Harnessing Molecular Photon Upconversion with Self-Assembled Multilayers

• Other Authors: Dilbeck, Tristan (author)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Chemistry
• Online Access: http://purl.flvc.org/fsu/fd/2019_Spring_Dilbeck_fsu_0071E_15045

Molecular photon upconversion via triplet−triplet annihilation (TTA-UC) combines two or more low energy photons to generate a higher energy excited state. It is an emerging strategy to potentially increase maximum solar cell efficiencies from 33% to greater than 43%, surpassing the Shockley-Queisser limit. In this dissertation, we introduce self-assembled bilayers on high surface area metal oxide films as a strategy to facilitate TTA-UC. Due to its modular nature, this formation strategy offers unique geometric and spatial control of donor−acceptor interactions at an interface. In Chapter 3, we discuss the use of self-assembled bilayers of acceptor and sensitizer molecules on high surface area metal oxides as a means of facilitating TTA-UC emission and generating an integrated TTA-UC dye-sensitized solar cell. The bilayer films generate photocurrent by two different mechanisms: (1) direct excitation and electron injection from the acceptor molecule and (2) low-energy light absorption by the sensitizer molecule followed by TTA-UC and electron injection from the acceptor upconverted state, as evidenced by intensity dependence and IPCE measurements. We also compare the energy transfer and photocurrent generation efficiency of the bilayer to a heterogeneous system, confirming the superior design of the bilayer structure. In Chapter 4, we explore the hypothesized mechanism for TTA-UC in a bilayer film. Steady-state and time-resolved emission/absorption spectroscopy were used to determine the rate constants of the processes involved. The rate constants indicate that sensitizer to acceptor triplet energy transfer as well as sensitizer and acceptor nonradiative decay rates are the primary efficiency limiting processes for TTA-UC at the interface. This information can help to guide the design of new self-assembled UC films, a critical step toward the long-term goal of generating a practical UC solar cell. The low solar energy conversion efficiency of TTA-UC solar cells can be attributed, at least in part, to the relatively narrow absorption features of the sensitizer molecule. In Chapter 5, we incorporate multiple sensitizers into a TTA-UC DSSC using bilayer and trilayer self-assembly to increase broadband light absorption. The sensitizers' work cooperatively to achieve peak TTA-UC efficiency at sub-solar irradiance (<1 sun or <100 mW cm2). The trilayer device exhibits a high efficiency of 1.2 x10-3%, nearing device relevance, due to the high sensitizer density and energy transfer cascade towards the charge separation interface. In conclusion, we outline improvements that must be made to produce a viable TTA-UC solar cell that can surpass the Shockley-Queisser limit. These improvements include engineering strategies, changing the sensitizer and acceptor, finding more effective redox mediators, understanding the structure of the bilayer film, and more. Efficient TTA-UC and photocurrent generation have the potential to increase the efficiency of existing record solar cells by more than 1%. === A Dissertation submitted to the Department of Chemistry and Biochemistry in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Spring Semester 2019. === April 5, 2019. === Includes bibliographical references. === Kenneth G. Hanson, Professor Directing Dissertation; William S. Oates, University Representative; Geoffrey Strouse, Committee Member; Lei Zhu, Committee Member.