Photophysics of Two‐Dimensional Perovskites—Learning from Metal Halide Substitution

Abstract

2D perovskites offers a rich playing field to explore exciton physics and they possess a great potential for a variety of opto-electronic applications. Whilst their photophysics shows intricate interactions of excitons with the lattice, most reports have so far relied on single compound studies. With the exception of variations of the organic spacer cations, the effect of constituent substitution on the photophysics and the nature of emitting species, in particular, have remained largely under-explored. Here PEA 2 PbBr 4 , PEA 2 PbI 4 , and PEA 2 SnI 4 (where PEA stands for phenylethylammonoium) are studied through a variety of optical spectroscopy techniques to reveal a complex set of excitonic transitions at low temperature. Weak high-energy features are attributed to vibronic transitions breaking Kasha's, for which the responsible phonons cannot be accessed through simple Raman spectroscopy. Bright peaks at lower energy are due to two distinct electronic states, of which the upper is a convolution of the free exciton and a localized dark state and the lower is attributed to recombination involving shallow defects. This study offers deeper insights into the photophysics of 2D perovskites through compositional substitution and highlights critical limits to the communities' current understanding of processes in these compounds.

Grain‐Specific Transitions Determine the Band Edge Luminescence in Dion–Jacobson Type 2D Perovskites

“…Simultaneously, the two shoulders observed in the room temperature luminescence move closer together, as observed above for PDMAPbI 4 and similarly reported for Ruddlesden-Popper compounds. [25] Following similar observations for RP structures based on PEA + , the temperature-dependent spectral shifts of PDMASnI 4 are more pronounced than for the Pb-based compound: [16] For example, the energy of the higher PL peak decreases from 2.01 to 1.93 eV over the temperature range (approx. 80 vs. 30 meV; Sn vs. Pb).…”

“…Metal substitution shifts the excitonic peak energy toward the red spectral region when using Sn instead of Pb, as similarly observed for RP or 3D structures. [16,24] In the current case, the absorbance of PDMAPbI 4 has a maximum around 2.41 eV, while that of the Sn-variant lies around 2.08 eV. The luminescence exhibits a relatively small Stokes shift in both cases (emission around 2.39 and 1.85 eV).…”

“…The overall band edge luminescence narrows, as expected from the suppression of thermal broadening, but it remains broad when compared to similar compounds based on primary amines. [14,16,19] While the absorbance spectra follow this general trend, cooling also allows for a clear distinction of two peaks at 2.39 and 2.44 eV at 5.4 K. Notably, the low energy shoulder of the PL seems to blue shift at lower temperature-also consider the absolute intensity in Figure S5, Supporting Information. PDMASnI 4 behaves similarly.…”

“…We note that the presence of such a pronounced second emission peak in Sn-based 2D perovskite structures seems to be a general observation. [16,26,27] At the same time, we would like to note that the limited intensity increase of PDMAPbI 4 at low temperature is also related to a further broad emission band at significantly lower energy, typical for leadbased 2D perovskites, as discussed further below. Additional data on the dynamics of the PL can be found in Figures S7 to S9, Supporting Information .…”

“…However, there is no consensus on the origin of the vibrational modes responsible for the observed spectral features and the actual mechanisms at play. [14][15][16] Slightly Stokes-shifted band edge emission and split sub-peaks have been analyzed in the framework of dark and bright excitons, [17,18] bright doublets, [19] localized defects, [16,20] and have been shown to depend on the angle of detection. [20,21] At even lower energy, broad emission bands are sometimes attributed to self-trapped excitons, but have recently been proposed to be defect-related.…”