Exciton Pair Research Articles

Photoluminescence studies of cubic GaN epilayers

AbstractThe luminescence properties of cubic GaN films grown upon 3C‐SiC/Si (001) substrates by MOCVD were investigated. The spectra show luminescence peaks which are associated with donor bound exciton recombination and donor acceptor pair recombination. A reduced peak energy for the D0X emission compared with values reported in the literature suggests a tensile‐strain‐reduced bandgap of approximately 3.27 eV, which is consistent with the absorption edge in photoluminescence‐excitation spectroscopy. The presence of hexagonal material introduces a broad emission band at 3.40 eV with a FWHM of 190 meV, extending to energies up to 3.60 eV. The intensity of this emission scales linearly with excitation power, its peak energy and width remaining unchanged. This band is associated with an absorption edge below 3.70 eV and therefore is not caused by absorption into phase‐pure cubic or hexagonal GaN. The photoluminescence lifetimes measured across this band reduce from 0.40 to 0.20 ns with increasing emission energy. All these observations can be explained by considering a type‐II‐band alignment adjacent to stacking faults within the cubic GaN.

The impact of long-range electron-hole interaction on the charge separation yield of molecular photocells.

We discuss the effects of charge carrier interaction and recombination on the operation of molecular photocells. Molecular photocells are devices where the energy conversion process takes place in a single molecular donor-acceptor complex attached to electrodes. Our investigation is based on the quantum scattering theory, in particular on the Lippmann-Schwinger equation; this minimizes the complexity of the problem while providing useful and non-trivial insight into the mechanism governing photocell operation. In this study, both exciton pair creation and dissociation are treated in the energy domain, and therefore there is access to detailed spectral information, which can be used as a framework to interpret the charge separation yield. We demonstrate that the charge carrier separation is a complex process that is affected by different parameters, such as the strength of the electron-hole interaction and the non-radiative recombination rate. Our analysis helps to optimize the charge separation process and the energy transfer in organic solar cells and in molecular photocells.

Influence of molar ratio of Sb/Zn on the crystal, electrical and optical properties of Sb-doped ZnO films

Sb-doped p-type ZnO films were grown on sapphire substrate by metal organic chemical vapor deposition method. The influence of the molar ratio of Sb/Zn on the crystal, electrical and optical properties of the Sb-doped ZnO films was investigated by X-ray diffraction (XRD), Hall measurement system, optical absorption and photoluminescence spectroscopy. XRD patterns revealed all Sb-doped ZnO films were polycrystalline structures with strong c-axis orientation preferred. Electrical measurement results indicated Sb-doped ZnO films exhibited p-type electrical properties, and carrier concentrations were in the range of 10−17–10−18 cm−3. Low temperature photoluminescence measurement confirmed the existence of Sb in Sb-doped ZnO film. The photoluminescence spectrum of the Sb-doped ZnO film revealed the transition between the free electrons and acceptors peak at 3.243 eV, the acceptor-bound exciton peak at 3.319 eV and the donor-acceptor pair exciton at 3.200 eV. The thermal binding energy of the Sb accepter was estimated to be 0.19 eV.

Tuning the role of charge-transfer states in intramolecular singlet exciton fission through side-group engineering.

Understanding the mechanism of singlet exciton fission, in which a singlet exciton separates into a pair of triplet excitons, is crucial to the development of new chromophores for efficient fission-sensitized solar cells. The challenge of controlling molecular packing and energy levels in the solid state precludes clear determination of the singlet fission pathway. Here, we circumvent this difficulty by utilizing covalent dimers of pentacene with two types of side groups. We report rapid and efficient intramolecular singlet fission in both molecules, in one case via a virtual charge-transfer state and in the other via a distinct charge-transfer intermediate. The singlet fission pathway is governed by the energy gap between singlet and charge-transfer states, which change dynamically with molecular geometry but are primarily set by the side group. These results clearly establish the role of charge-transfer states in singlet fission and highlight the importance of solubilizing groups to optimize excited-state photophysics.

Efficient singlet exciton fission in pentacene prepared from a soluble precursor

Carrier multiplication using singlet exciton fission (SF) to generate a pair of spin-triplet excitons from a single optical excitation has been highlighted as a promising approach to boost the photocurrent in photovoltaics (PVs) thereby allowing PV operation beyond the Shockley-Queisser limit. The applicability of many efficient fission materials, however, is limited due to their poor solubility. For instance, while acene-based organics such as pentacene (Pc) show high SF yields (up to200%), the plain acene backbone renders the organic molecule insoluble in common organic solvents. Previous approaches adding solubilizing side groups such as bis(tri-iso-propylsilylethynyl) to the Pc core resulted in low vertical carrier mobilities due to reduction of the transfer integrals via steric hindrance, which prevented high efficiencies in PVs. Here we show how to achieve good solubility while retaining the advantages of molecular Pc by using a soluble precursor route. The precursor fully converts into molecular Pc through thermal removal of the solubilizing side groups upon annealing above 150 °C in the solid state. The annealed precursor shows small differences in the crystallinity compared to evaporated thin films of Pc, indicating that the Pc adopts the bulk rather than surface polytype. Furthermore, we identify identical SF properties such as sub-100 fs fission time and equally long triplet lifetimes in both samples.

Quintet multiexciton dynamics in singlet fission

Singlet fission, in which two triplet excitons are generated from a single absorbed photon, is a key third-generation solar cell concept. Conservation of angular momentum requires that singlet fission populates correlated multiexciton states, which can subsequently dissociate to generate free triplets. However, little is known about electronic and spin correlations in these systems since, due to its typically short lifetime, the multiexciton state is challenging to isolate and study. Here, we use bridged pentacene dimers, which undergo intramolecular singlet fission while isolated in solution and in solid matrices, as a unimolecular model system that can trap long-lived multiexciton states. We combine transient absorption and time-resolved electron spin resonance spectroscopies to show that spin correlations in the multiexciton state persist for hundreds of nanoseconds. Furthermore, we confirm long-standing predictions that singlet fission produces triplet pair states of quintet character. We compare two different pentacene–bridge–pentacene chromophores, systematically tuning the coupling between the pentacenes to understand how differences in molecular structure affect the population and dissociation of multiexciton quintet states. Experiments show how molecular structure affects the interaction and dynamics of the triplet exciton pairs produced when an excited singlet exciton decays via singlet fission — a process that could be harnessed for optoelectronic applications.

Excitonic gap formation and condensation in the bilayer graphene structure

We have studied the excitonic gap formation in the Bernal Stacked, bilayer graphene (BLG) structures at half-filling. Considering the local Coulomb interaction between the layers, we calculate the excitonic gap parameter and we discuss the role of the interlayer and intralayer Coulomb interactions and the interlayer hopping on the excitonic pair formation in the BLG. Particularly, we predict the origin of excitonic gap formation and condensation, in relation to the furthermost interband optical transition spectrum. The general diagram of excitonic phase transition is given, explaining different interlayer correlation regimes. The temperature dependence of the excitonic gap parameter is shown and the role of the chemical potential, in the BLG, is discussed in details.

Lightwave-driven quasiparticle collisions on a subcycle timescale

Ever since Ernest Rutherford first scattered α-particles from gold foils1, collision experiments have revealed unique insights into atoms, nuclei, and elementary particles2. In solids, many-body correlations also lead to characteristic resonances3, called quasiparticles, such as excitons, dropletons4, polarons, or Cooper pairs. Their structure and dynamics define spectacular macroscopic phenomena, ranging from Mott insulating states via spontaneous spin and charge order to high-temperature superconductivity5. Fundamental research would immensely benefit from quasiparticle colliders, but the notoriously short lifetimes of quasiparticles6 have challenged practical solutions. Here we exploit lightwave-driven charge transport7–24, the backbone of attosecond science9–13, to explore ultrafast quasiparticle collisions directly in the time domain: A femtosecond optical pulse creates excitonic electron–hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes. The underlying wave packet dynamics, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands17–19 of the optical excitation. A full quantum theory explains our observations microscopically. This approach opens the door to collision experiments with a broad variety of complex quasiparticles and suggests a promising new way of sub-femtosecond pulse generation.

DNA and Protein Requirements for Substrate Conformational Changes Necessary for Human Flap Endonuclease-1-catalyzed Reaction

Human flap endonuclease-1 (hFEN1) catalyzes the essential removal of single-stranded flaps arising at DNA junctions during replication and repair processes. hFEN1 biological function must be precisely controlled, and consequently, the protein relies on a combination of protein and substrate conformational changes as a prerequisite for reaction. These include substrate bending at the duplex-duplex junction and transfer of unpaired reacting duplex end into the active site. When present, 5′-flaps are thought to thread under the helical cap, limiting reaction to flaps with free 5′-termini in vivo. Here we monitored DNA bending by FRET and DNA unpairing using 2-aminopurine exciton pair CD to determine the DNA and protein requirements for these substrate conformational changes. Binding of DNA to hFEN1 in a bent conformation occurred independently of 5′-flap accommodation and did not require active site metal ions or the presence of conserved active site residues. More stringent requirements exist for transfer of the substrate to the active site. Placement of the scissile phosphate diester in the active site required the presence of divalent metal ions, a free 5′-flap (if present), a Watson-Crick base pair at the terminus of the reacting duplex, and the intact secondary structure of the enzyme helical cap. Optimal positioning of the scissile phosphate additionally required active site conserved residues Tyr40, Asp181, and Arg100 and a reacting duplex 5′-phosphate. These studies suggest a FEN1 reaction mechanism where junctions are bound and 5′-flaps are threaded (when present), and finally the substrate is transferred onto active site metals initiating cleavage.

Microscopic analysis of polymer honeycomb thin film studied by PeakForce TUNA for organic solar cell application

New methodologies connecting molecular structure, self-organization, and nanoelectronics are important for the construction of better bulk heterojunction solar cells. In the present work, large area conjugated polymer honeycomb thin films were prepared from poly {2, 5-bis [3-N, N-diethylamino)-1-oxapropyl]-1, 4-phenylenevinylene} (P1) and EG-C60 (1:1 wt/wt). Further, surface morphology and conductivity of honeycomb thin films were studied using conductive-atomic force microscopy. The morphological studies clearly confirm that the EG-C60 molecules are uniformly present only at the nodes and frames of honeycomb structured blend film (which avoids the formation of exciton pair recombination of polymer), whereas the TUNA current map collected at positive and negative biases reveal holes and electrons collection networks corresponding to donor and acceptor phases at honeycomb thin films. This discovery could find applications in fully exploiting the potential of various material systems, and may open up new opportunities to improve the efficiency of organic solar cells.

Solvent‐induced surface morphology of polymer honeycomb thin films for power generating solar windows

Here, the role of two different solvents viz. chloroform (CHCl3) and chlorobenzene (CB) for the formation and dissolution of honeycomb‐structured conductive polymer poly[2,5‐bis [3‐N,N‐diethylamino)−1‐oxapropyl]‐1,4‐phenylenevinylene] (P1) thin films was demonstrated. Surface morphology of the films was investigated using atomic force microscopy (AFM) and it was confirmed that CB was a suitable solvent to obtain a better honeycomb structure over the CHCl3. Also, the film dissolution studies were carried out by immersing the honeycomb structured polymer films in their respective solvents for the specified interval of time, which show that the rate of film dissolution was prominent in CB than CHCl3. Based on the solvent selection, honeycomb structured blend as well as heterojunction (HJ) films were prepared from P1 and EG‐C60 (1:1 wt/wt). Here, morphological studies clearly confirmed that the EG‐C60 molecules were uniformly present only at the nodes and frames of the honeycomb structured blend film (which avoids the formation of exciton pair recombination of polymer), whereas in case of heterojunction film EG‐C60 molecules were unevenly distributed, this discovery could find use in fully exploiting the potential of various material systems, and open up new opportunities to improve the efficiency of OSCs. © 2016 American Institute of Chemical Engineers Environ Prog, 35: 1207–1214, 2016

Observation of a Biexciton Wigner Molecule by Fractional Optical Aharonov-Bohm Oscillations in a Single Quantum Ring.

The Aharonov-Bohm effect in ring structures in the presence of electronic correlation and disorder is an open issue. We report novel oscillations of a strongly correlated exciton pair, similar to a Wigner molecule, in a single nanoquantum ring, where the emission energy changes abruptly at the transition magnetic field with a fractional oscillation period compared to that of the exciton, a so-called fractional optical Aharonov-Bohm oscillation. We have also observed modulated optical Aharonov-Bohm oscillations of an electron-hole pair and an anticrossing of the photoluminescence spectrum at the transition magnetic field, which are associated with disorder effects such as localization, built-in electric field, and impurities.

Density of states and excitonic condensation in the double layer correlated systems

We consider the single-particle density of states (DOS) in the strongly correlated double layer (DL) system, without applied external fields. We demonstrate an unusual collapse effect in the spectrum of the normal single-particle spectral function at the particular high-symmetry point corresponding to the specific bunching-point solution of the chemical potential in the Frenkel channel. We show that at the low-temperature limit the anomalous spectral function obeys a concave like structure, which is directly related to the interlayer pair formation and condensation. We calculate the normal DOS functions, and we find their temperature dependence for different values of the interlayer Coulomb interaction parameter. We show that the normal electron and hole DOS functions demonstrate typical condensates double peak structures on the background of the excitonic pair formation quasiparticle spectra and we have found the evidence of the hybridization gap in the case of high-temperature limit, and small interlayer coupling parameter. Meanwhile, we show a possible crossover from the excitonic condensate regime into the band insulator state. The structure of the normal DOS spectra, in the Frenkel channel and for the strong interlayer coupling regime, is found gapless for all temperature limits, which clearly indicates the strong coherence effects in the DL structure, and the excitonic condensates therein. We have shown that the excitonic pair formation and pair condensation occur simultaneously in the DL system, in contrast with the purely three-dimensional (3D) or two-dimensional cases (2D), discussed previously.

Real-time observation of multiexcitonic states in ultrafast singlet fission using coherent 2D electronic spectroscopy.

Singlet fission is the spin-allowed conversion of a spin-singlet exciton into a pair of spin-triplet excitons residing on neighbouring molecules. To rationalize this phenomenon, a multiexcitonic spin-zero triplet-pair state has been hypothesized as an intermediate in singlet fission. However, the nature of the intermediate states and the underlying mechanism of ultrafast fission have not been elucidated experimentally. Here, we study a series of pentacene derivatives using ultrafast two-dimensional electronic spectroscopy and unravel the origin of the states involved in fission. Our data reveal the crucial role of vibrational degrees of freedom coupled to electronic excitations that facilitate the mixing of multiexcitonic states with singlet excitons. The resulting manifold of vibronic states drives sub-100 fs fission with unity efficiency. Our results provide a framework for understanding singlet fission and show how the formation of vibronic manifolds with a high density of states facilitates fast and efficient electronic processes in molecular systems.

Solid state photon upconversion utilizing thermally activated delayed fluorescence molecules as triplet sensitizer

The ability to upconvert light is useful for a range of applications, from biological imaging to solar cells. But modern technologies have struggled to upconvert incoherent incident light at low intensities. Here, we report solid state photon upconversion employing triplet-triplet exciton annihilation in an organic semiconductor, sensitized by a thermally activated-delayed fluorescence (TADF) dye. Compared to conventional phosphorescent sensitizers, the TADF dye maximizes the wavelength shift in upconversion due to its small singlet-triplet splitting. The efficiency of energy transfer from the TADF dye is 9.1%, and the conversion yield of sensitizer exciton pairs to singlet excitons in the annihilator is 1.1%. Our results demonstrate upconversion in solid state geometries and with non-heavy metal-based sensitizer materials.

Identification of a triplet pair intermediate in singlet exciton fission in solution

Singlet exciton fission is the spin-conserving transformation of one spin-singlet exciton into two spin-triplet excitons. This exciton multiplication mechanism offers an attractive route to solar cells that circumvent the single-junction Shockley-Queisser limit. Most theoretical descriptions of singlet fission invoke an intermediate state of a pair of spin-triplet excitons coupled into an overall spin-singlet configuration, but such a state has never been optically observed. In solution, we show that the dynamics of fission are diffusion limited and enable the isolation of an intermediate species. In concentrated solutions of bis(triisopropylsilylethynyl)[TIPS]--tetracene we find rapid (<100 ps) formation of excimers and a slower (∼ 10 ns) break up of the excimer to two triplet exciton-bearing free molecules. These excimers are spectroscopically distinct from singlet and triplet excitons, yet possess both singlet and triplet characteristics, enabling identification as a triplet pair state. We find that this triplet pair state is significantly stabilized relative to free triplet excitons, and that it plays a critical role in the efficient endothermic singlet fission process.

Phase coherence and spectral functions in the two-dimensional excitonic systems

The nonlocal correlation mechanism between excitonic pairs is considered for a two dimensional exciton system. On the base of the unitary decomposition of the usual electron operator, we include the electron phase degrees of freedom into the problem of interacting excitons. Applying the path integral formalism, we treat the excitonic insulator state (EI) and the Bose–Einstein condensation (BEC) of preformed excitonic pairs as two independent problems. For the BEC of excitons the phase field variables play a crucial role. We derive the expression of the local EI order parameter by integrating out the phase variables. Then, considering the zero temperature limit, we obtain the excitonic BEC transition probability function, by integrating out the fermions. We calculate the normal excitonic Green functions for the conduction and valence band electrons and we derive the excitonic spectral functions, both analytically and numerically. Different values of the Coulomb interaction parameter are considered.

A variational Monte Carlo study of exciton condensation

Exciton condensation in a two-band Hubbard model on a square lattice is studied with variational Monte Carlo method. We show that the phase transition from an excitonic insulator to a band insulator is induced by increasing the interband Coulomb interaction. To examine the character of the exciton condensation, the exciton pair amplitudes both in k-space and in real space are calculated. Using these quantities, we discuss the BCS-BEC crossover within the excitonic insulator phase.

Plasmonics: Hybrid Graphene–Giant Nanocrystal Quantum Dot Assemblies with Highly Efficient Biexciton Emission (Advanced Optical Materials 1/2015)

When giant nanocrystal quantum dots (g-NQDs) are coupled to graphene, pairs of excitons optically excited in the g-NQDs experience efficient recombination and subsequent simultaneous emission of photon pairs. This enhancement is provided by a localized plasmon supported by a charge puddle that forms underneath the photocharged g-NQD. On page 39, H. Htoon and co-workers reveal the tremendous potential of graphene-g-NQD hybrids for lasing and entangled photon source applications.

Solution-processable singlet fission photovoltaic devices.

We demonstrate the successful incorporation of a solution-processable singlet fission material, 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene), into photovoltaic devices. TIPS-pentacene rapidly converts high-energy singlet excitons into pairs of triplet excitons via singlet fission, potentially doubling the photocurrent from high-energy photons. Low-energy photons are captured by small-bandgap electron-accepting lead chalcogenide nanocrystals. This is the first solution-processable singlet fission system that performs with substantial efficiency with maximum power conversion efficiencies exceeding 4.8%, and external quantum efficiencies of up to 60% in the TIPS-pentacene absorption range. With PbSe nanocrystal of suitable bandgap, its internal quantum efficiency reaches 170 ± 30%.