Revealing the Importance of Energetic and Entropic Contributions to the Driving Force for Charge Photogeneration

Melissa P. Aplan, Jason M. Munro, Youngmin Lee, Alyssa N. Brigeman, Christopher Grieco, Qing Wang, Noel Christopher Giebink, Ismaila Dabo, John B. Asbury, Enrique Daniel Gomez

Research output: Contribution to journalArticle

3 Citations (Scopus)

Abstract

Despite significant recent progress, much about the mechanism for charge photogeneration in organic photovoltaics remains unknown. Here, we use conjugated block copolymers as model systems to examine the effects of energetic and entropic driving forces in organic donor-acceptor materials. The block copolymers are designed such that an electron donor block and an electron acceptor block are covalently linked, embedding a donor-acceptor interface within the molecular structure. This enables model studies in solution where processes occurring between one donor and one acceptor are examined. First, energy levels and dielectric constants that govern the driving force for charge transfer are systematically tuned and charge transfer within individual block copolymer chains is quantified. Results indicate that in isolated chains, a significant driving force of ∼0.3 eV is necessary to facilitate significant exciton dissociation to charge-transfer states. Next, block copolymers are cast into films, allowing for intermolecular interactions and charge delocalization over multiple chains. In the solid state, charge transfer is significantly enhanced relative to isolated block copolymer chains. Using Marcus Theory, we conclude that changes in the energetic driving force alone cannot explain the increased efficiency of exciton dissociation to charge-transfer states in the solid state. This implies that increasing the number of accessible states for charge transfer introduces an entropic driving force that can play an important role in the charge-generation mechanism of organic materials, particularly in systems where the excited state energy level is close to that of the charge-transfer state.

Original languageEnglish (US)
Pages (from-to)39933-39941
Number of pages9
JournalACS Applied Materials and Interfaces
Volume10
Issue number46
DOIs
StatePublished - Nov 21 2018

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Charge transfer
Block copolymers
Excitons
Electron energy levels
Electrons
Excited states
Molecular structure
Permittivity

All Science Journal Classification (ASJC) codes

  • Materials Science(all)

Cite this

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title = "Revealing the Importance of Energetic and Entropic Contributions to the Driving Force for Charge Photogeneration",
abstract = "Despite significant recent progress, much about the mechanism for charge photogeneration in organic photovoltaics remains unknown. Here, we use conjugated block copolymers as model systems to examine the effects of energetic and entropic driving forces in organic donor-acceptor materials. The block copolymers are designed such that an electron donor block and an electron acceptor block are covalently linked, embedding a donor-acceptor interface within the molecular structure. This enables model studies in solution where processes occurring between one donor and one acceptor are examined. First, energy levels and dielectric constants that govern the driving force for charge transfer are systematically tuned and charge transfer within individual block copolymer chains is quantified. Results indicate that in isolated chains, a significant driving force of ∼0.3 eV is necessary to facilitate significant exciton dissociation to charge-transfer states. Next, block copolymers are cast into films, allowing for intermolecular interactions and charge delocalization over multiple chains. In the solid state, charge transfer is significantly enhanced relative to isolated block copolymer chains. Using Marcus Theory, we conclude that changes in the energetic driving force alone cannot explain the increased efficiency of exciton dissociation to charge-transfer states in the solid state. This implies that increasing the number of accessible states for charge transfer introduces an entropic driving force that can play an important role in the charge-generation mechanism of organic materials, particularly in systems where the excited state energy level is close to that of the charge-transfer state.",
author = "Aplan, {Melissa P.} and Munro, {Jason M.} and Youngmin Lee and Brigeman, {Alyssa N.} and Christopher Grieco and Qing Wang and Giebink, {Noel Christopher} and Ismaila Dabo and Asbury, {John B.} and Gomez, {Enrique Daniel}",
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Revealing the Importance of Energetic and Entropic Contributions to the Driving Force for Charge Photogeneration. / Aplan, Melissa P.; Munro, Jason M.; Lee, Youngmin; Brigeman, Alyssa N.; Grieco, Christopher; Wang, Qing; Giebink, Noel Christopher; Dabo, Ismaila; Asbury, John B.; Gomez, Enrique Daniel.

In: ACS Applied Materials and Interfaces, Vol. 10, No. 46, 21.11.2018, p. 39933-39941.

Research output: Contribution to journalArticle

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T1 - Revealing the Importance of Energetic and Entropic Contributions to the Driving Force for Charge Photogeneration

AU - Aplan, Melissa P.

AU - Munro, Jason M.

AU - Lee, Youngmin

AU - Brigeman, Alyssa N.

AU - Grieco, Christopher

AU - Wang, Qing

AU - Giebink, Noel Christopher

AU - Dabo, Ismaila

AU - Asbury, John B.

AU - Gomez, Enrique Daniel

PY - 2018/11/21

Y1 - 2018/11/21

N2 - Despite significant recent progress, much about the mechanism for charge photogeneration in organic photovoltaics remains unknown. Here, we use conjugated block copolymers as model systems to examine the effects of energetic and entropic driving forces in organic donor-acceptor materials. The block copolymers are designed such that an electron donor block and an electron acceptor block are covalently linked, embedding a donor-acceptor interface within the molecular structure. This enables model studies in solution where processes occurring between one donor and one acceptor are examined. First, energy levels and dielectric constants that govern the driving force for charge transfer are systematically tuned and charge transfer within individual block copolymer chains is quantified. Results indicate that in isolated chains, a significant driving force of ∼0.3 eV is necessary to facilitate significant exciton dissociation to charge-transfer states. Next, block copolymers are cast into films, allowing for intermolecular interactions and charge delocalization over multiple chains. In the solid state, charge transfer is significantly enhanced relative to isolated block copolymer chains. Using Marcus Theory, we conclude that changes in the energetic driving force alone cannot explain the increased efficiency of exciton dissociation to charge-transfer states in the solid state. This implies that increasing the number of accessible states for charge transfer introduces an entropic driving force that can play an important role in the charge-generation mechanism of organic materials, particularly in systems where the excited state energy level is close to that of the charge-transfer state.

AB - Despite significant recent progress, much about the mechanism for charge photogeneration in organic photovoltaics remains unknown. Here, we use conjugated block copolymers as model systems to examine the effects of energetic and entropic driving forces in organic donor-acceptor materials. The block copolymers are designed such that an electron donor block and an electron acceptor block are covalently linked, embedding a donor-acceptor interface within the molecular structure. This enables model studies in solution where processes occurring between one donor and one acceptor are examined. First, energy levels and dielectric constants that govern the driving force for charge transfer are systematically tuned and charge transfer within individual block copolymer chains is quantified. Results indicate that in isolated chains, a significant driving force of ∼0.3 eV is necessary to facilitate significant exciton dissociation to charge-transfer states. Next, block copolymers are cast into films, allowing for intermolecular interactions and charge delocalization over multiple chains. In the solid state, charge transfer is significantly enhanced relative to isolated block copolymer chains. Using Marcus Theory, we conclude that changes in the energetic driving force alone cannot explain the increased efficiency of exciton dissociation to charge-transfer states in the solid state. This implies that increasing the number of accessible states for charge transfer introduces an entropic driving force that can play an important role in the charge-generation mechanism of organic materials, particularly in systems where the excited state energy level is close to that of the charge-transfer state.

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