Michael G. Helander

Universal energy-level alignment of molecules on metal oxides

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.

Chlorinated Indium Tin Oxide Electrodes with High Work Function for Organic Device Compatibility

Closer matching of the energy levels of transparent electrodes and active materials in organic light-emitting diodes improves efficiency. 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.

Highly Efficient Blue Phosphorescence from Triarylboron-Functionalized Platinum(II) Complexes of N-Heterocyclic Carbenes

The first examples of BMes(2)-functionalized NHC chelate ligands have been achieved. Their Pt(II) acetylacetonate complexes have been synthesized and fully characterized. These NHC-chelate Pt(II) compounds display highly efficient blue or blue-green phosphorescence in solution (Φ = 0.41-0.87) and the solid state (Φ = 0.86-0.90). Highly efficient electroluminescent devices based on these new Pt(II) compounds have also been fabricated.

Visible Colloidal Nanocrystal Silicon Light-Emitting Diode

We herein demonstrate visible electroluminescence from colloidal silicon in the form of a hybrid silicon quantum dot-organic light emitting diode. The silicon quantum dot emission arises from quantum confinement, and thus nanocrystal size tunable visible electroluminescence from our devices is highlighted. An external quantum efficiency of 0.7% was obtained at a drive voltage where device electroluminescence is dominated by silicon quantum dot emission. The characteristics of our devices depend strongly on the organic transport layers employed as well as on the choice of solvent from which the Si quantum dots are cast.

Highly simplified phosphorescent organic light emitting diode with >20% external quantum efficiency at >10,000 cd/m2

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.

Work function of fluorine doped tin oxide

Fluorine doped tin oxide (FTO) is a commonly used transparent conducting oxide in optoelectronic device applications. The work function of FTO is commonly cited as 4.4 eV, which is incommensurate with recent device performance results. Using x-ray photoelectron spectroscopy, the authors measured the work function of commercial FTO to be 5.0±0.1 eV. UV ozone treatment was found to increase the work function by ∼0.1 eV due to surface band bending. The origins of the much lower work function previously reported are also discussed and are found to be a result of carbon contamination and UV induced work function lowering.

N‐Heterocyclic Carbazole‐Based Hosts for Simplified Single‐Layer Phosphorescent OLEDs with High Efficiencies

Highly efficient single-layer organic light-emitting diodes (OLEDs) are demonstrated using new N-heterocyclic carbazole-based host materials. Phosphorescent OLEDs with a structure of ITO/MoO(3) /host/host:dopant/host/Cs(2) CO(3) /Al are fabricated in which the new materials act simultaneously as electron-transport, hole-transport, and host layer. Devices with maximum current and external quantum efficiencies of 92.2 cd A(-1) and 26.8% are achieved, the highest reported to date for a single-layer OLED.

A metallic molybdenum suboxide buffer layer for organic electronic devices

Molybdenum trioxide (MoO3) is commonly used as a buffer layer in organic electronic devices to improve hole-injection. However, stoichiometric MoO3 is an insulator, and adds a series resistance. Here it is shown that a MoO3 buffer layer can be reduced to form a metallic oxide buffer that exhibits more favorable energy-level alignment with N,N′-diphenyl-N,N′-bis-(1-naphthyl)-1-1′-biphenyl-4,4′-diamine (α-NPD) than does MoO3. This buffer layer thus provides the conductivity of a metal with the favorable energy alignment of an oxide. Photoemission shows the reduced oxide contains Mo4+ and Mo5+, with a metallic valence band structure similar to MoO2.

Band alignment at metal/organic and metal/oxide/organic interfaces

Charge injection at metal/organic interfaces dictates the performance, lifetime, and stability of organic electronic devices. We demonstrate that interface dipole theory, originally developed to describe Schottky contacts at metal/semiconductor interfaces, can also accurately describe the injection barriers in real organic electronic devices. It is found that theoretically predicted hole injection barriers for various archetype metal/organic and metal/oxide/organic structures are in excellent agreement with values extracted from experimental transport measurements. Injection barriers at metal/organic and metal/oxide/organic interfaces can therefore be accurately predicted based on the knowledge of only a few fundamental material properties of the oxide and organic layers.

C60:LiF blocking layer for environmentally stable bulk heterojunction solar cells.

Organic photovoltaics (OPVs) have attracted considerable interest due to their potential to be manufactured at low cost using solution processing. [ 1 ] Signifi cant effort has been devoted in recent years to improving the effi ciency of OPVs using a variety of different materials processing strategies. Power conversion effi ciencies (PCEs) greater than 5% are now routinely reported based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk-heterojunctions. [ 2–4 ] In addition to high effi ciency, a long lifetime is also equally as important to make OPVs a cost competitive and sustainable technology. [ 5 ] However, despite the overwhelming work dedicated to incremental improvements in effi ciency, there are very few studies that develop new pathways to improve device lifetime and environmental stability. There is therefore great interest in new materials that can deliver both high performance and long lifetime in OPVs. Here, we report on C 60 :LiF nanocomposites as electron transporting and hole-blocking layers for OPVs, with improved PCE and an impressive enhancement in device stability. The excellent environmental stability and high conductivity make the C 60 :LiF nanocomposite a versatile buffer layer to enable highperformance OPVs with long lifetime. The predominant degradation mechanism in OPVs is a result of the entrance of moisture and oxygen into the device. [ 6–9 ]