High External Quantum Efficiency Light-Emitting Diodes Enabled by Advanced Heterostructures of Type-II Nanoplatelets

Abstract

Colloidal quantum wells (CQWs), also known as nanoplatelets (NPLs), are exciting material systems for numerous photonic applications, including lasers and light-emitting diodes (LEDs). Although many successful type-I NPL-LEDs with high device performance have been demonstrated, type-II NPLs are not fully exploited for LED applications, even with alloyed type-II NPLs with enhanced optical properties. Here, we present the development of CdSe/CdTe/CdSe core/crown/crown (multi-crowned) type-II NPLs and systematic investigation of their optical properties, including their comparison with the traditional core/crown counterparts. Unlike traditional type-II NPLs such as CdSe/CdTe, CdTe/CdSe, and CdSe/CdSe_xTe_1-x core/crown heterostructures, here the proposed advanced heterostructure reaps the benefits of having two type-II transition channels, resulting in a high quantum yield (QY) of 83% and a long fluorescence lifetime of 73.3 ns. These type-II transitions were confirmed experimentally by optical measurements and theoretically using electron and hole wave function modeling. Computational study shows that the multi-crowned NPLs provide a better-distributed hole wave function along the CdTe crown, while the electron wave function is delocalized in the CdSe core and CdSe crown layers. As a proof-of-concept demonstration, NPL-LEDs based on these multi-crowned NPLs were designed and fabricated with a record high external quantum efficiency (EQE) of 7.83% among type-II NPL-LEDs. These findings are expected to induce advanced designs of NPL heterostructures to reach a fascinating level of performance, especially in LEDs and lasers.

Keywords: Type-II nanoplatelets; advanced heterostructures; colloidal quantum wells; external quantum efficiency; light-emitting diodes.

Figure 1

(a–c) Schematic sketch and lateral cross-sectional of views of (a) CdSe core, (b) CdSe/CdTe core/crown, and (c) CdSe/CdTe/CdSe multi-crowned NPLs. (d–f) STEM images of (d) CdSe core, (e) CdSe/CdTe core/crown, and (f) CdSe/CdTe/CdSe multi-crowned NPLs. (g–j) EDX elemental mapping spectra for all elements (g) combined and individual (h) Cd (red), (i) Se (green), and (j) Te (blue) elements of CdSe/CdTe/CdSe multi-crowned NPLs.

Figure 2

(a) Normalized absorption and (b) PL spectra of the core, core/crown, and multi-crowned NPLs. The inset picture in part a shows the emission from core, core/crown, and multi-crowned NPLs under UV lamp excitation (from left to right). (c) QY evolution of the core, core/crown, and multi-crowned NPLs throughout the crown growth processes. (d) PL decay curves of the core, core/crown, and multi-crowned NPLs at the emission peak wavelength. (e and f) Schematics of the band offsets between CdSe and CdTe layers and their respective charge transition interfaces for the (e) core/crown and (f) multi-crowned NPLs.

Figure 3

(a and c) Electron wave function for (a) the core/crown and (c) multi-crowned NPLs. (b and d) Hole wave function for (b) the core/crown and (d) multi-crowned NPLs. The core, core/crown, and multi-crown layers are indicated as bright green, red, and dark green rectangles, respectively, to guide the eye and for illustration purposes.

Figure 4

(a) Device structure of the multi-crowned NPL-LEDs. (b) EL spectrum of the multi-crowned NPL-LEDs. Inset is a photograph of NPL-LEDs under bias. (c) Position of the coordinates of (0.687, 0.311) in the CIE diagram. (d) EL spectra of the multi-crowned NPL-LEDs under different biases varied in the steps of 1 V. (e) EQE and CE of the multi-crowned NPL-LEDs. (f) Current density and luminance of the multi-crowned NPL-LEDs.