Chang-Ki Moon

Seoul National University | SNU · Department of Materials Science and Engineering

Although bismuth vanadate (BiVO4) has been promising as photoanode material for photoelectrochemical water splitting, its charge recombination issue by short charge diffusion length has led to various studies about heterostructure photoanodes. As a hole blocking layer of BiVO4, titanium dioxide (TiO2) has been considered unsuitable because of its r...

Space charge limited current theory is often used to analyze electrical characteristics of single‐layer organic semiconductor devices using Ohmic and Schottky contacts for injection of majority and minority carriers, respectively. However, significant deviations from this theory in the current density–voltage and capacitance–frequency characteristi...

Organic light-emitting diodes require efficient and inexpensive light extraction methods for applications toward lighting and displays. Here, a facile and effective method for fabricating random organic microstructures for efficient light extraction from blue OLEDs is presented. Simple drop casting of a TCTA and B4PyMPM mixed solution followed by U...

Deep‐blue triplet emitters remain far inferior to standard red and green triplet emitters in terms of exhibiting high‐color‐purity Commission International de l'Éclairage (CIE) y values of ≤0.1, external quantum efficiencies (EQEs), and high electroluminescent brightnesses in phosphorescent organic light‐emitting diodes. In fact, no deep‐blue tripl...

Developing renewable and sustainable energy sources is a compelling goal in materials science and engineering. Especially, the natural photosynthesis with its infinite energy reservoir gives a profound inspiration for the energy conversion and storage systems. Here, we report a multiplex living solar cell that offers a drastic power enhancement by...

Two-dimensional (2D) MoS2 nanosheets (NSs) modified 1D TiO2 nanorods/0D CdS nanocrystals (NCs) heterojunction has been fabricated by all solution process as a potential anode for photoelectrochemical (PEC) water splitting applications. This heterojunction photoanode shows high photocurrent density of 3.25 mA/cm² at 0.9 V vs. RHE (0 V vs. Ag/AgCl) c...

The excited charge-transfer (CT) complex (exciplex) has attracted much attention from researchers in the fields of OLEDs and OPVs. Upon excitation, intermolecular CT between donor (D) and acceptor (A) molecules forms a CT state at a heterojunction. The CT state then either dissociates into positive and negative polarons or relaxes into the ground s...

This thesis presented the principle and optical effects of the molecular orientation for thin films and OLEDs. It also introduced the application of molecular orientation in the analysis of the electronic structure of the bimolecular excited state.

Molecular orientation in organic semiconductor is an important factor influencing electrical and optical properties [1]. In OLEDs, emitting dipole orientation (EDO) is directly related to the outcoupling efficiency of the light. Therefore, employing horizontally oriented emitter is one of the effective methods to enhancing the outcoupling efficienc...

Light emission from organic materials has been an important research topic during the last few decades because of its scientific and technological importance, particularly due to the success of organic light emitting diodes (OLEDs).

High efficiency is important for successful deployment of any light sources. Continued efforts have recently made it possible to demonstrate organic light-emitting diodes with efficiency comparable to that of inorganic light-emitting diodes. However, such achievements were possible only with the help of a macroscopic lens or complex internal nanost...

Excited charge-transfer (CT) complexes, or exciplexes, have attracted significant attention due to their potential applications to improving the performance of organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). In solid states, exciplexes exhibit extraordinary characteristics, including broad emission spectra, multi-expone...

Emitting dipole orientation (EDO) is an important issue of emitting materials in organic light-emitting diodes (OLEDs) for an increase of outcoupling efficiency of light. The origin of preferred orientation of emitting dipole of spherically shaped iridium-based heteroleptic phosphorescent dyes doped in organic layers is revealed by simulation of va...

We report that the molecular orientation of a disk-shaped Pt(II) complex dopant in organic thin films is linearly proportional to the orientation of the host molecules. We ascribe this relationship to the parallel alignment of the Pt complex with the host molecules induced by a π-π interaction. This would be caused by their planar and conjugated st...

The relationship between the chemical structure and the dipole orientation of thermally activated delayed fluorescent (TADF) emitters was examined by synthesizing three TADF emitters with donor and acceptor moieties at ortho-, meta-, and para-positions of a phenyl linker. Two carbazolylcarbazole donor moieties and two CN acceptor moieties were atta...

We synthesized two new heteroleptic blue-emitting Ir(III) complexes tBuCN-FIrmMes and tBuCN-FIrdMes by using systematically tailored ancillary ligands 1-mesitylbutane-1,3-dione (mMes) and 1,3-dimesitylpropane-1,3-dione (dMes), respectively. Near-unity photoluminescence quantum yield (PLQY: 96 ± 3%) was achieved for tBuCN-FIrmMes in doped films with...

The singlet fraction of excitons on a conventional fluorescent emitter is increased by reverse intersystem crossing of exciplexes and the sensitization effect of phosphorescent materials. Consequently, a red fluorescent organic lightemitting diode (OLED) with an external quantum efficiency (EQE) of 23.7% is realized and an EQE of nearly 30% may be...

We present a comprehensive model for the quantitative analysis of factors influencing the efficiency of organic light-emitting diodes (OLEDs) as a function of the current density. The model takes into account the contribution made by the charge carrier imbalance, quenching processes, and optical design loss of the device arising from various optica...

New deep blue fluorescent emitters composed of anthracene as an electron rich unit, a diphenyltriazine as a strong electron acceptor unit, and phenyl or xylene as interconnecting units were synthesised. The interconnecting unit between chromophores increased the singlet transition energy and the ratio of horizontal emitting dipoles. As a result, a...

J.-J. Kim and co-workers achieve highly efficient blue organic light-emitting diodes (OLEDs) using a low-refractive-index layer. As described on page 4920, an external quantum efficiency over 34% is achieved, owing to the low refractive index of the materials. A milepost and a shining entrance of the castle are the metaphor indicating the way to hi...

Electrical doping is an important method in organic electronics to enhance device efficiency by controlling the Fermi level, increasing the conductivity, and reducing the injection barrier from the electrode. To understand the charge generation process of the dopant in doped organic semiconductors, it is important to analyze the charge transfer com...

The mixed co-hosts of 1,3-Bis(N-carbazolyl)benzene (mCP) and 2,8-bis(diphenylphosphoryl)dibenzothiophene (PO15) have been developed for a highly efficient blue fluorescent oragnic light emitting diode (OLED) doped with a thermally activated delayed fluorescence (TADF) emitter [4,5-di (9H-carbazol-9-yl) phthalonitrile (2CzPN)]. We have demonstrated...

Organic light-emitting diode with external quantum efficiency of 38.8% are realized using a Pt-based thin-film emitting layer with photoluminescence quantum yield of 96% and Θ of 93%. The emitting dipole orientation of the thin films fabricated using Pt complexes is investigated and discussed the structural relationship between X-ray structural ana...

The use of multiple donors in an active layer is an effective way to boost the efficiency of organic solar cells by broadening their absorption window. Here, we report an efficient vacuum-deposited ternary organic photovoltaic (OPV) using two donors, 2-((2-(5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophen-2-yl)thiazol-5-yl)methylene)malononitrile (...

Efficient vacuum-deposited tandem organic photovoltaic cells (TOPVs) composed of pristine fullerenes as the acceptors and two complementary absorbing donors, 2-((2-(5-(4-(diphenylamino)phenyl)thieno[3,2-b]thiophen-2-yl)thiazol-5-yl)methylene)malononitrile for the visible absorption and 2-((7-(5-(dip-tolylamino)thiophen-2-yl)benzo[c]-[1,2,5]thiadiaz...

Emitting dipole orientation (EDO) of phosphorescent iridium (III) complexes doped in various host layers are analyzed using angle-dependent PL spectra. Preferred orientation of emitting dipoles of a heteroleptic iridium complex (Ir(ppy)2tmd) varies from horizontal to isotropic, or even to vertical direction depending on host molecules. The degree o...

We present an optical model to describe the luminescence from oriented emitting dipoles in a birefringent medium and validate the theoretical model through its applications to a dye doped organic thin film and organic light emitting diodes (OLEDs). We demonstrate that the optical birefringence affects not only far-field radiation characteristics su...

Orientation of emission dipoles of heteroleptic iridium complexes in an emitting layer is controlled toward the horizontal direction by substituting the hydrogen atoms of the cyclometalated ligand by methyl groups and demonstrates an organic light-emitting diode with an unprecedentedly high external quantum efficiency of 34.1% and power efficiency...

Light extraction from organic light-emitting diodes that utilize phosphorescent materials has an internal efficiency of 100% but is limited by an external quantum efficiency (EQE) of 30%. In this study, extremely high-efficiency organic light emitting diodes (OLEDs) with an EQE of greater than 50% and low roll-off were produced by inserting a vacuu...

Two new heteroleptic blue emitting Ir(III) complexes, EtOCN-FIracac and tBuCN-FIracac, and a homoleptic Ir(III) complex, tBu-FCNIr, consisting of 2′,4″-difluorophenylpyridine (dfppy) with a cyano (CN) group at the 5′ position of the phenyl and an ethoxy (EtO-) or tert-butyl (tBu) group at the 4 position of the pyridine as the main ligands, are synt...

We report a highly enhanced light extraction from a top emission organic light emitting diode with little image blurring and color variation with viewing angle. Direct integration of a high refractive index micro lens array on the top of the transparent indium zinc oxide top electrode of a green phosphorescent OLED showed a significant enhancement...

A red fluorescent organic light-emitting device with an unprecedented external quantum efficiency of 10.6% is created by J.-J. Kim and co-workers using an exciplex-forming system as a host for a conventional fluorescent dopant, not showing delayed fluorescence. On page 895, quantitative analysis indicates that over 35% of the total excitons are con...

We report a high performance green color selective semitransparent inverted organic photodetectors (OPDs) with a detectivity of 2.1 × 1012 cm Hz1/2/W at the wavelength of 530 nm which has the comparable performance to the reported metal electrode based OPDs due to low dark current density of 3.8 × 10−10 A/cm2 at −1 V. The transparent OPD showed hig...

We report a highly efficient phosphorescent green inverted top emitting organic light emitting diode doped with Ir(ppy)2tmd [bis(2-phenylpyridine)iridium(III)(2,2,6,6-tetramethylheptane-3,5-diketonate)] as the horizontally oriented emitter in an exciplex forming cohost system. The device showed a maximum current efficiency of 120.7 cd/A, a maximum...

Organic light-emitting diodes (OLEDs) are among the most promising organic semiconductor devices. The recently reported external quantum efficiencies (EQEs) of 29-30% for green and blue phosphorescent OLEDs are considered to be near the limit for isotropically oriented iridium complexes. The preferred orientation of transition dipole moments has no...

Almost 100% internal quantum efficiency (IQE) is achieved with a green fluorescent organic light-emitting diode (OLED) exhibiting 30% external quantum efficiency (EQE). The OLED comprises an exciplex-forming co-host system doped with a fluorescent dye that has a strong delayed fluorescence as a result of reverse intersystem crossing (RISC); the exc...

A man walks through the desert leaving footsteps where green light emits from. J.‐J. Kim and co‐workers demonstrate on page 5684 such high EQEs achieved with a green fluorescent OLED where phosphorescent OLEDs have been considered to be very dominant. The authors address the challenging concept of taking advantage of an exciplexforming host doped w...

A high-efficiency blue-emitting organic light-emitting diode (OLED) approaching theoretical efficiency using an exciplex-forming co-host composed of N,N′-dicarbazolyl-3,5-benzene (mCP) and bis-4,6-(3,5-di-3-pyridylphenyl)- 2-methylpyrimidine (B3PYMPM) is fabricated. Iridium(III)bis[(4,6-difluorophenyl)- pyridinato-N,C2′]picolinate (FIrpic) is used...

Ancillary ligands in heteroleptic iridium complexes significantly influence the orientation of the transition dipole moments. Ir(ppy)3, a homoleptic iridium complex, exhibits isotropic dipole orientation, whereas the heteroleptic Ir complexes of Ir(ppy)2 tmd show a highly preferred dipole orientation (78%) in the horizontal direction. In addition,...

A new series of heteroleptic iridium(III) complexes, bis(2-phenylpyridinato-N,C20)iridium (2-(20,40-difluorophenyl)-4-methylpyridine), (ppy)2Ir(dfpmpy) and bis(2-(20,40-difluorophenyl)- 4-methylpyridinato-N,C20)iridium (2-phenylpyridine) (dfpmpy)2Ir(ppy), have been synthesized by using phenylpyridine as a main skeleton for bluish green phosphoresce...

We report a highly efficient phosphorescent green inverted top emitting organic light emitting diode with excellent color stability by using the 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile/indium zinc oxide top electrode and bis(2-phenylpyridine)iridium(III) acetylacetonate as the emitter in an exciplex forming co-host system. The device show...

A perfect ohmic contact is formed at the interface of indium tin oxide (ITO) and N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) using ReO3 as the interfacial layer. The hole injection efficiency is close to 100% at the interface, which is much higher than those for interfacial layers of 1,4,5,8,9,11-hexaazatripheylene hexacarbonitrile (HAT...

We report extremely high light out-coupling efficiency from a transparent organic light-emitting diode (OLED) integrated with microstructures on both sides of the device.[1] The OLED having a metal free structure offers dramatically reduced surface plasmonic loss and absorption loss. To extract the confined light inside the device, a high refractiv...

High-efficiency phosphorescent organic light-emitting diodes (OLEDs) doped with Ir(ppy)2(acac) [bis(2-phenylpyridine)iridium(III)-acetylacetonate] in an exciplex forming co-host have been optically analyzed. This emitter has a preferred orientation with the horizontal to vertical dipole ratio of 0.77:0.23 as compared to 0.67:0.33 in the isotropic c...

Extremely high light out-coupling efficiency from a transparent organic light-emitting diode integrated with microstructures on both sides of the device is reported. The metal free device offers dramatically reduced surface plasmonic and intrinsic absorption losses. Moreover, high refractive index micro patterns with optimal light extraction condit...