The Origin of Broad Emission in ⟨100⟩ Two-Dimensional Perovskites: Extrinsic vs Intrinsic Processes

Abstract

2D metal halide perovskites can show narrow and broad emission bands (BEs), and the latter's origin is hotly debated. A widespread opinion assigns BEs to the recombination of intrinsic selftrapped excitons (STEs), whereas recent studies indicate they can have an extrinsic defect-related origin. Here, we carry out a combined experimental−computational study into the microscopic origin of BEs for a series of prototypical phenylethylammonium-based 2D perovskites, comprising different metals (Pb, Sn) and halides (I, Br, Cl). Photoluminescence spectroscopy reveals that all of the compounds exhibit BEs. Where not observable at room temperature, the BE signature emerges upon cooling. By means of DFT calculations, we demonstrate that emission from halide vacancies is compatible with the experimentally observed features. Emission from STEs may only contribute to the BE in the wide-band-gap Br-and Cl-based compounds. Our work paves the way toward a complete understanding of broad emission bands in halide perovskites that will facilitate the fabrication of efficient narrow and white light emitting devices.

Spacer-Dependent and Pressure-Tuned Structures and Optoelectronic Properties of 2D Hybrid Halide Perovskites

“…36,46 However, in iodide-based 2D perovskites, broad emission may result from defect-related origins rather than STE. 47 Additionally, the asymmetric emission profile has been associated with the structural distortion of PbI 6 octahedra, which is believed to be an important structural factor for emission band shape. 9,48 Upon compression, the intensity of the major PL peak increases until 0.8 GPa, where the peak intensity is about 10 times stronger than that at ambient pressure (Figure 3b), which indicates a remarkable pressure-enhanced PL efficiency.…”

“…The PL spectrum at ambient pressure consists of a peak around 521 nm with an asymmetric emission profile (Figure S1a). The narrow, symmetric PL emission band of most 2D perovskites originates from free excitons, whereas a broad, asymmetric PL profile could be associated with the emission from self-trapped excitons (STE). , However, in iodide-based 2D perovskites, broad emission may result from defect-related origins rather than STE . Additionally, the asymmetric emission profile has been associated with the structural distortion of PbI 6 octahedra, which is believed to be an important structural factor for emission band shape. , …”

A2Bn-1PbnI3n+1 (A = BA, PEA; B = MA, n = 1, 2): Engineering Quantum-well Crystals for High Density and Fast Scintillators

“…The broadband of PEA2PbI4 crystal at 660 nm is possibly due to the radiative path of electron capture at a positive iodide vacancy with subsequent hole capture. 51 There is no emission band observed for (BA)2MAPb2I7 crystal at 532 nm excitation wavelength. The green emission band has full-width-at-half-maximum (FWHM) equal to 19 nm for (PEA)2PbI4, and…”

A2Bn–1PbnI3n+1 (A = BA, PEA; B = MA; n = 1, 2): Engineering Quantum-Well Crystals for High Mass Density and Fast Scintillators

“…On the other hand, when using the longer wavelength excitation of 532 nm, only one red emission band is observed at 620 nm (red) for (BA) 2 PbI 4 and a broad band at 660 nm (red) for (PEA) 2 PbI 4 . For (PEA) 2 MAPb 2 I 7 crystals, the red emission at 620 nm at its bandgap energy (2.0 eV) with an appreciable PL emission broad band at 748 nm can be originated from the edges of the exfoliated layers of perovskite crystal, as reported by Blancon et al The broadband of the PEA 2 PbI 4 crystal at 660 nm is possibly due to the radiative path of electron capture at a positive iodide vacancy with a subsequent hole capture . There is no emission band observed for the (BA) 2 MAPb 2 I 7 crystal at 532 nm excitation wavelength.…”