Structural and thermodynamic limits of layer thickness in 2D halide perovskites

Abstract

In the fast-evolving field of halide perovskite semiconductors, the 2D perovskites (A′)2(A)n−1MnX3n+1 [where A = Cs+, CH3NH3+, HC(NH2)2+; A′ = ammonium cation acting as spacer; M = Ge2+, Sn2+, Pb2+; and X = Cl−, Br−, I−] have recently made a critical entry. The n value defines the thickness of the 2D layers, which controls the optical and electronic properties. The 2D perovskites have demonstrated preliminary optoelectronic device lifetime superior to their 3D counterparts. They have also attracted fundamental interest as solution-processed quantum wells with structural and physical properties tunable via chemical composition, notably by the n value defining the perovskite layer thickness. The higher members (n > 5) have not been documented, and there are important scientific questions underlying fundamental limits for n. To develop and utilize these materials in technology, it is imperative to understand their thermodynamic stability, fundamental synthetic limitations, and the derived structure–function relationships. We report the effective synthesis of the highest iodide n-members yet, namely (CH3(CH2)2NH3)2(CH3NH3)5Pb6I19 (n = 6) and (CH3(CH2)2NH3)2(CH3NH3)6Pb7I22 (n = 7), and confirm the crystal structure with single-crystal X-ray diffraction, and provide indirect evidence for “(CH3(CH2)2NH3)2(CH3NH3)8Pb9I28” (“n = 9”). Direct HCl solution calorimetric measurements show the compounds with n > 7 have unfavorable enthalpies of formation (ΔHf), suggesting the formation of higher homologs to be challenging. Finally, we report preliminary n-dependent solar cell efficiency in the range of 9–12.6% in these higher n-members, highlighting the strong promise of these materials for high-performance devices.

Mixed Spacer Cation Stabilization of Blue‐Emitting n = 2 Ruddlesden–Popper Organic–Inorganic Halide Perovskite Films

“…Figure a shows that n = 1 phase has the lowest formation energy, but we can notice that the formation energies for different values of n are also quite similar. Our calculations agree with reported trend for the experimentally determined energies of formation of (BA) 2 (MA) n −1 Pb n I 3 n +1, which show that the formation energies for structures with an even number of layers are significantly more exothermic than those for odd values of n , i.e., the formation energy of n = 2 is more negative than that of n = 1 and n = 3 …”

Two‐Dimensional Halide Perovskites in Solar Cells: 2D or not 2D?

“…Either in films, powders, or single crystals, the thickest 2D perovskite thoroughly characterized is an n= 7 perovskite, and there is indirect evidence for an n= 9 phase. In fact, for the family of compounds (BA) 2 (MA) n −1 Pb n I 3n+1 , it has been shown that compounds with n >7 have unfavorable enthalpies of formation . Consequently, claims of 2D perovskites with n values greater than seven should be taken judiciously, as those would be in all likelihood mixed and/or passivated phases.…”

“… Bandgap and PL energy of the (BA) 2 (MA) n −1 Pb n I 3 n +1 family of perovskites as a function of n. n= ∞ values corresponds to MAPbI 3 . Data obtained from .…”

Manipulation of Dipolar Polarization at Steady States for a Quasi‐2D Organic–Inorganic Hybrid Perovskite with a Nanorod Network

“…Entirely layered 2D perovskite structures are built by sequential quantum wells with the chemical formula , in which A denotes a long chain of organic molecules, is a disordered small organic cation in the 3D framework, M and X represent a divalent metal cation and a halide anion, respectively, and n determines the number of layers of the octahedral . The structures feature strong quantum and dielectric confinements, leading to relatively larger bandgaps and binding energies depending on the value of n . Due to the enhancement of the binding energies for electron–hole recombination, they were more preferentially applied for perovskite light‐emitting devices (PeLEDs) at the beginning .…”