Squaraine Donor Based Organic Solar Cells.

Abstract

There are three main ongoing avenues to improve the power conversion efficiency of organic photovoltaics (OPV): 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. The microcrystal growth of SQ donors have been characterized by XRD, AFM and TEM