20.07.2013 Views

Organic Light Emitting Diodes

Organic Light Emitting Diodes

Organic Light Emitting Diodes

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

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 />

10

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!