Electrical conduction and low voltage blue electroluminescence in vacuum-deposited organic films

dc and pulsed electroluminescence (EL) in tetracene crystals have been studied at room temperature. The crystals are provided with Na/K alloy electron injecting contact and Au semitransparent layer as a hole injecting contact. dc EL intensity shows a superlinear relationship with crystal current. EL decay indicates that the electroluminescence in tetracene consists of two components: one prompt and a second delayed component. The results can be understood if the delayed component depends upon crystal current and upon some characteristics of the active traps in the crystal. The spectral distribution of the electroluminescence as compared to photoluminescence spectrum reveals the intensive long wavelength emission. It is presumably due to trapped excitons.

Electroluminescence in Tetracene Crystals

The growth by the Bridgman method of anthracene single crystals doped with certain electron acceptors

The electroluminescence (EL) study of anthracene and tetracene-doped anthracene films deposited by a vacuum evaporation onto a substrate at temperatureT0=−60 °C is reported. Such films show high resistence to electrical breakdown so that their EL behaviour at high electric fields up to above 106 V/cm could be investigated. The high-field EL is interpreted in terms of electron-hole recombination mechanisms with creation of emitting singlet excitons; the electron and hole concentration being limited by injection at metal contacts and by field-independent carrier velocities. The theoretical expression for current density follows the experimental data for fields above 4×105 V/cm at room temperature and the EL intensity is predicted to be a power function of the measured current. It is suggested that in pure anthracene layers the power is determined by the contact injection conditions, i.e. by barrier heights and the effective masses of holes and electrons. In the doped films a guest molecules-induced energy trap distribution leads to a decrease of the power as confirmed by the experimental plots of the host EL intensity versus current density.

High-field recombination electroluminescence in vacuum-deposited anthracene and doped anthracene films

Publisher Summary This chapter discusses present state of molecular organic light-emitting devices (OLEDs) science and technology. The chapter describes the structural properties of these devices and their implications on device performance. The simple OLED configuration is the single organic layer device consisting of an electron transport layer (ETL) or hole transport organic layer (HTL) sandwiched between an anode and a cathode. Besides serving to transport carriers, the organic thin film also acts as the electroluminescent layer. The new device structures demonstrated during the past several years together with conventional OLEDs are building blocks for many different organic light emitters and full-color displays. The development of a transparent, thin film organic light-emitting device demonstrates a significant advantage of organic materials in realizing high definition, full-color, and head-up displays. The chapter also describes the fabrication, physics, and state of the technology of vacuum deposited molecular (i.e., nonpolymeric) OLEDs.

Chapter 5 Molecular Organic Light-Emitting Devices

The steady state and pulsed current-voltage-electroluminescence intensity dependences have been measured for pentacene-doped anthracene crystals over the temperature range 300–200°K. Prompt and delayed electroluminescence from both host and guest are produced. The overall electroluminescence efficiency with respect to input power is much less than for neat anthracene crystals, though efficiency is still high, ∼10% with respect to crystal current.

N. Wakayama

Papers

Pulsed and Steady State Electroluminescence of Pentacene Doped Anthracene Crystals