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Figure 12: Photoluminiscence and electroluminiscence in conjugated polymers. a) Irradiation can<br />
excite an electron from the LUMO in the HOMO and two new energy states are generated. Both<br />
are filled with an electron of opposite spin (singlet exited state). Relaxation to the ground state<br />
leads to the emission of light of smaller frequency. b) In order to show electroluminescence,<br />
radical ions have to be produced in the polymer by the application of an electric field. When<br />
radical ions of opposite charge combine, so-called excitons (singlet or triplet excited state) are<br />
formed and the decomposition of this neutral excited state (recombination) leads to radiative<br />
emission.<br />
3 OLED device structure<br />
An OLED device consists of one or more semiconducting organic thin films sandwiched between two<br />
electrodes – one of which must be transparent. (Fig. 13).<br />
3.1 Basic PLED<br />
Figure 13: basic (two-layer) OLED structure<br />
The energy level diagram of a typical single layer PLED is shown in Figure 14. The device utilizes<br />
~100 nm of PPV with an ITO anode and calcium cathode [3,14].<br />
Figure 14: PLED device operation (energy diagram)<br />
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