Spacer Cation Alloying in Ruddlesden-Popper Perovskites for Efficient Red Light-Emitting Diodes with Precisely Tunable Wavelengths

Abstract

Perovskite light-emitting diodes (PeLEDs) have recently shown significant progress with external quantum efficiencies (EQEs) exceeding 20%. However, PeLEDs with pure-red (620-660 nm) light emission, an essential part for full-color displays, remain a great challenge. Herein, a general approach of spacer cation alloying is employed in Ruddlesden-Popper perovskites (RPPs) for efficient red PeLEDs with precisely tunable wavelengths. By simply tuning the alloying ratio of dual spacer cations, the thickness distribution of quantum wells in the RPP films can be precisely modulated without deteriorating their charge-transport ability and energy funneling processes. Consequently, efficient PeLEDs with tunable emissions between pure red (626 nm) and deep red (671 nm) are achieved with peak EQEs up to 11.5%, representing the highest values among RPP-based pure-red PeLEDs. This work opens a new route for color tuning, which will spur future developments of pure-red or even pure-blue PeLEDs with high performance.

Keywords: Ruddlesden-Popper perovskites; electroluminescence; light-emitting diodes; pure red; spacer cation.

Figure 1

a) Chemical structures of PBA, MBZA, and POEA. b) XRD patterns of n = 1 RPP films of (PBA _x MBZA_{1‐ x} )₂PbI₄. c–e) Finite potential QW superlattice model to fit resonance energies of x = 1 (c), x = 0.5 (d), and x = 0 (e) samples. Solid lines indicate the fitted energies and dashed lines indicate the fitted band gaps. The black dotted lines indicate the fitted bandgap (≈1.80 eV) of CsPbI₃.

Figure 2

a) Absorption spectra, b) normalized PL spectra, and c) PLQYs of (PBA _x MBZA_{1‐ x} )₂Cs _{n ‐1}Pb _n I_{3 n +1} RPP films. The inset is photos of the corresponding RPP films under 365 nm UV light. d–f) Gaussian peak fitting results of the PL spectra for the (PBA _x MBZA_{1‐ x} )₂Cs _{n ‐1}Pb _n I_{3 n +1} RPP films with x = 1 (d), x = 0.5 (e), and x = 0 (f).

Figure 3

a–c) Slices of TA spectra for the (PBA _x MBZA_{1‐ x} )₂Cs _{n ‐1}Pb _n I_{3 n +1} RPP films with x = 1 (a), x = 0.5 (b), and x = 0 (c). Dotted vertical lines indicate the photobleaching (PB) of n‐thickness QWs. Blue horizontal arrows in (b,c) indicate the redshift of the dominant PB peaks toward larger‐n QWs. d–f) Kinetics of PB peaks for the RPP films with x = 1 (d), x = 0.5 (e), x = 0 (f) with multi‐exponential fits. The olive data in (f) corresponds to n ≥ 5 PB at 652 nm. Details of lifetime fitting of kinetics can be found in Table S3, Supporting Information.

Figure 4

a) Normalized EL spectra, b) CIE 1931 chromatic coordinates, c) current density–voltage–luminance characteristics, and d) EQE values versus current density of the PeLEDs. e) Normalized EL spectra of PeLEDs under various bias voltages with x = 1, 0.5, and 0. f) Normalized EL spectra of PeLEDs under a constant bias voltage of 3.2 V with x = 1, 0.5, and 0.

Figure 5

a) Normalized PL spectra, b) EL spectra, and c) EQE values versus current density of (PBA _x POEA_{1‐ x} )₂Cs _{n ‐1}Pb _n I_{3 n +1} RPP films. d) Normalized PL spectra, e) EL spectra, and f) EQE values versus current density of (POEA _x MBZA_{1‐ x} )₂Cs _{n ‐1}Pb _n I_{3 n +1} RPP films.