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

Charge-based deep level transient spectroscopy has been used to study the d efect states that exist within poly(p-phenylene vinylene) (PPV), a semicond ucting polymer with a band gap of about 2.4 eV. The technique allows the de termination of activation energies, capture cross sections, and trap concen trations. In some circumstances, it is also possible to distinguish between minority and majority carrier traps. The structures investigated here cons isted of indium-tin-oxide (ITO)/PPV/MgAg light emitting diode (LED) devices . Two types of trapping centers were found. The first type has activation e nergies in the range 0.49-0.53 eV and capture cross sections of the order o f 10(-16)-10(-11) cm(2). It shows a Poole-Frenkel, field assisted-emission process. This level has been identified as a bulk acceptor-like majority ca rrier (i.e., hole) trap. The second type has activation energies in the ran ge 0.40-0.42 eV and capture cross sections of the order of 10(-19) cm(2). T his level has been identified as a minority carrier (i.e., electron) trap. This second trap type is therefore expected to limit minority carrier injec tion into the PPV layer within the LED, and hence reduce electroluminescenc e under forward bias conditions. (C) 2001 American Institute of Physics.