Morphology evolution towards ultra-stable mixed tin-lead perovskite via compositional engineering

Highlights

• Ultra-stable and highly luminescent mixed tin-lead 2D and 1D perovskite structures are synthesized.

• Varying lead to the tin ratio in the precursor solution, the dimensions of perovskites changes from 3D to quasi 2D and 1D.

• Multiple phases having different energy bandgaps lead to funnelling type of emission, resulting in high PLQY.

Abstract

Layered perovskites can be a substitute for bulk perovskites that can solve the stability issues in devices. In this work, we demonstrate the synthesis of ultra-stable and highly luminescent mixed tin-lead quasi two-dimensional (2D) and nanowire perovskite structures by compositional engineering. Ligand assisted reprecipitation method under ambient conditions leads to the formation of low dimensional perovskite nanostructures. By varying lead to the tin ratio in the precursor solution, the dimensions of perovskites vary from three dimensional (3D) to quasi 2D and one-dimensional (1D). Multiple phases (a mixture of 2D quasi 2D and 1D) having different energy bandgaps lead to charge concentration at the excited state of phase having the lowest bandgap (3D) and subsequent funneling type of emission, hence result in high photoluminescence quantum yield.

Introduction

Hybrid organic-inorganic perovskites have emerged as a promising materials in the optoelectronic research world. Methylammonium lead halides (MAPbX₃) are organic-inorganic perovskites commonly used for solar cells and other device applications. They are well known for their properties, such as tunable bandgaps, high carrier mobility, long diffusion lengths, ambipolar conductivity, easy and low-cost processing methods. They possess an absorption coefficient of one order higher than other direct band semiconductors such as GaAs. The extraordinary material characteristics of perovskites arise from their electronic structure [1,2]. They have a broad range of applications, including solar cells, light-emitting diodes, lasers, photodetectors, resistive memory, and field-effect transistors [[3], [4], [5], [6], [7], [8]].

Even though perovskites can outperform all other optoelectronic materials, they face obstacles in commercialization. The crucial drawbacks of perovskites are its long term instability in the presence of moisture, oxygen, heat, and ultraviolet radiation and the use of toxic metal lead [[9], [10], [11]]. There have been several attempts to replace the toxic Pb with elements Sn and Ge of the same group. But the substitution is not easy. Pb is stable in its 2+ oxidation state and germanium in the 4+ oxidation state. Sn lies between Pb and Ge in group 14 and exists in both 2+ and 4+ conditions. Sn gets oxidized to SnO₂ making, Sn-based perovskites more unstable than Pb perovskites. Low dimensional perovskites become a solution to this difficulty too. The density functional theory calculations predict that the synthesis of stable perovskites with different Pb to Sn ratios for all halogen groups is possible [12]. Perovskite bandgap varies between 1.55 eV and 1.3 eV in CH₃NH₃Sn_1-xPb_xI₃ prepared by mixing the precursors in aqueous HI/H₃PO₂, resulting in broadened spectral absorption [13]. Koh et al. introduced a formamidinium tin-based perovskite with a low energy gap of 1.41 eV for photovoltaic applications [14]. Spin coating tin halide perovskites resulted in near-infrared tunable emission [15].

3D organic-inorganic halide perovskites get easily decomposed into reactants in the ambient atmosphere and need stringent conditions for its growth. Besides, the exciton binding energy of 3D perovskites is only 20–50 meV [16,17]. Low dimensional perovskites such as 2D, quasi 2D, 1D, and 0D perovskites are stable counterparts applicable to devices. The addition of long-chain organic cation in perovskite 3D structure results in lower-dimensional perovskites. R₂MA_n-1Pb_nX_3n+1 is the general chemical formula for 2D perovskites. Here, n is the number of PbX₆⁴⁻ octahedral units enclosed inside two long-chain cations R. Bulk perovskites, MAPbX₃ result from n equal to infinity and n equal to 1 gives pure 2D perovskites, R₂PbX₄. Values of n ranging between these two extremities result in quasi 2D and oriented 1D structures with hybrid characteristics of both 3D and 2D. 2D perovskites are quantum-well structures with confinement effects, hence increasing exciton binding energy and photoluminescence quantum yield (PLQY) [17,18]. Low dimensional perovskites have considerable stabilities in moisture and high temperatures due to multiple interactions between organic and inorganic layers such as bonds between ammonium and lead and weak van der Waals force in an organic layer extending to a larger space compared to 3D perovskites [17].

Here, we synthesized stable, low-dimensional, Pb–Sn perovskites using the reprecipitation method. The method is well known for high-quality perovskite quantum dots and precipitates of larger sizes [19,20]. In this work, the oxidation of tin in the ambient atmosphere and Pb vacancies leads to quasi 2D and 1D perovskite colloidal suspensions. MA_n (OAm)_2nxn (Pb_ySn_1-x-y)Br_3n+1 is obtained as the chemical formula for the low dimensional perovskites. Here, MA is short organic cation, methylammonium, OAm is long chain organic cation oleylammonium, n is the number octahedral inorganic layers, x is quantity of Sn that get oxidized in the vacancy of which two OAm ions can enter. We discarded quantum dots and used larger sized precipitates for the current study. The photoluminescence quantum yield (PLQY) of these structures was very high, 79% for quasi 2D and 50% for the mixed 2D-1D, and remained stable for about three months. The mixed phases (2D, quasi 2D, and 1D) with different bandgaps populate the excited state of that phase having the lowest bandgap (3D) and result in an energy funneling type PL emission and hence possess high PLQY values. Hydrophobic long-chain cations, oleylamine intercalated into perovskite structure provide long term stability in ambient atmospheres.