Spectroscopic Investigation of Photoinduced Charge Separation and Recombination in Solid Polymers

Abstract

Charge generation in polymer films following photoexcitation of charge-transfer complexes and UV two-photon ionization of aromatic dopants was studied by transient absorption spectroscopy. Charge separation from photoinduced contact ion pairs is due to the hole migration away from the geminate radical anions during the lifetime of the exciplex, which occurs by a hopping mechanism. Hole trapping at dimeric sites and subsequent 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 primary charged species. The thermalization length of electrons ejected from perylene by 337-nm photons was measured as 36 Å in solid polystyrene. Within the first 1 ns, reactions of electrons with polymer matrixes compete with the geminate electron−cation recombination, which leads to electron trapping and charge separation, with trapped ions stable up to milliseconds of time. Among the polymers studied, polystyrene shows the lowest reactivity and therefore the lowest yield of charge separation, ψPS = 2.4%, whereas poly(vinylbenzyl chloride) scavenges nearly all the excess electrons and exhibits the highest charge separation yield, ψPVBC = 1 at 210 K. Subsequent recombination of charge carriers trapped at different depths in solid polymer matrixes is diffusion limited over a wide dynamic range. The ion neutralization kinetics gradually evolves from geminate in nature to a homogeneous second-order reaction. The diffusivities of charge carriers were measured as 7.5 × 10-10 cm2/s for Cl- in poly(vinylbenzyl chloride), 2.2 × 10-10 cm2/s for CO2-• in poly(benzyl methacrylate), and 7.0 × 10-12 cm2/s for CO2-• in poly(methyl methacrylate).