Electrical property heterogeneity at transparent conductive oxide/organic semiconductor interfaces: Mapping contact ohmicity using conducting-tip atomic force microscopy

Gordon A. MacDonald, P. Alexander Veneman, Diogenes Placencia, Neal R. Armstrong

Research output: Contribution to journalArticle

34 Scopus citations

Abstract

We demonstrate mapping of electrical properties of heterojunctions of a molecular semiconductor (copper phthalocyanine, CuPc) and a transparent conducting oxide (indium-tin oxide, ITO), on 20-500 nm length scales, using a conductive-probe atomic force microscopy technique, scanning current spectroscopy (SCS). SCS maps are generated for CuPc/ITO heterojunctions as a function of ITO activation procedures and modification with variable chain length alkyl-phosphonic acids (PAs). We correlate differences in small length scale electrical properties with the performance of organic photovoltaic cells (OPVs) based on CuPc/C60 heterojunctions, built on these same ITO substrates. SCS maps the "ohmicity" of ITO/CuPc heterojunctions, creating arrays of spatially resolved current-voltage (J-V) curves. Each J-V curve is fit with modified Mott-Gurney expressions, mapping a fitted exponent (γ), where deviations from γ = 2.0 suggest nonohmic behavior. ITO/CuPc/C60/BCP/Al OPVs built on nonactivated ITO show mainly nonohmic SCS maps and dark J-V curves with increased series resistance (R S), lowered fill-factors (FF), and diminished device performance, especially near the open-circuit voltage. Nearly optimal behavior is seen for OPVs built on oxygen-plasma-treated ITO contacts, which showed SCS maps comparable to heterojunctions of CuPc on clean Au. For ITO electrodes modified with PAs there is a strong correlation between PA chain length and the degree of ohmicity and uniformity of electrical response in ITO/CuPc heterojunctions. ITO electrodes modified with 6-8 carbon alkyl-PAs show uniform and nearly ohmic SCS maps, coupled with acceptable CuPc/C60OPV performance. ITO modified with C14 and C18 alkyl-PAs shows dramatic decreases in FF, increases in R S, and greatly enhanced recombination losses.

Original languageEnglish (US)
Pages (from-to)9623-9636
Number of pages14
JournalACS Nano
Volume6
Issue number11
DOIs
StatePublished - Nov 27 2012

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Keywords

  • conducting-tip atomic force microscopy
  • injection efficiency
  • ohmic contacts
  • organic semiconductor
  • organic solar cell

ASJC Scopus subject areas

  • Materials Science(all)
  • Engineering(all)
  • Physics and Astronomy(all)

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