Photophysics behind luminescence phenomenon in conducting conjugate polymer blends for application as an emissive layer in PLEDs

Abstract

Luminescence quenching by polarons is an important loss mechanism in polymer light-emitting diodes (PLEDs). Steady-state and time-resolved photoluminescence spectroscopy of polyaniline (PAni) thin films with varying polaron doping has helped us to realize polaron density-dependent photoluminescence quenching mechanisms inside the thin films. A sharp reduction in photoluminescence emission spectra has been observed at doping densities between 1017 and 1019 cm-3. This doping concentration-dependent photoluminescence phenomenon in PAni is modeled quantitatively using quenching of excitons by polarons through long-range Förster resonance energy transfer (FRET) and short-range charge transfer (CT) mechanisms. The results match well with the experimental findings that demonstrate that both models need to be considered to explain the mechanisms of luminescence quenching. FRET and CT phenomena inside an emissive layer of PLEDs have been demonstrated to play a pivotal role in the quantum efficiency roll-off phenomenon at high current density using experimentally obtained and theoretically calculated kinetic quenching parameters. To get rid of low luminescence in PAni, it has been blended with poly(methyl) methacrylate (PMMA) that enhances its luminescence manifold. The blending of PMMA leads to the introduction of a new photophysical phenomenon - donor PMMA concentration-dependent FRET contrary to original FRET theory proposed by Förster. Concentration-dependent FRET leads to a sharp drop in luminescence from the polymer blend after reaching a critical concentration of PMMA. Therefore, the present study explores the reason behind low luminescence in conducting polymers and demonstrates ways to mitigate it along with providing an account of the photo-physics of the conducting polymer as an emissive layer in PLEDs.