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Title: 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 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.
Authors:
; ;  [1] ;  [2]
  1. Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (United States)
  2. New Products R and D, Sigma-Aldrich Corporation, 6000 N. Teutonia Avenue, Milwaukee, Wisconsin 53209 (United States)
Publication Date:
OSTI Identifier:
22305731
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 116; Journal Issue: 12; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; BINDING ENERGY; CHARGE CARRIERS; CHARGE COLLECTION; CHARGE DENSITY; DECAY; ELECTRIC POTENTIAL; EQUIPMENT; EXTRACTION; HETEROJUNCTIONS; IMPEDANCE; PHOTOVOLTAIC CELLS; QUANTUM EFFICIENCY; REACTION KINETICS; RECOMBINATION; SPECTRA; SPECTROSCOPY; TRANSIENTS