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).

Afterglow Effects as a Tool to Screen Emissive Nongeminate Charge Recombination Processes in Organic Photovoltaic Composites

“…At this stage the mechanism responsible for the generation of free charges in the PS:PDI system remains obscure. 50 We note however that at high photoexcitation intensities autoionization effects caused by the annihilation of PDI excimer species is very likely to produce fully separated charges; these can then recombine for producing delayed excimer luminescence.…”

“…As such the PS:PDI system corresponds to the extreme case where the layer order supports the occurrence of free charge recombination from the very early time after photoexcitation. At this stage the mechanism responsible for the generation of free charges in the PS:PDI system remains obscure . We note however that at high photoexcitation intensities autoionization effects caused by the annihilation of PDI excimer species is very likely to produce fully separated charges; these can then recombine for producing delayed excimer luminescence.…”

Ion–electron recombination on silica gel surfaces: experiment and modelling

“…on a timescale of a few milliseconds), which has been observed in rigid media to take place by a mechanism involving electron tunnelling 54 or a combination of tunnelling and ion diffusion. 55 The deviation of the observed kinetics from geminate recombination at much longer times (minutes to hours) has been interpreted by us 3 as arising from motion of either the ion or electron resulting in a loss of the geminate identity of the radical ion-electron pairs, and the ion-electron recombination problem then becomes one of randomly distributed charge carriers. On the basis of previous studies in rigid media, 54,55 on the timescales involved here electron tunnelling would not be expected to be rate determining.…”

Dynamics of Radical Pair Processes in Bulk Polymers

“…Specifically excluded from this treatment will be processes of biradicals (i.e., single molecular species containing two radical centers separated by a saturated group), 2 radicals that are not spin 1 / 2 , and radical pairs generated from the polymer itself. The dynamics of charge recombination 3 and electron or hole migration 4 processes in bulk polymers are not treated here as well.…”

“…95 In the presence of H-donating substrates, the triplet excited state of B is able to abstract an H-atom directly (Equation 13.17) or via a sequential electron/H þ transfer mechanism (Equation 13.18) to produce triplet radical pairs, 3 …”