Determination of traps in poly(p-phenylene vinylene) light emitting diodes by charge-based deep level transient spectroscopy

Charge-based deep level transient spectroscopy has been used to study the defect states that exist within poly(p-phenylene vinylene) (PPV), a semiconducting polymer with a band gap of about 2.4 eV. The technique allows the determination of activation energies, capture cross sections, and trap concentrations. In some circumstances, it is also possible to distinguish between minority and majority carrier traps. The structures investigated here consisted of indium–tin–oxide (ITO)/PPV/MgAg light emitting diode (LED) devices. Two types of trapping centers were found. The first type has activation energies in the range 0.49–0.53 eV and capture cross sections on the order of 10−16–10−18 cm2. It shows a Poole–Frenkel, field assisted–emission process. This level has been identified as a bulk acceptor-like majority carrier (i.e., hole) trap. The second type has activation energies in the range 0.40–0.42 eV and capture cross sections on the order of 10−19 cm2. This level has been identified as a minority carrier (i.e., electron) trap. This second trap type is therefore expected to limit minority carrier injection into the PPV layer within the LED, and hence reduce electroluminescence under forward bias conditions.