Speaker
Description
Quasi-2D perovskites (L₂Aₙ₋₁PbₙI₃ₙ₊₁) are naturally forming quantum wells with highly tunable optoelectronic properties¹. While extensive research has explored the roles of spacer cation chemistry and halide substitution in governing carrier dynamics, the influence of A-site cation variation remains comparatively understudied¹⁻³. Here, we employ a fixed 4-fluorobenzylammonium (4-FBA) spacer paired with four representative A-site cations — methylammonium (MA), formamidinium (FA), cesium (Cs), and guanidinium (Gua) spanning a range of ionic sizes and organic/inorganic character, to systematically probe their effect on the optical properties and carrier dynamics of the resulting n=2 quasi-2D perovskite films.
Transient absorption (TA) measurements under 480 nm excitation reveal pronounced bleach features at 520 nm and 572 nm, assigned to the n = 1 and n = 2 phases, respectively, along with an additional bulk-like-phase bleach in the Cs- (700 nm), FA- (760 nm), and MA- (760 nm) based films (Figure 1). The presence of this bulk-phase signature and its apparent absence in Gua-based films, together with the extracted TA kinetics, points to clear A-site-dependent differences in carrier dynamics: the smaller Cs, MA, and FA cations favor 3D-like domain formation and efficient carrier funneling toward the bulk phase, whereas the bulkier Gua cation appears to suppress this process. Among the bulk-phase-forming compositions, the FA-based film shows the shortest n = 1 lifetime, indicating the most rapid and efficient downhill energy funneling, while Cs-, Gua-, and MA-based films exhibit longer n = 1 lifetimes, consistent with comparatively slower funneling. At the bulk phase, MA- and Cs-based films show the longest lifetimes, suggesting stronger carrier localization attributable to the formation of MAPbI₃-like domains and the lattice rigidity of the CsPbI₃-like framework, which promotes carrier self-trapping and thereby prolongs the observed lifetimes. These results identify A-site cation size and rigidity as a practical, spacer-independent lever for tuning carrier localization and emission energy in layered perovskite optoelectronics.