We report electroabsorption studies of electric fields in organic light emitting diodes made form substituted poly(para phenylene vinylene) derivatives and solar cells made form zinc phthalocyanine (ZnPc) and perylenetetracarboxylic diimide (PTCDI). The electric field in LEDs is not proportional to the applied bias due to the development of an internal electric field during operation that opposes the applied bias. This counter field is weaker for devices measured in vacuum than for those measured in an ambient atmosphere and is no longer apparent for devices that were prepared and tested under an inert atmosphere. We also observed that the built-in potential increased with operating time. The combination of these two processes leads to an increase in the turn-on voltage of organic LEDs with increasing operating time. We have detected an electric field at the electrode/organic LEDs with increasing operating time. We have detected an electric field at the electrode/organic interface of organic solar cells which is insensitive to the external DC bias. The interface field has a different spectral signature from that of the bulk of the two layers and is attributed to charged transfer-induced dipoles. Rectifying behavior due to the formation of a pn junction under illumination is observed in bilayer solar cells, but not single layer devices made from ZnPc or PTCDI.
We have investigated the hole injection characteristics from indium tin oxide (ITO) into 4,4',4''-tris[N,-(3- methylphenyl)-N-phenylamino] triphenylamine (m-MTDATA) and have measured the hole carrier drift mobility of this compound in single-layer ITO/m-MTDATA/Au structures. We have found that ITO is able to provide trap-free space-charge- limited currents over a wide range of film thicknesses and have established unambiguously that the ITO/m-MTDATA is an ideal ohmic contact at high electric fields. The drift mobility of the m-MTDATA molecular glass was found to be electric field dependent and a negative field dependence was detected at fields lower than 1 by 10<SUP>5</SUP> V/cm. Our observations clarify the role of m-MTDATA as a voltage- lowering hole-injecting buffer layer in organic light- emitting diodes.
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 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/cm<SUP>2</SUP> 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), 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 X 10<SUP>-5</SUP>% 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 X 10<SUP>-4</SUP>%, 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.