Computational determination of the physical-thermoelectric parameters of tin-based organomatallic halide perovskites (CH₃NH₃SnX₃, X = Br and I): Emerging materials for optoelectronic devices

Highlights

• Theoretical study of tin-based organomatallic halide perovskites.

• Electronic structure based band gap and density of states study.

• Dielectric constant, refractive index and absorption study.

• Thermoelectric properties have been investigated.

Abstract

The promising material hybrid Organic-Inorganic Perovskites (HOIP) has been frequently a subject of comprehensive and intensive experimental and theoretical studies. As Because of noxiousness and longtime environmental stability of heavy metal lead, since last few years non-lead based perovskites compounds either organic or inorganic are attracting attention in the solar cell formation. In this work, we have studied the fundamental properties of CH₃NH₃SnI₃ and CH₃NH₃SnBr₃ like structural, electronic, optical and thermoelectric. We have been used full-potential linearized augmented plane wave method (FP-LAPW) within density functional theory (DFT) and implement it in Wien2k. The generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) for the exchange-correlation functional has been used to relax the structural parameters of these materials. The electronic properties have been evaluated by using different GGA and LDA (Local density approximation) exchange correlation potential. We have obtained the values of band gap 1.172 eV and 1.902 eV for CH₃NH₃SnI₃ and CH₃NH₃SnBr₃ respectively by TB-mBJ exchange-correlation potential. According the optical properties, we have seen that because of high absorption coefficient of CH₃NH₃SnI₃ and CH₃NH₃SnBr₃perovskites, they may be strongly applied in photovoltaic device for the visible to ultra-violet wavelength region. Thermoelectric properties have shown that at room temperature they also may be used as thermoelectric device. Based on my knowledge, the investigations on tin based perovskites have been discussed for first time.

Introduction

Recently [[1], [2], [3]], organometallic halide perovskites have been used for high-performance photovoltaic (PV) devices application and in these references have been discussed about the lead iodide based hybrid perovskites emerged as light harvesters for mesoscopic hetero-junction solar cells. The efficiency of power conversion (PCE) of these solar cells is about 23% comparable to that of the commercial silicon solar cells. On the grounds that low processing outlay and highly bountiful raw materials perovskite solar cells are commercially attractive option. This exceptional performance of perovskites based solar cell is mainly because of the extraordinary semiconducting property and comprises earth-abundant elements [4,5]. Due to relatively long carrier diffusion length, tunable band gap, ambipolar transport, high charge carrier mobility and simple methods of fabrication, concentrated the researcher around hybrid organic-inorganic perovskites (HOIPs) as an auspicious candidate for thermoelectric and optoelectronic applications [[6], [7], [8], [9], [10]]. Suitable band gap and proper band alignment enable of these perovskites as a strong light absorber across IR to UV solar spectrum. Their band gap and proper band positioning enable strong light absorption across most of the solar spectrum [11]. Perovskite are expressed by the structure of empirical formula ABX₃ [12]; where B is a cation of +4 valence, X is an oxygen or halides and A has to be a cation of +2 valence (to achieve charge neutrality). In the present case, HOIP based on metal halides, espouses the similar structure as oxide based perovskites with B cation as Sn²⁺, X as Br¹⁻ and I¹⁻ and the A cation as methyl ammonium (MA or CH₃NH₃). Although methyl ammonium lead iodide, (CH₃NH₃PbI₃) has proven to be an effective active layer material [13], there remains a major toxicity concern associated with lead that has potential to wreak harm upon both the environment and human body [14]. In addition to that CH₃NH₃PbI₃ is relatively not so long-lasting, because CH₃NH₃PbI₃ at high temperatures decomposes into solid (PbI₂) and two gases CH₃NH₃ and HI. In order to swamp these complications, it is necessary to find a substitute of lead which maintains the same power conversion efficiencies (PCE) as previously discusses lead based solar cells. There are many options to replace lead with metal or nontoxic materials (i.e. Sn⁺² and Ge⁺²), among these Sn looks more auspicious candidate, possess narrowed optical band gap and high optical absorption coefficients; because Sn 5p orbital is less dispersive than Pb 6p orbital [15,16]. As regarded to effective mass of charge carriers, tin based perovskites has lower effective mass than that of lead based, consequently tin based perovskites exhibit higher charge carrier mobility [17].

Bernal et al [18], have used hybrid potentials (HSE06) for the band gap calculation, and also discussed that the position and orientation of organic cation (MA⁺or CH₃NH₃⁺) have no direct consequence on the valence band and conduction band formation. Moreover organic cation (MA) supplies single electron to neutralize the structure in MASnI₃ and MASnBr₃ perovskite structure. Ma et al [19] discussed that long diffusion length of electron and hole in the MASnI₃ based thin film, and the effect of doping of SnF₂ increases the fluorescence life time; which is much greater than the lead based HOIP solar cells. Mao et al [20] have discussed about the stability of perovskites based solar cell in moisture after the doping of chalcogen element like Oxygen, Sulphur or other group XVI elements along with halogen in the BX₆ octahedrons and also concluded that moisture have prevented by chalcogens. Hoye et al [21] have been prepared experimentally methyl ammonium bismuth iodide by solution processing and vapor assisted techniques. They have observed to be a suitable absorber layer, reduce the recombination time of charge carriers and have been stable in humid air environment unlike PbI₂ in case of lead based perovskites. The phase transformation is the essential property of HOIPs, similar to MAPbI₃, MASnI₃ and MASnBr₃ also exhibit a opulent phase transition as a function of temperature. Takahashi [22] has discussed that MASnI₃ changes its phase from cubic (space group-Pm-3m) to tetragonal (space group -P4/mmm) and then orthorhombic (space group -Pna21) by decreasing the temperature from 275K to 50 K. Huang et al [23] have prepared MASnI₃ single crystal using cooling crystallization method and evaluated crystal parameter and optical band gap. They have reported that MASnI₃ crystal is chemically unstable in air due to formation of SnI₄ and SnO, a limitation for its use in photoelectric fields. Zhao et al [24] have been investigated the fundamental properties of germanium based halide perovskite and studied the effect of replacement of I by Cl in the halide part by using first-principles calculations with HSE06 potential. They have shown that by increasing the doping ratio of chlorine atom, the optical and transport properties significantly improved. Qian et al [25] have also been studied a number of halide perovskites using DFT calculations, together with Shockley-Queisser Maximum Solar Cell Efficiency (S-Q) and Spectroscopic Limited Maximum Efficiency (SLME) mathematical models and shows that in germanium based perovskites have high absorption coefficient around the solar spectrum and in addition to that the electron and hole effective mass is also appropriate for germanium comparison to lead and tin based perovskites. Motivated from the above research on lead-free organic halide perovskites, we have studied the fundamental properties of MASnI₃ and MASnBr₃ perovskites by ab-initio calculations. The framework of the paper is as follows, in 2nd section a succinct analysis of the used computational strategy and description of the material used has been given. In section 3rd detailed analysis of structural, electronics, optical and thermoelectric properties are discussed; while in last 4th section conclusion of complete work and future aspects of proposed work have been discussed.