Reliability of the Transmission Line Method and Reproducibility of the Measured Contact Resistance of Organic Thin-Film Transistors

Using the transmission line method (TLM), we extracted the contact resistance of organic thin-film transistors (TFTs) based on five different vacuum-deposited small-molecule semiconductors fabricated on over 500 substrates. In the first part of this report, we illustrate how the reliability of the TLM analysis is affected by the statistical uncertainty that arises from the fitting procedure and by the systematic error that is introduced if the actual channel length of the TFTs deviates from the nominal channel length. In the second part, we show that the contact resistance of organic TFTs varies significantly from one fabrication run to the next (and even across substrates fabricated within the same fabrication run), no matter how much care is taken to keep all controllable fabrication-process parameters constant. A statistical analysis reveals no strong correlations between the contact resistance and environmental parameters present during TFT fabrication, such as the humidity in the laboratory or the base pressure of the vacuum during material depositions. This suggests that the observed variation in the contact resistance is mainly stochastic. For the TFTs based on the best-performing semiconductor, the contact resistance varies between 28 Ωcm and 1 kΩcm, with a median value of 160 Ωcm.