Small Cyclic Diammonium Cation Templated (110)-Oriented 2D Halide (X = I, Br, Cl) Perovskites with White-Light Emission

Abstract

Two-dimensional (2D) halide perovskites exhibit excellent potential for optoelectronics because of their outstanding physical properties and structural diversity. White-light emission is one property of 2D perovskites that originates from self-trapped excitons (STE) in the highly distorted structures. The so-called (110)-oriented 2D perovskites are generally distorted and believed to be good candidates for white-light emitting devices. Here, we report (110)-oriented 2D perovskites, C4N2H12PbX4 (X = I, Br, Cl), templated by the small cyclic diammonium cation, 3aminopyrrolidine (3APr). Structural characterization by single-crystal X-ray diffraction reveals that the distortion of the inorganic part of the structures is influenced by the stereochemical conformation of the cation between the perovskite layers. The experimental bandgaps follow the trend I < Br < Cl (2.56 eV, 3.29 eV, 3.85 eV, respectively). DFT calculations reveal a weak but significant electronic band dispersion along the stacking axis, suggesting a non-negligible interlayer electronic coupling caused by the short proximity of adjacent inorganic layers. The high level of distortion results in the emergence of white-light emission, rarely seen in iodide perovskites, as well as the bromide and chloride isostructural analogues, which provides perfect platform to compare the broad emission mechanism for all three halides. The bromide and chloride perovskites show longer lifetimes and higher color rendering index (CRI) (83 and 85), relevant to solid-state lighting. Temperature-dependent PL measurements confirm that the broad emission comes from different STE mechanism for different halides, with the peak broadening persisting even at low temperature for the chloride compound.

Single‐Component White‐Light Emission in 2D Hybrid Perovskites with Hybridized Halogen Atoms

“…These results clarified the effect of halogens on STEs emissions, and it is generally accepted that this effect is strongly correlated to the structural distortion of the perovskite layers in the ground state 7c. However, the precise WL emission mechanism in these materials is still under debate because only the role of halogens on STEs was investigated, while the effect of halogens on free excitons (FEs) and the transmissions between FEs and STEs were ignored 5b,7b,10. Thus, it is essential to study the luminescent mechanism to further understand the role of halogens on the intrinsic emissions.…”

“…The lifetime values at the broadband emission attributed to the STEs are close and continuous; thus, we propose that there are many self‐trapped energy levels, which is the fundamental reason for inducing broadband emission of STEs. Based on that observation, we provide a schematic diagram of the luminescent mechanism of the 2D hybrid perovskites, as shown in Figure a–c 7b,c. According to the effect of halogens on the energy levels of FEs rather than STEs,11a the energy level of FEs move toward the direction of higher energy due to the increasing trend of bandgaps with the increase of Cl ions.…”

“…Recently, 2D layered hybrid perovskites, including (C 6 H 5 C 2 H 4 NH 3 ) 2 PbBr 2 Cl 2 , (C 6 H 5 CH 2 NH 3 ) 2 PbBr 4− x Cl x , and (CH 3 CH 2 NH 3 ) 4 Pb 3 Br 10− x Cl x ,6b were also found to be WL emitters. These results clarified the effect of halogens on STEs emissions, and it is generally accepted that this effect is strongly correlated to the structural distortion of the perovskite layers in the ground state 7c. However, the precise WL emission mechanism in these materials is still under debate because only the role of halogens on STEs was investigated, while the effect of halogens on free excitons (FEs) and the transmissions between FEs and STEs were ignored 5b,7b,10.…”

“…Since then, several 2D hybrid perovskites, and some one‐dimensional (1D) and zero‐dimensional (0D) metal halide hybrids have been demonstrated to exhibit WL emission at room temperature 1b,5b,c,6. This broadband emission in low‐dimensional metal halides is usually associated with structural deformation in the crystal lattice, which induces the self‐trapped excitons (STEs) generated from recombination of excited electron–hole pairs through strong electron–phonon coupling 4e,6b,7. Recently, 2D layered hybrid perovskites, including (C 6 H 5 C 2 H 4 NH 3 ) 2 PbBr 2 Cl 2 , (C 6 H 5 CH 2 NH 3 ) 2 PbBr 4− x Cl x , and (CH 3 CH 2 NH 3 ) 4 Pb 3 Br 10− x Cl x ,6b were also found to be WL emitters.…”

Two-Dimensional Dion–Jacobson Hybrid Lead Iodide Perovskites with Aromatic Diammonium Cations

“…[11][12] For a larger cation to be incorporated into the structure, the 3D structures need to be broken down to lower dimensional structures, such as hollow perovskites (represented by (en)AMX 3 13-14 , (hea)AMX 3 15 and (tea)AMX 3 16 ; en= ethylenediamonium, hea = hydroxyethylammonium, tea = thioethylammonium) and two-dimensional (2D) structures with even larger organic cations intercalated between the inorganic layers acting as spacers. [17][18][19][20] The 2D perovskites have also shown promising performance in solar cells [21][22][23] and light emitting diodes (LEDs) [24][25][26][27][28] with their main attribute being the exceptional stability, which derives from their water repellent nature and lower ion migration, as well as optoelectronic tunability. The most common members of this class are the (100)-oriented perovskites, with the large spacers "cleaving" the parent 3D perovskites along the (100) plane.…”