In addition to its annual lectures, ChBE hosts a weekly seminar throughout the year with invited lecturers who are prominent in their fields. Unless otherwise noted, all seminars are held on Wednesdays in the Molecular Science and Engineering Building ("M" Building) in G011 (Cherry Logan Emerson Lecture Theater) at 4:00 p.m. Refreshments are served at 3:30 p.m. in the Emerson-Lewis Reception Salon.

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Polymer Solar Cell, Deconstructed 

We have successfully constructed polymer solar cells having bulk-heterojunction as well as bilayer structures by soft-contact lamination. This process entails fabricating and processing functional components individually; these separate components are then brought together in a final step to complete the devices. Physical contact occurs non-destructively at room temperature and ambient pressures so this process is particularly suitable for manipulating chemically and mechanically fragile organics. 

In the construction of inverted polymer solar cells having bulk-heterojunction structures, this process involves a substrate that supports the bottom electrode and the active layer of the polymer solar cells as well as an elastomeric substrate that supports the top electrodes. Lamination of the top substrate against the bottom substrate establishes electrical contact. Given the modularity of this process, the top electrodes can be readily removed after post-deposition processing and device testing so the active layer can be characterized. This interface is otherwise inaccessible in devices that are fabricated by conventional bottom-up approaches. Grazing-incidence x-ray diffraction carried out on the once-buried interface of inverted polymer solar cells of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) indicates that PCBM crystallinity, as opposed to P3HT crystallinity, increases significantly when the devices are annealed at higher temperatures. Quantification of two-dimensional x-ray patterns indicate PCBM readily adopts the triclinic crystal structure, with the (302) planes of the crystals preferentially oriented parallel to the substrate. This enhancement in PCBM crystallinity and its preferential orientation correlates positively with the measured short circuit current densities during device testing. We also find soft-contact lamination to be extremely robust; replacing the existing gold top electrodes with fresh gold electrodes results in quantitatively similar device characteristics.

In the same vein, we have demonstrated the successful construction of bilayer polymer solar cells by laminating thin films of polymer electron donors against those of electron acceptors.  The fabrication of bilayer polymer solar cells by conventional bottom-up approaches is challenging as it necessitates one organic semiconductor to be deposited directly on top of another. As such, organics having comparable solubilities cannot be employed in a single cell because the solution deposition of one species on top of the other will induce solvent damage of the underlying organic semiconductor. This lamination approach has effectively allowed us to isolate the deposition and processing of the electron donor and acceptor; individually deposited and processed organic layers are only brought together in the final step to construct bilayer polymer solar cells. With this approach, we have been able to independently control the properties of the individual constituents. Systematic examination of such devices has enabled the decoupling of morphological transformations that take place in the individual layers; we have thus been able to map out structure-function relationships of the electron donor and electron acceptor using this platform.