Modifying the chemical structures of organic semiconductors is challenging, which makes it difficult to control the turn-on voltage of transistors made of these materials, explains team member Yoonyoung Chung. "Our work shows that we can overcome this difficulty by controlling electric dipoles in the gate dielectric in these devices." These dipoles are equivalent to an additional voltage bias that is connected to the transistors, so the effects of applied external voltages can be varied.

Chung and colleagues used two different types of self-assembled monolayers (SAMs) inside the gate dielectric of their transistors. The first set was made from octadecylphosphonic acid (OPA) and the second from octadecylsilane (OTS). These two commonly used molecules have different "anchor" groups (silane and phosphonic acid) that bind to an oxide surface – a property that allowed the Stanford team to generate a electrical potential difference of around 0.35 V between the two SAMs. In nanoelectronics, where a low voltage is generally used, this potential difference is high enough to control the electrical characteristics of the device.

General technqiue
The researchers reckon that their technique is a general one and so could be applied to other classes of semiconductor materials, not just to organic semiconductors, and of course other types of electronic devices apart from transistors. "It will also have applications in light-emitting/detecting devices and solar cells for controlling the charge injection barrier at a metal and semiconductor junction," Chung told nanotechweb.org.

Although SAMs have already been used by several research groups to control the turn-on voltage of organic transistors, the new technique allows Chung and colleagues to make use of the same interface between the SAMs and the semiconductor. This is important because a small difference at an interface usually results in significant variations in the charge transport of the semiconductor. And, the cherry on the cake: controlling the dipoles in the gate dielectric makes n-channel organic transistors more stable in air by reducing the effects of unwanted "electron traps" from certain chemical groups.

The work was detailed in Nano Letters.