Transition Metal Ions in Methylammonium Chloride Perovskites.

Organic-inorganic perovskite materials have become star materials for future wide band gap optoelectronics due to their excellent optical and electrical properties. However, the lead ions inside perovskites have become a crucial environmental issue in the commercialization of wide band gap perovskite devices . This research tries to find the structure and properties of lead-free perovskite materials by screening Sn2+ and transition-metal ions to replace Pb2+ within the methylammonium (MA)-based chloride perovskite and find out a new two-dimensional structure of MA-based transition-metal ion chlorides. Overall, MAZnCl3 may be a potential ultraviolet-C luminescent material with a stable two-dimensional structure with a wide band gap of 5.64 eV, which is suitable for ultraviolet-C luminescence applications.

Introduction

Organic–inorganic perovskite materials have advantages such as high flexibility and low formation temperature and are therefore suitable for wide band gap optoelectronics.[1−4] With the probability of pandemic outbreaks increasing year by year,[5,6] research on convenient disinfection methods[7−10] and wide band gap devices and related materials deserves more attention. Due to their environmentally friendly nature and wide band gap applications, lead-free,[11−13] two-dimensional[14−17] perovskites merit more attention and investigation. Indeed, many transition metals are listed in the periodic table with the same positive divalent ion as Pb2+, making researchers doubt their possibilities. In the past, the Goldschmidt tolerance factor was a primary perovskite method to identify the possible cation according to the ionic radius. The value of the Goldschmidt tolerance factor of the structures should be between 0.85 and 1, so that the perovskite could remain in the cubic system, or deformation might happen.[18,19] However, the fine details of the possible structures and properties of the transition-metal ion perovskite need to be entirely investigated for the deformation of perovskite. The possible transition-metal ion perovskite could be revealed by the density functional theory calculation and the data of lead-free perovskite.

The structure or geometry optimization calculation of density functional theory is well known for estimating the possible crystal structures of unknown crystals and offering detailed information of the stable atom positions. Therefore, the DFT calculation is suitable for investigating the related electron[20−23] and phonon[24−26] properties from an optimized structure. This research figures out the phonon and electron properties of the unknown transition metals in methylammonium (MA) perovskite, with the halide set as chloride, through DFT, and elucidates the detailed information of crystals.

The phonon dispersion diagram and band structure were calculated to identify the optimized transition-metal ion perovskite structures with MA), stability, and semiconductor properties. Here, MA-based perovskites are chosen because of their better structure stability compared to the cesium (Cs+) and formamidinium (FA+) ions. The phonon dispersion diagram can reveal the dynamic crystal stability according to the number of imaginary phonon states, and dynamic stability relates to the phase stability. MAPbCl3, Pm3̅m, is a wide band gap perovskite (thin film with 3.17 eV[27]) with relatively stable property,[28,29] and the crystal template of this research uses MA-based chloride perovskites for the screening of transition-metal ions. Sn2+ and the stable same oxidation state ions of Pb2+ (such as Cd2+, Ni2+, Cu2+, Hg2+, Fe2+, Zn2+, Mn2+, and Co2+) were screened by this research.

Results and Discussion

The optimized structures of MAPbCl3, MASnCl3, and transition-metal ion perovskites, with the same stable oxidation state as that of the lead ion (MACdCl3, MANiCl3, MACuCl3, MAHgCl3, MAFeCl3, MAZnCl3, MAMnCl3, and MACoCl3), are shown in Figure a–j. The diameter difference of different cations deforms the Pm3̅m structure of the MAPbCl3 template shown in Figure a, and forms two-dimensional materials shown in Figure b (MASnCl3), Figure d (MANiCl3), Figure e (MACuCl3), Figure f (MAHgCl3), Figure g (MAFeCl3), Figure h (MAZnCl3), Figure i (MAMnCl3), and Figure j (MACoCl3). Only MACdCl3 in Figure c can retain the three-dimensional structure. This indicates that the MA molecular structure being too long for the Sn2+, Ni2+, Cu2+, Hg2+, Fe2+, Zn2+, Mn2+, and Co2+ ions undergoes deformation by breaking the three-dimensional structures at the (100) crystal plane and slicing the crystal into two-dimensional structures.

Figure 1

Optimized structures of MAPbCl3 (Pm3̅m) and the transition cation replacing structures (a) MAPbCl3, (b) MASnCl3, (c) MACdCl3, (d) MANiCl3, (e) MACuCl3, (f) MAHgCl3, (g) MAFeCl3, (h) MAZnCl3, (i) MAMnCl3, and (j) MACoCl3.

The transition-metal ions in perovskites might offer a potential research direction for discovering new 2D materials, and the optimized structure information is listed in Tables and 2. The radius of transition-metal ions is listed in Table . Indeed, Pb2+ has the largest ionic radius of 1.33 Å (Table ), compared to the lower ionic radii of Sn2+ (1.18 Å), Ni2+ (0.69 Å), Cu2+ (0.73 Å), Hg2+ (1.19 Å), Fe2+ (0.61 Å), Zn2+ (0.74 Å), Mn2+ (0.67 Å), and Co2+ (0.65 Å) cations, and so could not maintain the three-dimensional structures, thereby causing the MA molecule to break and slice the three-dimensional structures at the (100) surface to form two-dimensional structures. The Cd2+ ion has a radius of 0.95 Å, different from the radius of Pb2+, so MACdCl3 could retain the three-dimensional structure, as shown in Figure c. According to Table , the Goldschmidt factor of MACdCl3 is 0.98, the value of a normal three-dimensional structure, whereas the Goldschmidt factor between 0.85 and 1 is for a cubic structure. Therefore, the final structure can endure the MA molecular. This result increases the understanding of the ion replacement of MA-based perovskites and offers more information beyond the Goldschmidt tolerance rule of perovskites. These results could explain that the transition-metal ions could not be suitable doping elements, generally for high-efficiency perovskite solar cells and LED devices.

Table 1

Geometry-Optimized Crystal Parameters of the Structures given in Figure

Table 2

Geometry-Optimized Atomic Positions of the Structures in Figure , from Top to Down—x, y, and z

Table 3

Metal Ion Radius, Energy of the System (E0), Imaginary Part of Phonon DOS, and Debye Temperature (ΘD) of Different Structures

For MA-based transition-metal ion chlorides, the energy of the crystal system (free energy) is calculated with the same functional as that used for phonons, and the values are listed in Table . The free energies of the structures are MAPbCl3 (−64.25 eV), MASnCl3 (−54.22 eV), MACdCl3 (−50.94 eV), MANiCl3 (−53.22 eV), MACuCl3 (−51.57 eV), MAHgCl3 (−48.90 eV), MAFeCl3 (−55.80 eV), MAZnCl3 (−51.60 eV), MAMnCl3 (−56.61 eV), and MACoCl3 (−54.78 eV). MAPbCl3 has the lowest free energy and becomes the most stable crystal in the structures. The dynamic stability of the phonon should identify the phase stability of each crystal. The phonon dispersion diagram and the phonon density of states (DOSs) could identify the dynamic crystal bonding vibration condition and indicate the phase stability. The fewer imaginary phonon states show better dynamic stability of the crystal, and the structure has less possibility to undergo phase transition to another phase. This is due to the bonding vibration within the crystal not giving out energy at a certain point but absorbing the heat energy outside. If suppose the bonding vibration keeps giving out energy at a certain point, then the bonding will break by the vibrational energy loss in a certain direction, and the phase transition happens toward a stabilized bonding arrangement. Phonon dispersion and phonon DOS diagrams of the structures are shown in Figure a-j, and the imaginary phonon density of states (DOS) of the structures are listed in Table . Here, the imaginary part of phonon DOS, by counting the portion of imaginary phonon DOS in the whole phonon states, is used for the whole dynamic structure consideration. The Goldschmidt factors are also listed in Table according to the radius of the MA+ cation as 2.62 Å and the Cl– anion as 1.67 Å.

Figure 2

Phonon dispersion, phonon density of states, and temperature-dependent heat capacity diagrams of the structures. (a) MAPbCl3, (b) MASnCl3, (c) MACdCl3, (d) MANiCl3, (e) MACuCl3, (f) MAHgCl3, (g) MAFeCl3, (h) MAZnCl3, (i) MAMnCl3 (j) MACoCl3 and (k) temperature-dependent heat capacity diagram of the structures.

For two-dimensional MA-based transition-metal ion chlorides, MANiCl3 (Figure d) and MAMnCl3 (Figure i) might be the two most dynamic unstable structures, with abundant imaginary phonon states. Indeed, the imaginary part of phonon DOS is listed in Table , and the values of MANiCl3 and MAMnCl3 are 14.63 and 10.94%, respectively. Even though MACdCl3 has the three-dimensional structure, as shown in Figure c, 2.79% of imaginary phonon DOS indicates that the unstable dynamic conduction might decompose eventually. Two-dimensional MAZnCl3 has 0.97% imaginary part of phonon DOS, lower than that of MAPbCl3 (1.78%), as presented in Table , indicating that MAZnCl3 has better dynamic structural stability and has the potential for the device application. The temperature-dependent diagram in Figure k is calculated as the same functional as phonon and used to find out the Debye temperature of each crystal. For a solid-state material, the Debye temperature has a square mathematical relationship with the melting temperature, and a higher Debye temperature might accompany a higher melting temperature.

Furthermore, a higher melting temperature is accompanied by a higher phase formation temperature, and the Debye temperature calculation could offer the essential estimation of the crystal formation temperature. The Debye temperatures (ΘD) of each structure are MAPbCl3 (ΘD = 102 K), MASnCl3 (ΘD = 106 K), MACdCl3 (ΘD = 131 K), MANiCl3 (ΘD = 173 K), MACuCl3 (ΘD = 134 K), MAHgCl3 (ΘD = 93 K), MAFeCl3 (ΘD = 144 K), MAZnCl3 (ΘD = 106 K), MAMnCl3 (ΘD = 161 K), and MACoCl3 (ΘD = 123 K), which are listed in Table . The 2D MAZnCl3 structure, with comparable dynamic stable property, has a Debye temperature (106 K) nearly the same value as MAPbCl3 (102 K). In addition, the slight difference in the Debye temperature between MAZnCl3 and MAPbCl3 reveals that MAZnCl3 could be synthesized at the same temperature as MAPbCl3. The information could be helpful for future MAZnCl3 device applications.

The band structures and densities of states (DOSs) of MAPbCl3, MASnCl3, and transition-metal ion MA chlorides are shown in Figure , and the exchange–correlation functional of DFT is chosen as meta-GGA-MBJLDA. It is not only that meta-GGA-MBJLDA needs less expensive computational requirement with an accuracy similar to those of hybrid functionals or GW methods but also that the calculation result of MAPbCl3 by meta-GGA-MBJLDA is 3.17 eV, as shown in Figure a, which is the same as the MAPbCl3 thin-film experimental value of 3.17 eV.[27] Therefore, meta-GGA-MBJLDA is a suitable exchange–correlation functional of DFT for screening and estimating the band structures of MASnCl3, and the chemical composition of transition-metal ion MA chloride is similar to that of MAPbCl3. Figure shows the DFT band structures of MAPbCl3 (Eg = 3.17 eV, direct), MASnCl3 (Eg = 2.45 eV, direct), MACdCl3 (Eg = 3.70 eV, indirect), MANiCl3 (Eg = 0.45 eV, indirect), MACuCl3 (Eg = 0 eV), MAHgCl3 (Eg = 2.98 eV, indirect), MAFeCl3 (Eg = 2.28 eV, indirect), MAZnCl3 (Eg = 5.64 eV, direct), MAMnCl3 (Eg = 0 eV), and MACoCl3 (Eg = 0 eV). For MACuCl3, MAMnCl3, and MACoCl3 shown in Figure e,i,j, bands cross the Fermi level, and the electron can easily move up and down across the Fermi level. The three structures have semimetal-like band structures, and even using a different functional, GGAPBEsol, the three structures still have the same semimetal band structures, as shown in Figure S1e,i,j.

Figure 3

Band structure and density of states of the structures. (a) MAPbCl3, (b) MASnCl3, (c) MACdCl3, (d) MANiCl3, (e) MACuCl3, (f) MAHgCl3, (g) MAFeCl3, (h) MAZnCl3, (i) MAMnCl3, and (j) MACoCl3.

In Figure b,f,g, the Sn2+, Hg2+, and Fe2+ ion replacements of the Pb2+ ion will reduce the band gap, even though Hg2+ is not environmentally friendly, and the Sn2+ and Fe2+ ions could be good candidates for reducing the perovskite band gap in future two-dimensional optoelectronic device applications. In Figure c, the three-dimensional MACdCl3 structure has a slightly wider band gap of 3.70 eV, indicating that the Cd2+ cation replaces Pb2+ within MA-based perovskites, which might increase the band gap. In Figure d, two-dimensional MANiCl3 has a 0.45 eV band gap with a near-infrared range spectrum, emitting 2756 nm wavelength as an indirect band gap. The ultra-small band gap might be due to the isolated low conduction band structure above the Fermi level. This research might find a near-infrared material during 2D perovskite material screening and indicated that MANiCl3 could be used for making the near-infrared detector device. The two-dimensional structures of MACuCl3, MAMnCl3, and MACoCl3 have zero band gap, exhibiting metal-like properties, as shown in Figure e,i,j. The DOS diagrams in Figure e,i,j also show many states crossing the Fermi levels for MACuCl3, MAMnCl3, and MACoCl3, and the crossing bunch of states at the Fermi level are interesting electrical structures. It is a kind of bridge between the electron and hole states, exhibiting metal-like properties. Finally, MAZnCl3 might be the good predictable ultraviolet-C luminescent material with a wide direct band gap, 5.64 eV, at the gamma point, as shown in Figure h. Table lists the detailed band structure information, and N/A means no electron transition between the valence and conduction bands in two-dimensional MACuCl3, MAMnCl3, and MACoCl3. Due to the uncommon ultrawide band gap and near-infrared properties, the electrical properties and stabilities of two-dimensional MAZnCl3 and MANiCl3 need to be investigated further for ultraviolet-C and near-infrared device reference.

Table 4

Photon Behavior of the Structures in Figure

The whole temperature range of temperature-dependent electrical conductivity and carrier mobility of the structures is shown in Figure a–c. The calculation is performed by the GGA-PBEsol exchange–correlation functional in DFT. The optimum device working temperature is found to be in the range of 280–380 K, as shown in Figure d–f. The electrical conductivity and carrier mobility values are listed in Tables and 6. The experimental electrical conductivity and carrier mobility of thin-film MAPbCl3 are 1.8 × 10–6 (Ω –1 m–1) and 4.14 (cm2 V–1 S–1),[30] respectively, and the calculation results from this research of MAPbCl3 are 1.22 × 10–6 (Ω –1 m–1) and 3.1322 (cm2 V–1 S–1), listed in Tables and 6. The experimental measurements of electrical conductivity and carrier mobility of MAPbCl3 are conducted at room temperature, and the material screening calculation is set at room temperature. Therefore, the calculation result of MAPbCl3 is similar to the experimental measurement result and offers a relatively referable value of electrical conductivity and carrier mobility for the unknown perovskite-related structure screening. In Figure a, MACuCl3, MAMnCl3, and MACoCl3 have higher temperature-dependent electrical conductivities than other screening materials, around 10 000 to 100 times, and need a separate demonstration. In Figure b, the whole temperature ranges of temperature-dependent electrical conductivity of the semiconductor ranges, such as MAPbCl3, MASnCl3, MACdCl3, MANiCl3, MAHgCl3, MAFeCl3, and MAZnCl3, are shown. Indeed, the electrical conductivities of semiconductors will increase with the increase in temperature, and the carrier mobility values show a direct value jump when the temperature is higher than the activation temperature, and different screening materials have different activation temperatures. However, the whole temperature range diagram of electrical conductivities and carrier mobilities is not easy to compare. In Table , two-dimensional MACuCl3 and MACoCl3 have the highest two electrical conductivities as 4.68 × 104 (Ω–1 m–1) and 1.83 × 104 (Ω–1 m–1) at room temperature, much higher than the typical semiconductors. As a result, two-dimensional MAMnCl3 has an electrical conductivity of 110 (Ω–1 m–1) at room temperature, higher than the standard semiconductor value. For the potential ultraviolet-C luminescent material, MAZnCl3 has an electrical conductivity of 3.01 × 10–8 (Ω–1 m–1) in Table , which is lower than that of MAPbCl3, which is 1.22 × 10–6 (Ω–1 m–1). MAZnCl3 has a carrier mobility of 0.0396 (cm2 V–1 S–1), which is lower than that of MAPbCl3, which is 3.1322 (cm2 V–1 S–1). As a result, the two-dimensional MAZnCl3 is transformed to MAPbCl3, with Zn2+ replacing Pb2+, which has a band gap property of 5.64 eV and an luminescence emission wavelength of 220 nm, with an electrical conductivity and a carrier mobility of 3.01 × 10–8 (Ω–1 m–1) and 0.03961 (cm2 V–1 S–1), respectively.

Figure 4

Temperature-dependent electrical conductivity and carrier mobility of MaPbCl3 and transition cations replacing structures. Temperature-dependent electrical conductivities (0–1000 K) of (a) MACuCl3, MAMnCl3, and MACoCl3 and (b) MAPbCl3, MASnCl3, MACdCl3, MANiCl3, MAHgCl3, MAFeCl3, and MAZnCl3. (c) Temperature-dependent carrier densities (0–1000 K) of MaPbCl3 and transition cations replacing structures. The device working temperature range and temperature-dependent electrical conductivities of (d) MACuCl3, MAMnCl3, and MACoCl3 and (e) MAPbCl3, MASnCl3, MACdCl3, MANiCl3, MAHgCl3, MAFeCl3, and MAZnCl3. (f) Temperature-dependent carrier densities (280–380 K) of MaPbCl3 and transition cations replacing structures.

Table 5

Temperature-dependent Electrical Conductivity of the Structures in Figure d,e

Table 6

Temperature-dependent Carrier Mobilities of the Structures in Figure f

Conclusions

In summary, in this research, MAPbCl3 was chosen as a screening template and the Pb2+ ion was replaced with Sn2+ and transition-metal ions with the same stable oxidation state as Cd2+, Ni2+, Cu2+, Hg2+, Fe2+, Zn2+, Mn2+, and Co2+. The MA molecular became too long under lower ionic radius conditions, and the structures deformed to two-dimensional structures, except MACdCl3. Indeed, the DFT calculation shows that MAPbCl3 has a band gap of 3.17 eV, with an electrical conductivity of 1.22 × 10–6 (Ω–1 m–1) and a carrier mobility of 3.1322 (cm2 V–1 S–1), which are very close to the actual experimental values and show that the calculated values of this study could be taken as the reference. According to the lower imaginary states of two-dimensional MAZnCl3 phonon DOS with an ultra-wide band gap (5.64 eV), MAZnCl3 is a stable structure for future ultraviolet-C luminescence applications. The calculated electrical conductivity and carrier mobility of MAZnCl3 are 3.01 × 10–8 (Ω–1 m–1) and 0.0396 (cm2 V–1 S–1), respectively. Finally, this research points out that MAZnCl3 is a stable structure and has the potential for ultraviolet-C optoelectronic applications.

Methods

The calculations were processed by VASP (Vienna ab initio simulation package)[31,32] with different exchange–correlation functionals, GGA-PBEsol[33] and meta-GGA-MBJLDA,[34,35] for determining their semiconductor and thermodynamic properties. After processing, structure optimization was performed for new structures based on the GGA-PBE[36] with 520 eV plane-wave cut-off energy, k-spacing with a 5 × 5 × 5 mesh, and first-order Methfessel–Paxton smearing with a width of 0.2 eV. GGA-PBE calculated the phonon behaviors of the atomic models with a plane-wave cut-off energy of 500 eV, k-spacing as 5 × 5 × 5 mesh, and a linear tetrahedron method with Bloechl corrections to the energy. For the electronic property (electrical conductivity, carrier mobility, and carrier density) calculations, GGA-PBEsol was set a plane-wave cut-off energy of 400 eV and k-spacing with a 3 × 3 × 3 mesh and used with the chemical potential of 12 mu in total 30 mu. For electronic band structure results, meta-GGA-MBJLDA was used with a plane-wave cut-off energy of 500 eV, a k-spacing of 3 × 3 × 3 mesh, and a linear tetrahedron method.