A UO research team collaborating with several industry partners has identified a potential path toward improving the quality of nanomaterials for use in next-generation solar cells.
In their study, detailed in the Journal of Physical Chemistry Letters, researchers investigated electronic states of lead sulfide nanocrystals. By using a specially designed scanning tunneling microscope, researchers created atomic-scale maps of the density of states in individual nanocrystals. This allowed them to pinpoint the energies and localization of charge traps associated with defects in the nanocrystal surface structure that are detrimental to electron propagation.
To increase light-harvesting efficiency of solar cells beyond silicon's limit of about 29 percent, manufacturers have used layers of chemically synthesized semiconductor nanocrystals. Properties of quantum dots are manipulated by controlling the synthetic process and surface chemical structure.
This process creates imperfections at the surface-forming trap states that limit device performance. Until recently, improvements in production quality have relied on feedback provided by traditional characterization techniques that probe average properties of large numbers of quantum dots.
"We want to use these materials in real devices, but they are not yet optimized," said co-author Christian F. Gervasi, a UO doctoral student.
The microscope was designed in the lab of co-author George V. Nazin, a professor in the UO Department of Chemistry and Biochemistry. Its use was described in a previous paper in the same journal, in which Nazin's lab members were able to visualize the internal structures of electronic waves trapped by external electrostatic charges in carbon nanotubes.