Christoph Wolf

Overcoming the electroluminescence efficiency limitations of perovskite light-emitting diodes

Brighter perovskite LEDs Organic-inorganic hybrid perovskites such as methyl ammonium lead halides are attractive as low-cost light-emitting diode (LED) emitters. This is because, unlike many inorganic nanomaterials, they have very high color purity. Cho et al. made two modifications to address the main drawback of these materials, their low luminescent efficiency. They created nanograin materials lacking free metallic lead, which helped to confine excitons and avoid their quenching. The perovskite LEDs had a current efficiency similar to that of phosphorescent organic LEDs. Science, this issue p. 1222 Efficient organic-inorganic perovskite light-emitting diodes were made with nanograin crystals that lack metallic lead. Organic-inorganic hybrid perovskites are emerging low-cost emitters with very high color purity, but their low luminescent efficiency is a critical drawback. We boosted the current efficiency (CE) of perovskite light-emitting diodes with a simple bilayer structure to 42.9 candela per ampere, similar to the CE of phosphorescent organic light-emitting diodes, with two modifications: We prevented the formation of metallic lead (Pb) atoms that cause strong exciton quenching through a small increase in methylammonium bromide (MABr) molar proportion, and we spatially confined the exciton in uniform MAPbBr3 nanograins (average diameter = 99.7 nanometers) formed by a nanocrystal pinning process and concomitant reduction of exciton diffusion length to 67 nanometers. These changes caused substantial increases in steady-state photoluminescence intensity and efficiency of MAPbBr3 nanograin layers.

Planar CH3NH3PbI3 Perovskite Solar Cells with Constant 17.2% Average Power Conversion Efficiency Irrespective of the Scan Rate

J. H. Heo, D. H. Song, H. J. Han, Prof. S. H. Im Functional Crystallization Center (FCC) Department of Chemical Engineering Kyung Hee University 1732 Deogyeong-daero , Giheung-gu, Yongin-si , Gyeonggi-do 446-701 , Republic of Korea E-mail: imromy@khu.ac.kr S. Y. Kim, Prof. J. H. Kim Department of Physics Incheon National University 119 Academy-ro , Yeonsu-gu , Incheon 406-772 , Republic of Korea D. Kim, Dr. H. W. Shin, Prof. T. K. Ahn Department of Energy Science Sungkyunkwan University Seobu-ro 2066 , Jangan-gu , Suwon 440-746 , Republic of Korea C. Wolf, Prof. T.-W. Lee Department of Materials Science and Engineering Pohang University of Science and Technology (POSTECH) 77 Cheongam-Ro , Nam-Gu, Pohang , Gyungbuk 790-784, Republic of Korea

Efficient Visible Quasi‐2D Perovskite Light‐Emitting Diodes

Efficient quasi-2D-structure perovskite light-emitting diodes (4.90 cd A(-1) ) are demonstrated by mixing a 3D-structured perovskite material (methyl ammonium lead bromide) and a 2D-structured perovskite material (phenylethyl ammonium lead bromide), which can be ascribed to better film uniformity, enhanced exciton confinement, and reduced trap density.

Organic Non‐Volatile Resistive Photo‐Switches for Flexible Image Detector Arrays

A unique implementation of an organic image detector using resistive photo-switchable pixels is presented. This resistive photo-switch comprises the vertical integration of an organic photodiode and an organic resistive switching memory element. The photodiodes act as a photosensitive element while the resistive switching elements simultaneously store the detected light information.

Highly Efficient, Simplified, Solution‐Processed Thermally Activated Delayed‐Fluorescence Organic Light‐Emitting Diodes

Highly efficient, simplified, solution-processed thermally activated delayed-fluorescence organic light-emitting diodes can be realized by using pure-organic thermally activated delayed fluorescence emitters and a multifunctional buffer hole-injection layer, in which high EQE (≈24%) and current efficiency (≈73 cd A(-1) ) are demonstrated. High-efficiency fluorescence red-emitting and blue-emitting devices can also be fabricated in this manner.

Organometal Halide Perovskite Artificial Synapses.

Organometal halide perovskite synaptic devices are fabricated; they emulate important working principles of a biological synapse, including excitatory postsynaptic current, paired-pulse facilitation, short-term plasticity, long-term plasticity, and spike-timing dependent plasticity. These properties originate from possible ion migration in the ion-rich perovskite matrix. This work has extensive applicability and practical significance in neuromorphic electronics.

Highly Efficient Light-Emitting Diodes of Colloidal Metal-Halide Perovskite Nanocrystals Beyond Quantum Size

Colloidal metal-halide perovskite quantum dots (QDs) with a dimension less than the exciton Bohr diameter DB (quantum size regime) emerged as promising light emitters due to their spectrally narrow light, facile color tuning, and high photoluminescence quantum efficiency (PLQE). However, their size-sensitive emission wavelength and color purity and low electroluminescence efficiency are still challenging aspects. Here, we demonstrate highly efficient light-emitting diodes (LEDs) based on the colloidal perovskite nanocrystals (NCs) in a dimension > DB (regime beyond quantum size) by using a multifunctional buffer hole injection layer (Buf-HIL). The perovskite NCs with a dimension greater than DB show a size-irrespective high color purity and PLQE by managing the recombination of excitons occurring at surface traps and inside the NCs. The Buf-HIL composed of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) and perfluorinated ionomer induces uniform perovskite particle films with complete film coverage and prevents exciton quenching at the PEDOT:PSS/perovskite particle film interface. With these strategies, we achieved a very high PLQE (∼60.5%) in compact perovskite particle films without any complex post-treatments and multilayers and a high current efficiency of 15.5 cd/A in the LEDs of colloidal perovskite NCs, even in a simplified structure, which is the highest efficiency to date in green LEDs that use colloidal organic-inorganic metal-halide perovskite nanoparticles including perovskite QDs and NCs. These results can help to guide development of various light-emitting optoelectronic applications based on perovskite NCs.

High‐Efficiency Solution‐Processed Inorganic Metal Halide Perovskite Light‐Emitting Diodes

This paper reports highly bright and efficient CsPbBr3 perovskite light-emitting diodes (PeLEDs) fabricated by simple one-step spin-coating of uniform CsPbBr3 polycrystalline layers on a self-organized buffer hole injection layer and stoichiometry-controlled CsPbBr3 precursor solutions with an optimized concentration. The PeLEDs have maximum current efficiency of 5.39 cd A-1 and maximum luminance of 13752 cd m-2 . This paper also investigates the origin of current hysteresis, which can be ascribed to migration of Br- anions. Temperature dependence of the electroluminescence (EL) spectrum is measured and the origins of decreased spectrum area, spectral blue-shift, and linewidth broadening are analyzed systematically with the activation energies, and are related with Br- anion migration, thermal dissociation of excitons, thermal expansion, and electron-phonon interaction. This work provides simple ways to improve the efficiency and brightness of all-inorganic polycrystalline PeLEDs and improves understanding of temperature-dependent ion migration and EL properties in inorganic PeLEDs.

Improving the Stability of Metal Halide Perovskite Materials and Light-Emitting Diodes

Metal halide perovskites (MHPs) have numerous advantages as light emitters such as high photoluminescence quantum efficiency with a direct bandgap, very narrow emission linewidth, high charge-carrier mobility, low energetic disorder, solution processability, simple color tuning, and low material cost. Based on these advantages, MHPs have recently shown unprecedented radical progress (maximum current efficiency from 0.3 to 42.9 cd A-1 ) in the field of light-emitting diodes. However, perovskite light-emitting diodes (PeLEDs) suffer from intrinsic instability of MHP materials and instability arising from the operation of the PeLEDs. Recently, many researchers have devoted efforts to overcome these instabilities. Here, the origins of the instability in PeLEDs are reviewed by categorizing it into two types: instability of (i) the MHP materials and (ii) the constituent layers and interfaces in PeLED devices. Then, the strategies to improve the stability of MHP materials and PeLEDs are critically reviewed, such as A-site cation engineering, Ruddlesden-Popper phase, suppression of ion migration with additives and blocking layers, fabrication of uniform bulk polycrystalline MHP layers, and fabrication of stable MHP nanoparticles. Based on this review of recent advances, future research directions and an outlook of PeLEDs for display applications are suggested.

Synergistic Effects of Doping and Thermal Treatment on Organic Semiconducting Nanowires.

Doping of small molecular donors or acceptors on conjugated organic materials can be used to improve the performance of organic electronics. Here we demonstrate highly aligned poly(3-hexylthiophene) (P3HT) nanowires (NWs) doped with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) using electrohydrodynamic organic NW printing. The transistor based on p-doped NWs had an order of magnitude higher mobility than did the undoped NW device. This significant improvement resulted from the synergistic effects of p-type doping and thermal annealing on F4-TCNQ-doped P3HT NWs, which induce microstructure changes in P3HT chains.