Localized states and electronic transport in single component organic solids with diagonal disorder

There is growing interest in organic electroluminescence (EL). A great deal of progress has been made recently in improving the performance of various classes of organic EL devices. Some of these are now adequate for many applications. However, specialists focusing on selected aspects of organic EL devices have often lost contact with the general subject of EL. Therefore, a review covering all aspects of EL mechanisms and their experimental manifestation seemed necessary. This article is concerned with the new EL device physics that can be realized using crystals, or films made of organic materials, as electrically and optically active components, in devices ranging from simple single-component light emitting diodes (LEDs), through double- and multi-layer LEDs to light emitting electrochemical cells (LECs) and organic LED-based light transducers. The investigation of the properties of these devices has provided in turn a very effective method for studying the basic EL phenomena in these materials. Since the subject of the present review has generated a huge amount of literature, and it is impossible to mention here all that has been done, we have attempted to provide an outline of the background of the field of organic EL, and discussed in some detail those aspects most relevant to the EL device physics. Because of the diversity of the types of material and EL structure, there is no single, simple description of EL in organics. Therefore, the initial sections of the article are devoted to a discussion of the types of EL and related phenomena, such as carrier injection and recombination or nature of emitting states. Then, the fundamentals of the fabrication of various types of EL devices are discussed along with the most representative examples. In general, the reader will find in the article a brief historical review of the subject as well as a description of the latest trends in organic EL research covering all the new concepts and most important data which have become available before the time of publication.

https://iopscience.iop.org/article/10.1088/0022-3727/32/24/201/pdf

Electroluminescence in organics

Excimer emission of anthracene, perylene, coronene and pyrene microcrystals dispersed in water

Absorption spectra of tetracene and pentacene layers prepared by vapor deposition onto cooled substrates were measured as a function of sample formation temperature Tf, evaporation rate and recording temperature. Profile-analysis indicates that the main absorption peak is of gaussian shape. The width varies between 300 cm−1 and 650 cm−1, higher values referring to lower formation temperatures and/or higher evaporation rates and reflecting larger fluctuation of the site-energies. Appearance of a second electronic band equivalent to the upper Davydov component in the crystal spectrum indicates presence of short-range order. The low-energy behavior is dominated by a structural defect. No Urbacht-tail is observed. It is concluded that tetracene and pentacene layers represent examples for intermediate structural disorder.

W. Arden

Papers

Excimer emission from glassy films of some aromatic hydrocarbons