Date of Award

Spring 1-1-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Garry Rumbles

Second Advisor

Justin Johnson

Third Advisor

Obadiah Reid

Fourth Advisor

Sean Shaheen

Fifth Advisor

Oana Luca

Abstract

The boundary between molecular behavior and supramolecular behavior is not well-defined. A detailed understanding of molecular structure is necessary to understand and design new solid-state materials, but it is not sufficient. Solid-state materials display emergent properties not easily described by their individual molecular components. This work seeks to understand which solid-state properties directly follow from single-molecule predictions, and which properties arise from intermolecular interactions. Both types of properties can be leveraged for optoelectronic applications once the distinction between the two is made.

In the first part of this work, we explore the role of intermolecular coupling in both charge and exciton dynamics. We address a long-standing photophysical mystery: the mechanism of free charge generation in low-dielectric solid-state environments. Charge transfer in such environments should result in a tightly bound exciton from a coulombic standpoint. Instead, the result is often a large population of separated electron-hole pairs. Though many parameters modulate the dynamics of free charge carriers, this work identifies intermolecular coupling as the crucial parameter required for their creation. We extend this idea to exciton dynamics in singlet fission (SF), which has the potential to produce two electron-hole pairs per photon instead of one. However, instead of using intermolecular coupling to induce free charge generation, we use this feature to induce triplet exciton separation. Since SF triplets are born in pairs, this separation may prove crucial to ensuring eventual independent triplet dissociation.

In the second part of this work, we investigate molecular properties through the lens of driving force. This is informed by a study of SF chromophores on oxide surfaces, where we observe rapid singlet dissociation. This is an effective loss process in an SF solar cell, which relies on the dissociation of two triplet excitons. We show that this process can be circumvented in organic thin films. Even when charge transfer from triplet excitons is slow, singlet state dissociation can be mitigated through the presence of an “inverted” regime for charge transfer. When combined with insights on molecular coupling, this work provides a new set of guidelines for the design of next-generation organic optoelectronics.

Available for download on Wednesday, November 13, 2019

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