Effect of extrinsic traps on thermally stimulated luminescence in molecularly doped polymers

The low-temperature thermally stimulated luminescence (TSL) technique has been applied to study the influence of shallow and deep traps on localized state distribution in charge transport polymer systems. Effect of shallow traps is studied in molecularly doped polymer (MDP), as tri-p-tolyamine (TTA)-doped polystyrene in which a small concentration of di-p-anisyl-p-tolylamine (DAT), acting as a weak hole trap, was added (Et=0.15 eV). It was found that deep trap is created when DAT is doped into poly(N-epoxypropylcarbazole) (PEPC) (Et=0.5 eV) due to the difference in ionization potential of these materials. The results are described in terms of the Gaussian disorder model, which provides reasonable understanding of all trends observed in the TSL. The effect of extrinsic shallow trapping on TSL properties can be reasonably interpreted in terms of the effective energetic disorder and the TSL results are in good agreement with those obtained earlier by charge transport studies. The effect of percolative motion of charge carriers within manifold of trapping states on TSL properties of deep trap-containing polymers was found for the first time.