Thomas Riedl

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.

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.

Stability of transparent zinc tin oxide transistors under bias stress

Shifts in the threshold voltage (ΔVth) of transparent zinc tin oxide (ZTO) transistors under gate bias stress are studied. The effect of composition and processing temperature on the device stability has been investigated. Based on the research, highly stable transistors with ΔVth as small as 30mV after 1000min of operation have been fabricated with a composition of [Zn]:[Sn]=36:64. As current drivers in active matrix displays their stability renders ZTO thin film transistors (TFTs) a very attractive alternative to TFTs based on established technologies.

Efficient semitransparent inverted organic solar cells with indium tin oxide top electrode

We reported on highly efficient semitransparent polymer solar cells comprising a transparent sputtered indium tin oxide (ITO) top electrode. We used an inverted cell structure with titanium dioxide prepared by atomic layer deposition as electron selective layer and molybdenum oxide (MoO3) as hole extraction layer. Moreover, the MoO3 layer prevents damage to the organic active materials due to the ITO sputtering process. For the semitransparent device, power conversion efficiencies of 1.9% were achieved with a high transmittance of 80% in the red region of the visible spectrum.

Large Area Electronics Using Printing Methods

After the demonstration of the first organic FET in 1986, a new era in the field of electronic began: the era of organic electronics. Although the reported performance of organic transistors is still considerably lower compared to that of silicon transistors, a new market is open for organic devices, where the excellent performance of silicon technology is not required. Several commercial applications for organic electronics have been suggested: organic RFID tags, electronic papers, imagers, sensors, organic LED drivers, etc. The main advantage of organic technologies over silicon technologies is the possibility of making low-cost, large area electronics. The main processes which allow patterning with suitable resolution on a large areas are printing methods. Here we will provide an overview of methods that can be useful in the low-cost production of large area electronics.

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.

The influence of visible light on transparent zinc tin oxide thin film transistors

The characteristics of transparent zinc tin oxide thin film transistors (TTFTs) upon illumination with visible light are reported. Generally, a reversible decrease of threshold voltage Vth, saturation field effect mobility μsat, and an increase of the off current are found. The time scale of the recovery in the dark is governed by the persistent photoconductivity in the semiconductor. Devices with tuned [Zn]:[Sn] ratio show a shift of Vth of less 2V upon illumination at 5mW∕cm2 (brightness >30000cd∕m2) throughout the visible spectrum. These results demonstrate TTFTs which are candidates as pixel drivers in transparent active-matrix organic light emitting diode displays.

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.