Mark T. Greiner

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.

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 ]

Oxidized gold thin films: an effective material for high-performance flexible organic optoelectronics.

Adv. Mater. 2010, 22, 2037–204

Depleted-heterojunction colloidal quantum dot photovoltaics employing low-cost electrical contacts

With an aim to reduce the cost of depleted-heterojunction colloidal quantum dot solar cells, we describe herein a strategy that replaces costly Au with a low-cost Ni-based Ohmic contact. The resultant devices achieve 3.5% Air Mass 1.5 power conversion efficiency. Only by incorporating a 1.2-nm-thick LiF layer between the PbS quantum dot film and Ni, we were able to prevent undesired reactions and degradation at the metal-semiconductor interface.

The effect of UV ozone treatment on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

The interface between ultraviolet (UV) ozone treated poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and N,N′-diphenyl-N,N′-bis-(1-naphthyl)-1-1′-biphenyl-4,4′-diamine (α-NPD) was investigated using single carrier hole-only devices and in situ ultraviolet and x-ray photoelectron spectroscopy to elucidate the implications for device applications. It is found that although the work function of PEDOT:PSS is increased by UV ozone treatment, the injection barrier to α-NPD is in fact increased, resulting in lower current density in devices. The apparent increase in work function is attributed to a metastable surface dipole as a result of UV ozone treatment, which does not significantly influence the energy-level alignment.

Reverse Water–Gas Shift or Sabatier Methanation on Ni(110)? Stable Surface Species at Near-Ambient Pressure

The interaction of CO, CO2, CO + H2, CO2 + H2, and CO + CO2 + H2 with the nickel (110) single crystal termination has been investigated at 10(-1) mbar in situ as a function of the surface temperature in the 300-525 K range by means of infrared-visible sum frequency generation (IR-vis SFG) vibrational spectroscopy and by near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Several stable surface species have been observed and identified. Besides atomic carbon and precursors for graphenic C phases, five nonequivalent CO species have been distinguished, evidencing the role of coadsorption effects with H and C atoms, of H-induced activation of CO, and of surface reconstruction. At low temperature, carbonate species produced by the interaction of CO2 with atomic oxygen, which stems from the dissociation of CO2 into CO + O, are found on the surface. A metastable activated CO2(-) species is also detected, being at the same time a precursor state toward dissociation into CO and O in the reverse water-gas shift mechanism and a reactive species that undergoes direct conversion in the Sabatier methanation process. Finally, the stability of ethylidyne is deduced on the basis of our spectroscopic observations.

The Oxidation of Rhenium and Identification of Rhenium Oxides During Catalytic Partial Oxidation of Ethylene: An In-Situ XPS Study

Abstract Rhenium is catalytically active for many valuable chemical reactions, and consequently has been the subject of scientific investigation for several decades. However, little is known about the chemical identity of the species present on rhenium surfaces during catalytic reactions because techniques for investigating catalyst surfaces in-situ – such as near-ambient-pressure X-ray photoemission spectroscopy (NAP-XPS) – have only recently become available. In the current work, we present an in-situ XPS study of rhenium catalysts. We examine the oxidized rhenium species that form on a metallic rhenium foil in an oxidizing atmosphere, a reducing atmosphere, and during a model catalytic reaction (i.e. the partial-oxidation of ethylene). We find that, in an oxidizing environment, a Re2O7 film forms on the metal surface, with buried layers of sub-oxides that contain Re4+, Re2+ and Reδ+ (δ ∼ 1) species at the Re2O7/Re interface. The Re2+ containing sub-oxide is not a known bulk oxide, and is only known to exist on rhenium-metal surfaces. The Re2O7 film sublimes at a very low temperature (ca. 150 ℃), while the Re4+, Re2+ and Reδ+ species remain stable in oxidizing conditions up to at least 450 ℃. In a reducing atmosphere of H2, the Re2+ species remain on the surface up to a temperature of 330 ℃, while Reδ+ species can be detected even at 550 ℃. Under conditions for partial-oxidation of ethylene, we find that the active rhenium catalyst surface contains no bulk-stable oxides, but consists of mainly Re2+ species and small amounts of Re4+ species. When the catalyst is cooled and inactive, Re2O7 is found to form on the surface. These results suggest that Re2+ and Re4+ species may be active species in heterogeneous rhenium catalysts.

Reactivity of Carbon Dioxide on Nickel: Role of CO in the Competing Interplay between Oxygen and Graphene

The catalytic conversion of carbon dioxide to synthetic fuels and other valuable chemicals is an issue of global environmental and economic impact. In this report we show by means of X-ray photoelectron spectroscopy in the millibar range that, on a Ni surface, the reduction of carbon dioxide is indirectly governed by the CO chemistry. While the growth of graphene and the carbide-graphene conversion can be controlled by selecting the reaction temperature, oxygen is mainly removed by CO, since oxygen reduction by hydrogen is a slow process on Ni. Even though there is still a consistent pressure gap with respect to industrial reaction conditions, the observed phenomena provide a plausible interpretation of the behavior of Ni/Cu based catalysts for CO2 conversion and account for a possible role of CO in the methanol synthesis process.

Ethylene Epoxidation at the Phase Transition of Copper Oxides

Catalytic materials tend to be metastable. When a material becomes metastable close to a thermodynamic phase transition it can exhibit unique catalytic behavior. Using in situ photoemission spectroscopy and online product analysis, we have found that close to the Cu2O-CuO phase transition there is a boost in activity for a kinetically driven reaction, ethylene epoxidation, giving rise to a 20-fold selectivity enhancement relative to the selectivity observed far from the phase transition. By tuning conditions toward low oxygen chemical potential, this metastable state and the resulting enhanced selectivity can be sustained. Using density functional theory, we find that metastable O precursors to the CuO phase can account for the selectivity enhancements near the phase transition.