Abstract

Organic photovoltaics (OPVs) have continued to attract attention over the past two decades, promising solution processable and aesthetically pleasing solar energy harvesting devices. The power conversion efficiency of OPV has improved rapidly owing to the development of novel conjugated polymers and functional molecules. Recently, donor-acceptor push-pull type materials have been investigated ubiquitously for OPV applications due to their high extinction coefficients in the near-infrared region of the solar spectrum. At RIT, a series of donor-acceptor-donor type squaraine (SQ) materials have been systematically synthesized and investigated for their potential in bulk heterojunction (BHJ) OPV devices. This dissertation presents both experimental and theoretical work associated with these squaraines.

In the first part, the dependence of solar cell performance on BHJ morphology is discussed, with the emphasis on how SQ aggregation dominates the morphological behavior of the BHJ upon spin coating and post annealing treatments. SQ aggregates in the BHJ films represents crystalline domains which should benefit the charge transport toward the electrodes. At the same time, SQ aggregation induces phase separation and leads to formation of large SQ or PCBM domains. Domain size is a critical factor determining the solar cell efficiency as the exciton diffusion length in SQ films is believed to be small. The extent of phase separation can be controlled through varying SQ:PCBM weight ratio; a more homogeneously mixed BHJ morphology is obtained when PCBM content is high, leading to an improved solar cell efficiency. Film crystallinity and SQ aggregation is disrupted at high PCBM weight ratio but can be recovered via thermal annealing. Controlling the trade-off between crystallinity and phase separation of the BHJ is identified as critical for device optimization of SQ-based solar cells. In addition, different SQ molecules have been comparatively investigated to reveal the correlation between the molecular structure and the aggregation properties. In this way, this dissertation connects SQ structure to aggregation properties, then to BHJ morphology and finally to OPV performance.

The second half of this dissertation focuses on using an essential state model to fully understand the intermolecular interactions within the SQ aggregates. The model has been constructed based on three main charge resonant structures associated with the zwitterionic nature of the SQ conjugation backbone. Molecular aggregates of the SQ chromophores were built based on the experimentally obtained single crystal structures. Specifically, we found that, in as-cast BHJ films, the SQ-SQ interaction is dominated by Coulombic coupling (CC) while in annealed BHJ films the intermolecular charge transfer (ICT) strongly influences the electronic properties. The type of aggregation is shown to greatly influence the solar cell performance. Specifically, CC-aggregates formed in the as cast films yield better solar cell efficiency as compared to ICT-coupled aggregates (which is of higher ordered and more crystalline).

Finally, the sub-picosecond transient absorption spectroscopy results reveal how the excitons in the CC-aggregates are highly mobile, which rationalizes the high solar cell efficiency obtained from such aggregates.

Library of Congress Subject Headings

Photovoltaic cells--Materials; Organic compounds--Electric properties

Publication Date

6-2018

Document Type

Dissertation

Student Type

Graduate

Degree Name

Microsystems Engineering (Ph.D.)

Department, Program, or Center

Microsystems Engineering (KGCOE)

Advisor

Christopher Collison

Advisor/Committee Member

Jeremy A. Cody

Advisor/Committee Member

Jiandi Wan

Campus

RIT – Main Campus

Plan Codes

MCSE-PHD

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