Ultrafast and High-Yield Polaronic Exciton Dissociation in Two-Dimensional Perovskites

Abstract

Layered two-dimensional (2D) lead halide perovskites are a class of quantum well (QW) materials, holding dramatic potentials for optical and optoelectronic applications. However, the thermally activated exciton dissociation into free carriers in 2D perovskites, a key property that determines their optoelectronic performance, was predicted to be weak due to large exciton binding energy (E b , about 100−400 meV). Herein, in contrast to the theoretical prediction, we discover an ultrafast (<1.4 ps) and highly efficient (>80%) internal exciton dissociation in (PEA) 2 (MA) n−1 Pb n I 3n+1 (PEA = C 6 H 5 C 2 H 4 NH 3 + , MA = CH 3 NH 3 + , n = 2−4) 2D perovskites despite the large E b . We demonstrate that the exciton dissociation activity in 2D perovskites is significantly promoted because of the formation of exciton− polarons with considerably reduced exciton binding energy (down to a few tens of millielectronvolts) by the polaronic screening effect. This ultrafast and high-yield exciton dissociation limits the photoluminescence of 2D perovskites but on the other hand well explains their exceptional performance in photovoltaic devices. The finding should represent a common exciton property in the 2D hybrid perovskite family and provide a guideline for their rational applications in light emitting and photovoltaics.

Monochromatic light-enhanced photocatalytic CO2 reduction based on exciton properties of two-dimensional lead halide perovskites

“…The exciton dissociation activity in 2D perovskites is significantly promoted because of the formation of exciton-polarons with considerably reduced exciton binding energy (down to a few tens of meV) by the polaronic screening effect. 24 This finding explains their exceptional performance in photovoltaic devices, as is also proven by the work of Giulia Folpini 28 and Tao 35 et al In summary, compared with conventional 3D perovskite nanocrystals, 2D layered perovskites exhibit unique advantages, including the following: (1) the 2D perovskites exhibit unique and much better humidity resistance because of the long hydrophobic insulating ligands. 22,24,29,36 (2) 2D perovskites possess a larger surface-area-to-volume ratio in comparison with bulk materials, affording abundant active sites for CO 2 reduction.…”

“…24 This finding explains their exceptional performance in photovoltaic devices, as is also proven by the work of Giulia Folpini 28 and Tao 35 et al In summary, compared with conventional 3D perovskite nanocrystals, 2D layered perovskites exhibit unique advantages, including the following: (1) the 2D perovskites exhibit unique and much better humidity resistance because of the long hydrophobic insulating ligands. 22,24,29,36 (2) 2D perovskites possess a larger surface-area-to-volume ratio in comparison with bulk materials, affording abundant active sites for CO 2 reduction. 2,37,38 (3) The 2D dimensionality modifies the electronic structure of bulk materials due to the quantum confinement effect.…”

“…4,[19][20][21][22][23] The 2D perovskites can be described stoichiometrically by the formula L 2 A n−1 Pb n X 3n+1 , where L is ligand cations (e.g., BA = C 4 H 9 NH 3 + ), A is organic or inorganic cations (e.g., MA = CH 3 NH 3 + and Cs + ), X is halogen anions (e.g., Cl − , Br − or I − ), and the integer n presents the layer number of perovskite units. 24 The 2D network consists of inorganic perovskite layers of corner-sharing [PbX 6 ] 4− octahedra sandwiched by two insulating ligand layers to form multiple quantum wells. The unit layers are held together by a combination of Coulombic and hydrophobic interactions to maintain the integrity of the structure.…”

“…32,33 Despite the high binding energy, a thermally activated interconversion process between bound excitons and free carriers has been discovered in 2D perovskites at room temperature: recent works by Sun et al reported ultrafast (<1.4 ps) and highly efficient (>80%) internal exciton dissociation in 2D perovskites. 24 Due to the soft lattice and polar semiconductor characteristics of 2D perovskites, they exhibit strong exciton-phonon coupling, 34 which can readily lead to the formation of large exciton-polarons. The exciton dissociation activity in 2D perovskites is significantly promoted because of the formation of exciton-polarons with considerably reduced exciton binding energy (down to a few tens of meV) by the polaronic screening effect.…”

“…38 (5) The ultrafast and highyield exciton dissociation in 2D perovskites can promote electron-hole separation, charge transport, and collection. 24 These unique advantages are expected to improve the performance of photocatalytic CO 2 reduction. Photocatalytic CO 2 reduction is a complex process with multiple products and the selective conversion of CO 2 to valuable fuel remains challenging.…”