Temperature and field dependence of hole mobility in poly(9,9-dioctylfluorene)

Abstract

Detailed studies of hole mobility in the electroluminescent conjugated polymer poly($9,{9}^{\ensuremath{'}}$-dioctylfluorene) by the time-of-flight technique are reported. The hole mobility is studied in a number of samples over a wide range of temperatures and electric field strengths. Significant sample-to-sample variations are observed in both hole mobility and the character of hole transport in as-spun films of different thicknesses. Hole mobility is found to improve irreversibly as a result of thermal annealing at elevated temperatures. Analysis of the experimental results is carried out within the Gaussian disorder model (GDM), the polaronic correlated disorder model (PCDM), and the geometry fluctuation model based on torsional interactions (known here as the Los Alamos model). The PCDM is found to be more successful than the GDM in explaining the experimental data. Moreover, the Los Alamos model succeeds in explaining the data when modified to include polaron hopping. Analysis with both PCDM and Los Alamos models leads to the conclusion that the improvement in hole mobility upon annealing is consistent with reduced energetic disorder and reduced polaron binding energy, both of which may result from crystallization of the material. Spectroscopic ellipsometry is used to confirm that crystallization occurs upon annealing.