Edge stabilization in reduced-dimensional perovskites

Abstract

Reduced-dimensional perovskites are attractive light-emitting materials due to their efficient luminescence, color purity, tunable bandgap, and structural diversity. A major limitation in perovskite light-emitting diodes is their limited operational stability. Here we demonstrate that rapid photodegradation arises from edge-initiated photooxidation, wherein oxidative attack is powered by photogenerated and electrically-injected carriers that diffuse to the nanoplatelet edges and produce superoxide. We report an edge-stabilization strategy wherein phosphine oxides passivate unsaturated lead sites during perovskite crystallization. With this approach, we synthesize reduced-dimensional perovskites that exhibit 97 ± 3% photoluminescence quantum yields and stabilities that exceed 300 h upon continuous illumination in an air ambient. We achieve green-emitting devices with a peak external quantum efficiency (EQE) of 14% at 1000 cd m^-2; their maximum luminance is 4.5 × 10⁴ cd m^-2 (corresponding to an EQE of 5%); and, at 4000 cd m^-2, they achieve an operational half-lifetime of 3.5 h.

Fig. 1. Visualization of reduced-dimensional perovskites.

High-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images of the layered perovskites exhibiting domains with different number of layers (a–d), where in a, a four-layered structure was observed.

Fig. 2. Photo-induced degradation mechanisms and edge-stabilization strategy.

a Schematic illustrating imperfect edges in n = 3 reduced-dimensional perovskites of PEA₂Cs_n−1Pb_nBr_3n+1 and proposed reaction pathway of superoxide production under photoexcitation, including (1) missing Cs⁺ or PEA⁺ at the edge sites, (2) missing Cs⁺ or PEA⁺ at the corner sites, (3) desorbed CsBr or PEABr near the defect. b Energy level alignment obtained from DFT calculations. c PLQYs of the perovskites treated using different molecules.

Fig. 3. Photothermal stability.

a PL stability under continuous excitation under a 374-nm laser diode. The inset shows PL spectra of the untreated control and edge-stabilized sample before (in red and blue, respectively) and after (in gray) measurement. b Normalized fluorescence intensity of the superoxide probe solution. c Thermal stability of the untreated control and edge-stabilized perovskite under a continuous heat stress. d In situ GIWAXS of the untreated control and edge-stabilized perovskite. The films were gradually annealed up to 150 °C, let there for 30 min before cooling down and measured. The inset curves show the out-of-plane line profiles before and after heat stress.

Fig. 4. Origins of edge stabilization.

a Microscopic image of the mechanically exfoliated PEA₂CsPb₂Br₇ (n = 2) single crystal under continuous excitation by using a 374-nm laser diode. The inset shows PL spectra, from the both intrinsic (λ_em = 450 nm) and edge state (λ_em = 510 nm) emission. Scale bar is 10 μm. b–d Microscopic images as function of time after in situ addition of TPPO solution. e Normalized PL spectra extracted from the microscope images. Scale bar is 10 μm. f ³¹P-NMR spectra of TPPO only, TPPO-precursor (TPPO-PbBr₂) and TPPO-perovskites to monitor the interaction between P=O and Pb in perovskites.

Fig. 5. LED performance and operational stability.

a Cross-section focused ion beam (FIB) transmission electron microscope (TEM) image. b Energy band diagram based on literature and UPS measurements. c EQE versus current density and d luminance versus voltage characteristics of untreated controls and edge-stabilized perovskite LEDs. e Operational device stability of untreated controls and edge-stabilized perovskite LEDs at a starting luminance of 4000 cd m⁻². f Device operational stability of the perovskite LED with MPPO at a starting luminance of 4000 cd m⁻².