COMMUNICATION 1901956 (1 of 8) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.advmat.de Unprecedented Enhancement of Thermoelectric Power Factor Induced by Pressure in Small-Molecule Organic Semiconductors Wen Shi, Tianqi Deng, Gang Wu, Kedar Hippalgaonkar,* Jian-Sheng Wang, and Shuo-Wang Yang* Dr. W. Shi, Dr. T. Deng, Dr. G. Wu, Dr. S.-W. Yang Institute of High Performance Computing Agency for Science, Technology and Research 1 Fusionopolis Way, #16-16 Connexis Singapore 138632, Republic of Singapore E-mail: yangsw@ihpc.a-star.edu.sg Prof. K. Hippalgaonkar Institute of Materials Research and Engineering Agency for Science, Technology and Research 2 Fusionopolis Way, #08-03 Innovis Singapore 138634, Republic of Singapore E-mail: kedarh@imre.a-star.edu.sg Prof. K. Hippalgaonkar School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue, Singapore 639798, Republic of Singapore Prof. J.-S. Wang Department of Physics National University of Singapore 2 Science Drive 3, Singapore 117551, Republic of Singapore The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201901956. DOI: 10.1002/adma.201901956 generation and refrigeration. [1] The energy conversion efficiency of a TE material hinges on a dimensionless figure of merit, zT = (S 2 σT)/κ, where S, σ, κ, and T are Seebeck coefficient, electrical conductivity, total thermal conductivity, and absolute temperature, respectively. Unlike conven- tional inorganic TE materials, a unique characteristic of small-molecule organic semiconductors is their inherent softness, which gives them mechanical flexibility. [2] High-performance organic TE materials are an essential prerequisite for producing conformable, stretchable and large-area TE devices, which can be exploited as temperature and pressure sensors in self- powered wearable elements, electronic skins, biomonitors, etc. [3] In recent years, the scientific community has devoted much effort in developing high-efficiency small-molecule organic TE materials; impressive achievements have been made based on various strategies, such as controlling the carrier con- centration by field-effect modulation [4] or molecular doping, [5] and regulating the chemical structure. [6] However, in terms of performance, the TE efficiency of organic semiconductors still needs to be largely enhanced to meet practical applications. In organic semiconductors, the π-conjugated molecules are loosely assembled via van der Waals forces, and these weak intermolecular interactions are the origin of the large struc- tural deformations when small external mechanical forces are applied. As one efficient non-synthetic strategy, applying external mechanical force, such as solution shear, [7] off-center spin coating [8] and pressurization recrystallization, [9] has been used to modify the microscopic molecular packing structures, and thereby to regulate the carrier mobility. For example, an ultrahigh field-effect room-temperature hole mobility of 43 cm 2 V −1 s −1 was achieved in highly aligned 2,7-dioctyl[1] benzothieno[3,2-b][1]benzothiophene (C 8 -BTBT) films by off- center spin coating. [8] Moreover, using a solution-shearing method, the π–π stacking distance in 6,13-bis(triisopropylsilyl- ethynyl)pentacene films was decreased from 3.33 to 3.08 Å, consequently, improving field-effect hole mobility about six times from 0.8 to 4.6 cm 2 V −1 s −1 . [7] On the other hand, flexible organic electronics, such as wearable smart devices, [10] electronic skins, [11] and soft robots [12] that integrate field effect Establishing the relationship between pressure and heat–electricity interconversion in van der Waals bonded small-molecule organic semiconductors is critical not only in designing flexible thermoelectric materials, but also in developing organic electronics. Here, based on first-principles calculations and using naphthalene as a case study, an unprecedented elevation of p-type thermoelectric power factor induced by pressure is demonstrated; and the power factor increases by 267% from 159.5 µW m −1 K −2 under ambient conditions to 585.8 µW m −1 K −2 at 2.1 GPa. The underlying mechanism is attributed to the dramatic inhibition of lattice- vibration-caused electronic scattering. Furthermore, it is revealed that both restraining low-frequency intermolecular vibrational modes and increasing intermolecular electronic coupling are two essential factors that effectively suppress the electron–phonon scattering. From the standpoint of molecular design, these two conditions can be achieved by extending the π-conjugated backbones, introducing long alkyl sidechains to the π-cores, and substituting heteroatoms in the π-cores. Organic Electronics Thermoelectric (TE) materials enabling direct heat–electricity interconversion have immense potential for solid-state power Adv. Mater. 2019, 31, 1901956