Photoexcitation Control of Excitation Relaxation in Mixed‐Phase Ruddlesden‐Popper Hybrid Organic‐Inorganic Lead‐Iodide Perovskites

Abstract

The electronic states and exciton binding energies of layered Ruddlesden‐Popper (RP) metal‐halide perovskites can be tailored through changes of their chemical composition, yielding multi‐phase systems with complex energy cascades. Ultrafast photoexcitation relaxation with transfer dynamics into domains of increasing layer number has been reported for these materials. Here, ultrafast optical spectroscopy is used to report an unexpected excitation energy dependence of photoexcitation relaxation dynamics in mixed‐dimensional benzylammonium cesium lead iodide RP perovskite (BeA2CsPb2I7) thin films, which gives rise to spectrally broadband luminescence over the visible region. Using transient absorption and photoluminescence spectroscopy it is found that excitations, which are formed in the n = 2 RP‐phase after photoexcitation with ≈0.2 electron volt excess energy, transfer to higher layer number RP‐phases on unexpectedly slow timescales of tens of picoseconds. Further, it is observed that such excitations are initially optically passive. Notably, luminescence occurs under these conditions from multiple RP‐phases with optical bandgaps across the visible range, yielding broadband luminescence. The results hold potential for realization of broadband white‐light emitters and other light‐emitting devices.

Pressure-Induced Changes in the Phase Distribution and Carrier Dynamics of Quasi-Two-Dimensional Ruddlesden–Popper Perovskites

“…The resulting quasi-2D perovskite films contain mixed-phase structures with varying layer thicknesses from n = 1 to n = ∞, as illustrated in Figure b. This also creates an energy funnel from 2D to 3D perovskites. − The 2D perovskite PEA 2 PbBr 4 , 3D perovskite CsPbBr 3 , and quasi-2D perovskite (PEA) 2 Cs n –1 Pb n Br 3 n +1 films on quartz substrates were separately prepared to analyze their X-ray diffraction (XRD) characteristics (Figure c). By comparing the diffraction characteristic peaks of these films, we can clearly observe that the quasi-2D perovskite simultaneously exhibits the diffraction characteristic peaks of both 2D and 3D perovskites, thereby proving that the quasi-2D perovskite possesses a mixed phase of 2D and 3D structures.…”

“…Within <1 ps, the GSB signal of the n = 1 phase reaches a maximum and begins to decay, while the intensity of the GSB signal of the n = ∞ phase increases and continuously red-shifts within the first 10 ps. This suggests electron transfer from lower n phases to the n = ∞ phase in the film. ,,,− At 1.62 GPa, the GSB signals of the n = 1 and n = 2 phases red-shift to 424 and 447 nm, respectively, while the signals of the n = 3 and n = ∞ phases disappear due to phase transitions. At 3 GPa, the GSB signal of the n = 1 phase further red-shifts to 442 nm, and the phase transition of the n = 2 phase causes the disappearance of its GSB signal.…”

“…8−10 Their chemical formula is LA 2 A n−1 Pb n X 3n+1 , where LA serves as the molecular ligand for interlayer cations, such as butylammonium (BA) and phenethylammonium (PEA), A represents cesium (Cs), methylammonium (MA), or formamidinium (FA), X represents halide anions, and n denotes the number of metal layers sandwiched between organic layers. 11,12 When n = 1, pure two-dimensional (2D) perovskites are formed; when n = ∞, pure three-dimensional (3D) perovskites are formed. Intermediate values of n yield quasi-2D perovskites.…”

“…The quasi-two-dimensional (quasi-2D) Ruddlesden–Popper (RP) hybrid metal halide perovskites feature a natural quantum well structure with layered octahedral inorganic quantum wells separated by organic chains. This architecture offers enhanced stability and optoelectronic properties. − Their chemical formula is LA 2 A n –1 Pb n X 3 n +1 , where LA serves as the molecular ligand for interlayer cations, such as butylammonium (BA) and phenethylammonium (PEA), A represents cesium (Cs), methylammonium (MA), or formamidinium (FA), X represents halide anions, and n denotes the number of metal layers sandwiched between organic layers. , When n = 1, pure two-dimensional (2D) perovskites are formed; when n = ∞, pure three-dimensional (3D) perovskites are formed. Intermediate values of n yield quasi-2D perovskites.…”

“…As the pressure increases, the average lifetime decreases to 0.32 ns at 3.66 GPa. This reduction is attributed to the suppression of energy transfer processes and a decrease in the number of nonradiative transitions, which lead to an increased number of radiative transitions, shorter lifetimes, and enhanced luminescence. ,,,, After 3.98 GPa, an overlap between free exciton and STE emission is observed in the PL spectra, significantly affecting the average lifetime of the n = 1 phase (Figure b and Figure S9). Figure e shows that the average lifetime of the n = ∞ phase slightly increases from ambient conditions to 1 GPa, followed by a substantial increase from 1 to 1.5 GPa.…”