Correlation of Dielectric Confinement and Excitonic Binding Energy in 2D Layered Hybrid Perovskites Using Temperature Dependent Photoluminescence

Abstract

In 2D layered hybrid perovskite like (PEA) ₂PbX₄ (PEA = phenylethylammonium, X = Cl, Br, I), the high frequency dielectric constants for inorganic well layer (ε_w) and organic barrier (ε_b) are distinctly different. The dielectric contrast significantly influences their excitonic binding energy (E_ex^2D). Here we vary ε_w/ε_b by varying both the halide anion and the organic cation and then correlate the influence of the dielectric contrast on E_ex^2D. We estimate E_ex^2D by employing temperature (5.4–300 K) dependent photoluminescence (PL) and find that the change in E_ex^2D can be qualitatively monitored simply by measuring the PL lifetime at room temperature. E_ex^2D increases, and therefore, PL lifetime decreases by varying halide ions from Cl to Br to I for (PEA) ₂PbX₄. Notably, this trend is opposite the case of 3D Pb-halide perovskites, where the excitonic binding energy decreases for X = Cl to Br to I. The opposite trend for 2D perovskites is explained by dielectric confinement, where we find E_ex^2D ℃ (ε_w/ε_b)^m, with m as an unknown positive number and ε_w > ε_b. The dielectric confinement drastically diminishes with increasing ε_b. (EA) ₂PbI₄ (EA = ethanolammonium) with ε_b = 37.7 shows E_ex^2D = 65 meV, as opposed to (PEA) ₂PbI₄ with ε_b = 3.3 and E_ex^2D = 453 meV. This correlation of ε_w/ε_b with E_ex^2D is critical for optoelectronic applications of 2D layered perovskites.