Unsnarling Excitation, Relaxation and Scattering Dynamics in Multi-Chiral Distributions of Carbon Nanotubes
Ames, Jessica
2017
Abstract
This research begins the process of studying a polymer/carbon nanotube prototype solar cell. Inclusion of nanotubes in a photoactive polymer simultaneously overcomes the limited diffusion length of charge carriers and vastly extends the spectral sensitivity. Further improvements in these hybrid designs will now require a comprehensive understanding of the diverse contributions from, and limitations of, each component of these new solar cells. Research described in this dissertation focuses on the light harvesting component, a heterogenous distribution of individual carbon nanotubes in a photoactive polymer, with the goal of quantifying the contributions from each of the individual nanotubes to the harvested photocurrent. Absorption of light by photoactive polymers and nanotubes generates strongly bound excitons. To transform these short lived excitons into a harvestable photocurrent they must be efficiently dissociated and transported through the device, with minimal losses, before coupling to external leads. Designing an efficient solar cell then is aided by understanding every step in the evolution of charge carriers from excitation to harvested photocurrent. This research focuses on simultaneously measuring the excited states of a carbon nanotube distribution in comparison with the literature. The heterogenous distribution of carbon nanotubes provides a series of parallel detectors collectively contributing extended spectral sensitivity to the cell. Selectively observing carrier evolution from each nanotube species is accomplished through non-degenerate pump-probe measurements. The broadband measurements in spectrally congested neighborhoods of nanotubes limits selectivity needed to follow a single species. The challenge of simultaneously measuring the response of multiple nanotube species is addressed in two ways. First, a set of rate equations is assumed representing our current understanding of nanotube dynamics as a discrete set of energy levels for each relevant nanotube species. These dynamical equations are fit to multiple data sets for a range of excitation intensities. Second, additional relaxation measurements are taken for an atypical strong probe. The strong probe provides access to the dependence of nanotube dynamics on their excited state transitions. With this additional information the contributions from various nanotubes can be distinguished and quantified. Across all data sets we found, within 20% error, lowest energy bright exciton lifetimes of ~175fs, exciton-exciton annihilation rates of ~0.19/ps and that ~80% of second excited state excitons decayed non-radiatively into the first bright exciton. Fitting results, however, showed wide, unexpected variation in one bright state lifetime and the coupling between one of the second excitons to the first. This hot probe technique has, therefore, uncovered additional physics not fully described by a discrete-level rate equation model. Variations in rate equation solutions are partly attributed to coherent interaction of both pump and probe fields with the nanotubes. To test the coherence hypothesis an alternative density matrix approach is proposed that accounts for strong pump and probe as well as decay and dephasing. Results of this model are highly sensitive to both pump and probe intensities and can only generally reproduce the expected population behavior and differential absorption signatures seen in the data. The coherent model, however, predicts non-linear pump-probe interaction just beyond the measured power ranges providing for testable verification in the future. Both models therefore lead to the conclusion that the existing discrete-level models for carbon nanotube dynamics are incomplete, and a full microscopic theory of optical interaction will be required.Subjects
Excited state dynamics of carbon nanotubes
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