Extrinsic and intrinsic dc photoconductivity in a conjugated polymer

Steady-state photoconduction was studied in typically 100-nm-thick films of a polyphenylenevinylene derivative with ether linkages in the backbone. Samples were prepared in sandwich configuration between indium tin oxide (ITO) and Al electrodes. Forward photocurrents measured upon irradiating through the positively biased ITO electrode decreased by typically three orders of magnitude when a 8-10-nm-thick silicon monoxide (SiO) layer had been deposited on top of the ITO to prevent photoinjection of positive charge carriers. Insertion of a SiO layer can thus be used as a tool to separate extrinsic from intrinsic sources of optical charge-carrier generation. Analysis of the dependences of extrinsic and intrinsic photocurrents on temperature, electric field, and photon energy indicate that they can be described in terms of Onsager's theory of geminate pair dissociation in either one or three dimensions.