title: Seebeck effects in n-type and p-type polymers driven simultaneously by surface polarization and entropy differences based on conductor/polymer/conductor thin-film devices.
abstract: This paper reports Seebeck effects driven by both surface polarization difference and entropy difference by using photoinduced intramolecular charge-transfer states in n-type and p-type conjugated polymers, namely IIDT and IIDDT, respectively, based on vertical conductor/polymer/conductor thin-film devices. We obtain large Seebeck coefficients of -898 μV/K from n-type IIDT and 1300 μV/K from p-type IIDDT when the charge-transfer states are generated by a white light illumination of 100 mW/cm(2), compared with the values of 380 and 470 μV/K in dark condition, respectively. Simultaneously, the electrical conductivities are increased from almost insulating state in dark condition to conducting state under photoexcitation in both n-type IIDT and p-type IIDDT based devices. The large Seebeck effects can be attributed to the following two mechanisms. First, the intramolecular charge-transfer states exhibit strong electron-phonon coupling, which leads to a polarization difference between high and low temperature surfaces. This polarization difference essentially forms a temperature-dependent electric field, functioning as a new driving force additional to entropy difference, to drive the energetic carriers for the development of Seebeck effects under a temperature difference. Second, the intramolecular charge-transfer states generate negative or positive majority carriers (electrons or holes) in the n-type IIDT or p-type IIDDT, ready to be driven between high and low temperature surfaces for developing Seebeck effects. On the basis of coexisted polarization difference and entropy difference, the intramolecular charge-transfer states can largely enhance the Seebeck effects in both n-type IIDT and p-type IIDDT devices. Furthermore, we find that changing electrical conductivity can switch the Seebeck effects between polarization and entropy regimes when the charge-transfer states are generated upon applying photoexcitation. Therefore, using intramolecular charge-transfer states presents an approach to develop thermoelectric effects in organic materials-based vertical conductor/polymer/conductor thin-film devices.
authors: †Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States.
authors: Dehua
authors: D
authors: Hu
authors: †Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States.
authors: Qing
authors: Q
authors: Liu
authors: †Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States.
authors: Jeremy
authors: J
authors: Tisdale
authors: ‡Beijing National Laboratory for Molecular Science, The Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
authors: Ting
authors: T
authors: Lei
authors: ‡Beijing National Laboratory for Molecular Science, The Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
authors: Jian
authors: J
authors: Pei
authors: §Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
authors: Hsin
authors: H
authors: Wang
authors: ∥Air Force Research Laboratory, Wright Patterson Air Force Base, Dayton, Ohio 45433, United States.
authors: Augustine
authors: A
authors: Urbas
authors: ∞College of Science, Beijing Jiaotong University, Beijing 100044, China.|†Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States.
authors: Bin
authors: B
authors: Hu
keywords: Seebeck effects; charge-transfer states; entropy difference; n-type organic polymer; p-type organic polymer; surface polarization