Conduction and trapping in electroluminescent polymer devices

The current-voltage (JV) characteristics of ITO/polymer film/Al or Au devices of poly(phenylene vinylene) (PPV) and a dialkoxy PPV copolymer can be fitted at high applied bias to a power law of the form J = KVm where m increases with decreasing temperature, log(K) is proportional to m, and K is proportional to d-αm where d is the film thickness and α is a constant. α 2 and 1 for the Al and Au cathode devices respectively. Different single carrier space charge limited conduction (SCLC) theories, involving either an exponential trap distribution or a hopping transport field and temperature dependent mobility, are used to try and explain this behaviour. Both models are in good agreement with the general experimental results, but can also be criticized on a number of specific issues. Mixed SCLC models and the effect of dispersive transport are also explored. It is concluded that carrier mobility and trap measurements are required to distinguish between these models. To this end, initial trap measurements of ITO/PPV/Al devices using deep level transient spectroscopy (DLTS) are reported. Very deep positive carrier traps with emptying times >4 minutes have been detected. The non-exponential DLTS transients have been successfully modelled on an isoelectronic trap level emptying to a Gaussian distribution of transport states, with a trap depth and density of 0.8 eV and 4×1016 cm-3 respectively.