Semiconductor nanostructures are currently an active area of research, especially in the field of photovoltaics as they will play a major role in next generation solar devices that break the current theoretical limit for light-to-current conversion. For instance, the efficiency of the nanostructure-based solar cells can be increased due to carrier multiplication, or multiple exciton generation (MEG) process, where absorption of a single energetic photon results in the generation of several charge carriers. In order to design nanostructures with the desired properties, a detailed theoretical approach for studying photoexcited state processes is necessary. The approach developed in this work is based on many-body perturbation theory (MBPT) and the Boltzmann transport equation (BE) in combination with density functional theory (DFT) in order to compute quantum efficiency (QE). Conclusions about QE are made after studying all the major relaxation channels in a photoexcited system, such as exciton-to-biexciton decays, biexciton recombination and phonon-mediated exciton relaxation. In all calculations, excitonic effects have been included by solving the Bethe-Salpeter equation (BSE). Then, by including excitons in the MBPT calculations, the exciton-to- biexciton rates R1→2 as well as the biexciton-to-exciton rates R2→1 are computed by taking into account the singlet fission (SF) process. The methods developed here have been applied to various semiconductor nanostructures such as pristine chiral (6,2), (6,5) and (10,5) and functionalized (6,2) SWCNTs. We predict efficient MEG in the (6,2) and (6,5) SWCNTs within the solar spectrum range starting at the 2Eg energy threshold and with QE reaching ~ 1:6 at about 3Eg, where Eg is the electronic gap. Also, methods for MEG rates calculations have been improved by taking into account exciton-exciton interactions in the intermediate biexciton state, where results show a small (~ 40 meV) red-shift in the biexciton density of states. Finally, the MEG-BE technique is applied in studying charge transfer. Charge transfer has been studied in a doped silicon quantum dot (QD) - functionalized SWCNT system where it was found that an initial excitation localized on either the QD or CNT evolves into a transient CT state. / National Science Foundation (NSF CHE-1413614)
| Identifer | oai:union.ndltd.org:ndsu.edu/oai:library.ndsu.edu:10365/29401 |
| Date | January 2019 |
| Creators | Mihaylov, Deyan |
| Publisher | North Dakota State University |
| Source Sets | North Dakota State University |
| Detected Language | English |
| Type | text/dissertation, movingimage/video |
| Format | application/pdf, video/mp4 |
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