A thin-film transistor (TFT) is a device formed by depositing an active layer of semiconductor over a separate substrate such as glass or plastic, as opposed to more traditional transistors in which the semiconductor itself forms the substrate of the device. Furthermore, modern TFTs can be formed using organic semiconductors (OSCs) rather than the more traditional semiconductor materials such as silicon or metal oxides. These are referred to as organic thin-film transistors (OTFTs), and have found particular success in applications such as display screens for computers, televisions, mobile terminals and other appliances.
An example of an OTFT device is illustrated schematically in FIG. 1. A typical process of producing this device begins by defining source and drain electrodes 12 and 14 over the glass substrate 10 by means of a technique such as photolithography or shadow mask evaporation. Each of the source and drain electrodes 12, 14 comprises a suitable conductor such as gold. An organic semiconductor layer 20 is then formed over the substrate 10 and source and drain electrodes 12, 14, e.g. by spin coating. This is followed by a dielectric layer 30 formed over the semiconductor layer 20, and a gate electrode 32 formed over the dielectric layer 30. This arrangement may be referred to as a top-gate transistor.
In operation, charge carriers flow through a channel region between the source and drain electrodes 12, 14 in dependence on a signal applied at the gate 32. Charge carriers can either be negatively charged electrons (e−) in the case of an n-type semiconductor or positively-charged holes (h+) in the case of a p-type semiconductor.
An n-type organic semiconductor material comprises a small number of molecules in which an electron occupies the Lowest Unoccupied Molecular Orbital (LUMO). These electrons are able to move in-between LUMO orbitals of neighboring molecules, and hence act as negative charge carriers. A p-type semiconductor material comprises a small number of “holes”, each corresponding to a semiconductor molecule where one of the two electrons occupying the Highest Occupied Molecular Orbital (HOMO) is missing. These holes are able to accept electrons from adjacent molecules, and hence the holes can move like positively charged carriers.
One aspect of the performance of the device is its charge carrier mobility (the charge carrier drift velocity per unit electric field). Without thereby being limited by theory, in order to achieve a high mobility and therefore good performance, there are at least two properties of the device which it is desirable to control during production.                Firstly, a good ohmic contact should be ensured between the semiconductor layer 20 and each of the source and drain electrodes 12, 14. For this to be achieved, the work function of the electrodes 12,14 should be matched to the ionization potential of the semiconductor 20. The ionization potential is a measure of the amount of additional energy an electron occupying the Highest Occupied Molecular Orbital (HOMO) in an organic semiconductor material needs to be liberated from the solid semiconductor. Likewise, the work function is a measure of the amount of energy an electron needs to be liberated from a solid metal into free space. In case of an organic thin film transistor the difference between the work function of the metal contacts and the ionization potential of the organic semiconductor relates to the amount of energy a charge carrier needs to be injected from an electrode into the organic semiconductor 20. It will be appreciated that the ability of the electrodes to efficiently accept and donate charge carriers to and from the semiconductor 20 is relevant for forming ohmic contacts.        Secondly, the crystal morphology of the semiconductor 20 needs to be controlled during deposition. Small molecule (SM) organic semiconductors are commonly used as the active semiconductor layer 20 because they crystallize and thus provide high charge carrier mobility within the organic semiconductor film.        
The relevance of the work function is considered in more detail with reference to FIG. 2. This example shows a gold source electrode 12 injecting a hole into an organic semiconductor region 20. The work function φ of a solid is the difference between the highest energy level electron in the solid (the Fermi level F) and the energy that electron would have if just liberated from the solid into a vacuum (Evacuum). An untreated gold electrode 12 has a work function of about 5.0 eV, but the organic semiconductor 20 typically has a greater ionization potential, e.g. of about 5.4 eV.
That is, the gold 12 contains higher energy electrons than the semiconductor 20. This means there is a potential barrier resisting an electron being transferred from the semiconductor 20 into the gold 12, and hence a barrier resisting a hole being injected from the gold 12 into the semiconductor 20. When a signal is applied at the gate 32, the potential profile within the semiconductor channel region slopes as shown in FIG. 2 and a hole can quantum tunnel across the barrier. Nonetheless, if the work function of the gold electrode 12 can be increased then this barrier will be easier to overcome. Thus a higher work function of the electrodes results in better mobility, and it is desirable to increase the work function of the electrodes to a level close to (M) or even below (M′) that of the semiconductor 20.
Various surface treatments exist to modify the work function of the electrodes 12, 14 prior to deposition of the organic semiconductor film 20.
One such treatment is an initial pre-cleaning step of exposing the substrate 20 and electrodes 12, 14 to oxygen plasma.
This pre-cleaning removes contaminants from the surface of the gold and thus increases the work function. However, it also has an unwanted side effect of increasing the presence of ionic species on the surface of the glass substrate 10. These ionic species may lead to the formation of a conducting “back channel” that allows a source-drain current to flow even when the TFT is set to its “off state”. This increases the off current, reducing the on/off ratio and the sub-threshold swing and thus worsening performance.
To counter this side-effect, a treatment has been developed comprising the silanisation of the glass surface with organosilanes 16, as disclosed in international patent application publication no. WO 2010/015833. This treatment advantageously reduces the presence of ionic species on the surface of the glass substrate 10.
Self-assembled organo silane monoloayers (SAMs) have also been shown to improve morphologies and electrical transport layers properties in the case of pentacene films evaporated onto SiO2 surfaces [“Morphology and electrical transport in pentacene films on silylated oxide surfaces”, K Shankar & T N Jackson, Journal of Materials Research 19, p. 2003 (2004)].
However, the silanisation has a further side effect of somewhat reversing the increase in the work function of the electrodes 12, 14 which was achieved by the pre-cleaning step.
Therefore to increase the work function back to a more desirable level, yet another treatment may be employed. This comprises the application of a treatment substance comprising thiol SAM molecules to the metal source and drain contacts 12, 14. This results in the formation of an electrode contact layer 18 in the form of a self-assembled monolayer (SAM) on the electrodes 12, 14. This treatment is also disclosed in WO 2010/015833.