The temperature dependence of charge transport dramatically affects and even determines the properties and applications of organic semiconductors, but is challenging to effectively modulate. Here, we develop a strategy to circumvent this challenge through precisely tuning the effective height of the potential barrier of the grain boundary (i.e., potential barrier engineering). This strategy shows that the charge transport exhibits strong temperature dependence when effective potential barrier height reaches maximum at a grain size near to twice the Debye length, and that larger or smaller grain sizes both reduce effective potential barrier height, rendering devices relatively thermostable. Significantly, through this strategy a traditional thermo-stable organic semiconductor (dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene, DNTT) achieves a high thermo-sensitivity (relative current change) of 155, which is far larger than what is expected from a standard thermally-activated carrier transport. As demonstrations, we show that thermo-sensitive OFETs perform as highly sensitive temperature sensors. Controlling temperature-depending charge transport in organic semiconductors is key to tailoring their electronic properties. Here, the authors report a potential barrier engineering strategy for modulating thermally-activated charge transport in organic semiconductors.

/pdf/effectively-modulating-thermal-activated-charge-transport-in-4csmdjbup3.pdf

Effectively modulating thermal activated charge transport in organic semiconductors by precise potential barrier engineering.

The capacitance characteristics of pentacene metal-oxide-semiconductor (MOS) capacitors with a large uncovered pentacene area have been investigated. The capacitance measured was examined by assuming that the uncovered area is represented by a distributed constant circuit. The frequency dependence of the capacitance was reproduced by an equation derived based on the assumption. The sheet resistance for the uncovered area of a MOS capacitor was calculated as a function of the gate voltage from the capacitance measured. The mobility of a MOS capacitor with an uncovered area was estimated by fitting a curve to the gate voltage dependence of the sheet resistance, and was in the range of 0.48–0.64 cm2 V−1 s−1. In addition, the mobilities were compared with those calculated from the current–voltage characteristics of pentacene transistors fabricated on the same substrate.

Evaluation of organic metal-oxide-semiconductor capacitors based on a distributed constant circuit

We demonstrate a significant improvement in the gain of closed-loop organic amplifiers by systematically controlling the threshold voltage ( $V_{\mathrm {TH}})$ with double-gate structures. The use of an ultrathin parylene dielectric results in an operating voltage as low as 4 V with a supply voltage ( $V_{\mathrm {DD}})$ of 2 V. Double-gate structures were introduced for controlling $V_{\mathrm {TH}}$ of the transistor and the switching voltage of the inverter. The maximum dc gain of the inverter was recorded as 29.3 dB at a $V_{\mathrm {DD}}$ value of 2 V. In addition, the switching voltage of the circuits can be controlled from 0 V to $V_{\mathrm {DD}}$ without a significant performance degradation. The effects of the $V_{\mathrm {TH}}$ control on the closed-loop amplifier were systematically investigated and a significant gain improvement of up to 18.8 dB was demonstrated in the closed-loop gain by adjusting the switching voltage to half of $V_{\mathrm {DD}}$ .

Enhancement of Closed-Loop Gain of Organic Amplifiers Using Double-Gate Structures

The modulus spectra of organic field-effect transistors (OFETs) with electrode overlap and peripheral regions have been experimentally and theoretically investigated. The complex impedance of regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) OFETs with electrode overlap and peripheral regions was measured with a frequency response analyzer. The complex modulus was derived from an equivalent circuit of OFETs with overlap and peripheral regions using a four-terminal matrix approach. The modulus spectra of the P3HT OFETs were successfully fitted by those calculated using the expression derived from the equivalent circuit. Three structures were found in the modulus spectra of the P3HT OFETs owing to the dielectric properties of the gate insulator, transport properties of the organic semiconductor, and contact resistance from the low to high frequency ranges. The resistivity of the gate insulators and the field-effect mobility of working OFETs were determined using the values of the circuit components of the equivalent circuit obtained by fitting.

https://iopscience.iop.org/article/10.35848/1347-4065/ababb9/pdf

Interpretation of the modulus spectra of organic field-effect transistors with electrode overlap and peripheral regions: determination of the electronic properties of the gate insulator and organic semiconductor

Pentacene metal-oxide-semiconductor capacitors having a channel area uncovered with the top electrode have been examined by capacitance-voltage (C-V) and capacitance-frequency (C-f) measurements. The C-V and C-f characteristics were reproduced using an equivalent circuit based on a distributed constant circuit. The sheet resistance, which characterizes carrier transport in the pentacene film, was obtained as a sheet resistance included in the equivalent circuit reproducing the measured characteristics.

Voltage and Frequency Dependence of Capacitance Characteristics in Organic MOS Capacitors

Frequency-dependent carrier dynamics in dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT)-based organic field-effect transistors and metal-insulator-semiconductor capacitors were studied by using admittance measurements. Both the real (conductance) and imaginary (capacitance) components of admittance, which have a mutually complementary relationship, were analyzed in the same way. On the basis of an experimental study of devices with different contact geometries, peaks observed in the conductance spectra corresponding to slopes in the capacitance spectra were classified into two groups. The high-frequency peak was attributed to the carrier injection from the top contact to the DNTT/insulator interface just underneath the contact while the low-frequency peaks were attributed to the drift current spreading all over the interface. The model calculation of carrier diffusion reproduces the low-frequency peaks very well. It is shown that the field-effect mobility from the accumulation region to the subthreshold region can be estimated by the fitting.

Alternating current admittance of DNTT-based metal-insulator-semiconductor capacitors

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Alternating current admittance of DNTT-based metal-insulator-semiconductor capacitors