RESEARCH IN ONL
RESEARCH GOAL AND APPROACHES
"Understanding Fundamental Physics and Chemistry of Optoelectronic Materials and
Thier Utilization Toward High Performance Optoelectronic Devices"
WET CHEMISTRY OF SEMICONDUCTOR MATERIALS
Tracking, monitoring, and analyzing wet chemistry of semiconductor NCs
Carrier dynamics in optoelectronic materials and devices
Charge injection mechanism of organic-inorganic hybrid QLEDs
ONL focuses on fundamental principle of optoelectronic nanomaterials to apply them to novel, high performance optoelectronic devices, for instance, light-emitting diodes, solar cells, photo-detectors, and so forth. Our research efforts span across three research branches from wet chemistry of semiconductor materials (Thrust 1), optical spectroscopy (Thrust 2), to architecture design and physics of optoelectronic devices (Thrust 3). Not only in-depth research on each thrust, we also combine them in multilateral way to reach our ultimate goal.
THRUST 1: WET CHEMISTRY OF SEMICONDUCTOR MATERIALS
Manipulation of nucleation and growth behavior, surface chemistry, and collective behavior to surrounding media
Figure 1| Wet chemistry of semiconductor nanocrystals (NCs) and quantum confinement effect. (left) Colloidal semiconductor NCs are made in solution phase using specially designed setup, so-called Schlenk line. (middle) NCs exhibit unique quantized electronic states as their size becomes comparable or smaller than exciton Bohr radius, a barometer of exciton size. Decrease in size increases energy difference between lowest electron and hole states, which is translated into the increment of band gap of NCs. (right) As well as size, morphology of NCs also influences electronic structure of semiconductor NCs. Dots, rods and tetrapods are one of representative shapes successfully made by wet chemistry.
When size of semiconductor NCs is reduced below a certain level, they feature unique optical and electronic properties that are not observed in bulk materials. Most well-known and unique phenomenon is the quantum confinement effect, an increment of band gap with a decrement of size. Based on this phenomenon, controlling their chemical composition and shape open us new pathway to manipulate their optoelectronic properties as we demand. For example, we can full-color luminescence materials emitting from ultraviolet to near infrared emission just by controlling their size. Scientists have actively studied on this material to exploit it for next generation electronic and photonic devices, such as high quality displays, lightings, laser diodes, photovoltaics, photodetectors, and so on. Our research group has explored next generation light emitters featuring high efficiency, high color purity, and environmental safety.
Figure 2| Core/shell heterostructured quantu dots (QDs). (left) Schematic illustration of core/shell QDs and their typical type-I electronic structure for high quantum yield. (right) QDs-enhanced display and thier structure (image source: Samsung Display Inc.)
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Among numerous branches in wet chemistry of nanomaterials, we are particularly interested in heteroepitaxy of semiconductor NCs. Integration of various materials into the form of heterostructured NCs allows us to secure extended functionality and tunability on their optical and electrical properties. As a versatile optoelectronic material platform, we explore novel reaction pathways of heterostructured NCs with heavy metal-free II-VI, III-V or other compounds semiconductors for realizing high performance optoelectronic materials.
Core/shell quantum dots (QDs) are one of our major research topics. The QDs have demonstrated their values in the field of information display - high quantum yield, photochemical stability and narrow emission bandwidth. Despite their success and commercial use in displays, principle of chemical synthesis of heterostructured NCs still leaves a lot of question marks: adhesion and reaction of reaction precursors, ligand binding modes, ligand transfer pathways, and so on.
Figure 3| Unraveling ZnSe heteroepitaxy on InP QDs in wet chemistry. 1H-, 31P- and 77Se–NMR spectroscopy on intermediate species and optical spectroscopies (absorption, photoluminescence, and decay dynamics) on InP QDs allow to tracking whole sequence of shell growth process from surface ligand transfer to metal ion adsorption to ZnSe surface cluster formation reaction. This work is led by Mr. Youngho Choi.
Recently, we unravel details on InP/ZnSe heteroexpitaxy process in solution phase. Using multilateral probes like NMR, optical spectroscopy and elemental analysis, we track trasfer reactions occuring in shell growth. We clarify adhesion of precursors, metal ion transfer and shell formation reactions, which explains how suface-initiatd ZnSe shell growth is proceeded. Our finding will open new reaction pathways enabling highly efficient InP QDs for full-color displays. This work is supported by Samsung Display Inc. and NRF funds granted by Ministry of Science and ICT (NRF-2019R1C1C1006481 and NRF-2019M3D1A1078299).
THRUST 2: OPTICAL SPECTROSCOPY
Probing carriers and exciton species in optoelectronic nanomaterials
Figure 4| Recombination and transfer of excitons in QDs. (left) Radiative recombination of an exciton emitting a photon with energy equivalent to band gap. (right) Trap-involved nonradiative recombination process, negative trion pathway of nonradiative Auger recombination process and nonradiative resonant energy transfer occuring between QDs in proximity (from left to right).
Quantization of electronic states and enhanced electron-hole wavefunction overlap in semiconductor NCs brings about ultrafast decay dynamics of exciton species that is sensitive to the presence of extra charges, surroundings, mid-gap states, and so on. And how we can manipulate those carrier dynamics determines the electron - photon conversion efficiency. In order to understand carrier dynamics of semiconductor nanocrystals governed by their compositions, shapes, structures or ensemble phase (dispersion or assembly), we adopt static and transient optical spectroscopy. Static and transient absorption and photoluminescence spectroscopies allow us to probe the presence of charge or exciton species and their decay dynamics under various condition.
Figure 5| Investigation of exciton recombination dynamics in cg/A/B-QDs and quantification of exciton species in quantum dot-based light-emitting diodes (QLEDs). (top) Multi-shell QDs with continuously graded CdZnSe shell (cg), lattice adapting ZnSe shell (A) and injection barrier ZnSeS shell (B). Decay dynamics acquired at each shell growth period suggest accumulation of asymmetric compressive strain along increasing shell thicknesses. Pump-power dependent decay dynamics reveal biexciton and charged exciton recombination dynamics. (bottom) Based on information of exciton recombination dynamics obtained using spectroscopic probes, we enable to quantify quantum yield of multiexcitons and their contribution on internal quantum efficiency in QLEDs. This study was conducted by B. Kim.
One of our efforts in the field of QD-based light-emitting diodes (QLEDs) is to correlate unique photophysics of colloidal QDs and their electroluminescence characteristics. The QLEDs separately introduce electrons and holes into QDs and imbalanced charge injection results in formation of charged excitons, for example, negative trions with two electrons and one hole. As current density increases, QDs can possibly accept two or more excitons in a dot, so-called multiexcitons. Except for single exciton, such various exciton species have been regarded as efficiency-limiting factors.
In our recent research, we utilize TCSPC to quantify exciton fine structure of cg/A/B-QDs and decay dynamics thereof. We can monitor accumulation of compressive strain to CdSe core granted by thick CdZnSe/ZnSe/ZnSeS multi-shell ancitipated from accelerated exciton recombination rate. Electrochemical charging and pump-power dependent decay dynamics reveal radiative and Auger recombination pathways of excitons in the QDs. This quantification allows to induce the theoretical limit of QLED performance that is determined by summation of products, fraction of exciton speices and their quantum efficiency. More importantly, our calculation guide us to design novel device architecture challenging unprecedent high brightness. We are currently collaborating with a research group of prof. J. Kwak in Seoul National University. This work is supported by NRF funds granted by Ministry of Science and ICT (NRF-2019R1C1C1006481 and NRF-2019M3D1A1078299).
THRUST 3: OPTOELECTRONIC DEVICES
Exploring novel optoelectronic devices offering new functionalities and unprecedent performance
Figure 6| QD-based light-emitting diodes (QLEDs) and their emission spectra. (left) Typical QLED architecture employing organic-inorganic hybrid charge traport layers. Direct injection of electrons and holes to QDs form excitons in QDs. (middle) Comparison of emission spectra between typical QLEDs and organic LEDs. (right) Demonstration of high brightness QLEDs (Luminance > 100,000 nit at 624 nm).
Various optoelectronic nanomaterials synthesized in our lab are applied to electronic devices to utilize them for the electron-photon interconversion process. Our lab constructs a series of facilities enabling vertical integration from material preparation to device fabrication to performance evaluation. In combination with optical spectroscopies, we are particularly interested in scrutinizing operation principle of optoelectronic materials in a framework of electronic devices, such as carrier accumulation, injection and recombination dynamics. Qualitative and quantitative understanding on operation mechanism of nanomaterial-based devices would be the essential starting point to realize efficient, stable and high performance optoelectronic devices.
Based on the understanding on fundamental operation mechanism, our recent efforts focus on heavy metal-free colloidal QD-based light-emitting diodes (QLEDs). To realize vivid and natural full-color displays and to breakthrough operation limit of QLEDs, we have performed comprehensive research spanning from material chemistry to device physics.
Figure 7| Unraveling sub-band gap charge injection of hybrid QLEDs. (left) Current-voltage-luminance characteristics of QLEDs with different organic hole transport layers. For some cases, early device turn-on below band gap (Eg) of QDs (1.97 eV) is observed. (right, top) Schematic on the alignment of energy landscape derived by surface states of core/shell QDs. Fermi level (EF) of QDs is defined by surface states of ZnS shell and surrounding charge transport layers deform their energy band along the EF of QDs. Resulting energy band structure provide electrostatic potential gain for charge injection. (bottom, left) EF pinning of QDs on various substrates monitored by UV photoelectron spectroscopy (UPS). Pinned EF correlates to that of ZnS NCs with surface Zn sites. (bottom, right) HOMO level bending with respect to the QD emissive layer that minimizes the hole injection barrier less than 0.1 eV. This work was performed by H. Lee.
The past decade has witnessed remarkable progress in the efficiency of QLEDs comprising organic hole transport layers (HTLs) and ZnO electron transport layer (ETL). The striking improvement notwithstanding, the following conundrum remains underexplored: state-of-the-art devices with seemingly unfavorable energy landscape exhibit barrierless hole injection initiated even at sub-band gap voltages. And it appears that ZnO “electron” transport layer in the hybrid QLEDs trigger barrierless “hole” injection regardless of the band discontinuity. In the line of reasoning, QDs and charge transport layers (CTLs) are treated as dielectrics for the simplicity of argument, perhaps to a fault. To rationalize the barrierless carrier injection in hybrid QLEDs, however, it is necessary to examine the characteristics of QDs and to make sense of carrier injection in the framework of prolific QLED device structures.
Motivated from this conundrum, we investigate the energetics of carriers in the hybrid QLEDs based on the energy landscape at ZnO ETL–QD and QD–organic HTL junctions. It turns out that Fermi level alignment derived by surface states of QDs is responsible for macroscopic energy band reshaping in hybrid QLEDs. Such energy level alignment becomes the main driving force for barrierless hole injection. Strong electrostatic potential gain established at the QD–HTL junction enhances the hole injection process for a variety of HTLs, which cannot be rationalized by flat band model widely accepted in the field of QLEDs. Our analysis provides a general insight into the operational mechanism of the QLEDs from both macroscopic and microscopic perspectives. This work is supported by Samsung Research Funding & Incubation Center of Samsung Electronics under Project Number SRFC-TB1903-02 Inc. and NRF funds granted by Ministry of Science and ICT (NRF-2019R1C1C1006481 and NRF-2019M3D1A1078299).