Luminescence Phenomena

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Light emitting plastics is one of the major applications of organic semiconductors. This involves the process of luminescence.

Luminescence is the process of light emission by a solid when given some form of energy. In order to provide visual information, materials are usually put in conditions that allow them to emit light. There are a several types of energy that can stimulate different molecules into an excited state.

Types of Luminescence

The energy can be provided by a several methods depending on the excitation process.

• photoluminescence: excitation comes from absorption of photons. Glow in the dark paint is an example.

• cathodoluminescence: phosphor excitation due to bombardment by an electron beam (in old-fashioned displays for TVs and desktop computers)

• electroluminescence: excitation due to application of an electric field and injection of charge carriers (electrical current). This is the basis for the new generation of LED and OLED TVs and monitors.

Cause of luminescence

Whatever the source of excitation the final step is an electronic transition between two states of energy E₁ and E₀ with emission of a photon of a specific wavelength and color that depends on the energy levels.

<math>h\nu = E_1 - E_0 = \frac{hc}{\lambda}\,\!</math>

<math>E_1 - E_0\,\!</math> is the difference in energy from the excited state to a lower state.

Luminescence vs phosphoresence

Within the luminescence processes there is also a distinction between two processes that occur after the excitation stops.

• Fluorescence is luminescence that persists only on a time scale typical of the lifetime of the excited state. This is on the or der 10^-9 - 10^-8 seconds.

• Phosphorescence is luminescence that persists for a longer period. Phosphorescence is usually related to the presence of metastable electronic state(s) E₁ itself or states with energy lower than E₁.

Activators and coactivators

In conventional inorganic “phosphors” that are used in conventional TV or computer displays much of the action is due to dopant or impurities within the host material known as activators. Activators are chosen to have electronic levels that are in the gap of the host material. This brings electrons from the valence band to the conduction band of the host. The electron then drops down from the conduction band into a forbidden or metastable state from where it gives of light in the form of phosphorescence. You can also have two different impurities known as an activator and co-activator. The activator has energy levels closer to the conduction band of the host and the coactivator has energy band closer to the valence band. The electron jumps from the activator to conduction band and then down to the coactivator energy band.

Persistance of phoshorescence

The persistence of the luminescence of the phosphorescence is the time it takes for the electron to go back up into the conduction band from the activator and then down to the coactivator. This is known as detrapping.

The probability of detrapping is related to the difference in energy level of the electron and the energy of the bottom of the conduction band.

<math>P=Q \quad exp\bigg[-\frac{(E_c-E_d)}{kT}\bigg]\,\!</math>

Resulting in;

<math>E_c \equiv E\,\!</math>at the bottom of the conduction band

<math>E_d \equiv E\,\!</math>at the trap level for the electron

<math>Q \sim 10^{+8}s^{-1}\,\!</math>

So for example if Ec – Ed ~ 0.4 eV the probability of detrapping will be 10 per second.

Red green blue phosphors in CRT

A conventional TVs needs three colored phosphors to get full color – red, green and blue

• blue ZnS:Ag

where “:” means “doped with”, i.e., Ag is the activator

• green: Znx Cd1-x S : Cu

• red: Y₂O₂S : Eu, Tb (yttrium oxysulfide doped with europium and terbium)

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