Transition-metal oxides improve power conversion efficiencies in organic photovoltaics and are used as low-resistance contacts in organic light-emitting diodes and organic thin-film transistors. What makes metal oxides useful in these technologies is the fact that their chemical and electronic properties can be tuned to enable charge exchange with a wide variety of organic molecules. Although it is known that charge exchange relies on the alignment of donor and acceptor energy levels, the mechanism for level alignment remains under debate. Here, we conclusively establish the principle of energy alignment between oxides and molecules. We observe a universal energy-alignment trend for a set of transition-metal oxides--representing a broad diversity in electronic properties--with several organic semiconductors. The trend demonstrates that, despite the variance in their electronic properties, oxide energy alignment is governed by one driving force: electron-chemical-potential equilibration. Using a combination of simple thermodynamics, electrostatics and Fermi statistics we derive a mathematical relation that describes the alignment.
In organic light-emitting diodes (OLEDs), a stack of multiple organic layers facilitates charge flow from the low work function [~4.7 electron volts (eV)] of the transparent electrode (tin-doped indium oxide, ITO) to the deep energy levels (~6 eV) of the active light-emitting organic materials. We demonstrate a chlorinated ITO transparent electrode with a work function of >6.1 eV that provides a direct match to the energy levels of the active light-emitting materials in state-of-the art OLEDs. A highly simplified green OLED with a maximum external quantum efficiency (EQE) of 54% and power efficiency of 230 lumens per watt using outcoupling enhancement was demonstrated, as were EQE of 50% and power efficiency of 110 lumens per watt at 10,000 candelas per square meter.
White organic light‐emitting diodes (WOLEDs) are currently under intensive research and development worldwide as a new generation light source to replace problematic incandescent bulbs and fluorescent tubes. One of the major challenges facing WOLEDs has been to achieve high energy efficiency and high color rendering index simultaneously to make the technology competitive against other alternative technologies such as inorganic LEDs. Here, an all‐phosphor, four‐color WOLEDs is presented, employing a novel device design principle utilizing molecular energy transfer or, specifically, triplet exciton conversion within common organic layers in a cascaded emissive zone configuration to achieve exceptional performance: an 24.5% external quantum efficiency (EQE) at 1000 cd/m2 with a color rendering index (CRI) of 81, and an EQE at 5000 cd/m2 of 20.4% with a CRI of 85, using standard phosphors. The EQEs achieved are the highest reported to date among WOLEDs of single or multiple emitters possessing such high CRI, which represents a significant step towards the realization of WOLEDs in solid‐state lighting.
A fluorinated phenoxy boron subphthalocyanine (BsubPc) is shown to function as a fluorescent dopant emitter in small molecule organic light emitting diodes (OLEDs). Narrow electroluminescence (EL) emission with a full width at half-maximum of ∼30 nm was observed regardless of the host used, indicating that this narrow EL is intrinsic to the BsubPc molecule. A bathochromic shift and the growth of a new EL peak at higher wavelengths with increasing doping concentration were found to be a result of molecular aggregation. Excitation of BsubPc by direct charge trapping as well as Förster resonant energy transfer were shown using different host molecules. A maximum efficiency of 1.5 cd/A was achieved for a 4,4'-N,N'-dicarbazole-biphenyl (CBP) host.
Phosphorescent
copper(I) complexes show great promise as emitters in organic light-emitting
diodes (OLEDs). However, most copper(I) complexes are neither soluble
nor stable toward sublimation and, hence, not amenable to the typical
methods to fabricate OLEDs. In this work, a compound 3-(carbazol-9-yl)-5-((3-carbazol-9-yl)phenyl)pyridine
(CPPyC) was designed as both a good ligand and host matrix. Codeposition
of CPPyC and copper iodide (CuI) gives luminescent films with photoluminescent
quantum yields (PLQY) as high as 100%. A dimeric copper(I) complex
Cu2I2(CPPyC)4 is formed in the thin
film, characterized by X-ray absorption spectroscopy. A series of
simple, highly efficient green-emitting OLEDs were demonstrated by
using the codeposited film as an emissive layer. A device comprised
of only CPPyC and CuI gave an external quantum efficiency (EQE) of
12.6% (42.3 cd/A) at 100 cd/m2, while a device with tailored
hole and electron transporting layers gave an efficiency of 15.7%
(51.6 cd/A) at the same brightness.
A simplified trilayer green phosphorescent organic light emitting diode with high efficiency and an ultralow efficiency roll-off has been demonstrated. In particular, the external quantum efficiency drops <1% from 100 to 5,000 cd/m2 and remains as high as ∼21.9% at 10,000 cd/m2. The power efficiency is also significantly improved, reaching 78.0 lm/W at 100 cd/m2, 50.5 lm/W at 5,000 cd/m2, and 42.8 lm/W at 10,000 cd/m2. The working mechanism of this simple device structure with an unprecedented high efficiency is also discussed.
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