Electrons, Excitons, and Phonons in Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties

Abstract

Two-dimensional (2D) hybrid perovskites are stoichiometric compounds consisting of alternating inorganic metal-halide sheets and organoammonium cationic layers. This materials class is widely tailorable in composition, structure, and dimensionality and is providing an intriguing playground for the solid-state chemistry and physics communities to uncover structure-property relationships. In this Perspective, we describe semiconducting 2D perovskites containing lead and tin halide inorganic frameworks. In these 2D perovskites, charges are typically confined to the inorganic framework because of strong quantum and dielectric confinement effects, and exciton binding energies are many times greater than kT at room temperature. We describe the role of the heavy atoms in the inorganic framework; the geometry and chemistry of organic cations; and the "softness" of the organic-inorganic lattice on the electronic structure and dynamics of electrons, excitons, and phonons that govern the physical properties of these materials.

Two‐Dimensional Halide Perovskites in Solar Cells: 2D or not 2D?

“…First, and similar to 3D perovskites, 2D perovskites exhibit a valence band (VB) made up predominantly of halide p‐orbitals hybridized with some metal s orbitals and a conduction band (CB) where metal p‐orbitals dominate. In particular, for lead iodide perovskites, the relevant orbitals are I 5p as well as Pb 6s and 6p . Compared with 3D halide perovskites, 2D perovskites have enhanced exciton binding energies ( E b ) due to a dielectric confinement effect in the layers and thus the excited electrons are strongly attracted to the holes; in other words, they have a higher E b .…”

“…In particular,f or lead iodide perovskites, the relevant orbitals are I5pa sw ell as Pb 6s and 6p. [50] Comparedw ith 3D halide perovskites, 2D perovskites have enhanced exciton binding energies (E b )due to adielectric confinement effect in the layers and thus the excited electrons are strongly attracted to the holes;i no ther words, they have a higher E b .T his can also be explained when considering that E b is proportionalt ot he effective mass (m)o ft he chargec arriers and dielectric constant(e). Thus, ad ielectric confinement effect originates from the alternation of low e ascribed to the organic moiety and ah igh e of the lead(II)h alide layers [23,51] (Scheme 5).…”

Lead-free hybrid perovskites for photovoltaics

“…Simultaneously, the formation of interlayer voids or intermediate layers of other semiconductors/dielectrics may result in a regular quantum-well structure that is strongly beneficial for the photoinduced charge separation between the separated HP sheets [207–208]. Finally, a combination of single layers of two and more different lead-free HPs into a composite material may offer unprecedented variability of optical properties and band design.…”

Rational Core–Shell Design of Open Air Low Temperature In Situ Processable CsPbI3 Quasi‐Nanocrystals for Stabilized p‐i‐n Solar Cells

“…Irrespective of the film quality, XRD patterns in Figure 3b demonstrate high qualities of α phases without δ phases in any sample. [43] FPEAI with a higher dielectric constant than OAI will decrease such coulomb force between charges in the nanocrystals, thereby increasing the coulomb screening effect. The fitting results are shown in Figure S15 in the Supporting Information, where a similar variation trend of crystal size was compared to that of SEM images.…”

“…[42] Notably, there is a little blue-shift of PL peak after introducing FPEAI and Mn 2+ together, which might originate from the following reasons: In terms of ligands, coulomb force between charges in the mutual crystal lattices can increase due to the generation of electric field by charge extending into ligands. [43] FPEAI with a higher dielectric constant than OAI will decrease such coulomb force between charges in the nanocrystals, thereby increasing the coulomb screening effect. In this case, a more efficient radiative band-to-band recombinations with an increased defect tolerance will be achieved, thus inducing a blue-shifted PL peak.…”