Charge trapping in polymer electroluminescent devices

We report studies focusing on the nature of trap states present in single layer ITO/polymer/metal devices of poly(p- phenylene vinylene) and its soluble derivative poly(2,5- dialkoxy-p-phenylene vinylene). In the high applied bias regime the IV characteristics from 11 to 290K can be successfully modeled by space charge limited current (SCLC) theory with an exponential trap distribution, giving a trap density of between 1018 and 4 X 1017 cm-3 and a characteristic energy Et of 0.15 eV. Measured conductance transients of PPV are non-exponential and follow a power-law relationship with time whose decay rate decreases with decreasing temperature. This can be directly related to the emptying of the trap distribution deduced from the SCLC analysis. Due to variations in structure, conformation and environment, the polymer LUMO and HOMO density of states form a Gaussian distribution of chain energy sites. The sites involved in carrier transport are those towards the center of the distribution. The deep sites in the tail of the distribution in the carrier energy gap are the observed traps for both positive and negative carriers. The same deep sites dominate the photo- and electroluminescence emission spectra. The model implies that the emissive material in organic light emitting diodes should be made as structurally disordered as possible.