Axially Bound Ruthenium Phthalocyanine Monolayers on Indium Tin Oxide: Structure, Energetics, and Charge Transfer Properties

Ramanan Ehamparam, Luis E. Oquendo, Michael W. Liao, Ambjorn K. Brynnel, Kai Lin Ou, Neal R. Armstrong, Dominic V. McGrath, S. Scott Saavedra

Research output: Research - peer-reviewArticle

Abstract

The efficiency of charge collection at the organic/transparent conducting oxide (TCO) interface in organic photovoltaic (OPV) devices affects overall device efficiency. Modifying the TCO with an electrochemically active molecule may enhance OPV efficiency by providing a charge-transfer pathway between the electrode and the organic active layer, and may also mitigate surface recombination. The synthesis and characterization of phosphonic acid-ruthenium phthalocyanine (RuPcPA) monolayer films on indium tin oxide (ITO), designed to facilitate charge harvesting at ITO electrodes, is presented in this work. The PA group was installed axially relative to the Pc plane so that upon deposition, RuPcPA molecules were preferentially aligned with the ITO surface plane. The tilt angle of 22° between the normal axes to the Pc plane and the ITO surface plane, measured by attenuated total reflectance (ATR) spectroscopy, is consistent with a predominately in-plane orientation. The effect of surface roughness on RuPcPA orientation was modeled, and a correlation was obtained between experimental and theoretical mean tilt angles. Based on electrochemical and spectroelectrochemical studies, RuPcPA monolayers are composed predominately of monomers. Electrochemical impedance spectroscopy (EIS) and potential modulated-ATR (PM-ATR) spectroscopy were used to characterize the electron-transfer (ET) kinetics of these monolayers. A rate constant of 4.0 × 103 s-1 was measured using EIS, consistent with a short tunneling distance between the chromophore and the electrode surface. Using PM-ATR, ks,opt values of 2.2 × 103 and 2.4 × 103 s-1 were measured using TE and TM polarized light, respectively; the similarity of these values is consistent with a narrow molecular orientation distribution and narrow range of tunneling distances. The ionization potential of RuPcPA-modified ITO was measured using ultraviolet photoelectron spectroscopy and the results indicate favorable energetics for hole collection at the RuPcPA/ITO interface, indicating that this type of TCO modification may be useful for enhancing charge collection efficiency in OPV devices.

LanguageEnglish (US)
Pages29213-29223
Number of pages11
JournalACS Applied Materials and Interfaces
Volume9
Issue number34
DOIs
StatePublished - Aug 30 2017

Fingerprint

Tin oxides
Ruthenium
Indium
Charge transfer
Monolayers
indium tin oxide
ruthenium phthalocyanine
Electrodes
Oxides
Electrochemical impedance spectroscopy
Spectroscopy
Molecules
Ultraviolet photoelectron spectroscopy
Molecular orientation
Ionization potential
Light polarization
Chromophores
Rate constants
Monomers
Surface roughness

Keywords

  • ATR spectroelectrochemistry
  • electron-transfer kinetics
  • indium tin oxide
  • molecular orientation
  • organic photovoltaics
  • phosphonic acid
  • ruthenium phthalocyanine

ASJC Scopus subject areas

  • Materials Science(all)

Cite this

Axially Bound Ruthenium Phthalocyanine Monolayers on Indium Tin Oxide : Structure, Energetics, and Charge Transfer Properties. / Ehamparam, Ramanan; Oquendo, Luis E.; Liao, Michael W.; Brynnel, Ambjorn K.; Ou, Kai Lin; Armstrong, Neal R.; McGrath, Dominic V.; Saavedra, S. Scott.

In: ACS Applied Materials and Interfaces, Vol. 9, No. 34, 30.08.2017, p. 29213-29223.

Research output: Research - peer-reviewArticle

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