Electron mobility characterization in OLEDs from ac small signal optical modulation

This paper investigates the field dependence of electron mobility in tris(8-hydroxyquinolinato)aluminium (Alq₃) and bathocuproine (BCP) through ac small signal optical modulation on green light (ITO/PEDOT/NPD/ Alq₃/Ba/Ag) and blue light (ITO/PEDOT/NPD/BCP/Alq₃/Ba/Ag) OLED. The electroluminescence (EL) transient time delay for the blue light OLED is much longer than for the green one. The electron mobility in BCP was extracted based on a Poole-Frenkel-like equation and EL transient time delay measurement, which is in the range (7-9) × 10^-8 cm² V^-1 s^-1 at an external electric field of 1530-1830 (V cm^-1)^1/2, comparable to the results from other published reports (Muckl et al 2000 Synth. Met. 111-112 91; Barth et al 2001 J. Appl. Phys. 89 3711; Nakamura H et al 1996 Int. Symp. on Inorganic and Organic Electroluminescence ed R H Mauch and H-E Gumlich (Berlin: Wissenschaft und Technik) p 95; Xie et al 2002 Appl. Phys. Lett. 80 1477). The difference in EL transient time delay and electron mobility for green and blue light OLEDs was demonstrated by the results of direct modulation. The electron transit time shows similar field dependence in both Alq₃ layers in green and blue OLEDs. Unlike Alq₃, the field dependence of electron mobility in BCP did not fit the conventional organic semiconductor characteristics μ∞exp(βE^1/2), and the excitons formation at the NPD/BCP interface for the blue OLEDs was demonstrated through the EL spectrum.