A numerical study of operational characteristics of organic light-emittingdiodes

We propose a simple model for the device characteristics of organic single layer molecular or polymeric light-emitting diodes. The model is based on P oisson's equation and the conservation law of charges. A bimolecular recomb ination process is incorporated phenomenologically, and boundary conditions are given by carrier injection functions. Equations fur the electric field and carrier concentrations are formulated for single-carrier and double-ca rrier injection cases. The equations are solved for different parameters in cluding carrier mobility and energy barrier height at the electrodes. Curre nt-voltage characteristics, relative quantum efficiency, and emission distr ibution are obtained. The results show that (1) at least one barrier height should be low for device operation at low voltage, (2) high mobility is es sential for devices with high brightness, (3) low electron mobility confine s the emission region near the cathode and should be avoided to prevent ele ctrode quenching, We also discuss the effects from persistent charged traps . The calculation was compared with experimental results obtained by single layer conjugated polymer and dye-doped polymer devices. Fairly good agreem ent between experiments and calculations on the current-voltage characteris tics and relative quantum efficiency were obtained using reasonable physica l parameters. (C) 1998 American Institute of Physics. [S0021-8979(98)01221- 3].