Improvement of hole mobility in PTB7 polymer film with a low temperature electric field treatment

Abstract

Improved charge carrier mobility of polymer semiconductors is often a critical factor in achieving superior performance for organic electronic devices. A novel low-temperature electric-field (LTEF) treatment on poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) films, which helps in increasing hole mobility in these films, is presented herein. The zero-field hole mobility at room temperature of the polymer film in a “hole-only” device configuration is found to increase by more than seven times from 1.1 × 10−4 to 7.8 × 10−4 cm2 V−1 s−1 following the application of an electric field strength of 2 × 105 V cm−1 across the films at low temperature (80 K). On the other hand, a room temperature electric field treatment of the devices does not appear to have any effect on the hole mobility in the PTB7 films. LTEF treatment is shown to help decrease the average barrier energy for charge carriers between adjacent molecular energy levels in the films from 302 to 266 meV, energy disorder from 53.2 to 49.3 meV, and position disorder from 2.1 to 1.7 in PTB7 films. The hole mobility improvement is attributed to the field induced enhanced order in those films by dipolar alignment, which is facilitated by higher polarization at low temperatures.