Study of trap states in polyspirobifluorene based devices: Influence of chromophore addition

Abstract

The defect states in spiro copolymer based light emitting diodes were investigated by charge based deep level transient spectroscopy (Q-DLTS). Two types of polymers have been studied: blue emitting spiro copolymer and white emitting spiro blend polymer. The white emitting spiro polymer was obtained by adding green and red chromophores into the host blue copolymer. The devices are composed of indium-tin oxide–polyethylene dioxythiophene:polystyrene sulfonate-spiro copolymer-Ba–Al. Q-DLTS measurements were performed on these diodes with various conditions of charging time, charging voltage, and temperature aiming at determining the role of chromophores in the defect formation process. Analysis of the Q-DLTS spectra obtained in both devices revealed at least five trap levels. The mean activation energies of traps are distributed in the range 0.17–0.85 eV within the band gap of the copolymers with capture cross sections of the order of 10−16–10−20 cm2. The trap densities are in the range of 1015–1016 cm−3. The results show that incorporation of dyes into the copolymer resulted in creation of an additional electron trap level and an increase in the density of the existing trap levels, indicating a more disordered state of the emitting material containing chromophores