Role of acceptor concentration-dependent FRET behind luminescence tuning in polymer capped ZnO for application in hybrid LEDs

Abstract

Surface modification of the ZnO nanoparticles using polymer can be an important way to tune their optoelectronic properties. On capping the ZnO nanoparticles (NPs) using the polymer polyvinylpyrrolidone (PVP), a synchronous change in the photoluminescence emission has been observed from different defect states with different formation energies, which raises doubts about the decade-old proposed theory of passivation of defect states upon surface capping. The doubt has been confirmed by X-ray photoelectron spectroscopy (XPS) of the samples. Blue shifting of the band edge absorption on polymer capping has been attributed to a change in exciton dynamics due to the tuning of band alignment on surface modification. Polymer concentration-dependent tunability for emission from defect states has been explored using two new photophysical process – acceptor (PVP)-concentration-dependent Förster resonance energy transfer (FRET), and generation of superoxide (O2-) charge-transfer states. Surface capping has been found to improve spectral purity at the cost of a decrease in quantum efficiency. Therefore, to design a very bright hybrid light-emitting layer using polymer capped ZnO, we must consider the emission and absorption spectral overlap between the ZnO nanoparticles and the polymer, FRET, exciton diffusion and balling efficiency of the polymer.