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Proceedings Paper

Probing charge transfer and hot carrier dynamics in organic solar cells with terahertz spectroscopy
Author(s): Paul D. Cunningham; Paul A. Lane; Joseph S. Melinger; Okan Esenturk; Edwin J. Heilweil
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Paper Abstract

Time-resolved terahertz spectroscopy (TRTS) was used to explore charge generation, transfer, and the role of hot carriers in organic solar cell materials. Two model molecular photovoltaic systems were investigated: with zinc phthalocyanine (ZnPc) or alpha-sexathiophene (α-6T) as the electron donors and buckminsterfullerene (C60) as the electron acceptor. TRTS provides charge carrier conductivity dynamics comprised of changes in both population and mobility. By using time-resolved optical spectroscopy in conjunction with TRTS, these two contributions can be disentangled. The sub-picosecond photo-induced conductivity decay dynamics of C60 were revealed to be caused by auto-ionization: the intrinsic process by which charge is generated in molecular solids. In donor-acceptor blends, the long-lived photo-induced conductivity is used for weight fraction optimization of the constituents. In nanoscale multilayer films, the photo-induced conductivity identifies optimal layer thicknesses. In films of ZnPc/C60, electron transfer from ZnPc yields hot charges that localize and become less mobile as they thermalize. Excitation of high-lying Franck Condon states in C60 followed by hole-transfer to ZnPc similarly produces hot charge carriers that self-localize; charge transfer clearly precedes carrier cooling. This picture is contrasted to charge transfer in α-6T/C60, where hole transfer takes place from a thermalized state and produces equilibrium carriers that do not show characteristic signs of cooling and self-localization. These results illustrate the value of terahertz spectroscopic methods for probing charge transfer reactions.

Paper Details

Date Published: 29 April 2016
PDF: 8 pages
Proc. SPIE 9856, Terahertz Physics, Devices, and Systems X: Advanced Applications in Industry and Defense, 98560Y (29 April 2016); doi: 10.1117/12.2228379
Show Author Affiliations
Paul D. Cunningham, U.S. Naval Research Lab. (United States)
Paul A. Lane, U.S. Naval Research Lab. (United States)
Joseph S. Melinger, U.S. Naval Research Lab. (United States)
Okan Esenturk, Middle East Technical Univ. (Turkey)
Edwin J. Heilweil, National Institute of Standards and Technology (United States)


Published in SPIE Proceedings Vol. 9856:
Terahertz Physics, Devices, and Systems X: Advanced Applications in Industry and Defense
Mehdi F. Anwar; Thomas W. Crowe; Tariq Manzur, Editor(s)

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