Thermally activated motion of cold excitons in conjugated polymers

Thermally activated migration of localized cold excitons were investigated in archetypal conjugated polymers; trans-polyacetylene, polythiophene, and poly(p-phenylene vinylene). Room temperature thermal energy has been found to provide sufficient energy to avail substantial mobility for excitons. A simple undistorted but displaced harmonic oscillator model has been assumed to predict the activation energy for exciton hopping between identical sites in perfectly planar conjugated chains, promoted by vibrational modes with Bu symmetry. The activation energies of exciton hopping has been found to increase in the order of trans-polyacetylene (6.4 meV), polythiophene (29.5 meV), and poly(p-phenylene vinylene) (147.4 meV). A rough Monte-Carlo simulation based on incoherent hopping mechanism reveals excitons can displace in an average of 2.17 nm in polythiophene whereas those distances are much smaller in poly(p-phenylene vinylene) (0.24 nm) and trans-polyacetylene (0.37 nm). The simulation results partially explain why polythiophene based polymers are better suited for organic photovoltaic applications whereas poly(p-phenylene vinylene) are more appropriate for organic light emitting diode applications.