LUMO Density Research Articles

Charge trapping in polymer diodes

We report studies focusing on the nature of trap states present in single layer ITO/poly(phenylene vinylene)/Al light emitting diodes. At high applied bias the IV characteristics from 11 to 290 K can be successfully modelled by space charge limited current (SCLC) theory with an exponential trap distribution, giving a trap density H t of 4(±2) × 10 17 cm −3, μ p , between 10 −6 and 5 × 10 −8 cm 2 V −1 s −1 and a characteristic energy E t of 0.15 eV at high temperatures. The transient conductance follows 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 found in the SCLC analysis. Due to variations in structure, conformation and environment, the polymer LUMO and HOMO density of states form Gaussian distributions of chain sites. The deep sites in the tail of the distributions are the observed traps for both positive and negative carriers. The same sites dominate the photo- and electroluminescence emission. This implies that the emissive layer in organic LED's should be made as structurally disordered as possible.

Study of the origin of superstructure patterns in the scanning tunneling images of perylene-3,4,9,10-tetracarboxylic-dianhydride on graphite by electronic structure calculations

Partial electron density plots were calculated for various arrangements of perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) molecules on graphite to understand why scanning tunneling microscopy (STM) images of PTCDA on graphite exhibit superstructure contrast variations. In agreement with experiment, the contrast of the partial electron density plot depends strongly on the orientation and position of PTCDA on graphite. This observation originates from the fact that the overlap between the orbitals of the adsorbate and substrate is strongly affected by their relative arrangement. The HOMO or LUMO density of an adsorbate molecule can be inadequate in interpreting STM images of adsorbate molecules.