Molecular interactions and excited-state dynamics in MA free efficient and stable 2D/3D perovskite solar cells

Abstract

Mixed 2D/3D perovskite solar cells (PSCs) achieve high efficiency and long-term stability, primarily driven by the critical role of functional groups on large organic molecules, which influence the underlying chemical interaction mechanisms. In this study, we explore the molecular interactions and excited-state dynamics of mixed 2D/3D PSCs based on a methylammonium-free (MA-free) FA-Cs perovskite structure, aimed at enhancing device performance and stability. We introduced benzamidine hydrochloride (BzCl) and 3,5-difluorobenzamidine hydrochloride (DFBzCl) molecules to form a 2D perovskite layer on top of the 3D perovskite absorber. Remarkably, the incorporation of DFBzCl resulted in a significant increase in power conversion efficiency (PCE), rising from 18.09% in the control device to 21.21%. In-depth excited-state dynamics analysis using ultrafast transient absorption spectroscopy revealed that BzCl and DFBzCl incorporation slows hot carrier cooling, facilitates charge transfer, mitigates phase segregation, and extends carrier recombination lifetimes. These effects are particularly pronounced in DFBzCl-modified devices due to the higher dipole moment of the molecule, which enhances internal electric fields, improves energy band alignment, and facilitates better charge transport at the interface. The study emphasizes the importance of dipole moment, and excited-state dynamics in optimizing the performance and stability of 2D/3D PSCs, providing crucial insights for future design.