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

Abstract

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.