Devices based on organic materials have long been heralded as the route to achieve fully flexible and bendable electronic and optoelectronic applications. This capability is afforded by their inherent flexibility and solution processability, both stemming from the weak intermolecular van der Waals interactions. Manufacturing at or near room temperature opens the realm of soft materials for use as viable substrates. Although there have been significant strides in the performance of organic semiconductors through the introduction of new chemistries, crystal engineering, and film microstructure control, the resistance to injection from a metal electrode into the semiconductor layer has remained a major hurdle in the pursuit of the promised potential of these materials. Moreover, with increasing effective mobilities of the organic semiconductor layer and reduced channel dimensions, the problem becomes even more pressing. This resistance to injection, or contact resistance, places limits on the operating voltage and switching speed such that even devices with superb peripheral characteristics (e.g. charge-carrier mobility) are no longer suitable for commercial applications. Various means have been utilized to minimize the contact effects in organic devices and while these efforts have been largely successful the issue remains far from concluded.
Through a simple approach, we have shown how the contact resistance in OFETs can be drastically reduced. The properties of evaporated metal films depend highly on the deposition parameters, and the parameter we focus on is the rate of deposition. It is often beneficial to use a fast evaporation when working with organic materials to reduce the amount of time that these soft materials are under thermal stress, however in our case we use silicon oxide or glass as the substrate to form bottom-contact transistors. The use of these thermally resilient materials allowed us to drop the Au deposition rate considerably, resulting in larger grains at the injection interface. These larger grains allow the formation of highly ordered domains in the self-assembled monolayer placed at the surface of the electrodes. Consequently, high work-function regions form which promote channels of enhanced injection and therefore a reduction in contact resistance.
This method of lowering the evaporation rate is similar to the annealing techniques that are often used when working with metallic films, however in this case the annealing and the deposition are simultaneous. Adoption of this method led to contact resistances of 200 Ωcm and device charge carrier mobilities of 20 cm2V-1s-1 independent of the applied gate voltage. The proposed approach is efficient for both small molecule and polymeric thin-film transistor devices and can be generally applied in all common processes and device architectures. In addition to enabling high-mobility transistors with near ideal current-voltage characteristics, the use of this method will also lead to accurate measurement of the charge carrier mobility, a critical step in a rational material design.
The manuscript can be found here:
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