Emmanuelle Deleporte1
ENS Paris-Saclay1
Halide perovskites have proven recently to be excellent materials for optoelectronics, showing relevant properties both in the photovoltaic and light emitting devices context. In particular, perovskite-based solar cells, using three-dimensional (3D) perovskites, have reached power conversion efficiencies (PCE) up to 25.7%, close to silicon technology. However, it is still limited for commercialization by the instability of the 3D perovskites when exposed to light, temperature, oxygen, and moisture. Thanks to properties like surface passivation, reduction of nonradiative recombination, or inhibition of ionic migration, remarkable achievements have been made on PCE and stability by realizing 2D/3D heterostructures, using new cations and innovative architectures. Nevertheless, the understanding of the phenomena driving the formation of the 2D/3D interface remains to be deepened as it is a crucial challenge for the optimization of devices. In order to study the formation mechanisms occuring at the interface between the 2D and 3D perovskites, we focus here on phenylethylammonium (PEA) based amino-groups, commonly used for synthesizing 2D-3D perovskites film with optimum optical properties for solar cells and light-emitting devices.<br/><br/>We first report on the synthesis of a 2D/3D perovskite heterostructure by depositing a solution containing the para-fluorophenylethylammonium iodide (4FPEAI) cation salt on a triple cation 3D perovskite layer, commonly used in efficient solar cells. This leads to the formation of a thin layer of 2D perovskite crystallized on top of the 3D bulk perovskite layer. In order to optimize the 2D layer, we tested different synthesis parameters, such as the concentration of 2D cation solution, the time soaking between the 2D cation solution and the 3D perovskite, and the lead iodide content of the 3D perovskite. We studied more particularly the formation mechanism of the 2D perovskite and the chemical and physical phenomena occurring at this interface, by performing a large panel of characterizations, ranging from the analysis of extreme surface (XPS) and morphology (SEM) to the optical and structural characterization (PL, XRD). We find that the 3D perovskite composition affects the stoichiometry of the 2D layer formed on top. We thus conclude that the 2D layer formation results from at least two concomitant mechanisms: the reaction of the PEA-based organic spacer with the excess PbI<sub>2</sub> from the 3D layer as commonly observed and a cation exchange reaction between the 3D phase and 4-FPEAI [1]. The presence of this second mechanism is confirmed by forming a 2D perovskite phase even after removing the PbI<sub>2</sub> excess from the 3D perovskite stoichiometry. We revealed that these mechanisms occur concurrently to form the 2D layer until all the 4-FPEAI cation is consumed, making it the limiting reactant of the process.<br/><br/>We then report the use of PEA in the synthesis of CsPbBr3 nanocrystals (NCs) to reach a very high monodispersity without the need for several centrifugation steps [2]. In this method, some polydisperse CsPbBr3 NCs are first synthesized with usual protocols. Then, their size and size distribution are reduced with the help of PEA acting as a pair of scissors cutting the preformed NCs. From optical and TEM characterizations, we propose a mechanism involving a cation exchange reaction between the 3D perovskite cation of the preformed NCs and the PEA , leading to a structural modification of 3D perovskite in a 2D perovskite structure. This step is then followed by a reorganization of the remaining CsPbBr3 NCs in presence of the reactants present in the solution.<br/><br/>[1] T. Campos et al,<i>J. Phys. Chem. C</i> 2022, 126, 31, 13527–13538. https://doi.org/10.1021/acs.jpcc.2c04957<br/><br/>[2] C. R. Mayer et al, <i>Chem. Commun.</i>, 2022, 58, 5960-5963. https://doi.org/10.1039/D2CC01028C