Zinc Phthalocyanine–Phosphonic Acid Monolayers on ITO: Influence of Molecular Orientation, Aggregation, and Tunneling Distance on Charge-Transfer Kinetics

Oquendo, Luis E.; Ehamparam, Ramanan; Armstrong, Neal R.; Saavedra, S. Scott; McGrath, Dominic V.

doi:10.1021/acs.jpcc.8b10301

Title: Zinc Phthalocyanine–Phosphonic Acid Monolayers on ITO: Influence of Molecular Orientation, Aggregation, and Tunneling Distance on Charge-Transfer Kinetics

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Abstract

Efficient charge harvesting at electrodes is critical for the effective performance of organic photovoltaics and is strongly influenced by the first molecular monolayer at the transparent conducting oxide electrode. In this work, we introduce a study of the relationship between molecular orientation and tunneling distance on charge-transfer rates between a tethered sub-monolayer/monolayer of phosphonic acid (PA)-functionalized Zn phthalocyanines (ZnPcs) and an indium tin oxide (ITO) surface by a combination of electrochemical techniques and waveguide tools such as attenuated total reflectance (ATR) UV–vis spectroscopy and potential-modulated ATR (PM-ATR) spectroelectrochemistry. The distance between the main chromophore and PA anchoring group was modulated by an aliphatic carbon of various lengths (n = 3, 9, 10, or 11) that resulted in an extended distance of 7–18 Å between the ZnPcs and the ITO surface. Modified ITO surfaces were composed of monomeric and aggregated subpopulations of ZnPcs with molecular orientations predominantly in-plane (36–39°) and out-of-plane (72–75°), respectively. Charge-transfer rate constants (k_s,opt) were measured using PM-ATR. For a given tether length, the aggregated subpopulations exhibit higher ks,opt values compared to the monomeric subpopulations. Furthermore, the observed k_s,opt values had an exponential dependence on the effective tunneling distance with a decay constant (β) that ranged from 0.32more »« less

Authors:

Oquendo, Luis E. ^[1]; Ehamparam, Ramanan ^[1]; Armstrong, Neal R. ^[1];

^[1];

^[1]

Univ. of Arizona, Tucson, AZ (United States)

Publication Date:: Thu Feb 28 00:00:00 EST 2019

Research Org.:: Univ. of Texas, Austin, TX (United States). Energy Frontier Research Center (EFRC); Univ. of Arizona, Tucson, AZ (United States)

Sponsoring Org.:: USDOE Office of Science (SC), Basic Energy Sciences (BES)

OSTI Identifier:: 1566524

Grant/Contract Number:: SC0001084

Resource Type:: Accepted Manuscript

Journal Name:: Journal of Physical Chemistry. C

Additional Journal Information:: Journal Volume: 123; Journal Issue: 12; Journal ID: ISSN 1932-7447

Publisher:: American Chemical Society

Country of Publication:: United States

Language:: English

Subject:: 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; solar (photovoltaic); electrodes - solar; charge transport; synthesis (novel materials); synthesis (self-assembly); synthesis (scalable processing)

Citation Formats


                    Oquendo, Luis E., Ehamparam, Ramanan, Armstrong, Neal R., Saavedra, S. Scott, and McGrath, Dominic V. Zinc Phthalocyanine–Phosphonic Acid Monolayers on ITO: Influence of Molecular Orientation, Aggregation, and Tunneling Distance on Charge-Transfer Kinetics.  United States: N. p., 2019. 
Web.  doi:10.1021/acs.jpcc.8b10301.

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                    Oquendo, Luis E., Ehamparam, Ramanan, Armstrong, Neal R., Saavedra, S. Scott, & McGrath, Dominic V. Zinc Phthalocyanine–Phosphonic Acid Monolayers on ITO: Influence of Molecular Orientation, Aggregation, and Tunneling Distance on Charge-Transfer Kinetics.  United States.  https://doi.org/10.1021/acs.jpcc.8b10301

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                    Oquendo, Luis E., Ehamparam, Ramanan, Armstrong, Neal R., Saavedra, S. Scott, and McGrath, Dominic V. Thu .  
"Zinc Phthalocyanine–Phosphonic Acid Monolayers on ITO: Influence of Molecular Orientation, Aggregation, and Tunneling Distance on Charge-Transfer Kinetics".  United States.  https://doi.org/10.1021/acs.jpcc.8b10301.  https://www.osti.gov/servlets/purl/1566524.

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@article{osti_1566524,

  title        = {Zinc Phthalocyanine–Phosphonic Acid Monolayers on ITO: Influence of Molecular Orientation, Aggregation, and Tunneling Distance on Charge-Transfer Kinetics},

  author       = {Oquendo, Luis E. and Ehamparam, Ramanan and Armstrong, Neal R. and Saavedra, S. Scott and McGrath, Dominic V.},

  abstractNote = {Efficient charge harvesting at electrodes is critical for the effective performance of organic photovoltaics and is strongly influenced by the first molecular monolayer at the transparent conducting oxide electrode. In this work, we introduce a study of the relationship between molecular orientation and tunneling distance on charge-transfer rates between a tethered sub-monolayer/monolayer of phosphonic acid (PA)-functionalized Zn phthalocyanines (ZnPcs) and an indium tin oxide (ITO) surface by a combination of electrochemical techniques and waveguide tools such as attenuated total reflectance (ATR) UV–vis spectroscopy and potential-modulated ATR (PM-ATR) spectroelectrochemistry. The distance between the main chromophore and PA anchoring group was modulated by an aliphatic carbon of various lengths (n = 3, 9, 10, or 11) that resulted in an extended distance of 7–18 Å between the ZnPcs and the ITO surface. Modified ITO surfaces were composed of monomeric and aggregated subpopulations of ZnPcs with molecular orientations predominantly in-plane (36–39°) and out-of-plane (72–75°), respectively. Charge-transfer rate constants (ks,opt) were measured using PM-ATR. For a given tether length, the aggregated subpopulations exhibit higher ks,opt values compared to the monomeric subpopulations. Furthermore, the observed ks,opt values had an exponential dependence on the effective tunneling distance with a decay constant (β) that ranged from 0.32 to 0.47 Å–1, depending on the chromophore orientation and aggregation state. The fastest charge-transfer rate constants were found for the chromophores with the smallest tunneling distance (n = 3). A ks,opt of 3.9 × 104 s–1 represents the fastest rate constant measured by PM-ATR for a PA-functionalized ZnPc chromophore tethered to an ITO waveguide electrode.},

  doi          = {10.1021/acs.jpcc.8b10301},

  journal      = {Journal of Physical Chemistry. C},

  number       = 12,

  volume       = 123,

  place        = {United States},

  year         = {Thu Feb 28 00:00:00 EST 2019},

  month        = {Thu Feb 28 00:00:00 EST 2019}

}

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