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Balzer D, Kassal I. Even a little delocalization produces large kinetic enhancements of charge-separation efficiency in organic photovoltaics. SCIENCE ADVANCES 2022; 8:eabl9692. [PMID: 35960797 PMCID: PMC9374333 DOI: 10.1126/sciadv.abl9692] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/17/2021] [Accepted: 06/28/2022] [Indexed: 05/25/2023]
Abstract
In organic photovoltaics, charges can separate efficiently even if their Coulomb attraction is an order of magnitude greater than the available thermal energy. Delocalization has been suggested to explain this fact, because it could increase the initial separation of charges in the charge-transfer (CT) state, reducing their attraction. However, understanding the mechanism requires a kinetic model of delocalized charge separation, which has proven difficult because it involves tracking the correlated quantum-mechanical motion of the electron and the hole in large simulation boxes required for disordered materials. Here, we report the first three-dimensional simulations of charge-separation dynamics in the presence of disorder, delocalization, and polaron formation, finding that even slight delocalization, across less than two molecules, can substantially enhance the charge-separation efficiency, even starting with thermalized CT states. Delocalization does not enhance efficiency by reducing the Coulomb attraction; instead, the enhancement is a kinetic effect produced by the increased overlap of electronic states.
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2
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Upreti T, Tormann C, Kemerink M. Can Organic Solar Cells Beat the Near-Equilibrium Thermodynamic Limit? J Phys Chem Lett 2022; 13:6514-6519. [PMID: 35822430 PMCID: PMC9310094 DOI: 10.1021/acs.jpclett.2c01565] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/15/2023]
Abstract
Despite an impressive increase over the past decade, experimentally determined power conversion efficiencies of organic photovoltaic cells still fall considerably below the theoretical upper bound for near-equilibrium solar cells. Even in otherwise optimized devices, a prominent yet incompletely understood loss channel is the thermalization of photogenerated charge carriers in the density of states that is broadened by energetic disorder. Here, we demonstrate by extensive numerical modeling how this loss channel can be mitigated in carefully designed morphologies. Specifically, we show how funnel-shaped donor- and acceptor-rich domains in the phase-separated morphology that are characteristic of organic bulk heterojunction solar cells can promote directed transport of positive and negative charge carriers toward the anode and cathode, respectively. We demonstrate that in optimized funnel morphologies this kinetic, nonequilibrium effect, which is boosted by the slow thermalization of photogenerated charges, allows one to surpass the near-equilibrium limit for the same material in the absence of gradients.
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Affiliation(s)
- Tanvi Upreti
- Complex
Materials and Devices, Department of Physics, Chemistry and Biology
(IFM), Linköping University, 581 83 Linköping, Sweden
- Centre
for Advanced Materials, Heidelberg University, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
| | - Constantin Tormann
- Centre
for Advanced Materials, Heidelberg University, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
| | - Martijn Kemerink
- Complex
Materials and Devices, Department of Physics, Chemistry and Biology
(IFM), Linköping University, 581 83 Linköping, Sweden
- Centre
for Advanced Materials, Heidelberg University, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
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3
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Fu Z, Zhang X, Zhang H, Li Y, Zhou H, Zhang Y. On the Understandings of Dielectric Constant and Its Impacts on the Photovoltaic Efficiency in Organic Solar Cells. CHINESE J CHEM 2020. [DOI: 10.1002/cjoc.202000289] [Citation(s) in RCA: 19] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Zihao Fu
- School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University No. 37 Xueyuan Road Beijing 100191 China
| | - Xuning Zhang
- School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University No. 37 Xueyuan Road Beijing 100191 China
| | - Hong Zhang
- Key Laboratory of Nanosystem and Hierachical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology Beijing 100190 China
| | - Yanxun Li
- Key Laboratory of Nanosystem and Hierachical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology Beijing 100190 China
| | - Huiqiong Zhou
- Key Laboratory of Nanosystem and Hierachical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology Beijing 100190 China
| | - Yuan Zhang
- School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University No. 37 Xueyuan Road Beijing 100191 China
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4
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Emmerich S, Hedwig S, Arnoldi B, Stöckl J, Haag F, Hemm R, Cinchetti M, Mathias S, Stadtmüller B, Aeschlimann M. Ultrafast Charge-Transfer Exciton Dynamics in C 60 Thin Films. THE JOURNAL OF PHYSICAL CHEMISTRY. C, NANOMATERIALS AND INTERFACES 2020; 124:23579-23587. [PMID: 33193941 PMCID: PMC7659033 DOI: 10.1021/acs.jpcc.0c08011] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/02/2020] [Revised: 09/25/2020] [Indexed: 06/11/2023]
Abstract
The high flexibility of organic molecules offers great potential for designing the optical properties of optically active materials for the next generation of optoelectronic and photonic applications. However, despite successful implementations of molecular materials in today's display and photovoltaic technology, many fundamental aspects of the light-to-charge conversion in molecular materials have still to be uncovered. Here, we focus on the ultrafast dynamics of optically excited excitons in C60 thin films depending on the molecular coverage and the light polarization of the optical excitation. Using time- and momentum-resolved photoemission with femtosecond extreme ultraviolet (fs-XUV) radiation, we follow the exciton dynamics in the excited states while simultaneously monitoring the signatures of the excitonic charge character in the renormalization of the molecular valence band structure. Optical excitation with visible light results in the instantaneous formation of charge-transfer (CT) excitons, which transform stepwise into Frenkel-like excitons at lower energies. The number and energetic position of the CT and Frenkel-like excitons within this cascade process are independent of the molecular coverage and the light polarization of the optical excitation. In contrast, the depopulation times of the CT and Frenkel-like excitons depend on the molecular coverage, while the excitation efficiency of CT excitons is determined by the light polarization. Our comprehensive study reveals the crucial role of CT excitons for the excited-state dynamics of homomolecular fullerene materials and thin films.
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Affiliation(s)
- Sebastian Emmerich
- University
of Kaiserslautern and Research Center OPTIMAS, Erwin-Schrödinger-Straße
46, Kaiserslautern 67663, Germany
- Graduate
School of Excellence Materials Science in Mainz (MAINZ), Gottlieb-Daimler-Straße 46, Kaiserslautern 67663, Germany
| | - Sebastian Hedwig
- University
of Kaiserslautern and Research Center OPTIMAS, Erwin-Schrödinger-Straße
46, Kaiserslautern 67663, Germany
| | - Benito Arnoldi
- University
of Kaiserslautern and Research Center OPTIMAS, Erwin-Schrödinger-Straße
46, Kaiserslautern 67663, Germany
| | - Johannes Stöckl
- University
of Kaiserslautern and Research Center OPTIMAS, Erwin-Schrödinger-Straße
46, Kaiserslautern 67663, Germany
| | - Florian Haag
- University
of Kaiserslautern and Research Center OPTIMAS, Erwin-Schrödinger-Straße
46, Kaiserslautern 67663, Germany
- Graduate
School of Excellence Materials Science in Mainz (MAINZ), Gottlieb-Daimler-Straße 46, Kaiserslautern 67663, Germany
| | - Ralf Hemm
- University
of Kaiserslautern and Research Center OPTIMAS, Erwin-Schrödinger-Straße
46, Kaiserslautern 67663, Germany
| | - Mirko Cinchetti
- Experimentelle
Physik VI, Technische Universität
Dortmund, Dortmund 44221, Germany
| | - Stefan Mathias
- I.
Physikalisches Institut, Georg-August-Universität
Göttingen, Friedrich-Hund-Platz
1, Göttingen 37077, Germany
- International
Center for Advanced Studies of Energy Conversion (ICASEC), Georg-August-Universität Göttingen, Göttingen 37077, Germany
| | - Benjamin Stadtmüller
- University
of Kaiserslautern and Research Center OPTIMAS, Erwin-Schrödinger-Straße
46, Kaiserslautern 67663, Germany
- Graduate
School of Excellence Materials Science in Mainz (MAINZ), Gottlieb-Daimler-Straße 46, Kaiserslautern 67663, Germany
| | - Martin Aeschlimann
- University
of Kaiserslautern and Research Center OPTIMAS, Erwin-Schrödinger-Straße
46, Kaiserslautern 67663, Germany
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5
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Felekidis N, Melianas A, Kemerink M. The Role of Delocalization and Excess Energy in the Quantum Efficiency of Organic Solar Cells and the Validity of Optical Reciprocity Relations. J Phys Chem Lett 2020; 11:3563-3570. [PMID: 32301322 DOI: 10.1021/acs.jpclett.0c00945] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
The photon energy dependence of long-range charge separation is studied for two prototypical polymer:fullerene systems. The internal quantum efficiency (IQE) of PCDTBT:PC61BM is experimentally shown to be independent of the excitation energy. In contrast, for TQ1:PC71BM the IQE is strongly energy-dependent for excitation energies close to charge transfer (CT) electroluminescence peak maximum while it becomes energy-independent at higher excitation energies. Kinetic Monte Carlo simulations reproduce the experimental IQE and reveal that the photon energy-dependence of the IQE is governed by charge delocalization. Efficient long-range separation at excitation energies corresponding to the CT electroluminescence peak maximum or lower requires an initial separation of the hole-electron pair by ∼4-5 nm, whereas delocalization is less important for charge separation at higher photon energies. Our modeling results suggest that a phenomenological reciprocity between CT electroluminescence and external quantum efficiency does not necessarily prove that commonly employed reciprocity relations between these spectra are valid from a fundamental perspective.
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Affiliation(s)
- N Felekidis
- Complex Materials and Devices, Department of Physics, Chemistry and Biology, Linköping University, 58183 Linköping, Sweden
| | - A Melianas
- Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States
| | - M Kemerink
- Complex Materials and Devices, Department of Physics, Chemistry and Biology, Linköping University, 58183 Linköping, Sweden
- Centre for Advanced Materials, University of Heidelberg, 69120 Heidelberg, Germany
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6
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Fratini S, Nikolka M, Salleo A, Schweicher G, Sirringhaus H. Charge transport in high-mobility conjugated polymers and molecular semiconductors. NATURE MATERIALS 2020; 19:491-502. [PMID: 32296138 DOI: 10.1038/s41563-020-0647-2] [Citation(s) in RCA: 261] [Impact Index Per Article: 65.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/10/2019] [Accepted: 02/20/2020] [Indexed: 06/11/2023]
Abstract
Conjugated polymers and molecular semiconductors are emerging as a viable semiconductor technology in industries such as displays, electronics, renewable energy, sensing and healthcare. A key enabling factor has been significant scientific progress in improving their charge transport properties and carrier mobilities, which has been made possible by a better understanding of the molecular structure-property relationships and the underpinning charge transport physics. Here we aim to present a coherent review of how we understand charge transport in these high-mobility van der Waals bonded semiconductors. Specific questions of interest include estimates for intrinsic limits to the carrier mobilities that might ultimately be achievable; a discussion of the coupling between charge and structural dynamics; the importance of molecular conformations and mesoscale structural features; how the transport physics of conjugated polymers and small molecule semiconductors are related; and how the incorporation of counterions in doped films-as used, for example, in bioelectronics and thermoelectric devices-affects the electronic structure and charge transport properties.
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Affiliation(s)
| | - Mark Nikolka
- Cavendish Laboratory, University of Cambridge, Cambridge, UK
| | - Alberto Salleo
- Department of Materials Science and Engineering, Stanford University, Stanford, CA, USA
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Melianas A, Kemerink M. Photogenerated Charge Transport in Organic Electronic Materials: Experiments Confirmed by Simulations. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2019; 31:e1806004. [PMID: 30719756 DOI: 10.1002/adma.201806004] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/15/2018] [Revised: 10/29/2018] [Indexed: 06/09/2023]
Abstract
The performance of organic optoelectronic devices, such as organic photovoltaic (OPV) cells, is to a large extent dictated by their ability to transport the photogenerated charge, with relevant processes spanning a wide temporal (fs-µs) and spatial (1-100 nm) range. However, time-resolved techniques can access only a limited temporal window, and often contradict steady-state measurements. Here, commonly employed steady-state and time-resolved techniques are unified over an exceptionally wide temporal range (fs-µs) in a consistent physical picture. Experimental evidence confirmed by numerical simulations shows that, although various techniques probe different time scales, they are mutually consistent as they probe the same physical mechanisms governing charge motion in disordered media-carrier hopping and thermalization in a disorder-broadened density of states (DOS). The generality of this framework is highlighted by time-resolved experimental data obtained on polymer:fullerene, polymer:polymer, and small-molecule blends with varying morphology, including recent experiments revealing that low donor content OPV devices operate by long-range hole tunneling between non-nearest-neighbor molecules. The importance of nonequilibrium processes in organic electronic materials is reviewed, with a particular focus on experimental data and understanding charge transport physics in terms of material DOS.
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Affiliation(s)
- Armantas Melianas
- Department of Materials Science and Engineering, Stanford University, Stanford, CA, 94305, USA
| | - Martijn Kemerink
- Complex Materials and Devices, Department of Physics, Chemistry and Biology, Linköping University, 58183, Linköping, Sweden
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8
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Shih YC, Wang L, Hsieh HC, Lin KF. Effect of Fullerene Passivation on the Charging and Discharging Behavior of Perovskite Solar Cells: Reduction of Bound Charges and Ion Accumulation. ACS APPLIED MATERIALS & INTERFACES 2018; 10:11722-11731. [PMID: 29557169 DOI: 10.1021/acsami.8b03116] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Ion accumulation of organometal halide perovskites (OHPs) induced by electrode polarization of perovskite solar cells (PSCs) under illumination has been intensely studied and associated with a widely observed current-voltage hysteresis behavior. This work is dedicated to the investigation of the behavior of charged species at the compact TiO2/OHP interface with respect to electrode polarization in PSC devices. By providing a comprehensive discussion of open-circuit voltage ( VOC) buildup and VOC decay under illumination and in the dark for the PSCs modified with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) at the TiO2/OHP interface and their corresponding electrochemical impedance spectroscopies (EISs), a justified mechanism is proposed attempting to elucidate the dynamics of interfacial species with respect to the time and frequency domains. Our results demonstrate that the retarded VOC buildup and decay observed in PSC devices are related to the formation of bound charges in TiO2, which is essential to neutralize the oppositely charged ions accumulating at the OHP side. Besides, inserting a thicker PCBM at the TiO2/OHP interface as a passivation layer can alleviate the electrode polarization more efficiently as verified by the low dielectric constant measured from EIS. Moreover, photoluminescence measurements indicate that PCBM at the TiO2/OHP interface is capable of passivating a trap state and improving charge transfer. However, with respect to the time scale investigated in this work, the reduction of the hysteresis behavior on a millisecond scale is more likely due to less bound charge formation at the interface rather than shallow trap-state passivation by PCBM. After all, this work comprehensively demonstrates the interfacial properties of PSCs associated with PCBM passivation and helps to further understand its impact on charging/discharging as well as device performance.
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9
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Zhu H, Johansson MB, Johansson EMJ. The Effect of Dopant-Free Hole-Transport Polymers on Charge Generation and Recombination in Cesium-Bismuth-Iodide Solar Cells. CHEMSUSCHEM 2018; 11:1114-1120. [PMID: 29372625 DOI: 10.1002/cssc.201702169] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/14/2017] [Revised: 01/10/2018] [Indexed: 06/07/2023]
Abstract
The photovoltaic characteristics of CsBi3 I10 -based solar cells with three dopant-free hole-conducting polymers are investigated. The effect on charge generation and charge recombination in the solar cells using the different polymers is studied and the results indicate that the choice of polymer strongly affects the device properties. Interestingly, for the solar cell with poly[[2,3-bis(3-octyloxyphenyl)-5,8-quinoxalinediyl]-2,5-thiophenediyl] (TQ1), the photon-to-current conversion spectrum is highly improved in the red wavelength region, suggesting that the polymer also contributes to the photocurrent generation in this case. This report provides a new direction for further optimization of Bi-halide solar cells by using dopant-free hole-transporting polymers and shows that the energy levels and the interaction between the Bi-halide and the conducting polymers are very important for solar cell performance.
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Affiliation(s)
- Huimin Zhu
- Department of Chemistry-Ångström Laboratory, Institution of Physical Chemistry, Uppsala University, Box 523, SE-751 20, Uppsala, Sweden
| | - Malin B Johansson
- Department of Chemistry-Ångström Laboratory, Institution of Physical Chemistry, Uppsala University, Box 523, SE-751 20, Uppsala, Sweden
| | - Erik M J Johansson
- Department of Chemistry-Ångström Laboratory, Institution of Physical Chemistry, Uppsala University, Box 523, SE-751 20, Uppsala, Sweden
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10
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Gluchowski A, Gray KLG, Hood SN, Kassal I. Increases in the Charge Separation Barrier in Organic Solar Cells Due to Delocalization. J Phys Chem Lett 2018; 9:1359-1364. [PMID: 29494769 DOI: 10.1021/acs.jpclett.8b00292] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Because of the low dielectric constant, charges in organic solar cells must overcome a strong Coulomb attraction in order to separate. It has been widely argued that intermolecular delocalization would assist charge separation by increasing the effective initial electron-hole separation in a charge-transfer state, thus decreasing their barrier to separation. Here we show that this is not the case: including more than a small amount of delocalization in models of organic solar cells leads to an increase in the free-energy barrier to charge separation. Therefore, if delocalization were to improve the charge separation efficiency, it would have to do so through nonequilibrium kinetic effects that are not captured by a thermodynamic treatment of the barrier height.
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Affiliation(s)
- Adam Gluchowski
- School of Mathematics and Physics and Centre for Engineered Quantum Systems , The University of Queensland , St. Lucia , QLD 4072 , Australia
| | - Katherine L G Gray
- School of Mathematics and Physics and Centre for Engineered Quantum Systems , The University of Queensland , St. Lucia , QLD 4072 , Australia
| | - Samantha N Hood
- School of Mathematics and Physics and Centre for Engineered Quantum Systems , The University of Queensland , St. Lucia , QLD 4072 , Australia
| | - Ivan Kassal
- School of Chemistry and the University of Sydney Nano Institute , The University of Sydney , Sydney , NSW 2006 , Australia
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11
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Analysis of collision-controlled time dependence of diffusion coefficient of polaron pairs from transient absorption spectra of conducting polymers. CHEMICAL PAPERS 2018. [DOI: 10.1007/s11696-018-0439-1] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 10/17/2022]
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12
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Rozzi CA, Troiani F, Tavernelli I. Quantum modeling of ultrafast photoinduced charge separation. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2018; 30:013002. [PMID: 29047450 DOI: 10.1088/1361-648x/aa948a] [Citation(s) in RCA: 20] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
Phenomena involving electron transfer are ubiquitous in nature, photosynthesis and enzymes or protein activity being prominent examples. Their deep understanding thus represents a mandatory scientific goal. Moreover, controlling the separation of photogenerated charges is a crucial prerequisite in many applicative contexts, including quantum electronics, photo-electrochemical water splitting, photocatalytic dye degradation, and energy conversion. In particular, photoinduced charge separation is the pivotal step driving the storage of sun light into electrical or chemical energy. If properly mastered, these processes may also allow us to achieve a better command of information storage at the nanoscale, as required for the development of molecular electronics, optical switching, or quantum technologies, amongst others. In this Topical Review we survey recent progress in the understanding of ultrafast charge separation from photoexcited states. We report the state-of-the-art of the observation and theoretical description of charge separation phenomena in the ultrafast regime mainly focusing on molecular- and nano-sized solar energy conversion systems. In particular, we examine different proposed mechanisms driving ultrafast charge dynamics, with particular regard to the role of quantum coherence and electron-nuclear coupling, and link experimental observations to theoretical approaches based either on model Hamiltonians or on first principles simulations.
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Abramavicius D, Chorošajev V, Valkunas L. Tracing feed-back driven exciton dynamics in molecular aggregates. Phys Chem Chem Phys 2018; 20:21225-21240. [DOI: 10.1039/c8cp00682b] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
Abstract
Excitation, exciton transport, dephasing and energy relaxation, and finally detection processes shift molecular systems into a specific superposition of quantum states causing localization, local heating and finally excitonic polaronic effects.
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Affiliation(s)
| | | | - Leonas Valkunas
- Institute of Chemical Physics
- Vilnius University
- Vilnius
- Lithuania
- Center for Physical Sciences and Technology
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14
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