SPECTROSCOPIC INVESTIGATION OF PHOTOINDUCED CHARGE SEPARATION AND RECOMBINATION IN SOLID POLYMERS

Charge generation in polymer films following photoexcitation of charge -transfer complexes and UV two-photon ionization of aromatic dopants w as studied by transient absorption spectroscopy. Charge separation fro m photoinduced contact ion pairs is due to the hole migration away fro m the geminate radical anions during the lifetime of the exciplex, whi ch occurs by a hopping mechanism. Hole trapping at dimeric sites and s ubsequent charge recombination give rise to delayed exterplex emission , in resonant two-photon photoionization experiments, radical cations of aromatic molecules and excess electrons were produced as the primar y charged species. The thermalization length of electrons ejected from perylene by 337-nm photons was measured as 36 Angstrom in solid polys tyrene. Within the first 1 ns, reactions of electrons with polymer mat rixes compete with the geminate electron-cation recombination, which l eads to electron trapping and charge separation, with trapped ions sta ble up to milliseconds of time. Among the polymers studied, polystyren e shows the lowest reactivity and therefore the lowest yield of charge separation, psi(PS) = 2.4%, whereas poly(vinylbenzyl chloride) scaven ges nearly all the excess electrons and exhibits the highest charge se paration yield, psi(PVBC) = 1 at 210 K. Subsequent recombination of ch arge carriers trapped at different depths in solid polymer matrixes is diffusion limited over a wide dynamic range. The ion neutralization k inetics gradually evolves from geminate in nature to a homogeneous sec ond-order reaction. The diffusivities of charge carriers were measured as 7.5 x 10(-10) cm(2)/s for Cl- in poly(vinylbenzyl chloride), 2.2 x 10(-10) cm(2)/s for CO2-. in poly(benzyl methacrylate), and 7.0 x 10( -12) cm(2)/s for CO2-. in poly(methyl methacrylate).