Bipyridiniums, also known as viologens, are particularly useful in photoinduced electron transfer (PET) processes and are therefore of interest in photovoltaic applications that typically rely on electron-rich donors like polythiophene (PTh). However, the PET mechanism and relaxation dynamics between interfacing PTh and viologen-based thin films has not been well studied in terms of film thickness of the acceptor layer. In my presentation, I describe the fabrication of a novel, bilayered thin film composite using spin-coating and curing polyviologen (PV)-based micron-sized films of variable thicknesses (0.5–11.3 μm) on top of a PTh layer. The electron-transfer mechanism and relaxation dynamics from the PTh sublayer into the upper PV film were investigated using femtosecond transient absorption (fTA) spectroscopy and electrochemistry to understand how charge-transfer/relaxation lifetimes could be extended using thicker PV acceptor films. In collaboration with the Loomis group at WashU, fTA experiments were performed under inert N2 conditions as well as in ambient O2. Contact angle measurements using H2O and MeI were performed to measure changes in surface free energy that would aid the assessment of efficiency relating to light penetration, photoexcitation, electron mobility, and relaxation within the bilayered thin films. Insights gained from this work will support the development of future devices that employ viologen-based materials as an alternative electron-acceptor that is both easily processable and scalable.

Photoinduced electron transfer and changes in surface free energy in polythiophene-polyviologen bilayered thin films

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