Models of organic light-emitting diodes

Based upon the recognition that bimolecular charge recombination in random organic systems is in accord with the Langevin formalism, a simple model has been developed to treat the recombination of charge carriers in light-emitting diodes (LEDs). A more elaborate analytic theory will be presented that is able to account for the recombination of majority and minority carriers in bilayer LEDs in which internal energy barriers give rise to charge accumulation. It relates the LED performance to cell parameters, notably the external and internal energy barriers. It can also account for the observation of an electroluminescence overshoot in spin-coated bilayer LEDs upon turning off the drive voltage. Model predictions concerning (i) the dependence of the majority current on the presence of an internal blocking layer, (ii) the dependence of the electroluminescence yield on the thickness of the blocking layer and the electric field, and (iii) the form of the electroluminescence decay when turning off the cell voltage have been verified by experiment. (C) 1997 Elsevier Science S.A.