CARRIER TUNNELING AND DEVICE CHARACTERISTICS IN POLYMER LIGHT-EMITTING-DIODES

In this paper it is demonstrated that the characteristics of light-emi tting diodes based upon MEH-PPV [more fully known as thoxy,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 b and offset between the polymer and the electrodes. It is shown that ma nipulating these offsets can control the useful operating voltage of t he device as well as its efficiency. A model is developed that clearly explains the device characteristics of a wide range of diodes based u pon MEH-PPV. The turn-on voltage for an ideal device is shown to be eq ual to the band gap, i.e., 2.1 eV for MEH-PPV, and is slightly lower a t 1.8 eV for an indium-tin oxide/MEH-PPV/Ca device. If there is a sign ificant difference in the barrier height, the smaller of the two barri ers controls the I-V characteristics, while the larger barrier determi nes 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 injec tion is only 0.1 eV. This combination of electrodes is close to optima l for MEH-PPV, but lowering the hole barrier can still lead to a doubl ing of the device efficiency.