Download Polymer Solar Cells, Deconstructed Yueh-Lin (Lynn) Loo Chemical Engineering Department, Princeton University

Polymer Solar Cells, Deconstructed
Yueh-Lin (Lynn) Loo
Chemical Engineering Department, Princeton University
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.