<title>Charge trapping in polymer electroluminescent devices</title>

Abstract

ABSTRACT We report studies focusing on the nature of trap states present in single layer ITO! polymer/metal devices ofpoly(p-phenylene vinylene) and its soluble derivative poly(2,5-dialkoxy-p-phenylene vinylene). In the highapplied bias regime the IV characteristics from 1 1 to 290K can be successfully modelled by space charge limited current (SCLC) theory with an exponential trap distribution, giving a trap density of between 8 and 4x10'7cm3 and a characteristic energy Et of 0.15eV. 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 SCLCanalysis. Due to variations in structure, conformation and enviroment, the polymer LUMO and HOMO densityof states form a Gaussian distribution of chain energy sites. The sites involved in carrier transport are thosetowards the centre of the distribution. The deep sites in the tail of the distribution in the carrier energy gap are