Spatial Charge Separation as the Origin of Anomalous Stark Effect in Fluorous 2D Hybrid Perovskites

Abstract

2D hybrid perovskites (2DP) are versatile materials, whose electronic and optical properties can be tuned through the nature of the organic cations (even when those are seemingly electronically inert). Here, it is demonstrated that fluorination of the organic ligands yields glassy 2DP materials featuring long-lived correlated electron-hole pairs. Such states have a marked charge-transfer character, as revealed by the persistent Stark effect in the form of a second derivative in electroabsorption. Modeling shows that electrostatic effects associated with fluorination, combined with the steric hindrance due to the bulky side groups, drive the formation of spatially dislocated charge pairs with reduced recombination rates. This work enriches and broadens the current knowledge of the photophysics of 2DP, which will hopefully guide synthesis efforts toward novel materials with improved functionalities.

Keywords: 2D perovskite; Stark effect; electroabsorption; transient absorption.

Figure 1

a) Cartoon showing the structure of (Fluo)₂PbI₄; b,c) Linear absorption and photoluminescence spectra (upon exciting at 420 nm) of (Fluo)₂PbI₄ versus temperature as indicated in the legend. d) Sketch of the device employed in electroabsorption (EA) experiments done at room temperature. e) EA spectra of the (Fluo)₂PbI₄ device at different biases. f) EA spectra at 10 V with a simulation composed of a linear combination of derivatives of the absorption bands.

Figure 2

a) ns transient absorption (nsTA) spectra at selected delays (as indicated in the legend) for the (Fluo)₂PbI₄ thin film upon excitation at 420 nm, carrier density 10¹⁸ cm⁻³ b) nsTA signal at 1 ns probe delay with the simulated EA spectra (as in Figure 1f).

Figure 3

Results of frozen glass simulation. a) Lead‐iodide radial distribution function computed for (Fluo)₂PbI₄ and (BUA)₂PbI₄. b) Dipole moment for BUA and Fluo cations, with the NH₃ ⁺ group substituted by a hydrogen atom to preserve the charge neutrality.

Figure 4

a) Electrostatic potential along the thickness of (BUA)₂PbI₄ and (Fluo)₂PbI₄ slabs; b) Partial density of electronic states (pDOS) for the two materials referred to the electrostatic potential in (a). The ionization potential (IP) is indicated. c) Total density of state (black curve) and density of state (red line) of one iodine which has been pulled out by 0.6 Å (vide infra) with respect to the central PbI₄ plane. The spatial localization of the valence band edge orbital is reported in the inset.