Interpreting impedance spectra of organic photovoltaic cells—Extracting charge transit and recombination rates

Impedance spectroscopy has been widely used to extract the electron-hole recombination rate constant in organic photovoltaic cells (OPVs). This technique is typically performed on OPVs held at open-circuit. Under these conditions, the analysis is simplified with recombination as the only pathway for...

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Bibliographic Details
Published in:Journal of applied physics 2014-09, Vol.116 (12), p.124513
Main Authors: Mullenbach, Tyler K., Zou, Yunlong, Holst, James, Holmes, Russell J.
Format: Article
Language:English
Subjects:
Applied physics
BINDING ENERGY
CHARGE CARRIERS
CHARGE COLLECTION
CHARGE DENSITY
Charge efficiency
CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY
Current carriers
DECAY
ELECTRIC POTENTIAL
EXTRACTION
HETEROJUNCTIONS
Holes (electron deficiencies)
IMPEDANCE
Impedance spectroscopy
Oxidation
PHOTOVOLTAIC CELLS
QUANTUM EFFICIENCY
Rate constants
REACTION KINETICS
RECOMBINATION
SPECTRA
SPECTROSCOPY
Spectrum analysis
TRANSIENTS
Transit
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Summary:Impedance spectroscopy has been widely used to extract the electron-hole recombination rate constant in organic photovoltaic cells (OPVs). This technique is typically performed on OPVs held at open-circuit. Under these conditions, the analysis is simplified with recombination as the only pathway for the decay of excess charge carriers; transit provides no net change in the charge density. In this work, we generalize the application and interpretation of impedance spectroscopy for bulk heterojunction OPVs at any operating voltage. This, in conjunction with reverse bias external quantum efficiency measurements, permits the extraction of both recombination and transit rate constants. Using this approach, the transit and recombination rate constants are determined for OPVs with a variety of electron donor-acceptor pairings and compositions. It is found that neither rate constant individually is sufficient to characterize the efficiency of charge collection in an OPV. It is demonstrated that a large recombination rate constant can be accompanied by a large transit rate constant, thus fast recombination is not necessarily detrimental to OPV performance. Extracting the transit and recombination rate constants permits a detailed understanding of how OPV architecture and processing conditions impact the transient behavior of charge carriers, elucidating the origin of optimum device configurations.
ISSN:0021-8979
1089-7550
DOI:10.1063/1.4896269