Colloidal quantum dot solids and devices 1

Colloidal quantum dot solids and devices

Our interest in colloidal semiconductor quantum dots (QDs) stems from their size-tunable properties, unique photophysics (e.g., multiple exciton generation), and ability to be self-assembled from solution into functional films for optoelectronics. Our work is vertically integrated, encompassing QD synthesis, film fabrication via self-assembly, ligand and surface chemistry studies, structural characterization, fundamental investigations of charge transport using a variety of experimental approaches (field-effect transistors, Hall effect), and fabrication, characterization and modeling of QD-based devices such as solar cells, photodetectors and transistors. A major goal of the QD sub-group is to fabricate QD solids with sufficient spatial and energetic order to trigger the emergence of electronic mini-bands that offer high carrier mobility and long diffusion length with controlled doping, as is necessary for making high-performance QD optoelectronic devices.

The slide above summarizes some of our ongoing work in this field. The videos below show (left) the conversion of an oleate-capped PbSe QD superlattice into an epitaxially-fused superlattice and (right) an electron tomography reconstruction of a 120 × 38 nm disc-shaped region of a PbSe QD epi-superlattice at 0.65 nm resolution.

photoelectrochemical solar fuels 1

Photoelectrochemical solar fuels

  • photoelectrochemical solar fuels 1

The discovery of efficient, stable and inexpensive photoanode materials for use in tandem photoelectrochemical water splitting devices remains a major challenge to the realization of practical solar-driven production of green hydrogen and liquid hydrocarbons. We carry out in-depth investigations of promising photoanode candidates to establish their fundamental properties (light absorption, charge transport, catalytic activity, stability) and understand how their OER performance depends on composition, morphology, phase purity, nanostructuring, surface chemistry, heterostructuring, the use of surface protective and catalytic layers, and other factors. Members of this sub-group develop refined syntheses of target complex oxide semiconductors in bulk and nanostructured thin-film form and use a host of spectroscopy, microscopy, and (photo)electrochemical techniques to evaluate them.

The slide above summarizes some of our ongoing work in this field.

plasmonics and plasmonic photocatalysis 1

Plasmonics and plasmonic photocatalysis

  • plasmonics and plasmonic photocatalysis 1

The excitation of localized surface plasmons (collective oscillations of conduction electrons) in metal nanostructures produces enhanced local electromagnetic fields, hot carriers and heat that can be used for surface-enhanced Raman spectroscopy (SERS) and photocatalysis. We are exploring novel strategies to deterministically assemble gold and silver nanocrystals into monodisperse colloidal dimers with well-defined hot spots and more complicated, reconfigurable structures with interesting plasmonic properties. We are also studying the photophysics of multi-metallic nanocrystals and utilizing supported nanocrystals as catalysts in benchtop photoreactor investigations of plasmon-driven chemical reactions.

The slide above summarizes some of our ongoing work in this area. The video below show the serial assembly of gold nanocrystals into chiral nanopropellers and nanorings via SEM nanomanipulation on a silicon substrate.

earth abundant thin film absorbers 1

Earth-abundant absorbers

  • earth abundant thin film absorbers 1 1

This slide highlights our work on one particular earth-abundant absorber, iron pyrite. Pyrite solar cells can produce large photocurrent but suffer from very low photovoltage, the origin of which had long remained a mystery. We used systematic transport studies of pyrite single crystals and thin-films made by gas-phase and molecular ink approaches to demonstrate that this material possesses a conductive, hole-rich surface layer (an inversion layer on n-type crystals) that limits the photovoltage to small values by enabling electrons to tunnel through most of the surface potential barrier instead of going over it. Our work suggests that passivation of this conductive surface layer is key to attaining reasonable photovoltage and power conversion efficiency from pyrite photovoltaics. Controlled bulk doping of pyrite is another major challenge, especially for p-type material. Our combination of comparative studies of high-quality single crystals and thin-films with emphasis on detailed transport measurements, modeling, surface modification and doping is powerful and can be extended to understand other promising but underperforming absorber materials.