Device physics of organic light-emitting diodes based on molecular materials

Considerable research and development means have been focused in the past decade on organic semiconductor thin films and devices with applications to full color displays, flexible electronics and photovoltaics. Critical areas of these thin films are their interfaces with electrodes, with other organic films and with dielectrics, as these interfaces control charge injection and transport through the device. Full understanding of the mechanisms that determine the electronic properties of these interfaces, i.e. the relative position of molecular levels and charge carrier transport states, is an important goal to reach for developing reliable device processing conditions. This report provides an extensive, although probably somewhat biased, review of polymer– and small molecule–metal interface work of the past few years, with emphasis placed specifically on (i) the electronic structure and molecular level alignment at these interfaces, (ii) the perceived differences between small molecule and polymer interfaces, (iii) the difference between organic-on-metal and metal-on-organic interfaces, and (iv) the role played by electrode surface contamination in establishing interface energetics. Environmental conditions, e.g. vacuum vs. ambient, are found to be critical parameters in the processing of polymer and small molecule interfaces with metals. With similar processing conditions, these two types of interfaces are found to obey very similar molecular level alignment rules.

Energetics of metal–organic interfaces: New experiments and assessment of the field

Charge injection at the interface between metallic electrodes and organic semiconductors plays a crucial role in the performance of organic (opto-)electronic devices. This article discusses the current understanding of the formation of the metal–organic contact and the parameters which control the injection current. Organic semiconductors differ significantly from their inorganic counterparts, primarily because they are amorphous van der Waals solids. As a result the electronic states are highly localized, and charge transport is by site-to-site hopping. Organics can also form clean interfaces with many metals, free of interface states in the gap. Nevertheless, there is generally found to be a significant vacuum level offset, the origins of which are not yet fully understood. Organic semiconductors are frequently free of donor and acceptor dopants, and as a result the depletion depth is larger than the organic layer thickness. Thus the Fermi level in the organic and the charge injection barriers depend mo...

Metal–organic interface and charge injection in organic electronic devices

A numerical study of space charge effects in multilayer organic light-emitting diodes (OLEDs) is presented. The method of solving the coupled Poisson and continuity equations, previously established for single-layer polymer LEDs, has been extended to treat internal organic interfaces. In addition, we consider the transient current and electroluminescence response. We discuss the accumulation of charges at internal interfaces and their signature in the transient response as well as the electric field distribution. Comparison to experimental transient data of a typical bilayer LED based on tris(8-hydroxyquinolinato)aluminum (Alq3) is provided and good agreement is found. Our results are consistent with commonly assumed operating principles of bilayer LEDs. In particular, the assumptions that the electric field is predominantly dropped across the Alq3 layer and that the electroluminescence delay time is determined by electrons passing through Alq3 to the internal interface are self-consistently supported by ...

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Transient and steady-state behavior of space charges in multilayer organic light-emitting diodes

Surface and interface analytical studies have generated critical insight of the fundamental processes at interfaces involving organic semiconductors. I will review surface analytical studies of interface formation of organic semiconductors with different materials. Metal/organic interface is a focus of both device engineering and basic science, since it is a key factor in nearly all important aspects of device performances, including operation voltages, degradation, and efficiency. I will discuss metal–organic interface dipole formation, charge transfer, chemical reaction, energy level alignment, in-diffusion, quenching of luminescence and possible recovery of it. The effect of the insertion of ultra-thin interlayers such as LiF and doping by alkali metals will also be discussed. In organic/organic interface, the energy offset between the two dissimilar organic materials is vitally important to efficient device operation of organic light emitting diodes (OLED), as well as change separation at donor–acceptor interface in organic photovoltaic devices (OPV). I will discuss the interface energy level alignment, band bending, Debye screening, and charge separation dynamics as observed in surface analytical studies, and the implications to OLED and OPV. The interfaces of OSCs with other inorganic materials are also important. For organic thin film transistors (OTFT), the electronic properties of the interface formed between the organic and the dielectric strongly influences the current–voltage characteristics, as the electronic activity has been shown to occur primarily at the interface between the dielectric and the organic materials. I will review the interface formation of OSCs with dielectric materials and with indium-tin-oxide (ITO), a material whose transparency and conductivity make it indispensable for a number of opto-electronic applications and whose electronic properties and energy level alignment with organics have proven dramatically altered by surface treatments.

Surface analytical studies of interfaces in organic semiconductor devices

We present internal photoemission, photocurrent versus bias voltage, and current–voltage measurements of metal/tris-(8-hydroxyquinoline) aluminum [Alq]/metal structures. Internal photoemission and photocurrent versus bias measurements were used to determine metal/Alq Schottky energy barriers for a range of contact metals with work functions from 2.7 eV (Sm) to 5.6 eV (Pt). The electron Schottky barrier for low work-function metals ( about 3.6 eV) the ideal Schottky model is generally accurate. A previously established device model was used to describe the current–voltage characteristics using the measured Schottky barriers. The results imply comparable electron and hole mobilities of about 2×10−5 cm2/V s at an electric field of 106 V/cm.

I. H. Campbell

Papers

Schottky energy barriers and charge injection in metal/Alq/metal structures