Photochemical Sciences Ph.D. Dissertations

Sensitizer Molecule Engineering: The Development Of Novel Ru(II) Polypyridyl Complexes for Application in Dye Sensitized Solar Cells

Date of Award

2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Chemistry

First Advisor

Felix N. Castellano

Second Advisor

William K. Ingle (Committee Member)

Third Advisor

Michael Y. Ogawa

Fourth Advisor

Thomas H. Kinstle

Abstract

Three series of hydrophobic ruthenium(II) polypyridyl sensitizers for DSSC application with the molecular structure Ru(N^N)(Hdcbpy)X2, [Ru(N^N^N)(H2dcbpy)X]+/2+ and [Ru(N^N)2(H2dcbpy)]2+ were designed and synthesized in this dissertation. Their absorption maxima and molar extinction coefficients were tuned with the variation of the coordinated polypyridyl ligands. Fundamental photophysical and electrochemical studies were carried out by using a battery of available techniques including absorption, luminescence spectroscopy, lifetime, and cyclic and differential pulse voltammetry to clarify the effect of ligand electronic structure on photophysical properties and redox properties of relevant spectroscopic orbitals of these complexes. Initial photovoltaic performance of their associated solar cells, including the incident photon to current efficiency (IPCE), the short circuit current density (Isc), and the open circuit voltage (Voc), were evaluated upon 514.5 nm illumination (7.3 W/m2). Density functional theory (DFT) calculations were carried out for three particular sensitizers to explore the relationship between the photovoltaic performance and the sensitizers’ molecular structure. From the combination of the theoretical and experimental approaches, the “antenna” in the HOMO, such as NCS- or CN-, is critical to the design of sensitizers for efficient photovoltaic performance in DSSC application. Regardless of their strong absorption in the visible region of the spectrum, the Ru(II)/pyrene chromophores yield low single wavelength conversion efficiency due to postulated competing intramolecular processes such as triplet energy transfer to pyrene.

Among the selected promising sensitizers, the synthetically facile heteroleptic ruthenium(II) sensitizer (NBu4)[Ru(4,7-dpp)(Hdcbpy)(NCS)2], coded as YS5-b, produces broad, high extinction coefficient MLCT bands spanning the visible spectrum. In operational liquid junction-based DSSCs under simulated global AM 1.5 sunlight (100 mW/cm2), this sensitizer routinely outperforms all “N719” cells measured in parallel, producing an optimum efficiency of 7.21% with a maximum IPCE value 73% at 540 nm.

Furthermore, this dissertation attempts to demonstrate the plausibility of applying scalable, economical, and “green” microwave-assisted chemistry to ruthenium(II) sensitizer synthesis, yielding complexes with high purity which can be prepared in minutes with minimal or no purification required. The efficient and rapid synthetic methodologies under mild microwave irradiation operating at atmospheric pressure were developed for the heteroleptic ruthenium(II) sensitizers or synthons for DSSC applications. These methods remarkably shorten the reaction time and lower the reaction temperature of all procedures investigated. Additionally, the simplified purification procedures and several multi-step reactions proceeding flawlessly in a single pot were also realized. The combined results suggest that microwave-assisted chemistry is indeed a valuable tool as far as ruthenium(II) coordination chemistry is concerned and can likely be applied in the combinatorial pursuit of new dyes bearing sensitive functionalities.

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