You Zhai

High Efficiency and Optical Anisotropy in Double-Heterojunction Nanorod Light-Emitting Diodes

Recent advances in colloidal quantum dot light-emitting diodes (QD-LEDs) have led to efficiencies and brightness that rival the best organic LEDs. Nearly ideal internal quantum efficiency being achieved leaves light outcoupling as the only remaining means to improve external quantum efficiency (EQE) but that might require radically different device design and reoptimization. However, the current state-of-the-art QD-LEDs are based on spherical core/shell QDs, and the effects of shape and optical anisotropy remain essentially unexplored. Here, we demonstrate solution-processed, red-emitting double-heterojunction nanorod (DHNR)-LEDs with efficient hole transport exhibiting low threshold voltage and high brightness (76,000 cd m(-2)) and efficiencies (EQE = 12%, current efficiency = 27.5 cd A(-1), and power efficiency = 34.6 lm W(-1)). EQE exceeding the expected upper limit of ∼ 8% (based on ∼ 20% light outcoupling and solution photoluminescence quantum yield of ∼ 40%) suggests shape anisotropy and directional band offsets designed into DHNRs play an important role in enhancing light outcoupling.

Double-heterojunction nanorods.

As semiconductor heterostructures play critical roles in todays electronics and optoelectronics, the introduction of active heterojunctions can impart new and improved capabilities that will enable the use of solution-processable colloidal quantum dots in future devices. Such heterojunctions incorporated into colloidal nanorods may be especially promising, since the inherent shape anisotropy can provide additional benefits of directionality and accessibility in band structure engineering and assembly. Here we develop double-heterojunction nanorods where two distinct semiconductor materials with type II staggered band offset are both in contact with one smaller band gap material. The double heterojunction can provide independent control over the electron and hole injection/extraction processes while maintaining high photoluminescence yields. Light-emitting diodes utilizing double-heterojunction nanorods as the electroluminescent layer are demonstrated with low threshold voltage, narrow bandwidth and high efficiencies.

Double-heterojunction nanorod light-responsive LEDs for display applications

Multifunctional displays As we head toward the “Internet of things” in which everything is integrated and connected, we need to develop the multifunctional technology that will make this happen. Oh et al. developed a quantum dot-based device that can harvest and generate light and process information. Their design is based on a double-heterojunction nanorod structure that, when appropriately biased, can function as a light-emitting diode or a photodetector. Such a dual-function device should contribute to the development of intelligent displays for networks of autonomous sensors. Science, this issue p. 616 A dual-function device capable of light harvesting and emission is demonstrated for intelligent displays. Dual-functioning displays, which can simultaneously transmit and receive information and energy through visible light, would enable enhanced user interfaces and device-to-device interactivity. We demonstrate that double heterojunctions designed into colloidal semiconductor nanorods allow both efficient photocurrent generation through a photovoltaic response and electroluminescence within a single device. These dual-functioning, all-solution-processed double-heterojunction nanorod light-responsive light-emitting diodes open feasible routes to a variety of advanced applications, from touchless interactive screens to energy harvesting and scavenging displays and massively parallel display-to-display data communication.

Cu2S/ZnS Heterostructured Nanorods: Cation Exchange vs. Solution–Liquid–Solid‐like Growth

Cu2 S/ZnS heterostructured nanorods (HNRs) with uncommon morphologies are achieved through single-pot and multi-batch synthetic strategies. In both cases, Cu2 S NRs form first, which then undergo partial cation exchange and solution-liquid-solid (SLS)-like growth catalyzed by the remaining Cu2 S parts of the NRs. The location and the volume of ZnS achieved through partial cation exchange control the size of the Cu2 S catalysts, which in turn determine whether tapered rod-rod, body/tail, or barbell-like structure results from subsequent SLS-like growth. Concurrent cation exchange can sometimes cause Cu2 S catalysts to be lost during SLS-like growth, leading to further diversity in achievable morphologies of Cu2 S/ZnS HNRs. Additional insights are gained on how parameters such as Zn precursor, ligand choice, and concentration alter cation exchange and SLS-like growth steps.