Do the defects make it work? Defect engineering in pi-conjugated polymers and their solar cells

The charged defect density in common π-conjugated polymers such as poly(3-hexylthiophene), P3HT, is around 10¹⁸ cm^-3. Despite, or perhaps because of, this huge defect density, bulk heterojunction solar cells made from these polymers and a C60 derviative such as PCBM exhibit some of the highest efficiencies (∼5%) yet obtained in solid state organic photovoltaic cells. We discuss defects in molecular organic semiconductors and in π-conjugated polymers. These defects can be grouped in two categories, covalent and noncovalent. Somewhat analogous to treating amorphous silicon with hydrogen, we introduce chemical methods to modify the density and charge of the covalent defects in P3HT by treating it with electrophiles such as dimethyl sulfate and nucleophiles such as sodium methoxide. The effects of these treatments on the electrical and photovoltaic properties and stability of organic PV cells is discussed in terms of the change in the number and chemical properties of the defects. Finally, we address the question of whether the efficiency of OPV cells requires the presence of these defects which function as adventitious p-type dopants. Their presence relieves the resistance limitations usually encountered in cleaner organic semiconductors and can create built-in electric fields at junctions. These considerations lead toward the design of a new generation of OPV materials that replaces the covalent defects with purposely added dopants.