LIGHT­ING TECH­NOL­OGY – The next gen­er­a­tion

DEMM Engineering & Manufacturing - - FRONT PAGE - By Robert Mor­ris.

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

Con­tin­ued on page 32Ê

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