Metal Halide Perovskite

1. Metal Halide Perovskite Light Emitters

Metal halide perovskites (MHPs) have become a promising class of light-emissive materials for next-generation displays due to numerous excellent optical/electrical properties such as high photoluminescence quantum efficiency, very narrow emission spectrum (FWHM <20 nm, which is smaller than the FWHM of InP QDs (FHWM ~ 35 nm)), high charge carrier mobility, low energetic disorder, solution processability, simple color tuning, and low material cost.

With these advantages, MHPs have great potential in realizing more immersive and realistic AR/VR displays that could satisfy almost 100% of Rec.2020 in the near future.

However, the following challenges must be overcome to realize the successful commercialization of perovskite emitters.

Toxicity of lead (Pb)
Low material/device stability
Low device efficiency of blue perovskite LEDs
Photo-induced halide segregation

Our group aims to overcome these challenges by combining Surface Chemistry and Device Engineering.

We envision our research on perovskite emitters will lead to a new mainstream in the display industry: "Perovskite Displays" or "PeLED Displays".

2. Metal Halide Perovskite Polycrystalline Films

In our groups, we aim to overcome these challenges by combining interfacial defect engineering, surface chemistry, and our own strategies (as follows) to create a new mainstream in the display industry.

1) Film morphology control:

Advanced nanocrystal pinning: effective control of grain size and morphology by using crystal growth inhibitor.
Post additive treatment: Effective passivation agent which effectively suppresses increment of the grain and defect formation.

The additive- based nanocrystal pinning process using the electron transporting organic material solutions in a volatile antisolvent improve the radiative recombination rate by hindering trap assisted non-radiative recombination at the grain boundaries due to In-situ fabricated nanocrystals structure. Furthermore, the growth inhibitor-based grain size tuning and morphology control is the breakthrough for high-quality polycrystalline perovskite thin films.

2) Advanced Quasi-2D Structure (Material's Dimension Control)

The inherent instability problems of perovskite materials and devices can be mitigated by controlling materials' dimensionality with bulky ammonium moieties, which convert perovskite crystal structure from 3D to so-called "Quasi-2D" (e.g., Ruddlesden-Popper phase). The energy transfer induced by confined natural quantum well structure enhances the radiative recombination rate. However, low mobility due to organic bulky ammonium suppresses device efficiency. In our lab, we aim to directly control the orientation of the perovskite to get higher mobility as well as stability by combining the dimensional control and advanced nanocrystal pinning of the perovskite materials.

3) Surface Chemistry and Interfacial Defect Engineering

The trap-meditated non-radiative recombination is the dominant problem for high quality emission layer. We focused on the surface chemistry to passivate interfacial defect which effectively suppress non-radiative recombination. The proper passivation strategies can enhance the efficiency and stability by the low defect concentration and mitigation of residual stress in the crystal structure.

3. Colloidal Metal Halide Perovskite Nanocrystals

1) Ligand Modification

The low formation energy facilitates solution processing, enabling cost-effective fabrication of large-area optoelectronic devices with high uniformity. Colloidal MHP nanocrystals exhibit ligand-capped structure, with nanometer-sized MHPs surrounded by long-chain organic capping ligands (e.g., carboxylates, alkylammonium). As high quality MHP NCs are achieved after liquid-phase synthesis with subsequent purification process they work as an effective light emitter with efficient charge carrier confinement resulting in high optical properties.

As the MHP NCs possess high surface-to-volume ratio, the surface defects and ligand properties greatly affect the final quality MHP NCs. For the realization of the new emissive material mainstream in the display industry, our group focuses on further researches including enhanced colloidal stability, and introducing potential functionalities (e.g., higher conductivity) through optimized ligand exchange mechanisms.

2) Colloidal Synthesis of Environmentally-benign MHP NCs

H.Cho et.al. ACS Nano, 2024, XX

The inherent toxicity of lead ions presents a barrier to industrial applications, necessitating the exploration of environmentally friendly alternatives. To address the inherent toxicity of lead ions, our group focuses on developing lead-free PNCs that maintain the advantageous photophysical properties of traditional lead-based PNCs (Pb-PNCs). Our group has recently proposed divalent europium-based PNCs (Eu-PNCs) as a viable alternative. By replacing Pb2+ with Eu2+ ions, we have demonstrated that Eu-PNCs exhibit deep-blue emission attributed to dipole-allowed 4f–5d electronic transitions and are characterized by narrow emission line widths. These properties align with the requirements for display applications that demand an enhanced color gamut, with reported color coordinates close to the Rec. 2020 standard for blue emitters. Additionally, Eu2+ has an ionic radius (117 pm) closely matching that of Pb2+ (119 pm), satisfying the Goldschmidt tolerance factor for stable ABX3 perovskite structures. Our research highlights Eu-PNCs as promising candidates for environmentally benign blue-emitting PNCs with the potential to advance display technology.