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

Abstract

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...

Tuning the effective spin-orbit coupling in molecular semiconductors

“…4c). For comparison, simulations of intramolecular vibrational modes of DNTT predict frequencies of 6–50 THz for stretching modes along the molecule's symmetry axes, distortions of the phenyl rings and vibrations of the C-H bonds, in ascending order24. They exceed the Zeeman splitting by orders of magnitude and are therefore less effective at flipping the spin.…”

Charge-transfer excited states in the donor/acceptor interface from large-scale GW calculations

“…41 Although the implementation at the GW level is straightforward, this manuscript focused on the monomer states because of the larger computational cost for GW self-energies of fragment dimers. In addition, the delocalized excited states can be computed by constructing the excited-state Hamiltonian 73,74 of an entire system. The combined effects of charge delocalization and state-specific polarization will be investigated in a future study.…”

Thousand-atom ab initio calculations of excited states at organic/organic interfaces: toward first-principles investigations of charge photogeneration

“…Third, charge photogeneration should be understood as a dynamical process, and the role of delocalized ICT states in the excited-state dynamics remains to be elucidated. A real-time simulation of the charge separation dynamics will be achieved by combining a large-scale numerical technique [71][72][73] with a quantum dynamics theory, [74][75][76] which is left for a future study.…”