Improving Out‐of‐Plane Charge Mobility and Phase Stability of Dion‐Jacobson Lead‐Free Perovskites via Intercalating π‐Conjugated Aromatic Spacers

Abstract

Currently, the perovskite solar cells (PSCs) with CsSnI 3 as solar energy absorbers hold a PCE ≈10%, [10] which is far below the theoretical efficiency of CsSnI 3 PSCs exceeding 30% [11] and the record PCE of lead-based perovskite solar cells. [12] Moreover, the PCE of α-CsSnI 3 perovskite suffers a significant reduction because the crystal structure undergoes a phase transition. [13] When exposed to air or organic solvents, the divalent Sn 2+ is prone to oxidize and change to Sn 4+ . Then the α-CsSnI 3 perovskite readily transforms into the low-symmetry B-γ CsSnI 3 phase, followed by the thermodynamically more stable but photon inert Y-phase (yellow with a bandgap of 2.6 eV), [14][15][16] and finally deforms to Cs 2 SnI 6 with weak light absorption ability. [7,9,17,18] Inspiringly, the interface engineering strategy of converting 3D bulk perovskites into 2D layered perovskites can significantly enhance the photovoltaic performance and stability of 3D Sn-based PSCs. [19][20][21] Two-dimensional perovskites can be constructed by intercalating the charged spacer cations into the 3D structure along various crystal planes, such as the (100), (110), and (111), etc. [22][23][24] The layered Dion-Jacobson phase perovskites hold the stoichiometric formula A″A n−1 B n X 3n+1 (A″: denotes the divalent organic cations; A: represent the monovalent cation (such as MA + , FA + , and Cs + ); B: represent the divalent metal (denoted as Pb 2+ , Sn 2+ ); X: denotes the halide anion (like I − , Br − , and Cl − ); n: represent the number of the inorganic layer). The inorganic layer number (n) not only enriches the structural complexity of the perovskite family but also allows fine-tuning of the optoelectronic properties, such as tunable natural quantum wells. [23,[25][26][27] However, the photogenerated excitons are confined within the inorganic wells because of the dielectric constant mismatch between the organic and inorganic components of 2D perovskites, leading to a significantly improved exciton binding energy. [28,29] Conventional 2D lead-halide perovskites, normally display exciton binding energies of several hundred meV because of the nonpolar nature of the organic cations. [30] Compared with 3D perovskites, 2D perovskites exhibit larger exciton binding energy (E b ), [31] smaller dielectric constant (ε r ), [32] and higher open-circuit voltage (V OC ). [33] Meanwhile, the enhanced carrier effective masses suggest the charge mobility of the perovskite is enormously impeded by the organic layer. [32,34] The layered quasi-2D perovskites are recognized as one of the effective strategies to resolve the big problem of intrinsic phase instability of the perovskites. However, in such configurations, their performance is fundamentally limited due to the correspondingly weakened out-of-plane charge mobility. Herein, the π-conjugated p-phenylenediamine (PPDA) is introduced as organic ligand ions for rationally designing lead-free and tin-based 2D perovskites with the aid of theoretical computation. It is evidenced that both out-of-pla...