Determination of carrier mobility of amorphous organic electronic material by thin film transistor configuration

We demonstrate that organic thin film transistors (OTFTs) can act as an alternative tool for carrier mobility evaluation in amorphous organic electronic materials. OTFT is a three terminal device which can be operated with an active layer of film thickness thinner than 100 nm. The materials under investigation are phenylamine-based (PA) compounds, which are amorphous hole transporting materials widely used in organic light emitting diodes (OLEDs). The field effect (FE) mobilities of PA compounds (hole) were determined in a TFT configuration. For the case of N,N′-diphenyl-N,N′-bis(1-naphthyl) (1,1′biphenyl)-4,4′diamine (NPB), the FE mobility was found to be 2 × 10^-5 cm²/Vs. It is about one order of magnitude smaller than that obtained from independent time-of-flight (TOF) technique (2 × 10^-4 cm²/Vs) using a thick film of ∼ 5 μm. Temperature dependent measurement was performed under temperature ranging from 235 to 360 K. The extracted energetic disorder by means of the Gaussian Disorder Model from OTFT was 85meV, which was larger than that of TOF (∼74 meV). Similar observations were found in other PA compounds. The increase in the extracted disorder parameter in TFT configuration was one of the origins of the discrepancy between the FE and TOF mobility. OTFTs can be regarded as a useful tool for carrier mobility evaluation with little material consumption.