Jin Woo Choi

High-Efficiency Polycrystalline Perovskite Light-Emitting Diodes Based on Mixed Cations

We have achieved high-efficiency polycrystalline perovskite light-emitting diodes (PeLEDs) based on formamidinium (FA) and cesium (Cs) mixed cations without quantum dot synthesis. Uniform single-phase FA1- xCs xPbBr3 polycrystalline films were fabricated by one-step formation with various FA:Cs molar proportions; then the influences of chemical composition on film morphology, crystal structure, photoluminescence (PL), and electroluminescence (EL) were systematicallyxa0investigated. Incorporation of Cs+ cations in FAPbBr3 significantly reduced the average grain size (to 199 nm for FA:Cs = 90:10) and trap density; these changes consequently increased PL quantum efficiency (PLQE) and PL lifetime of FA1- xCs xPbBr3 films and current efficiency (CE) of PeLEDs. Further increase in Cs molar proportion from 10 mol % decreased crystallinity and purity, increased trap density, and correspondingly decreased PLQE, PL lifetime, and CE. Incorporation of Cs also increased photostability of FA1- xCs xPbBr3 films, possibly due to suppressed formation of light-induced metastable states. FA1- xCs xPbBr3 PeLEDs show the maximum CE = 14.5 cd A-1 at FA:Cs = 90:10 with very narrow EL spectral width (21-24 nm); this is the highest CE among FA-Cs-based PeLEDs reported to date. This work provides an understanding of the influences of Cs incorporation on the chemical, structural, and luminescent properties of FAPbBr3 polycrystalline films and a breakthrough to increase the efficiency of FA1- xCs xPbBr3 PeLEDs.

Synthesis and organic field effect transistor properties of isoindigo/DPP-based polymers containing a thermolabile group

(E)-6,6′-Dibromo-1,1-bis(2-octyldodecyl)-(3,3′-biindolinylid-ene)-2,2′-dione and/or 2,5-bis(2-octyldodecyl)-3,6-di(5-bromothien-2-yl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione and their tBoc-counterparts were propagated with 2,5-bis(tributylstannyl)thiophene in a molar ratio of 0.8u2006:u20060.2u2006:u20061.0 to release P(ODIDT-BID), P(ODIDT·BDPP), P(ODDPPT·BID) and P(ODDPPT·BDPP) as a new series of random conjugated polymers (RCPs) bearing a large number of octyldodecyl chains to ensure solubility and a small number of thermocleavable tBoc function to cast H-bonding upon heating up to 220 °C. All new polymers were synthesised via Pd catalysed Stille cross-coupling methodology in high yields and reasonable average molecular weights. The cast polymer films exhibited considerable red-shifted UV-vis absorption spectra and a further red-shift was also obtained in the thermal annealed films (at 220 °C for 30 min), which reflected the increasing of crystalline structure. The formation of H-bonding in these polymers was investigated using X-ray diffractometry (XRD) measurements. The field-effect mobilities of these polymers were investigated in the configuration of bottom-gate and bottom-contact (BGBC) field-effect transistors (FETs). The results from FETs indicated that the crystalline structure of RCPs exhibited reasonable FET mobilities with 1.17 × 10−3 cm2 V−1 s−1 for P(ODDPPT·BID) and 1.41 × 10−3 cm2 V−1 s−1 for P(ODDPPT·BDPP).

Molecular ordering of A(D–A′–D)2-based organic semiconductors through hydrogen bonding after simple cleavage of tert-butyloxycarbonyl protecting groups

We have successfully achieved the molecular ordering of semiconducting small molecules comprising a newly designed A(D–A′–D)2 system. The A(D–A′–D)2 small molecules have two different acceptors based on isoindigo and diketopyrrolopyrrole along with tert-butoxycarbonyl (t-Boc) groups and hexyl chains, which improve the solubility of the materials. After simple thermal annealing, the t-Boc groups were removed to allow strong hydrogen bonds between N–H⋯CO to form, and this resulted in improved molecular ordering of the organic semiconductors. The crystalline morphology was confirmed by X-ray diffraction coupled with high-voltage electron microscopy, and the resulting materials showed improved hole mobilities. In this work, the effect of the incorporation of t-Boc groups onto A(D–A′–D)2-based organic semiconductors on their morphological and electrical properties was evaluated.

Preparation of perovskite-embedded monodisperse copolymer particles and their application for high purity down-conversion LEDs

Perovskite nanoparticle (NP)-embedded copolymer particles are prepared via self-emulsion polymerization (SEP) and a vapor-assisted solution process (VASP). The prepared particles are self-assembled to form photonic bandgaps (PBs), which modified the photoluminescence (PL) spectrum of the embedded perovskite NPs to have an enhanced PL intensity and a reduced full width at half maximum (FWHM).

Temperature-Dependent Photoluminescence of CH3NH3PbBr3 Perovskite Quantum Dots and Bulk Counterparts

Organic-inorganic lead halide perovskite is emerging as a potential emissive material for light emitting devices, such as, light emitting diodes (LEDs) and lasers, which has emphasized the necessity of understanding its fundamental opto-physical properties. In this work, the temperature-dependent photoluminescence of CH3NH3PbBr3 perovskite quantum dots (QDs), polycrystalline thin film (TF), and single crystal (SC) has been studied. The optophysical properties, such as exciton-phonon scattering, exciton binding energy, and exciton decay dynamics, were investigated. The exciton-phonon scattering of perovskite is investigated, which is responsible for both PL line width broadening and nonradiative decay of excitons. The exciton binding energy of QDs, TF, and SC were estimated to be 388.2, 124.3, and 40.6 meV, respectively. The observed main exciton decay pathway for QDs is the phonon assisted thermal escape, while that for TF and SC was the thermal dissociation due to low exciton binding energy.

Introducing paired electric dipole layers for efficient and reproducible perovskite solar cells

Elimination of charge trapping at defects is highly challenging for poly-crystalline organometal halide perovskites. Here, we report a new architecture for reinforcing the built-in electric field (Ein) across the photoactive layer with a pair of strong electric dipole layers (EDLs). The paired EDLs significantly intensify the Ein across the perovskite layer, resulting in suppressed charge trapping of photogenerated charges. As a result, our low-temperature processed P–I–N planar PeSC devices using the paired EDLs exhibit a higher power conversion efficiency (ηmax ∼ 19.4%) and a smaller device-to-device variation with a standard deviation (S.D.) of 0.70%, which far surpass those (ηmax ∼ 17.8%, S.D. ∼ 1.1%) of the devices with typical charge transport layers.