17 research outputs found
Exact Wave Packet Dynamics of Singlet Fission in Unsubstituted and Substituted Polyene Chains within Long-Range Interacting Models
Singlet fission (SF) is a potential pathway for significant enhancement of
efficiency in organic solar cells (OSC). In this paper, we study singlet
fission in a pair of polyene molecules in two different stacking arrangements
employing exact many-body wave packet dynamics. In the non-interacting model,
the SF yield is absent. The individual molecules are treated within Hubbard and
Pariser-Parr-Pople (PPP) models and the interaction between them involves
transfer terms, intersite electron repulsions and site-charge--bond-charge
repulsion terms. Initial wave packet is constructed from excited singlet state
of one molecule and ground state of the other. Time development of this wave
packet under the influence of intermolecular interactions is followed within
the Schr\"odinger picture by an efficient predictor-corrector scheme. In
unsubstituted Hubbard and PPP chains, excited singlet state leads to
significant SF yield while the state gives negligible fission yield.
On substitution by donor-acceptor groups of moderate strength, the lowest
excited state will have sufficient character and hence results in
significant SF yield. Because of rapid internal conversion, the nature of the
lowest excited singlet will determine the SF contribution to OSC efficiency.
Furthermore, we find the fission yield depends considerably on the stacking
arrangement of the polyene molecules.Comment: 13 pages, 8 figures, 4 table
Model for Triplet State Engineering in Organic Light Emitting Diodes
Engineering the position of the lowest triplet state (T1) relative to the
first excited singlet state (S1) is of great importance in improving the
efficiencies of organic light emitting diodes and organic photovoltaic cells.
We have carried out model exact calculations of substituted polyene chains to
understand the factors that affect the energy gap between S1 and T1. The
factors studied are backbone dimerisation, different donor-acceptor
substitutions and twisted geometry. The largest system studied is an eighteen
carbon polyene which spans a Hilbert space of about 991 million. We show that
for reverse intersystem crossing (RISC) process, the best system involves
substituting all carbon sites on one half of the polyene with donors and the
other half with acceptors.Comment: 7 Pages; 10 Figure
Spin relaxation of electron and hole polarons in ambipolar conjugated polymers.
The charge-transport properties of conjugated polymers have been studied extensively for opto-electronic device applications. Some polymer semiconductors not only support the ambipolar transport of electrons and holes, but do so with comparable carrier mobilities. This opens the possibility of gaining deeper insight into the charge-transport physics of these complex materials via comparison between electron and hole dynamics while keeping other factors, such as polymer microstructure, equal. Here, we use field-induced electron spin resonance spectroscopy to compare the spin relaxation behavior of electron and hole polarons in three ambipolar conjugated polymers. Our experiments show unique relaxation regimes as a function of temperature for electrons and holes, whereby at lower temperatures electrons relax slower than holes, but at higher temperatures, in the so-called spin-shuttling regime, the trend is reversed. On the basis of theoretical simulations, we attribute this to differences in the delocalization of electron and hole wavefunctions and show that spin relaxation in the spin shuttling regimes provides a sensitive probe of the intimate coupling between charge and structural dynamics
Efficient energy transport in an organic semiconductor mediated by transient exciton delocalization.
Efficient energy transport is desirable in organic semiconductor (OSC) devices. However, photogenerated excitons in OSC films mostly occupy highly localized states, limiting exciton diffusion coefficients to below ~10-2 cm2/s and diffusion lengths below ~50 nm. We use ultrafast optical microscopy and nonadiabatic molecular dynamics simulations to study well-ordered poly(3-hexylthiophene) nanofiber films prepared using living crystallization-driven self-assembly, and reveal a highly efficient energy transport regime: transient exciton delocalization, where energy exchange with vibrational modes allows excitons to temporarily re-access spatially extended states under equilibrium conditions. We show that this enables exciton diffusion constants up to 1.1 ± 0.1 cm2/s and diffusion lengths of 300 ± 50 nm. Our results reveal the dynamic interplay between localized and delocalized exciton configurations at equilibrium conditions, calling for a re-evaluation of exciton dynamics and suggesting design rules to engineer efficient energy transport in OSC device architectures not based on restrictive bulk heterojunctions
Symmetrized density matrix renormalization group algorithm for low-lying excited states of conjugated carbon systems: Application to 1,12-benzoperylene and polychrysene
The symmetry adapted density matrix renormalization group (SDMRG) technique has been an efficient method for studying low-lying eigenstates in one-and quasi-one-dimensional electronic systems. However, the SDMRG method had bottlenecks involving the construction of linearly independent symmetry adapted basis states as the symmetrymatrices in theDMRGbasis were not sparse. We have developed a modified algorithm to overcome this bottleneck. The newmethod incorporates end-to-end interchange symmetry (C-2), electron-hole symmetry (J), and parity or spin-flip symmetry (P) in these calculations. The one-to-one correspondence between direct-product basis states in the DMRG Hilbert space for these symmetry operations renders the symmetry matrices in the new basis with maximum sparseness, just one nonzero matrix element per row. Using methods similar to those employed in the exact diagonalization technique for Pariser-Parr-Pople (PPP) models, developed in the 1980s, it is possible to construct orthogonal SDMRG basis states while bypassing the slow step of the Gram-Schmidt orthonormalization procedure. Themethod together with the PPP modelwhich incorporates long-range electronic correlations is employed to study the correlated excited-state spectra of 1,12-benzoperylene and a narrow mixed graphene nanoribbon with a chrysene molecule as the building unit, comprising both zigzag and cove-edge structures
From monomer sequence to charge mobility in semiconductor polymers via model reduction
We present a model reduction scheme for polymer semiconductors which can be utilized to compute intrachain charge-carrier mobility from the monomer sequence. The reduced model can be used in conjunction with any quantum dynamics approach, but it is explored here assuming that transport takes place through incoherent hopping events between states of different degrees of delocalization. The procedure is illustrated by considering 28 realistic polymers for which we establish a quantitative correlation between charge localization characteristics and charge mobility. The data set helps in establishing plausible ranges for all the microscopic parameters of the model and it can therefore be used to determine the maximum plausible improvement in mobility. The reduced model is also used to provide some insight on the observation that the highest mobility polymers do not have very broad valence bands: there is indeed a range of the inter-monomer coupling for which this parameter has little effect on the mobility
Correlated Electronic Properties of a Graphene Nanoflake: Coronene
We report studies of the correlated excited states of coronene and substituted coronene within the Pariser-Parr-Pople (PPP) correlated -electron model employing the symmetry-adapted density matrix renormalization group technique. These polynuclear aromatic hydrocarbons can be considered as graphene nanoflakes. We review their electronic structures utilizing a new symmetry adaptation scheme that exploits electron-hole symmetry, spin-inversion symmetry, and end-to-end interchange symmetry. The study of the electronic structures sheds light on the electron correlation effects in these finite-size graphene analogues, which diminishes going from one-dimensional to higher-dimensional systems, yet is significant within these finite graphene derivatives