Electron transit time and reliable mobility measurements from thick film hydroxyquinoline-based organic light-emitting diode

The time delay (τd) in the transient electroluminescence (EL) signal of a bilayer organic light-emitting diode with a structure of indium-tin oxide /N,N′-diphenyl-N,N′-bis(3methylphenyl)-(1,1′-biphenyl)-4,4′-diamine /tris(8-hydroxyquinoline) aluminum (Alq3)/Al has been measured and analyzed as a function of the thickness (D) of the Alq3 layer. For a thin layer of Alq3 (D<180 nm), it is found that τd is affected by both the charging effect and carrier transit time through the Alq3 layer. For a thicker layer of Alq3 (D>200 nm), τd approaches the intrinsic electron transit time through Alq3. Electron mobility of Alq3 can be evaluated for the thick-film devices and the results are in excellent agreement with independent time-of-flight measurements. The application of transient EL in mobility measurement for C540-doped Alq3 is discussed.