High-performance optoelectronic device developed using non-toxic colloidal quantum dots

High-performance solar cells and infrared sensors enabled by inorganic ligand ensemble surface modification

Study by Prof. Baek Se-woong’s team featured on inside cover of Advanced Energy Materials

▲ (From left) Professor Baek Se-woong (corresponding author, Department of Chemical and Biological Engineering), Professor Kim Han-seul (corresponding author, Chungbuk National University’s Department of Advanced Materials Engineering), 

Kim Dong-eon (first author, integrated master-doctoral degree program), Cho Ga-eun (first author, integrated master-doctoral degree program)

Professor Baek Se-woong of the Department of Chemical and Biological Engineering and Professor Kim Han-seul’s team developed a ligand ensemble, enabling the control of defects in non-toxic AgBiS2 quantum dots and enhancing the efficiency of optoelectronic devices.

The research team achieved successful surface control of the less-explored charge-neutral surface of AgBiS2 quantum dots. Utilizing AgBiS2 quantum dots, they achieved the lowest open-circuit voltage loss in solar cells and the fastest response time among infrared sensors. The development of this high-performance, non-toxic quantum dot optoelectronic device not only contributes to energy conversion but also opens new doors for infrared materials/components essential in areas such as autonomous driving, advanced robotics, and immersive displays. The study was published online on January 5 in the top-tier global academic journal in the materials field, Advanced Energy Materials (IF 27.8, JCR 2.8%).

* Title of paper: Multi-Facet Passivation of Ternary Colloidal Quantum Dot Enabled by Quadruple-Ligand Ensemble toward Efficient Lead-Free Optoelectronics

Colloidal quantum dots, nano-sized particles with physical properties such as absorption and emission varying based on particle size, were celebrated by the Nobel Prize in Chemistry in 2023 and achieved commercial success in applications like QLEDs. Notably, colloidal quantum dots exhibit distinctive features compared to other solution-process-based semiconductor materials, extending their optical response beyond the near-infrared range that traditional organic semiconductors or perovskites find challenging to absorb. These quantum dots, owing to their large surface area-to-volume ratio, significantly depend on their surface state for their electrical properties. As such, there has been extensive research on the surface chemistry of colloidal quantum dots to enhance the performance of quantum dot-based optoelectronic devices. Because widely used quantum dot absorption materials contain lead sulfide (PbS), there is a growing demand for the development of eco-friendly quantum dots, unrestricted by regulations such as the Restricted Hazardous Substances (RoHS) directive.

The research team examined non-toxic quantum dots, specifically AgBiS2, which is lead-free and complies with the RoHS directive. AgBiS2 quantum dots have attracted interest due to their affordable costs, high absorption coefficients, and suitability for solution processing. However, current research on AgBiS2 quantum dot-based optoelectronic devices exhibits efficiency below 60%, lower than devices incorporating lead (Pb). Notably, innovations in ligand exchange methods, transitioning from solid-state ligand exchange to solution-phase ligand exchange, have improved the surface and electrical properties of quantum dots, especially those based on lead. Yet, in the case of AgBiS2 quantum dots, the majority of studies have employed solid-state ligand exchange.

To enhance the surface and electrical properties of AgBiS2 quantum dots, the team developed a solution-phase ligand exchange method. By introducing alkaline cations into the quantum dot ligands, they successfully improved the charge-neutral surface of AgBiS2 quantum dots that were previously challenging to modify. This resulted in a significantly higher ink stability than for quantum dots with unmodified charge-neutral surfaces. Professor Han-seul Kim's team elucidated the mechanism by which alkaline cations act on the quantum dot surface based on Density Functional Theory (DFT), demonstrating that alkaline cations can increase the attachment rate of surface ligands on the quantum dots. The quantum dots with modified charge-neutral surfaces were confirmed to have a low defect density. Ultimately, the research team proposed a AgBiS2 quantum dot-based optoelectronic device with the highest solar energy conversion efficiency of 8.1% and the fastest response time of 400 ns of existing solution-phase ligand-exchanged AgBiS2 quantum dot devices. This development opens up new possibilities for lead-free quantum dot optoelectronic devices.

Professor Baek Se-woong, the corresponding author, said, “Our quantum dot surface substitution technology exhibits exceptional versatility and has the potential to significantly increase the efficiency of devices. We expect it to have varied applications in research fields related to photovoltaic energy conversion devices. With further research, the biocompatible infrared sensor can be applied to key areas of the fourth industrial revolution, such as autonomous driving, security, healthcare, and AR/VR.”

The study was funded by Korea University, National Research Foundation’s New Researcher Support Program (No.2021R1C1C101043413), and Future Technology Research Laboratory Program (No. NRF-2022M3H4A1A03076626).