Characterization of ZnO Interlayers for Organic Solar Cells

Correlation of Electrochemical Properties with Thin-Film Morphology and Device Performance

Kai Lin Ou, Ramanan Ehamparam, Gordon Macdonald, Tobias Stubhan, Xin Wu, R. Clayton Shallcross, Robin E Richards, Christoph J. Brabec, Steven S Saavedra, Neal R Armstrong

Research output: Contribution to journalArticle

8 Citations (Scopus)

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.

Original languageEnglish (US)
Pages (from-to)19787-19798
Number of pages12
JournalACS Applied Materials and Interfaces
Volume8
Issue number30
DOIs
StatePublished - Aug 3 2016

Fingerprint

Tin oxides
Electrochemical properties
Indium
Thin films
Zinc Oxide
Organic solar cells
indium tin oxide
Oxidation
Dark currents
Electrochemistry
Zinc oxide
Contacts (fluid mechanics)
Oxides
Electrolytes
Oxide films
Sol-gels
Heterojunctions
Charge transfer
Solar cells
Porosity

Keywords

  • charge selective interlayers
  • impedance spectroscopy
  • organic solar cells
  • phase separation
  • voltammetry
  • zinc oxide

ASJC Scopus subject areas

  • Materials Science(all)

Cite this

Characterization of ZnO Interlayers for Organic Solar Cells : Correlation of Electrochemical Properties with Thin-Film Morphology and Device Performance. / Ou, Kai Lin; Ehamparam, Ramanan; Macdonald, Gordon; Stubhan, Tobias; Wu, Xin; Shallcross, R. Clayton; Richards, Robin E; Brabec, Christoph J.; Saavedra, Steven S; Armstrong, Neal R.

In: ACS Applied Materials and Interfaces, Vol. 8, No. 30, 03.08.2016, p. 19787-19798.

Research output: Contribution to journalArticle

Ou, Kai Lin ; Ehamparam, Ramanan ; Macdonald, Gordon ; Stubhan, Tobias ; Wu, Xin ; Shallcross, R. Clayton ; Richards, Robin E ; Brabec, Christoph J. ; Saavedra, Steven S ; Armstrong, Neal R. / Characterization of ZnO Interlayers for Organic Solar Cells : Correlation of Electrochemical Properties with Thin-Film Morphology and Device Performance. In: ACS Applied Materials and Interfaces. 2016 ; Vol. 8, No. 30. pp. 19787-19798.
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