Seunghyup Yoo

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.

Efficient thin-film organic solar cells based on pentacene/C60 heterojunctions

We have fabricated an efficient organic photovoltaic cell based on a heterojunction of pentacene and C60. Photocurrent action spectra exhibit broad light-harvesting throughout the visible spectrum with a peak external quantum efficiency (EQE) of 58±4% at short-circuit condition. Modeling studies indicate that this high EQE can be partly attributed to the large exciton diffusion length in the pentacene film as well as efficient dissociation of excitons at the pentacene/C60 heterojunction.

Workfunction-Tunable, N-Doped Reduced Graphene Transparent Electrodes for High-Performance Polymer Light-Emitting Diodes

Graphene is a promising candidate to complement brittle and expensive transparent conducting oxides. Nevertheless, previous research efforts have paid little attention to reduced graphene, which can be of great benefit due to low-cost solution processing without substrate transfer. Here we demonstrate workfunction-tunable, highly conductive, N-doped reduced graphene film, which is obtainable from the spin-casting of graphene oxide dispersion and can be successfully employed as a transparent cathode for high-performance polymer light-emitting diodes (PLEDs) as an alternative to fluorine-doped tin oxide (FTO). The sheet resistance of N-doped reduced graphene attained 300 Ω/□ at 80% transmittance, one of the lowest values ever reported from the reduction of graphene oxide films. The optimal doping of quaternary nitrogen and the effective removal of oxygen functionalities via sequential hydrazine treatment and thermal reduction accomplished the low resistance. The PLEDs employing N-doped reduced graphene cathodes exhibited a maximum electroluminescence efficiency higher than those of FTO-based devices (4.0 cd/A for FTO and 7.0 cd/A for N-doped graphene at 17,000 cd/m(2)). The reduced barrier for electron injection from a workfunction-tunable, N-doped reduced graphene cathode offered this remarkable device performance.

Selective Electron‐ or Hole‐Transport Enhancement in Bulk‐Heterojunction Organic Solar Cells with N‐ or B‐Doped Carbon Nanotubes

Doping improves performance. N- or B-doped carbon nanotubes (CNTs) uniformly dispersed in the active layer of P3HT/PCMB (poly (3-hexylthiophene/[6,6]-phenyl-C61-butyric acid methyl ester) bulk-heterojunction solar cells selectively enhance electron or hole transport and eventually help carrier collection. Specifically, the incorporation of 1.0 wt% B-doped CNTs results in balanced electron and hole transport and accomplishes a power conversion efficiency improvement from 3.0% (without CNTs) to 4.1%.

Origin of the open-circuit voltage in multilayer heterojunction organic solar cells

From temperature dependent studies of pentacene/C60 solar cells in the dark, the reverse saturation current is found to be thermally activated with a barrier height that corresponds to the difference in energy between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor corrected for vacuum level misalignments and the presence of charge-transfer states. From the reverse saturation current in the dark and the short-circuit current under illumination, the open-circuit voltage can be predicted. Examination of several donor materials supports the relationship between reverse saturation current, this barrier height, and open-circuit voltage.

Intensity-dependent equivalent circuit parameters of organic solar cells based on pentacene and C60

We present studies of the current–voltage characteristics of organic solar cells based on heterojunctions of pentacene and C60 as a function of illumination intensity. The photovoltaic response at a given illumination level is parameterized and modeled using the equivalent circuit model developed for inorganic pn-junction solar cells. Reduction in shunt resistance and increase in diode reverse saturation current density are observed upon increase of the light intensity. We demonstrate that this effect can be modeled by a refined equivalent circuit model that contains an additional shunt resistance and an additional diode the properties of which are functions of the light intensity. The effects of these additional components on the overall photovoltaic performance are discussed.

Resistive Switching Characteristics of Sol–Gel Zinc Oxide Films for Flexible Memory Applications

Unipolar resistive switching devices are investigated for nonvolatile memory applications in a metal-insulator-metal structure in which the insulator layer is based on sol-gel-derived zinc oxide (ZnO) films prepared by a simple spin-coating process followed by thermal annealing. Fast programming ( les 50 ns) and a high off-to-on resistance ratio ( ges 104) is demonstrated. The influences on the switching behaviors according to the crystallinity of the ZnO films are studied as a function of the annealing temperature. In addition, the devices are fabricated on a flexible plastic substrate and exhibit excellent durability upon repeated bending tests, demonstrating their potential for flexible low-cost memory devices.

Optical Outcoupling Enhancement in Organic Light‐Emitting Diodes: Highly Conductive Polymer as a Low‐Index Layer on Microstructured ITO Electrodes

Organic light-emitting diodes (OLEDs) are now entering mainstream display markets and are also being explored for next-generation lighting applications. In both types of applications, high external quantum efficiency (EQE) is of premium importance for both low power consumption and long lifetime. It is well known that one of the bottlenecks in achieving high EQE in OLEDs is the low light-extraction efficiency, which is limited to <20%, mostly because total internal reflections occurring at interfaces between optically distinctive layers confine a significant portion of the light within the substrate (1⁄4 ‘‘substrate-confined’’ mode) or within the organic/indium tin oxide (ITO) layers (1⁄4 ‘‘wave-guided’’ mode). Hence, many device structures have been proposed to extract light that would not normally be outcoupled: some have attempted to extract the light that is confined in a substrate by introducing structures such as microlens array (MLA) or pyramidal arrays on the backside of the substrate, where other research groups have tried to extract the light that is confined within organic/ITO layers by introducing optical structures such as photonic crystals or low-index grids that can disrupt the wave-guiding of the light within the organic/ITO layers. The lattermay be carried out in a direct way by converting wave-guided modes directly into outcoupled modes or in an indirect way by converting wave-guided modes into substrate-confined modes and then extracting them with structures mentioned in the former approach. Criteria for choosing a specific method or structure over others depend on the target applications: for display applications, methodologies such as MLA and substrate structuring are often avoided due to their optical blurring effect; for lighting applications, such methods are readily accepted, but complex processes that add too much cost are generally not welcomed. In both cases, compatibility with a common fabrication technique and large-area fabrication is strongly preferred, and the Lambertian angular dependence and the absence of spectral dependence are also preferred in most situations. Here we introduce a novel anode structure based on micropatterned ITOs coated with high-conductivity (HC)-grade poly(3,4ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) layers. This proposed electrode structure can improve the outcoupling efficiency of OLEDs in a relatively simple way without severe spectral dependence, blurring (optional), or deviation from the normal angular dependence. Figure 1 illustrates a tilted top-view and cross-section of the proposed anode structure and its working principle. ITO layers are patterned so that the square opening (Wo Wo) repeats in a square lattice layout with a spatial period ofWt. For simplicity, we consider a situation where Wt1⁄4 2 Wo. In this case, the width of ITO strips (WITO (1⁄4Wt–Wo)) next to each opening equalsWo, and the ITO-less portion is 25% per each unit cell. The spatial period and the dimension of openings are chosen to be sufficiently larger than the emission wavelength so that a geometric optic approach can be valid and spectral dependence may be ignored. Each pattern may have a taper angle utaper, as defined in Figure 1a. A high-conductivity PEDOT:PSS (Baytron PH 500, HC Starck, Inc.) layer is coated throughout the anode area over the patterned anode. Organic layers and metal cathodes are then deposited to complete the device. Note that the light emitted with a small angle within the emission layer, which would normally be wave-guided throughout the organic/ ITO layers, is now guided either solely within organic layers (the ray in red coming from the left side in Figure 1b) or solely within ITO layers (the ray in blue coming from the right side in Figure 1b), because the refractive index ( 1.42 at l1⁄4 550 nm) of PEDOT:PSS is lower than those of ITOs and typical organic layers used in OLEDs. Upon hitting the structured region once or multiple times, the guided light will change its direction so that it can be directly coupled out. Some portion of the wave-guided mode can also be converted to a substrate-confined mode (see Figure S1 in Supporting Information). It has to be noted that patterned ITO electrodes alone without the PEDOT:PSS overcoat would not work effectively, because organic layers and ITO layers are optically almost identical due to their similar refractive indices. The low refractive index of a PEDOT:PSS layer is indeed the characteristic that enables internal reflections at the structured interfaces, which are among the key processes that must occur in order for the guided light to be converted to an outcoupled or a substrate-confined mode. In addition to the optical benefits noted above, it is also critical that, in the region without ITO, the HC-grade PEDOT:PSS layer provides an electrical sheet conduction and works as an anode independently so that there is no inactive area in the devices. In fact, we note each anode region consisting solely of PEDOT:PSS is surrounded by ITO electrodes, resembling the conductive grid structure suggested by Leo and his coworkers that can ensure

Encapsulation of pentacene/C60 organic solar cells with Al2O3 deposited by atomic layer deposition

Organic solar cells based on pentacene/C60 heterojunctions were encapsulated using a 200-nm-thick film of Al2O3 deposited by atomic layer deposition (ALD). Encapsulated devices maintained power conversion efficiency after exposure to ambient atmosphere for over 6000h, while devices with no encapsulation degraded rapidly after only 10h of air exposure. In addition, thermal annealing associated with the ALD deposition is shown to improve the open-circuit voltage and power conversion efficiency of the solar cells.

Nanoscale Electronics: Digital Fabrication by Direct Femtosecond Laser Processing of Metal Nanoparticles

For various applications in the electronics industry, the fabrication of electrically conductive nanoand micropatterns has become important. Conventional vacuum metal deposition and photolithography processes are widely used for high-resolution metal patterning of microelectronics. However, those conventional approaches require expensive vacuum conditions, high processing temperatures, many steps, and toxic chemicals to fabricate one layer of a metal pattern. Furthermore, it is almost impossible to change the design of the expensive photomask once it is fabricated. For these reasons, the development of alternative maskless, direct, high-resolution patterning techniques to fabricate conductive microand nanopatterns at atmospheric pressure and low temperature without using vacuum deposition or photolithography has attracted wide attention in recent years. One of the most promising alternatives is the direct patterning of solution-deposited metal nanoparticles (NPs). The development of metal NP solution ink enabled 1) an inexpensive solution-based metal deposition approach without using expensive vacuum deposition and 2) a low-temperature metal deposition process, which allows using heat-sensitive and inexpensive polymer as the substrate. Examples of NP-inkbased direct metal patterning include screen printing, [ 1 ] direct nanoimprinting, [ 2 , 3 ] microcontact printing, [ 4 , 5 ] inkjet printing, [ 6 , 7 ]