Optical properties of multinary copper chalcogenide semiconductor nanocrystals and their applications in electroluminescent devices

Abstract

In the past few decades, semiconductor nanocrystals (semiconductor quantum dots) have received widespread attention due to their high color purity and size-dependent optical properties as well as their potential applications in the next generation of lightening and displays. At present, most of the emitting layer materials in the quantum-dot electroluminescent devices are focused on cadmium-based chalcogenide quantum dots, but cadmium is harmful to the environment and human health, which faces great challenges in the future commercialization. Therefore, it is necessary to develop an environment-friendly, cadmium-free semiconductor quantum dot with excellent optoelectronic performance. In recent years, multinary copper chalcogenide semiconductor nanocrystals have attracted extensive attention due to their low toxicity, tunable luminescent properties and potential applications in optoelectronic devices. Ternary copper-based chalcogenide semiconductor nanocrystals (such as CuInS2, etc.) can be considered as the counterparts of the II-VI compounds, in which the divalent metal ions (such as Zn2+, etc.) are replaced by monovalent copper ions (Cu+) and trivalent indium ions (In3+). As a result, the crystal structure of ternary I-III-VI nanocrystals is similar to that of II-VI nanocrystals. Up to date, ternary CuInS2 nanocrystals are the most widely studied among different multinary copper chalcogenide semiconductor nanocrystals. The crystal structure of ternary CuInS2 nanocrystals mainly includes sphalerite, wurtzite and chalcopyrite phase, and different-phase CuInS2 nanocrystals often have different optical band gaps. With the development of colloidal chemical synthetic technique, most of the multinary copper chalcogenide semiconductor nanocrystals can be synthesized through a colloidal chemical method. The reaction parameters including temperature, reaction precursor activity and surface ligand in the synthesis process play an important role in the tuning of the size, morphology and composition of the resultant nanocrystals. Generally, ternary CuInS2 nanocrystals have a low photoluminescence quantum yield due to their internal and surface defects. Since most of the multinary copper chalcogenide semiconductor nanocrystals exhibit non-stoichiometric characteristics, the number of copper vacancies also affects their luminescence performance to some extent. In order to improve the luminescence performance, it is a common strategy to introduce Zn2+ into the Cu-In-S system to form a quaternary Cu-In-Zn-S alloyed structure or grow a wide-band gap semiconductor material (such as ZnS) onto its surface to form a core-shell structure. However, due to higher ion mobility of copper ions at a relatively high temperature and the similar radius of Cu+ ion and Zn2+, cation exchange reaction is likely to occur in the epitaxial growth of ZnS shells. Therefore, it is difficult to distinguish the alloyed and core-shell structure for the multinary copper chalcogenide semiconductor nanocrystals. With the in-depth understanding of the relationship between the nanostructure and luminescence properties of multinary copper chalcogenide semiconductor nanocrystals, these nanocrystals have been widely used as light-emitting layers in electroluminescent devices, and a significant progress has been made. At present, different strategies have been exploited to improve the device performance of the electroluminescent devices based on multinary copper chalcogenide semiconductor nanocrystals, such as changing the device structure, regulating the nanostructure of materials, and device interfacial engineering. This review gives a detailed summary on the relationship between the composition, surface ligands, crystal structure as well as the nanostructures and optical properties of the multinary copper chalcogenide semiconductor nanocrystals. Moreover, the progress of electroluminescence devices based on the multinary copper chalcogenide semiconductor nanocrystals is highlighted, and finally the prospect of the development of multinary copper chalcogenide semiconductor nanocrystals is concluded.