What are the application developments of quantum dots in photoluminescence and electroluminescence?

Following the discovery of many new physical phenomena of semiconductor nanocrystals [1-5], many potential applications using quantum dots (QD) have been discovered. Due to the quantum confinement effect and quantum size effect, semiconductor quantum dots have the characteristics of wide excitation spectrum, narrow half-value width, adjustable wavelength and solution processing, etc., which have received extensive attention [6-9]. After more than 30 years of development, quantum dot materials have achieved a "green synthesis route", and their performance has gradually improved, enabling the production and supply of industrialized products. Currently, photoluminescent devices for commercial applications have been developed, and this series of devices have been used in LED lighting. And display field [10-12]. Especially in the display field, quantum dots such as cdse have extremely narrow line widths, high color saturation, and strong color reproduction capabilities for objects, which can reach more than 120% of the ntsc color gamut [13,14], causing most TVs in the world Sought after by manufacturers and mobile phone screen suppliers. Since SONY released the first quantum dot TV in 2013, companies such as TCL, Samsung, LG and Nanojing Technology have released several quantum dot products, covering many fields such as TVs, mobile phones, and computer monitors [15,16]. At the 2018 CES exhibition in the United States, TCL, Hisense and other companies pushed for quantum dot display technology, which will surely promote the further development and growth of quantum dot display. It is estimated that in 2025, quantum dot displays can occupy more than 30% of the market [17].

This review focuses on the application of quantum dots to photoluminescence and electroluminescence applications in high-color quality lighting and display technologies. The development of quantum dots has entered the stage of commercial application from the initial lighting to today's photoluminescence high color gamut display backlight. In the next stage, it can be imagined that the gradual reduction in the cost of quantum dots, the large-scale synthesis, and the efforts made by manufacturers such as QD Vision, Samsung, and LG in quantum dot electroluminescence, will further promote the large-area electricity of quantum dots. Commercialization of electroluminescent devices.

2. Colloidal quantum dots

Colloidal quantum dots usually refer to nano-sized crystals synthesized and processed in solution [18], which can be uniformly dispersed in the solution. The surface of the quantum dots is covered with a layer of organic ligands, and the ligands are connected by coordination bonds. To the surface of the quantum dot. The most common quantum dots are semiconductor nanoparticles composed of II-VII group (cdse, cds, Znse, cds, Pbs, Pbse), III-VI group (InP, InAs) or I-III-VII group (cuIns2, AgIns2). By combining different elements and ligands in the synthesis, quantum dots with different morphologies and properties can be obtained [19,20].

Due to the quantum size effect and quantum confinement effect, by simply adjusting the size of the prepared quantum dots, the spectrum can cover all wavelengths from blue to near-infrared [21-23]. For example, cadmium selenide quantum dots, when the synthetic particle size increases from 2nm to 8nm, under ultraviolet light, its color can transition from blue to red [24]. At present, cadmium-based quantum dots have proved to have excellent performance [25], and quantum dots composed of cadmium, zinc, selenium, sulfur and other elements have entered the application stage. At the same time, cadmium-free quantum dots such as InP [26] are also in the process of research progress; perovskite quantum dots are also currently a popular research system, but the stability of perovskite quantum dots is still a problem. This review focuses on colloidal semiconductor quantum dots.

2.1. Development of quantum dot synthesis

The synthesis of quantum dots has played a decisive role in the development of quantum dots. Only stable and reliable quantum dots can be obtained to lay the foundation for research and industrial applications. According to the synthesis system of quantum dots, it is divided into water phase and oil phase system, but the stability of the quantum dots synthesized in the water phase is poor, the quantum yield is low, the size distribution is wide, and it is easy to agglomerate and precipitate, and has been gradually eliminated [27]. In the oil phase system, usually including in the organic high boiling point solvent at a temperature of 120-360°C, the precursor reacts to form the nuclei of quantum dots and stops the growth by subsequent cooling [25,28,29]. In 2001, Peng [29] and others successfully prepared high-quality cdse, cds, and cdte quantum dots using cadmium oxide, which has low toxicity and reactivity. Then in 2002, a non-coordinating solvent system [30] was proposed, which is currently the most used system. The wide octadecene system, this low-melting, high-boiling solution, successfully prepared cds quantum dots in an Ar atmosphere. This synthesis system does not need to be reacted in an anhydrous environment, and the reaction is mild, the quality of the crystal nucleus is easy to control, the experiment reproducibility is good, the preparation process is simplified, and it is known as the "green synthesis route". It is now academically and industrially They are all synthesized using this method.

In the past 10 years, the micro-reaction method has also been improved. This method can be used to produce nanocrystals on a large scale and have good control over physical and chemical properties. Due to the increase in the controllability of the reactor, the integration of sensors that can be analyzed in real time in each process step and the optimization of the algorithm to increase the output have made this improvement possible [31,32]. Nanocrystalline colloids have been successfully synthesized in microreactors, such as cdte, cdse, InP[33,34], and even cdse/Zns and Znse/Zns core/shell quantum dots [35]. Although the microreactor method can replace batch synthesis, additional improvements are needed to synthesize nanocrystals with more complex compositions, shapes, and controllable fluorescence properties.