Non-Conjugated Polymers for Organic Photovoltaics: Physical and Optoelectronic Properties of Poly(perylene diimides)

Most polymers employed in organic photovoltaic cells have π-conjugated backbones. Here we investigate the physical and optoelectronic properties of three nonconjugated polymers based on perylene diimides. The optical properties, exciton diffusion length, band edge positions, conductivity, and carrier mobility of these polymers are described. They are electron-conducting and highly photostable. The film structure and optoelectronic properties vary with deposition conditions and annealing procedures. One of the polymers has an unusually long exciton diffusion length of 22 nm. The free electron density resulting from n-type charged defects is ∼3 ×10¹⁵ cm ^-3 in one polymer and is even lower in the others. Simple bilayer solar cells are limited by their series resistance suggesting that these semiconductor films would benefit from doping.