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

Published

May 10, 2016

Author(s)

Paul D. Cunningham, Paul A. Lane, Joseph S. Melinger, Okan Esenturk, Edwin J. Heilweil

Abstract

Time-resolved terahertz spectroscopy (TRTS) was used to explore charge generation, transfer, and the role of hot carriers in organic solar cell photovoltaic 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 to 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/C60, electron transfer from ZnPc yields hot charges that localize and become less mobile as they thermalize. Excitation of high-lying Frank 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.

Proceedings Title

Terahertz Spectroscopy II (#9856)

Volume

9856

Conference Dates

April 17-21, 2016

Conference Location

Baltimore, MD, US

Conference Title

SPIE Defense and Commercial Sensing

Pub Type

Conferences

Download Paper

https://doi.org/10.1117/12.2228379

Local Download

Keywords

terahertz spectroscopy, carrier dynamics, organic solar cells

Citation

Cunningham, P. , Lane, P. , Melinger, J. , Esenturk, O. and Heilweil, E. (2016), Probing Charge Transfer and Hot Carrier Dynamics in Organic Solar Cells with Terahertz Spectroscopy, Terahertz Spectroscopy II (#9856), Baltimore, MD, US, [online], https://doi.org/10.1117/12.2228379, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=920502 (Accessed October 11, 2025)