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

The active material in an organic photovoltaic is typically a mixture of electon donors and electron acceptors. An electron in the material is promoted to an excited electronic state upon the absorption of a photon. That electon will be transferred to an electron acceptor, leaving a hole in the electron 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. These films are most economically formed by casting them from solutions. As the solvent vaporizes, the donor and acceptor molecules come closer together and can aggregate into different structures. These different aggregate structures have different amounts of intermolecular coupling, which can dramatically affect the charge transfer efficiency in unpredictable ways.

In this project, the REU student will investigate how the efficiency of charge separation at the donor/acceptor interface changes while the donor and acceptor molecules are aggregating to form a film. The REU student will learn basic film formation techniques; learn to build instruments that can perform absorption and fluorescence measurements in situ, during film formation; and learn to use a femtosecond laser to perform transient absorption measurements to determine the rate of charge transfer during the process of molecular aggregation.