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

Dehua Hu; Qing Liu; Jeremy Tisdale; Ting Lei; Jian Pei; Hsin Wang; Augustine Urbas; Bin Hu

doi:10.1021/acsnano.5b00589

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

ACS Nano. 2015 May 26;9(5):5208-13. doi: 10.1021/acsnano.5b00589. Epub 2015 Apr 20.

Authors

Dehua Hu¹, Qing Liu¹, Jeremy Tisdale¹, Ting Lei², Jian Pei², Hsin Wang³, Augustine Urbas⁴, Bin Hu^{5

1}

Affiliations

¹ †Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States.
² ‡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.
³ §Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
⁴ ∥Air Force Research Laboratory, Wright Patterson Air Force Base, Dayton, Ohio 45433, United States.
⁵ ∞College of Science, Beijing Jiaotong University, Beijing 100044, China.

PMID: 25877512
DOI: 10.1021/acsnano.5b00589

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.

Keywords: Seebeck effects; charge-transfer states; entropy difference; n-type organic polymer; p-type organic polymer; surface polarization.

Publication types

Research Support, U.S. Gov't, Non-P.H.S.