Impact of Halogen Substitution on the Electronic and Optical Properties of 2D Lead-Free Hybrid Perovskites

Abstract

Development of hybrid organic−inorganic lead halide perovskites is still hampered by the toxicity of lead and their structural stability in air. Hence, the research of lead-free alternatives is extremely important to promote the development of perovskite-based photovoltaics. Here, we construct lead-free tin (Sn)-and germanium (Ge)-based two-dimensional (2D) hybrid organic−inorganic perovskites, (BA) 2 GeX 4 and (BA) 2 SnX 4 (X = I, Br, and Cl), and explore their geometrical, electronic, and light absorption characteristics via first-principles calculations. The lattice constants will increase from (BA) 2 MCl 4 , (BA) 2 MBr 4 to (BA) 2 MI 4 (M = Ge, and Sn), owing to the increase in halogen ionic radius in the order of Cl − (1.81 Å) < Br − (1.96 Å) < I − (2.20 Å). Meanwhile, (BA) 2 GeX 4 materials show less lattice constants by ∼3−4% along a axis and ∼2−4% along b axis, respectively, than those of (BA) 2 SnX 4 materials. Moreover, both the 2D tin and germanium halide perovskites have direct band gaps; (BA) 2 GeX 4 exhibits values of 1.54, 2.10, and 2.55 eV, while (BA) 2 SnX 4 exhibits values of 1.53, 1.98, and 2.49 eV, for X = I, Br, and Cl, respectively. In particular, the estimated effective masses of electrons/holes in 2D-(BA) 2 GeX 4 and (BA) 2 SnX 4 are 0.15−0.23/0.16−0.27 m e and 0.14−0.18/0.14− 0.19 m h at Γ−X and 0.16−0.29/0.12−0.14 m e and 0.22−0.35/0.12−0.14 m h at Γ−Y, respectively, about half of those in BA 2 PbI 4 and BA 2 PbBr 4 . Furthermore, the density of states (DOS) and partial DOS results suggest that the contributions to the valence and conduction band are mostly made by the components of halogen p state orbitals and metal p state orbitals, respectively. Furthermore, the band-decomposed charge densities of the conduction band minimum and valence band maximum results imply that the distorted inorganic [GeX 6 ] 4− and [SnX 6 ] 4− octahedra have a direct influence on the band gaps. Our results indicate that this series of homologous 2D tin and germanium halide perovskites are a promising class of stable and efficient light-absorbing materials for photovoltaic and/or photoelectronic applications.

Photophysical and Magnetic Properties in Zero-Dimensional (H2DABCO)MX4·nH2O (M = Mn and Cu; X = Cl and Br; n = 0, 1, and 4)

“…Recently, the class of organic–inorganic hybrid (OIH) halide compounds has emerged as a bigwig in the field of energy harvesting. − OIH halides incorporate the organic cations and inorganic halide frameworks leading to exciting optoelectronic and photovoltaic properties and potential for several applications like light-emitting diodes, photodetectors, solar cells, and so on. − The organic cation and inorganic metal halide sublattices independently or synergistically play a pivotal role in displaying dielectric, ferroelectricity, piezoelectricity, magnetism, second harmonic generation, and luminescence properties. − Although much progress has been made in lead-based hybrid halide perovskites, they are detrimental owing to their toxicity . Consequently, numerous lead-free hybrid halides have been designed, yet their low stability is their primary shortcoming. − Though researchers employed a transition metal doping strategy to minimize the toxicity, there is doubt regarding its application due to the structural deformation and charge variation . Hence transition metal (Mn 2+ , Cu + , and Cu 2+ ) based hybrid halides have gained substantial attention owing to their low cost and nontoxic nature and their advantage of a large Stokes shift, unlike lead halide perovskites, which suffer from self-absorption. − …”

Two-Dimensional Layered Simple Aliphatic Monoammonium Tin Perovskite Thin Films and Potential Applications in Field-Effect Transistors

“…As compared to (BA) 2 PbI 4 with the exciton binding energy varying from 370 to 490 meV, 44,49,52−54 the (BA) 2 SnI 4 film has a relatively low exciton binding energy due to the fact that Sn perovskites have larger dielectric constants. 17,19,20 This suggests that (BA) 2 SnI 4 has greater potential for developing 2D layered organic−inorganic fieldeffect or even light-emitting transistors because of the larger exciton binding energy than those of 3D perovskites, in which strong electroluminescence has been achieved in the corresponding transistors. 55 The temperature-dependent PL spectra of the optimized (BA) 2 SnI 4 thin film were further analyzed to get information 39,44,46 about phonons that impact the charge transport via phonon scattering processes.…”

“…5,17 Besides, Sn perovskites with larger dielectric constants have much smaller exciton binding energies than Pb perovskites, facilitating charge transport. 17,19,20 These three factors therefore promote the transport properties of 2D layered Sn perovskites, 21 which are conducive to transistor applications. Furthermore, A-site organic cations, which are either simple aliphatic and aromatic monoammonium cations or complex and conjugated monoammonium cations, 15,17,20,22−24 can also finely tailor the physical and chemical properties of 2D layered organic−inorganic perovskites, ultimately determining the performance and stability of the relevant perovskite optoelectronic devices.…”

“…The intrinsic reasons for the better performance of 2D Sn perovskite transistors than 2D Pb ones lie in the differences in the B-site metals. Heavy metals are known to possess a relativistic effect; thus, the weaker relativistic effect of the s orbital of Sn results in smaller effective masses of charge carriers in Sn perovskites . In polar semiconductors such as organic–inorganic perovskites, the Fröhlich interactions are also the main mechanism that limits the carrier mobility at room temperature .…”

Switchable Dielectric Two-Dimensional Lead-Free Perovskite with Reversible Thermochromic Response

“…Stimuli-responsive switchable hybrid perovskite materials display reversible color changes, fluorescence, phase transition, or other physical properties under external stimulations, including temperature, light, pressure, and electricity, and play an important role in energy-saving and environmental protection. − Besides, these materials also have great potential for applications in smart windows, storage devices, sensors, optoelectronic devices, switch operations, and so on. − Among them, thermochromic hybrid perovskites, possessing the interesting stimulation response of color change under varying temperatures, have emerged as good candidates due to their wide applications in smart temperature coatings, colored clothing, smart stealth materials, and temperature sensors. − Generally, thermochromism, caused by phase transition or vibronic coupling, is a notable color-change phenomenon and is often present in Cu-based and Pb-based hybrid perovskites. − More glaringly, compared with the high toxicity of Pb-based perovskites, Cu-based perovskites are excellent candidates for thermochromic materials. The 2D [CuX 4 ] 2– layers were separated by the organic cations in 2D OIHPs. , As a result of the Jahn–Teller effect, two of the six Cu–X bonds are longer than the others combining with the distorted [CuX 4 ] 2– layer, which result in additional flexibility of the inorganic framework .…”