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 x Te1–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.

Tailoring Electronic Properties of Colloidal Quantum Dots for Efficient Optoelectronics

“…[232] The characterization results displayed in Figure 10e,f are associated with a typical EL device involving CQWs heterostructures and charge-transport layers. [233] In short, CQWs are dimensionally unique analogs of CQDs sharing a similar set of electronic and optical properties. The approaches introduced to modify CQDs can expectedly be useful for CQWs as well, though diverging outcomes are also probable.…”

“…[ 232 ] The characterization results displayed in Figure 10e,f are associated with a typical EL device involving CQWs heterostructures and charge‐transport layers. [ 233 ]…”

Orientation‐Dependent Photoconductivity of Quasi‐2D Nanocrystal Self‐Assemblies: Face‐Down, Edge‐Up Versus Randomly Oriented Quantum Wells

“…[ 1,2 ] These superior features are a direct consequence of the NPL's strong quantum confinement along the vertical and atomically precise thickness, allowing them to have an ultra‐narrow light emission, large absorption cross‐section, and giant oscillator strength with their extended lateral size. [ 3,4 ] These unique properties enable such quasi‐2D NPLs to favorable building blocks of future optoelectronic and electronic devices including light‐emitting diodes, [ 5–8 ] lasers, [ 9,10 ] photovoltaics, [ 11,12 ] photodetectors, [ 13,14 ] and transistors. [ 15,16 ] So far, the main focus of the NPL‐based devices is directed toward photoemission applications due to their superior emissive properties, while very few studies have explored the diverse electrical applications thus far.…”

On the Rational Design of Core/(Multi)-Crown Type-II Heteronanoplatelets

“…Most recently, the growth of multiple crowns has also been demonstrated on seed CdSe NPLs. ,,, CdSe/CdTe/CdSe double-crowned NPLs were used to demonstrate bicolor power-tunable emission and LEDs with a high external quantum efficiency (EQE). , CdSe/CdSe 1– x Te x /CdS core/multi-crown NPLs were shown to exhibit ultralow amplified spontaneous emission and enhanced photostability as well as reduced stacking which is significantly important for attaining uniform film formation . In addition, multiexcitonic emission and two-photon fluorescence upconversion were demonstrated using CdSe/CdS/CdTe core/multicrown NPLs .…”