Tuning charge transfer and recombination in exTTF/CNT nanohybrids by choice of chalcogen: A time-domain density functional analysis

Carbon nanotubes (CNTs) have emerged as a promising material due to large surface areas, high absorption cross sections, excellent electron accepting properties, and outstanding carrier mobilities.¹ CNTs constitute the subject of intense research efforts toward multiple applications, including molecular electronics,^2,3 photo-catalysis,^4,5 hydrogen storage,^6,7 electrochemical biosensing,^8,9 chemical sensing,^10–12 displays,¹³ conducting films,¹⁴ and computing.¹⁵ Design of versatile nano-sized electron donor–acceptor CNT structures draws particular attention. Such structures are produced either via covalent functionalization of CNTs or by a supramolecular approach. The latter is based on π–π interactions between CNTs and organic molecules that are able to attach strongly to the CNT surface. The large surface-to-volume ratio of the sp²-hybridized all-carbon lattice of CNTs enables π–π stacking with other π-conjugated systems, which bind to CNTs by van der Waals forces. In contrast to covalent functionalization, the supramolecular approach preserves the electronic properties of pristine CNTs, and for this reason, it has been widely adopted for the generation of donor–acceptor nanohybrids. Noncovalent attachment of a variety of aromatic species, such as conjugated polymers,¹⁶ pyrenes,¹⁷ anthracenes,^18,19 and porphyrins,¹⁷ to CNT surfaces has been achieved experimentally.

Depending on the energetic alignment with the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of the photo-sensitizing partners, CNTs can serve as either electron acceptors or electron donors. When CNTs were coupled with porphyrins,²⁰ phthalocyanines,^21,22 metal complexes,²³ CdX (X = S/Se/Te) nanoparticles,²⁴ and conjugated polymers, such as poly (3-hexylthiophene) (P3HT),²⁵ they functioned as electron acceptors due to their relatively lower conduction band (CB) energy. On the other hand, there were only a few reports on the use of CNTs as electron donors, with sensitizers including fullerene,^26–28 perylenebisdiimides,²⁹ phenylenevinylene oligomers,³⁰ and 11,11,12,12-tetracyano-9,10-anthraquinodimethane (TCAQ) derivatives.³¹ In such instances, CNTs acted as electron donors due to the lower lying LUMO levels of the light absorbers compared to the CNT CB.

When CNTs act as electron donor in a composite system, photo-excitation of CNTs generates holes that act as charge carriers in the CNT valence band (VB). Recent photo-electrochemical studies suggest that CNTs act as hole acceptors when combined with ZnO.³² It has been further demonstrated that ultrafast (<1 ps) charge separation occurs in CNT-TiO₂ hybrids by hole transfer (HT) from the VB of TiO₂ to the VB of CNTs.³³ A strong evidence has been found of a direct hole transfer from photo-excited CNTs to the P3HT polymer.³⁴ Recent transient absorption measurements demonstrate that photo-induced HT occurs at the heterojunction between (6,5) CNTs and perylene diimide (PDI) based electron acceptors.³⁵ Furthermore, it is demonstrated that CNTs enable fast (sub-picosecond) hole extraction from a prototypical perovskite absorber layer.³⁶ Transient absorption analysis revealed sub-picosecond charge generation at interfaces of CNTs with MoS₂, including both electron transfer (ET) to MoS₂ after selective CNT excitation and HT to the CNT layer following selective MoS₂ excitation.³⁷ 

Tetrathiafulvalene (TTF) is a S-rich molecule, well known for its electron donating ability.³⁸ The electron-rich structure of TTF is attractive for noncovalent interactions. The better shape complementarily of TTF extended with aromatic polycyclic compounds, termed exTTF, have been widely used to form electro-active and photo-active architectures when linked to π-surfaces of carbon-based materials, such as graphene,^39,40 fullerenes,^41–43 and CNTs.^38,44,45 exTTFs have been successfully utilized for preparing photo-induced charge transfer systems for solar energy conversion devices.^38,46

Lifetimes of the charge separated states of the exTTF/CNT and TTF/CNT nanoconjugates were measured by Herranz et al. using time-resolved spectroscopy.⁴⁷ The lifetimes were longer in the case of exTTFs compared to the analogous TTF nanohybrids. Interaction of exTTFs with CNT surfaces using a pyrene tether moiety was demonstrated, and a very rapid intra-hybrid ET was observed.⁴⁸ Ehli et al. prepared supramolecular nanohybrids formed by a pyrene-TTF dyad and different types of CNTs.⁴⁹ The study provided evidence of stable charge-separated states.

Simultaneous ET and HT occurring in opposite directions between two materials can enhance performance of solar materials and devices. For example, both CNTs and organic molecules have low bandgaps accessible by UV/Vis photo-excitation. Composites formed by CNTs and organic molecules with appropriately aligned energy levels can exhibit both electron and hole transfer that occur by excitation of either CNT or molecule. Often, ET is faster than HT in such CNT/molecule hybrids, because CNTs, acting as electron acceptors, have higher densities of states (DOS).⁵⁰ Still, sub-picosecond HT is always desired along with ultrafast ET. Therefore, one aims to improve the HT rate without affecting ET. The key to exploitation of the full potential of CNT based hybrids by achieving higher photovoltaic conversion efficiencies lies in a detailed understanding of the interfacial charge separation and recombination processes. Since interfacial charge separation depends strongly on properties of electron and hole acceptors, search for suitable charge carrier acceptors remains an active area of investigation.

Figure 1 illustrates a variety of scenarios for photo-induced excited state dynamics in the exTTF/exTSeF-CNT hybrids. Photo-excitation of exTTF/exTSeF results in ET to the CNT CB. In contrast, selective photo-excitation of the CNT leads to HT from the CNT VB to the exTTF/exTSeF HOMO. Once the charge separation is achieved in either way, the separated carriers eventually undergo electron–hole (e–h) recombination at the interface.

The current work presents time-domain atomistic simulation of charge transfer and recombination in the exTTF/exTSeF-CNT hybrids. The simulation demonstrates a strong influence of the chalcogen atom on the excited state dynamics in the nanohybrid. The simulations show that upon photo-excitation of the molecule, an electron is transferred from the excited state of the donor exTTF/exTSeF into the CNT CB on a sub-picosecond time scale, in agreement with the experimental observation.⁵¹ When the S atom of exTTF is replaced with Se (exTSeF), the ET rate remains nearly identical. However, the rate of HT that follows photo-excitation of the CNT is accelerated by a factor of 100 for the exTSeF nanohybrid. Both ET and HT produce a long-lived charge separated state. The factors responsible for the faster photo-induced HT in the exTSeF/CNT system include a smaller donor-acceptor energy gap, a stronger NA charge-phonon interaction, and a longer-lived quantum coherence between the donor and acceptor states. Coherence is shortened in the exTTF/CNT hybrid due to coupling to a broader range of low frequency modes. While ET and HT occur on femto- and pico-second timescales, the nonradiative recombination of the charge separated state takes place on a nanosecond time scale. The e–h pair in the exTTF system recombines at a lower rate compared to exTSeF, due to weaker NA coupling and faster coherence loss. The coupling is weaker because of the reduced participation of high-frequency phonons, and the coherence is shorter because of involvement of a broader range of lower frequency modes. The fast charge separation and slow recombination make exTTF/exTSeF-CNT hybrids promising candidates for solar energy applications.

The simulation cells for the systems under investigation include a unit cell of the (6,5) CNT and a exTTF/exTSeF molecule, as shown in Fig. 2. The CNT and the molecule are noncovalently attached through the van der Waals force of attraction and are 3.6 Å apart from each other in both hybrids. To avoid spurious interactions between simulation cell replicas, 50 Å of vacuum are added in the x- and y-directions. The CNT is periodic in the z-direction with the optimized cell length of 41 Å and the optimized tube diameter is 7.57 Å. The total number of atoms in the simulation cell is 400 in both hybrids.

This simulation is carried out by the combination of the self-consistent-charge density-functional tight-binding (SCC-DFTB) and nonadiabatic molecular dynamics (NAMD) methodologies,⁵² developed by our group. The details are provided in the supplementary material. The method has been applied to a variety of large systems, including CNTs,^20,53,54 CdS/Se quantum dots,⁵⁵ CdS/Se nanoplatelets,^56,57 and a porphyrin macromolecular system.⁵⁸ The previous simulations were well corroborated with the experimental charge transfer dynamics carried out by time-resolved spectroscopies. Yang et al. studied the dynamics of photo-induced charge separation in CNT/fullerene hybrids using DFT combined with NAMD.⁵⁹ They showed that the ET from CNT to fullerene depends on the excitation energy and fullerene type. A broad range of nanoscale materials^60–68 has been studied by the closely related methodology based on ab initio density functional theory.

Geometry optimization, electronic structure, and adiabatic MD are performed with the SCC-DFTB method as implemented in the DFTB+ code.^69,70 The parameter set (Slater–Koster files) used in the SSC-DFTB calculations has been tested extensively for a broad class of systems.⁷¹ The DOS has been computed using a 1 × 1 × 256 Monkhorst–Pack k-point mesh. After the geometry relaxation, the systems have been heated to 300 K using velocity rescaling. Then, 3 ps microcanonical MD trajectories have been generated using the Verlet algorithm⁷² with a 1 fs time step. The energy of the Kohn–Sham molecular orbitals and the NA coupling matrix elements have been calculated at each time step. Then, the generated time-dependent information has been utilized to perform the NAMD simulations using the fewest switching surface hopping (FSSH)^73,74 and decoherence-induced surface hopping (DISH)⁷⁵ methods, as implemented within the PYXAID program.^76,77

Generation of a long-lived charge separated state upon absorption of a photon is required in order to have an efficient photovoltaic device. The time-domain atomistic calculations of the charge separation and recombination provide a complete description of the electron/hole transfer and e–h recombination. Since optical selection rules lead to photo-excited states in which electron and hole wavefunctions overlap, photo-excitation has to be followed by efficient and ultrafast charge separation, in order to avoid recombination that has to occur at a slower rate than charge separation. In the following section, we focus first on the geometric and electronic structure of the exTTF/exTSeF-CNT composites. Later on, we discuss the inelastic and elastic electron-phonon interactions that drive the photo-induced dynamics. Finally, we explicitly analyze the charge separation and recombination processes at the interfaces under study.

The relative positions of the components in the interface determine the strength of the donor–acceptor coupling and govern the charge transfer mechanisms. Therefore, knowledge of the geometry of the system constitutes the first step for the analysis of the photo-induced charge transfer dynamics. Figures 2(a)–2(d) represent the side and top views of the optimized geometries of the exTTF/exTSeF-CNT composites, respectively. Figure S1 in the supplementary material demonstrates snapshots of geometries of the two systems obtained from the MD trajectories at room temperature. At both zero and ambient temperatures, the anthracene unit of exTTF/exTSeF is mildly curved along the CNT surface, indicating that its π-electron system remains intact and interacts strongly with that of the CNT.

The key elements of a system's electronic structure that govern the photo-induced dynamics are relative energies of the donor and acceptor levels, and localization of the corresponding wavefunctions. To identify the components that contribute to the band edges, projected density of states (PDOS) of the exTTF/exTSeF-CNT nanohybrid has been investigated, as shown in Figs. 3(a) and 3(b). The DOS and PDOS can provide qualitative insights into interfacial interactions between the components. The energies of the CNT band edge states and the molecular frontier orbitals indicate that the system forms a type-II heterostructure. Since exTTF/exTSeF is a molecule, it exhibits discrete energy levels, while the CNT DOS contains sharp van Hove singularities separated by regions of relatively low and continuous DOS. Note that the van Hove singularities and the discrete molecular levels characteristic of each component are maintained in the composite system, indicating that the components are bound by a noncovalent interaction. The electronic structure of the pristine CNT tube is well conserved in the presence of the π-electronically anchored exTTF/exTSeF molecule, and vice versa. Comparing the PDOS of the two systems, we observe that the CNT DOS is at a lower energy in exTTF/CNT than exTSeF/CNT, relative to the molecular DOS. This is explained by the asymmetric charge distribution between the CNT and the molecule. The Mulliken analysis shows a more significant ET from the molecule to the CNT in exTTF/CNT compared to exTSeF/CNT, 0.0017 vs 0.0010 a.u., respectively. Because of the asymmetric charge distribution, a local electro-static interaction is present between the CNT and the molecule. The stronger electro-static interaction shifts the CNTs states to lower energies.⁷⁸ Photo-excitation of either the molecule, or the CNT, or both simultaneously can be achieved by tuning the photon energy. Photo-excitation of exTTF/exTSeF promotes an electron from its HOMO to LUMO. Subsequently, the ET to the CNT, where it relaxes vibrationally to the CB minimum (CBM). On the other hand, excitation of the CNT leads to HT from the VB maximum (VBM) of the CNT to the molecule's HOMO. The charge recombination eventually occurs between the CNT's CBM and the molecule's HOMO. Note that the CNT DOS is higher than the molecule DOS, as the CNT contains more atoms. Therefore, the density of final states is higher for the ET than the HT. In general, a small gap between the donor and acceptor states for charge transfer leads to larger NA coupling and brings electronic and vibrational quanta closer to resonance, accelerating the transfer.

The key electron and hole orbitals that participate in the charge separation and recombination dynamics of the exTTF/exTSeF-CNT composites are shown for the optimized geometries in Figs. 4(a) and 4(b), respectively. All the key orbitals are localized within the corresponding species, indicating that the donor–acceptor interaction is relatively weak. The ET occurs from the molecule's LUMO into CNT's CB states. The HT happens from the CNT VBM and VBM-1 into the molecule's HOMO. The e–h recombination takes place between the CNT's CBM and the molecule's HOMO.

The knowledge of electron-phonon interactions is of fundamental importance in advancing the understanding of carrier dynamics in nanoscale systems. Electron–phonon interactions drive the photo-induced charge transfer dynamics by both elastic and inelastic scattering. Elastic scattering randomizes phases of the electronic wavefunctions and leads to loss of quantum coherence between superpositions of electronic states formed during quantum transitions. Rapid loss of quantum coherence slows down quantum dynamics, as exemplified by the quantum Zeno effect.⁷⁹ Moderate decoherence can also accelerate dynamics.⁸⁰ Inelastic electron–phonon scattering is responsible for loss of electronic energy to heat during the charge transfer processes. Although the molecule and the CNT exhibit a broad spectrum of vibrational motions, not all vibrational modes couple to the electronic subsystem, as governed by electron-vibrational coupling selection rules. In order to establish the active modes that participate in the carrier dynamics, we compute Fourier transform (FT) of the energy gap for a particular process. The amplitude of the signals in the influence spectrum characterizes the strength of the electron-phonon coupling for the mode of the corresponding frequency. Note that the NA electron–phonon coupling is directly proportional to the second derivative of the energy gap along the nuclear trajectory. Vibrational modes that generate largest oscillations in the energy gap are most strongly coupled to the electronic transition. Similarly, the rate of coherence loss due to elastic electron–phonon scattering is proportional to the amplitude of the energy gap fluctuation.⁸¹ 

CNTs exhibit two types of vibrations: low-frequency acoustic radial breathing modes (RBM) and the high-frequency optical G-modes. Collective inwards and outwards movement of CNT atoms corresponds to RBMs that appear at frequencies below 300 cm⁻¹.⁵⁰ The G-mode occurs at 1700 cm⁻¹ and arises from C–C stretches within the π-conjugated CNT surfaces.⁸² As seen from Figs. 5(a) and 5(b), the ET is guided by both low- and high-frequency phonons in each systems. The low-frequency vibration at 125 cm⁻¹ is ascribed to the RBM of the CNT. A distinct phonon peak around 300 cm⁻¹ is attributed to the exTTF/exTSeF molecule. The high-frequency mode at 1750 cm⁻¹ can be assigned to the G band of the tube. In semiconducting CNTs, the in-plane G-mode is split into G⁺ and G⁻ modes due to the rehybridization of valence orbitals of the carbon atoms from sp² to sp³-like because of CNT curvature. In the ET process, exTTF/CNT exhibits a sharp G⁻ peak along with a weaker G⁺ peak. The RBM peaks at low frequencies are weak. Participation of the intense G⁻ and weak RBM band results in a strong NA electron–phonon coupling between the electron donor and acceptor states, as seen in Table I. The phonon mode participation scenario is different during the ET in exTSeF/CNT. The RBMs participate strongly, whereas the G⁺ and G⁻ modes participate rather weakly, leading to a weaker NA electron–phonon coupling compared to exTTF/CNT. Nevertheless, the ET is ultrafast in both cases and occurs in less than 200 fs. The vibrational modes participating in the HT are presented in Figs. 5(c) and 5(d) for the exTTF and exTSeF hybrids, respectively. High-frequency modes are present in both hybrids at ∼1600–1750 cm⁻¹. The high frequency G-mode and a broad range of RBMs are involved for exTTF/CNT, but the intensities of the peaks are small. On the other hand, a strong G-mode peak, along with moderate RBM peaks, appears for exTSeF/CNT. The electron–phonon coupling strength is characterized by the height of the phonon peaks, and the exTSeF/CNT shows a much stronger NA coupling compared to exTTF/CNT. As a result, the photo-induced HT occurs on a sub-picosecond time scale (688 fs) in exTSeF/CNT, whereas it takes 70 ps in exTTF/CNT.

An electronic transition necessarily involves development of a coherent superposition of the initial and final states. Elastic electron–phonon scattering randomizes phases of electronic wavefunctions of the two states, leading to coherence loss. Typically coherence loss slows down the transition, as demonstrated by the quantum Zeno effect in the limit of infinitely fast decoherence.⁷⁹ In present, the decoherence times are estimated as the pure-dephasing times of the optical response theory.⁸³ The details of the computation protocol are presented in the supplementary material. The pure-dephasing times can be measured experimentally through time-resolved photon-echo measurements⁸⁴ or as inverse of the homogeneous linewidth.⁸⁵ Generally, decoherence should be slow during charge separation and fast during charge recombination to favor photovoltaic activity. The simulated pure-dephasing times (τ_gau) are obtained from the Gaussian fitting, $y=exp(−0.5[t/τgau]2)$⁠, of the pure-dephasing functions and are presented in Table I. Figures 6(a) and 6(b) show the pure-dephasing functions to the ET and HT, respectively. All coherence times are less than 100 fs. Coherence is longest for the ET process (65–70 fs) and is shorter for the HT and e–h recombination (15–20 fs). Longer coherence for the HT in the exTSeF hybrid assists in faster HT, compared to the exTTF system. The computed pure-dephasing times are several times longer for the ET than the HT. Correspondingly, the ET is faster than the HT.

The efficiency of a photovoltaic cell is determined by an interplay of charge separation and recombination, which follow light absorption. Photon absorption by the exTTF/exTSeF molecule leads to ET from the molecular LUMO to the CNT CB, Fig. 3. The dynamics of the photo-induced ET processes are characterized in Fig. 7, which display decay of population of the molecule's LUMO. The dynamics are not purely exponential and contain a notable Gaussian component. Thus, the data were fitted to the combined Gaussian and exponential function: $y=Aexp(−t/τexp)+(1−A)exp(−t/τgau)2$⁠, and the time scales reported in Table I were obtained as the weighted average: $τ=Aτexp+(1−A)τgau$⁠. The simulated 149 fs ET time for the exTTF/CNT hybrid agrees with the experimental observation.⁵¹ The ET is bit slower (159 fs) in exTSeF/CNT. Replacement of a lighter S atom with a heavier Se atom decreases the NA charge-phonon coupling (Table I) and slightly slows down the dynamics. Also, the stronger involvement of the low-frequency phonon modes in the exTSeF hybrid, Fig. 5, slows down the ET.

After photo-excitation of the CNT in the hybrid, a hole is created at the CNT VB, Fig. 1. The created hole migrates to the molecule HOMO. The HT dynamics at the molecule–CNT interface is demonstrated in Figs. 8(a) and 8(b). The HT time scale is obtained by fitting the data with a linear combination of Gaussian and exponential functions, $y=Aexp(−t/τexp)+(1−A)exp(−t/τgau)2+B$⁠, and computing the weighted average, $τ=Aτexp+(1−A)τgau$⁠. The constant B is set to 0 for fitting the data in Fig. 8(a), while it is needed for Fig. 8(b), because the HT in the exTSeF/CNT composite is incomplete. The donor and acceptor states are close in energy, Fig. 3(b), and occasionally cross at ambient temperature. The simulated time of the hole migration is 70 ps for the exTTF/CNT nanohybrid. It is quite long because of the appreciable gap between the HT donor and acceptor states, Fig. 3(a). The rate is accelerated by a factor of 100 in the exTSeF/CNT hybrid, with a time constant of 688 fs. The simulation demonstrates the strong influence of the choice of the chalcogen atoms (S and Se) on the hole migration. Replacement of S with Se reduces the energy gap between the HT donor and acceptor states, Fig. 3(b). The NA hole relaxation dynamics depends strongly on the magnitude of the NA coupling, which is inversely proportional to the energy gap. The exTSeF system exhibits a much larger NA coupling (43.4 meV) than exTTF (2.7 meV), Table I. The faster rate for the Se system also is reflected in the longer-lived quantum coherence between the donor and acceptor states in the exTSeF/CNT hybrid, Table I and Fig. 6(b). In summary, the Se-rich exTSeF molecule accepts the hole, photo-generated in the CNT, at a much faster rate compared to the analogous exTTF molecule.

Once the charge separation is achieved, it is important to maintain a long-lived charge separated state. Thus, nonradiative e–h recombination is a crucial factor influencing solar cell performance. It constitutes the major source of charge and energy losses in photovoltaic applications, and therefore, slow e–h recombination is a primary criterion for a high quality photovoltaic material. The e–h recombination dynamics across the exTTF/exTSeF–CNT interface are analyzed in Fig. 9. The CNT's CBM and molecule's HOMO correspond to the initial and final states for the charge recombination, Fig. 1. Figure 9(a) presents evolution of the populations of the charge-separated states in the two hybrids. The linear plots correspond to the short-time expansion of the exponential decay: $y=Aexp(−t/τ)≈1−t/τ$⁠. The corresponding time constants are given in Table I. The e–h lifetime (27.5 ns) computed for the exTTF/CNT composite is an order of magnitude longer than the reported value.⁵¹ Most likely, the discrepancy arises because the simulation considers perfect systems, while CNTs in the experimental samples can contain defects or an admixture of metallic CNTs. Furthermore, the simulation includes only one e–h pair and does not consider Auger-type processes involving trions or multiple excitons. Auger channels can create fast routes for charge annihilation.^53,86,87

The excited electron in the CNT's CBM undergoes nonradiative recombination with the hole remaining in the exTTF's HOMO across the energy gap of 0.57 eV. During the relaxation, this amount of energy is dissipated in the form of heat that is accommodated by the phonon modes. Because the energy gap is larger in the exTSeF/CNT hybrid (0.80 eV) than the exTTF/CNT hybrid (0.57 eV), one can anticipate that the exTSeF/CNT hybrid should exhibit a slower e–h recombination. However, it is important to keep in mind that other factors, including the NA coupling, participation of particular phonon modes, and decoherence between the initial and final states, also influence the e–h recombination. The vibrational modes contributing to the nonradiative e–h recombination in the two composites are characterized in Figs. 9(b) and 9(c), which presents FTs of the phonon-induced fluctuations of the energy gaps between the CBM and the HOMO. A broad range of primarily low-frequency phonon modes couples with the electronic transition in the exTTF system. In comparison, the range of active frequencies is more narrow, and the contribution of the high frequency modes is larger in the exTSeF system. High-frequency vibrations are particularly efficient at facilitating the electron–phonon energy exchange during the e–h recombination across large energy gaps, because high-frequency phonon quanta better match electronic energy gaps. In addition, the NA coupling matrix element is directly proportional to the atomic velocity, and the atomic velocity is larger for higher frequency modes at a given temperature. The stronger participation of the high-frequency modes results in a stronger NA coupling in the exTSeF/CNT hybrid (0.60 meV) compared to the exTTF/CNT hybrid (0.13 meV), Table I. Participation of the broader range of phonons in the exTTF/CNT system, Fig. 9(b), results in faster coherence loss, inset in Fig. 9(a), and faster decoherence slows down the nonradiative transition. The simulation reveals that the lifetime of the photo-generated e–h pair is longer in the S-rich system by an order of magnitude. As a consequence, the energy and voltage losses are minimized in the exTTF/CNT nanohybrid.

In summary, we have investigated photo-induced charge transfer and recombination dynamics at the exTTF/exTSeF–CNT interfaces using our recently developed methodology combining NAMD with SCC-DFTB. The simulation explores the role of chalcogen atoms (S/Se) on the excited state dynamics of the molecule/CNT nanohybrid. We show that, upon photo-excitation, an electron is transferred from the molecule's LUMO to the CNT's CB on a sub-picosecond time scale, in agreement with the experimental observation. When the S atom of exTTF is replaced with Se, the ET time is found to be comparable. At the same time, the HT dynamics is accelerated by factor of 100 for the exTSeF/CNT nanohybrid. The simulation unravels the factors responsible for such a significant rate enhancement. The donor–acceptor energy gap is smaller, and the nonadiabatic charge–phonon coupling is stronger in the Se composite, thereby promoting faster HT. The longer-lived quantum coherence also favors faster HT in the Se system. In comparison, recombination of the charge-separated state in the exTTF/exTSeF-CNT composites takes place on a nanosecond time scale. The e–h pair in the S system recombines at a slower rate compared to the Se system. The slower decay of the excited e–h pair to recover the ground state population is due to the weaker NA charge–phonon interaction between the frontier (band-edge) states of the exTTF/CNT composite. The faster time of coherence loss additionally slows down the e–h recombination. The current time-domain atomistic simulation elucidates the influence of the chalcogen atoms on the photo-induced charge carrier dynamics and provides a detailed understanding of the charge separation and recombination mechanisms. The insights generate valuable guidelines for optimization of photovoltaic efficiency for the next generation energy management.

See the supplementary material for the details of the simulation methodology and representative configurations of the systems at room temperature.

R.S. is grateful to CSIR for Senior Research Fellowships. S.P. acknowledges financial support from CSIR [01(2956)/18/EMR-II], Government of India. O.V.P. acknowledges financial support from the U.S. National Science Foundation (NSF) under Grant No. CHE-1900510.

The data that support the findings of this study are available from the corresponding authors upon reasonable request.