We present a generic theory of primary photoexcitations in low band gap donor-acceptor conjugated copolymers. Because of the combined effects of strong electron correlations and broken symmetry, there is considerable mixing between a charge-transfer exciton and an energetically proximate triplet-triplet state with an overall spin singlet. The triplet-triplet state, optically forbidden in homopolymers, is allowed in donor-acceptor copolymers. For an intermediate difference in electron affinities of the donor and the acceptor, the triplet-triplet state can have stronger oscillator strength than the charge-transfer exciton. We discuss the possibility of intramolecular singlet fission from the triplet-triplet state, and how such fission can be detected experimentally.PACS numbers: 78.66. Qn, 71.20.Rv, 71.35.Cc,The primary photophysical process in polymer solar cells is photoinduced charge transfer, whereby optical excitation at the junction between a donor conjugated polymer and acceptor molecules creates a charge transfer (CT) exciton whose dissociation leads to charge carriers. The donor polymeric materials used to be homopolymers such as polythiophene which absorb in the visible range of the solar spectrum [1]. Homopolymers have recently been replaced by block copolymers whose repeat units consist of alternating donor (D) and acceptor (A) moieties [2][3][4][5][6][7][8][9][10][11]. This architecture reduces the optical gap drastically, and the DA copolymers absorb in the near infrared, where the largest fraction of the photons emitted by the Sun lie. The power conversion efficiencies (PCEs) of organic solar cells with DA copolymers as donor materials have exceeded 10% [11], and there is strong interest in the development of structure-property correlations that will facilitate further enhancement of the PCE. Clearly, this requires precise understanding of the nature of the primary photoexcitations of DA copolymers.Existing electronic structure calculations of DA copolymers are primarily based on the density functional theory (DFT) approach or its time-dependent version (TD-DFT) [12][13][14][15][16][17][18]. The motivations behind these calculations have largely been to understand the localized versus delocalized character of the excited state reached by ground state absorption. Experimentally, DA copolymers exhibit a broad low energy (LE) absorption band at ∼ 700 − 800 nm and a higher energy (HE) absorption band at ∼ 400 − 450 nm [2][3][4]. There is agreement between the computational studies that the LE band is due to CT from D to A, and the HE band is a higher π-π * excitation.Recent optical studies indicate that the above simple characterization of the LE band might be incomplete, and as in the homopolymers [19], electron correlations play a stronger role in the photophysics of the DA copolymers than envisaged within DFT approaches. Grancini et al. determined from ultrafast dynamics studies that the broad LE band in PCPDT-BT (the Supplemental Material [20] for the structures of this and other DA copolymers) is c...

A necessary condition for superconductivity (SC) driven by electron correlations is that electron-electron (e-e) interactions enhance superconducting pair-pair correlations, relative to the noninteracting limit. We report high-precision numerical calculations of the ground state within the frustrated two-dimensional (2D) Hubbard Hamiltonian for a wide range of carrier concentration ρ (0 < ρ < 1) per site. We find that long range superconducting pair correlations are enhanced only for ρ 0.5. At all other fillings e-e interactions mostly suppress pair correlations. The enhancement of pair correlations is driven by the strong tendency to local singlet bond formation and spin gap (SG) in ρ = 0.5, in lattices with quantum fluctuation [1][2][3] . We also report determinantal quantum Monte Carlo (DQMC) calculations that are in strong agreement with our ground state results. Our work provides a key ingredient to the mechanism of SC in the 2D organic charge-transfer solids (CTS), and many other unconventional superconductors with frustrated crystal lattices and ρ 0.5, while explaining the absence of SC in structurally related materials with substantially different ρ.The possibility that e-e interactions can be the driving force behind SC in correlated-electron systems has been intensely investigated since the discovery of SC in the high T c cuprates. The minimal requirements for a complete theory are, (i) the superconducting pair correlations are enhanced by e-e interactions, and (ii) pair correlations are long range. For moderate to large e-e interactions, pair correlations are perhaps best calculated numerically, which however can be done only for finite clusters. The simplest model incorporating e-e interactions is usually assumed to be the Hubbard model, which in quite general form can be written asAll terms in Eq. 1 have their standard meaning. The first sum is the kinetic energy of noninteracting electrons within a 2D tight-binding model with hopping matrix elements t ij ; U and V ij are onsite and nearest neighbor (n.n.) Coulomb interactions respectively. Existing numerical calculations within Eq. 1 on 2D lattices have failed to find enhancement of pair-pair correlations relative to the noninteracting model without making severe approximations 4 . It has sometimes been surmised that correlated-electron SC might evolve upon doping a spin-gapped semiconductor, as would occur in toy models such as a 2D lattice consisting of weakly coupled even-leg ladders 5,6 . Finding realistic 2D models with SG and enhanced pair correlations however remains challenging. In the present work we demonstrate from explicit numerical calculations on frustrated 2D lattices enhanced pair correlations evolving from a spin-gapped state at a carrier density ρ 0.5, far from the region most heavily investigated until now (0.7 < ρ < 1.0). We further point out the strong relevance of the resulting theoretical picture to real materials, in particular the 2D CTS superconductors, which were discovered earlier than the cuprates 7 but are still not un...

We present a generalized adaptive time-dependent density matrix renormalization group

In this paper, we address the role of electron-electron interactions on the velocities of spin and charge transport in one-dimensional systems typified by conjugated polymers. We employ the Hubbard model to model electron-electron interactions. The recently developed technique of time dependent Density Matrix Renormalization Group (tdDMRG) is used to follow the spin and charge evolution in an initial wavepacket described by a hole doped in the ground state of the neutral system. We find that the charge and spin velocities are different in the presence of correlations and are in accordance with results from earlier studies; the charge and spin move together in noninteracting picture while interaction slows down only the spin velocity. We also note that dimerization of the chain only weakly affects these velocities.

We develop a correlated-electron minimal model for the normal state of charged phenanthrene ions in the solid state, within the reduced space of the two lowest antibonding molecular orbitals of phenanthrene. Our model is general and can be easily extended to study the normal states of other polycyclic aromatic hydrocarbon superconductors. The main difference between our approach and previous correlated-electron theories of phenacenes is that our calculations are exact within the reduced basis space, albeit for finite clusters. The enhanced exchange of electron populations between these molecular orbitals, driven by Coulomb interactions over and above the bandwidth effects, gives a theoretical description of the phenanthrene trianions that is very different from previous predictions. Exact many-body finite cluster calculations show that while the systems with molecular charges of −1 and −2 are one-and two-band Mott-Hubbard semiconductors, respectively, molecular charge −3 gives two nearly 3 4 -filled bands, rather than a completely filled lower band and a 1 2 -filled upper band. The carrier density per active molecular orbital is thus nearly the same in the normal state of the superconducting aromatics and organic charge-transfer solids, and may be the key to understanding unconventional superconductivity in these molecular superconductors.