Influence of $\pi$-conjugated cations and halogen substitution on the optoelectronic and excitonic properties of layered hybrid perovskites

Low-cost chemical engineering of two-dimensional layered hybrid halide perovskite structures allows for the design of hybrid semiconductor quantum wells with tailored room-temperature excitonic optical absorption, emission, and charge carrier transport properties. Here density functional theory and the Bethe-Salpeter equation are used to predict the electronic structure and optical response of layered perovskites with two representative single-ring conjugated organic spacers, ammonium-propyl-imidazole (API) and 2-phenethylammonium (PEA). The inorganic perovskite quantum well properties are further tuned by analyzing the effect of halogen ($X$ = I, Br, Cl) substitution. We found that visible light absorption occurs primarily within the perovskite layer and that UV light absorption induces partial electron-hole separation between layers. In addition, a strong exciton binding energy and influence on absorption spectrum is found by solving the Bethe-Salpeter equation. Our results suggest that further engineering is necessary beyond the single-ring limit, by introducing more conjugated rings and/or heavier nuclei into the organic spacer. This is a promising future direction to achieve photoinduced charge separation and more generally hybrid heterostructures with attractive optoelectronic properties.

Figure 1

The atomic structure of ammonium-propyl-imidizole (API)-${\mathrm{PbBr}}_4$ projected along the (a) [100] direction and (b) [010] direction and 2-phenethylammonium (${\mathrm{PEA}}_2$)-${\mathrm{PbBr}}_4$ projected along the (c) [100] and (d) [010] axes. The stacking direction is [001]. Atomic color coding: Pb (red), Br (blue), C (black), N (green), and H (pink). Tilting angle $\delta$ and bond angle $\eta$ are indicated (see text).

Figure 2

The ion-projected band structures, computed using PBE+SOC+${\mathrm{\Delta }}_{\mathrm{HSE}06}$, of API-${\mathrm{PbBr}}_4$ (a)–(c) and ${\mathrm{PEA}}_2-{\mathrm{PbBr}}_4$ (d)–(f). The color bar indicates the contribution of Pb (red), Br (blue), and API/PEA (magenta) to each state as a percentage.

Figure 3

The PBE+SOC+${\mathrm{\Delta }}_{\mathrm{HSE}06}$ ion-projected DOS of (a) API-${\mathrm{PbI}}_4$, API-${\mathrm{PbBr}}_4$, and API-${\mathrm{PbCl}}_4$ and (b) ${\mathrm{PEA}}_2-{\mathrm{PbI}}_4, {\mathrm{PEA}}_2-{\mathrm{PbBr}}_4$, and ${\mathrm{PEA}}_2-{\mathrm{PbCl}}_4$. The energies of the Pb $5d$ states at $-15.3$ eV are used for alignment, and the valence band maxima of API-${\mathrm{PbI}}_4$ and ${\mathrm{PEA}}_2-{\mathrm{PbI}}_4$ are used as energy zero. Dashed lines mark the band extrema. Color coding: Pb (red), halide (blue), organic layer (shaded brown).

Figure 4

The ion-resolved imaginary dielectric functions of (a) API-${\mathrm{PbI}}_4$, (b) API-${\mathrm{PbBr}}_4$, (c) API-${\mathrm{PbCl}}_4$, (d) ${\mathrm{PEA}}_2-{\mathrm{PbI}}_4$, (e) ${\mathrm{PEA}}_2-{\mathrm{PbBr}}_4$, and (f) ${\mathrm{PEA}}_2-{\mathrm{PbCl}}_4$. The color coding indicates the resolution of ${\varepsilon }_2$ along specific ion-to-ion transitions, which are (in valence ions $\to$ conduction ions notation) perovskite$\to$perovskite (blue, solid), organic$\to$organic (red, solid), organic$\to$perovskite (blue, dashed), perovskite$\to$organic (red, dashed), and all (black, dotted). The bottom panels show the percent that each type of transitions contributes to the total ${\varepsilon }_2$ as a function of photon energy.

Figure 5

Imaginary part of the dielectric function of API-${\mathrm{PbI}}_4$ (top), API-${\mathrm{PbBr}}_4$ (middle), and API-${\mathrm{PbCl}}_4$ (bottom) with (red lines) and without (black lines) excitonic effects. The PBE+SOC Kohn-Sham energies and states are used as input to the BSE and the single-particle band gap is shifted to the HSE06+SOC value. That band gap and the lowest excitation from the BSE are marked by vertical black and red dashed lines, respectively.