Exciton Diffusion in Nanocrystal Solids

This dissertation work is focused on exploring the unique optical and electronic properties of several semiconductor nanostructures and devices based on them. The present research demonstrates the near-field confinement of light achieved through the use of small-diameter Au nanoparticles embedded into a PbS nanocrystal solid. Using this strategy, we developed plasmonic solar cells that can harness the emission of Au nanoparticles by transferring the plasmon energy to band gap transitions of PbS semiconductor nanocrystals. The contribution of Au near-field emission toward the charge carrier generation was successfully proved through the observation of an enhanced short circuit current and improved power conversion efficiency of mixed (Au, PbS) solar cells, compare to PbS-only devices. Moreover, unique behavior of semiconductor nanocrystals makes them to be promising candidates not only for photovoltaics but for light-emitting applications as well. In light-emitting devices NC solids are designed to have large interparticle gaps that minimize exciton diffusion to dissociative sites. This strategy reduces electrical coupling between nanoparticles in a film, making the injection of charges inefficient. We demonstrated that bright luminescence from nanocrystal solids can be achieved without compromising their electrical conductivity. Our research showed that solids featuring low absorption-emission spectral overlap exhibit slower exciton diffusion to recombination centers, promoting longer exciton lifetimes. As a result, enhanced emission is achieved despite a strong electronic coupling. The inverse correlation between film luminescence and absorption-emission spectral overlap was verified by the comparison of CdSe/CdS and ZnSe/CdS solids and further confirmed in two control systems (ZnTe/CdSe and Mn2+-doped ZnCdSe/ZnS). Another challenging task in the development of quantum dot based solids lies in the studying the motion of neutral excitons. The nature of the exciton dissociation mechanism as well as exciton diffusion trajectories in nanocrystal solids remain poorly understood. We developed an experimental technique for mapping the motion of excitons in semiconductor nanocrystal films. This was accomplished by doping PbS nanocrystal solids with metal nanoparticles that force the exciton dissociation. By correlating the metal-metal interparticle distance in the film with corresponding changes in the emission lifetime, we could obtain important transport characteristics, including the exciton diffusion length and the exciton diffusivity