Realizing low voltage-driven bright and stable quantum dot light-emitting diodes through energy landscape flattening

Abstract

Solution-processed quantum dot light-emitting diodes (QLEDs) hold great potential as competitive candidates for display and lighting applications. However, the serious energy disorder between the quantum dots (QDs) and hole transport layer (HTL) makes it challenging to achieve high-performance devices at lower voltage ranges. Here, we introduce "giant" fully alloy CdZnSe/ZnSeS core/shell QDs (size ~ 19 nm) as the emitting layer to build high-efficient and stable QLEDs. The synthesized CdZnSe-based QDs reveal a decreased ground-state band splitting, shallow valence band maximum, and improved quasi-Fermi level splitting, which effectively flatten the energy landscape between the QD layer and hole transport layer. The higher electron concentration and accelerated hole injection significantly promote the carrier radiative recombination dynamics. Consequently, CdZnSe-based device exhibits a high power conversion efficiency (PCE) of 27.3% and an ultra-low efficiency roll-off, with a high external quantum efficiency (EQE) exceeding 25% over a wide range of low driving voltages (1.8-3.0 V) and low heat generation. The record-high luminance levels of 1,400 and 8,600 cd m^-2 are achieved at bandgap voltages of 100% and 120%, respectively. Meanwhile, These LEDs show an unprecedented operation lifetime T₉₅ (time for the luminance to decrease to 95%) of 72,968 h at 1,000 cd m^-2. Our work points to a novel path to flatten energy landscape at the QD-related interface for solution-processed photoelectronic devices.

Fig. 1. Structural and morphological properties of QDs.

Schematic illustrations of QDs with corresponding energy levels and electron/hole wavefunctions for a CdSe-based QDs and b CdZnSe-based QDs. HAADF scanning transmission electron microscopy images of c CdSe-based QDs and d CdZnSe-based QDs, insets: the corresponding size distribution histograms. EDS elemental mapping of cadmium and zinc for e CdSe-based QDs and f CdZnSe-based QDs. EDS elemental line scanning for g CdSe-based QDs and h CdZnSe-based QDs

Fig. 2. Spectroscopic analysis of two QDs with different valence-band state degeneracy.

a Absorption and PL spectra. b The second derivatives of absorption spectra. c ΔCPD evolution using excitation at 590 and 480 nm, the bars at the top represent the on–off status of the excitation light. d, e EETA spectra at different voltages. f The extracted ΔA of different bleaching signal peaks for CdSe-based and CdZnSe-based devices

Fig. 3. Device structure and performance.

a Schematic of the device structure. b J–V–L curves. c EQE−L−CE curves. d EQE−J curves. e PCE–V curves. f L−Time−V curves for two devices using CdSe and CdZnSe QDs

Fig. 4. The accelerated radiative recombination of excitons and the evolution of device temperature.

a C-V curves. b J–V characteristics of electron-only and hole-only devices. c Normalized TrEL with a 3.0 V bias for QLED with CdSe-based and CdZnSe-based QDs. The variation of surface temperature over time under different driving voltages for d CdSe-based and e CdZnSe-based QDs. f Device temperatures under different brightnesses at 10 min