Seebeck Coefficient Measurement of High Mobility Organic Semiconductors

<b>Abstract</b><br/>Thin film-based organic materials are widely used in field effect transistors (FET), thermoelectric device applications, and solar cells. Recently, small molecule 2,9-Didecyldinaphtho [2,3-b:2',3'-f]thieno[3,2-b]thiophene (C₁₀-DNTT) has drawn a lot of attention in the organic FET (OFET) community due to its high mobility up to 15 cm²V^-1s^-1 and ultrathin single molecular layer thickness. To date, the contact resistance between electrode and semiconductor as well as the cross-plane resistance of semiconductors are still the major research directions.<br/>To improve the performance of FET, and explore the intrinsic mobility of organic semiconductors (OSCs), Seebeck coefficient or thermoelectric effect, electromotive force that generate across a material with temperature difference, can provide valuable information on estimating the carrier density, intrinsic mobility and other electric properties of the thin film. By comparing hole mobility and effect mobility measured by field effect measurement, it not only allows us to determine the trap states in OSCs, but also help us to optimize the molecule design for future FET and thermoelectric devices.<br/>In this work, we prepared C₁₀-DNTT monolayer organic field effect transistor (OFET) by solution shearing method on SiO₂ substrates. The Seebeck measurement is performed via providing a temperature gradient from 0.3K to 1.6K along the channel with a length of 135 μm. The results show that Seebeck coefficient of monolayer C₁₀-DNTT reaches 450 μV/K when no gate voltage is applied. With applying gate voltage gradually into -90V, the Seebeck coefficient shows descent trend regularity down to 300 μV/K. We evaluated the carrier concentration and noticed the Seebeck coefficient exhibited a logarithmic decrement dependence as an increase in carrier concentration. Eventually, further improvement in measuring Seebeck mobility and field effect mobility will be done to estimate traps states in OSC, and provide attractive approach for designing OSC materials in FET and thermoelectric devices.<br/>KEYWORDS: Seebeck coefficient, mobility, trap states