Designing molecular materials with very large exciton diffusion lengths would remove some of the intrinsic limitations of present-day organic optoelectronic devices. Yet, the nature of excitons in these materials is still not sufficiently well understood. Here we present Frenkel exciton surface hopping, an efficient method to propagate excitons through truly nano-scale materials by solving the time-dependent Schrödinger equation coupled to nuclear motion. We find a clear correlation between diffusion constant and quantum delocalization of the exciton. In materials featuring some of the highest diffusion lengths to date, e.g. the non-fullerene acceptor Y6, the exciton propagates via a transient delocalization mechanism, reminiscent to what was recently proposed for charge transport. Yet, the extent of delocalization is rather modest, even in Y6, and found to be limited by the relatively large exciton reorganization energy. On this basis we chart out a path for rationally improving exciton transport in organic optoelectronic materials.
A series of contorted tetrabenzo[a,d,j,m]coronenes (TBCs)
substituted
with four fluoro-, chloro-, or methyl groups at 2,7,12,17-positions
were synthesized and characterized. Except for the one with methyl
substituents, which exhibits a shifted π–π stacking,
the rest all show cofacial π–π stacking with small
parallel displacements. One-dimensional growth along the stacking
direction was observed in the single crystals for all derivatives.
A systematic comparison of the crystal packing and the calculated
electronic coupling/mobility with the measured field-effect mobility
for single-crystal field-effect transistors shows a good correlation.
Time-dependent electronic structure methods are growing in popularity as tools for modeling ultrafast and/or nonlinear processes, for computing spectra, and as the electronic structure component of mean-field molecular dynamics simulations. Time-dependent configuration interaction (TD-CI) offers several advantages over the widely used real-time time-dependent density functional theory: namely, that it correctly models Rabi oscillations; it offers a spin-pure description of open-shell systems; and a hierarchy of TD-CI methods can be defined that systematically approach the exact solution of the time-dependent Schrodinger equation (TDSE). In this work, we present a novel TD-CI approach that extends TD-CI to large complete active-space configuration expansions. Such extension is enabled by use of a direct configuration interaction approach that eliminates the need to explicitly build, store, or diagonalize the Hamiltonian matrix. Graphics processing unit (GPU) acceleration enables fast solution of the TDSE even for large active spaces-up to 12 electrons in 12 orbitals (853776 determinants) in this work. A symplectic split operator propagator yields long-time norm conservation. We demonstrate the applicability of our approach by computing the response of a large molecule with a strongly correlated ground state, decacene (CH), to various pulses (δ-function, transform limited, chirped). Our simulations predict that chirped pulses can be used to induce dipole-forbidden transitions. Simulations of decacene using the 6-31G(d) basis set and a 12 electrons/12 orbitals active space took 20.1 h to propagate for 100 fs with a 1 attosecond time step on a single NVIDIA K40 GPU. Convergence with respect to time step is found to depend on the property being computed and the chosen active space.
Visibly transparent luminescent solar concentrators (TLSC) have the potential to turn existing infrastructures into net-zero-energy buildings. However, the reabsorption loss currently limits the device performance and scalability. This loss is typically defined by the Stokes shift between the absorption and emission spectra of luminophores. In this work, the Stokes shifts (SS) of near-infrared selective-harvesting cyanines are altered by substitution of the central methine carbon with dialkylamines. We demonstrate varying SS with values over 80 nm and ideal infrared-visible absorption cutoffs. The corresponding TLSC with such modification shows a power conversion efficiency (PCE) of 0.4% for a >25 cm2 device area with excellent visible transparency >80% and up to 0.6% PCE over smaller areas. However, experiments and simulations show that it is not the Stokes shift that is critical, but the total degree of overlap that depends on the shape of the absorption tails. We show with a series of SS-modulated cyanine dyes that the SS is not necessarily correlated to improvements in performance or scalability. Accordingly, we define a new parameter, the overlap integral, to sensitively correlate reabsorption losses in any LSC. In deriving this parameter, new approaches to improve the scalability and performance are discussed to fully optimize TLSC designs to enhance commercialization efforts.
Quantum dynamical simulations are essential for a molecular-level
understanding of light-induced processes in optoelectronic materials,
but they tend to be computationally demanding. We introduce an efficient
mixed quantum-classical nonadiabatic molecular dynamics method termed
eXcitonic state-based Surface Hopping (X-SH), which propagates the
electronic Schrödinger equation in the space of local excitonic
and charge-transfer electronic states, coupled to the thermal motion
of the nuclear degrees of freedom. The method is applied to exciton
decay in a 1D model of a fullerene–oligothiophene junction,
and the results are compared to the ones from a fully quantum dynamical
treatment at the level of the Multilayer Multiconfigurational Time-Dependent
Hartree (ML-MCTDH) approach. Both methods predict that charge-separated
states are formed on the 10–100 fs time scale via multiple
“hot-exciton dissociation” pathways. The results demonstrate
that X-SH is a promising tool advancing the simulation of photoexcited
processes from the molecular to the true nanomaterials scale.
A series of nonplanar tetrabenzo-fused acenes exhibited a hole mobility from 0.044 cm2 V−1 s−1 up to 0.81 cm2 V−1 s−1 in their single crystal field-effect transistors.
We examine the performance of the asymptotically corrected model potential scheme on the two lowest singlet excitation energies of acenes with different number of linearly fused benzene rings (up to 5), employing both the real-time time-dependent density functional theory and the frequency-domain formulation of linear-response time-dependent density functional theory. The results are compared with the experimental data and those calculated by long-range corrected hybrid functionals and others. The long-range corrected hybrid scheme is shown to outperform the asymptotically corrected model potential scheme for charge-transfer-like excitations.
We review recent efforts to model nonradiative recombination in semiconductor nanoparticles through conical intersections, focusing on the reasons for and consequences of the locality of such intersections.
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