Excitons and biexcitons in poly(para-phenylenevinylene)

The Hückel model fails to describe optical absorption in PPV and its derivatives, and explicit inclusion of moderate electron-electron interactions is essential for a proper theoretical description. The lowest optical state is an exciton with a binding energy estimated as 0.90 ± 0.15 eV, in excellent agreement with experiment. Such a large exciton binding energy strongly suggests stable biexcitons. The concept of biexcitons in π-conjugated polymers is introduced, and picosecond photoinduced absorption is explained as excited state absorption from the exciton to the biexciton.