Probing Charge Transfer and Hot Carrier Dynamics in Organic Solar Cells with Terahertz Spectroscopy

2016-04-19
CUNNINGHAM, Paul D.
LANE, Paul A.
MELINGER, Joseph S.
Esentürk, Okan
HEILWEIL, Edwin J.
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 (alpha-6T) as the electron donors and buckminsterfullerene (C-60) 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 C-60 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 multi layer films, the photo-induced conductivity identifies optimal layer thicknesses. In films of ZnPc/C-60, electron transfer from ZnPc yields hot charges that localize and become less mobile as they theunalize. Excitation of high-lying Franck Condon states in C-60 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 alpha-6T/C-60, where hole transfer takes place from a theunalized 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.
Conference on Terahertz Physics, Devices, and Systems X - Advanced Applications in Industry and Defense

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Citation Formats
P. D. CUNNINGHAM, P. A. LANE, J. S. MELINGER, O. Esentürk, and E. J. HEILWEIL, “Probing Charge Transfer and Hot Carrier Dynamics in Organic Solar Cells with Terahertz Spectroscopy,” Baltimore, MD, 2016, vol. 9856, Accessed: 00, 2020. [Online]. Available: https://hdl.handle.net/11511/39935.