Jens Meyer

A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics

A Sturdy Electrode Coating To operate efficiently, organic devices—such as light-emitting diodes—require electrodes that emit or take up electrons at low applied voltages (that is, have low work functions). Often these electrodes are metals, such as calcium, that are not stable in air or water vapor and have to be protected from environmental damage. Zhou et al. (p. 327; see the Perspective by Helander) report that a coating polymer containing aliphatic amine groups can lower the work functions of various types of electrodes by up to 1.7 electron volts and can be used in a variety of devices. Air-stable, physisorbed polymers containing aliphatic amine groups can improve the efficiency of organic electronic devices. Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low–work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.

Transition Metal Oxides for Organic Electronics: Energetics, Device Physics and Applications

During the last few years, transition metal oxides (TMO) such as molybdenum tri-oxide (MoO(3) ), vanadium pent-oxide (V(2) O(5) ) or tungsten tri-oxide (WO(3) ) have been extensively studied because of their exceptional electronic properties for charge injection and extraction in organic electronic devices. These unique properties have led to the performance enhancement of several types of devices and to a variety of novel applications. TMOs have been used to realize efficient and long-term stable p-type doping of wide band gap organic materials, charge-generation junctions for stacked organic light emitting diodes (OLED), sputtering buffer layers for semi-transparent devices, and organic photovoltaic (OPV) cells with improved charge extraction, enhanced power conversion efficiency and substantially improved long term stability. Energetics in general play a key role in advancing device structure and performance in organic electronics; however, the literature provides a very inconsistent picture of the electronic structure of TMOs and the resulting interpretation of their role as functional constituents in organic electronics. With this review we intend to clarify some of the existing misconceptions. An overview of TMO-based device architectures ranging from transparent OLEDs to tandem OPV cells is also given. Various TMO film deposition methods are reviewed, addressing vacuum evaporation and recent approaches for solution-based processing. The specific properties of the resulting materials and their role as functional layers in organic devices are discussed.

Role of the deep-lying electronic states of MoO3 in the enhancement of hole-injection in organic thin films

The electronic structures of vacuum-deposited molybdenum trioxide (MoO3) and of a typical MoO3/hole transport material (HTM) interface are determined via ultraviolet and inverse photoelectron spectroscopy. Electron affinity and ionization energy of MoO3 are found to be 6.7 and 9.68 eV, more than 4 eV larger than generally assumed, leading to a revised interpretation of the role of MoO3 in hole injection in organic devices. The MoO3 films are strongly n-type. The electronic structure of the oxide/HTM interface shows that hole injection proceeds via electron extraction from the HTM highest occupied molecular orbital through the low-lying conduction band of MoO3.

MoO3 Films Spin‐Coated from a Nanoparticle Suspension for Efficient Hole‐Injection in Organic Electronics

MoO3 films spin-coated from a suspension of nanoparticles, which offers energetic properties nearly identical to those of thermally evaporated MoO3 films, are reported. It is demonstrated that our solution-based MoO3 acts as a very efficient hole-injection layer for organic devices.

Highly efficient simplified organic light emitting diodes

The authors report on highly efficient organic light emitting diodes (OLEDs) consisting of only two organic layers. The key to the simplification is the direct injection of holes into the wide band gap hole transport material 4,4′,4″-tris(N-carbazolyl)-triphenyl amine (highest occupied molecular orbital is 5.9eV) through an indium tin oxide/tungsten oxide (WO3) anode. Kelvin probe analysis has revealed an extremely high work function of 6.4eV for WO3. The efficiencies of the simplified OLEDs exceed 40lm∕W and 45cd∕A at a brightness of 100cd∕m2, unsurpassed by other comparably simple OLED devices. Therefore, our OLED architecture demonstrates highly efficient, yet easy to fabricate devices.

Charge generation layers comprising transition metal-oxide/organic interfaces: Electronic structure and charge generation mechanism

The energetics of an archetype charge generation layer (CGL) architecture comprising of 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA), tungsten oxide (WO3), and bathophenanthroline (BPhen) n-doped with cesium carbonate (Cs2CO3) are determined by ultraviolet and inverse photoemission spectroscopy. We show that the charge generation process occurs at the interface between the hole-transport material (TCTA) and WO3 and not, as commonly assumed, at the interface between WO3 and the n-doped electron-transport material (BPhen:Cs2CO3). However, the n-doped layer is also essential to the realization of an efficient CGL structure. The charge generation mechanism occurs via electron transfer from the TCTA highest occupied molecular orbital level to the transition metal-oxide conduction band.

Electronic structure of Vanadium pentoxide: An efficient hole injector for organic electronic materials

The electronic structure of Vanadium pentoxide (V2O5), a transition metal oxide with an exceedingly large work function of 7.0 eV, is studied via ultraviolet, inverse and x-ray photoemission spectroscopy. Very deep lying electronic states with electron affinity and ionization energy (IE) of 6.7 eV and 9.5 eV, respectively, are found. Contamination due to air exposure changes the electronic structure due to the partial reduction of vanadium to V+4 state. It is shown that V2O5 is a n-type material that can be used for efficient hole-injection into materials with an IE larger than 6 eV, such as 4,4′-Bis(N-carbazolyl)-1,1′-bipheny (CBP). The formation of an interface dipole and band bending is found to lead to a very small energy barrier between the transport levels at the V2O5/CBP interface.

Effect of contamination on the electronic structure and hole-injection properties of MoO3/organic semiconductor interfaces

The electronic structure and hole-injection properties of ambient contaminated molybdenum trioxide (MoO3) surfaces are studied by ultraviolet and inverse photoemission spectroscopy, and current-voltage measurements. Contamination reduces the work function (WF), electron affinity (EA) and ionization energy by about 1 eV with respect to the freshly evaporated film, to values of 5.7 eV, 5.5 eV, and 8.6 eV, respectively. However, the WF and EA remain sufficiently large that the hole-injection properties of MoO3 are not affected by contamination. The results are of particular importance in view of potential applications of transition metal oxides for low-cost manufacturing of devices in low-vacuum or nonvacuum environment.

p-type doping efficiency of MoO3 in organic hole transport materials

We report on the p-type doping efficiency of molybdenum trioxide (MoO3) in the ambipolar organic charge transport material 4,4′-Bis(carbazol-9-yl)-biphenyl (CBP). Kelvin probe analysis is used to study the work function with increasing thickness of doped CBP layers with varied MoO3 concentration deposited on indium tin oxide (ITO). Based on the model of a one-sided abrupt (n+p) junction between ITO and the MoO3 doped CBP layer, the density of free holes has been determined. A surprisingly low p-type doping efficiency of less than 2% has been derived. Segregation and clustering of the MoO3 dopant could explain these results.

Metal oxide induced charge transfer doping and band alignment of graphene electrodes for efficient organic light emitting diodes.

The interface structure of graphene with thermally evaporated metal oxide layers, in particular molybdenum trioxide (MoO3), is studied combining photoemission spectroscopy, sheet resistance measurements and organic light emitting diode (OLED) characterization. Thin (<5 nm) MoO3 layers give rise to an 1.9 eV large interface dipole and a downwards bending of the MoO3 conduction band towards the Fermi level of graphene, leading to a near ideal alignment of the transport levels. The surface charge transfer manifests itself also as strong and stable p-type doping of the graphene layers, with the Fermi level downshifted by 0.25 eV and sheet resistance values consistently below 50 Ω/sq for few-layer graphene films. The combination of stable doping and highly efficient charge extraction/injection allows the demonstration of simplified graphene-based OLED device stacks with efficiencies exceeding those of standard ITO reference devices.