Exciton trapping at heterojunctions in polymer blends

Abstract

Optoelectronic devices made from semiconductor polymers often employ partially phase-separated binary polymer blends with "distributed heterojunctions" in the polymer film, and the migration of bulk excitons towards these heterojunctions crucially influences the device performance. Here, we investigate exciton migration in blend films of two polyfluorene derivatives. Localized exciplex states form in electron-hole capture at the heterojunction between the two polymers and these can be thermally excited to transfer to bulk excitons. Rapid radiative emission from these excitons can then allow efficient light-emitting diode operation. We show here that when these excitons migrate to another heterojunction site within their lifetime they are re-trapped at the interface and again form exciplex states or dissociate completely. We demonstrate that in polymer blend light-emitting diodes this can reduce the exciton population by more than 54% and can strongly influence the emission spectrum. We then analyze exciton re-trapping in detail using time-resolved photoluminescence spectroscopy on blends with different morphologies and find that for nanometer-scale phases exciton emission is completely suppressed. We show that the data agree well with a simple kinetic model which confirms the importance of the blend morphology for the exciton trapping efficiency.

Monte Carlo Simulation of Exciton Bimolecular Annihilation Dynamics in Supramolecular Semiconductor Architectures

“…[12][13][14][15] Furthermore, exciton diffusion away from such heterojunctions is essential after charge recombination in LEDs to prevent retrapping and possible charge separation. 16,17 In this paper, we model exciton dynamics, derived from ultrafast spectroscopic measurements, within isolated semiconductor nanostructures applying Monte Carlo simulations of RET processes. We explore exciton bimolecular signatures as a probe of such dynamics.…”

Fluorescence light emission at 1eV from a conjugated polymer

“…The localization of the excitation upon geometry relaxation is a prerequisite for the manifestation of such environmental effects since a cancellation of attractive and repulsive forces is expected to prevail for the delocalized excitations initially generated upon photoexcitation [19]. However, Coulomb interactions in the attractive regime should also give rise to chains emitting at lower energy compared to solution; this discrepancy can be reconciled with the experimental measurements by conjecturing at this stage that such excitations are actually converted into inter-chain exciplexes that are known to be generally weakly emissive [20] in agreement with the low quantum efficiency recorded. More experiments are in progress to verify such hypotheses.…”

Optical properties of polyimides films treated by nanosecond pulsed electrical discharges in water

“…Recent studies of polymer blends have shown that the Coulomb interaction between the charge distribution of an excited state chain and that of an adjacent neutral chain can lead to a blue shift of the emission in the repulsive regime. 32,33) The collapse of intramolecular hydrogen bonding in blended polymers was found to be responsible for the blue shifts of the fluorescence peak.…”