title: Characterization of ZnO Interlayers for Organic Solar Cells: Correlation of Electrochemical Properties with Thin-Film Morphology and Device Performance.
abstract: This report focuses on the evaluation of the electrochemical properties of both solution-deposited sol-gel (sg-ZnO) and sputtered (sp-ZnO) zinc oxide thin films, intended for use as electron-collecting interlayers in organic solar cells (OPVs). In the electrochemical studies (voltammetric and impedance studies), we used indium-tin oxide (ITO) over coated with either sg-ZnO or sp-ZnO interlayers, in contact with either plain electrolyte solutions, or solutions with probe redox couples. The electroactive area of exposed ITO under the ZnO interlayer was estimated by characterizing the electrochemical response of just the oxide interlayer and the charge transfer resistance from solutions with the probe redox couples. Compared to bare ITO, the effective electroactive area of ITO under sg-ZnO films was ca. 70%, 10%, and 0.3% for 40, 80, and 120 nm sg-ZnO films. More compact sp-ZnO films required only 30 nm thicknesses to achieve an effective electroactive ITO area of ca. 0.02%. We also examined the electrochemical responses of these same ITO/ZnO heterojunctions overcoated with device thickness pure poly(3-hexylthiophehe) (P3HT), and donor/acceptor blended active layers (P3HT:PCBM). Voltammetric oxidation/reduction of pure P3HT thin films on ZnO/ITO contacts showed that pinhole pathways exist in ZnO films that permit dark oxidation (ITO hole injection into P3HT). In P3HT:PCBM active layers, however, the electrochemical activity for P3HT oxidation is greatly attenuated, suggesting PCBM enrichment near the ZnO interface, effectively blocking P3HT interaction with the ITO contact. The shunt resistance, obtained from dark current-voltage behavior in full P3HT/PCBM OPVs, was dependent on both (i) the porosity of the sg-ZnO or sp-ZnO films (as revealed by probe molecule electrochemistry) and (ii) the apparent enrichment of PCBM at ZnO/P3HT:PCBM interfaces, both effects conveniently revealed by electrochemical characterization. We anticipate that these approaches will be applicable to a wider array of solution-processed interlayers for "printable" solar cells.
authors.affiliation: Department of Chemistry & Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Kai-Lin
authors.initials: KL
authors.lastname: Ou
authors.affiliation: Department of Chemistry & Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Ramanan
authors.initials: R
authors.lastname: Ehamparam
authors.affiliation: Department of Chemistry & Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Gordon
authors.initials: G
authors.lastname: MacDonald
authors.affiliation: Institute of Materials for Electronics and Energy Technology, Friedrich-Alexander-University Erlangen-Nuremberg , Martensstrasse 7, 91058 Erlangen, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Stubhan
authors.affiliation: Department of Chemistry & Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Xin
authors.initials: X
authors.lastname: Wu
authors.affiliation: Department of Chemistry & Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: R Clayton
authors.initials: RC
authors.lastname: Shallcross
authors.affiliation: Department of Chemistry & Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Robin
authors.initials: R
authors.lastname: Richards
authors.affiliation: Institute of Materials for Electronics and Energy Technology, Friedrich-Alexander-University Erlangen-Nuremberg , Martensstrasse 7, 91058 Erlangen, Germany.
authors.forename: Christoph J
authors.initials: CJ
authors.lastname: Brabec
authors.affiliation: Department of Chemistry & Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: S Scott
authors.initials: SS
authors.lastname: Saavedra
authors.affiliation: Department of Chemistry & Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Neal R
authors.initials: NR
authors.lastname: Armstrong
keywords: charge selective interlayers; impedance spectroscopy; organic solar cells; phase separation; voltammetry; zinc oxide