Improving the Stability and Efficiency of Perovskite Solar Cells by a Bidentate Anilinium Salt

We report improvement in the perovskite solar cell efficiency and stability after passivation with an organic molecule decorated with two anilinium cations. We compare this salt with its neutral analog and found that the change in the electron density distribution upon protonation and the presence of the halide anion are key to explaining the better passivation ability of the salt. In addition, we show that the counteranion has a significant impact on the performance of the device.


Section 1. Experimental Procedures
H nuclear magnetic resonance (NMR): The spectra were collected in a Bruker Avance III 400 MHz (9.4 THz), with a 1 H frequency of 400. 13 MHz.
Atomic force and Kelvin force probe microscopy measurements: The morphology of the films was investigated using an NX10 atomic force microscope (AFM) from Prak Systems. Together with surface imaging, the surface potential was mapped using a Kelvin probe. A point probe-plus Silicon-SPM-Sensor (PPP-EFM-W) with a resistivity of 0.01-0.02 Ω cm produced by nanosensors was used for this purpose.
Contact angle measurements: Photographs of the drop of water on the perovskite film were acquired immediately after the dripping. Such measurements were performed using an Attension optical tensiometer, Theta model. The angle was measured in the software of the equipment.
Device assembly: FTO glass substrates were cleaned with Hellmanex (2%), deionized water, acetone, and isopropanol (IPA) for 20 min each in an ultrasonic bath. The FTO glasses were dried using nitrogen and treated in a UV-ozone chamber for 30 min. For the electron transport material (ETL) SnO 2 colloidal solution (15wt%) was diluted in 1:3 (v:v) in deionized water, and this solution was spin-cast on the precleaned FTO glasses at 3000 rpm for 30 s, followed by annealing at 150°C for 30 minutes in the air.
The substrates were then treated with UV-ozone for 30 min before transferring into the glovebox. For the fabrication of perovskite films, the gas quenching method was used.
The perovskite Cs 0.05 FA 0.95 PbI 2.95 Br 0.05 (1.5 M, 6% excess of PbI 2 ) solution was prepared by the dissolution of CsBr, formamidinium iodide (FAI), and PbI 2 in a mixture of DMF:NMP (4:1, v:v). The solution was stirred for 30 minutes at room temperature and then 40% of methylammonium chloride (MACl) was added. After an additional 30 min of stirring, the solution was spin-coated on SnO 2 films at 1000 rpm for 10 seconds, and then 4000 rpm for 25 s. After 5 seconds at 4000 rpm, a blow of nitrogen was used to remove the solvent. Immediately after the spin coating, the films were annealed at 100°C for 1 minute inside the glovebox. In sequence, all the films were removed to the air and thermally annealed at 100°C for 60-90 minutes in air ambient with a relative humidity of 30-40%. Following the films were placed again inside the glovebox.
The post-treatment of the perovskite was performed using a solution of the salt (FM-NH 3 X) or the neutral molecule (FM-NH 2 ) in isopropanol or chlorobenzene, S6 respectively. For the deposition, 60 μL of the solution of the passivating molecule was dynamically spin-cast onto the perovskite using 3000 rpm for 30 s.
The preparation of the perovskite films used in the characterization was the same, with the exception that they were deposited onto a pre-cleaned glass substrate. To Fourier transform infrared (FTIR) spectroscopy: the data were acquired using a Cary 630 FTIR spectrometer. For the powder, the Attenuated Total Reflectance (ATR) acquisition mode was used. In the case of the films, they were deposited in a Si <100> substrate and the spectra were acquired in the transmission mode.
Single crystal X-ray diffraction (XRD): X-ray diffraction data collections on single crystals of FM-2Br and FM-NH 2 were performed with an Oxford-Diffraction GEMINI-Ultra using Mo-Kα radiations (0.71073 Å). Measurements were performed at 293 K. Data integration and scaling of the reflections for all compounds were performed with the CRYSALIS suite. 1 Final unit cell parameters were based on the fitting of all reflections positions. Analytical absorption corrections and the space group identification were performed using CRYSALIS suite. 1 The structures of the compound were solved by direct methods using the SUPERFLIP program. 2 The positions of all atoms could be unambiguously assigned on consecutive difference Fourier maps.
Refinements were performed using SHELXL 3 based on F 2 through a full-matrix leastsquare routine. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. All hydrogen atoms were located in difference maps and included as fixed contributions according to the riding model. 4 Values C-H = 0.97 Å and U iso (H) = 1.2 U eq (C/N) for the aromatic carbon atoms and amine groups.
Stability upon aging of the devices: to the stability test, the devices were stored at room temperature in a box filled with N 2 , with humidity <30%, and protected from UV light.
They were removed from the box to the electrical characterization and then stored again.
Time-resolved photoluminescence (TRPL): The decay curves were measured in an Edinburg Analytical Instruments FL 900 spectrofluorometer with an MCP-PMT (Hamamatsu R3809U-50; 50 ps). These measurements were carried out with a PicoQuant LDH-D-C-440 pulsed diode laser with λ exc =440 nm (bandwidth of 5 nm, pulse width = 80 ps, F = 9.7 nJ cm -2 ). The decays were collected at 800 nm of each sample on a time scale of 10 μs.
X-ray diffraction (XRD): The diffractograms were obtained on a Shimadzu diffractometer, model XRD-6000 with Cu Kα radiation of 0.154 nm wavelength.
X-ray photoelectron spectroscopy (XPS): X-ray photoelectron spectroscopy (XPS) measurements were carried out using a monochromatic Al Kα source on a K-alpha spectrometer (Thermo Fisher Scientific) equipped with a 180° double-focusing hemispherical analyzer with 128-channel detector. Survey and selected atomic signals were obtained with a pass energy of 50 eV and a step of 0.02 eV. Casa XPS software package was used to analyze the obtained spectra and the peaks were fitted using a mixed Gauss-Lorentz function and Shirley background. All the measured spectra were corrected by setting the reference binding energy of carbon (C 1s) at 284.8 eV.

Section 2. Synthesis
The materials were purchased from Sigma-Aldrich and TCI chemicals and used as received. Tetrakistriphenylphosphine Pd (0) was synthesized according to the protocol described below. The synthetic route adopted is shown in Fig. S1.

2.1.
Synthesis of 9-(dibromomethylene)-9H-fluorene (FM-2Br) In the first step, 9-fluorenone (2.00 g, 11. In sequence, it was cooled to room temperature, and the solvent evaporated. The product was extracted with CHCl 3 (3x 50 mL) and washed with brine (2x 50 mL). The organic phases were dried over Na 2 SO 4 and the solvent evaporated. The crude product was purified in a chromatographic column using hexane as eluent, affording 3.00 g of FM-2Br ( Fig. S1)

Synthesis of 3,3'-((9H-fluoren-9-ylidene)methylene)dibenze-naminium (FM-NH 3 X, X: Cl, Br, I)
To a round-bottom flask, 150 mg of FM-NH 2 (0.416 mmol, 1 eq.) and 5 mL of dioxane were added. Then, 2.2 equivalent of HCl (37 wt%), HBr (48 wt%), or HI (57wt%) were added dropwise for 10 minutes, and the reaction was kept at 27 °C for 6 h under N 2 . The solvent was partially evaporated, toluene (20 mL) was poured into the solids, and the solvent was evaporated; this step was repeated 3x. Toluene helps to remove DMSO and water. In sequence, 10 mL of toluene was added and the suspension was heated with stirring until reflux and kept for 30 minutes. Following, it was cooled to room temperature, the solids were filtered and washed with diethyl ether (7x 20 mL).
The spectra were obtained for the powder of the materials in the ATR mode. In stark contrast, when thin films of FM-NH 2 were prepared we observed brightly yellowish emissions (Fig. S5a); the same is true for the solid-state emission.

Section 4. Characterization of the Aggregation-Induced Emission
Probably, the increased emission of the thin films compared to the solution is due to the restriction of the intramolecular rotation of the aniline groups caused by physical hindrance. From Fig. S5b, we can see that the emission of the film is bathochromically shifted compared to the solution, which is related to an increase in the energy barrier for molecular motion, triggered by the steric hindrance between neighboring molecules. 6 The enhanced Stoke shift and the emission in the aggregated state suggest that FM-NH 2 presents the phenomenon of aggregation-induced emission (AIE). 6 To check  that the molecules were more rigid than when surrounded by EtOAc. This can be linked S19 to the H-bonding interaction between the amine groups in FM-NH 2 and the solvent EtOH, which helps to preclude the rotation of the aniline moieties.
We were also able to verify the effects of H-bonding in the PL emission spectra of FM-NH 2 in a solution of dimethylsulfoxide, DMSO, with the addition of EtOH (Fig.   S5d). We note that the emission intensity in DMSO is more intense and structured compared to the EtOAc: an effect of the higher viscosity of the medium. As ethanol is added, the emission intensity increases, as a result of the H-bonding interaction between solute and solvent, and the poor solubility of the molecule in ethanol.
Such results evidenced that the FM-NH 2 is a typical example of AIE-luminogen.

Finite Size Systems Theoretical Approach and Computational Details
All ab initio total-energy calculations were based on spin-polarized DFT calculations within the screened exchange-correlation functional as proposed by Heyd-Scuseria-Ernzerhof (HSE06), 7,8 using the Fritz-Haber Institute ab initio molecular simulations package (FHI-aims), 9,10 which employs numeric atom-centered basis functions (NAOs), 9,10,11 whereas the second improvement level from a free-atom orbital minimal basis, i.e., light-tier2 (adopting FHI-aims terminology), was considered. The electrons were treated by the scalar-relativistic framework within the atomic zerothorder relativistic approximation (atomic ZORA). 12 The self-consistent electronic density (SCF) for the FM-NH 2 and FM-NH 3 molecules was obtained once the following criteria were reached, i.e., 1.0 .10 -6 eV (total energy) and 1.0 .10 -4 eV/Å (atomic forces). The criteria of 1.0x10 -3 eV/Å of the atomic forces were used to obtain the equilibrium geometries. Furthermore, for all total energy calculations, we employed a Gaussian broadening parameter of 10 meV to achieve the correct occupation of the electronic states.
The vibrational frequency calculations were performed upon the harmonic approximation and the finite-difference algorithm for all lowest energy configurations, S21 using 10 -1 tighter parameter to calculate the Hessian Matrix, atomic displacements of 0.0025 Å were used. Finally, those molecules were verified as true local minima by the absence of negative eigenvalues in the Hessian matrix.

Optimal SCF and Relaxation Parameter
Using the Perdew-Burke-Ernzerhof (PBE) functional at light-tier2 level, we calculate the optimized structure of the FM-NH 2 and FM-NH 3 , as well as the correspondent energy levels (HOMO and LUMO), and the energy gap (E g ) between HOMO-LUMO. We tested different criteria to obtain the SCF convergence: energy, density, and forces. We also compare the spin none and spin colinear configurations.
The results are expressed in Table S1.
We started the geometry optimization using two different guesses: one with the aniline rings on the same plane of the fluorene, and the other with the aniline rings torsioned ('t') regarding the fluorene. This strategy allows us to verify if the torsioned structure is in fact the most stable configuration. Our results demonstrate that regardless of the initial configuration the optimized structure always has the aniline rings torsioned. We also tested another hybrid functional: the HSE06 with the same basis set level, i.e., light-tier2. Using this functional we performed two sets of calculations: In the first one, the geometry was optimized, and the energy was calculated using the HSE06 functional. In the second, we used the optimized geometry obtained from PBE/light-tier2 and we ran a single-point energy calculation. The results are shown in Table S2. Fig. S6 compares the optimized structure obtained from PBE vs. HSE06. We found that the geometry optimized by HSE06 presented a better match with the experimental results in what concerned the energy of the HOMO and LUMO (values signed with * in Table S2). Thus, due to the better accuracy, all structural, energetic, and electronic properties were obtained with the HSE06 optimized geometry. Table S2. Energetic and electronic properties for the optimization of FM-NH 2 and FM-NH 3 at HSE06/light-tier2, considering the lowest energy configuration from the previous torsionated initial (-t) structures obtained at PBE/light-tier2 level. E tot DFT is the DFT total energy; q n+ is the molecule total charge where n = 0-2; m tot is the total magnetic moment;  HOMO and  LUMO are the energies of HOMO and LUMO, respectively; and E g is the LUMO-HOMO energy gap.  Going further, we compare in Table S3 the bond lengths and dihedral angles obtained from the optimized geometry structure of FM-NH 2 and FM-NH 3

S25
In both FM-NH 2 and FM-NH 3, the aniline/anilinium group can assume different conformations, yielding distinct isomers, as shown in Fig. S7. In this sense, we study from an energetic point of view the energy associated with them (Table S4). We observe that the isomers of FM-NH 2 are almost degenerate in energy, while for FM-NH 3 , the energetic difference is slightly higher, ~100 meV. We can conclude that all configurations change their initial conformation, e.g., ring torsion angle, upon geometric optimization aiming to maximize the distance between the neighboring -NH 2 or -NH 3 .  The experimental data evinces the preference of NH 2 at an up-down position which agrees with the DFT calculations. However, the electronic properties are not largely affected by the position of -NH 2 and -NH 3 groups, as observed in Table S4.

Experimental vs Theoretical Structure
We were able to obtain a single crystal of both FM-2Br and FM-NH 2    We were able to obtain a single crystal of FM-NH 2 by the slow-evaporation method from a concentrated solution of the molecule in ethyl acetate. We then resolve the crystal structure of this molecule by XRD and found that the aniline rings present structural disorder, being treated as double split positions. Fig. S9b shows the representation of the molecular structure of FM-NH 2 obtained from the XRD experiment. The main crystallographic information is summarized in Table S5.

S30
To check the validity of our calculations in predicting the optimized structure of FM-NH 2 we compare in Table S6 the experimental and calculated bond lengths. We found an excellent correlation between theoretical and experimental results, evidencing the validity of our DFT calculations.  18 for implicit solvent representation. The results are shown in Fig. S10.

Energy Level Distribution and Charge Analysis
We calculated the energy level distribution and performed Hirshfeld charges analysis using the HSE06/light-tier2 level. For the case of the neutral molecule, FM-NH 2 , we found that the aniline groups were twisted about 20° about the plane of the fluorene. In the case of the salt, FM-NH 3 , the torsion angle between the anilinium groups and the fluorene decreases to ~16°. Changes can also be seen in the distribution of the energy levels over the molecule (Fig. S11a). Whereas in the neural molecule both HOMO and LUMO are spread out over the entire backbone, in the protonated the S35 HOMO is mainly concentrated on the fluorene moiety, while the LUMO experiences a similar distribution founded for FM-NH 2 . This is an indication that an internal charge transfer from the fluorene to the anilinium groups could be occurring in the case of the salt. In addition, the charge distribution is significantly affected by protonation (Fig.   S11b): the charge on the nitrogen atoms changes from -0.192 to +0.027. Also, the carbon atom attached directly to the N becomes more positively charged upon protonation.

Description of the Models
To better understand the interaction between FM-NH 2 and FM-NH 3  For each model, we considered two possible configurations for the adsorbed molecule (up-up / down-down), which are differentiated by the rotation angle between the aniline/anilinium group and the fluorene. The geometry of these structures was previously determined and is shown in Section 5. These arrangements will affect the interaction between the molecules and the perovskite surface.
Additionally, we evaluated two different possible positions the molecules can occupy on the perovskite surface, to evaluate if some arrangement was preferential. To summarize, considering the two molecules, two models, the two possible molecule configurations, and the two possible molecule positions, we have eight models to study.

Total Energy Calculations
Our theoretical calculations were based on DFT, within the semilocal

Analyses
To quantify the interaction between molecules and surface, we calculated two properties: (i) the interaction energy, E i , which is the difference between the energy of the equilibrium geometry of the perovskite+molecule system and the sum of the energy of the respective parts, separated and frozen; (ii) the adsorption energy, E a , which is the difference between the energy of the equilibrium geometry of the perovskite+molecule system and the sum of the energy of the respective parts, separated but optimized. This allows the understanding of the role of the distortions in the adsorption process, and the energy associated with these distortions, E d , can be used to correlate the adsorption and interaction energies via E a = E i + E d . Additionally, we evaluated the dispersion energy of the different systems and their respective components, as well as the dispersion energy associated only with the interaction between the molecules and the perovskite slab, Edisp, which can be defined as E disp = E disp A+B -E disp A -E disp B.

Results
The eight models constructed are illustrated in Figure S12. Following the structural optimization, their resulting configurations are illustrated in Figure S13, and the energy results are summarized in Tables S7 and S8.

Section 13. Discussion About the Kelvin Probe Force Microscopy Measurements
In Kelvin probe force microscopy (KFPM), a voltage is applied to an AFM tip to nullify the charges on the surface. The samples were placed in a resistor and connected by metallic contact (silver ink) (Fig. S19), allowing the electrons to flow between one sample and another. In this sense, KFPM will provide information about the local changes in contact potential difference (V CPD ) in the sample, measured between the sample and the tip. Interestingly, for metals and semiconductors, it is possible to relate the V CPD with the Fermi energy (E F ) of the materials: V CPD ∝ E F, sample -E F, tip . Note that we used a proportionality signal and not equality. This is because V CPD is affected by the electrostatic charges adsorbed on the surface, contamination, and degradation of the sample. These parameters will influence the value measured, making it slightly different from the real E F .
In this sense, we only can probe the trends on the E F of the surface and not the absolute value. To do so, we must use the same tip to run all the measurements, avoiding changes in the E F,tip , which otherwise will affect the comparison between the E F of the samples in a given batch of the experiment. We minimize the interfering parameters cited above using this strategy while keeping the same E F,tip . As a result, since the samples are in short-circuit, the electrons of the lower E F material will flow to the higher ones. When the equilibrium is reached, the E F will be equalized, and the higher E F material becomes negatively charged, whereas the lower E F , is positively charged.
Consequently, the tip-applied potential necessary to nullify the charges on the former will be lower compared to the latter. In this sense, the measured V CPD of the sample with the higher E F will be lower compared to the lower E F material. Thus, we can monitor the variation on the E F position from sample to sample. Figure S19. Scheme of the sample preparation for the KPFM measurements. The samples were placed onto a steel dis, and silver ink was used to electrically connect them and allow electron flux.