Abbreviations

MAI Methylammonium iodide

FTO Fluor-doped tin oxide glass substrate

XRD X-ray diffraction analysis

SEM Scanning electron microscopy

DMF N, N-dimethylformamide

XPS X-ray photoelectron spectroscopy

UPS Ultraviolet photoelectron spectroscopy

RPM Rotations per minute

VB Valence band

WF Work function

E_B Binding energy

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1. Introduction

Group 14 halometalates of the composition AMX₃ represent a large group of semiconducting materials. In these compounds, MX₆ octahedra form one-dimensional (1D) edge-sharing or three-dimensional (3D) corner-sharing (perovskite) structures. The formation of a defined polymorph depends on preparation method and temperature ^[1,2]. The valence band of such semiconductors primarily stems from the X⁻ halogen anion fully occupied valence p-orbitals and the conduction band from the empty M²⁺ metal cation valence p-orbitals, while the uppermost occupied and lowest unoccupied states of the A⁺ cation are off the band edges ^[3]. Thus, the variation of the A species does not directly influence the energy gap, but may influence it indirectly by acting on the MX₆ configuration in the lattice. For RbPbI₃ and CsPbI₃ band structures, calculations show increased dispersion in the 3D phase that leads to a smaller band gap as compared to the 1D-phase. Organic-inorganic iodide perovskites, such as CH₃NH₃PbI₃, have recently drawn attention ^[4,5,6,7] due to functional qualities as light-absorbers in solar cells, achieving average rates of 15% ^[8,9,10,11] and certified rates up to 20% ^[10]. As for the cause of some of the MAPbI₃ electronic peculiarities, the methyl-ammonium ion (MA) with its dipole moment has been previously examined ^[11]. The contribution of the inorganic counterparts also deserves proper investigation. Assessments of Alkali-PbI₃ systems have shown energy gaps in the optical spectrum, suggesting an advantageous role as photovoltaic absorbers. While the perovskite phase (3D) of CsPbI₃ is black with a band gap around 1.67 eV, ^[12] the 1D-phases are yellow with energy gaps above 2 eV and may also be potentially suitable for photovoltaic applications, e.g., in tandem cell configurations or as luminescence materials.

Perovskites have been the subject of great interest for many years, due to the large number of compounds which crystallize in this structure. Many studies have been published on the substances with the formula AMX₃, such as BaTiO₃, NaNbO₃ and CsPbCl₃ ^[13,14]. Besides these inorganic perovskites, various amines may be placed on the A-site, such as ammonium (NH₄⁺) ^[15], methylammonium (CH₃NH₃⁺) ^[16,17,18], and formamidinium (NH₂CH=NH₂⁺) ^[19,20]. These organic cations are particularly common in compounds based around lead and tin halides. In these compounds, the perovskite aristotype is a cubic Pm3m framework structure of composition AMX₃, where A (Wyckoff position 1a) is commonly a large cation coordinated to 12X (3c) anions, M (1b) is a smaller metal bonded to six X anions, and MX₆ octahedra are corner-connected to form a three-dimensional framework. The hinged octahedra allow for wide adjustment of the M-X-M bond angle, and several sets of cooperative rotations, known as tilt transitions, promote symmetry reduction of the aristotype ^[21]. The black phase of CsPbI₃ can be grown as nanoparticles in a high-temperature solution process using a boiling solvent ^[22], whereas the low-temperature aqueous solution process applied in the current study leads to growth of the yellow modification in needle-shaped crystals in accordance with the 1D-arrangement of the PbI₆ octahedra ^[23].

The main purpose of our work targets the synthesis and characterization of APbI₃ compounds with systematically varied A cations from the alkali series and the organic ammonium cation NH₄⁺ in comparison to the well studied CH₃NH₃PbI₃ using a modified simple self-organizing solution based-method. The crystallographic (XRD), morphologic (SEM), and optical properties were measured on the primary APbI₃ systems. In addition, thin films were prepared from solutions of the primary materials dissolved in dimethylformamide (DMF) and photoelectron-spectroscopically characterized using XPS and UPS. Basic semiconductor properties such as energy gap, work functions, and absolute ionization energies have been derived.

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2. Materials and Method

APbI₃ materials were prepared by a self-organization process, using precursor solutions that were prepared by commercially available compounds. In particular, LiI (lithium iodide), NaI (sodium iodide), KI (potassium iodide), NH₄I (ammonium iodide), Pb(CH₃COO)₂*2H₂O (lead acetate dihydrate), HI (47% wt., iodic acid stabilized with 1.5% wt. hypophosphorous acid), and CsI (cesium iodide) were used as delivered by Alfa Aesar without further purification. Tri-halogeno-plumbates (II) with potassium, ammonium, and cesium were synthesized through a modified self-organization process, as previously reported ^[16]. The materials were prepared in a concentrated aqueous solution of iodic acid at 110 °C, which contained Pb²⁺ ions [from lead (II) acetate] and a respective amount of cesium, potassium, or ammonium-iodide.

The methylammonium-containing system was prepared with a respective amount of CH₃NH₃⁺ (by adding a 40% solution of CH₃NH₂ in water) ^[16,24]. As these perovskites are grown from aqueous solutions, crystallization water is to be expected ^[25]. For A = Na and Li, severe preparation conditions such as concentrated and hot alkaline solutions are necessary in order to obtain high yields of pure products. Moreover, the complex liquid-solid equilibrium in the solutions requires very careful control of the NaI (or LiI)/PbI₂ starting stoichiometry. Therefore, lithium-and sodium-containing tri-iodo-plumbates were prepared by adding the corresponding amount of PbI₂ to solutions containing lithium and sodium ions from LiI and NaI, respectively, in acetone at 40 °C, in which both salts are soluble. The microcrystalline precipitates formed spontaneously while cooling to room temperature, subsequently separated by filtration, and dried at room temperature in air. Finally, the materials were stored in a glove-box under inert atmosphere to minimize reactions with ambient atmosphere, particularly moisture.

The structural analysis was performed by X-ray powder diffraction (XRD) on thoroughly grinded samples using a D8 Bruker powder diffractometer (Cu Kα₁ + Cu Kα₂ radiation) with a theta/ 2 theta Bragg-Brentano configuration. The diffractometer is equipped with an Energy Dispersion Detector Si(Li) to minimize fluorescence effects. A scanning electron microscope (SEM) Philips XL 30 FEG was used to investigate the morphology of the samples.

Optical diffuse-reflectance measurements were performed on the prepared powders using a Perkin Elmer UV/VIS/NIR double-monochromator spectrometer operating from 200 to 2500 nm. BaSO₄ was used as a non-absorbing reflectance reference. The generated reflectance-versus-wavelength data were used to estimate the band gap of the material by converting reflectance to absorbance data, according to the Kubelka−Munk equation: α/S = (1− R)²/2R, where R is the reflectance and α and S are the absorption and scattering coefficients, respectively ^[26,27].

For X-ray and ultra-violet photoelectron spectroscopy (XPS and UPS), the primary synthesized crystals were grinded, dissolved in dimethylformamide (33% wt), recrystallized from a spin-coated (in air) liquid layer on a transparent conductive oxide FTO (fluorine doped SnO₂) on glass substrates (Pilkington TEC15), and dried at 80 °C. The reference system MAPbI₃ was deposited in a two-step process. In the first step, a 37% wt solution of PbI₂ was spin-coated at 1000 RPM and dried at 90 °C for 30 min. In the second step, the PbI₂ films were dipped in analcoholic solution (2-propanol) of MAI (10mg/ml), then rinsed with 2-propanol and finally dried out at 90 °C for 30 min. The films were stored in a glove box under dry atmosphere (<5ppm water). All photoemission spectra were collected under a pressure of 10⁻⁹ mbar and in normal emission. The spectrometer (PhiVersaprobe II) was calibrated to the Fermi-edge of a silver reference and the Ag3d_5/2 core level at a binding energy E_B = 368.26 eV. To perform XPS measurements, a monochromatic AlKα X-ray-gun with an excitation energy of 1486.6 eV was used, and the pass energy of the hemispherical analyzer was set to 11.5 eV. In addition to the emissions of the examined species, Sn and oxygen emissions were also observed, due to partial coverage of the FTO substrate and residuals of the solvent. A helium gas discharge lamp was used to generate He-I (21.22 eV) for UPS measurements. As the UPS valence band (VB) spectrashow structures due to incompletely covered FTO, the valence band spectra were also taken with AlKαXPS excitation, for which the relative cross section for orbitals of lead and iodine, which form the valence band, are pronounced compared to UV excitation ^[54]. As the FTO work function (WF) is larger than the observed WF, the onset of the secondary electrons is initiated by secondary electrons emitted from the film, which occurs on incompletely covered FTO substrates, as well. Thus, the WFs are derived from the secondary electron emission onset in He-I spectra taken with an applied bias voltage of 3 eV between the substrate and the spectrometer. All measurements were performed at room temperature. Chemical states and electronic states (valence band maximum position, work function, and ionization potential) of the samples were derived.

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3. Results and Discussion

The primary crystalline powders were prepared using a self-organizing process from solution; the details of this process have been previously reported ^[24]. Crystallographic stability and probable structure of perovskites can be deduced by considering the (t) (Goldschmidt’s tolerance factor) and (μ) (Li’s octahedral factor) factors ^[28]. The tolerance factor evaluates ionic size mismatches of the perovskite-structure until a different structure-type is formed in the perovskite AMX₃. The evaluation methods of the tolerance factors t and µ have been discussed in the Supporting Information of Ref. ^[28]. The values of t factors have been summarized in Table 1 in order of the ionic radii ^[29]. Analysis of Table 1 indicated that the Cs-containing system fulfills the conditions for a perovskite structured material (for halide-containing systems, the lowest value is 0.813), whereas the ammonium-containing compound may lead to distorted perovskites ^[28]. Below ~0.80, other structures, such as the ilmenite-type (FeTiO₃), are more stable due to the similar sizes of the cations A and M. The materials formed from aqueous solution, for which crystallization occurs by cooling slowly from 100-110 °C to room temperature without stirring, show discrete needle-like structures when A= Cs, K, and NH₄⁺, in contrast to polyhedral structures for CH₃NH₃⁺, shown in Figure 1A-D. The A = Li and Na based materials grown from acetone solution form polycrystalline aggregates, as clearly shown in Figure 1E and Figure 1F, respectively. The morphological investigation of the APbI₃ systems, shown in Figure 1A-D, revealed that, for A = Cs, the crystallites generally grow as narrow platelets with lengths up to 3-4 mm, which is a typical crystal habit of an orthorhombic lattice (space group Pnma) (Figure 1A). A similar habit can be observed in the ammonium-containing system, as shown in Figure 1C; on the other hand, the crystallites for A = K demonstrated a thin stripe-like structure (Figure 1B) with lengths up to 5-6 mm, which is a typical crystalline habit of a tetragonal lattice. Finally, unlike the alkaline metals and ammonium-containing compounds, for A = MA, the formation and growth of dodecahedral crystallites (Figure 1D) were observed (space group I 4/mcm).

The powder XRD analysis confirmed the crystal structures initially drawn from SEM images. The X-ray diffractograms of the perovskites grown from aqueous solution, i.e., systems containing Cs, K, NH₄⁺, and CH₃NH₃⁺, are shown in Figure 2A-F. For Cs, K, and NH₄⁺, the room temperature crystalline reflections can be indexed on the basis of an orthorhombic structure (space group Pnma), by comparison to the data presented previously by Trots ^[30] and Bedlivy ^[31]. Upon evaporation or cooling of the solutions, APbI₃·nH₂O is obtained, where A = Li, Na, K, NH₄, Rb, or Cs and n = 0, 2, 4, or 6, depending on the alkali ion and on the conditions of formation [32 and references there in]. Of this family, only the structures of the two isotopic anhydrous salts RbPbI₃ and low temperature CsPbI₃ phases are known. On the other hand, Fan ^[32] recently reported the preparation of anhydrous NH₄PbI₃ by solid-state reaction in a sealed and evacuated quartz tube. The obtained prism-shaped yellow crystals were used to determine the crystalline structure and properties of the system. In the ammonium-containing system, the XRD-patterns suggest the presence of reflections due to secondary phase(s) (indicated with * in Figure 2B). Although all systems crystallize as orthorhombic structures, the intensity differences resulting from the same diffractions can be explained by the different crystalline habits of the specimens, which are influenced by thermodynamic and kinetic factors during the formation and growth of the crystallites. For A = MA⁺ (Figure 2D), the crystalline reflections indicate the formation of a tetragonal structure at room temperature (space group I 4/m or I 4/mcm), i.e., a 3D perovskite structure ^[16]. Zieger ^[33] reported an attempt to obtain the Na-containing compound by the interaction of an aqueous NaI solution (conc. 1 g ml⁻¹) with PbI₂, which was dissolved in the solution up to saturation. After drying out, a pale yellow hygroscopic powder was obtained. The diffraction pattern revealed the presence of unreacted precursors PbI₂, NaI, and a new phase (NaPbI₃). Very similar results were obtained experimentally, and by comparing the results of the XRD analysis, we find similar reflections of an additional crystalline phase, indicated by asterisks in Figure 2E. This phase can be identified as NaPbI₃·xH₂O (the content of water was not measured), in which x can tend to 4 or even 6 for NaI : PbI₂ = 50:50 (mol/mol), as reported by Roger ^[34], when exposed to high relative humidity (ca. 60%) for several days. A similar argument can be used to explain the data in the Li-containing system. Similarly, the formation of LiPbI₃·4H₂O as solid phase in LiI concentrated aqueous solutions (@ 60% wt.) has been reported ^[27]. In our case, the presence of this phase as well as the presence of unreacted PbI₂ and LiI have been confirmed (Figure 2F) by comparison to data previously reported ^[35,36] . Indeed, for A = Li, detailed reports of crystalline structure could not be found in the literature. Consequently, we cannot attribute any Miller indexes to XRD patterns in Figure 2F. For better analysis, we improved the visual representation of the XRD patterns by enlarging Figures 2A-2F and reporting them in Figures S2-S4 of the Supplementary Information. The Li-containing material is highly hygroscopic even at low relative humidity, thus, storage under inert atmosphere is mandatory.

Both structural and morphological features can dramatically influence the physical properties of the prepared materials. The optical absorption spectra of the APbI₃ systems are displayed in Figure 3 and the corresponding Tauc plots (explained below) in Figure 4. The main optical absorption is attributed to electron excitations from the valence band to the conduction band. The values of the band-gaps are given by the absorption edge, obtained by fitting the linear part of the absorbance spectrum ^[37]. The values of the band-gaps have been summarized in Table 1 in the order of the ionic radii ^[29]. Data of PbI₂ without any further cations are added for comparison and are in accordance with published data ^[1,38]. The relationship between the absorption coefficient (α) near the absorption edge and the optical band gap (E_g) approximately obeys the following formula ^[39,40]

$\alpha \text{ }=\text{ }A{{(h\nu ~-~{{E}_{g}})}^{n}}/h\nu$

(Eq. 1)

where A is a parameter that relates to the effective masses associated with the valence and conduction bands, hν is the photon energy, and n is related to the nature of the fundamental optical transition, i.e., for direct allowed transitions n = ${1}/{2}\;$, while for indirect allowed transitions n = 2 ^[41]. Assuming direct transitions, the value of the energy gaps are obtained by extrapolating the linear section of (αhν)² to α = 0, as shown in the Tauc plots in Figure 4. The respective values for the energy gaps are also reported in Table 2.

Several important points are to note. The energy band gap values for CsPbI₃ in Table 2 are lower than those reported by Clark ^[42] (E_g = 2.76 eV) for samples obtained by cooling melts containing the appropriate proportions of CsI and anhydrous PbI₂. CsPbI₃ is a well known and thoroughly investigated perovskite system; its orthorhombic lattice (space group Pnma) with a three-dimensional network. The orthorhombic lattice is retained up to temperatures of 563-602 K; at higher temperatures, it turns into cubic phase ((red arrow) according to previous data), whereas monoclinic phase occurs upon temperature decreases ((blue arrow) according to later detailed studies ^[30]) Cubic ← Tetragonal ← Orthorhombic ® Monoclinic. The higher energy band gap values have been explained with the formation and growth of Cs₄PbI₆ as secondary phase because the two compounds have very similar melting points ^[42,44].

Potassium-containing perovskites crystallize as KPbI₃·2H₂O solid phase with an octahedral crystalline structure (space group Pnma) and form thin flexible pale-yellow needles. An investigation of the phase diagram from solutions was already carried out by Talmadge ^[45]. Salau ^[46] investigated the optical absorption properties at room temperature of PbI₂-KI thin films prepared by crystals that were subsequently crushed into powders and used for evaporation purposes. Assuming a direct energy gap, calculations suggest (eq.1) E_g values of 2.32 eV for PbI₂ and 2.4 eV for KPbI₃, which are higher than the values obtained (2.19 eV) following preparation in aqueous solutions (Table 2).

Na-Pb-I systems, with a very low Pb content (with 10^-3 in weight), may be functionally used as phosphors; Baltog ^[47] reported a dependence of the luminescence properties as a function of Pb-content in the system. The introduction of Pb induced the formation of impurity aggregation. When the amount of the impurity aggregation reaches a critical concentration, the aggregation gives birth to a precipitation phase of the type PbI₂:nNaI. Consequently, an exciton-like emission appears in the luminescence spectrum with a corresponding shift of the absorption peak to a wavelength value above 500 nm, as confirmed by our absorption data and direct band gap model (Figure 4D).

Unlike Cs- and K-containing systems, NH₄⁺-, Na-, and Li-containing systems have been poorly investigated as possible materials for solar cells. The ammonium-containing system (NH₄PbI₃·2H₂O) crystallizes in space group Pnma as well ^[31,32], and forms thin pale yellow needles. The crystals are very similar to anhydrous CsPbI₃, the only obvious difference being the pronounced yellow color of the latter ^[32]. NH₄PbI₃·2H₂O is iso-structural to KPbI₃·2H₂O and demonstrates only minor differences in bond lengths and angles. The structures contain [PbI₃⁻] double chains of edge-sharing PbI₆ octahedra; the repeat periods of the [PbI₃⁻] chains and, consequently, certain I-Pb-I angles, vary considerably with the alkali ion. Since the alkali ions have their shortest distances parallel to the chain directions, the repulsion between them, which increases with the size of the ions, is most likely responsible for this effect. The anion chain has no significant hydrogen-bonding interactions with the cations. Previously only poorly investigated, the optical absorption properties of the NH₄PbI₃·2H₂O system have now been examined experimentally and are reported in Table 2. Recently, Filip ^[48] obtained a band gap of 1.9 eV for the NH₄PbI₃ system by using the single particle Green’s function G and the screened Coulomb interaction W (GW method) and Wannier functions to calculate the quasiparticle band gaps, band structures, and effective masses of lead iodide perovskites. The different band gap values may be explained by the crystalline structure of the material, as influenced by the presence of water during preparation.

The powder XRD diffractogram of the Li-Pb-I system revealed the formation of LiPbI₃·nH₂O (water content was not determined), but some PbI₂ remained unreacted under the current experimental conditions. This may explain the very similar profiles of the Tauc plots as well as the slight difference between the values of the optical energy gaps (2.32 eV and 2.36 eV for PbI₂ and LiPbI₃·nH₂O, respectively), as presented in Table 2. The collected data also illuminate novel properties of the LiPbI₃·4H₂O system, detailed in Table 2. Notably, the optical energy gap value and the XRD pattern in Figure 2F (enlarged in Figure S4, even with secondary phases) represent the first reported measurements for the Li-containing lead iodide system. Further analysis remains to be done to contribute to a deeper understanding of these systems.

Furthermore, the electronic structure of these materials as thin films in photoemission spectroscopy were studied. The films were prepared by dissolving the synthesized powder in di-methyl-formamide (DMF), spin-coating on FTO coated glass substrates, and heating at T = 80 °C for a few minutes in air. During the evaporation of the solvent, re-crystallization of the corresponding APbI₃ phase took place on the substrates. This was confirmed by the XRD analysis and optical measurements of the APbI₃-layers, as shown in Figure S1A and Figure S1B for the MAPbI₃ system. Indeed, the crystalline reflections of the deposited layer (Figure S1B) match the reflections of the starting powder (S1A). The higher intensity can be explained by higher crystallinity of the recrystallized material. For quantitative evaluation of the stoichiometry in these films, the most intense photoelectron line of every element was applied ^[49]. Detailed Pb4f and I3d XPS core level spectra are shown in Figure 5A and Figure 5B, respectively, while the core level spectra of the cations (Cs 3d, K 2p, Na 1s and N 1s) ^[52] are displayed in Figure 6.

The relative atomic concentrations were calculated from the integrated photoemission intensities using the atomic sensitivity factors according to Wagner and machine-specific parameters ^[52]. The spectral backgrounds were subtracted using a Tougaard-type function ^[50]. The Li1s photoelectron emission is very weak and, because of the overlap with the I4d emission line, the Li1s photoemission line could not be resolved. Besides the expected elements of the films tin, oxygen, and carbon, photoelectron lines were also detected. Tin and, in part, oxygen originate from the FTO-coating of the substrate due to imperfect coverage of the studied film. Oxygen may, in addition, stem from surface phases formed through interaction with humidity, whereas carbon may be introduced from ubiquitously adsorbed hydrocarbons in the atmosphere during preparation at ambient atmosphere. The results of the measured chemical compositions are summarized in Table 3. The experimental values of I/Pb and A/Pb atomic ratios in the APbI₃ thin films differ from the stoichiometry compositions of I/Pb = 3:1 and A/Pb = 1/1 used in solution. For I : Pb < 3 and A : Pb < 1 molar ratios, the presence of PbI₂ occurs as an additional phase, whereas for I : Pb > 3 and A : Pb > 1 ratios, the presence of A-iodide may be assumed. This could be explained by supposing that the fast evaporation of the solvent changes the thermodynamic and/or kinetic conditions of the crystallization process of the APbI₃ phases on the substrate, which favor secondary processes like formation of secondary phases.

In photoemission spectroscopy, the Fermi level is intrinsically used as the reference energy. As the Fermi level was found at slightly varied positions for individual samples of the same material, the core level spectra in Figure 5 and Figure 6 are displayed with respect to the valence band maximum for better comparison. On the other hand, the valence band spectra and the secondary edges displayed in Figure 7 are in reference to the Fermi level of the respective sample to illustrate the respective doping level. MAPbI₃ consistently displays a typical n-type behavior across all batches, whereas the materials with Cs, K, and NH₄⁺ demonstrate weak p-type and those with Na and Li present strong n-type behavior. According to these different positions of the Fermi level, it seems possible to shift the position of the Fermi level in the band gap by changing the process parameters, particularly the stoichiometry. Emara ^[51] found a relationship between the stoichiometry and the position of the Fermi level in MAPbI₃ system. The authors reported a wide variety of ionization energies, ranging from 5.67 to 6.4 eV due to changes in film preparation and film stoichiometry.

Moreover, Miller ^[56] and Schulz ^[57] reported independently that the position of the Fermi level energy can be influenced by the nature of the substrate even by depositing thick layers of MAPbI₃. In order to minimize the influence of the substrate, all samples have been deposited on F:SnO2-layered glass substrates. XRD measurements confirm that the excess material is incorporated not as secondary phases but rather as interstitials incorporated into the perovskite structure, without showing additional diffraction patterns. Such interstitials and variations in composition can be assumed to be the reason for the widely differing values of ionization energies and perovskite device efficiencies reported in the literature. The core level binding energy values are listed in Table 4. The Pb4f doublet spectra show the Pb4f_7/2 emission around 137 eV below the respective valence band edge. The spin orbit splitting of 4.87 eV is in agreement with the value in literature of 4.86 eV ^[52]. The measured binding energies E_B confirm that the lead ions are in ionization state +2 ^[53] as expected for the APbI₃ systems; however, weak emissions shifted to lower binding energies have been observed for the Na- and Li-containing systems (135 eV and 140 eV, respectively), tentatively attributed to elemental lead (Pb⁰) and Pb⁴⁺, respectively. Although the growth and the effects of Pb⁰ are not completely understood, its presence can be explained by the exposure to moisture, as reported by Lindblad ^[49] for CH₃NH₃PbI₃, and also thermic decomposition by formation of lead clusters ^[58]. In the Li compound, a weak and broad emission doublet at 2.4 eV higher binding energy is observed. This emission is assigned to a shake-up satellite due to excitation of an electron from the valence to the conduction band in the process of emitting the Pb4f photo-electrons. The binding energy of the I3d_5/2 and I3d_3/2 states are 618 eV and 629.5 eV, respectively, indicating an oxidation state of (−1). The splitting is independent of the respective cation A and with 11.50 eV close to standard values for iodine ions ^[52]. In the Li-system, weak, broad side emissions at 2.2 eV higher binding energy are found, similarly attributed to shake-up satellites of the I3d photo-electrons, as observed for the Pb4f emission in this system. The binding energy differences between Pb4f_7/2 and I3d_5/2 (Table 3) are very similar (481 eV), indicating that the partial charges of Pb and I are nearly independent on the respective cation A. The XPS spectra of the alkaline metal ions are shown in Figure 6. Due to spin-orbit coupling, the XPS spectra show two emission lines (doublet) for Cs3d and K2p and one emission line (singlet) for Na1s and N1s. Furthermore, all cations reveal valence state +1, as expected ^[53]. On the other hand, the photoelectron emission of Li1s (at 56 eV) is much weaker by a factor of about 100 than that of I4d_3/2 (51 eV); for this reason, a resolution of the Li1s photoelectron emission in the XPS spectra could not be performed and analyzed.

The XPS valence band spectra are displayed in Figure 7. The line shape of the upper valence bands is quite similar, and their width is about 4.5 eV. These states are formed mostly by the iodide 5p orbitals and in minor content of Pb6s states of the PbI₆ octahedra ^[55], which also explains the constant energy distance of the iodine core level with respect to the valence band maximum. For CH₃NH₃PbI₃, it is known from calculation and IPES measurements that the PbI₆ octahedra are responsible for the valence band and the conduction band ^[57,59]. The band gap is primarily dependent on the orientation of these octahedra to each other ^[1]: face- and edge-sharing materials (as PbI₂) have a band gap >2 eV, while corner-sharing materials, such as CH₃NH₃PbI₃ or CsPbI₃, have a smaller band gap <2 eV.Unfortunately, it was impossible to resolve the structure to such a level to confidently conclude the electronic structure.

The work functions were measured via the secondary electron edge by He(I) excitation energy (21.22 eV). Studies ^[59,60] which determined the band gap for CH₃NH₃PbI₃ found a slightly higher band gap (1.7 eV) if IPES/UPS is used compared to optical measurements and EQE measurements (1.5-1.6 eV); a similar behavior may also be observed for the alkali systems. But, the studied materials are direct semiconductors as the Tauc-plots confirm. Therefore the band gaps, determined by the optical measurements are reliable and were usedto draw the band structure. The measured values are summarized in Figure 8. While the ionization energy values are quite similar (6.15 ± 0.25 eV), the work function varies due to variation in the Fermi-level positions of the unintentionally doped materials for different cations and different batches with the same cation. These first data suggest that p-type systems can be prepared for NH₄, Cs, and K, and n-type for Na and Li.

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4. Conclusions

This work presents an investigation of layers of APbI₃ systems (A = alkali metals, ammonium, methylammonium (MA⁺) (as reference)) prepared by self-organization processes from solution at low temperature. The morphological analysis revealed the influence of the respective cation on the crystalline habit of the materials. For A = Cs and NH₄⁺, the crystallites generally demonstrated needle-like morphology with lengths up to 3-4 mm, whereas for A = K, they exhibited a thin stripe-like structure with lengths up to 5-6 mm. On the other hand, the crystallites of MAPbI₃ generally formed dodecahedra, which is a typical crystalline habit of a body-centered tetragonal lattice. By decreasing the ionic radius, the APbI₃ systems tend to form polycrystalline aggregates, such as in the sodium- and lithium-containing systems. In the X-ray diffractograms, the crystalline reflections were indexed on the basis of an orthorhombic structure (space group Pnma), whereas for A = MA⁺, the crystalline reflections indicate the formation of a tetragonal structure at room temperature (space group I 4/m or I 4/mcm).

Optical absorption measurements on the primary synthesized materials indicate that Cs-, NH₄-, K-, and Na-containing systems behave like direct-gap semiconductors with energy band gaps ranging between 2.19 eV and 2.40 eV at room temperature. As the energy gap is determined mainly by the PbI₆ octahedra and their crystal structure, common to all investigated systems (except the MAPbI₃), the similar values of the energy band gaps are related to a minor effect of the cation A on the valence and conduction band structure. The XPS spectra show the iodine and lead ions in formal oxidation state −1 in +2, respectively. As the Pb4f_7/2 I3d_5/2 binding energy differences are very similar (481 eV), the partial charges of Pb and I are only weakly dependent on the respective cation A. All cations A are in +1 oxidation state. The valence bands measured with X-ray excitation show very similar line shapes from the valence band maximum (VBM) to 4.5 eV below, in agreement with common PbI₆ octahedra in the investigated systems. The ionization energies are around 6.1 ± 0.2 eV; unintentionally doped films formed from dissolved primary synthesized materials show degenerate n-type for the A = MA_, strong n-type for A = Na and Li, and weak p-type for A = NH₄, Cs, and K.

This work investigated the structural, microstructural, optical, and electronic features of APbI₃ systems with different positive monovalent cations “A”, which were prepared under very similar conditions. According to our results, the positive cations begin to influence system properties as early as the preparation of the materials.

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Acknowledgements

The Authors thank the Federal Ministry of Research and Development (BMBF) for the financial support during this work. Many thanks are due to Mr. J.-C. Jaud for the technical assistance in the XRD analysis and Mrs. C. Spanheimer for the technical assistance in the SEM analysis. Also thanks to Julianne McCall for editing the manuscript.

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Conflict of Interest

The authors declare that there is no conflict of interest regarding the publication of this manuscript.

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