MODELS OF ORGANIC LIGHT-EMITTING-DIODES

Based upon the recognition that bimolecular charge recombination in ra ndom organic systems is in accord with the Langevin formalism, a simpl e model has been developed to treat the recombination of charge carrie rs in light-emitting diodes (LEDs). A more elaborate analytic theory w ill be presented that is able to account for the recombination of majo rity and minority carriers in bilayer LEDs in which internal energy ba rriers give rise to charge accumulation. It relates the LED performanc e to cell parameters, notably the external and internal energy barrier s. It can also account for the observation of an electroluminescence o vershoot in spin-coated bilayer LEDs upon turning off the drive voltag e. Model predictions concerning (i) the dependence of the majority cur rent on the presence of an internal blocking layer, (ii) the dependenc e of the electroluminescence yield on the thickness of the blocking la yer and the electric field, and (iii) the form of the electroluminesce nce decay when turning off the cell voltage have been verified by expe riment. (C) 1997 Elsevier Science S.A.