Perovskites are generally in the shape of orthorhombic, of which the structural formula is ABX3 (A =organic ammonium cation acted as ligand with metal in complex, CH3NH3+; B = Sn2+ or other metals; X = Cl-, Br-, I-) [1-3]. The electrical and optical features of Pb-based perovskites has expressed approximately perfect solar cells [4, 5]. Perovskites solar cell's latest efficiency has reached 23.7% [6, 7], which exceeds Cu(In,Ga)(Se,S)2, CdTe and Si-based solar cells [8, 9]. Nevertheless, two main concerns of Pb-based perovskites solar cells are: Poor stability and high toxicity [10, 11]. The stability challenge of these perovskites is undertaken by using two-dimensional (2D) perovskites, as well as advanced device encapsulation and engineeringapplications [12, 13]. The development of less toxic Pb-free materials is certainly always a priority in the solar cell market even if performance is not compromised much. Ideal Pd-free candidates as solar cell absorbers should have less toxicity, narrow direct band gaps , high optical-absorption coefficients , high mobility , low exciting-binding energies , longer charge-carrier life times, and good stability [18, 19]. Further they have interesting features including less-toxicity constituents for making perovskite structures, such as Sn/Ge-based halides , some double perovskites [21, 22], and some Bi/Sb-based halides  with perovskite-like structure.
Among these contestants, Sn-based perovskites have attracted the most attention because of their custom-made properties and the most promising performance achieved by their devices . The direct band gaps of around 1.20 eV, 1.41 eV and 1.3 eV for the representative methylammonium tin iodide (MASnI3) , formamidinium tin iodide (FASnI3) , and cesium tin iodide (CsSnI3) , respectively, which are even narrower and more attractive than those of their Pb-based analogs. After being exposed to the air in the atmosphere, Sn-based Perovskites Sn4+ became SnO2 that is an eco-friendly material . A quick inspection of all the basic physical features of Sn-based perovskites reveals a significant similarity by making a comparison with Pb-based ones . Therefore, at least in principle these materials should be able to match the photovoltaic properties of the ABX3 (such as APbI3) elements.
After the ﬁrst report on Sn-based perovskite solar cells which achieved about 6% power conversion efficiency (PCE) , many reports have focused on Sn-based perovskites solar cells. It has been reported that 9.6% is the highest efficiency of Sn-based perovskites solar cells. Keeping an eye on photovoltaic parameters, Sn-based perovskites solar cells usually have high short-circuit current densities of 20 to 25 mA cm-2 due to their low band gaps . However, the average of open-circuit voltage (Voc) of the Pb-based analogs is 1.1 V whereas the average of open-circuit voltage (Voc) of the solar cells is only around 0.5 V, which is much lower in value . Heavy P-type doping of materials is the major reason for the lower Voc of Sn-based perovskites solar; heavy doping is responsible for the highly complex and unwanted oxidization of Sn2+ to Sn4+ [33, 34], which works as a P-type dopant in the structure resulting in excessively high dark-carrier concentration and very high photo carrier recombination.
It has been recently found that, adding ethylenediammonium (EN) cations gives birth to a new type of three- dimensional (3D) so-called hollow perovskites of SN-based perovskites , which can tune the band gap from 1.3 to 1.9 eV . As a result, with the of addition 10% EN the 3D hollow Sn-based perovskites solar cells achieved a much higher Voc’s and PCEs than the standard ASnI3 perovskites (0.48 V and 7.14% vs. 0.15 V and 1.4%) .
A recent study achieved 9.6% recorded efficiency by adding 1% en combining 20% guanidinium (GA) . Other cations, such as phenylethylammonium (PEA) and butylammonium (BA), are also attractive in low-dimensional structures. The representatives of 2D (PEA)2(MA)n-1SnnI3n+1 and (BA)2(MA)n-1SnnI3n+1 perovskites have much better stability due to protecting the organic gap levels. In particular, recent reports have described that high performance typically uses 2D Sn-based perovskites with a reverse device structure, with the highest PCE providing 9.4%. In the Sn-based perovskites solar cells, using both 3D hollow and 2D perovskites in the same films may significantly progress the stability. The most stable of Sn-based perovskites is Cs2SnX6 (X =I, Br) which is a peculiar compound having a stable Sn4+. Therefore, it does not oxidize. The major disadvantage is that it does not have a fully 3D perovskites structure. Cs2SnI6 has a narrow direct band gap of 1.3 to 1.6 eV .
These materials have been used in different kinds of optoelectronic devices, including solar cells [20, 38], light- emitting diodes (LED) , lasers [40, 41], photo detectors , photocatalysis [43-46], electric device [44, 47-51] and even chemo sensors . Comparing with commercial silicon-based photo detectors, the perovskites photo detectors have demonstrated superior performance and application prospects. Recent research shows an ultra-high response exceeding 104 AW-1 and a short response time of 4 electrons per photon [42, 53].
No theoretical studies are available for characterization of CH3NH3SnBr3, according to the best of our knowledge. This paper covers the complementary study of CH3NH3SnBr3 including the detailed investigation of structural, electronic and optical and thermodynamic properties using first principles-based Density Functional Theory (DFT). We also report, for the first time, the main features including band gap energy, ionized donor density, etc. which are studied on the basis of the absorption spectra and polarization curves.
In this study, band structure has been studied using GGA with PBE method which has been considered the most accurate functional from computational tools. Secondly, it illustrates that how the s, p and d orbitals contribute in valance band and conduction band even band gap. Finally, it can be said that CH3NH3SnBr3 is an eco-friendly material and its optical properties are accounted as best supportive properties for light absorption and can be used as photovoltaic cell.
Material and methods
Initially, the geometric optimization was achieved before energy calculation where the convergence criterion for the force between atoms was 3×10−6 eV/A˚, and the maximum displacement, the total energy and the maximal stress were at 1×10−3A˚, 1×10−5 eV/atom and 5×10−2 GPa, respectively. After geometry stabilization through molecular optimization, the method of GGA with PBE functional was used from CASTEP code of the material studio version 8.0 (54) for calculating the electronic band structure, total density of state (TDOS) and partial density of state (PDOS), because this functional has been considered the most feasible method for calculating the electronic and structural properties (55). In this condition, the band structure and density of state were enumerated using the cut off at 500 eV, and k point at 2×2×2 with norm-conserving pseudopotentials. Next, the optical features, such as refractive index, reflectivity, absorption, conductivity and loss function, were similarly simulated for calculation. In addition, for the comparative study of band gaps for CH3NH3SnBr3, the Generalized Gradient Approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) (56) functional was investigated using required cut off energy at 500 eV, and k point at 2×2×2 with nom-conserving pseudopotentials of required structures.
Result and Dissection
Geometry of optimized structure
The lattice parameters value for CH3NH3SnBr3 are a = 8.422 Å, b = 8.547 Å, c = 12.184 Å and angels between them as α= 90.000°, β= 90.000°, γ= 90.000°. The CH3NH3SnBr3 crystal and the space group is Hermanna Mauguin Pnma , orthorhombic crystal system, point group mmm, hall -P 2ac 2n, density 3.63 g/cm3 shown in figure 1.The optimize structure of materials showing in from the materials studio after optimizing their crystal structure.
The electronic band gap plays significant role of semiconductor for determining the color and conductivity of materials to reveal their absorption capacity of visible or UV and ultraviolet light. The maximum valence band (MVB) and the minimum conduction band (MCB) is closely attribute to the Lowest Unoccupied Molecular Orbital (LUMO) and Highest Occupied Molecular Orbital (HUMO) are shown in figure 2 and figure 3. The energy range of the band gap 1.1 eV to 1.5 eV that absorb all color of visible light. The Fermi energy level has been set at 0 eV for determining the electronic band structure of CH3NH3SnBr3 as shown in figure 4. The band gap has been reported 1.421 eV using the most accurate GGA with PBE method. It has been observed that higher energy state of valence band was in found in G symmetry point as well as the lower energy state of conduction was obtained in G point that reveals it follows the nature of direct band gap so that the electron can easily shift maximum valence band to the minimum conduction band without changing the momentum.
Density of states and Partial density of state
The method GGA with PBE has been applied to evaluate the density of states (DOS) or partial density of states (PDOS) of CH3NH3SnBr3 compound for determining the nature of electronic structure properly as well as scattering of orbital. Figures 5 to 10 depict the total density of states and partial density of states as a function of photon energy that are closely related the Highest Occupied Molecular Orbital (HOMO), Lowest Unoccupied Molecular Orbital (LUMO) and EHOMO-ELUMO gap. In Figure 6, for PDOS of CH3NH3SnBr3 it consists of 2p2 for C, 1s1 for H, 2p3 for N, 5p2 for Sn, and 4p5 for Br atoms. It has been found that the highest electron density in the valence band at about 55 electrons/ eV for p orbital and s orbital provide some contribution bellow Fermi energy. In the conduction band, the strong contribution of hybridization provides 5p2 for Sn atom in the photon energy range between 1 eV to 5 eV, also 2p3 orbital for N atom is provide moderate contribution. So that Sn metal is responsible for lower energy band gap which is a crucial point in this research. Finally, it can be seen that the p orbital is the higher contributor for both valence and conduction band of CH3NH3SnBr3 lead free perovskites. Figures 7 to 10 represent the comparative study of s, p, and d orbitals for CH3NH3SnBr3.
To compare its opto-electronic properties for CH3NH3SnBr3 in case of electronic structure is very close to CsSnCl3, CsPbBr3, CH3NH3PbBr3, NH2CHNH2SnBr3 which was estimated as opto-electronic materials .
Figure 1: Optimized structure of CH3NH3SnBr3
Figure 2: LUMO (CB)
Figure 3: HOMO (VB)
Figure 4: Electronic structure for CH3NH3SnBr3
- Ø For HOMO, Blue color indicates the positive node and yellow color mentions the negative color of orbital.
- Ø In case of LUMO, deep radish color is positive and deep orange is negative.
- Ø CB = Conduction band and VB = Valance band.
Figure 5: Total density of states
Figure 6: Partial density of states
Figure 7: C atom for CH3NH3SnBr3
Figure 8: N atom for CH3NH3SnBr3
Figure 9: Sn atom for CH3NH3SnBr3
Figure 10: Br atom for CH3NH3SnBr3
Photovoltaic and chemical properties
As one of the most important properties of photovoltaic, maximum open circuit voltage (Voc) may be defined as electrical potential difference between two ends of an electric device or materials. When it has been disconnected from any circuit, it has to be related the term of HOMO and LUMO of the organic solar cell. In addition, it is the difference of HOMO and LUMO or energy gap between valance band and conduction band into account the energy lost during the photo-charge generation. The mathematical expression for calculating the magnitudes of open-circuit voltage (Voc) has been calculated from the following equation:
In addition, according to Koopman’s theorem for energy gap, Egap is defined as the difference between HOMO and LUMO energy . The ionization potential (I), and electron affinity (A) can be estimated from the HOMO and LUMO energy values. The HOMO and LUMO energies are used for the determination of global reactivity descriptors. The electrophilicity (ω), chemical potential (μ), electronegativity (χ), hardness (η) and softness (S) are calculated
To explain the chemical properties of this crystal, the LUMO-HOMO gap is to be mentioned for their chemical stability. For the organic compound, the LUMO-HOMO gap is 7.0 to 9.0 eV (59-64), indicating chemical stable configuration or molecule while this crystal illustrating the LUMO-HOMO gap is 1.426 eV as shown in Table 1. It is well known that the smaller LUMO-HOMO gap indicates the greater chemical stability of molecules. Moreover, the chemical potential, hardness and softness are also indicator of chemical stability while softness provides how easily dissociated or break down a molecule while hardness is completely opposite of softness. From Table 1, the large value of softness mentions its high chemical stability whereas the value of hardness inverses to softness.
Understanding about the interaction of incident electromagnetic radiation to a material belonging to towering applicable and informative tools is for proper use in optoelectronic devices. So, the optical analysis of various features, optical reflectivity, optical absorption, optical reflective index, optical conductivity, optical dielectric function and loss function, enable us to appraise the compound repercussion with the electromagnetic wave and its electronic structure.
To explain the nature of optoelectronic materials, the optical reflectivity plays a significant role about passing light through the materials when light falls on these materials that is explained in the following way. Firstly, UV light is exposed to these materials, it can absorb the light, and it can be considered that a smaller value of reflectivity illustrated greater absorption of light or UV. In this section, it was observed that the reflectivity of CH3NH3SnBr3 crystal showed (Figure 11) that the value of reflectivity near photon energy 0 eV was around 0.18, representing no band gap was found in the Fermi energy level that confirmed CH3NH3SnBr3 has a semi-conductor nature as organic perovskites cell. It is illustrated that the values of reflectivity for CH3NH3SnBr3 maintain the high reflectivity around the photon energy range about 1.40 eV at 0.22.
Figure 11: Optical Reflectivity
Figure 12 illustrates that the study of the optical absorption for CH3NH3SnBr3 is essential to provide information about polycrystalline polarization technique. It is revealed that absorption peaks are attributed to the transition energy from the highest energy states of valence band to the lowest energy state of conduction band under UV or visible light illumination. The maximum absorption was found for CH3NH3SnBr3 crystal in the mid visible range between 3.10 eV and 8.20 eV.
Figure 12: Optical Absorption
Optical reflective index
The refractive index has two segment real portion and imaginary portion. The real part of refractive index indicates the phase velocity whereas the imaginary segment is mass attenuation coefficient. Figure 13 portrays the theoretical learning of refractive index as a function of photon energy reveals that the values for the imaginary part are almost zero until photon energy 6 eV for CH3NH3SnBr3, although, at initial point of photon energy, the values of refractive index are at around 2.7 for real part for CH3NH3SnBr3 organic crystal.