CBNU News

Image title: Multifaceted passivation strategy for developing AgBiS₂ colloidal quantum dots

Image caption: This innovative strategy uses a unique quadruple-ligand ensemble to suppress the surface defects of AgBiS₂ CQDs, resulting in a stable CQD ink and a substantial increase in the power conversion efficiency of CQD solar cells.

Image credit: Han Seul Kim from Chungbuk National University

License type: Original Content

Usage restrictions: Cannot be reused without permission

By utilizing an innovative new strategy, researchers significantly enhance the efficiency of solar cells made from AgBiS₂ colloidal quantum dots.

As an alternative to toxic lead-based colloidal quantum dots (CQDs), AgBiS₂ CQDs have garnered considerable attention for the development of non-toxic solar cells. However, these solar cells suffer from lower power conversion efficiency than their lead-based counterparts. To address this, researchers have developed a novel strategy to develop high-quality AgBiS₂ CQDs and stable CQD inks. This strategy holds great potential for developing high-efficiency solar cells and ultrafast optoelectronics.

Solution-processed semiconductors are a special class of semiconductors that can be synthesized using chemical solutions. They have gained significant attention in the fields of energy conversion and optoelectronics. Among these are colloidal quantum dots (CQDs), which are nanoscale materials that can be used to make optoelectronic semiconductors. In these semiconductors, light is converted to electricity through the generation of charge carriers. CQDs offer a unique advantage for developing solar cells with high optical-to-electrical power conversion efficiency (PCE), thanks to the quantum confinement effect that allows tuning of their absorption spectrum from visible to infrared.

Conventional CQDs are typically made from lead-based compounds, which limits their commercial viability due to the high toxicity of lead. As a result, there has been a growing interest in nontoxic CQDs made from ternary compounds, such as Bismuth Silver Sulfide (AgBiS₂) CQDS. Despite their high absorption coefficient, AgBiS₂ CQD solar cells have a relatively lower PCE than their lead-based counterparts. This is due to higher nonradiative recombination losses, where charge carriers become trapped in defects within the material, dissipating energy as heat instead of electricity. Many strategies have been proposed to solve this issue, however, the surface properties of AgBiS₂ CQDs are still not understood and an efficient solution-based and surface-tailored method for developing stable CQD inks is yet to be discovered.

To address this gap, a team of researchers led by Assistant Professor Han Seul Kim from the Department of Advanced Materials Engineering at Chungbuk National University, Korea, developed a novel method for synthesizing high-quality AgBiS₂ CQDs. In this joint research effort, Chungbuk National University researchers revealed and designed the mechanism through simulation and a Korea University research team produced it through experimentation.

“Conventional methods to develop AgBiS₂ CQD films introduce defects on their surface, increasing the trapping of charge carriers and consequently the nonradiative combination loss. We devised a novel quadruple-ligand ensemble using a solution-phase ligand exchange strategy to passivate CQD surfaces and address this issue” explains Dr. Kim. Their study was made available online on January 05, 2024, and published in Volume 14, Issue 7 of the journal Advanced Energy Materials on February 15, 2024.

At the core of their method is an innovative multi-faceted passivation strategy, in which a unique quadruple-ligand ensemble consisting of Silver Iodide (AgI), Sodium Iodide (NaI) Silver Bromide (AgBr) and Sodium Bromide (NaBr) is utilized in a special solution-phase ligand exchange process (SPLE). During this process, the ligands effectively replace the defects on the polar (100) and nonpolar (111) facets of AgBiS₂ CQDs. This results in the multifaceted passivation of the CQD surface, ultimately resulting in the formation of a stable CQD ink.

To understand the role of each ligand, the researchers utilized the powerful Density Functional Theory for simulations. Their analysis revealed that the Ag ligands initially deposit on the two facets and further induce additional adsorption of Na ligands, resulting in multifaced passivation. This synergistic passivation results in lower trapping of charge carriers, thereby boosting PCE.

The researchers teamed up with a group from Korea University to implement this strategy in fabricating thin film AgBiS₂ CQDs solar cells and photodetectors. The solar cells demonstrated a remarkable PCE of 8.1%, representing a substantial 53% increase compared to conventional CQD solar cells. Moreover, the photodetectors exhibited the fastest reported response time of 400 nanoseconds.

“These results highlight the potential of our strategy for fabricating high-quality AgBiS₂ CQDs and therefore realizing high-performance optoelectronic devices. Moreover, our method is generally applicable to quantum dot material synthesis,” remarks Dr. Kim. “Our research opens the door to several real-life applications, including highly efficient solar energy harvesting, ultrafast sensors, and high-end imaging systems for medical diagnosis.”

Overall, this groundbreaking strategy contributes to the development of high-efficiency solar cells and enhanced optoelectronic devices, paving the way for improved environmental sustainability and potentially transforming energy consumption and electronics usability.

Reference

About the institute

Chungbuk National University, located in Cheongju, South Korea, is a distinguished institution renowned for its commitment to academic excellence, research innovation, and community engagement. Founded in 1951, the university offers a wide spectrum of undergraduate, graduate, and doctoral programs spanning diverse fields and maintaining rigorous academic standards. It is a hub of cutting-edge research, with numerous research centers and institutes dedicated to technological advancements, healthcare, agriculture, and cultural studies.

About Assistant Professor Han Suel Kim

Han Seul Kim is currently an Assistant Professor at the Department of Advanced Materials Engineering at Chungbuk National University. Her group is focusing on utilizing atomic-level simulations to explore and theoretically elucidate the properties of next-generation nanomaterials for semiconductor and energy applications. Beyond theoretical work, the group also employs data science and experimental methodologies to validate these theories in practical scenarios. This integrative approach combines advanced computational methods with real-world experimentation to advance the understanding and development of innovative materials. Before coming to Chungbuk National University in March 2023, she worked at the Korea Institute of Science and Technology Information as a senior researcher. In 2017, Han Seul Kim received a PhD in the graduate school of energy, environment, water, and sustainability (EEWS) from Korea Advanced Institute of Science and Technology (KAIST).