Blue Single‐Layer Organic Light‐Emitting Diodes Using Fluorescent Materials: A Molecular Design View Point

Abstract

technology is mastered, it suffers from a real complexity, a high cost, and is time-consuming. Simplifying the multilayer structure with the so-called single-layer OLEDs (SL-OLEDs), the simplest device only made of the electrodes and the EML has hence rapidly appeared as a promising and appealing solution in this technology (Scheme 2, left).However, removing the functional organic layers of an OLED stack often leads to a dramatic decrease of the performance and reaching high efficiency blue SL-OLEDs has required intense researches and especially in term of materials design. Indeed, if one removes all the organic layers surrounding the EML, the efficient injection, transport, and recombination of charges within the device, should be insured by the EML itself and therefore the fluorophore. However, this appears to be a real obstacle to cross for blue emitting materials. Indeed, blue fluorophores usually possess a high HOMO and a low LUMO energy rendering the injection of charges in such materials very difficult. This has been one of the main issue to address in this field. Common properties required for high-performance blue emitting materials can be summarized as follows: 1) blue light emission between 380 and 500 nm with maximum wavelength around 450 nm; 2) a narrow full width at half maximum less than 60 nm; which produces low Commission Internationale de l'Eclairage (CIE) y-coordinate of less than 0.10 for TVs and cell phones; 3) thermal and morphological stability for stable device operation and a long device lifetime; 4) balance charge transport between electrons and holes flow and 5) adequate HOMO/LUMO energy levels for charge recombination and injection.Gathering all these properties in a single molecule is far from being an easy task and requires very precise designs. Indeed, as soon as one tries to adjust the HOMO/ LUMO energy levels of an organic semiconductor (OSC) with the Fermi levels of OLED electrodes, its gap is contracted and the emission wavelength is bathochromically shifted. This design strategy can hence lead to a material in which the charges can be more easily injected (the gap is contracted) but with an emission wavelength no more in the blue region. Trying to find the best compromise between adequate HOMO/LUMO energy and a blue emission has been the main challenge to address in the molecular design of efficient fluorophores for blue SL-OLEDs. This particularity has strongly restrained the development of such materials. However, for the last 30 years, research groups have developed many different molecular design strategies, which have led in some cases to high performance blue SL-OLEDs. Reaching high performance SL-OLEDs by molecular engineering of the fluorophore and Cyril Poriel is

Highly Efficient Single-Layer Phosphorescent Organic Light-Emitting Diodes Based on Co-Host Structure

“…Furthermore, only one organic EML was used to realize the light emitting and the charge transport. The devices A-D were only made of electrodes and one EML according with the characteristics of single-layer configuration [21]. Further, the test of device performance was a very crucial step in data analysis.…”

Designing Host Materials for the Emissive Layer of Single‐Layer Phosphorescent Organic Light‐Emitting Diodes: Toward Simplified Organic Devices

“…Reaching efficient and stable blue emission in OLED technology has been a very active research field for more than 20 years and for the three generations of OLEDs, fluorescent, phosphorescent, and thermally activated delayed fluorescent (TADF) OLEDs. [ 5,9,24,29,30 ] Developing high‐performance host materials for blue SL‐PhOLEDs represent the most difficult challenge to address. This is due to the high E T (above 2.6 eV) and the large HOMO/LUMO gap of blue phosphors.…”

“…As far as we know, there is to date only one review dealing with single‐layer devices and this review is fully dedicated to blue fluorescent materials. [ 24 ] The present review is therefore the first dedicated to single‐layer phosphorescent OLEDs (several reviews have been published on ML‐PhOLEDs in the last 10 years). [ 10,23,25,26 ]…”

“…Herein, will be first reviewed the SL‐PhOLEDs possessing the following architecture: Anode/EML (host+guest)/cathode (see SL‐PhOLED architecture in Figure 1, top middle). As neat electrodes (both anode and cathode) are rarely used in OLED technology, [ 24,27 ] a thin hole and/or electron injection layers almost always cover the anode and the cathode respectively. In fact, most of the time, the anode is indium tin oxide (ITO) either in its neat form or covered either by a thin film of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) this allows to induce a better organization of the interface and a decrease of the anode work function (WF) [ 28 ] ) or by MoO 3 or 4,4′,4″‐tris(3‐methyl)phenylphenylamino)triphenylamine (mt‐DATA).…”

Suppression of external quantum efficiency rolloff in organic light emitting diodes by scavenging triplet excitons

“…ver the last few decades, a wide variety of materials and methods have been developed for improving performance of organic light-emitting diodes (OLEDs) [1][2][3][4][5] . Important advantages of OLEDs are easy fabrication, mechanical flexibility, light weight, emission colour tunability, low cost, and compactness [1][2][3][4][5] . One of the biggest issues frequently observed in OLEDs is that external quantum efficiencies (EQE) significantly decrease at high current densities which is termed as "efficiency rolloff".…”