Research Summary
The Asbury group seeks to understand and control molecular species that determine electrical properties of emerging low temperature solution processed photovoltaic (PV) materials based on conjugated molecules and nanocrystalline systems. The group developsultrafast infrared electro-optical spectroscopies to probe the electrical properties of emerging PV materials while simultaneously revealing the nature of molecular species that give rise to those properties through the vibrational modes of the molecules. The fundamental spectroscopic studies are deeply nested in on-going collaborations with device developers such that functional electronic materials are examined enabling direct correlation of device performance, molecular properties, and fundamental charge transport, trapping, and recombination dynamics. Fundamental insights about how molecular species influence electrical properties are used to inform on-going work in next-generation PV research aimed at disruptive technologies to overcome current cost and performance limitations of silicon and thin film PV.
Many emerging electronic materials targeting high throughput low temperature processing are molecular in nature. Examples include organic semiconductors for organic solar cells in which the active electronic materials consist of conjugated molecules or polymers. Inorganic material systems involving molecular moieties include colloidal quantum dot or nanocrystalline PV systems in which molecular ligands are used to passivate dangling bonds and facilitate strong electronic coupling between nanocrystals. The utilization of molecular species either as active materials or as precursors causes the structure and characteristics of molecules to figure prominently in the electronic properties of the materials. Electrical techniques have long existed to characterize with high sensitivity charge carrier mobilities, defect state densities and their energetic distributions in materials. However, the techniques do not provided detailed information about the molecular species that give rise to these properties. The electro-optical techniques we develop uniquely fill this void. We focus on several classes of materials including organic and colloidal quantum dot photovoltaics as well as novel materials for flexible electronics and display technologies.
Many of the unique capabilities the Asbury group has demonstrated take advantage of the sensitivity of vibrational modes of molecules to their local molecular environments. This is why we take the counter-cultural position that vibrational spectroscopy provides unique opportunities to study electronic processes in materials. For example, the ultrafast solvatochromism assisted vibrational spectroscopy (SAVS) technique we demonstrated uses this sensitivity to directly measure electron transfer and charge transfer state dissociation dynamics at buried interfaces in organic PV materials. The sensitivity also allows us to identify molecular species on surfaces of inorganic nanocrystals that are related to the transport states in colloidal quantum dot PV materials. Th sensitivity of vibrational modes to their environments derives largely from the influence that local electrostatic potentials have on the frequencies of molecular vibrations. We also take advantage of this influence to probe local electrostatic potentials in the active sites of proteins.
For more details, you can read about:
Free carrier formation in organic photovoltaics
and in colloidal quantum dot solids
Sensitivity of vibrational frequencies to their local
environments