If measurements fail to account for these divergent behaviours in ‘organic field-effect transistors’ (OFETs), the resulting estimates of how fast electrons or other charge carriers travel in the devices may be more than 10 times too high, the researchers claim. The team's measurements implicate an overlooked source of electrical resistance as the root of inaccuracies that can inflate estimates of organic semiconductor performance.
Already used in LEDs, electrically conductive polymers and small molecules are being developed for applications in flexible displays, flat-panel TVs, sensors, smart textiles, solar cells and IoT applications. Besides flexibility, a key selling point is that the organic devices can be manufactured in large volumes and less expensively than silicon-based devices.
A key sticking point, however, is the challenge of achieving the high levels of charge-carrier mobility that these applications require. In the semiconductor arena, the general rule is that higher mobility is always better, enabling faster, more responsive devices. So chemists have set out to hurry electrons along. Working from a large palette of organic materials, they have been searching for chemicals that will up the speed limit in their experimental devices.
Just as for silicon semiconductors, assessments of performance require measurements of current and voltage. In the basic transistor design, a source electrode injects charge into the transistor channel leading to a drain electrode. In between sits a gate electrode that regulates the current in the channel by applying voltage, functioning much like a valve.
Typically, measurements are analysed according to a longstanding theory for silicon field-effect transistors. Plug in the current and voltage values and the theory can be used to predict properties that determine how well the transistor will perform in a circuit.
Results are rendered as a series of ‘transfer curves’. Of particular interest in the study are curves showing how the drain current changes in response to a change in the gate electrode voltage. For devices with ideal behaviour, this relationship provides a good measure of how fast charge carriers move through the channel to the drain.
"Organic semiconductors are more prone to non-ideal behaviour because the relatively weak intermolecular interactions that make them attractive for low-temperature processing also limit the ability to engineer efficient contacts as one would for state-of-the-art silicon devices," said electrical engineer David Gundlach, who leads NIST's Thin Film Electronics Project. "Since there are so many different organic materials under investigation for electronics applications, we decided to step back and do a measurement check on the conventional wisdom."
Using what Gundlach describes as the semiconductor industry's ‘workhorse’ measurement methods, the team scrutinised an OFET made of single-crystal rubrene. Their measurements revealed that electrical resistance at the source electrode significantly influences the subsequent flow of electrons in the transistor channel, and hence the mobility.
In effect, contact resistance at the source electrode creates the equivalent of a second valve that controls the entry of current into the transistor channel. Unaccounted for in the standard theory, this valve can overwhelm the gate and become the dominant influence on transistor behaviour.
At low gate voltages, this contact resistance at the source can overwhelm device operation. Consequently, model-based estimates of charge-carrier mobility in organic semiconductors may be more than 10 times higher than the actual value, the research team reported.
The researchers say the aim of the study is to improve "understanding of the source of the non-ideal behaviour and its impact on extracted figures of merit," especially charge-carrier mobility. This knowledge can inform efforts to develop accurate, comprehensive measurement methods for benchmarking organic semiconductor performance, as well as guide efforts to optimise contact interfaces.
