Photoconduction in thin films of a ladder-type poly-para-phenylene

Stationary photoconductivity has been measured on typically 100 nm thick films of a methyl-substituted ladder-type poly-para-phenylene sandwiched between indium–tin oxide (ITO) and aluminium (Al) electrodes. Three contributions to photocarrier production have been identified: (1) hole injection from the ITO and the Al-anode via dissociation of singlet excitons; (2) intrinsic carrier production via relaxed S1 excitons; and (3) intrinsic photogeneration via non-relaxed electronic states excited by photons with an energy of 1.1 eV above the S1←S0 0–0 absorption band. The intrinsic photoionization yield increases strongly with electric field but is virtually independent of temperature, at variance with the prediction of the conventional Onsager theory of geminate pair dissociation.