Influence of polaron doping and concentration dependent FRET on luminescence of PAni–PMMA blends for application in PLEDs
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.