Optical properties light-emitting diodes LEDs

Abstract. This article reviews mainly the results of our recent research on the relationship between the structure and the luminescence properties of PPV derivatives. PPV derivatives are particularly useful in an effort toward the establishment of such relationship because their chemical structures can be manipulated very systematically. Attachment of a wide variety of substituents, inclusion of kinky structural units, modification of main chain structures by inclusion of hole- and/or electron-transferring structures, and blending of polymers having different optical and electronic properties are representative approaches. The device characteristics of the light-emitting diodes (LEDs) fabricated from these polymers are discussed in relation to their structures. In certain cases, their photoluminescence (PL) properties are compared with their electroluminescence (EL) properties. 

The physical mechanism by which semiconductor light-emitting diodes (LEDs) emit light is spontaneous recombination of electron-hole pairs and simultaneous emission of photons. The spontaneous emission process is fundamentally different from the stimulated emission process occurring in semiconductor lasers and superluminescent LEDs. The characteristics of spontaneous emission that determine the optical properties of LEDs will be discussed in this section. 

The complete optical setup of induced fluorescence involves an excitation part and an emission part. The intersection point of the two parts is the detection window in the microdevice. The excitation part starts from the light source and ends at the microdevice, while the emission part originates from the microdevice and stops at the detector. In free-space fluorescence detection, the common excitation sources include the laser, light-emitting diode (LED), and mercury or xenon arc lamp, and each light source has its distinctive spectroscopic property and practical benefits. Laser-induced fluorescence (LIE) is a highly sensitive optical detection method and is able to perform even single molecule detection. LIE has been introduced into the... 

The importance of these amorphous layers derives from their electronic structure. There are no longer sharp bands characterized by a definite band gap, but quasi-continuous changes in the density of states are observed leading to differences between the optical gap and the mobility gap . Thus, interesting optoelectronic properties and applications are reported, e.g., photoconductivity and solar cells [204, 205], optical vnndows for solar cells [206, 207], electroluminescence and light emitting diodes (LED) [207, 208], or thermistors for IR sensors [209]. 

Poly(phenylene vinylene) (PPV) is another valuable conjugated polymer, as the synthetic process of PPV is simple and low cost. PPV possesses excellent photoluminescent (PL) and electroluminescent (EL) properties, as well as photovoltaic (PV) and nonlinear optical properties [57]. These properties have led to its being used in broad applications, in such areas as light-emitting diodes (LEDs) and flat-panel displays and photonics applications such as wave-guiding and all-optical switching. 

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