Anomalous temperature-dependent behaviors of electroluminescence phenomena in disordered ITO/PEDOT/PF/Ca/Al polymer light-emitting diodes

In this study we systematically investigated the anomalous Berthelot-type optical property dependence of the electroluminescence (EL) that occurs in polymer light-emitting diode (PLED) structures. Devices were operated at 0.1 and 0.5 mA over a temperature range from 300 to 20 K. Initially, the emission intensity increased as the temperature decreased, until a maximum was reached at the turning-point temperature Tt. After this, the intensity decreased with further decrease of temperature down to 20 K. A smaller reduction in intensity was exhibited by the device with 0.5 mA, a larger one exhibited by the device with 0.1 mA. The strong electron–electron scattering arising from high excitation, high microcrystalline randomization and high hopping frequency led to the corresponding dominance of the Berthelot-type process by a large Tt. It can be seen in the Arrhenius plot of the integrated EL intensity that a smaller activation energy can be expected to facilitate the redistribution of the thermal carriers, overcoming the energy barrier and the large degradation of luminescence. This results in not only better than carrier confinement, but also enhances the quantum efficiency of PLED structures requiring lower excitation.