Carrier tunneling and device characteristics in polymer light-emitting diodes

In this paper it is demonstrated that the characteristics of light-emitting diodes based upon MEH-PPV more fully known as poly(2-methoxy,5-(2′-ethyl-hexoxy)-1,4-phenylene- vinylene) are determined by tunneling of both the holes and the electrons through interface barriers caused by the band offset between the polymer and the electrodes. It is shown that manipulating these offsets can control the useful operating voltage of the device as well as its efficiency. A model is developed that clearly explains the device characteristics of a wide range of diodes based upon MEH-PPV. The turn-on voltage for an ideal device is shown to be equal to the band gap, i.e., 2.1 eV for MEH-PPV, and is slightly lower at 1.8 eV for an indium-tin oxide/MEH-PPV/Ca device. If there is a significant difference in the barrier height, the smaller of the two barriers controls the I–V characteristics, while the larger barrier determines the device efficiency. In indium-tin-oxide/MEH-PPV/Ca devices, the barrier to hole injection is 0.2 eV and the barrier to electron injection is only 0.1 eV. This combination of electrodes is close to optimal for MEH-PPV, but lowering the hole barrier can still lead to a doubling of the device efficiency.