1
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Zheng Y, Venkatesh R, Rojas-Gatjens E, Reichmanis E, Silva-Acuña C. Exciton Bimolecular Annihilation Dynamics in Push-Pull Semiconductor Polymers. J Phys Chem Lett 2024; 15:272-280. [PMID: 38166236 PMCID: PMC10788955 DOI: 10.1021/acs.jpclett.3c03094] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/03/2023] [Revised: 12/17/2023] [Accepted: 12/28/2023] [Indexed: 01/04/2024]
Abstract
Exciton-exciton annihilation is a ubiquitous nonlinear dynamic phenomenon in materials hosting Frenkel excitons. In this work, we investigate the nonlinear exciton dynamics of an electron push-pull conjugated polymer by fluence-dependent transient absorption and excitation-correlation photoluminescence spectroscopy, where we can quantitatively show the latter to be a more selective probe of the nonlinear dynamics. Simulations based on a time-independent exciton annihilation model show a decreasing trend for the extracted annihilation rates with excitation fluence. Further investigation of the fluence-dependent transients suggests that the exciton-exciton annihilation bimolecular rates are not constant in time, displaying a t-1/2 time dependence, which we rationalize as reflective of one-dimensional exciton diffusion, with a diffusion length estimated to be 9 ± 2 nm. In addition, exciton annihilation gives rise to a long-lived species that recombines on a nanosecond time scale. Our conclusions shed broad light onto nonlinear exciton dynamics in push-pull conjugated polymers.
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Affiliation(s)
- Yulong Zheng
- School
of Chemistry and Biochemistry, Georgia Institute
of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States
| | - Rahul Venkatesh
- School
of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, United States
| | - Esteban Rojas-Gatjens
- School
of Chemistry and Biochemistry, Georgia Institute
of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States
| | - Elsa Reichmanis
- Department
of Chemical & Biomolecular Engineering, Lehigh University, 124 E. Morton Street, Bethlehem, Pennsylvania 18015, United States
| | - Carlos Silva-Acuña
- School
of Chemistry and Biochemistry, Georgia Institute
of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States
- Institut
Courtois & Département de physique, Université de Montréal, 1375 Avenue Thérèse-Lavoie-Roux, Montréal H2V 0B3, Québec, Canada
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2
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Zheng Y, Venkatesh R, Callaway CP, Viersen C, Fagbohungbe KH, Liu AL, Risko C, Reichmanis E, Silva-Acuña C. Chain Conformation and Exciton Delocalization in a Push-Pull Conjugated Polymer. CHEMISTRY OF MATERIALS : A PUBLICATION OF THE AMERICAN CHEMICAL SOCIETY 2023; 35:10258-10267. [PMID: 38107193 PMCID: PMC10720347 DOI: 10.1021/acs.chemmater.3c02665] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 10/17/2023] [Accepted: 10/24/2023] [Indexed: 12/19/2023]
Abstract
Linear and nonlinear optical line shapes reveal details of excitonic structure in polymer semiconductors. We implement absorption, photoluminescence, and transient absorption spectroscopies in DPP-DTT, an electron push-pull copolymer, to explore the relationship between their spectral line shapes and chain conformation, deduced from resonance Raman spectroscopy and from ab initio calculations. The viscosity of precursor polymer solutions before film casting displays a transition that suggests gel formation above a critical concentration. Upon crossing this viscosity deflection concentration, the line shape analysis of the absorption spectra within a photophysical aggregate model reveals a gradual increase in interchain excitonic coupling. We also observe a red-shifted and line-narrowed steady-state photoluminescence spectrum along with increasing resonance Raman intensity in the stretching and torsional modes of the dithienothiophene unit, which suggests a longer exciton coherence length along the polymer-chain backbone. Furthermore, we observe a change of line shape in the photoinduced absorption component of the transient absorption spectrum. The derivative-like line shape may originate from two possibilities: a new excited-state absorption or Stark effect, both of which are consistent with the emergence of a high-energy shoulder as seen in both photoluminescence and absorption spectra. Therefore, we conclude that the exciton is more dispersed along the polymer chain backbone with increasing concentrations, leading to the hypothesis that polymer chain order is enhanced when the push-pull polymers are processed at higher concentrations. Thus, tuning the microscopic chain conformation by concentration would be another factor of interest when considering the polymer assembly pathways for pursuing large-area and high-performance organic optoelectronic devices.
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Affiliation(s)
- Yulong Zheng
- School
of Chemistry and Biochemistry, Georgia Institute
of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States
| | - Rahul Venkatesh
- School
of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, United States
| | - Connor P. Callaway
- Department
of Chemistry and Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40506, United States
| | - Campbell Viersen
- School
of Chemistry and Biochemistry, Georgia Institute
of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States
| | - Kehinde H. Fagbohungbe
- Department
of Chemistry and Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40506, United States
| | - Aaron L. Liu
- School
of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive NW, Atlanta, Georgia 30332, United States
| | - Chad Risko
- Department
of Chemistry and Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40506, United States
| | - Elsa Reichmanis
- Department
of Chemical & Biomolecular Engineering, Lehigh University, 124 East Morton Street, Bethlehem, Pennsylvania 18015, United States
| | - Carlos Silva-Acuña
- School
of Chemistry and Biochemistry, Georgia Institute
of Technology, 901 Atlantic Drive, Atlanta, Georgia 30332, United States
- School
of Physics, Georgia Institute of Technology, 837 State Street, Atlanta, Georgia 30332, United States
- School
of Materials Science and Engineering, Georgia
Institute of Technology, North Avenue, Atlanta, Georgia 30332, United States
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3
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Kumar S, Dunn IS, Deng S, Zhu T, Zhao Q, Williams OF, Tempelaar R, Huang L. Exciton annihilation in molecular aggregates suppressed through qu antum interference. Nat Chem 2023:10.1038/s41557-023-01233-x. [PMID: 37337112 DOI: 10.1038/s41557-023-01233-x] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/06/2022] [Accepted: 05/05/2023] [Indexed: 06/21/2023]
Abstract
Exciton-exciton annihilation (EEA), an important loss channel in optoelectronic devices and photosynthetic complexes, has conventionally been assumed to be an incoherent, diffusion-limited process. Here we challenge this assumption by experimentally demonstrating the ability to control EEA in molecular aggregates using the quantum phase relationships of excitons. We employed time-resolved photoluminescence microscopy to independently determine exciton diffusion constants and annihilation rates in two substituted perylene diimide aggregates featuring contrasting excitonic phase envelopes. Low-temperature EEA rates were found to differ by more than two orders of magnitude for the two compounds, despite comparable diffusion constants. Simulated rates based on a microscopic theory, in excellent agreement with experiments, rationalize this EEA behaviour based on quantum interference arising from the presence or absence of spatial phase oscillations of delocalized excitons. These results offer an approach for designing molecular materials using quantum interference where low annihilation can coexist with high exciton concentrations and mobilities.
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Affiliation(s)
- Sarath Kumar
- Department of Chemistry, Purdue University, West Lafayette, IN, USA
| | - Ian S Dunn
- Department of Chemistry, Columbia University, New York, NY, USA
| | - Shibin Deng
- Department of Chemistry, Purdue University, West Lafayette, IN, USA
| | - Tong Zhu
- Laser Micro/Nano Fabrication Laboratory, School of Mechanical Engineering, Beijing Institute of Technology, Beijing, People's Republic of China
| | - Qiuchen Zhao
- Department of Chemistry, Purdue University, West Lafayette, IN, USA
| | | | - Roel Tempelaar
- Department of Chemistry, Northwestern University, Evanston, IL, USA.
| | - Libai Huang
- Department of Chemistry, Purdue University, West Lafayette, IN, USA.
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4
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Glaab F, Süβ J, Engel V. Third-order pump-probe spectroscopy applied to molecular dimers: characterization of relaxation dynamics and exciton–exciton annihilation. Phys Chem Chem Phys 2022; 24:25316-25326. [DOI: 10.1039/d2cp03435b] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Abstract
Exciton–exciton annihilation in a dimer, described within the basis of localizes monomer states.
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Affiliation(s)
- Fabian Glaab
- Universität Würzburg, Institut für Physikalische und Theoretische Chemie, Emil-Fischer-Str. 42, 97074 Würzburg, Germany
| | - Jasmin Süβ
- Universität Würzburg, Institut für Physikalische und Theoretische Chemie, Emil-Fischer-Str. 42, 97074 Würzburg, Germany
| | - Volker Engel
- Universität Würzburg, Institut für Physikalische und Theoretische Chemie, Emil-Fischer-Str. 42, 97074 Würzburg, Germany
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5
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Wei A, Lv S, Zhang Y, Xia C, Wang L. The configuration effect on the exciton dynamics of zinc chlorin aggregates. Phys Chem Chem Phys 2021; 23:25769-25775. [PMID: 34755738 DOI: 10.1039/d1cp03127a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022]
Abstract
Excitonic energy transfer among the zinc chlorin molecules is significant for the photovoltaic process because of their high sensitivities to harvesting sunlight. Zinc chlorin monomers and dimers can be synthesized experimentally, and they can form various self-assembled structures. Using the realistic parameters of zinc chlorin molecules, we assume that 20 molecules with J-, H- or J-H aggregation are arranged in a line and we investigate their dipole configuration effect on exciton dynamics. The expectation value approximation of operators is applied to derive the equations of motion of multi-exciton states. The temporal evolution of multi-exciton states is analyzed in the scheme of density matrix theory. Our simulations show that the inter-molecular coupling results in an exciton band and the wave-packet progressing excited by the resonant laser pulse exhibits attractive or repulsive behavior at the exciton level due to the dipole configuration effect. In the defined J-H coupling, the coherent wave-packet cannot overcome the configuration barrier to the no-excited part. The exciton dynamics revealed here might be helpful to better understand the energy transfer process in organic photovoltaic devices.
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Affiliation(s)
- An Wei
- Institute of Theoretical Physics, Department of Physics, University of Science and Technology, Beijing, Beijing 100083, China.
| | - Siyuan Lv
- Institute of Theoretical Physics, Department of Physics, University of Science and Technology, Beijing, Beijing 100083, China.
| | - Yuan Zhang
- Henan Key Laboratory of Diamond Optoelectronic Materials and Devices, Key Laboratory of Material Physics, Ministry of Education, School of Physics and Microelectronics, Zhengzhou University, Zhengzhou 450052, China.
| | - Caijuan Xia
- School of Science, Xi'an Polytechnic University, Xi'an 710048, China
| | - Luxia Wang
- Institute of Theoretical Physics, Department of Physics, University of Science and Technology, Beijing, Beijing 100083, China.
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6
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Süß J, Engel V. Exciton-exciton annihilation in a molecular trimer: Wave packet dynamics and 2D spectroscopy. J Chem Phys 2020; 153:164310. [PMID: 33138437 DOI: 10.1063/5.0027837] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
We theoretically study the exciton-exciton annihilation (EEA) in a molecular trimer MMM. The system is treated within a model of electronic states, and the coupling to a bath is incorporated using the quantum jump method. Two situations of initial excitation are compared. In the first one, a two-photon process populates configurations M*M*M and MM*M* so that two excitons reside on neighboring monomers M. Then, EEA can immediately proceed. In contrast, if the trimer initially is in the local configuration M*MM*, exciton diffusion must occur before the annihilation process can take place. For the trimer, this excitonic motion takes place on a very short time scale. In both cases, wave packets are prepared which show a different quantum dynamics where the latter depends on the couplings and decay rates. It is documented how fifth-order coherent two-dimensional spectroscopy can be used to directly map the EEA as a function of time.
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Affiliation(s)
- J Süß
- Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Emil-Fischer-Str. 42, 97074 Würzburg, Germany
| | - V Engel
- Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Emil-Fischer-Str. 42, 97074 Würzburg, Germany
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7
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Süß J, Engel V. A wave packet picture of exciton-exciton annihilation: Molecular dimer dynamics. J Chem Phys 2020; 152:174305. [PMID: 32384841 DOI: 10.1063/1.5134534] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Abstract
The usual view of exciton-exciton annihilation (EEA) processes in molecular aggregates is based on locally excited states of the monomer units. However, the corresponding localized configurations can only be assumed if the system is in a coherent superposition of eigenstates, i.e., a wave packet. We study a molecular dimer and focus on the characterization of EEA by a wave packet motion induced in the system by ultrashort pulse excitation. Here, coherences that appear are destroyed by dissipation processes. We discuss the influence of interband and intraband relaxation on the dynamics. The states that participate in the annihilation process are directly accessible by fifth-order optical two-dimensional spectroscopy. Such spectra are calculated, and spectral features are related to the annihilation process.
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Affiliation(s)
- J Süß
- Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Emil-Fischer-Str. 42, 97074 Würzburg, Germany
| | - V Engel
- Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Emil-Fischer-Str. 42, 97074 Würzburg, Germany
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8
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Heshmatpour C, Hauer J, Šanda F. Interplay of exciton annihilation and transport in fifth order electronic spectroscopy. Chem Phys 2020. [DOI: 10.1016/j.chemphys.2019.110433] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/26/2022]
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9
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Süß J, Wehner J, Dostál J, Brixner T, Engel V. Mapping of exciton–exciton annihilation in a molecular dimer via fifth-order femtosecond two-dimensional spectroscopy. J Chem Phys 2019; 150:104304. [DOI: 10.1063/1.5086151] [Citation(s) in RCA: 16] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
Affiliation(s)
- J. Süß
- Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Emil-Fischer-Str. 42, 97074 Würzburg, Germany
| | - J. Wehner
- Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Emil-Fischer-Str. 42, 97074 Würzburg, Germany
| | - J. Dostál
- Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany and Center for Nanosystems Chemistry (CNC), Theodor-Boveri-Weg, 97074 Würzburg, Germany
| | - T. Brixner
- Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany and Center for Nanosystems Chemistry (CNC), Theodor-Boveri-Weg, 97074 Würzburg, Germany
| | - V. Engel
- Institut für Physikalische und Theoretische Chemie, Universität Würzburg, Emil-Fischer-Str. 42, 97074 Würzburg, Germany
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10
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Schreck MH, Breitschwerdt L, Marciniak H, Holzapfel M, Schmidt D, Würthner F, Lambert C. fs–ps Exciton dynamics in a stretched tetraphenylsquaraine polymer. Phys Chem Chem Phys 2019; 21:15346-15355. [DOI: 10.1039/c9cp02900a] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
A squaraine polymer shows surprisingly fast light induced energy transfer between two different structural sections on the ps/fs time scale.
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Affiliation(s)
- Maximilian H. Schreck
- Institute of Organic Chemistry
- Center for Nanosystems Chemistry
- Universität Würzburg
- D-97074 Würzburg
- Germany
| | - Lena Breitschwerdt
- Institute of Organic Chemistry
- Center for Nanosystems Chemistry
- Universität Würzburg
- D-97074 Würzburg
- Germany
| | - Henning Marciniak
- Institute of Organic Chemistry
- Center for Nanosystems Chemistry
- Universität Würzburg
- D-97074 Würzburg
- Germany
| | - Marco Holzapfel
- Institute of Organic Chemistry
- Center for Nanosystems Chemistry
- Universität Würzburg
- D-97074 Würzburg
- Germany
| | - David Schmidt
- Institute of Organic Chemistry
- Center for Nanosystems Chemistry
- Universität Würzburg
- D-97074 Würzburg
- Germany
| | - Frank Würthner
- Institute of Organic Chemistry
- Center for Nanosystems Chemistry
- Universität Würzburg
- D-97074 Würzburg
- Germany
| | - Christoph Lambert
- Institute of Organic Chemistry
- Center for Nanosystems Chemistry
- Universität Würzburg
- D-97074 Würzburg
- Germany
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11
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Tempelaar R, Jansen TLC, Knoester J. Exciton-Exciton Annihilation Is Coherently Suppressed in H-Aggregates, but Not in J-Aggregates. J Phys Chem Lett 2017; 8:6113-6117. [PMID: 29190421 PMCID: PMC5742477 DOI: 10.1021/acs.jpclett.7b02745] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
We theoretically demonstrate a strong dependence of the annihilation rate between (singlet) excitons on the sign of dipole-dipole couplings between molecules. For molecular H-aggregates, where this sign is positive, the phase relation of the delocalized two-exciton wave functions causes a destructive interference in the annihilation probability. For J-aggregates, where this sign is negative, the interference is constructive instead; as a result, no such coherent suppression of the annihilation rate occurs. As a consequence, room temperature annihilation rates of typical H- and J-aggregates differ by a factor of ∼3, while an order of magnitude difference is found for low-temperature aggregates with a low degree of disorder. These findings, which explain experimental observations, reveal a fundamental principle underlying exciton-exciton annihilation, with major implications for technological devices and experimental studies involving high excitation densities.
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Affiliation(s)
- Roel Tempelaar
- Zernike
Institute for Advanced Materials, University
of Groningen, Nijenborgh
4, 9747 AG Groningen, The Netherlands
- Department
of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, United States
- E-mail:
| | - Thomas L. C. Jansen
- Zernike
Institute for Advanced Materials, University
of Groningen, Nijenborgh
4, 9747 AG Groningen, The Netherlands
| | - Jasper Knoester
- Zernike
Institute for Advanced Materials, University
of Groningen, Nijenborgh
4, 9747 AG Groningen, The Netherlands
- E-mail:
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12
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Hader K, Consani C, Brixner T, Engel V. Mapping of exciton–exciton annihilation in MEH-PPV by time-resolved spectroscopy: experiment and microscopic theory. Phys Chem Chem Phys 2017; 19:31989-31996. [DOI: 10.1039/c7cp05168a] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Transient absorption traces taken on samples of the polymer MEH-PPV are measured as a function of the laser intensity.
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Affiliation(s)
- Kilian Hader
- Institut für Physikalische und Theoretische Chemie
- Universität Würzburg
- 97074 Würzburg
- Germany
| | - Cristina Consani
- Institut für Physikalische und Theoretische Chemie
- Universität Würzburg
- 97074 Würzburg
- Germany
- Center for Nanosystems Chemistry (CNC)
| | - Tobias Brixner
- Institut für Physikalische und Theoretische Chemie
- Universität Würzburg
- 97074 Würzburg
- Germany
- Center for Nanosystems Chemistry (CNC)
| | - Volker Engel
- Institut für Physikalische und Theoretische Chemie
- Universität Würzburg
- 97074 Würzburg
- Germany
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13
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Hader K, May V, Lambert C, Engel V. Identification of effective exciton–exciton annihilation in squaraine–squaraine copolymers. Phys Chem Chem Phys 2016; 18:13368-74. [DOI: 10.1039/c6cp01780k] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Within a microscopic model, exciton annihilation processes in squaraine–squaraine copolymers are identified and the unusual time-dependence of transient absorption time traces is explained.
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Affiliation(s)
- Kilian Hader
- Universität Würzburg
- Institut für Physikalische und Theoretische Chemie
- 97074 Würzburg
- Germany
| | - Volkhard May
- Institut für Physik
- Humboldt Universität
- D-12489 Berlin
- Germany
| | - Christoph Lambert
- Universität Würzburg
- Institut für Organische Chemie
- 97074 Würzburg
- Germany
| | - Volker Engel
- Universität Würzburg
- Institut für Physikalische und Theoretische Chemie
- 97074 Würzburg
- Germany
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14
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Pensack RD, Tilley AJ, Parkin SR, Lee TS, Payne MM, Gao D, Jahnke AA, Oblinsky DG, Li PF, Anthony JE, Seferos DS, Scholes GD. Exciton Delocalization Drives Rapid Singlet Fission in Nanoparticles of Acene Derivatives. J Am Chem Soc 2015; 137:6790-803. [DOI: 10.1021/ja512668r] [Citation(s) in RCA: 171] [Impact Index Per Article: 19.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Affiliation(s)
- Ryan D. Pensack
- Department
of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
| | - Andrew J. Tilley
- Department
of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
| | - Sean R. Parkin
- Department
of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States
| | - Tia S. Lee
- Department
of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
| | - Marcia M. Payne
- Department
of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States
| | - Dong Gao
- Department
of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
| | - Ashlee A. Jahnke
- Department
of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
| | - Daniel G. Oblinsky
- Department
of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
| | - Peng-Fei Li
- Department
of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
| | - John E. Anthony
- Department
of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States
| | - Dwight S. Seferos
- Department
of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
| | - Gregory D. Scholes
- Department
of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada
- Department
of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
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15
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Savoie BM, Jackson NE, Chen LX, Marks TJ, Ratner MA. Mesoscopic features of charge generation in organic semiconductors. Acc Chem Res 2014; 47:3385-94. [PMID: 25051395 DOI: 10.1021/ar5000852] [Citation(s) in RCA: 68] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/15/2023]
Abstract
CONSPECTUS: In the past two decades, organic materials have been extensively investigated by numerous research groups worldwide for implementation in organic photovoltaic (OPV) devices. The interest in organic semiconductors is spurred by their potential low cost and facile tunability, making OPV devices a potentially disruptive technology. To study OPV operating mechanisms is also to explore a knowledge gap in our general understanding of materials, because both the time scales (femtosecond to microsecond) and length scales (nanometer to micrometer) relevant to OPV functionality occupy a challenging and fascinating space between the traditional regimes of quantum chemistry and solid-state physics. New theoretical frameworks and computational tools are needed to bridge the aforementioned length and time scales, and they must satisfy the criteria of computational tractability for systems involving 10(4)-10(6) atoms, while also maintaining predictive utility. While this challenge is far from solved, advances in density functional theory (DFT) have allowed researchers to investigate the ground- and excited-state properties of many intermediate sized systems (10(2)-10(3) atoms) that provide the outlines of the larger problem. Results on these smaller systems are already sufficient to predict optical gaps and trends in valence band energies, correct erroneous interpretations of experimental data, and develop models for charge generation and transport in OPV devices. The active films of high-efficiency OPV devices are comprised of mesoscopic mixtures of electron donor (D) and electron acceptor (A) species, a "bulk-heterojunction" (BHJ) device, subject to variable degrees of structural disorder. Depending on the degree of intermolecular electronic coupling and energy level alignment, the spatial delocalization of photoexcitations and charge carriers can affect the dynamics of the solar cell. In this Account, we provide an overview of three pivotal characteristics of solar cells that possess strong delocalization dependence: (1) the exciton binding energy, (2) charge transfer at the D-A heterojunction, and (3) the energy landscape in the vicinity of the D-A heterojunction. In each case, the length scale dependence can be assessed through DFT calculations on reference systems, with a view to establishing general trends. Throughout the discussion, we draw from the experimental and theoretical literature to provide a consistent view of what is known about these properties in actual BHJ blends. A consistent interpretation of the results to date affords the following view: transient delocalization effects and resonant charge transfer at the heterojunction are capable of funneling excitations away from trap states and mediating exciton dissociation; these factors alone are capable of explaining the remarkably good charge generation currently achieved in OPV devices. The exciton binding energy likely plays a minimal role in modern OPV devices, since the presence of the heterojunction serves to bypass the costly exciton-to-free-charge transition state.
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Affiliation(s)
- Brett M. Savoie
- Department
of Chemistry, the Materials Research Center, and the Argonne-Northwestern Solar Energy Research
Center, Northwestern University, Evanston, Illinois 60208, United States
| | - Nicholas E. Jackson
- Department
of Chemistry, the Materials Research Center, and the Argonne-Northwestern Solar Energy Research
Center, Northwestern University, Evanston, Illinois 60208, United States
| | - Lin X. Chen
- Department
of Chemistry, the Materials Research Center, and the Argonne-Northwestern Solar Energy Research
Center, Northwestern University, Evanston, Illinois 60208, United States
| | - Tobin J. Marks
- Department
of Chemistry, the Materials Research Center, and the Argonne-Northwestern Solar Energy Research
Center, Northwestern University, Evanston, Illinois 60208, United States
| | - Mark A. Ratner
- Department
of Chemistry, the Materials Research Center, and the Argonne-Northwestern Solar Energy Research
Center, Northwestern University, Evanston, Illinois 60208, United States
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Völker SF, Schmiedel A, Holzapfel M, Böhm C, Lambert C. Charge transfer dynamics in squaraine-naphthalene diimide copolymers. Phys Chem Chem Phys 2013; 15:19831-44. [PMID: 24145596 DOI: 10.1039/c3cp53455c] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Abstract
The synthesis of an alternating squaraine-naphthalene diimide donor-acceptor low band gap polymer (1.14-1.40 eV) as well as its monomolecular analogue is presented. Spectroelectrochemistry experiments and transient absorption spectroscopy in the fs-time regime reveal an ultrafast population of a charge separated state for both polymer and monomer. Local excitation of the squaraine moiety is followed by population of intermediate states, presumably charge transfer states, followed by full charge separation, which occurs within a ca. 2 ps. Charge recombination takes place within 5.2 ps, probably because the system is close to the Marcus optimal region for barrierless ET. For the polymer, measurements of the transient absorption anisotropy show that neither charge nor does energy transfer take place within the lifetime of the charge separated state, indicating that this state is essentially confined within one donor-acceptor pair.
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Affiliation(s)
- Sebastian F Völker
- Institut für Organische Chemie and Center for Nanosystems Chemistry, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany.
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