Temperature dependence of the space–charge distribution in injection limited conjugated polymer structures

Temperature dependent pyroelectric measurements have been carried out on high barrier devices of conjugated polymers. At room temperature we find significant buildup of space charge at the metal/polymer interface as well as in the bulk despite the very low device current. At lower temperature (180 K) the charge profile is “frozen in” and shows nearly no bias dependence, while above room temperature the space charge seems to become more mobile and leads to broadening of the charge profile. By applying an injection model that accounts for charge backflow via interface recombination we demonstrate that the high amount of space charge detected in our films indicates the existence of deep traps in the surface region that can effectively collect charge from the electrode. In the bulk of the film we find space charge of different sign, which is assumed to be due to separation in the field of charge from intrinsic defect states. These bulk charges are also immobile at low temperatures, indicating that they correspond to electrons or holes that are transported by hopping or tunneling between traps.