Abstract
The field-dependent hole mobility of 4,4′,4″-tris(1-naphthylphenylamino)triphenylamine (1-NaphDATA) has been measured on hole-only samples with layer thicknesses between 100 and 500 nm. To determine the mobility we employed three different methods: steady-state space-charge limited currents (SCLC), transient dark injection SCL currents (DI-SCLC), and admittance spectroscopy (AS). The results, especially the field dependence of the mobility, depend on the measuring method. The SCLC mobility increases with the layer thickness and becomes constant above 200 nm, which indicates a transition from contact-limited to bulk-limited conduction. Compared to AS and SCLC we found that DI-SCLC is the most reliable method to measure the mobility on thin samples because it is hardly affected by carrier trapping nor by the layer thickness. Using DI-SCLC an organic layer of 200 nm was sufficient to determine the bulk hole mobility of 1-NaphDATA (μ0=6.57×10−6 cm2/V s and γ=2.74×10−3 (cm/V)1/2, which is very thin compared to time-of-flight samples whose thickness is in the range of micrometers.
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