arXiv subjects

Suryoday Prodhan

Publications and source records attributed to Suryoday Prodhan.

6 recordsLinked to original sources

arXiv2021

Efficient Energy Transport in an Organic Semiconductor Mediated by Transient Exciton Delocalization

Efficient energy transport is highly desirable for organic semiconductor (OSC) devices such as photovoltaics, photodetectors, and photocatalytic systems. However, photo-generated excitons in OSC films mostly occupy highly localized states over their lifetime. Energy transport is hence thought to be mainly mediated by the site-to-site hopping of localized excitons, limiting exciton diffusion coefficients to below ~10^{-2} cm^2/s with corresponding diffusion lengths below ~50 nm. Here, using ultrafast optical microscopy combined with non-adiabatic molecular dynamics simulations, we present evidence for a new 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. In films of highly-ordered poly(3-hexylthiophene) nanofibers, prepared using living crystallization-driven self-assembly, we show that this enables exciton diffusion constants up to 1.1+-0.1 cm^2/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 the basic picture of exciton dynamics. This establishes new design rules to engineer efficient energy transport in OSC films, which will enable new devices architectures not based on restrictive bulk heterojunctions.

cond-mat.mtrl-sci↗

arXiv2019

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 $π$-electron model employing 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. Study of the electronic structures sheds light on the electron correlation effects in these finite-size graphene analogues, which diminishes on going from one-dimensional to higher-dimensional systems, yet is significant within these finite graphene derivatives.

physics.chem-ph↗

arXiv2018

A model Exact Study of the properties of low-lying electronic states of Perylene and Substituted Perylenes

There is a resurgence of interest in the electronic structure of perylene for its applications in molecular devices such as organic photovoltaics and organic light emitting diodes. In this study, we have obtained the low-lying singlet states of perylene by exactly solving the Parisar-Parr-Pople model Hamiltonian of this system with 20 sites and 20 electrons, in the VB basis where dimensionality is $\sim$ 5.92 billion. The triplet states of perylene are obtained using a DMRG scheme with symmetry adaptation. The one and two photon states are very close in energy $\sim$ 3.2 eV while the lowest triplet state is slightly below 1.6 eV indicating that perylene is a good candidate for singlet fission. To explore the tunability of the electronic states, we have studied donor-acceptor substituted perylenes. The two donors and two acceptors are substituted symmetrically either at the four bay sites or four peri sites. In all the bay substitution and one peri substitution at moderate D/A strength, the optical gap is lowered to about 2.8 eV. These molecules can be used as blue emitters. We have also reported bond orders in all the cases and perylene as well as substituted perylenes can be viewed as two weakly coupled naphthalene in the singlet states but in triplets these bonds tend to be comparable to other bonds in strength. The charge densities in substituted perylenes are mostly localized around the substitution sites in the ground state. The positive spin densities in triplets are concentrated around the peri and bay sites with the remaining sites having small spin densities of either sign.

cond-mat.str-el↗

arXiv2017

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 Schrödinger picture by an efficient predictor-corrector scheme. In unsubstituted Hubbard and PPP chains, $2{}^1A$ excited singlet state leads to significant SF yield while the $1{}^1B$ state gives negligible fission yield. On substitution by donor-acceptor groups of moderate strength, the lowest excited state will have sufficient $2{}^1A$ 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.

physics.chem-ph↗

arXiv2016

Correlated Electronic Properties of Some Graphene Nanoribbons: A DMRG Study

The significant electron-electron interactions that characterize the π-electrons of graphene nanoribbons (GNRs) necessitate going beyond one-electron tight-binding description. Existing theories of electron-electron interactions in GNRs take into account one electron-one hole interactions accurately but miss higher order effects. We report highly accurate density matrix renormalization group (DMRG) calculations of the ground state electronic structure, the relative energies of the lowest one-photon versus two-photon excitations and the charge gaps in three narrow graphene nanoribbons (GNRs) within the correlated Pariser-Parr-Pople model for π-conjugated systems. We have employed the symmetrized DMRG method to investigate the zigzag nanoribbon 3-ZGNR and two armchair nanoribbons 6-AGNR and 5-AGNR, respectively. We predict bulk magnetization of the ground state of 3-ZGNR, and a large spin gap in 6-AGNR in their respective thermodynamic limits. Nonzero charge gaps and semiconducting behavior, with moderate to large exciting binding energies are found for all three nanoribbons, in contradiction to the prediction of tight-binding theory. The lowest two-photon gap in 3-ZGNR vanishes in the thermodynamic limit, while this gap is smaller than the one-photon gap in 5-AGNR. However, in 6-AGNR the one-photon gap is smaller than the two-photon gap and it is predicted to be fluorescent.

cond-mat.str-el↗

arXiv2014

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.

physics.chem-ph↗