arXiv subjects

Chang Woo Myung

Publications and source records attributed to Chang Woo Myung.

23 records · Page 2Linked to original sources

arXiv2020

Anharmonicity-driven Rashba co-helical excitons break quantum efficiency limitation

Closed-shell light-emitting diodes (LEDs) suffer from the internal quantum efficiency (IQE) limitation imposed by optically inactive triplet excitons. Here we show an undiscovered emission mechanism of lead-halide-perovskites (LHPs) APbX$_3$ (A=Cs/CN$_2$H$_5$; X=Cl/Br/I) that circumvents the efficiency limit of closed-shell LEDs. Though efficient emission is prohibited by optically inactive $J=0$ in inversion symmetric LHPs, the anharmonicity arising from stereochemistry of Pb and resonant orbital-bonding network along the imaginary A$^+\cdots$X$^-$ (T$_{1u}$) transverse optical (TO) modes, breaks the inversion symmetry and introduces disorder and Rashba-Dresselhaus spin-orbit coupling (RD-SOC). This leads to bright co-helical and dark anti-helical excitons. Many-body theory and first-principles calculations affirm that the optically active co-helical exciton is the lowest excited state in organic/inorganic LHPs. Thus, RD-SOC can drive to achieve the ideal 50 $\%$ IQE by utilizing anharmonicity, much over the 25 $\%$ IQE limitation for closed-shell LEDs.

cond-mat.mtrl-sci↗

arXiv2018

La-Doped BaSnO$_3$ Electron Transport Layer for Perovskite Solar Cells

Due to the photo-instability and hysteresis of TiO$_2$ electron transport layer (ETL) in perovskite solar cells (PSCs), novel electron transport materials are highly demanded. Here, we show ideal band alignment between La-doped BaSnO$_3$ (LBSO) and methyl ammonium (MA) lead iodide perovskite (MAPbI$_3$). The CH$_3$NH$_3$PbI$_3$/La$_x$Ba$_{(1-x)}$SnO$_3$ interface forms a stable all-perovskite heterostructure. The selective band alignment is manipulated with band gap renormalization by La-doping on the Ba site. LBSO shows high mobility, photo-stability, and structural stability, promising the next generation ETL materials.

cond-mat.mtrl-sci↗

arXiv2018

Rashba-Dresselhaus Effect in Inorganic/Organic Lead Iodide Perovskite Interfaces

Despite the imperative importance in solar-cell efficiency, the intriguing phenomena at the interface between perovskite solar-cell and adjacent carrier transfer layers are hardly uncovered. Here we show that PbI$_2$/AI-terminated lead-iodide-perovskite (APbI$_3$; A=Cs$^+$/ methylammonium(MA)) interfaced with the charge transport medium of graphene or TiO2 exhibits the sizable/robust Rashba-Dresselhaus (RD) effect using density-functional-theory and ab initio molecular dynamics (AIMD) simulations above cubic-phase temperature. At the PbI$_2$-terminated graphene/CsPbI3(001) interface, ferroelectric distortion towards graphene facilitates an inversion breaking field. At the MAI-terminated TiO$_2$/MAPbI$_3$(001) interface, the enrooted alignment of MA$^+$ towards TiO$_2$ by short-strong hydrogen-bonding and the concomitant PbI$_3$ distortion preserve the RD interactions even above 330 K. The robust RD effect at the interface even at high temperatures, unlike in bulk, changes the direct-type band to the indirect to suppress recombination of electron and hole, thereby letting these accumulated carriers overcome the potential barrier between perovskite and charge transfer materials, which promotes the solar-cell efficiency.

cond-mat.mtrl-sci↗

arXiv2017

A New Perspective on the Role of A-site Cation in Perovskite Solar Cells

As the race towards higher efficiency for inorganic/organic hybrid perovskite solar cells (PSCs) is becoming highly competitive, a design scheme to maximize carrier transport towards higher power efficiency has been urgently demanded. Here, we unravel a hidden role of A-site cation of PSCs in carrier transport which has been largely neglected, i.e., tuning the Frohlich electron-phonon (e-ph) coupling of longitudinal optical (LO) phonon by A-site cations. The key for steering Frohlich polaron is to control the interaction strength and the number of proton (or lithium) coordination to halide ion. The coordination to I alleviates electron-phonon scattering by either decreasing the Born effective charge or absorbing the LO motion of I. This novel principle discloses lower electron-phonon coupling by several promising organic cations including hydroxyl-ammonium cation (NH$_3$OH$^+$) and possibly Li$^+$ solvating methylamine (Li$^+$NH$_2$CH$_3$) than methyl-ammonium cation. A new perspective on the role of A-site cation could help in improving power efficiency and accelerating the application of PSCs.

cond-mat.mtrl-sci↗

arXiv2016

Two-dimensional Excitonic Photoluminescence in Graphene on Cu surface

Despite having outstanding electrical properties, graphene is unsuitable for optical devices because of its zero band gap. Here, we report two-dimensional excitonic photoluminescence (PL) from graphene grown on Cu(111) surface, which shows an unexpected remarkably sharp and strong emission near 3.16 eV (full-width at half-maximum $\leq$ 3meV) and multiple emissions around 3.18 eV. As temperature increases, these emissions blue-shift, showing the characteristic negative thermal coefficient of graphene. Observed PLs originate from significantly suppressed dispersion of excited electrons in graphene caused by hybridization of graphene $π$ and Cu d orbitals of the 1st and 2nd Cu layers at a shifted saddle point 0.525(M+K) of Brillouin zone. This finding provides a new pathway to engineering novel optoelectronic graphene devices, whilst maintaining the outstanding electrical properties of graphene.

cond-mat.mes-hall↗