Near-field electroluminescence probe of polymer light-emitting diodes

We used the luminescence induced in a polymer light-emitting diode (LED) by the biased tip of a scanning tunneling microscope (STM) to study under air the local factors affecting polymer electroluminescence. We have observed the light emission induced by the scanned platinum:iridium tip of an STM in 100 nm thick PVK films doped with a Coumarin dye, spin-coated on a transparent ITO electrode. The light is detected through indium tin oxide (ITO) electrode by photon-counting and photon-maps are acquired simultaneously with STM images, in the Z (i.e. constant-current) mode. Two emission regimes are evidenced: the first emission was observed when increasing tip bias, it occurs through discharges appearing as emission segments along the scan direction in photon maps. At higher voltages a more regular emission regime is observed. These two regimes appear at different locations, that can be correlated to the characteristics of the surface profile measured simultaneously by STM. (C) 1999 Elsevier Science B.V. All rights reserved.