Organic Spacers in 2D Perovskites: General Trends and Structure‐Property Relationships from Computational Studies

Abstract

Addition of large organic molecules to halide perovskites has been shown to provoke dimensionality reduction and formation of two‐dimensional phases that demonstrate improved long‐term stabilities. Optoelectronic properties of the resulting 2D layered perovskites are strongly influenced by the chemical nature of the additive molecules, which opens immense possibilities for preparation of materials with tailored properties. However, given the huge chemical space of possible organic spacers, a systematic and exhaustive search for optimal compounds is impossible and general structure–property relationships that could guide a rational design are still largely absent. Here, we provide an overview of a series of recent computational studies from our group on different types of spacers. We first develop a simplified universal monovalent cation model to map out approximate structural stability maps as a function of the van der Waals radius and the magnitude of dispersion interactions to monitor the possible emergence of 2D phases. We further provide structural and photophysical insights from classical and first‐principles molecular dynamics simulations and density functional theory calculations on 2D hybrid perovskites based on a wide range of spacers with different chemical nature and varying conformational properties. Our computational predictions are validated through comparison with powder diffraction, conductivity and optical measurements. Such comparative study allows for providing some general structure–property correlations, which can serve as design guidelines in the search for optimal 2D and mixed 2D/3D perovskite photovoltaic materials.

Layered Hybrid Formamidinium Lead Iodide Perovskites: Challenges and Opportunities

“…(a) Schematic energy alignment of the organic layer (defined by the corresponding HOMO and LUMO levels) and the conduction band (CB) and valence band (VB) edges of the inorganic layer of 2D perovskites defining the band gap ( E g ). (b) Energy band gap estimates for representative 2D compositions ( n ) defining 2D, 2D/3D, and 3D phases based on optical properties and DFT calculations at the PBE0 level including spin–orbit coupling. ,, (c, d) UV–vis absorption spectra of thin films of (c) (ADAM)­FA n –1 Pb n I 3 n +1 and (d) (PDMA)­FA n –1 Pb n I 3 n +1 ( n = 1–3) on FTO/mp-TiO 2 and FTO/mp-Al 2 O 3 substrates, respectively. Dashed lines (d, right) show the corresponding PL spectra.…”

The Role of Alkyl Chain Length and Halide Counter Ion in Layered Dion−Jacobson Perovskites with Aromatic Spacers

“…The XRD pattern of PDA-I (Figure a, black) corresponds to the (PDA)­PbI 4 ·2H 2 O dihydrate (Figure a, green), which is a one-dimensional (1D) perovskite, indicating that a layered phase has not formed . This is likely due to the lack of flexibility of the PDA spacer, as well as its lower level of penetration into the neighboring perovskite layers, which prevents the formation of a layered phase. , In addition, the PDA-I precursor solution changes its properties over time, which is likely due to the oxidation and subsequent polymerization of (PDA)­I 2 (Supporting Information, Section S2, Figures S1 and S2). A significant difference in the behavior of the spacer is observed when the alkyl chain length of the spacers is extended by one methylene (−CH 2 −) group; the PDMA-I films show typical layered perovskite diffractograms with a sharp diffraction peak at 7.19°, which corresponds to the (001) reflection with a layer spacing of 12.3 Å, along with higher order diffraction peaks of the (00 l ) Bragg planes family (Figure a, red). , Further extension of the spacer to PDEA led to a larger lattice parameter, evidenced by the diffraction peak located at 6.98° (Figure a, blue), corresponding to a layer spacing of 12.7 Å.…”

Naphthalenediimide/Formamidinium-Based Low-Dimensional Perovskites

“…As a result, the effective masses in the in-plane directions decrease upon increasing n for both electrons and holes, while significantly higher values for electrons prevail in the perpendicular direction to the inorganic layer (Table S2, SI). Although some of the in-plane calculated effective hole masses tend to be somewhat lower than those of 1,4-phenyldimethanammonium (PDMA)-based Dion–Jacobson structures, the effective masses for electrons are significantly increased with respect to other 2D perovskites due to the localized nature of the spacer-dominated CB, which could affect charge transport in these hybrid materials.…”

“…The level of disorder may also be the result of stronger noncovalent interactions in the spacer layer, which could compromise its adaptability in the formation of 2D perovskites, leading to mixed low-dimensional phases and their orientations. 37 In addition, the highly localized nature of the CB revealed by theoretical analysis is likely to affect the charge transport. This was further confirmed by trial tests of photovoltaic performances in conventional solar cell devices (detailed in Section S8, SI) that suggest lower performances under several conditions as compared to other 2D and 2D/3D perovskites (Tables S3−S5, Figure S10, SI).…”