The role of quantum mechanical phenomena such as polaron-exciton quenching interaction and concentration-dependent FRET in determining the luminescence efficiency of PAni-PMMA polymer blends has been investigated. PAni samples prepared in different environments using different acids and bases show different absorbance and emission profiles indicating a direct relation between generated polarons in PAni by acid-base doping-dedoping and photoluminescence spectra of PAni. The observed low luminescence in PAni has been modeled using exciton quenching by polarons through charge transfer. The investigation also reveals that the effect of exciton quenching by polarons becomes pronounced when the polaron concentration in PAni reaches a density of ∼1017-1018 polarons cm-3. To overcome the low emission efficiency of PAni, poly(methyl methacrylate) (PMMA) has been blended with PAni. The blending of donor PMMA (D) with acceptor PAni (A) gives rise to another quantum phenomenon - donor PMMA concentration dependent FRET between PAni (A) and PMMA (D). It is experimentally observed from the photoluminescence measurements of blends that at high donor PMMA concentration above a critical value in the PAni-PMMA polymer blend the emission profile of blends drops sharply. Donor concentration dependent FRET is a contradictory observation with respect to standard concentration independent FRET theory due to competition between inter-layer donor-acceptor and donor-donor intra-layer energy transfer within the donor layer. At high donor concentration intra-donor interaction gradually overtakes inter-layer donor-acceptor FRET which modifies the lifetime of the donor. The modification decreases the quantum yield of the donor and hence emission efficiency of blends above a critical concentration of PMMA by reducing inter donor-acceptor FRET. Thus, polaron exciton quenching and concentration dependent FRET are two dominant physical phenomena controlling luminescence in PAni-PMMA polymer blends. Therefore, optimization of luminescence of PAni-PMMA should be achieved by tuning the factors like reduction of spectral overlap between polarons and excitons in PAni, the density of PAni, diffusion of excitons in blends, and intra donor FRET within the PMMA layer before consideration of the blend being used as an emissive layer in PLEDs.
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.
Excitation wavelength-dependent visible emissions from ZnO nanostructures demonstrate that defect states are insufficient to explain their optical properties.
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