Light Emitting Diode and Organic Laser

Abstract

The development of semiconductor lasers is accompanied by the development of light emitting diodes (LEDs). An LED, based on spontaneous electron-hole recombination, does not require a cavity and can therefore have a simpler design than a laser. We discuss properties of diodes and mention various areas of applications. There is a growing market of LEDs as lighting sources. Superluminescent semiconductor diodes can reach high efficiencies. Stimulated emission just below laser threshold is favorable for a high efficiency.

Problems

34.1

Estimate the quantum efficiency of an OLED.

34.2

The actual efficiency of an OLED can be enhanced if three organic layers are used instead of one. In this case, the third layer is embedded in two organic layers. The third layer (e.g., a layer with a dye) has two energy levels in the gap of the organic layers. The upper level is trap of electrons and the lower level is a trap of holes. Electron-hole recombination gives rise to fluorescence radiation. Illustrate the principle of the three-layer system in a sketch.

34.3

Treat the organic laser as a two-quasiband laser. Assume that the density of states in the upper band as well as the density of states in the lower band have Gaussian distributions and that the widths of the distributions are equal.

• (a) Determine the Fermi energy as a function of the density of excited molecules.

• (b) What is the condition of gain?

• (c) Estimate the gain coefficient of a system of molecules ($N_0 = 10^{24}$ m$^{-3}$); spontaneous lifetime of excited molecules = 10 ms.