Control of Resistance Temperature Change of Solution-Processed Conductive Polymers for Strain Sensor Applications

Organic materials are used in flexible electronics due to their excellent flexibility and simple fabrication using solution processes. Because the resistance of organic materials changes with strain and temperature, flexible strain^[1] and temperature^[2] sensors using organic materials have been studied. In such sensors, control of conductivity and resistance temperature change is important. When the device is used as a strain sensor, the resistance temperature change needs to be small to make the sensor less susceptible to temperature. In contrast, when the device is used as a temperature sensor, the resistance temperature change should be large to increase sensitivity. Low conductivity makes the device more sensitive to electronic noise. In this study, we evaluated the influence of the fabrication conditions on the conductivity and temperature characteristics of devices using conductive polymers. Then, we selected a condition and applied it to a strain sensor.<br/>Chromium and gold were evaporated on substrates as electrodes. Conductive polymers were spin-coated on the substrates followed by annealing under a nitrogen atmosphere. Conductive polymers consisted of a polymer material and a p-type dopant. The doping concentration in this experiment was 12%, which means that twelve dopant molecules were added per one hundred monomeric units of the polymer. The standard conditions in this experiment were P3HT for the polymer, BCF for the dopant, and 70 °C for the annealing temperature, and the effects of changing these conditions were evaluated. Temperature coefficient of resistance (TCR) is defined as the ratio of resistance change to temperature change. In this study, TCR was calculated using the resistances at 25 and 40 °C. When the device was applied to the strain sensor, a 1-µm-thick parylene was deposited on the device as a passivation layer. A gauge factor, defined as a ratio of resistance change to applied strain, was measured.<br/>By adding BCF to P3HT, the conductivity increased by four orders of magnitude, and the TCR changed from -1.3%/K to -0.9%/K. Increasing the annealing temperature from 70 to 150 °C reduced the conductivity by two orders of magnitude and changed the TCR to -2.0%/K. Similar results were obtained when F4-TCNQ was used as a dopant, indicating that the influence of the dopant type on the characteristics was small. When other polymers such as DPP-DTT, PTB7 and PBDTT-DPP were mixed with F4-TCNQ, the TCRs were -2.0%/K, -2.3%/K, and -2.7%/K, respectively. Subsequently, we fabricated a strain sensor using P3HT/BCF annealed at a low temperature, which was a condition that exhibited a small resistance temperature change and high conductivity. The gauge factor of the sensor was 5.9 when a tensile strain of 6.6% was applied. The sensor was attached to an arm and detected resistance change during arm bending.<br/>&lt;Reference&gt;<br/>[1] S. Watanabe <i>et al. Adv. Sci. </i><b>8</b>, 2002065 (2021) [2] Y. Wang <i>et al. Scientific Reports </i><b>10</b>, 2467 (2020)<br/>&lt;Abbreviation of materials&gt;<br/>P3HT: poly(3-hexylthiophene-2,5-diyl), BCF: tris(pentafluorophenyl)borane, F4-TCNQ: 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane, DPP-DTT: poly[[2,3,5,6-tetrahydro-2,5-bis(2-octyldodecyl)-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-2,5-thiophenediylthieno[3,2-b]thiophene-2,5-diyl-2,5-thiophenediyl], PTB7: poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]], PBDTT-DPP: poly{2,6'-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,4-b]dithiophene-alt-5-dibutyloctyl-3,6-bis(5-bromothiophen-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione}