Photoconductivity in Frans-Poly(Phenylacetylene) and its Charge-Transfer Complexes

Steady-state and pulsed photoconductivities were explored in 4-8-µm-thick films of poly-(phenylacetylene) (PPA) and PPA doped with some inorganic and organic electron-accepting compounds, particularly iodine and DDQ (2,3-dichloro-5,6-dicyano-p-benzoquinone). Data are reported on the dependence of the photocurrent density (J_ph) on the exciting wavelength (λ), the dopant concentration (Y), the light intensity (I), and the electric field (E) in frans-PPA. J_ph(λ) shows two regions of response, one at the π-π_⋆ transition at λ <525 nm, the other in the red and near-IR in doped, as well as undoped, PPA. The latter response is associated with photodetrapping from shallow electron traps in the undoped polymer and with charge carrier generation in the charge-transfer band in the doped polymer. The field dependence is superlinear at E ~ 10⁴-10⁵ V/cm and J_ph/ph ∝ I^ax with a ≤ 0.5. Carrier lifetimes and mobilities are reported for some conditions. Charge transport occurs by a bandlike mechanism that is modulated by shallow electron traps in the undoped polymer and by trapping in the charge-transfer complex in the doped polymer. The control of photoconductivity by the carrier recombination kinetics is an important characteristic of this semiconducting photoconductor.