Hole transport in blue and white emitting polymers

Abstract

Hole transport in a blue emitting polyspirobifluorene polymer and in a white emitting polymer consisting of a polyspirobifluorene backbone and two dyes (green and red) was studied. The hole mobility was measured using the time-of-flight method as a function of the electric field and temperature in the range 105 - 106 V/cm and 285-335 K, respectively. The observed temperature and electric field dependence of the hole mobility was analyzed in the framework of the Bässler disorder model. Also, steady-state current-voltage characteristics were measured over a wide range of electric fields and temperatures and the hole mobility was determined. Our measurements have shown that the hole mobility in the white emitting polymer is the same as in the blue emitting polymer. The performed disorder model analysis gives the same values for the effective energetic disorder (115 meV) and for the positional disorder (1.85) for both polymers. Therefore, we have concluded that the added green and red dyes do not act as hole traps as they have no influence on the hole mobility. It can therefore be concluded that their highest occupied molecular orbital (HOMO) levels are aligned with the HOMO level of the polyspirobifluorene backbone. © 2008 American Institute of Physics.