Long Carrier Diffusion Length and Efficient Charge Transport in Thick Quasi-Two-Dimensional Perovskite Solar Cells Enabled by Modulating Crystal Orientation and Phase Distribution

The applications of quasi-two-dimensional (quasi-2D) perovskite solar cells (PSCs) are limited by their insufficient charge transport, and the underlying charge transport mechanism is not clear yet. Herein, we report a simple strategy for achieving efficient charge transport in the quasi-2D perovskite FPEA 2 MA n –1 Pb n I 3 n +1 (⟨ n ⟩ = 4) by combining the crystal orientation control via the hot-casting method and the regulation of phase composition and distribution using methylammonium chloride (MACl) additives. The steady and transient optical spectra reveal that the synergistic effect of hot-casting processing and MACl additive leads to the formation of 3D-like phases, acting as highly conductive pathways to boost charge collection and transport in quasi-2D perovskite films. These specific phase heterogeneities result in an extraordinary long carrier diffusion length of 1.2 μm. The resultant quasi-2D PSCs show a high power conversion efficiency of 17.5%. Notably, this remarkable efficiency is achieved for a relatively thick film (∼623 nm), which is much thicker than most reported efficient quasi-2D PSCs (200–500 nm). Moreover, our PSCs have demonstrated excellent environmental stability and thermal stability. Our results highlight that controlling the crystal orientation and multiphase distribution of quasi-2D perovskites is very promising to push the efficiency of quasi-2D PSCs approaching their three-dimensional (3D) counterparts.