Showing papers on "Exciton published in 2009"

Excitonic Effects on the Optical Response of Graphene and Bilayer Graphene

Abstract: We present first-principles calculations of many-electron effects on the optical response of graphene, bilayer graphene, and graphite employing the GW-Bethe Salpeter equation approach We find that resonant excitons are formed in these two-dimensional semimetals The resonant excitons give rise to a prominent peak in the absorption spectrum near 45 eV with a different line shape and significantly redshifted peak position from those of an absorption peak arising from interband transitions in an independent quasiparticle picture In the infrared regime, our calculated optical absorbance per graphene layer is approximately a constant, 24%, in agreement with recent experiments; additional low frequency features are found for bilayer graphene because of band structure effects

Determining exciton bandwidth and film microstructure in polythiophene films using linear absorption spectroscopy

Abstract: We analyze the linear absorption spectrum of regioregular poly(3-hexylthiophene) films spun from a variety of solvents to probe directly the film microstructure and how it depends on processing conditions. We estimate the exciton bandwidth and the percentage of the film composed of aggregates quantitatively using a weakly interacting H-aggregate model. This provides a description of the degree and quality of crystallites within the film and is in turn correlated with thin-film field-effect transistor characteristics.

Triplet states in organic semiconductors

Abstract: Today's technology is not possible without optoelectronic devices such as light-emitting diodes, transistors and solar cells. These basic units of modern electronic appliances may be made not only from traditional inorganic semiconductors, but also from organic semiconductors, i.e. hydrocarbon molecules that combine semiconducting properties with some mechanical properties such as easy processability and flexibility. The weak van der Waals forces that bind the molecules to a solid imply a low dielectric constant, so that coulomb and exchange interactions between electrons are significant. As a result, photoexcitation or electrical excitation results in strongly bound electron–hole pairs, so-called excitons. Depending on the relative orientation of the electron and hole spin, the exciton may be of a overall singlet or triplet spin state. While the fluorescent singlet state has been investigated intensively since the first reports of organic electroluminescence, research into the properties of the phosphorescent triplet state has intensified mainly during the last decade. In this review we give an overview on the photophysical processes associated with the formation of triplet states and their decay, as well as the energy levels and energy transfer processes of triplet states. We aim to give a careful introduction for those new to this particular research area as well as to highlight some of the current research issues and intriguing questions for those familiar with the field. The main focus of this review is on molecular assemblies and polymer films, though relevant work on molecular crystals is also included where it assists in forming a larger picture.

Exciton diffusion lengths of organic semiconductor thin films measured by spectrally resolved photoluminescence quenching

Abstract: We demonstrate spectrally resolved photoluminescence quenching as a means to determine the exciton diffusion length of several archetype organic semiconductors used in thin film devices. We show that aggregation and crystal orientation influence the anisotropy of the diffusion length for vacuum-deposited polycrystalline films. The measurement of the singlet diffusion lengths is found to be in agreement with diffusion by Forster transfer, whereas triplet diffusion occurs primarily via Dexter transfer.

Coherent Multidimensional Optical Spectroscopy of Excitons in Molecular Aggregates; Quasiparticle versus Supermolecule Perspectives

Abstract: Molecular aggregates are abundant in nature; they form spontaneously in concentrated solutions and on surfaces and can be synthesized by supramolecular chemistry techniques.1-3 Assemblies of chromophores play important roles in many biological processes such as light-harvesting and primary * To whom correspondence should be addressed. E-mail: smukamel@uci.edu. † University of California Irvine. ‡ Universität Würzburg. § Charles University. Chem. Rev. 2009, 109, 2350–2408 2350

Bimolecular Crystals of Fullerenes in Conjugated Polymers and the Implications of Molecular Mixing for Solar Cells

Abstract: The performance of polymer:fullerene bulk heterojunction solar cells is heavily influenced by the interpenetrating nanostructure formed by the two semiconductors because the size of the phases, the nature of the interface, and molecular packing affect exciton dissociation, recombination, and charge transport. Here, X-ray diffraction is used to demonstrate the formation of stable, well-ordered bimolecular crystals of fullerene intercalated between the side-chains of the semiconducting polymer poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene. It is shown that fullerene intercalation is general and is likely to occur in blends with both amorphous and semicrystalline polymers when there is enough free volume between the side-chains to accommodate the fullerene molecule. These findings offer explanations for why luminescence is completely quenched in crystals much larger than exciton diffusion lengths, how the hole mobility of poly(2-methoxy-5-(3′,7′-dimethyloxy)-p-phylene vinylene) increases by over 2 orders of magnitude when blended with fullerene derivatives, and why large-scale phase separation occurs in some polymer:fullerene blend ratios while thermodynamically stable mixing on the molecular scale occurs for others. Furthermore, it is shown that intercalation of fullerenes between side chains mostly determines the optimum polymer:fullerene blending ratios. These discoveries suggest a method of intentionally designing bimolecular crystals and tuning their properties to create novel materials for photovoltaic and other applications.

Near-IR femtosecond transient absorption spectroscopy of ultrafast polaron and triplet exciton formation in polythiophene films with different regioregularities.

Abstract: The formation dynamics of polaron pairs, polarons, and triplet excitons in regiorandom and regioregular poly(3-hexylthiophene) (RRa-P3HT and RR-P3HT) films was comprehensively studied by transient absorption spectroscopy over the wide wavelength region from 500 to 1650 nm under various excitation intensities. In both RRa-P3HT and RR-P3HT films, polaron pairs were generated not from relaxed singlet exciton states but from hot excitons on a time scale of <100 fs and decayed monomolecularly by geminate recombination. In RRa-P3HT films, triplet excitons were rapidly generated on a picosecond time scale from higher exciton states produced by the singlet exciton−exciton annihilation as well as from the lowest singlet exciton states by the normal intersystem crossing. In RR-P3HT films, no triplet excitons were observed; polarons were also generated not from relaxed singlet exciton states but from hot excitons in competition with the formation of polaron pairs. The polarons formed in RR-P3HT can freely migrate an...

Charge-transfer excitons at organic semiconductor surfaces and interfaces.

Abstract: When a material of low dielectric constant is excited electronically from the absorption of a photon, the Coulomb attraction between the excited electron and the hole gives rise to an atomic H-like quasi-particle called an exciton. The bound electron-hole pair also forms across a material interface, such as the donor/acceptor interface in an organic heterojunction solar cell; the result is a charge-transfer (CT) exciton. On the basis of typical dielectric constants of organic semiconductors and the sizes of conjugated molecules, one can estimate that the binding energy of a CT exciton across a donor/acceptor interface is 1 order of magnitude greater than k(B)T at room temperature (k(B) is the Boltzmann constant and T is the temperature). How can the electron-hole pair escape this Coulomb trap in a successful photovoltaic device? To answer this question, we use a crystalline pentacene thin film as a model system and the ubiquitous image band on the surface as the electron acceptor. We observe, in time-resolved two-photon photoemission, a series of CT excitons with binding energies < or = 0.5 eV below the image band minimum. These CT excitons are essential solutions to the atomic H-like Schrodinger equation with cylindrical symmetry. They are characterized by principal and angular momentum quantum numbers. The binding energy of the lowest lying CT exciton with 1s character is more than 1 order of magnitude higher than k(B)T at room temperature. The CT(1s) exciton is essentially the so-called exciplex and has a very low probability of dissociation. We conclude that hot CT exciton states must be involved in charge separation in organic heterojunction solar cells because (1) in comparison to CT(1s), hot CT excitons are more weakly bound by the Coulomb potential and more easily dissociated, (2) density-of-states of these hot excitons increase with energy in the Coulomb potential, and (3) electronic coupling from a donor exciton to a hot CT exciton across the D/A interface can be higher than that to CT(1s) as expected from energy resonance arguments. We suggest a design principle in organic heterojunction solar cells: there must be strong electronic coupling between molecular excitons in the donor and hot CT excitons across the D/A interface.

Quantitative modeling of the role of surface traps in CdSe/CdS/ZnS nanocrystal photoluminescence decay dynamics

Abstract: Charge carrier trapping is an important phenomenon in nanocrystal (NC) decay dynamics because it reduces photoluminescence (PL) quantum efficiencies and obscures efforts to understand the interaction of NC excitons with their surroundings. Particularly crucial to our understanding of excitation dynamics in, e.g., multiNC assemblies, would be a way of differentiating between processes involving trap states and those that do not. Direct optical measurement of NC trap state processes is not usually possible because they have negligible transition dipole moments; however, they are known to indirectly affect exciton photoluminescence. Here, we develop a framework, based on Marcus electron transfer theory, to determine NC trap state dynamics from time-resolved NC exciton PL measurements. Our results demonstrate the sensitivity of PL to interfacial dynamics, indicating that the technique can be used as an indirect but effective probe of trap distribution changes. We anticipate that this study represents a step toward understanding how excitons in nanocrystals interact with their surroundings: a quality that must be optimized for their efficient application in photovoltaics, photodetectors, or chemical sensors.

Exciton-Dissociation and Charge-Recombination Processes in Pentacene/C60 Solar Cells: Theoretical Insight into the Impact of Interface Geometry

Abstract: The exciton-dissociation and charge-recombination processes in organic solar cells based on pentacene/C60 heterojunctions are investigated by means of quantum-mechanical calculations. The electronic couplings and the rates of exciton dissociation and charge recombination have been evaluated for several geometrical configurations of the pentacene/C60 complex, which are relevant to bilayer and bulk heterojunctions. The results suggest that, irrespective of the actual pentacene−fullerene orientation, both pentacene-based and C60-based excitons are able to dissociate efficiently. Also, in the case of parallel configurations of the molecules at the pentacene/C60 interface, the decay of the lowest charge-transfer state to the ground state is calculated to be very fast; as a result, it can compete with the dissociation process into mobile charge carriers. Since parallel configurations are expected to be found more frequently in bulk heterojunctions than in bilayer heterojunctions, the performance of pentacene/C6...

Trion Decay in Colloidal Quantum Dots

Abstract: Using charged films of colloidal CdSe/CdS core/shell quantum dots of ∼3.5 to 4.5 nm core diameters and 0.6 to 1.2 nm thick CdS shells, the radiative and nonradiative decay of the negatively charged exciton, the trion T−, are measured. The T− radiative rate is faster than the exciton by a factor of 2.2 ± 0.4 and estimated at ∼10 ns. The T− lifetime is ∼0.7−1.5 ns for the samples measured and is longer than the biexciton lifetime by a factor or 7.5 ± 1.7.

Two-quantum 2D FT electronic spectroscopy of biexcitons in GaAs quantum wells.

Abstract: The motions of electrons in solids may be highly correlated by strong, long-range Coulomb interactions. Correlated electron-hole pairs (excitons) are accessed spectroscopically through their allowed single-quantum transitions, but higher-order correlations that may strongly influence electronic and optical properties have been far more elusive to study. Here we report direct observation of bound exciton pairs (biexcitons) that provide incisive signatures of four-body correlations among electrons and holes in gallium arsenide (GaAs) quantum wells. Four distinct, mutually coherent, ultrashort optical pulses were used to create coherent exciton states, transform these successively into coherent biexciton states and then new radiative exciton states, and finally to read out the radiated signals, yielding biexciton binding energies through a technique closely analogous to multiple-quantum two-dimensional Fourier transform (2D FT) nuclear magnetic resonance spectroscopy. A measured variation of the biexciton dephasing rate indicated still higher-order correlations.

Determining exciton coherence from the photoluminescence spectral line shape in poly(3-hexylthiophene) thin films.

Abstract: The photoluminescence (PL) spectral line shape of regioregular poly(3-hexylthiophene) thin films is analyzed using a model which treats the polymer pi-stacks as H-aggregates with exciton-vibrational coupling and spatially correlated site disorder The Stokes shift, linewidth, and relative vibronic peak intensities in the low-temperature PL spectrum (T=10 K) are accurately reproduced, allowing the coherence function corresponding to the lowest energy (emitting) exciton to be determined from the ratio of the 0-0 to 0-1 peak intensities The exciton migration length is determined from the N-dependent Stokes shift, where N is the number of segments comprising the stack Based on the temperature dependence of the PL spectrum it is concluded that emission arises from a low concentration of aggregates which are more disordered than the dominant species responsible for absorption The emissive aggregates are characterized by shorter average conjugation lengths and hence greater exciton bandwidths The coherence length of the emitting exciton is estimated to be only three lattice spacings ( approximately 11 nm) along the pi-stacking direction By contrast, the exciton migration length for incoherent hopping between coherent domains is estimated to be approximately 15 nm

Variations in the Quantum Efficiency of Multiple Exciton Generation for a Series of Chemically Treated PbSe Nanocrystal Films

Abstract: We study multiple exciton generation (MEG) in two series of chemically treated PbSe nanocrystal (NC) films. We find that the average number of excitons produced per absorbed photon varies between 1.0 and 2.4 (±0.2) at a photon energy of ∼4Eg for films consisting of 3.7 nm NCs and between 1.1 and 1.6 (±0.1) at hν ∼ 5Eg for films consisting of 7.4 nm NCs. The variations in MEG depend upon the chemical treatment used to electronically couple the NCs in each film. The single and multiexciton lifetimes also change with the chemical treatment: biexciton lifetimes increase with stronger inter-NC electronic coupling and exciton delocalization, while single exciton lifetimes decrease after most treatments relative to the same NCs in solution. Single exciton lifetimes are particularly affected by surface treatments that dope the films n-type, which we tentatively attribute to an Auger recombination process between a single exciton and an electron produced by ionization of the dopant donor. These results imply that ...

Morphology effectively controls singlet-triplet exciton relaxation and charge transport in organic semiconductors.

Abstract: We present a comparative study of ultrafast photoconversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photoinduced absorption in Tc and Pc crystals is activated and temperature-independent, respectively, demonstrating dominant singlettriplet exciton fission. In Pc films (as well as C60-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.

Size and mobility of excitons in (6, 5) carbon nanotubes

Abstract: Knowledge of excited-state dynamics in carbon nanotubes is determinant for their prospective use in optoelectronic applications. It is known that primary photoexcitations are quasi-one-dimensional excitons, the electron–hole correlation length (‘exciton size’) of which corresponds to a finite volume in the phase space. This volume can be directly measured by nonlinear spectroscopy provided the time resolution is short enough for probing before population relaxation. Here, we report on the experimental determination of exciton size and mobility in (6, 5) carbon nanotubes. The samples are sodium cholate suspensions of nanotubes (produced by the CoMoCat method) obtained by density-gradient ultracentrifugation. By using sub-15 fs near-infrared pulses to measure the nascent bleach of the lowest exciton resonance, we estimate the exciton size to be 2.0±0.7 nm. Exciton–exciton annihilation in our samples is found to be rather inefficient so that many excitons can coexist on a single nanotube. An accurate determination of the size and diffusion length of excitons generated with single-walled nanotubes supports the Wannier–Mott picture of their behaviour, and improves the outlook for the use of nanotubes in optoelectronics and biosensing applications.

Coherent coupling between surface plasmons and excitons in semiconductor nanocrystals

Abstract: We present an experimental demonstration of strong coupling between a surface plasmon propagating on a planar silver substrate, and the lowest excited state of CdSe nanocrystals. Variable-angle spectroscopic ellipsometry measurements demonstrated the formation of plasmon-exciton mixed states, characterized by a Rabi splitting of $\sim$ 82 meV at room temperature. Such a coherent interaction has the potential for the development of plasmonic non-linear devices, and furthermore, this system is akin to those studied in cavity quantum electrodynamics, thus offering the possibility to study the regime of strong light-matter coupling in semiconductor nanocrystals at easily accessible experimental conditions.

Uniform exciton fluorescence from individual molecular nanotubes immobilized on solid substrates

Abstract: Self-assembled quasi one-dimensional nanostructures of π-conjugated molecules1,2,3,4,5,6,7,8,9,10,11,12,13,14,15 may find a use in devices owing to their intriguing optoelectronic properties, which include sharp exciton transitions1,2,3,4,5, strong circular dichroism5,6,7, high exciton mobilities8,9 and photoconductivity10. However, many applications require immobilization of these nanostructures on a solid substrate, which is a challenge to achieve without destroying their delicate supramolecular structure. Here, we use a drop-flow technique to immobilize double-walled tubular J-aggregates of amphiphilic cyanine dyes without affecting their morphological or optical properties. High-resolution images of the topography and exciton fluorescence of individual J-aggregates are obtained simultaneously with polarization-resolved near-field scanning optical microscopy. These images show remarkably uniform supramolecular structure, both along individual nanotubes and between nanotubes in an ensemble, demonstrating their potential for light harvesting and energy transport. Individual double-walled tubular aggregates are immobilized on a solid substrate out of solution using a drop-flow technique. Using near-field scanning optical microscopy, these aggregates are shown to have a remarkably uniform supramolecular structure.

High efficiency organic multilayer photodetectors based on singlet exciton fission

Abstract: We employ an exciton fission process that converts one singlet exciton into two triplet excitons to increase the quantum efficiency of an organic multilayer photodetector beyond 100%. The photodetector incorporates ultrathin alternating donor-acceptor layers of pentacene and C60, respectively. By comparing the quantum efficiency after separate pentacene and C60 photoexcitation we find that singlet exciton fission in pentacene enhances the quantum efficiency by (45±7)%. In quantitative agreement with this result, we also observe that the photocurrent generated from pentacene excitons is decreased by (2.7±0.2)% under an applied magnetic field of H=0.4 T, while the C60 photocurrent is relatively unchanged.

Tunable magnetic exchange interactions in manganese-doped inverted core–shell ZnSe–CdSe nanocrystals

Abstract: Magnetic doping of semiconductor nanostructures is actively pursued for applications in magnetic memory and spin-based electronics. Central to these efforts is a drive to control the interaction strength between carriers (electrons and holes) and the embedded magnetic atoms. In this respect, colloidal nanocrystal heterostructures provide great flexibility through growth-controlled 'engineering' of electron and hole wavefunctions in individual nanocrystals. Here, we demonstrate a widely tunable magnetic sp-d exchange interaction between electron-hole excitations (excitons) and paramagnetic manganese ions using 'inverted' core-shell nanocrystals composed of Mn(2+)-doped ZnSe cores overcoated with undoped shells of narrower-gap CdSe. Magnetic circular dichroism studies reveal giant Zeeman spin splittings of the band-edge exciton that, surprisingly, are tunable in both magnitude and sign. Effective exciton g-factors are controllably tuned from -200 to +30 solely by increasing the CdSe shell thickness, demonstrating that strong quantum confinement and wavefunction engineering in heterostructured nanocrystal materials can be used to manipulate carrier-Mn(2+) wavefunction overlap and the sp-d exchange parameters themselves.

Structure–Property Relationship of Pyridine-Containing Triphenyl Benzene Electron-Transport Materials for Highly Efficient Blue Phosphorescent OLEDs

Abstract: Three triphenyl benzene derivatives of 1,3,5-tri(m-pyrid-2-yl-phenyl)benzene (Tm2PyPB), 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (Tm3PyPB) and 1,3,5-tri(m-pyrid-4-yl-phenyl)benzene (Tm4PyPB), containing pyridine rings at the periphery, are developed as electron-transport and hole/exciton-blocking materials for iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C2′)picolinate (FIrpic)-based blue phosphorescent organic light-emitting devices. Their highest occupied molecular orbital and lowest unoccupied molecular orbital (LUMO) energy levels decrease as the nitrogen atom of the pyridine ring moves from position 2 to 3 and 4; this is supported by both experimental results and density functional theory calculations, and gives improved electron-injection and hole-blocking properties. They exhibit a high electron mobility of 10−4–10−3 cm2 V−1 s−1 and a high triplet energy level of 2.75 eV. Confinement of FIrpic triplet excitons is strongly dependent on the nitrogen atom position of the pyridine ring. The second exponential decay component in the transient photoluminescence decays of Firpic-doped films also decreases when the position of the nitrogen atom in the pyridine ring changes. Reduced driving voltages are obtained when the nitrogen atom position changes because of improved electron injection as a result of the reduced LUMO level, but a better carrier balance is achieved for the Tm3PyPB-based device. An external quantum efficiency (EQE) over 93% of maximum EQE was achieved for the Tm4PyPB-based device at an illumination-relevant luminance of 1000 cd m−2, indicating reduced efficiency roll-off due to better confinement of FIrpic triplet excitons by Tm4PyPB in contrast to Tm2PyPB and Tm3PyPB.

Making tracks: electronic excitation roles in forming swift heavy ion tracks

Abstract: Swift heavy ions cause material modification along their tracks, changes primarily due to their very dense electronic excitation. The available data for threshold stopping powers indicate two main classes of materials. Group I, with threshold stopping powers above about 10 keV nm −1 , includes some metals, crystalline semiconductors and a few insulators. Group II, with lower thresholds, comprises many insulators, amorphous materials and high Tc oxide superconductors. We show that the systematic differences in behaviour result from different coupling of the dense excited electrons, holes and excitons to atomic (ionic) motions, and the consequent lattice relaxation. The coupling strength of excitons and charge carriers with the lattice is crucial. For group II, the mechanism appears to be the self-trapped exciton model of Itoh and Stoneham (1998 Nucl. Instrum. Methods Phys. Res. B 146 362): the local structural changes occur roughly when the exciton concentration exceeds the number of lattice sites. In materials of group I, excitons are not self-trapped and structural change requires excitation of a substantial fraction of bonding electrons, which induces spontaneous lattice expansion within a few hundred femtoseconds, as recently observed by laser-induced time-resolved x-ray diffraction of semiconductors. Our analysis addresses a number of experimental results, such as track morphology, the efficiency of track registration and the ratios of the threshold stopping power of various materials. (Some figures in this article are in colour only in the electronic version) This paper celebrates the contribution made by Dr Richard Palmer to IOP Publishing, and especially to Journal of Physics: Condensed Matter.

Exciton condensation and charge fractionalization in a topological insulator film.

Abstract: An odd number of gapless Dirac fermions is guaranteed to exist at a surface of a strong topological insulator. We show that in a thin-film geometry and under external bias, electron-hole pairs that reside in these surface states can condense to form a novel exotic quantum state which we propose to call ``topological exciton condensate'' (TEC). This TEC is similar in general terms to the exciton condensate recently argued to exist in a biased graphene bilayer, but with different topological properties. It exhibits a host of unusual properties including a stable zero mode and a fractional charge $\ifmmode\pm\else\textpm\fi{}e/2$ carried by a singly quantized vortex in the TEC order parameter.

Size Dependence of the Spontaneous Emission Rate and Absorption Cross Section of CdSe and CdTe Quantum Dots

Abstract: In this paper, the size dependence of the band gap, of the spontaneous emission rate, and of the absorption cross section of quantum dots is systematically investigated over a wide size range, using colloidal CdSe and CdTe QDs as model systems (diameters ranging from 12 to 8 nm and from 2 to 95 nm, respectively) The size dependence of the band gap is well-described by theoretical models, and is dominated by the quantum confinement contribution (1/d2 scaling) The spontaneous emission rate increases linearly with the emission frequency for both CdSe and CdTe QDs, in good agreement with theoretical predictions By extrapolating the frequency dependence of the emission rates to the bulk band gap values, the exciton radiative lifetime in bulk CdSe and CdTe could be estimated for the first time (viz, 18 and 20 ns, respectively) Comparison between the empirical trends and theoretical predictions provides new fundamental insights into the size dependence of the 1S(e)1S3/2(h) oscillator strengths of QDs, bot

Carrier Dynamics and Quantum Confinement in type II ZB-WZ InP Nanowire Homostructures

Abstract: We use time-resolved photoluminescence from single InP nanowires containing both wurtzite (WZ) and zincblende (ZB) crystalline phases to measure the carrier dynamics of quantum confined excitons in a type-II homostructure. The observed recombination lifetime increases by nearly 2 orders of magnitude from 170 ps for excitons above the conduction and valence band barriers to more than 8400 ps for electrons and holes that are strongly confined in quantum wells defined by monolayer-scale ZB sections in a predominantly WZ nanowire. A simple computational model, guided by detailed high-resolution transmission electron microscopy measurements from a single nanowire, demonstrates that the dynamics are consistent with the calculated distribution of confined states for the electrons and holes.

Electrical control of spontaneous emission and strong coupling for a single quantum dot

Abstract: We report the design, fabrication and optical investigation of electrically tunable single quantum dots—photonic crystal defect nanocavities operating in both the weak and strong coupling regimes of the light–matter interaction. Unlike previous studies where the dot–cavity spectral detuning was varied by changing the lattice temperature, or by the adsorption of inert gases at low temperatures, we demonstrate that the quantum-confined Stark effect can be employed to quickly and reversibly switch the dot–cavity coupling simply by varying a gate voltage. Our results show that exciton transitions from individual dots can be tuned by ~4 meV relative to the nanocavity mode before the emission quenches due to carrier tunneling escape. This range is much larger than the typical linewidth of the high-Q cavity modes (~100 μeV) allowing us to explore and contrast regimes where the dots couple to the cavity or decay by spontaneous emission into the two-dimensional photonic bandgap. In the weak-coupling regime, we show that the dot spontaneous emission rate can be tuned using a gate voltage, with Purcell factors ≥7. New information is obtained on the nature of the dot–cavity coupling in the weak coupling regime, and electrical control of zero-dimensional polaritons is demonstrated for the highest-Q cavities (Q≥12 000). Vacuum Rabi splittings up to ~120 μeV are observed, larger than the linewidths of either the decoupled exciton (γ≤40 μeV) or cavity mode. These observations represent a voltage switchable optical nonlinearity at the single photon level, paving the way towards on-chip dot-based nano-photonic devices that can be integrated with passive optical components.

An inverted organic solar cell with an ultrathin Ca electron-transporting layer and MoO3 hole-transporting layer

Abstract: An inverted organic solar cell based on poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61 (PCBM) was fabricated with an ultrathin Ca electron-transporting layer and MoO3 hole-transporting layer. The 1 nm Ca on indium tin oxide (ITO) electrode modifies the work function of ITO suitable for electron extraction. An appropriate thickness of MoO3 hole extraction layer is also essential to effectively prevent exciton quenching at the Ag anode, yet not introduce much voltage loss and series resistance. The optical field distribution across the active layer was also simulated to discuss the effect of MoO3 thickness on the photocurrent. The maximum power conversion efficiency obtained was 3.55% under simulated 100 mW/cm2 (AM 1.5G) solar irradiation.