LIGHTING TECHNOLOGY – The next generation

How many of you have heard of, or maybe even seen an OLED? An OLED, or organic light emitting diode can best be described as a solid-state device composed of thin films of organic molecules that create light when voltage is applied to the device.

Amazingly, an OLED is only 100 to 500 nanometres thick, or about 200 times smaller than a human hair. OLEDs use red, green and blue diodes to produce colour images that can provide brighter, crisper displays on electronic devices and use less power than conventional light-emitting diodes (LEDs) or liquid crystal displays (LCDs) that are commonly used today.

An OLED is generally made up of six parts:

• Substrate – the base of the OLED and made of clear plastic, glass or foil. • Anode – the positive terminal • Conducting layer – composed of organic plastic molecules or polymers.

• Emissive layer – composed of organic plastic molecules or polymers and layer where light is formed. • Cathode – the negative terminal. • Seal – protective covering of glass or plastic. A voltage is applied across the OLED by connecting the cathode and anode plates to a power supply. This results in the cathode receiving and the anode losing electrons. Electrons in the atoms of the anode move resulting in a flow of current across the OLED circuit and causing the anode to be depleted of electrons. This can be thought of as creating positive holes or the anode receiving ‘holes'.

The additional electrons in the cathode cause the emissive layer to become negatively charged (increase in electrons). Similarly, the conductive layer next to the anode becomes positively charged (increase in holes).

The holes jump across the boundary, between the conductive and the emissive layer, and combine with the excess electrons in the emissive layer. When this happens, it forms a burst of light. The process occurs continuously and generates a steady stream of light, as long as a voltage is applied to the OLED.

The type of organic molecules used in the emissive layer will determine the colour of the light produced. Several types of organic films are placed on the same OLED to make the colour displays.

The intensity or brightness of the light can then be adjusted based on the amount of electrical current applied; the more current that is applied, the brighter the light.

Types of OLEDs There are a variety of OLED technologies, each with its advantages and disadvantages depending on use and application.

PMOLED – Passive matrix OLED. This type of OLED is primarily used to display text and icons on small screens found on a cellphone, PDA or

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