Temperature-dependent electron and hole transport in disordered semiconducting polymers: Analysis of energetic disorder

We have used space-charge limited current measurements to study the mobility of holes and electrons in two fluorene-based copolymers for temperatures from 100 to 300 K. Interpreting the results using the standard analytical model produced an Arrhenius-type temperature dependence for a limited temperature range only and mobility was found to be apparently dependent on the thickness of the polymer film. To improve on this, we have interpreted our data using a numerical model that takes into account the effects of the carrier concentration and energetic disorder on transport. This accounted for the thickness dependence and gave a more consistent temperature dependence across the full range of temperatures, giving support to the extended Gaussian disorder model for transport in disordered polymers. Furthermore, we find that the same model adequately describes both electron and hole transport without the need to explicitly include a distribution of electron traps. Room-temperature mobilities were found to be in the region of 4 x 10(-8) and 2 x 10(-8) cm(2) V-1 s(-1) in the limit of zero field and zero carrier density with disorders of 110+/-10 and 100+/-10 meV for polymers poly{9,9-dioctylfluorene-co-bis[N,N'-(4-butylphenyl)]bis(N, N'-phenyl-1,4-phenylene)diamine} and poly(9,9-dioctylfluorene-co-benzothiadiazole), respectively.