Phase Modulation by Vacuum Poling toward Enhanced Performance of Quasi-Two-Dimensional Bromide Perovskite-Based Light-Emitting Diodes

Modulation Phase Distribution of Ruddlesden-Popper Quasi-2D Perovskites with a Similarly Spaced Dion-Jacobson Phase

Quasi-two-dimensional (quasi-2D) perovskites exhibit excellent performance when applied to light-emitting diodes (LEDs). However, quasi-2D perovskite films generally have nonuniform n phases and irregular internal crystal structures, which degrade the device's performance. Here, we propose using a Dion-Jacobson (DJ)-type organic spacer to modulate the phase distribution of the Ruddlesden-Popper (RP) quasi-2D perovskite. A DJ-type organic spacer cation, 1.6-hexamethylenediamine (HDABr2), was introduced into the perovskite as the second spacer cation with propylamine hydrobromide (PABr). As DJ-type and RP-type perovskites have similar spacings, RP-DJ style does not cause a chaotic crystalline structure; instead, it modulates the perovskite crystallization and narrows the phase distribution. In parallel, there is a substantial improvement in the maximum luminance, current efficiency, external quantum efficiency, and device stability of the quasi-2D perovskite LEDs. This work provides a novel concept for combining the organic spacer cations for quasi-2D perovskites.

Stable Yellow Light-Emitting Diodes Based on Quasi-Two-Dimensional Perovskites

Perovskite light-emitting diodes (PeLEDs) show great potential in display and lighting because of their tunable wavelength, narrow emission bandwidths, and high color purity. Currently, the external quantum efficiency (EQE) of red and green PeLEDs has reached >23%. However, yellow PeLEDs are still rarely reported because of phase separation in mixed-halide perovskites and the coexistence of multiple phases in quasi-two-dimensional (quasi-2D) perovskites L₂A_n-1B_nX_3n+1 (n = 1, 2, 3, ...), where L is a bulky organoammonium ligand. Here, we fabricate stable yellow PeLEDs by manipulating the phase distribution and incorporating rubidium cations (Rb⁺) in quasi-2D perovskites. The transient absorption results confirm that alkylammonium ligand butyl ammonium (BA) has a narrower phase distribution than phenylethyl ammonium (PEA) in the quasi-2D perovskites, resulting in a more blue-shifted emission peak. We further incorporate a proper molar ratio of Rb⁺ in the (BA)₂CsPb₂I₇ perovskite to blue-shift the emission peak to the yellow range. Finally, the yellow PeLEDs exhibit an EQE of 3.5%, and the stable emission peak is located at 595 nm. Our work provides a useful approach for the fabrication of highly efficient and stable yellow PeLEDs.

Ultrafast and High-Yield Polaronic Exciton Dissociation in Two-Dimensional Perovskites

Layered two-dimensional (2D) lead halide perovskites are a class of quantum well (QW) materials, holding dramatic potentials for optical and optoelectronic applications. However, the thermally activated exciton dissociation into free carriers in 2D perovskites, a key property that determines their optoelectronic performance, was predicted to be weak due to large exciton binding energy (E_b, about 100-400 meV). Herein, in contrast to the theoretical prediction, we discover an ultrafast (<1.4 ps) and highly efficient (>80%) internal exciton dissociation in (PEA)₂(MA)_n-1Pb_nI_3n+1 (PEA = C₆H₅C₂H₄NH₃⁺, MA = CH₃NH₃⁺, n = 2-4) 2D perovskites despite the large E_b. We demonstrate that the exciton dissociation activity in 2D perovskites is significantly promoted because of the formation of exciton-polarons with considerably reduced exciton binding energy (down to a few tens of millielectronvolts) by the polaronic screening effect. This ultrafast and high-yield exciton dissociation limits the photoluminescence of 2D perovskites but on the other hand well explains their exceptional performance in photovoltaic devices. The finding should represent a common exciton property in the 2D hybrid perovskite family and provide a guideline for their rational applications in light emitting and photovoltaics.