On February 23, Yueh-Lin (Lynn) Loo, Theodora D. ’78 and William H. Walton III ’74 Professor in Engineering and Professor of Chemical and Biological Engineering delivered an invited lecture, entitled “Small Molecules for Large-Area Applications” in the Frontiers of Chemistry Lecture Series in the Department of Chemistry at Case Western Reserve University (CWRU).  Established in 1941, these distinguished lectures are held on selected occasions during the academic year and are sponsored by local industrial and government laboratories, as well as CWRU.

ABSTRACT

Small Molecules for Large-Area Applications

Yueh-Lin (Lynn) Loo

Director, Andlinger Center for Energy and the Environment

Theodora D. ’78 & William H. Walton III ’74 Professor in Engineering

Department of Chemical and Biological Engineering

Princeton University

Energy use in residential and commercial buildings comprises about 40% of energy demand today and 30% of energy-related carbon emissions in the United States, with heating accounting for nearly twice the energy required for cooling and lighting combined. Increasing building energy efficiency will shave peak demands for electricity. In my talk, I will highlight our development of solar-powered electrochromic windows that can be integrated into buildings to reduce electricity consumption.

The solar-powered electrochromic window comprises a polyelectrochromic conducting polymer that is optically transparent in its reduced state and dark blue in its oxidized state. Integration with a semitransparent organic solar cell provides the necessary power to switch between its transparent and colored states. Unique to this approach is our use of a single-junction organic solar cell. By designing and using materials that absorb exclusively in the ultra-violet and near-visible as the photoactive layers in our solar cells, the resulting devices exhibit open-circuit voltages that are unprecedented for single-junction organic devices (> 1.4 V). As such, we are able to drive switching of the electrochromic window without the need to construct solar cells with complex tandem architectures. With this pairing, we demonstrate the intelligent management of the solar spectrum, with near-UV photons powering the regulation of visible and near-infrared photons for natural lighting and heating purposes. Importantly, the active layers of our solar cells are pinhole- and defect-free. Coupled with inherently low resistive power losses, the photocurrents are scalable with the footprint of solar cells while our devices retain high fill factors. Whereas device optimization of typical solar cells to increase photovoltage has almost always come at the expense of decreasing photocurrents, the scalability of photocurrents in our devices has enabled us to break this paradigm. We can tune device photovoltage per application needs through judicious selection of the donor/acceptor materials pair whose optoelectronic properties can be tuned via molecular design, and access the necessary photocurrents by fabricating arbitrarily large devices.