Contactless charge carrier mobility measurement in organic field-effect transistors
Introduction
The performance of organic field-effect transistors (OFET) increases remarkably, and an increasing number of organic semiconductors is being reported with a charge carrier mobility over that of a-Si, ∼1 cm2/V s [1], [2]. Especially n-type and ambipolar polymers recently made strong progression [3], [4], [5], [6], [7], [8]. However, as the channel resistance decreases due to the increase of organic semiconductor mobilities, contact resistances often become the bottleneck for the total device performance [1], [9], [10]. Many strategies have been reported to decrease the contact resistance for either holes or electrons [1], [10], [11], using self-assembled monolayers [12], [13], doping [14], [15], interlayers [16], [17], or by changing the device lay-out [9], [18]. Contact resistance is an (even) more fundamental problem in ambipolar transistors, where both electrons and holes need to be injected [5], [19]. The electron and hole injection barriers at any contact always sum up to the semiconductor band gap. Hence the injection barrier cannot be negligible for both holes and electrons simultaneously, leading to a contact resistance for at least one of the two charges. By using different contact materials for source and drain this problem can partially be solved, but for ease of fabrication it is preferred to have a single electrode material that is able to inject both types of charge carriers [20].
For research purposes one is often only interested in the transport properties of the semiconductor. The charge carrier mobility is then typically obtained by measuring the current in an OFET [21]. When the transistor suffers from contact resistance, a lower current and concomitantly a lower mobility is found. Correction for contact resistance is possible by estimating its value [22], [23], [24], [25], [26]. The most popular way to do so is by the transfer line method in which the resistance of OFETs as function of the channel length is measured. The extrapolated resistance at zero channel length is a measure for (twice) the contact resistance. This method however requires good device reproducibility [27], [28].
In view of the above there is a clear need for a tool to determine the charge carrier mobility that is insensitive to contact resistance. Here we present such a technique. The basic idea is that an alternating current (AC) is capacitively generated and probed in an OFET channel. To this end two additional finger gates are placed near the accumulation layer. Conventional source and drain contacts are still required to fill the accumulation layer but do not need to absorb or inject any current and are therefore decoupled from the mobility measurement.
Section snippets
Device fabrication
Devices were fabricated on cleaned glass substrates. The finger gates were created by evaporating 40 nm thick Au electrodes through a shadow mask substrate, preceded by a 2 nm Cr adhesion layer. Subsequently a 870 nm thick Cytop™ dielectric was applied by spin coating, followed by annealing for 30 min at 150 °C. On top of that, 40 nm Au source and drain electrodes were evaporated through a shadow mask. The active layer,
Theory
In order to determine the relation between characteristic frequency and mobility, the system will first be analyzed analytically for which it is simplified to the four capacitor circuit as drawn in Fig. 1b. Furthermore, the charge flow away from the finger gate into the rest of the transistor channel is neglected. On basis of a simple dimension analysis the RC-time τRC of the system may be expected to scale as:with R the resistance of the accumulation channel, CG the gate
Transistor characteristics
Before turning to the AC mobility measurement, we shall first illustrate the urgency to avoid injection barriers by conventionally measuring the mobility in transistors with source and drain contacts having different work functions, but based on the same DPP-polymer (PDPPTPT). Different work functions were established by using plasma cleaned gold and solvent cleaned gold. Rinsing gold contacts by a solvent is known to decrease their work function from the range of −(5.0–5.5) eV for plasma
Conclusion
We have presented a general applicable method to measure charge carrier mobilities in organic field-effect transistors, independent of contact resistance. By using capacitive generation and detection of alternating currents there is no need to inject and/or extract charges during the mobility measurement and contact resistance effects are avoided. For that, two additional finger gates were positioned near the accumulation channel. By performing impedance spectroscopy on the finger gates the
Acknowledgment
We appreciate financial support from NanoNextNL, project 06D.03.
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