charge and support the structural integrity of the 2D perovskites. Depending on the cut in the 3D framework, the 2D perovskites can be subcategorized as <100>, <110>, and <111>-oriented perovskites. What effectively determines this crystallographic cut is the size and shape of the spacer cation. Cations employed for this purpose to date include, but not limited to, ammonium, [START_REF] Stoumpos | High Members of the 2D Ruddlesden-Popper Halide Perovskites: Synthesis, Optical Properties, and Solar Cells of (CH 3 (CH 2 )[END_REF][START_REF] Stoumpos | Ruddlesden-Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors[END_REF][START_REF] Papavassiliou | Structural, optical and related properties of some natural three-and lower-dimensional semiconductor systems[END_REF][START_REF] Sourisseau | Reduced Band Gap Hybrid Perovskites Resulting from Combined Hydrogen and Halogen Bonding at the Organic-Inorganic Interface[END_REF][START_REF] Billing | Inorganic-organic hybrid materials incorporating primary cyclic ammonium cations: The lead iodide series[END_REF][START_REF] Mitzi | Organic-Inorganic Perovskites Containing Trivalent Metal Halide Layers: The Templating Influence of the Organic Cation Layer[END_REF] amidinium, [START_REF] Wang | Synthesis and Characterization of [NH 2 C(I):NH 2 ] 3 MI 5 (M = Sn, Pb): Stereochemical Activity in Divalent Tin and Lead Halides Containing Single <110> Perovskite Sheets[END_REF][START_REF] Mousdis | Preparation, structure and optical properties of[END_REF] imidazolium [START_REF] Zimmermann | Crystal structures of isotypic poly[bis(benzimidazolium) [tetra-[mu]-iodido-stannate(II)]] and poly[bis(5,6difluorobenzimidazolium) [tetra-[mu]-iodido-stannate(II)]][END_REF] and phosphonium [START_REF] Apostolico | The reaction of tin(iv) iodide with phosphines: formation of new halotin anions[END_REF] ions. Among the <100> perovskites, the Ruddlesden-Popper (RP) perovskites, [START_REF] Ruddlesden | The compound Sr 3 Ti 2 O 7 and its structure[END_REF][START_REF] Ruddlesden | New compounds of the K 2 NiF 4 type[END_REF] are by far the most common structural type with less common types being the Dion-Jacobson (DJ), [START_REF] Dion | Nouvelles familles de phases M(I)M(II) 2 Nb 3 O 10 a feuillets "perovskites[END_REF][START_REF] Jacobson | Interlayer chemistry between thick transition-metal oxide layers: synthesis and intercalation reactions of K[Ca 2 Na n-3 Nb n O 3n+1 ] (3≤n≤7)[END_REF] and Aurivillius (AV) [START_REF] Aurivillius | Mixed bismuth oxides with layer lattices. The structure type of CaNb 2 Bi 2 O 9[END_REF][START_REF] Aurivillius | Mixed bismuth oxides with layer lattices[END_REF][START_REF] Aurivillius | Mixed bismuth oxides with layer lattices[END_REF] phases both of which finding paradigms only in oxide perovskite families. The less common DJ phases are chemically different from the RP phases in that the two spacer cations (M) 2 of the (M) 2 A n-1 B n O 3n+1 RP structure are replaced by a single alkali metal M' spacer in the M'A n-1 B n O 3n+1 DJ structure. The AV perovskites have a closer relationship to the RP compounds, but they are distinctively different because the individual M cations are replaced by extended cationic [(Bi 2 O 2 ) 2+ ] ∞ layers, which drastically change the physical properties of the compounds. [START_REF] Hervoches | A Variable-Temperature Powder Neutron Diffraction Study of Ferroelectric Bi4Ti3O12[END_REF][START_REF] Kendall | Recent Developments in Oxide Ion Conductors: Aurivillius Phases[END_REF] From the crystallographic viewpoint, the DJ phases are accompanied by a relative shift of the layers along the ab-plane (½ , 0 shift, KLaNb 2 O 7 -type) [START_REF] Sato | Structure determination of KLaNb 2 O 7 exhibiting ion exchange ability by X-ray powder diffraction[END_REF] or no shift at all (0, 0 shift, CsLaNb 2 O 7 -type) [START_REF] Kumada | CsLaNb 2 O 7[END_REF] in the perovskite layers, contrary to the RP phases that universally exhibit a (½, ½) shift. The (½, 0) case results in an 'eclipsed' configuration of the layers along the stacking direction. The structural chemistry of the DJ oxide perovskites has recently undergone a resurgence after the predictions that such structures can exhibit ferroelectricity [START_REF] Strayer | Emergent Noncentrosymmetry and Piezoelectricity Driven by Oxygen Octahedral Rotations in n = 2 Dion-Jacobson Phase Layer Perovskites[END_REF][START_REF] Benedek | Understanding ferroelectricity in layered perovskites: new ideas and insights from theory and experiments[END_REF] using certain material design principles. [START_REF] Benedek | Origin of Ferroelectricity in a Family of Polar Oxides: The Dion-Jacobson Phases[END_REF] Despite the abundance of DJ perovskites in oxides, [START_REF] Schaak | Perovskites by Design: A Toolbox of Solid-State Reactions[END_REF] this structural type is hitherto unknown in the hybrid halide perovskites.

Herein we report on the (C(NH 2 ) 3 )(CH 3 NH 3 ) n Pb n I 3n+1 (n = 1-3) family of 2D perovskites, new type of 2D halide perovskite which feature two different alternating cations in the interlayer space (ACI). This results is cation-ordering in the interlayer space with the two spacer cations being guanidinium (C(NH 2 ) 3 , GA) + and methylammonium (CH 3 NH 3 ), MA) + . This unique ordering leads to the formation of an unprecedented structure type which blends the chemical formula of the RP perovskites with the structural features of the DJ perovskites. The ordering of the cations leads to a (½, 0) shift of the layers reminiscent of the DJ perovskites but at the same differing in the occupation of the interlayer sites due to charge balance restrictions, thus leading to a different centering of the unit cell (Table 1). GA was chosen as a spacer since it has a structural formula reminiscent of the formamidinium (HC(NH 2 ) 2 , FA) + and iodoformamidinium (IC(NH 2 ) 2 , IFO) + cations, which stabilize the perovskite structure, either in its 3D form [START_REF] Stoumpos | Semiconducting Tin and Lead Iodide Perovskites with Organic Cations: Phase Transitions, High Mobilities, and Near-Infrared Photoluminescent Properties[END_REF] or in the <110> 2D perovskite, [START_REF] Mitzi | Conducting Layered Organic-inorganic Halides Containing -Oriented Perovskite Sheets[END_REF] respectively. Conceptually, GA is obtained by substituting the C-H hydrogen atom in FA or C-I iodine atom in IFO with an -NH 2 group. The present study expands on the series of simple 2D guanidinium compounds that adopt the <110>-oriented RP perovskite type, originally obtained by Szafrański and coworkers [START_REF] Grottel | NMR Study of Cation Motion in Guanidinium Iodoplumbates[END_REF][START_REF] Szafrański | Phase transitions in the layered structure of diguanidinium tetraiodoplumbate[END_REF][START_REF] Szafrański | Pressure-induced decoupling of the order-disorder and displacive contributions to the phase transition in diguanidinium tetrachlorostannate[END_REF] and recently revisited by the Hillebrecht's group. [START_REF] Daub | Synthesis, Crystal Structures, Optical Properties, and Phase Transitions of the Layered Guanidinium-Based Hybrid Perovskites[END_REF] The ACI perovskites reported in this work find join the very recently reported GACs n Pb n X 3n+1 compounds which adopt a similar crystal structure. [START_REF] Nazarenko | Luminescent and Photoconductive Layered Lead Halide Perovskite Compounds Comprising Mixtures of Cesium and Guanidinium Cations[END_REF] Here the ACI phase GAMA n Pb n I 3n+1 series (n = 1, 2 and 3) has been structurally characterized by means of single-crystal X-ray diffraction. The compounds exhibit lower bandgaps and slightly reduced photoluminescence (PL) intensity compared to the known Ruddlesden-Popper (RP) perovskites having butylammonium (BA) spacers. The observed trend in the bandgap within the ACI series can be captured by DFT calculations. They predict that ACI have slightly narrower (n = 1, 3) or comparable (n = 2) bandgap compared to the RP perovskites for their corresponding n-member.

We also investigated the photovoltaic performance of the highest member of the series GAMA 3 Pb 3 I 10 (n = 3), which adopts the centrosymmetric Imma space group and has the smallest optical gap in the series, E g = 1.73 eV, making it a good candidate for a light absorber material in both single-junction and tandem solar cells. Smooth, compact high-quality thin films were obtained by employing the solvent-engineering method with anti-solvent dripping at an optimized delay time. While the morphology improves drastically for the films before and after anti-solvent treatment, the crystallinity is found to remain unaffected. Planar solar cells comprising the optimized n = 3 films exhibit a promising power-conversion-efficiency (PCE) of 7.26% (average 6.69 ± 0.47%), with a high Voc of 0.974 V and an impressive fill factor (FF) of ~80%, which is the highest value reported for 2D perovskite solar cells.

EXPERIMENTAL

Materials. Lead oxide (PbO, 99%), methylamine hydrochloride (CH 3 NH 3 Cl, 99%), guanidine hydrochloride ((C(NH 2 ) 3 Cl, ≥ 98%), hydriodic acid (HI, 57 wt% in H 2 O), hypophosphorous acid (H 3 PO 2 , 50 wt% in H 2 O), N,N-dimethylformamide (DMF, anhydrous, 99.8%), dimethylsulfoxide (DMSO, anhydrous, ≥ 99.9%), and chlorobenzene (CB, anhydrous, 99.8%) were used as received from Sigma-Aldrich without further purification. Poly(3,4ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and phenyl-C61-butyric acid methyl ester (PC 61 BM, > 99%) were purchased from Clevios and American Dye Source Inc., respectively.

Materials Synthesis. ((C(NH 2 )

3 ) 2 PbI 4 . PbO (670 mg, 3 mmol) was dissolved in a mixture of 20.0 ml of aqueous HI and 1.7 ml of aqueous H 3 PO 2 by heating to boiling under constant magnetic stirring for at least 15 min. To the hot bright yellow solution was added solid C(NH 2 ) 3 Cl (573 mg, 6 mmol), and the reaction was kept under constant heating and stirring for a few hours until ~25% of the acid solution has evaporated. The stirring was then discontinued, and the hot supersaturated solution was slowly quenched to room temperature. Bright yellow plate-like crystals precipitated after the solution was allowed to stand for 2 days. The crystals were isolated by suction filtration and thoroughly dried at 60 °C in a vacuum oven (Yield: 0.66g, 26% based on total Pb).

(C(NH 2 ) 3 )(CH 3 NH 3 )PbI 4 . PbO (670 mg, 3 mmol) was dissolved in a mixture of 20.0 ml of aqueous HI and 1.7 ml of aqueous H 3 PO 2 by heating to boiling under constant magnetic stirring for at least 15 min. To the hot bright yellow solution were added solids CH 3 NH 3 Cl (203 mg, 3 mmol) and C(NH 2 ) 3 Cl (287 mg, 3 mmol), and the reaction was kept under constant heating and stirring for a few hours until ~25% of the acid solution has evaporated. The stirring was then discontinued, and as the hot supersaturated solution was slowly cooled to room temperature. Red crystals precipitated after the solution was allowed to stand for 2 days. The crystals were isolated by suction filtration and thoroughly dried at 60 °C in a vacuum oven (Yield: 0.54g, 22% based on total Pb).

(C(NH 2 ) 3 )(CH 3 NH 3 ) 2 Pb 2 I 7 . PbO (2232 mg, 10 mmol) was dissolved in a mixture of 20.0 ml of aqueous HI and 1.7 ml of aqueous H 3 PO 2 by heating to boiling under constant magnetic stirring for at least 15 min. To the hot bright yellow solution were added solids CH 3 NH 3 Cl (675 mg, 10 mmol) and C(NH 2 ) 3 Cl (478 mg, 5 mmol), and the reaction was kept under constant heating until all the the reactants dissolved. The stirring was then discontinued, and as the hot supersaturated solution was slowly quenched to room temperature, upon which the precipitation of dark red crystals commenced. The crystals were allowed to settle for further 2 h, before the crystals were isolated by suction filtration and thoroughly dried at 60 °C in a vacuum oven (Yield: 1.64g, 23% based on total Pb).

(C(NH 2 ) 3 )(CH 3 NH 3 ) 3 Pb 3 I 10 . PbO (2232 mg, 10 mmol) was dissolved in a mixture of 20.0 ml of aqueous HI and 1.7 ml of aqueous H 3 PO 2 by heating to boiling under constant magnetic stirring for at least 15 min. To the hot bright yellow solution were added solids CH 3 NH 3 Cl (675 mg, 10 mmol) and C(NH 2 ) 3 Cl (318 mg, 3.33 mmol), and the reaction was kept under constant heating until all the the reactants dissolved. The stirring was then discontinued, and as the hot supersaturated solution was slowly quenched to room temperature, upon which the precipitation of black crystals commenced. The crystals were allowed to settle for further 2 h, before the crystals were isolated by suction filtration and thoroughly dried at 60 °C in a vacuum oven (Yield: 1.50g, 22% based on total Pb). Materials Characterization. High resolution synchrotron powder X-ray diffraction (SXRD) data were collected at beamline 11-BM-B of the Advanced Photon Source (APS), Argonne National Laboratory using λ = 0.517040 Å. Discrete detectors covering an angular range from 0 to 4º 2θ are scanned over a 26 o 2θ range, with data points collected every 0.001 o 2θ and a scan speed of 0. 1º/s. Scanning Electron Microscopy (SEM) images of the perovskite crystals were obtained by using a Hitachi SU3400 equipped with an electron beam accelerated at 10-20 kV.

A Shimadzu UV-3600 PC double-beam, double-monochromator spectrophotometer was used to acquire room-temperature optical diffuse reflectance spectra of the powdered samples in the range of 200-2500 nm. BaSO 4 was used as a non-absorbing reflectance reference, and reflectance data were converted to absorbance data via Kubelka-Munk transformation. [START_REF] Gate | Comparison of the Photon Diffusion Model and Kubelka-Munk Equation with the Exact Solution of the Radiative Transport Equation[END_REF][START_REF] Chondroudis | Chemistry in Molten Alkali Metal Polyselenophosphate Fluxes. Influence of Flux Composition on Dimensionality. Layers and Chains in APbPSe4, A4Pb(PSe4)2 (A = Rb, Cs), and K4Eu(PSe4)2[END_REF][START_REF] Mccarthy | Synthesis in Molten Alkali Metal Polyselenophosphate Fluxes: A New Family of Transition Metal Selenophosphate Compounds, A 2 MP 2 Se 6 (A = K, Rb, Cs; M = Mn, Fe) and A 2 M' 2 P 2 Se 6 (A = K, Cs; M' = Cu, Ag)[END_REF] Photoluminescence spectra were collected on oriented crystals using Horiba LabRam Evolution high-resolution confocal Raman microscope spectrometer (600 g/mm diffraction grating) equipped with a diode CW laser (473 nm, 25 mW) and a Synapse CCD camera.

Single-crystal X-ray diffraction experiments were performed using a STOE IPDS II or IPDS 2T diffractometer with Mo Kα radiation (λ = 0.71073 Å) and operating at 50 kV and 40 mA.

Integration and numerical absorption corrections were performed using the X-AREA, X-RED, and X-SHAPE programs. The structures were solved by charge flipping and refined by fullmatrix least squares on F 2 using the Jana2006 package. [START_REF] Petříček | Crystallographic Computing System JANA2006: General features[END_REF] The PLATON software [START_REF] Spek | Structure validation in chemical crystallography[END_REF] was used to identify the twinning domains and validate the space groups of the compounds.

Theoretical calculations. First-principles calculations are based on density functional theory (DFT) as implemented in the Siesta package. [START_REF] José | The SIESTA method for ab initio order-N materials simulation[END_REF] Calculations have been carried out with the GGA functional in the PBE form, using the experimentally determined crystal structures. [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] Spin-orbit coupling is taken into account through the on-site approximation as proposed by Fernández-Seivane et al. [START_REF] Fernández-Seivane | On-site approximation for spin-orbit coupling in linear combination of atomic orbitals density functional methods[END_REF] Core electrons are described with Troullier-Martins pseudopotentials. [START_REF] Troullier | Efficient pseudopotentials for plane-wave calculations[END_REF] The valence wavefunction is developed over a double-ζ polarized basis set of finite-range numerical pseudoatomic orbitals. [START_REF] Artacho | Linear-Scaling abinitio Calculations for Large and Complex Systems[END_REF] An energy cutoff of 150 Ry for real-space mesh size is used as well as a 1×4×4 k-point sampling grid for sampling the Brillouin zone.

Thin Film Characterization. AFM images were acquired using a Dimensional Icon Scanning Probe Microscope (Veeco) in tapping mode. Powder X-ray diffraction measurements were carried out on a Rigaku MiniFlex600 X-ray diffractometer (Cu Kα radiation, λ = 1.5406 Å) operating at 40 kV and 15 mA. GIWAXS measurements were performed at Beamline 8-ID-E of the Advanced Photon Source (APS) at Argonne National Laboratory. Samples prepared on Si/PEDOT:PSS substrates were exposed to an X-ray beam (λ = 1.6868Å) at an incident angle of 0.20° for 5 s, and the scattered light was collected by a Pilatus 1 M pixel array detector at 204 mm from the sample. Absorbance data were collected on Cary 6000i UV-Vis-NIR spectrophotometer in the 175-1800 nm region.

Solar Cell Device Fabrication. The FTO-coated glass substrates (TEC7, 2.2 mm, Hartford Glass Co. Inc.) were cleaned by sequential sonication in aqueous detergent, deionized water, isopropanol (IPA), and acetone (ACE) for 15 min each, followed by a 5 min oxygen plasma treatment. A thin layer of PEDOT:PSS was deposited on the pre-cleaned FTO substrates by spincoating at 6,000 rpm for 30 s and annealed at 150 °C for 30 min in air. The substrates were then transferred to a nitrogen-filled glove box to complete the rest of the device fabrication. Solutions of GAMA 3 Pb 3 I 10 were prepared by dissolving 300 mg of the oven-dried crystals in 1 ml of DMF and 100 µl of DMSO solvents. For perovskite thickness study, various concentrations of the compound (20 mg, 25 mg, 30 mg, 35 mg, and 40 mg) in 100 µl:10 µl DMF:DMSO were used.

The perovskite layer was deposited by spin-casting 60 µl of the as-prepared solution on the ITO/PEDOT:PSS substrates at 500 rpm. for 3s, followed by 4,000 rpm. for 60 s. For solventengineered films, 300 µl of CB anti-solvent was dropped on the spinning substrate at various delay times (3 s, 5 s, 10 s, 20 s, 30 s, 40 s, and 50 s), and the films turned brown instantaneously.

The substrates were then annealed at 80 °C for 15 min to ensure removal of any remaining solvent residuals. PC 61 BM layer was deposited on the perovskite film from a 20 mg/ml PC 61 BM solution in CB at 5,000 rpm. for 30 s. Finally, 150 nm of Al was thermally evaporated through a shadow mask at a pressure of ~ 2 × 10 -7 Torr. The active area of the device was 0.09 cm 2 .

Solar Cell Device Characterization. The current density vs. voltage (J-V) characteristics were collected in air without any encapsulation using a Keithley 2400 source meter under simulated AM 1.5G irradiation (100 mW/cm 2 ) generated by a standard solar simulator (Abet Technologies). The light intensity was calibrated by using an NREL-certified monocrystalline Si reference cell to reduce the spectral mismatch.

RESULTS AND DISCUSSION

Materials synthesis. The synthesis of the series of (C(NH 2 ) 3 ) 2 PbI 4 and (C(NH 2 ) 3 )(CH 3 NH 3 ) n Pb n I 3n+1 compounds was accomplished by mixing stoichiometric ratios of pre-dried guanidinium chloride (GACl, C(NH 2 ) 3 Cl), methylammonium chloride (MACl, CH 3 NH 3 Cl), and lead oxide (PbO) in a hot HI solution. As soon as the supersaturated, clear, bright yellow solution was removed from the hot plate and left to cool to ambient temperature, crystals of the title compounds started forming. Bright yellow plates of (C(NH 2 ) 3 ) 2 PbI 4 , red needles of (C(NH 2 ) 3 )(CH 3 NH 3 )PbI 4 , dark red needles of (C(NH 2 ) 3 )(CH 3 NH 3 ) 2 Pb 2 I 7 , and black needles of (C(NH 2 ) 3 )(CH 3 NH 3 ) 3 Pb 3 I 10 were obtained, Figure 1. Unlike the RP perovskites, ACI perovskites obey a different synthetic protocol requiring a fully stoichiometric reaction in order to to crystallize in pure form. This is presumably because of the higher solubility of guanidinium salts in the reaction medium, in contrast to the long aliphatic ammonium cations used in the synthesis of the RP perovskites. The synthetic procedure finds an additional obstacle in the form of the reaction by-product the yellow (C(NH 2 ) 3 )PbI 3 , 37 a relatively insoluble one-dimensional (1D) "perovskitoid" of the NH 4 CdCl 3 structure-type which competes with the precipitation of the ACI perovskite phases. [START_REF] Stoumpos | Structure-Band Gap Relationships in Hexagonal Polytypes and Low-Dimensional Structures of Hybrid Tin Iodide Perovskites[END_REF] The co-formation of two reaction products was also observed in the Snbased GA systems [START_REF] Stoumpos | Structure-Band Gap Relationships in Hexagonal Polytypes and Low-Dimensional Structures of Hybrid Tin Iodide Perovskites[END_REF] with the 2D perovskite (C(NH 2 ) 3 ) 2 SnI 4 ) being the kinetic reaction product and the 3D ((C(NH 2 ) 3 )SnI 3 ) perovskite being the thermodynamic product. However, in the case of the (C(NH 2 ) 3 ) 2 PbI 4 /(C(NH 2 ) 3 )PbI 3 system this tendency inverts with the (C(NH 2 ) 3 ) 2 PbI 4 being the thermodynamic product and GAPbI 3 the kinetic one, possibly due to the lower dimensionality of the 1D perovskitoid, which makes it easier to precipitate. With the hindsight of the behavior of the pure guanidinium systems, we conducted the reactions to synthesize the (C(NH 2 ) 3 )(CH 3 NH 3 ) n Pb n I 3n+1 (n = 1-3) perovskites under relatively low concentrations below the saturation point of the solution, in order to avoid the formation of the 1D by-product.

All the ACI perovskites crystallize in high purity but their yield is relatively low (~25%) due to the special synthetic considerations discussed above, Figure 1b. An exception is found for (C(NH 2 ) 3 )(CH 3 NH 3 )PbI 4 (the n = 1) member which shows a photo-instability (in solution and in the solid state) towards decomposition to the higher n = 2 member, presumably through a disproportionation mechanism. The photoreaction is not severe and it occurs only on the surface of the crystallites. A similar photosensitivity has been observed for the similar cation iodoformamidinium cation n = 1 perovskite which degrades upon ambient light irradiation. [START_REF] Mitzi | Synthesis and Characterization of [NH 2 C(I)NH 2 ] 2 ASnI 5 with A = Iodoformamidinium or Formamidinium: The Chemistry of Cyanamide and Tin(II) Iodide in Concentrated Aqueous Hydriodic Acid Solutions[END_REF] Another important characteristic of the ACI perovskites that differentiates them from the RP perovskites is their mechanical behavior. All ACI perovskites are brittle and tend to crack when mechanical force is applied, as opposed to the RP perovskites that are more malleable and tend to undergo a plastic deformation. As will be discussed below, the unique mechanical properties of the ACI perovskites arise from their tight, more 3D-like crystal structure, which locks the layers in place with respect to one another in contrast to the RP perovskites in which the floppy organic chains of the spacers lend the materials additional flexibility.

Description of the crystal structures.

The crystal structures of the homologous ACI perovskite series, (GA)(MA) n Pb n I 3n+1 (n = 1-3), are illustrated in Figure 2; selected crystallographic data are presented in Table 2, and selected bonding parameters are provided in Table 3. All compounds crystallize in orthorhombic space groups with their unit cell dimensions being [a x 2a x 2(a . n+x)], where a ~ 6.3 Å is the lattice parameter of the idealized cubic 3D perovskite and x ~ 3.1 Å is the interlayer spacing (Table 2). The odd n-members (n = 1, n = 3) adopt the centrosymmetric Imma space group, whereas the even n-member (n = 2) adopts the polar noncentrosymmetric Bmm2 space group. The difference in odd and even members of homologous 2D perovskites series is expected as it arises from the changes in the crystallographic symmetry element that bisects the perovskite layers themselves along the abplane. For even n-members a mirror plane is present whereas for odd n-numbers this is replaced by a glide plane. [START_REF] Aleksandrov | Structural Phase Transitions in Layered Perovskitelike Crystals[END_REF] As a consequence of this trend all even-numbered halide 2D perovskites that bear MA as the "perovskitizer" cation tend to be noncentrosymmetric due to the incompatibility of the CH 3 NH 3 + cation of sitting on a mirror plane. [START_REF] Stoumpos | Ruddlesden-Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors[END_REF][START_REF] Mercier | HO 2 C(CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 )Pb 2 I 7 : a predicted non-centrosymmetrical structure built up from carboxylic acid supramolecular synthons and bilayer perovskite sheets[END_REF] Even though this assignment is ambiguous because of the dynamic disorder present in most of the halide perovskites, [START_REF] Szafrański | Mechanism of Pressure-Induced Phase Transitions, Amorphization, and Absorption-Edge Shift in Photovoltaic Methylammonium Lead Iodide[END_REF] strictly crystallographic criteria suggest the noncentrosymmetric structure to be the most favorable one. [START_REF] Hamilton | Significance tests on the crystallographic R factor[END_REF][START_REF] Rogers | On the application of Hamilton's ratio test to the assignment of absolute configuration and an alternative test[END_REF] The crystal structures of the (GA)(MA) n Pb n I 3n+1 ACI perovskites are unique among 2D perovskites because they are characterized by a very unusual ordering of the GA and MA cations which act as spacers between the perovskite layers. The GA/MA ordering leads to a doubling of the unit cell along the b-axis relative to the ideal undistorted cell, which coincides with the preferred growth direction of the crystals which adopt a needle-shaped morphology. The doubled b-axis hosts two individual (PbI 6 ) 4-units, whereas the GA and MA cations form individual stacks along the crystallographic a-axis to engulf a single (PbI 6 ) 4-unit. In order to accommodate the ordering of the spacers, the perovskite layers deviate from their idealized tetragonal symmetry, undergoing a strong octahedral tilting along the cation ordering axis (b-axis). Each layer compresses to engulf the compact MA spacer and expands to allow the incorporation of the bulky GA cation at the layer edges, forming rhombic and hexagonal channels, respectively in the process, Figure 3. The other two dimensions of the perovskite, along the a and c axes, remain relatively undistorted as there is no mismatch in the crystal packing of the organic cations. The MA cations that reside within the cuboctahedral perovskite cavities inside the layer adopt their usual orientation, by aligning along the crystallographic c-axis in a very similar arrangement to that observed ion the 3D perovskite MAPbI 3 .

In order to better highlight the special features of the ACI perovskites it is important to compare them with the common structure archetypes. The general distinctive features of the DJ perovskites over the RP perovskites are: i) the number of cations spanning the layers is reduced to one in DJ perovskites vs. two in the RP perovskites; ii) the change in the relative arrangement of two adjacent layers which in the case of the DJ perovskites involves a shift by half a unit cell in one direction (½, 0) or none at all (0, 0), but in RP perovskites involves a universal display of a half unit cell shift in both a and b crystallographic axes (½, ½). 58 In oxides, the crystal structure of DJ perovskites is strongly dependent on the size of the spacer cation and hinges between the two archetypes; the (0,0) modification (P4/mmm archetype) is obtained for large cations (Rb, Cs) which favor the occupation of the 8-coordinate sites between the layers, locking the layers on top of one another. Small cations (Li, Na) on the other hand, adopt lower coordination numbers resulting in the sliding of adjacent layer to the (½, 0) configuration (I4/mmm archetype). [START_REF] Fukuoka | Crystal Structure of a Layered Perovskite Niobate KCa 2 Nb 3 O 10[END_REF] All members of the ACI (GA)(MA) n Pb n I 3n+1 homology adopt the latter (½, 0) type of the DJ-like perovskite structure albeit in a lower symmetry modification which corresponds to the "amphoteric" type of oxide DJ perovskites as obtained when 6-coordinated K is used as a spacer and lies between the two archetypes. [START_REF] Sato | Structure determination of KLaNb 2 O 7 exhibiting ion exchange ability by X-ray powder diffraction[END_REF][START_REF] Fukuoka | Crystal Structure of a Layered Perovskite Niobate KCa 2 Nb 3 O 10[END_REF] The fact that the (GA)(MA) n Pb n I 3n+1 compounds adopt the offset layer structure is somewhat surprising given that both GA and MA are significantly larger than Cs. [START_REF] Kieslich | Solid-state principles applied to organicinorganic perovskites: new tricks for an old dog[END_REF] An explanation to this may lie in the fact that the term "coordination" does not strictly apply to the organic cations and trying to compensate for this by means of H-bonding fails since the closest N … I distances are at ~ 3.6 Å for both GA and MA implying only very weak hydrogen bonding. [START_REF] Steiner | The Hydrogen Bond in the Solid State[END_REF] It is perhaps more sensible to assess this effect based on the shape of the two cations which have distinctive triangular (GA) and rod (MA) shapes and due to this "steric hindrance" the perovskite layers offset to accommodate both cations between the two layers in a close-packed arrangement.

Note that this DJ-like structural rearrangement observed in the ACI halide perovskites is necessary in order to maintain charge balance, since there is no conceivable charge redistribution between the M 2+ metal and the A + cation to satisfy the genuine DJ structure expressed by the hypothetical "(GA)(MA) n-1 Pb n I 3n+1 " formula. By contrast, transition metal oxide DJ perovskites can maintain charge equilibrium and even interconvert to the RP structure simply by changing the oxidation state of the constituent M x+ metal (typically x = 4-5) to compensate for the loss of the A + cation. [START_REF] Schaak | Perovskites by Design: A Toolbox of Solid-State Reactions[END_REF] The close-packing of the GA and MA cations can be seen when the arbitrary "coordination" polyhedra around the GA (cutoff distance 4. Because of their specific characteristics, the crystal structures of the ACI perovskites are distinctively different from the corresponding RP perovskites. Using the RP (BA) 2 (MA) n- 1 Pb n I 3n+1 series as a reference example, [START_REF] Stoumpos | Ruddlesden-Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors[END_REF] the most distinctive differences can be summarized as follows: i) RP perovskites exhibit a significant distortion in both a and b crystallographic directions owing to the distortive nature of the bulky BA spacers which exert a strong disruptive force at the perovskite edges. In ACI perovskites, however, the combination of the MA/GA cations which pack tightly between the layers causes the distortion to be minimized in only one direction. Using the crystallographic nomenclature (Glazer [START_REF] Glazer | The classification of tilted octahedra in perovskites[END_REF] in parenthesis or Aleksandrov [START_REF] Aleksandrov | Structural Phase Transitions in Layered Perovskitelike Crystals[END_REF] in square brackets), the individual layers in the BA series is characterized by a (a -a 0 a -) [Φ0Φ]

tilting mode whereas the GA/MA series by a (a + a 0 a 0 ) [Ψ00] mode. Remarkably, none of these two series has any significant distortion along the stacking direction, unlike the corresponding PEA series (PEA is phenylethylammonium) [START_REF] Smith | A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability[END_REF][START_REF] Calabrese | Preparation and characterization of layered lead halide compounds[END_REF] which shows a tilting preference along the stacking axis adopting a (a 0 a 0 a + ) [00Ψ] tilting mode. ii) As a result of the more tidy packing of the cations, ACI perovskites are more symmetric as reflected in the shape of the unit cell [a x 2a

x 2(a . n+x)] which is closer to the α-phase of the 3D perovskite as opposed to the RP perovskites [√a x √a x 2(a . n+x)]. The latter are better described as a derivative of the β-MAPbI 3 phase at room temperature.

Optical Properties. It is very interesting to investigate in the new structure types in 2D halide perovskites how these small structural changes affect the electronic structure of the material visà-vis the RP phases and consequently their photophysical properties. Figures 4a-c shows the absorption spectra of the (GA)(MA) n Pb n I 3n+1 perovskites (n = 1-3). The spectra of all three compounds exhibit a progressive decrease in the bandgap starting from E g = 2.27 eV for n = 1 to E g = 1.99 eV for n = 2 and E g = 1.73 eV for n = 3. The bandgap value is estimated by extrapolating the high-energy edge to the energy axis, since the presence of excitonic peaks in all spectra precludes the observation of a clear band edge. Similar to the RP perovskites, the intensity of the excitonic peaks also decreases with increasing n in ACI perovskites as a consequence of the decrease in the dielectric confinement. [START_REF] Stoumpos | Ruddlesden-Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors[END_REF] However, the absorption edge and the exciton peak position is red shifted in ACI perovskites when compared to the RP perovskites. This is also reflected in the color of the (GA)(MA) n Pb n I 3n+1 compounds which changes from red (n = 1) to dark red (n = 2) to black (n = 3), whereas the more established BA-based RP series display a corresponding color change from orange (n = 1) to red (n = 2) to dark red (n = 3), Figures 4a-c.

A plausible, but not conclusive, explanation for this behavior is the symmetry (degree of distortion) in the corresponding perovskites. [START_REF] Pedesseau | Advances and Promises of Layered Halide Hybrid Perovskite Semiconductors[END_REF] As mentioned above, the ACI perovskites are distorted only along one dimension (the GA/MA ordering axis), whereas the RP perovskites are distorted along both directions in the ab-plane. The higher symmetry of the ACI perovskites is [START_REF] Stoumpos | Ruddlesden-Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors[END_REF] This symmetry argument, stating that the less distorted crystal structure will have the narrowest bandgap that applies to the 3D halide perovskites, 10 likely applies to the 2D perovskites as well, with the ACI and RP perovskites being the primary examples. In order to better illustrate the bandgap reduction effect, the absorption edges of the n = 1 members of the ACI, RP and <110>-sliced perovskites are plotted in Figure 4d. The observed trend reinforces the symmetry argument since it shows that the most distorted structure of the <110>-sliced perovskite GA 2 PbI 4 (P2 1 /n, E g = 2.49 eV) [START_REF] Daub | Synthesis, Crystal Structures, Optical Properties, and Phase Transitions of the Layered Guanidinium-Based Hybrid Perovskites[END_REF] has by far the widest bandgap. This is due to the larger distortion and the dihedral angle dependence introduced by the <110> cleavage of the perovskite sheets. [START_REF] Stoumpos | Structure-Band Gap Relationships in Hexagonal Polytypes and Low-Dimensional Structures of Hybrid Tin Iodide Perovskites[END_REF] This can be compared to to the regular absorption of the less distorted RP perovskite BA 2 PbI 4 at E g = 2.35 eV 6 and the GAMAPbI 4 ACI perovskite at E g = 2.27 eV.

The reason that the symmetry argument discussed above may not hold in full in the 2D perovskites is that other factors such as the contribution of spacer cations to the dielectric confinement or their close proximity of the perovskite layers can be significant. As first pointed out by Ishihara, [START_REF] Hong | Dielectric confinement effect on excitons in PbI 4 -based layered semiconductors[END_REF] the dielectric constant of the spacer can dramatically influence the excitons in in 2D perovskites pinpointing the effect of conjugated organic molecules to the binding energy of the confined exciton. The photo-physical puzzle of the 2D materials becomes even more complex if one considers that the precise determination of the band edge continuum relative to the exciton energy level may be challenging, [START_REF] Blancon | Extremely efficient internal exciton dissociation through edge states in layered 2D perovskites[END_REF][START_REF] Ishihara | Optical properties due to electronic transitions in two-dimensional semiconductors (C n H 2n+1 NH 3 ) 2 PbI 4[END_REF] and that the mechanism of the exciton expression also shows a strong temperature dependence. [START_REF] Yaffe | Excitons in ultrathin organic-inorganic perovskite crystals[END_REF] Nevertheless, on first approximation, the excitonic emission as this gets expressed using photoluminescence (PL) spectroscopy does not seem to be affected by these considerations, Figure 5.

The PL spectra of the (GA)(MA) n Pb n I 3n+1 ACI perovskite family (Figure 5) follows a similar trend as the absorption spectra, with the PL emission wavelength increasing with increasing n. The red shift in absorption observed for the ACI perovskites, relative to the RP perovskites, holds firmly in the PL spectra as well. The marked difference between the spectra of the two 2D families is that the ACI perovskites show significantly reduced PL emission intensity relative to the RP series under comparable excitation conditions suggesting that the effect of the dielectric environment due to the conjugated nature of the GA cations or the close proximity between the perovskite layers has a significant effect. Although the present work is not an exhaustive study of the optical properties of the new ACI perovskites, it is already obvious that the properties of all 2D halide perovskites are significantly influenced by subtle structural changes and thus offer a wide field for further investigations. observed in halide perovskites because of strong Rashba splitting caused by the noncentrosymmetric nature of its structure (Figure S1). The calculated bandgaps are underestimated within their typical margins at this level of theory but they nicely agree with the experimental trends for the ACI (GA)(MA) n Pb n I 3n+1 perovskites of E g = 1.12 eV (n = 1), E g = 0.75 eV (n = 2) and E g = 0.12 eV (n = 3). Noteworthy, at the same level of calculation, the corresponding bandgaps for the RP (BA) 2 (MA) n-1 Pb n I 3n+1 perovskites amount to E g = 1.17 eV (n = 1), E g = 0.74 eV (n = 2) and E g = 0.49 eV (n = 3) again predicting the experimental trend (with the exception of n=2).

A remarkable feature observed in the ACI perovskites comes from the fact that they show a small but significant dispersion along the perovskite layer stacking direction (Figure 6a and Table S7; along W-T for n = 1 and Y-Γ for n = 2), suggesting that the structures of the ACI perovskites have a weak but noticeable 3D character. The effect is greater at the valence band maximum (VBM) than at conduction band minimum (CBM; Table S7) and is likely due to the short spacing between the layers (~ 3.1 Å) that is comparable with the Pb-I bonding within the perovskite layers (~ 3.2 Å). In fact, the short spacing allows for additional anti-bonding interaction between iodine p-orbitals (Figure 6b), leading to a greater destabilization of the VBM at the edge of the Brillouin zone. Interestingly, the bands stop being dispersive between the layers for n = 3 (< 1meV) resulting in the typical 2D perovskite picture which has nearly flat bands along the layer stacking direction (e.g. < 1meV for the n = 1-3 RP (BA) 2 (MA) n-1 Pb n I 3n+1 ).

However, the reasons for the almost vanishing dispersion are different in the two cases. In the typical 2D RP perovskites the interlayer distance is large leading to a vanishing electronic coupling along the stacking direction. In addition, for the n = 3 ACI perovskite, the VBM becomes localized on the middle perovskite layer with no weight on the iodides pointing towards the spacing layer (Figure 6b). This localization, in turn, leads to the absence of inter-perovskite layer electronic coupling. Interestingly, the CBM for the n = 3 localizes on the two outer perovskite layers. By contrast, in the n = 1 perovskite both VBM and CBM reside on the same unique perovskite layer not allowing the charges to be efficiently separated. The localized density of states for the n = 2 ACI perovskite evidences a distinctive feature related to its noncentrosymmetric Bmm2 space group and the VBM and CBM are forced to localize on each individual layer finding themselves in a state of "charge-frustration".

In order to assess the effect of the spacer dielectric constant on the dielectric confinement of the ACI perovskites, we calculated the dielectric constant of the (GA)(MA) n Pb n I 3n+1 perovskites (n = 1 and 3) of this study to compare to those of the RP (BA) 2 (MA) n-1 Pb n I 3n+1 perovskites (n = 1 and 3, Figure S2). [START_REF] Sapori | Quantum confinement and dielectric profiles of colloidal nanoplatelets of halide inorganic and hybrid organic-inorganic perovskites[END_REF][START_REF] Even | Electronic surface states and dielectric selfenergy profiles in colloidal nanoscale platelets of CdSe[END_REF] We avoid the inclusion of the n = 2 members because the permanent dipole related to the lack of inversion symmetry mentioned above bias the calculation. The calculated dielectric profiles indicate that the dielectric contrast difference between the inorganic and the spacer layers is significantly reduced for the ACI perovskites by a factor of 2 compared to those obtained for the RP perovskites. Using a previously proposed composite approach, [START_REF] Even | Understanding Quantum Confinement of Charge Carriers in Layered 2D Hybrid Perovskites[END_REF] we demonstrate that the higher dielectric constant in the spacer layer in ACI perovskites can be attributed to the leakage of electron density from the perovskite layers due to the shorter interlayer spacing, as the contribution of the more polarizable GA/MA pairs is only marginally larger than that of the (BA) 2 bilayers (Figure 2d). The reduction of the dielectric confinement effect is expected to reduce the exciton binding energy. This trend holds universally throughout the two perovskite families in which the dielectric constants of the materials increase progressively to reach the maximum value of the dielectric constants found in the n = ∞ member (MAPbI 3 ).

Thin film fabrication and morphology. For solar cell applications, we focused on the compound with the smallest bandgap, GAMA 3 Pb 3 I 10 (n = 3). First, thin films of this compound were prepared from 10:1 DMF:DMSO solutions. The synthesized compounds readily dissolve in such solvents, yielding a clear bright yellow solution. It was found that a typical post-annealing method yields a very rough, dull-colored film with a root-mean-square (rms) roughness of 37 nm, Figure 7a. The pinholes found in this type of film are unfavorable for planar solar cell device architectures, creating undesired contacts between the two electrodes that results in poor device performance. [START_REF] Soe | Room Temperature Phase Transition in Methylammonium Lead Iodide Perovskite Thin Films Induced by Hydrohalic Acid Additives[END_REF] To improve the film quality, we applied the previously reported solventengineering method where anti-solvent, specifically chlorobenzene, is dripped during the spinning of the substrate to expedite the solvent removal process and subsequently promotes faster perovskite crystallization. [START_REF] Jeon | Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells[END_REF][START_REF] Xiao | A Fast Deposition-Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin-Film Solar Cells[END_REF] This method enables precise control of the perovskite crystallization rate by changing the delay time before the commencement of the spin-casting process and the anti-solvent dropping.

Here we investigated the optimal crystallization rate for GAMA 3 Pb 3 I 10 by varying the delay time between 3 s and 50 s. The obtained films exhibit a desirable compact morphology with nm-scale crystallites, a dramatic improvement from the films without the solvent treatment.

We find that 20-30 s delay times yield the smoothest, most compact films. Upon the introduction of anti-solvent drip, the film surface rms roughness decreases markedly down to 10 nm for 10-20 s delay times, beyond which it begins to increase again and eventually reaches 35 nm at 50 s, Figure 7b. The PXRD of these films are shown in Figure 7c. All the films were polycrystalline, consistent with the Bragg diffraction rings in the 2D GIWAXS data (Figure S3). Unlike RP perovskites, 2,[START_REF] Cao | 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications[END_REF][START_REF] Smith | A Layered Hybrid Perovskite Solar-Cell Absorber with Enhanced Moisture Stability[END_REF] the present films from ACI perovskites do not orient vertically, presumably due to the shorter spacing between the perovskite layers provided by GA in comparison to BA cations.

Even though the perovskite slab orientation on the substrate is still suboptimal, we fabricated solar cells with high efficiency. We constructed planar device structures sandwiched between a PEDOT:PSS hole transport layer and a PCBM electron transport layer, Figure 8a, since the desirable vertical growth of the layered perovskites is more favorable on flat substrates for efficient charge transport. 2 The perovskite layer was deposited using the fast-crystallization method with anti-solvent dripping, as described above. Following the trend shown in the AFM images of the films, Figure 6a, which show the rms roughness of the films steadily decreasing down to ~20s anti-solvent dripping delay time and then increasing again after the 30s delay mark, we fabricated devices at the same dripping delay time intervals to study the effect of film morphology on the device performance. Here the delay time is defined as the time between the commencement of the spin-casting process and the dripping of the anti-solvent while the substrate is spinning. The devices without the solvent treatment were not functional because of the high roughness and protruding features on the rough perovskite film which create shorting pathways. We observed a dramatic change in device parameters depending on when the antisolvent is dropped. As expected, there is a distinct correlation between the device performance and the perovskite film smoothness and compactness. At very short delay times, low photocurrents and low V oc are observed, which improve upon increasing the anti-solvent delay time, Figure 8.

Remarkably, a significant improvement in the overall performance is observed at 10-20 sec delay due to the formation of the compact, smooth film morphology. In particular, at 20 s delay time, a high Voc of ~1V, high Jsc of 9.84 mA/cm 2 , and an impressively high FF of 79.68% are obtained. The high V oc illustrates an excellent interface between the perovskite layer and the charge extraction layers. Intriguingly, while 2D perovskites typically suffer from low FF due to the relatively higher charge recombination rate than their 3D counterparts, here we obtain a superior FF, which we attribute to the high-quality film morphology with full surface coverage and minimal pinholes. These qualities are in good agreement with previous studies on MAPbI 3

where GA was used as an additive. [START_REF] Marco | Guanidinium: A Route to Enhanced Carrier Lifetime and Open-Circuit Voltage in Hybrid Perovskite Solar Cells[END_REF] The main difference between the devices fabricated from the bulk materials as described here vs. devices made by mixing the constituent salts as described by Marco et al. [START_REF] Marco | Guanidinium: A Route to Enhanced Carrier Lifetime and Open-Circuit Voltage in Hybrid Perovskite Solar Cells[END_REF] can be seen from the optical absorption spectra and the PXRD patterns of the films (Figures S3 andS4). Our data clearly identify the active solar absorber to be the title 2D compound, whereas the additive approach yields MAPbI 3 as the active absorber. [START_REF] Marco | Guanidinium: A Route to Enhanced Carrier Lifetime and Open-Circuit Voltage in Hybrid Perovskite Solar Cells[END_REF] Our results suggest that the ACI perovskites are very promising in giving a competitive device efficiency even without the benefits of preferred orientation, 2,[START_REF] Cao | 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications[END_REF] which is an obvious path towards improving device performance in the future.

The present solar cells also show high reproducibility with an average PCE from 10 devices 6.69 ± 0.47%. For longer anti-solvent delay times, the device performance begins to decrease, as evidenced by large drops in V oc and FF on going from 20 sec to 50 sec (Table 4).

This may be caused by the increased surface inhomogeneity and roughness as crystallites begin to aggregate and form nanostructures compromising the compactness of the films. It is, however, interesting that the photocurrent did not change significantly for longer delay times, likely because the intrinsic film crystallinity remains unaltered regardless of the delay time.

To ensure fabrication of high efficiency solar cells, it is essential to optimize the light absorber thickness, since the light harvesting ability and carrier diffusion length must be balanced for optimum solar cell efficiency. Thus, we conducted a systematic study of device efficiency as a function of precursor concentration, which in turn determines the perovskite thickness and find an optimized condition at 30 mg in 110 µL solvent mixture (Figure S3). device performance. The question arises as to whether the ACI perovskites also exhibit similar edge states and will be the subject of future investigations.

Supporting Information

Additional DFT plots for GAMA

  8 Å) and MA (cutoff distance 4.2 Å) cation are drawn, centered on the carbon atom of each cation, Figure 3. Both cations are 8coordinate with MA lying in the center of a compressed square prism and GA in the center of an expanded gyrobifastigium. The parallelogram equatorial planes of the two polyhedra reflect clearly the distortion of the perovskite lattice to accommodate the two cations. The equatorial plane of both cations is defined by two parallel vertices that coincide with the crystallographic axes along the a-direction and another two along the b-direction. Along a-axis the vertices coincide with unit cell parameter (~6.3 Å), whereas along the b-axis the vertices deviate with the one around GA expanding to ~7.3 Å whereas the one around MA gets compressed to ~5.2 Å. The geometrical "sum" of these two octahedra has proper dimensions to allow for the doubling of the perovskite layer along the b-axis (~12.5 Å) and it is probably responsible for the offset of the adjacent layers.

  clearly reflected in the corresponding Pb-I-Pb angles which show the tilting along the a-axis to be marginal (176-180 o ), along the b-axis large (~160 o ) and intermediate along the stacking c-axis (165-167 o ). The trend is markedly different from the RP perovskites which see a more uniform deformation with the corresponding Pb-I-Pb angles ranging between 165-170 o for both the ac-plane and the stacking b-axis.

  Figure 6a. The n = 2 member shows a slight deviation from the typical direct bandgap behavior,

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 45

  Compared to BA-based n = 3 cells, the photocurrent generated in this GA-based n = 3 devices is comparatively low, likely due to non-optimally oriented nature of the 2D GAMA 3 Pb 3 I 10 . Work is in progress to further engineer the vertical orientation of the microstructure to further improve device performance.CONCLUSIONSA new family of 2D halide perovskites of the formula (C(NH 2 ) 3 )(CH 3 NH 3 ) n Pb n I 3n+1 can be created which contains alternating cations in the interlayer space (ACI), by employing guanidinium cations as spacers. The red crystals of (C(NH 2 ) 3 )(CH 3 NH 3 )PbI 4 (n = 1) have an optical gap of 2.27 eV; dark red crystals of (C(NH 2 ) 3 )(CH 3 NH 3 ) 2 Pb 2 I 7 (n = 2) have a 1.99 eV optical gap, and black crystals of (C(NH 2 ) 3 )(CH 3 NH 3 ) 3 Pb 3 I 10 (n = 3) have a bandgap of 1.73 eV.These materials have narrower bandgaps in comparison to typical RP perovskites, exhibiting a clear red shift in their optical gaps by 80, 90 and 230 meV, respectively for the corresponding n = 1-3 members. We attribute the reduced bandgap of the ACI perovskites to their less distorted crystal structure, behaving in an analogous manner to the 3D halide perovskites as the A-cation changes from Cs to FA. Theoretical calculations successfully confirm this trend providing further insights in the dielectric confinement of these QW electronic systems, and suggesting a small exciton binding energy, which is beneficial for charge-transport related applications. Our initial attempts to employ these compounds as photo-absorbers in planar solar cells for the higher member of the ACI perovskites, GAMA 3 Pb 3 I 10 with the smallest bandgap within the present series, have yielded solar cell devices with a maximum η = 7.26% power conversion efficiency due to an impressive FF of ~ 80%. The superior optoelectronic properties of the 2D ACI perovskites suggest that this new class of compounds can advance photovoltaic research on 2D halide perovskites, offering the possibility of breaching the 15% efficiency threshold.Previously, we reported that the photo-physics of thin Ruddlesden-Popper perovskites films with n>2 are dominated by lower energy states associated with the local intrinsic electronic structure of the edges of the perovskite layers. These states appear to provide a direct pathway for dissociating excitons into longer-lived free-carriers that significantly enhance optoelectronic

Figure 1 .

 1 Figure 1. (a) Representative SEM images of (GA)(MA) n Pb n I 3n+1 (n = 1-3) crystals showing a plate-like crystal morphology. The plates are irregular parallelepipeds growing preferentially along one dimension. The scale bars represent a 50µm length. (b) High resolution synchrotron Xray data (11-BM, APS, λ = 0.517040 Å) for (GA)(MA) n Pb n I 3n+1 (n = 1-3). The experimental data are in excellent agreement with the theoretical patterns determined by single-crystal X-ray diffraction analysis. The impurity observed in the n = 1 perovskite that grows on the surface of the crystal was identified to arise from the n = 2 member and amounts to ~10% of the sample.

Figure 2 .Figure 3 .

 23 Figure 2. View of the unit cells of the (GA)(MA) n Pb n I 3n+1 (n = 1-3) perovskites along (a) the crystallographic b-and along (b) the crystallographic a-axis highlighting the ordered crystal packing of the GA and MA cations between the perovskite layers Figure 3. (a) Projected view of a supercell of (GA)(MA) 3 Pb 3 I 10 along the crystallographic a-axis emphasizing on the polyhedral voids between the perovskite layers. (b-c) The "coordination" polyhedra of the GA and MA cations. (b) GA resides in the barycenter of a gyrobifastigium and (c) MA resides in the center of a square prism cavity. (d-f) The crystal packing of the respective polyhedra. (d) Packing of the GA cations along the a-axis. (e) Packing of GA and MA along the b-axis. (f) Side view of the packing of MA cations along the a-axis.

Figure 4 .

 4 Figure 4. (a-c) Optical absorption spectra of the (GA)(MA) n Pb n I 3n+1 (n = 1-3) perovskites. (d) Selected absorption spectra of n = 1 member of the ACI perovskite (GA)(MA)PbI 4 , the RP perovskite (BA) 2 PbI 4 and the (110)-cleaved perovskite (GA) 2 PbI 4 highlighting the importance of the perovskite structure-type on the optical properties of the materials.

Figure 5 .

 5 Figure 5. (a-c) Steady-state PL spectra and (d) graphical representation of the (GA)(MA) n Pb n I 3n+1 (n = 1-3) ACI perovskites (solid lines) and their comparison with the corresponding n-members of the (BA) 2 (MA) n-1 Pb n I 3n+1 series of RP perovskites.

Figure 6 .

 6 Figure 6. Electronic structure of the (GA)(MA) n Pb n I 3n+1 (n = 1-3) perovskites. (a) Electronic band structure of the n = 1-3 members highlighting the progressive narrowing of the bandgap and the splitting of the band extrema as the number of 2D layers increases. (b) The localized density of states around the conduction band minimum (CBM) and the valence band maximum (VBM), illustrating the propensity of the terminal 2D layers to host the CBM and the central 2D layers to stabilize the VBM.

Figure 7 .

 7 Figure 7. (a) AFM images of solution-processed thin films of (GA)(MA) 3 Pb 3 I 10 as a function of delay time for anti-solvent chlorobenzene (CB) dripping. The scale bar is 2 µm. (b) change in

Figure 8 .FiguresFigure 1 Page 34 of 45 ACS

 8145 Figure 8. (a) Planar heterojunction solar cell structure employing (GA)(MA) 3 Pb 3 I 10 as a light absorber. (b) J-V curves of (GA)(MA) 3 Pb 3 I 10 as a function of delay time for anti-solvent chlorobenzene (CB) dripping, and (c) the corresponding plots J sc , V oc , FF and power conversion efficiency as a function of delay time for anti-solvent dripping.

10 a

 10 in the aristotype, ie. ignoring octahedral tilts b The n = 2 is chosen for clarity; for the n = 3 member see reference

  2 Pb 2 I 7 highlighting the noncentrosymmetric features (Rashba splitting) of the compound, dielectric constant calculations, and PXRD and GIWAXS patterns of GAMA 2 Pb 2 I 7 and GAMA 3 Pb 3 I 10 are provided in the Supporting Information. Detailed crystallographic tables and crystallographic data in .cif format can be found as supplementary materials. This material is available free of charge via the Internet at http://pubs.acs.org. User Facility operated for the US DOE Office of Science. LANL, an affirmative-action equal opportunity employer, is operated by Los Alamos National Security for the National Nuclear Security Administration of the US DOE under contract DE-AC52-06NA25396. C.K. and B.T.

	acknowledge high-performance computing resources from Grand Equipment National de Calcul
	Intensif (CINES/IDRIS, grant 2016-[x2016097682]). DFT calculations were performed at the
	Institut des Sciences Chimiques de Rennes, which received funding from the European Union's
	Horizon 2020 Programme for Research and Innovation under grant 687008. This work made use
	of the SPID (confocal microscopy) and EPIC (scanning electron microscopy) facilities of
	Northwestern University's NUANCE Center, which has received support from the Soft and
	Corresponding Author Hybrid Nanotechnology Experimental Resource (NSF ECCS-1542205), the Materials Research
	*m-kanatzidis@northwestern.edu

Science and Engineering Centers (NSF DMR-1121262), the International Institute for Nanotechnology (IIN), the Keck Foundation, and the State of Illinois through the IIN. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the Basic Energy Sciences program of the US DOE Office of Science under contract DE-AC02-06CH11357.

Table 1 .

 1 Structural configurations of the three types of <100> 2D perovskites.n+1 B n X 3n+1 A′ A n-1 B n X 3n+1 (Bi 2 O 2 )A n-1 B n X 3n+1 (A′A) n+1 B n X 3n+1 Stacking sequence [(A′X)(ABX) n ] [(A′)(A n-1 BX) n ] [(Bi 2 O 2 )(A n-1 BX) n ] [(A′X)(ABX) n ] Centering of perovskite slabs a

		RP	DJ	AV	ACI
	Formula	(A′A)

Table 2 .

 2 Crystal data and structure refinement for GAMA n Pb n I 3n+1 (n = 1-3) at 293 K. Σ||F o |-|F c || / Σ|F o |, wR = {Σ[w(|F o | 2 -|F c | 2 ) 2 ] / Σ[w(|F o | 4 )]} 1/2 and w=1/(σ 2 (I)+0.0004I 2 )

	Formula	GAMAPbI 4	GAMA 2 Pb 2 I 7	GAMA 3 Pb 3 I 10
	FW	807.0	1426.9	2046.9
	Temperature (K)		293
	Wavelength		0.71073 Å
	Refinement method		Full-matrix least-squares on F 2
	Crystal system		orthorhombic
	Space group	Imma	Bmm2	Imma
	Unit cell dimensions (Å)	a = 6.4286(6) b = 12.4577(9) c = 18.8258(13)	a = 6.3729(4) b = 12.5435(8) c = 31.376(2)	a = 6.3520(7) b = 12.4378(9) c = 43.970(3)
	Volume (Å 3 )	1507.7(2)	2508.2(3)	3473.8(5)
	Z	4	4	4
	ρ calc (g/cm 3 )	3.5551	3.7788	3.9138
	µ (mm -1 )	19.349	22.017	23.395
	F(000)	1392	2424	3464
	Crystal size (mm 3 )	0.1111 x 0.0225 x 0.0167 0.0511 x 0.179 x 0.0100 0.0733 x 0.0372 x 0.0193
	θ range (°)	3.27 to 29.23	2.6 to 29.27	2.78 to 29.29
		-8<=h<=8	-8<=h<=8	-8<=h<=8
	Index ranges	-17<=k<=16	-17<=k<=17	-17<=k<=16
		-25<=l<=25	-42<=l<=42	-57<=l<=60
	Reflections collected	7172	11855	12119
	Independent reflections	784 [R int = 0.1277]	2595 [R int = 0.0563]	1810 [R int = 0.1261]
	Completeness to θ	99%	99%	99%
	Data / restraints / parameters	784 / 3 / 28	2595 / 9 / 82	1810 / 5 / 59
	GOF	3.18	2.19	2.80
	R indices [I>2σ(I)] R indices [all data] Largest diff. peak and hole (e•Å -3 )	R obs = 0.0575 wR obs = 0.1356 R all = 0.0677 wR all = 0.1361 1.84 and -2.31	R obs = 0.0555 wR obs = 0.1099 R all = 0.0898 wR all = 0.1126 5.69 and -1.65	R obs = 0.0597 wR obs = 0.1254 R all = 0.0863 wR all = 0.1270 3.32 and -3.20
	R =

Table 3 .

 3 Bond lengths [Å] and angles (degrees) for GAMAPbI 4 , GAMA 2 Pb 2 I 7 , and GAMA 3 Pb 3 I 10 at 293 K with estimated standard deviations in parentheses.

	GAMAPbI 4

Table 4 .

 4 Solar cell device parameters for GAMA 3 Pb 3 I 10 as a function of the delay in the antisolvent dripping. The average PCE is obtained from at least ten devices at each condition.

	Delay time (s)	Jsc [mA/cm 2 ]	Voc [V] FF [%]	η[%]
	3 s	6.781	0.863	69.07	4.04 (3.90 ± 0.18)
	5 s	7.446	0.925	71.86	4.95 (4.33 ± 0.40)
	10 s	8.668	1.033	76.08	6.81 (6.14 ± 0.36)
	20 s	9.357	0.974	79.68	7.26 (6.69 ± 0.47)
	30 s	8.833	0.970	78.01	6.68 (6.30 ± 0.59)
	40 s	8.821	0.695	65.44	4.01 (3.84 ± 0.19)
	50 s	8.989	0.554	60.83	3.03 (2.72 ± 0.28)

TOC Image Page 45 of 45 ACS Paragon Plus Environment Journal of the American Chemical Society

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Acknowledgements

Work on the synthesis and characterization of the materials was supported by the U.S.

Department of Energy

, Office of Science (Grant No. SC0012541). Work on the solar cells fabrication and evaluation was supported as part of the ANSER Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences (award no. DE-SC0001059). Work at Los Alamos National Laboratory (LANL) was supported by the Laboratory Directed Research & Development program. This work was performed in part at the Center for Integrated Nanotechnologies, an Office of Science

Author Contributions

# These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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Journal of the American Chemical Society

Properties and Photovoltaic Performance

Chan Myae Myae Soe, 1# Constantinos C. Stoumpos, 1# Mika𝑒𝑒l Kepenekian, 2 Boubacar Traoré, 2 Hsinhan Tsai, [START_REF] Blancon | Extremely efficient internal exciton dissociation through edge states in layered 2D perovskites[END_REF] Wanyi Nie, [START_REF] Blancon | Extremely efficient internal exciton dissociation through edge states in layered 2D perovskites[END_REF] Binghao Wang, [START_REF] Stoumpos | High Members of the 2D Ruddlesden-Popper Halide Perovskites: Synthesis, Optical Properties, and Solar Cells of (CH 3 (CH 2 )[END_REF] Claudine Katan, 2 Ram Seshadri, [START_REF] Cao | 2D Homologous Perovskites as Light-Absorbing Materials for Solar Cell Applications[END_REF][START_REF] Mao | Role of Organic Counterion in Lead-and Tin-based Two-dimensional Semiconducting Iodide Perovskites and Application in Planar Solar Cells[END_REF][START_REF] Stoumpos | Ruddlesden-Popper Hybrid Lead Iodide Perovskite 2D Homologous Semiconductors[END_REF] Aditya D.

Mohite, [START_REF] Blancon | Extremely efficient internal exciton dissociation through edge states in layered 2D perovskites[END_REF] Jacky Even, [START_REF] Mao | White-light Emission and Structural Distortion in New Corrugated 2D Lead Bromide Perovskites[END_REF] Tobin J. Marks, [START_REF] Stoumpos | High Members of the 2D Ruddlesden-Popper Halide Perovskites: Synthesis, Optical Properties, and Solar Cells of (CH 3 (CH 2 )[END_REF]

-5000 2500 498


Ludivine Fadel
email: fadel@ixl.u-bordeaux.fr

Isabelle Dufour

Frédéric Lochon

O Français

Microcantilever: a chemical resonant sensor

teaching and research institutions in France or abroad, or from public or private research centers.

The process to achieve the microstructures has been done at ESIEE group [START_REF]ELMI-Groupe ESIEE-Cité Descartes-BP 99 -93162 Noisy Le Grand[END_REF] using silicon technologies. Five mask levels (Fig. 2) were necessary for the fabrication of silicon microcantilevers: the first two are used to define strain gauges, another one is dedicated to electric contacts and the last two are required to etch both the top and bottom silicon sides by DRIE. In order to chose accurately structures thicknesses, n-type SOI (Silicon On Insulator) wafers have been used. The oxide thickness separating SOI to silicon is about 0.5µm and acts as stop layer for DRIE etching. First, a 4500Ǻ silicon oxide is deposited by wet oxidation, used later as a protect layer when boron diffusion is set up on silicon. This step is followed by a photolithography which permits to define strain gauges areas. After cleaning, the structure is doped with boron implantation at 975°C during 20min and annealed inducing the boron diffusion on silicon substrate (a). The second mask draws contact strain gauges aperture, then the oxide is etched with HF 10% (b). At this stage, 1µm aluminum thick is deposited by pulverization followed by the third photolithography which specifies contact pads and coils for magnetic actuation (c). Aluminum etching is later realized in wet. Using the fourth mask, the next step consists on etching the top side of the SOI by DRIE. Before the DRIE, considering the weak SOI thickness, a photoresist has to be deposited as protective layer, then the passivation oxide and the bottom side oxide are removed (d). The bottom side etching requires to protect the top side with thick resin, then a 5000Ǻ aluminum layer, deposited by pulverization, is used on the back side as protective layer. The last photolithography needs a bottom side alignment in order to achieve the complete etching of the wafer by DRIE. The total substrate Key Words: microcantilever, sensitive coating, resonant frequency, mass effect

I. INTRODUCTION

Micromechanical freestanding microstructures, and microcantilevers in particular, are gaining interest as new applications for gas detection. The sorption of specific species by a sensitive layer deposited on silicon microcantilevers modifies the mechanical properties of the structure. In fact, perturbations of the sensitive coating properties induce frequency drift (Fig. 1), thus making chemical detection possible [START_REF] Baller | A cantilever array-based artificial nose[END_REF][START_REF] Betts | Selectivity of chemical sensors based on micro-cantilevers coated with thin polymer films[END_REF]. This physical effect is used to realize a chemical sensor and the sensitive coating is chosen for its high affinities with molecules to detect.

Fig. 1. Resonant frequency variation

In this paper, experimental measurements performed with microcantilevers having different geometries are presented. In fact, the measurement of the mass dependence of the resonant frequency allows to predict the gas sensor response, especially the gas concentration sensitivity. 525µm) is etched up to the oxide. Finally, the last and most important step is the release of the freestanding microstructures obtained by wet etching (BHF 10%) (e). At last, the cheap dicing is realized before the cleaning so that the resin protects the structures.

Flexural modulus

Mass density

This polymer is deposited on one side of the cantilever by "spray coating" using a time controller, to obtain a reproducible layer (Fig. 3). Before spraying, the PEUT is dissolved in methylene dichloride (25mg/ml) and is continuously agitated to maintain a better homogenisation. Then, the solution is sprayed onto the cleaned devices with an airbrush system (Dosage 2000, model Valmemate TM 7040) using pure nitrogen as carrier gas and a shadow mask.

III. MASS EFFECT MEASUREMENTS II.2 COATING MATERIAL

III.1. MEASUREMENT PROCEDURE

Among a wide range of polymeric coating materials, partial selectivity and sorption properties can be investigated. Sensor sensitivities and selectivities are controlled by tailoring the chemical and physical properties of the coating material to maximize particular solubility interactions. The coating selection is made through a systematic variation of the coating material solubility properties, so that the sensor is selective for a different balance of solubility interactions [START_REF] Grate | Solubility interactions and the design of chemically selective sorbent coatings for chemical sensors and arrays[END_REF]. According to our application, in order to detect humidity and organic volatile compounds (alcohols, toluene, n-octane…) the microsensors are coated with a slightly polar poly(etherurethane) PEUT film. This polymer, received from Thermedics Inc. (Tecoflex EG-80A), presents a high porosity and its physical data are summarized in Table 1.

Targeting a full compact microsystem, the excitation of the cantilever in the resonant mode is performed by using piezoelectric material or a magnetic field. The structure oscillations are detected with the strain gauges etched on the device surface. Then, the microcantilever is inserted in the retroaction loop of an amplifier to constitute an oscillator (Fig. 4) and, according to the Barkhausen conditions, the resonant frequency is monitored. Using this electronic circuit, shifts in the frequency are observed when the structure mass changes, e.g by addition of a sensitive coating or later by the sorption of vapor into surface coating.

The variation of the resonant frequency with the deposition time is due to the cantilever mass sensitivity which will be determined in the following section.

III.2. EXPERIMENTAL RESULTS

The measurements have been realised with four structures labelled IC5 , IC8 , IB5 , IB6 (Fig. 5 and Table 2) [START_REF] Dufour | Resonant microcantilever type chemical sensors: analytical modeling in view of optimization[END_REF].

with m the microstructure mass and k the microstructure stiffness.

For a cantilever composed of two different materials, the mass and stiffness expressions are:

b 1 L 1 b 2 L 2 (top view ) b 1 L 1 b 2 L 2 (top view ) ( )Σ ρ + ρ = 2 2 1 1 h h m and         + + + + Σ α = ) E ĥ E ĥ ( 4 ) h h ( E Ê ĥ h 12 h E 12 h E L k 2 2 1 1 2 2 1 2 1 2 1 3 2 2 3 1 1

Fig. 5. Microcantilever geometry

with α a constant depending on the shape of the microcantilever, Σ the upper-surface of the microcantilever, L the length, h 1 and h 2 the thicknesses of the two materials, ρ 1 and ρ 2 the mass densities, )

1 ( E E ˆ2 1 1 1 ν - = and ) 1 ( E E ˆ2 2 2 2 ν - =
with E 1 and E 2 the Young's moduli and ν 1 and ν 2 the Poisson's coefficients. During the sensitive coating deposit, the structure stiffness can be considered constant and only the mass is modified [START_REF] Fadel | Coupled determination of gravimetric and elastic effects on two resonant chemical sensors: Love-wave and microcantilevers platforms[END_REF]. Consequently, the frequency expression f with coating is linked to the frequency without coating, f 0 , as follows:

Table 2. Device dimensions L 1 b 1 b 2 L 2 IC5 1mm 200µm 1mm 1mm IC8 1mm 200µm 2mm 2mm IB5 500µm 200µm 1mm 1mm IB6 500µm 200µm 2mm 2mm
Hence, the experiments consist in recording the frequency shift of each structures in oscillator configuration (Fig. 6). Ten sprays of dissolved polymer (duration: 1s) have been made with a polymer stabilisation waiting between each spray. Using this expression, the polymer thickness is extracted from experimental frequency shift (Fig. 6) and is plotted versus time deposition (Fig. 7). These results, obtained with different structures measurements, allow to characterise the spray coating (the PEUT thickness increases linearly with the deposition time in a reproducible way) and to determine the mass sensitivities for each microcantilevers: S m (Table 3). Then, using the partition coefficient K (relative to the sensitive coating and gas) and the measured mass sensitivities (∆f/∆m), (Table 3) the gas concentration sensitivity is estimated [START_REF] Dufour | Resonant microcantilever type chemical sensors: analytical modeling in view of optimization[END_REF]: Both experimental results (Fig. 8) and theoretical model show a sensitivity S Cg f proportional to resonant frequency f 0 and polymer thickness h 2 . However, the oscillator noise usually increases with the frequency and consequently the limit of detection could be less performing for high resonant frequency system. Moreover, large PEUT thickness induces important diffusion time (sorption/desorption) and so a long sensor response time. Consequently, the choice of resonant frequency and polymer thickness for gas detection will result from comprises concerning sensitivity, response time and limit of detection which is directly linked to the oscillator noise.

S Cg f = ∆f /∆C g = K h 2 Σ (∆f/∆m)

V. CONCLUSION

SOI microcantilevers with integrated excitation (piezoelectric, electromagnetic) and piezoresistive measurement have been realized. A spray coating system has also been developed. Then, the first measurements of resonant frequency variation show the coating thickness reproducibility and allow to calculate the mass sensitivity. Thanks to this experimental results, the gas sensor response has been studied by estimating the gas concentration sensitivity. Actually, a gas line test bench is in process which will permit to compare the results to gas measurements.

Fig. 3 .

 3 Fig. 3. Coating system Fig. 2. Technological steps

Fig. 4 .

 4 Fig. 4. Block diagram of the electrical oscillator with piezoelectric excitation and piezoresistive measurement

Fig. 6 .Fig. 7 .

 67 Fig. 6. Resonant frequency versus deposition time

Fig. 8 .

 8 Fig. 8. Gas concentration sensitivity versus PEUT thickness

Table 1 .

 1 Physical Data thickness (

Table 3 .

 3 Mass sensitivities

	IC5	S m =0.025 Hz/ng
	IC8	S m =0.003 Hz/ng
	IB5	S m =0.06 Hz/ng
	IB6	S m =0.008Hz/ng

IV.2. STRUCTURES COMPARISON