SKPM scanning schematic

SKPM scanning schematic

Organic field-effect transistors (OFETs) are an emerging display and sensing technology with cheaper and lower energy processing. Their threshold voltage (VT), the voltage where the OFET is decidedly “on,” should be precisely tuned. Scanning Kelvin probe microscopy (SKPM) gives a surface voltage profile across the entire span of an organic field-effect transistor (OFET), including gate, insulator, semiconductor, and source-drain electrodes before and after operation, which can be converted to a charge density profile. The results show very different behaviors among three representative polymers and point to which materials and interfaces stabilize charge and prevent extraneous charging during operation.

A sideways transistor geometry correlated static charge quantitatively to changes in threshold voltage (VT) without disturbing transistor materials or interfaces, and also showed striking differences among different polymer gate materials. This will indicate more effective gate insulator choices for printed transistors with stabilized, defined VT, and inspire new material structures for other applications.

Scan data showing relative surface potentials on color scales depending on sample driving and charging

Scan data showing relative surface potentials on color scales depending on sample driving and charging

A VT shift in OFETs is routinely observed during normal device operation, a phenomenon known as bias stress. A major consequence of this phenomenon is poor performance—and ultimately, failure—of circuitry that relies on precisely-tuned voltages for operation. The physical origin of this VT instability has been widely debated in the literature, with agreement on charge trapping as the prevalent mechanism but disagreement on whether mobile charges were being trapped in the OSC or in the dielectric. This motivates the mapping of interfacial potentials in the OFET to identify static charge trapped in the gate material, and ultimately the design of new material structures where this trapping can be controlled.

OFETs are usually made of stacked films. Here, we made lateral OFETs so differences in electrical potential and charge density could be visualized. We used pentacene semiconductor and polystyrene as gate insulator. Materials were polarized from bias stress during OFET operation or in a charging step. Charge storage was indicated, without altering the OFET, as a surface voltage with SKPM, and correlated to VT shift. Charge densities calculated from capacitance-VT shift products agreed with those from SKPM and the application of Poisson’s equation.

In comparison, poly(2-trifluoromethylstyrene) had more stability to bias stress, and poly(methyl methacrylate) showed more leakage current. Thus, polystyrene-based materials are more likely to allow charge penetration for the intentional tuning of VT,while fluorinated polymers present stronger barriers to the unwanted trapping of static charge in dielectrics. These baseline characteristics are informing the design of new polymer heterostructures where static charge can be placed exactly where desired for certain functions, and inhibited from causing performance instabilities.

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