Spectroscopic characterisation of polymer photodiodes

We have studied the influence of alkoxy side-chain length (methoxy to hexadecyloxy) on photodiodes made from a series of poly(p-phenylene-co-2, 5-dialkoxy phenylene vinylene)s [PPV-co-DAOPV]. The current-voltage curves of unilluminated devices indicate that the conductivity of the polymer drops as the side-chain length increases. We interpret this as a drop in the hole mobility in the polymer, due to an increase in the average separation of transport-active sites. The quantum efficiency of the short-circuit photocurrent under 0.25 mW/cm2 illumination at 2.48 eV drops by an order of magnitude from about 0.3% for the polymer with the shortest side chains to about 0.03% for the polymer with the longest side chains. We consider that this is primarily a reduction in the efficiency of exciton dissociation. We have also studied poly(p-phenylene-2, 3′ bis(3, 2′ diphenyl) quinoxaline-7-7′-diyl) [PPQ], which is of interest as an electron transport material. We find that it is indeed primarily an electron transporting material, but that the mobility of both carrier species is low. The quantum efficiency of the short-circuit photocurrent is very small - only 1.7×10-5 % under 10 mW illumination at 420 nm. Doping the polymer with tetra(1-dimethylamino-phenyl)-ethynylene [TDPE] increases the quantum efficiency of the short-circuit current to 4×10-4 %, probably due to enhanced exciton dissociation. Even with the TDPE dopant, the limiting factor for the photocurrent appears to be charge transport to the contacts.