Photovoltaic and optoelectronic materials are often assembled from nanoscale building blocks, such as small organic molecules, quantum dots, or polymers. Different methods can be used to put these building blocks together, but one of the most common and cost-effective methods is deposition from a solution. As solvent evaporates, the individual building blocks get closer together, start to interact, and end up in particular physical arrangements. As components of a system couple together, these physical arrangements can result in disorder and defects, and the group of particles can exhibit collective phenomena that alter the behavior of excitons and carriers in unexpected ways.

Research in our lab seeks to adapt time-resolved exciton spectroscopies to the measurement of nanoscale building blocks during their self-assembly. We will measure electronic structure and exciton dynamics in situ and in real-time as irreversible processes occur, such as crystallization, self-assembly, and chemical bond formation. By measuring and comparing how exciton behavior changes during self-assembly using various solution deposition techniques, we develop strategies to control self-assembly to create materials with designer excitonic properties.


In situ transient absorption spectroscopy

We have developed a single-shot transient absorption spectrometer that allows us to measure exciton dynamics in situ during materials formation and other non-equilibrium processes. Transient absorption uses an ultrafast laser pulse to ‘pump’ the sample, creating excited states, then a second laser pulse after a controlled delay time to ‘probe’ the sample. By changing the delay time, we can measure the dynamics of the excited states. We use tilted beams to spatially encode the time delay in our sample, allowing us to measure the dynamics in a single shot, dramatically decreasing the time needed to complete a measurement. This allows us to measure systems that are changing in time, like aggregating organic molecules, or crystallizing perovskites. Our technique can provide insight into the complex processes involved in materials formation, and will show us how we can steer materials to have particular excited state dynamics by changing environmental conditions while the material is being made. We are continually developing and improving the design of our instruments and building new reaction chambers and film deposition stages so we can measure materials formation in our laser lab, and provide valuable feedback for rational materials design.


Dynamic dynamics during molecular aggregation into thin films

Molecular aggregation during the formation of a thin film can impact the suitability of a material for particular applications. Excited state dynamics are rarely measured during the film formation process, so it is not known whether photophysics smoothly evolves from those of monomers to those of aggregates, or if intermediate states with unique optoelectronic properties play a role. We are measuring these evolving excited state dynamics during the aggregation of small organic molecules using our single-shot transient absorption spectrometer, and correlating the measured dynamics to in situ absorption and fluorescence measurements. By determining how the electronic structure and dynamics change during the film formation process, we gain insight into how we can modify our film formation conditions to kinetically trap aggregates with particular desired photophysics.


Simulations of molecular aggregation

Electronic structure can change dramatically upon self-assembly from monomers into a thin film of molecular aggregates. Absorption, fluorescence, and transient absorption measurements performed in situ, during thin film formation, contain a wealth of information on how molecular aggregates form and how they impact photophysics. We are simulating these evolving spectra using a site-based Hamiltonian, and fitting in situ absorption and fluorescence spectra to linear combinations of n-mers to provide insight into the evolving composition of a material during molecular aggregation. We are also pursuing Monte Carlo simulations of the process of molecular aggregation. The absorption and fluorescence spectra of a system of coarse-grained molecules are simulated as they undergo a biased random walk. By correlating simulated in situ spectra to experimental spectra, we can provide molecular-level insight into the process of thin film formation.


Evolving photophysics during the synthesis of organic-inorganic hybrid perovskites

Organic-inorganic hybrid perovskites are a class of emerging materials with fascinating electronic properties and the potential for wide-ranging, large scale applications in photovoltaics, solid state lighting, and optoelectronics. We aim to understand how the excited state dynamics of these materials change during their formation and if synthetic conditions can be varied to controllably change the electronic structure for a desired application. We use in situ transient absorption spectroscopy to study the photogeneration of excitons and free charge carriers in these materials and observe these excited species immediately following excitation. Understanding these processes will provide new strategies for controlling the photophysical behavior of these promising materials. Currently, we are investigating the effect of different synthetic techniques on the formation of methylammonium lead halide perovskites using a homebuilt transient absorption spectrometer and other in situ techniques.


Intriguing nanohoop photophysics

Cycloparaphenylene “nanohoops” are the shortest possible armchair carbon nanotube. Their fluorescence spectra exhibit a counter-intuitive size-dependence, with larger nanohoops emitting fluorescence at shorter wavelengths than smaller nanohoops. We are developing a deeper understanding of these unique samples and their intriguing photophysics using transient absorption spectroscopy. In collaboration with the Jasti group, we seek to understand the excited state dynamics of these hoops in solution and as they crystallize into aggregates.