Ultrafast Charge Separation in Organic Photovoltaics Enhanced by Charge Delocalization and Vibronically Hot Exciton Dissociation

Tamura, Hiroyuki; Burghardt, Irène

doi:10.1021/ja4093874

Cited by 312 publications

(490 citation statements)

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“…Localization of wave functions due to the nuclear vibrations works toward decreasing the e-h separations. Our finding is in line with the observations by Tamura and Burghardt [52,53] that charge separation at the donor/acceptor interface can be enhanced by charge delocalization.In the time-resolved spectroscopy experiment by Ishino et al. [48], the pump pulses excite to exciton manifolds at 3.1 eV, which leads to the optical excitation of the blue edge of the absorption maximum.…”

supporting

confidence: 93%

“…Although the degree of CT character in excited states has been reported as described in the earlier paragraph, its role in excitation dy- * Electronic address: t.fujita@kuchem.kyoto-u.ac.jp † Electronic address: aspuru@chemistry.harvard.edu namics has remained unclear. Furthermore, there has been growing interest in the effects of excited-state delocalizations on charge photogeneration processes [35,[50][51][52][53][54][55]. Therefore, we studied the excited-state dynamics in DNTT as a model system to provide insight into the role of the CT states in OSCs.…”

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confidence: 99%

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Coherent Dynamics of Mixed Frenkel and Charge-Transfer Excitons in Dinaphtho[2,3-b:2′3′-f]thieno[3,2-b]-thiophene Thin Films: The Importance of Hole Delocalization

Fujita

Atahan-Evrenk

Sawaya

et al. 2016

J. Phys. Chem. Lett.

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Charge transfer states in organic semiconductors play crucial roles in processes such as singlet fission and exciton dissociation at donor/acceptor interfaces. Recently, a time-resolved spectroscopy study of dinaphtho [2,3-b:2 ′ 3 ′ -f]thieno[3,2-b]-thiophene (DNTT) thin films provided evidence for the formation of mixed Frenkel and charge-transfer excitons after the photoexcitation. Here we investigate optical properties and excitation dynamics of the DNTT thin films by combining ab initio calculations and a stochastic Schrödinger equation. Our theory predicts that the low-energy Frenkel exciton band consists of 8 to 47% CT character. The quantum dynamics simulations show coherent dynamics of Frenkel and CT states in 50 fs after the optical excitation. We demonstrate the role of charge delocalization and localization in the mixing of CT states with Frenkel excitons as well as the role of their decoherence.Organic semiconductors (OSCs) are widely investigated as candidates for inexpensive and flexible materials for photovoltaics and other optoelectronic applications. [1][2][3]. In recent years, new OSCs have been designed and improved through computational modeling [4][5][6] and virtual high-throughput screening [7][8][9]. Modeling energy and charge transport properties is also essential for understanding structure-property relationships and thus for rationally designing novel OSCs [10][11][12][13][14][15][16].The low-energy optical excitations in OSCs lead to the formation of a bound electron-hole (e-h) pair, a Frenkel exciton. Excitonic properties of organic crystals are substantially different from those of isolated molecules, owing to excitonic couplings, near-field interactions between electronic transitions [17][18][19][20]. Interactions with a charge-transfer (CT) statea state in which the electron and hole are located on spatially separated regions-can also affect the optical properties of an exciton, as has been demonstrated by several theoretical studies [21][22][23][24][25][26]. Experimentally, the degree of CT character has been studied by momentum-dependent electron-loss spectroscopy [27][28][29] or electroabsorption spectroscopy [30][31][32]. CT states can act as precursors for interfacial CT states [33][34][35]; mixing of CT states with Frenkel excitons would facilitate charge separation in photovoltaic materials. They have also gained recent attention due to their relevance to singlet fission [36][37][38], in which singlet to triplet conversion can proceed via sequential CT steps [39][40][41].Here we focus on dinaphtho[2,3-b:, a p-type OSC originally introduced by Takimiya and co-workers [42]. DNTT and its derivatives [5,[43][44][45][46][47][48][49] have gained attention due to their high hole mobility values and air stability. A recent time-resolved spectroscopy study by Ishino et al. [48] concludes that mixed Frenkel and CT excitons are formed after the optical excitation. Although the degree of CT character in excited states has been reported as described in the earlier paragraph, its role in...

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supporting

confidence: 93%

mentioning

confidence: 99%

Coherent Dynamics of Mixed Frenkel and Charge-Transfer Excitons in Dinaphtho[2,3-b:2′3′-f]thieno[3,2-b]-thiophene Thin Films: The Importance of Hole Delocalization

Fujita

Atahan-Evrenk

Sawaya

et al. 2016

J. Phys. Chem. Lett.

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“…On the other hand, energetic disorder in the regions away from the interface would provide an entropic driving force by increasing the density of localized polaron states away from the interfacial region, allowing the polarons to hop or diffuse away from the interface before recombination could take place 16 . It has also been suggested that in polymer/fullerene blends, interfacial exciton fission is facilitated by chargedelocalization alone the interface, which provide a lower barrier for fission with the excess energy provided by thermally-hot vibronic dynamics 17 . Finally, a report by Bakulin et al 18 indicates that when relaxed charge-transfer excitons are pushed with an infrared pulse, they increase photocurrent via delocalized states rather than by energy gradient-driven hopping.…”

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confidence: 99%

Noise-induced quantum coherence drives photo-carrier generation dynamics at polymeric semiconductor heterojunctions

2014

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Here we report on an exciton/lattice model of the electronic dynamics of primary photo excitations in a polymeric semiconductor heterojunction that includes both polymer p-stacking, energetic disorder and phonon relaxation. Our model indicates that that in polymer/fulerene heterojunction systems, resonant tunnelling processes brought about by environmental fluctuations couple photo excitations directly to photocurrent producing charge-transfer states on o100 fs time scales. Moreover, we find this process to be independent of the location of energetic disorder in the system, and hence we expect exciton fission via resonant tunnelling to polarons to be a ubiquitous feature of these systems.

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“…[88] Therefore, it seems reasonable to conclude that especially polaronic transport levels in the vicinity of interfaces in amorphous thin films are subject to a considerable amount of disorder. In view of the recently highlighted influence of long-range order and charge delocalization on the efficiency of exciton dissociation at organic heterojunctions, [17,89,90] this result suggests potential efficiency limits arising from low-charge mobilities in the realm of the interface.From Figure 7 (upper panel), it becomes furthermore evident that a correlation between the disorder of excitation energies and the disorder of ionization potentials exists. Surprisingly, in contrast to the above-given state energy disorders, those molecules with the largest polarizability (eg, the squaraine, diketopyrrolopyrrole, or tri(biphenyl-4-yl)amine (TBA)) generally show rather small disorders in their excitonic and polaronic states (see Figure 7 and Table 1).…”

mentioning

confidence: 92%

QM/MM calculations combined with the dimer approach on the static disorder at organic‐organic interfaces of thin‐film organic solar cells composed of small molecules

Brückner

Stolte²,

Würthner³

et al. 2017

J of Physical Organic Chem

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A QM/MM approach using ωB97X-D combined with AMOEBA calculations was used to analyze the energetic disorder in the vicinity of interfaces in amorphous organic heterostructures. Distributions of ground-, excited-, and cationic-state energies as well as of ionization potentials and excitation energies, all being relevant quantities for the transport properties of thin films, were calculated. As already found for bulk amorphous organic semiconductors, local densities of states at molecular interfaces possess Gaussian-shaped profiles with a significant amount of disorder. Assuming a disorder-limited activation of exciton and charge transport, a decrease in the amount of disorder could improve the transport properties. Relating calculated disorders to molecular parameters revealed that in line with the Bässler model, especially the molecular polarity, its change upon electronic excitation, and the molecular polarizability are relevant quantities leading to energetically largely disordered films. Moreover, because of the different mechanisms of exciton and polaron delocalization, a given morphology with disordered charge-transport levels gives not necessarily rise to disordered exciton-transport levels and vice versa. | INTRODUCTIONDisorder in molecular organic semiconductors has been recognized as a key parameter limiting the semiconductor's transport properties for both charges and excitons. [1] A random distribution of the molecules in a disordered amorphous film results in considerable variations in local intermolecular interactions. This gives rise to a distribution of ionization and excitation energies, the relevant quantities for charge and exciton transport, which depend among others on these intermolecular interaction energies. [2] According to the central limit theorem, resulting densities of states (DOSs) for excitons and charges have a Gaussian shape because intermolecular interaction energies are composed of many independently varying contributions. [3] The width of the Gaussian-shaped DOSs, ie, the standard deviation of the Gaussian normal distribution σ, is referred to as the disorder parameter and characterizes the amount of site energy disorder, ie, of static disorder, in the semiconducting solid-state system. [2] Based on this Gaussian DOS, the Bässler model describes charge transport in organic semiconductors as an incoherent hopping process between the normally distributed sites. [4] The expression "disorder" can be connected with geometrical and energetical disorder. In line with the Bässler nomenclature, in the following the expression, "disorder" is always used to characterize the energetic disorder unless otherwise noted. Although experimentally detected field-and temperature-dependent charge carrier mobilities confirm the predictions of the Bässler model in principle, no direct experimental proof for the Gaussian-shaped DOS, particularly of charge carriers, in disordered molecular semiconductors † This article is published as part of a special issue to celebrate the 80 th birthday of P...

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Ultrafast Charge Separation in Organic Photovoltaics Enhanced by Charge Delocalization and Vibronically Hot Exciton Dissociation

Cited by 312 publications

References 31 publications

Coherent Dynamics of Mixed Frenkel and Charge-Transfer Excitons in Dinaphtho[2,3-b:2′3′-f]thieno[3,2-b]-thiophene Thin Films: The Importance of Hole Delocalization

Coherent Dynamics of Mixed Frenkel and Charge-Transfer Excitons in Dinaphtho[2,3-b:2′3′-f]thieno[3,2-b]-thiophene Thin Films: The Importance of Hole Delocalization

Noise-induced quantum coherence drives photo-carrier generation dynamics at polymeric semiconductor heterojunctions

QM/MM calculations combined with the dimer approach on the static disorder at organic‐organic interfaces of thin‐film organic solar cells composed of small molecules

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