Short-Wave Infrared Light-Emitting Diodes Using Colloidal CuInS2 Quantum Dots with ZnI2 Dual-Passivation.

Short-wave infrared (SWIR) light-emitting diodes (LEDs) have emerged as promising technologies for diverse applications such as optical communication, biomedical imaging, surveillance, and machine vision. Colloidal quantum dots (QDs) are particularly attractive for SWIR LEDs due to their solution processability, compatibility with flexible substrates, and tunable absorption and luminescence. However, the presence of toxic elements or precious metals in most SWIR-emitting QDs poses health, environmental, and cost challenges. In this context, CuInS2 (CIS) QDs are known for low toxicity, cost-effective fabrication, and SWIR-light emitting capability. However, CIS QDs have not yet been directly utilized to fabricate SWIR LEDs to date, which is due to low particle stability, inefficient charge carrier recombination, and significantly blue-shifted luminescence after integrating into LED devices. To address challenges, we propose a dual-passivation strategy using ZnI2 as a chemical additive to enhance both the optical property of plain CIS QDs and charge carrier recombination upon LED device implementation. The resulting CIS-QD-based LEDs exhibit a stable SWIR electroluminescence (EL) peak (over 1000 nm) with a high EL radiance and a record external quantum efficiency in the SWIR region. Our study represents a significant step forward in SWIR-QLED technology, offering a pathway for the development of high-performance, low-cost, and nontoxic SWIR light sources.

Effect of aggregation configuration of molecular fluorophore CZA on photoluminescence properties of carbon dots.

The effect of aggregation configuration of molecular fluorophore citrazinic acid (CZA) on the photoluminescence (PL) properties of carbon dots (CDs) has been investigated using first-principles method. The structural stability of all aggregates has been analyzed, and the results show that the most stable structures are J-type CZA aggregates with head-to-tail configurations and the CZA/CD aggregates are bonded by replacing H atoms on the CD edges with de-OH from the pyridine ring of CZA. The luminescent properties of CZA/CD aggregates are mainly affected by the binding modes and binding sites. When the sites belong to electron-donating groups, electron-withdrawing groups or sp2 domain, the PL spectra of CDs are shifted and the luminescent intensities are significantly enhanced. The results suggest that covalently bonded CZA/CD aggregates are responsible for the high fluorescence quantum yield of CD. Moreover, the distance between the centers of the two pyridine rings in H-type CZA dimers less than 3.5 Å is prone to π-π stacking, leading to fluorescence quenching of aggregates. The present work is helpful in understanding the effect of molecular fluorophores on the PL properties of CDs and provides theoretical guidance for the controllable synthesis of CDs.

Effect of conjugation length on fluorescence characteristics of carbon dots.

The influence of sp2- and sp3-hybridized carbon coexisting in carbon cores on fluorescence characteristics of carbon dots (CDs) was revealed by density functional theory calculations. Based on the constructed coronene-like structures, the fluorescence emission spectra, transition molecular orbital pairs and several physical quantities describing the distribution of electrons and holes were investigated. The results indicate that due to the interaction between sp2 and sp3 carbon atoms, two main factors including the hyperconjugative effect and the separation of sp2 domain by sp3 carbon atoms can regulate the fluorescence wavelength. By analyzing the transition molecular orbital pairs, it was found that the fluorescence wavelength has a close correlation with the conjugation length, suggesting that the conjugation length can predict the shift of the emission spectra of CDs. The theoretical results provide a comprehensive understanding of fluorescence mechanism and help to synthesize CDs with expected fluorescence wavelength.

Unveiling the roles of halogen ions in the surface passivation of CsPbI3 perovskite solar cells.

Halide ion passivation is an effective way to improve the stability and the power conversion efficiency (PCE) of perovskite solar cells. In this work, the passivation mechanism of the surface iodine vacancies of inorganic perovskite CsPbI3 films by halogen ions (F-, Cl-, and Br-) has been studied using the first-principles method. Due to its high electronegativity, the F ion withdraws electron density out of its neighboring atoms, readily forms ionic bonds with Pb atoms and has a coupling effect with the nearest neighbor Cs atoms, which can alleviate the generation of cation vacancy and ion migration to locally stabilize the structure of the perovskite. The fluorinated CsPbI3 (001) surface has a lower surface energy, which improves the grain growth of perovskite films. Different from F-, the passivation via Cl- or Br- ions can effectively prevent the charge accumulation on the film surface, reduce the exciton binding energy of CsPbI3, and eliminate the loss of optical absorption intensity in the visible light range caused by iodine vacancies. These results provide a deep understanding about surface passivation by halogen ions for perovskite solar cells.