Stability of the J–V characteristics of (BEHP-PPV)-co-(MEH-PPV) based light-emitting diodes

This paper reports main characteristics of copolymer (BEHP-PPV)-co-(MEH-PPV) based PLEDs with PEDOT:PSS as anode and Mg as cathode. Polymeric light-emitting diodes, PLEDs, were prepared by spin-coating of the polymeric layers. Barrier at the anode is around 0.2 eV corresponding to a copolymer HOMO of around 5.3 eV. The energy gap is around 2.3 eV. Photoemission peak is at 547 nm with a refractive index of 1.44. The onset of visible electroluminescence was around J = 100 A/m2. The study of the stability of J–V curves showed that for current densities between 620 and 3100 A/m2, Joule heating effect associated causes segregation of the PSS from the bulk PEDOT:PSS that increases the anode–copolymer barrier height. For current densities higher than 1500 A/m2 and for longer operation times, a delamination process in the cathode metal is observed associated to the formation of bubble-like structures that appear as dark spots, leading to final failure of the LED.