Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy

Thomson, Stuart A. J.; Niklas, Jens; Mardis, Kristy L.; Mallares, Christopher; Samuel, Ifor D. W.; Poluektov, Oleg G.

doi:10.1021/acs.jpcc.7b08217

Title: Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy

Abstract

Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh₂)₂, DTS(F2BTTh₂)₂, DTS(PTTh₂)₂, DTG(FBTTh₂)₂ and DTG(F2BTTh₂)₂) with the fullerene derivative PC₆₁BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh₂)₂ blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh₂)₂ blend wasmore »« less

Authors:

Thomson, Stuart A. J. ^[1];

^[2]; Mardis, Kristy L. ^[3]; Mallares, Christopher ^[3];

^[1]; Poluektov, Oleg G. ^[2]

Univ. of St. Andrews, St. Andrews (United Kingdom)
Argonne National Lab. (ANL), Argonne, IL (United States)
Chicago State Univ., Chicago, IL (United States)

Publication Date:: Wed Sep 13 00:00:00 EDT 2017

Research Org.:: Argonne National Laboratory (ANL), Argonne, IL (United States)

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

OSTI Identifier:: 1421951

Grant/Contract Number:: AC02-06CH11357

Resource Type:: Accepted Manuscript

Journal Name:: Journal of Physical Chemistry. C

Additional Journal Information:: Journal Volume: 121; Journal Issue: 41; Journal ID: ISSN 1932-7447

Publisher:: American Chemical Society

Country of Publication:: United States

Language:: English

Subject:: 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats


                    Thomson, Stuart A. J., Niklas, Jens, Mardis, Kristy L., Mallares, Christopher, Samuel, Ifor D. W., and Poluektov, Oleg G. Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy.  United States: N. p., 2017. 
Web.  doi:10.1021/acs.jpcc.7b08217.

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                    Thomson, Stuart A. J., Niklas, Jens, Mardis, Kristy L., Mallares, Christopher, Samuel, Ifor D. W., & Poluektov, Oleg G. Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy.  United States.  https://doi.org/10.1021/acs.jpcc.7b08217

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                    Thomson, Stuart A. J., Niklas, Jens, Mardis, Kristy L., Mallares, Christopher, Samuel, Ifor D. W., and Poluektov, Oleg G. Wed .  
"Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy".  United States.  https://doi.org/10.1021/acs.jpcc.7b08217.  https://www.osti.gov/servlets/purl/1421951.

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@article{osti_1421951,

  title        = {Charge Separation and Triplet Exciton Formation Pathways in Small-Molecule Solar Cells as Studied by Time-Resolved EPR Spectroscopy},

  author       = {Thomson, Stuart A. J. and Niklas, Jens and Mardis, Kristy L. and Mallares, Christopher and Samuel, Ifor D. W. and Poluektov, Oleg G.},

  abstractNote = {Organic solar cells are a promising renewable energy technology, offering the advantages of mechanical flexibility and solution processability. An understanding of the electronic excited states and charge separation pathways in these systems is crucial if efficiencies are to be further improved. Here we use light induced electron paramagnetic resonance (LEPR) spectroscopy and density functional theory calculations (DFT) to study the electronic excited states, charge transfer (CT) dynamics and triplet exciton formation pathways in blends of the small molecule donors (DTS(FBTTh2)2, DTS(F2BTTh2)2, DTS(PTTh2)2, DTG(FBTTh2)2 and DTG(F2BTTh2)2) with the fullerene derivative PC61BM. Using high frequency EPR the g-tensor of the positive polaron on the donor molecules was determined. The experimental results are compared with DFT calculations which reveal that the spin density of the polaron is distributed over a dimer or trimer. Time-resolved EPR (TR-EPR) spectra attributed to singlet CT states were identified and the polarization patterns revealed similar charge separation dynamics in the four fluorobenzothiadiazole donors, while charge separation in the DTS(PTTh2)2 blend is slower. Using TR-EPR we also investigated the triplet exciton formation pathways in the blend. The polarization patterns reveal that the excitons originate from both intersystem crossing (ISC) and back electron transfer (BET) processes. The DTS(PTTh2)2 blend was found to contain substantially more triplet excitons formed by BET than the fluorobenzothiadiazole blends. As a result, the higher BET triplet exciton population in the DTS(PTTh2)2 blend is in accordance with the slower charge separation dynamics observed in this blend.},

  doi          = {10.1021/acs.jpcc.7b08217},

  journal      = {Journal of Physical Chemistry. C},

  number       = 41,

  volume       = 121,

  place        = {United States},

  year         = {Wed Sep 13 00:00:00 EDT 2017},

  month        = {Wed Sep 13 00:00:00 EDT 2017}

}

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