Wong

Project 1:  Transient Absorption Spectroscopy during Nanocrystal Growth and Decay

Organo-metal halide perovskite nanocrystals show great potential for high-efficiency solid-state lighting devices since they have tunable luminescence spectra and exhibit high quantum yields. These nanocrystals are typically colloidal, with the surface of the luminescent core being capped by organic ligands. If part of the nanocrystal is not capped by a ligand and is instead exposed to the surrounding environment, it can become the location of a so-called ‘surface trap’. An electron or hole can get trapped at this location and not be able to cause photoluminescence. This decreases the brightness of the nanocrystal. These traps may also increase the rate of ions migrating within the nanocrystal, changing color of its luminescence. These two effects both limit the potential of these nanocrystals for lighting applications.

In this project, the REU student will learn to synthesize these nanocrystals and participate in time-resolved laser spectroscopy measurements to determine the impact of surface states on the luminescent efficiency and luminescence spectrum. In particular, the student will work closely with a graduate student to perform spectroscopic measurements of the dynamics of excited states while the synthesis is occuring. These are time-sensitive measurements that require a concerted team effort to execute properly. The REU student will get a chance to align and perform measurements with the femtosecond laser in the research lab.

 

Project 2:  Transient Absorption Spectroscopy during Organic Film Formation

Semiconducting organic molecules can form the active layer in flexible solar cells, and with further improvements in the chemistry and materials science, these materials solar cells have the potential to be efficient and affordable. The active material in an organic solar cell is typically a mixture of electron donors and electron acceptors. An electron in the material is promoted to an excited electronic state upon the absorption of a photon, creating an exciton. The exciton undergoes energy transfer to get to an interface between the electron donor and acceptor, where the exciton can separate to yield an excited electron in the acceptor and a hole in the donor. These two carriers can then migrate to electrodes where they can be extracted. The efficiency of the photovoltaic depends on the efficiency of each step. Films of these organic molecules are typically thermally annealed to improve charge transport rates, but this processing can also have an impact on exciton transport. In this project, the REU student will learn to cast organic films and develop a protocol for thermally annealing. The student will learn how to use homebuilt absorption and fluorescence spectrometers to characterize their films, and use an ultrafast laser system to measure energy transfer during thermal annealing.