When 9:30 AM - 11:30 AM Mar 01, 2012
Where 1005 EECS
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Squaraine Donor Based Organic Solar Cells

Guodan Wei
Thesis Defense

Stephen R. Forrest, advisor


Organic photovoltaics (OPV) have been considered as a potential low-cost solar energy conversion solution for a clean, safe, affordable and sustainable energy future. They have attracted tremendous academic and industrial interests in recent years, which stems from the deposition flexibility of lightweight organic thin films on a variety of very-low-cost substrates such as glass, plastic or metal foils. Currently, there are three main ongoing avenues to improve the power conversion efficiency of OPVs: the development of new organic materials, improved process control and novel device architecture design. In this thesis, through molecular design with chemical modification of functional organic molecules, a family of new highly absorptive solution processable squaraine (SQ) materials have been systematically synthesized and explored to improve the sunlight harvesting and charge transport. The spin-cast SQ donors are then coated with fullerene acceptors to form a unique nanocrystalline heterojunction (NcHJ) OPV device. This combination of a novel and efficient family of SQ donors, a unique NcHJ device architecture and optimized fabrication processes leads to high efficiency solar cells. For example, solar cells with efficiencies of ~5.7 % and a fill factor ~0.74 are achieved.

We find a correlation between solar cell fill factor with the SQ thin film density, providing support for the molecular design concept that planar end groups result in close intermolecular stacking, and hence improved charge transport and exciton diffusion.  Finally, thermal annealing of the films results in the formation of nanocrystalline morphologies that lead to further improvements in device performance. These results suggest that SQs have promise for achieving further increases in solar cell efficiency in a more ordered nanocrystalline OPV, as will provide an ideal material system and device architecture for low-cost large-area OPVs using roll-to-roll printing technique.