UNVEILING THE MECHANISM OF 2D, QUASI-2D AND 3D HALIDE PEROVSKITE THIN FILMS FORMATION 

Metal halide perovskites (PVK) have emerged as one of the most promising family of materials for applications in various fields such as lasers, light emitting diodes, photodetectors, scintillation and photovoltaic solar cells.[1,2] This material family exhibits excellent optoelectronic properties, tunable bandgap, long charge carrier lifetime, low-cost and low-temperature solution processibility. PVK thin films can be printed on flexible substrates for lightweight devices. In PVK-based optoelectronic devices, especially in photovoltaic solar cells, the growth control and the optimization of the film material quality are key to reach high efficiency. [1,2] We have investigated the solvent and perovskite elements’ location and evolution upon the film processing steps. For this study, we focused on the technique employed for the preparation of efficient solar cells: spin coating, anti-solvent dripping and annealing.[3] 2D, Quasi-2D and 3D perovskite films formations with various compositions have been investigated. We show that the solvent always accumulates in the upper part of the initial layer, while lead is concentrated in its lower part. Moreover, solvent is eliminated in two steps upon annealing. Importantly, we show the key role of solvent elimination. We propose a taxonomy which distinguishes three types for the growth direction of perovskite crystals: downward, upward and lateral.[4] Among them, high efficiency and high stability always occur in samples with a lateral growth that can be governed with additives.