UCLA

Organic photovoltaics (OPVs) represent a promising alternative to silicon based photovoltaics because of they are lower cost and easier to process then current technology. A typical OPV consists of a semiconducting polymer that acts as the electron donor and primary photoabsorber combined with an electron acceptor, often a C60 fullerene derivative. Due to the low exciton diffusion length in organic blends, the two components must be blended on a fine enough length scale to enable efficient electron transfer. However, the components still need to be separated enough to maintain high conductivity in the pure domains for the extraction of the carriers. It can be difficult to create such a demanding structure and optimal morphology reproducibly. This work focuses on controlling the semiconducting polymer through self-assembly as a way to tune the overall nanoscale architecture of a blended system.

In the first part of this dissertation, the effect of changing the crystallinity and orientation of semiconducting polymers will be examined as a way to tune the nanoscale morphology of a polymer/fullerene blend. The resulting device performance will be analyzed to gain further understanding of the optimal morphology required to maximize efficiency. Three polymer systems are used to explore how small changes in the crystallinity, regioregularity, and chain conformation can dramatically alter the resulting OPV device performance. Additionally, the changes in polymer morphology with two fullerene incorporation techniques, blend casting and sequential processing, will be compared. Since polymer morphology and characteristics can be determined, it will be shown that the processing method can be selected to match each intrinsic polymer, which are often more difficult to control.

In the second part, self-assembly will be used as a method to precisely direct the polymer morphology in films and solution. It will be shown that through the use of an inorganic host, individual chains can be straightened to dramatically increase the hole mobility in the polymer backbone. The ability to straighten polymer chains will be further expanded upon by using solution self-assembly to create a 3D conductive polymer network. Upon the addition of fullerene to this network, an energy cascade can be created to enable efficient charge separation through the formation of stable polarons.