Dispersive electron transport in an electroluminescent polyfluorene copolymer measured by the current integration time-of-flight method

Here, we report results of both traditional current mode and current integration mode time-of-flight (TOF) measurements on the electroluminescent polyfluorene copolymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (BT). Current mode TOF shows a strong but dispersive electron transport signal. The mobility derived from the current integration mode transit time $(tQ)$ increases with decreasing film thickness as expected for dispersive transport. The fastest carriers in the photogenerated carrier packet are estimated to have mobilities of order $10−3 cm2/V s$ at applied fields of 0.5 MV/cm. Holes are heavily trapped close to the interface at which they are photogenerated. The TOF signals decay with repeated measurement and $tQ$ remains constant with applied field. The transport properties of BT are thus in complete contrast to those of other polyfluorenes which show high-mobility nondispersive hole transport but weak and highly dispersive electron transport.