Photophysics of Photovoltaic Polymers

Polymers that are comprised of extended conjugated chromophore sequences show considerable potential as the active materials for organic solar cells. Obtaining a detailed understanding of the light induced energy and electron transport processes occurring in these materials is essential for the development of optimised molecular structures and conditions for their use. We report here photophysical studies of a series of variable conjugation length polymers based on p-phenylene vinylene (figure 1) at both the ensemble and single chain levels. The objective is to investigate the role of molecular structure, conjugation length and molecular environment on photoinduced energy and charge transfer in these materials. Solutions of the polymers PT-3-PT6 in chloroform exhibit high fluorescence quantum yields (φ fl > 0.5) compared to the fully conjugated polymer Alt-Co-MEH-PPV (φ fl = 0.32). In films the emission of all polymers is quenched in the presence of the fullerene derivative PCBM and transient spectroscopy confirms that this quenching can be attributed to photo- induced charge separation. 1 While the transient kinetics are similar for all polymers, the fully conjugated polymer exhibits the highest yields of polaron formation. Single polymer chain fluorescence trajectories of Alt-Co-MEH-PPV dispersed in poly(methyl methacrylate) films exhibit 'blinking' on both short (sub-millisecond) and long (10's of milliseconds to seconds) time-scales. The longer time-scale intermittencies are attributed to the reversible formation of non-fluorescent polymer polarons indicating that photo-induced charge separation processes