Theoretical study on the cooperative exciton dissociation process based on dimensional and hot charge-transfer state effects in an organic photocell

Shimazaki, Tomomi; Nakajima, Takahito

doi:10.1063/1.4953905

Cited by 19 publications

(26 citation statements)

References 56 publications

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“…Overall, the low-frequency phonons may play central roles in the OPV operations at room temperature because such disorder motions in the polymers and also in the PC 61 BM fullerene clusters may enhance the density of the states for the separated electron−hole pairs coupled with the dense phonon bath, contributing to the efficient photocurrents. 21…”

Section: Electron−hole Dissociation After Photoinduced Charge Separationmentioning

confidence: 99%

“…This meant that the Coulomb binding potentials are overcome by the entropy, as represented in Figure B bottom. The above two different mechanisms (hot exciton mechanism vs entropy mechanism) have been in strong discussions in current researches by experiments and by theoretical approaches. , As mentioned above, the connection among the nuclear motions, the electronic coupling and geometries in Figure B is required to be clarified to further understand the charge generation mechanism by the entropy. This entropy role is thus definitely distinguished from the conventional exciton model in Figure B.…”

Section: Dynamic Excitonmentioning

confidence: 99%

“…The intermolecular libration modes and the disorder alkyl side chain motions were concluded to be responsible for the intermolecular dissociations in Figure E, as demonstrated on the charge conductions in the above P3HT-Ph n -C 60 linked system (Figure A). Overall, the low-frequency phonons may play central roles in the OPV operations at room temperature because such disorder motions in the polymers and also in the PC 61 BM fullerene clusters may enhance the density of the states for the separated electron–hole pairs coupled with the dense phonon bath, contributing to the efficient photocurrents …”

Section: Dynamic Excitonmentioning

confidence: 99%

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Manipulation of Charge-Transfer States by Molecular Design: Perspective from “Dynamic Exciton”

2021

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Conspectus “Dynamic exciton”, an umbrella term concept in photochemistry, plays an important role in nature, science, and technology, especially in photoinduced and electrically induced electron transfer (ET) between a donor (D) and an acceptor (A). Typically, an exciton in molecular D–A systems is considered a locally excited (LE) state of a donor (D*) or an acceptor (A*) molecule, but let us extend the terminology of “exciton” to an integrated class of LE, charge-transfer (CT), and charge-separated (CS) states. The degree of CT (0–100%), spin multiplicity, and D–A interaction (i.e., electronic coupling) are pivotal factors in the “exciton”. Another important aspect of the “exciton” is strongly related to the “dynamic” aspect of the “exciton” by movement of atomic nuclei (i.e., vibration, rotation, and fluctuation) and their collective motions controlling behaviors of electrons and spins by the passage of time. The concept of “dynamic exciton” should cover a wide variety of the photochemical phenomena that are all essential for the energy conversion devices and processes including various kinds of living systems. In these, a huge amount of the nuclear motional modes may cooperatively be entangled to electronic orbitals. Thus, the idea behind “dynamic exciton” includes usage of the cooperation between nuclear motions and the spin–orbital, as electron–phonon coupling for innovative designs of the energy conversion materials and assemblies. For this, it is particularly important to examine how this wide variety of the motions plays roles on electronic couplings, intermediate geometries and mobilities, exciton energies, CT characters, and so on. We draw researchers’ attention to this aspect of the vibronic effect, like the entropy role by collective movement of side chains of the conjugated polymer to modulate the electronic coupling time-dependently at the charge delocalization and dissociation, which is reinforced three-dimensionally at the D/A domain interface of organic photovoltaics (OPVs). In addition, to realize the ultimate high-performance OPVs, the voltage loss must be diminished; i.e., the energy difference between the band gap and the open circuit voltage is caused in part by interfacial charge recombination via vibrational relaxation. Attaining efficient excited-state migration with prevention of the vibrational relaxation at the D/A interface would possibly overcome the voltage loss issues at the primary CT event, when we understand the roles of low-frequency disorder movements by the phonon on the vibrational relaxation in the solid state. Importantly, conversion of the excited state-to-CS state in organic OPVs is an opposite process to that of the CS state-to-excited state in organic light-emitting diodes (OLEDs). Specifically, intramolecular D–A linked systems and intermolecular D–A systems hold a prominent position in both OPVs and OLEDs, and an intrinsic similarity is seen in the mutual interplay among LE, CT, and CS states of such systems facilitated by the dynamic effects. These facts encourage ...

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Section: Electron−hole Dissociation After Photoinduced Charge Separationmentioning

confidence: 99%

Section: Dynamic Excitonmentioning

confidence: 99%

Section: Dynamic Excitonmentioning

confidence: 99%

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Manipulation of Charge-Transfer States by Molecular Design: Perspective from “Dynamic Exciton”

2021

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“…The CT mechanisms have gone through heated discussions in the past several years. − Notably, some reports have emphasized the importance of hot CT states, that is, excess energy in the promotion of charge separation. ,− While other works combined with experimental and theoretical data suggest that the direct excitations are also important in promoting charge separation. ,,, Normally, direct excitations have not been considered for P3HT / PC 61 BM because of the weak absorption of PC 61 BM and small f of CT states. However, it is different for P3HT /nonfullerene systems owing to larger f of their CT states presented in Figures and S4–S6.…”

Section: Resultsmentioning

confidence: 99%

Theoretical Insight into Multiple Charge-Transfer Mechanisms at the P3HT/Nonfullerenes Interface in Organic Solar Cells

Zhao

Pan

Geng

et al. 2019

ACS Sustainable Chem. Eng.

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Poly(3-hexylthiophene) (P3HT)-based organic solar cells (OSCs) have been developed in recent years because of their advantages such as easy production, low cost, and large-area manufacture. However, fewer nonfullerene acceptors with higher power conversion efficiency (PCE) than PC 61 BM have been explored in P3HT-based OSCs. In this contribution, the excited states were in-depth analyzed toward probing the particularities of superior P3HT/nonfullerene systems. Multiple charge-transfer (CT) mechanisms involving intermolecular electric field (IEF), hot CT states, and direct excitation of CT states were found, which suggests that more favorable CT pathways exist at these P3HT/ nonfullerene interfaces. Accordingly, the calculations on CT rates of all of the investigated donor/acceptor interfaces further verified the positive effect of multiple CT pathways. In addition, the interesting hybrid Frenkel-CT states were first found to be relevant with the stronger electrostatic surface potential (ESP) differences on the donor and acceptor for these P3HT/nonfullerene systems, which may provide a strategy for the design of high-efficiency OSCs.

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“…The LUMO energy difference (offset) between the donor and acceptor materials plays an essential role in the dissociation of such strongly bound excitons. [2][3][4][5][6][7][8][9] The energy offset at the donoracceptor junction can give excess energy to the exciton and assist the dissociation process required to generate a free electron and a hole. 8,9 Several experiments have demonstrated that the energy conversion efficiency of an organic photocell device is suppressed in small offset cases.…”

Section: Introductionmentioning

confidence: 99%

Application of the dielectric-dependent screened exchange potential approach to organic photocell materials

Shimazaki

Nakajima

2016

Phys. Chem. Chem. Phys.

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This paper discusses the fundamental features of the dielectric-dependent screened exchange potential approach for organic molecules and photocell materials. The energy difference (gap) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is a key factor when designing organic photocell materials. The magnitude of this energy gap strongly depends on the ratio of the Hartree-Fock (HF) exchange term combined with hybrid density functional theory (DFT) functionals. In ordinary hybrid DFT functionals, the HF exchange term ratio is often determined empirically and a constant value is employed for all materials. Conversely, the dielectric-dependent exchange potential approach employs a system-dependent value for this parameter, which is proportional to the inverse of the dielectric constant. The organic materials examined in this paper take different dielectric constant values according to their material types and unit lengths; therefore, their HF exchange ratios are also different. This paper discusses the relationship between the energy gaps, the dielectric constants, and the HF exchange ratios in the dielectric-dependent screened exchange potential approach. For a series of acene-type compounds, it was found that the HOMO-LUMO gap decreased when their conjugation systems are extended. The dielectric-dependent screened exchange potential approach demonstrated that the values of the dielectric constants of the compounds increased; consequently, the HF exchange ratio decreased. Similar results were obtained for a series of thiophene-based donor molecules. The calculated values were compared with those obtained via experimental measurements. We found that although the theoretical calculations of the HOMO-LUMO gap usually overestimate experimental optical gaps, a better agreement between the experimental and calculated values can be obtained if we correct for the exciton binding energy.

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Theoretical study on the cooperative exciton dissociation process based on dimensional and hot charge-transfer state effects in an organic photocell

Cited by 19 publications

References 56 publications

Manipulation of Charge-Transfer States by Molecular Design: Perspective from “Dynamic Exciton”

Manipulation of Charge-Transfer States by Molecular Design: Perspective from “Dynamic Exciton”

Theoretical Insight into Multiple Charge-Transfer Mechanisms at the P3HT/Nonfullerenes Interface in Organic Solar Cells

Application of the dielectric-dependent screened exchange potential approach to organic photocell materials

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