Showing papers on "Exciton published in 2011"
TL;DR: This ab initio study characterizes the low-lying excited states in acene molecular crystals in order to describe how SF occurs in a realistic crystal environment and shows how intermolecular interactions are shown to localize the initially delocalized bright state onto a pair of monomers.
Abstract: Singlet fission (SF) could dramatically increase the efficiency of organic solar cells by producing two triplet excitons from each absorbed photon. While this process has been known for decades, most descriptions have assumed the necessity of a charge-transfer intermediate. This ab initio study characterizes the low-lying excited states in acene molecular crystals in order to describe how SF occurs in a realistic crystal environment. Intermolecular interactions are shown to localize the initially delocalized bright state onto a pair of monomers. From this localized state, nonadiabatic coupling mediated by intermolecular motion between the optically allowed exciton and a dark multi-exciton state facilitates SF without the need for a nearby low-lying charge-transfer intermediate. An estimate of the crossing rate shows that this direct quantum mechanical process occurs in well under 1 ps in pentacene. In tetracene, the dark multi-exciton state is uphill from the lowest singlet excited state, resulting in a d...
420 citations
TL;DR: If the MEG efficiency can be further enhanced and charge separation and transport can be optimized within QD films, then QD solar cells can lead to third-generation solar energy conversion technologies.
Abstract: Multiple exciton generation in quantum dots (QDs) has been intensively studied as a way to enhance solar energy conversion by utilizing the excess energy in the absorbed photons. Among other useful properties, quantum confinement can both increase Coulomb interactions that drive the MEG process and decrease the electron–phonon coupling that cools hot excitons in bulk semiconductors. However, variations in the reported enhanced quantum yields (QYs) have led to disagreements over the role that quantum confinement plays. The enhanced yield of excitons per absorbed photon is deduced from a dynamical signature in the transient absorption or transient photoluminescence and is ascribed to the creation of biexcitons. Extraneous effects such as photocharging are partially responsible for the observed variations. When these extraneous effects are reduced, the MEG efficiency, defined in terms of the number of additional electron–hole pairs produced per additional band gap of photon excitation, is about two times bet...
418 citations
TL;DR: This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells.
Abstract: We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ∼80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha’s Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased effic...
400 citations
TL;DR: In this paper, the femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots were conducted to provide the first quantitative measure of excitonic state-to-state transition rates.
Abstract: The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax. The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton–phonon coupling between the vario...
347 citations
TL;DR: The results demonstrate the importance of ultrafast free carrier generation and suppression of interfacial CT-state formation and question the applicability of the often used Braun-Onsager model to describe the bias dependence of the photocurrent in polymer:fullerene organic photovoltaic devices.
Abstract: The precise mechanism and dynamics of charge generation and recombination in bulk heterojunction polymer:fullerene blend films typically used in organic photovoltaic devices have been intensively studied by many research groups, but nonetheless remain debated. In particular the role of interfacial charge-transfer (CT) states in the generation of free charge carriers, an important step for the understanding of device function, is still under active discussion. In this article we present direct optical probes of the exciton dynamics in pristine films of a prototypic polycarbazole-based photovoltaic donor polymer, namely poly[N-11''-henicosanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), as well as the charge generation and recombination dynamics in as-cast and annealed photovoltaic blend films using methanofullerene (PC(61)BM) as electron acceptor. In contrast to earlier studies we use broadband (500-1100 nm) transient absorption spectroscopy including the previously unobserved but very important time range between 2 ns and 1 ms, which allows us not only to observe the entire charge carrier recombination dynamics but also to quantify the existing decay channels. We determine that ultrafast exciton dissociation occurs in blends and leads to two separate pools of products, namely Coulombically bound charge-transfer (CT) states and unbound (free) charge carriers. The recombination dynamics are analyzed within the framework of a previously reported model for poly(3-hexylthiophene):PCBM (Howard, I. A. J. Am. Chem. Soc. 2010, 132, 14866) based on concomitant geminate recombination of CT states and nongeminate recombination of free charge carriers. The results reveal that only ~11% of the initial photoexcitations generate interfacial CT states that recombine exclusively by fast nanosecond geminate recombination and thus do not contribute to the photocurrent, whereas ~89% of excitons create free charge carriers on an ultrafast time scale that then contribute to the extracted photocurrent. Despite the high yield of free charges the power conversion efficiency of devices remains moderate at about 3.0%. This is largely a consequence of the low fill factor of devices. We relate the low fill factor to significant energetic disorder present in the pristine polymer and in the polymer:fullerene blends. In the former we observed a significant spectral relaxation of exciton emission (fluorescence) and in the latter of the polaron-induced ground-state bleaching, implying that the density of states (DOS) for both excitons and charge carriers is significantly broadened by energetic disorder in pristine PCDTBT and in its blend with PCBM. This disorder leads to charge trapping in solar cells, which in turn causes higher carrier concentrations and more significant nongeminate recombination. The nongeminate recombination has a significant impact on the IV curves of devices, namely its competition with charge carrier extraction causes a stronger bias dependence of the photocurrent of devices, in turn leading to the poor device fill factor. In addition our results demonstrate the importance of ultrafast free carrier generation and suppression of interfacial CT-state formation and question the applicability of the often used Braun-Onsager model to describe the bias dependence of the photocurrent in polymer:fullerene organic photovoltaic devices.
281 citations
TL;DR: Time-resolved studies of J-aggregate-Au nanoshell complexes when the nanoshel plasmon and J- Aggregate exciton energies are degenerate probe the dynamical behavior of this coupled system.
Abstract: Coherently coupled plasmons and excitons give rise to new optical excitations- plexcitons − due to the strong coupling of these two oscillator systems Time-resolved studies of J-aggregate-Au nanoshell complexes when the nanoshell plasmon and J-aggregate exciton energies are degenerate probe the dynamical behavior of this coupled system Transient absorption of the interacting plasmon-exciton system is observed, in dramatic contrast to the photoinduced transmission of the pristine J-aggregate An additional, transient Fano-shaped modulation within the Fano dip is also observable The behavior of the J-aggregate-Au nanoshell complex is described by a combined one-exciton and two-exciton state model coupled to the nanoshell plasmon
266 citations
TL;DR: A nanoengineering-based approach is demonstrated that provides control over EI while maintaining nearly constant emission energy in core–shell CdSe/CdS NCs with a variable shell width and can be applied to other nanostructures with variable electron–hole overlap.
Abstract: A strong electron-hole exchange interaction (EI) in semiconductor nanocrystals (NCs) gives rise to a large (up to tens of meV) splitting between optically active ('bright') and optically passive ('dark') excitons. This dark-bright splitting has a significant effect on the optical properties of band-edge excitons and leads to a pronounced temperature and magnetic field dependence of radiative decay. Here we demonstrate a nanoengineering-based approach that provides control over EI while maintaining nearly constant emission energy. We show that the dark-bright splitting can be widely tuned by controlling the electron-hole spatial overlap in core-shell CdSe/CdS NCs with a variable shell width. In thick-shell samples, the EI energy reduces to <250 μeV, which yields a material that emits with a nearly constant rate over temperatures from 1.5 to 300 K and magnetic fields up to 7 T. The EI-manipulation strategies demonstrated here are general and can be applied to other nanostructures with variable electron-hole overlap.
241 citations
TL;DR: In this article, the structure and energy of photogenerated electrons and holes in the bulk and at the (101) surface of anatase TiO2 were investigated using hybrid functional electronic structure calculations.
Abstract: Using hybrid functional electronic structure calculations, we have investigated the structure and energetics of photogenerated electrons and holes in the bulk and at the (101) surface of anatase TiO2. Excitons formed upon UV irradiation are found to become self-trapped, consistent with the observation of temperature-dependent Urbach tails in the absorption spectrum and a large Stokes shift in the photoluminescence band of anatase. Electron and hole polarons are localized at Ti3+ and O– lattice sites, respectively. At the surface, the trapping sites generally correspond to undercoordinated Ti3+5c and O–2c surface atoms or to isolated OH species in the case of a hydroxylated surface. The polaron trapping energy is considerably larger at the surface than in the bulk, indicating that it is energetically favorable for the polarons to travel from the bulk to the surface. Computed one-electron energy levels in the gap and hyperfine coupling constants compare favorably with oxidation potential and EPR measurements.
237 citations
TL;DR: A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode.
Abstract: A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0–0) vibronic band of only −32 cm−1, far smaller than the measured value of 631 cm−1 and of the wrong sign—a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0–0 DS of 601 cm−1 and a nearly quantitative reproduction of the relative spectral intensities of the 0–n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications...
235 citations
TL;DR: It is demonstrated that the driving force for exciton dissociation at the fullerene-fullerene heterointerface is optimized for diameters <1.0 nm, which will guide the development of next-generation high-performance carbon nanotube-based solar cells and photosensitive devices.
Abstract: We have employed thin films of highly purified semiconducting carbon nanotubes as near-infrared optical absorbers in heterojunction photovoltaic and photodetector devices with the electron acceptor C60. In comparison with previous implementations of more electrically heterogeneous carbon nanotube/C60 devices, we have realized a 10× gain in zero-bias quantum efficiency (QE) and even more substantial gains in power conversion efficiency (ηp). The semiconducting nanotube/C60 heterojunctions are highly rectifying with a peak external QE, internal QE, and ηp of 12.9 ± 1.3, 91 ± 22, and 0.6%, respectively, in the near-infrared. We show that the device efficiency is determined by the effective length scale for exciton migration in the nanotube films, confirm the high internal QE via photoluminescence quenching, and demonstrate that the driving force for exciton dissociation at the fullerene-fullerene heterointerface is optimized for diameters <1.0 nm. These results will guide the development of next-generation h...
214 citations
TL;DR: Investigation of the impact of vibronic couplings on the electronic structures and relaxation mechanisms of two cyanobacterial light-harvesting proteins suggests that the distinct behaviors of these closely related proteins are understood on the same footing only in a basis of joint electronic-nuclear states.
Abstract: Transport processes and spectroscopic phenomena in light harvesting proteins depend sensitively on the characteristics of electron-phonon couplings. Decoherence imposed by low-frequency nuclear motion generally suppresses the delocalization of electronic states, whereas the Franck-Condon progressions of high-frequency intramolecular modes underpin a hierarchy of vibronic Coulombic interactions between pigments. This Article investigates the impact of vibronic couplings on the electronic structures and relaxation mechanisms of two cyanobacterial light-harvesting proteins, allophycocyanin (APC) and C-phycocyanin (CPC). Both APC and CPC possess three pairs of pigments (i.e., dimers) that undergo electronic relaxation on the subpicosecond time scale. Electronic relaxation is ~10 times faster in APC than in CPC despite the nearly identical structures of their pigment dimers. We suggest that the distinct behaviors of these closely related proteins are understood on the same footing only in a basis of joint electronic-nuclear states (i.e., vibronic excitons). A vibronic exciton model predicts well-defined rate enhancements in APC at realistic values of the site reorganization energies, whereas a purely electronic exciton model points to faster dynamics in CPC. Calculated exciton sizes (i.e., participation ratios) show that wave function delocalization underlies the rate enhancement predicted by the vibronic exciton model. Strong vibronic coupling and heterogeneity in the pigment sites are the key ingredients of the vibronic delocalization mechanism. In contrast, commonly employed purely electronic exciton models see heterogeneity as only a localizing influence. This work raises the possibility that similar vibronic effects, which are often neglected, may generally have a significant influence on energy transport in molecular aggregates and photosynthetic complexes.
TL;DR: Two-photon-pumped blue lasing was observed in these organic waveguiding nanostructures above a threshold of 60 nJ, excited with a 750 nm near-infrared femtosecond pulse laser at 77 K.
Abstract: Single-crystal organic nanowires were fabricated with a soft-template-assisted self-assembly method in liquid phase. These nanowires with rectangular cross section can serve as resonators for exciton-photon coupling, leading to a microcavity effect and a relatively low threshold of laser actions. Two-photon-pumped blue lasing was observed in these organic waveguiding nanostructures above a threshold of 60 nJ, excited with a 750 nm near-infrared femtosecond pulse laser at 77 K.
TL;DR: In this article, the biexciton to exciton (X) to ground photoluminescence cascade of single colloidal semiconductor nanocrystals (NCs) under weak excitation in a g(2) photon correlation measurement is observed and the normalized amplitude of the cascade feature is equal to the ratio of the BX to X fluorescence quantum yields.
Abstract: Biexciton properties strongly affect the usability of a light emitter in quantum photon sources and lasers but are difficult to measure for single fluorophores at room temperature due to luminescence intermittency and bleaching at the high excitation fluences usually required. Here, we observe the biexciton (BX) to exciton (X) to ground photoluminescence cascade of single colloidal semiconductor nanocrystals (NCs) under weak excitation in a g(2) photon correlation measurement and show that the normalized amplitude of the cascade feature is equal to the ratio of the BX to X fluorescence quantum yields. This imposes a limit on the attainable depth of photon antibunching and provides a robust means to study single emitter biexciton physics. In NC samples, we show that the BX quantum yield is considerably inhomogeneous, consistent with the defect sensitivity expected of the Auger nonradiative recombination mechanism. The method can be extended to study X,BX spectral and polarization correlations.
TL;DR: Organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C(60) are reported, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.
Abstract: Singlet exciton fission is an efficient multiexciton generation process in organic molecules. But two concerns must be satisfied before it can be exploited in low-cost solution-processed organic solar cells. Fission must be combined with longer wavelength absorption in a structure that can potentially surpass the single junction limit, and its efficiency must be demonstrated in nanoscale domains within blended devices. Here, we report organic solar cells comprised of tetracene, copper phthalocyanine, and the buckyball C(60). Short wavelength light generates singlet excitons in tetracene. These are subsequently split into two triplet excitons and transported through the phthalocyanine. In addition, the phthalocyanine absorbs photons below the singlet exciton energy of tetracene. To test tetracene in nanostructured blends, we fabricate coevaporated bulk heterojunctions and multilayer heterojunctions of tetracene and C(60). We measure a singlet fission efficiency of (71 ± 18)%, demonstrating that exciton fission can efficiently compete with exciton dissociation on the nanoscale.
TL;DR: The first observation of trions (charged excitons), three-particle bound states consisting of one electron and two holes, in hole-doped carbon nanotubes at room temperature is reported.
Abstract: We report the first observation of trions (charged excitons), three-particle bound states consisting of one electron and two holes, in hole-doped carbon nanotubes at room temperature. When p-type dopants are added to carbon nanotube solutions, the photoluminescence and absorption peaks of the trions appear far below the E11 bright exciton peak, regardless of the dopant species. The unexpectedly large energy separation between the bright excitons and the trions is attributed to the strong electron-hole exchange interaction in carbon nanotubes.
TL;DR: In this article, a non-equilibrium condensates of exciton-polaritons are used as a platform for exploring the physics of vortex-antivortex pairs.
Abstract: Bound pairs consisting of a vortex and an antivortex are expected to dominate the low-temperature physics in a variety of two-dimensional systems. The observation of such bound pairs, however, remains elusive. A study now establishes non-equilibrium condensates of exciton-polaritons as a platform for exploring the physics of vortex–antivortex pairs.
TL;DR: In the presence of disorder and for T > 0 K, λ2S(R) is closely approximated by the exciton coherence number N( coh), thereby providing a simple and direct way of extracting N(coh) from the photoluminescence spectrum.
Abstract: Exciton coherence in a J-aggregate with exciton−phonon coupling involving a single intramolecular vibration is studied. For linear aggregates with no disorder and periodic boundary conditions, the 0−0 to 0−1 line strength ratio, SR, corresponding to the low-temperature photoluminescence spectrum is rigorously equal to N/λ2, where N is the number of chromophores comprising the aggregate and λ2 is the Huang−Rhys factor of the coupled vibrational mode. The result is independent of exciton bandwidth and therefore remains exact from the weak to strong exciton−phonon coupling regimes. The simple relation between SR and N also holds for more complex morphologies, as long as the transition from the lowest exciton state to the vibrationless ground state is symmetry-allowed. For example, in herringbone aggregates with monoclinic unit cells, the line strength ratio, defined as SR ≡ Ib0−0/Ib0−1 (where Ib0−0 and Ib0−1 correspond to the b-polarized 0−0 and 0−1 line strengths, respectively) is rigorously equal to N/λ2....
TL;DR: In this article, the signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory using a single BN-sheet.
Abstract: The signature of defects in the optical spectra of hexagonal boron nitride (BN) is investigated using many-body perturbation theory. A single BN-sheet serves as a model for different layered BN nanostructures and crystals. In the sheet we embed prototypical defects such as a substitutional impurity, isolated boron and nitrogen vacancies, and the divacancy. Transitions between the deep defect levels and extended states produce characteristic excitation bands that should be responsible for the emission band around 4 eV, observed in luminescence experiments. In addition, defect bound excitons occur that are consistently treated in our ab initio approach along with the “free” exciton. For defects in strong concentration, the coexistence of both bound and free excitons adds substructure to the main exciton peak and provides an explanation for the corresponding feature in cathodo- and photoluminescence spectra.
TL;DR: Transient absorption (TA) spectroscopy of solution-phase mixtures of colloidal CdS quantum dots (QDs) with acid-derivatized viologen molecules indicates electron transfer occurs from the conduction band of the QD to the LUMO of V(2+) after photoexcitation of a band-edge exciton in theQD.
Abstract: Transient absorption (TA) spectroscopy of solution-phase mixtures of colloidal CdS quantum dots (QDs) with acid-derivatized viologen molecules, N-[1-heptyl],N′-[3-carboxypropyl]-4,4′-bipyridinium dihexafluorophosphate (V2+), indicates electron transfer occurs from the conduction band of the QD to the LUMO of V2+ after photoexcitation of a band-edge exciton in the QD. Analysis of the magnitude of the ground state bleach of the QD as a function of the molar ratio QD:V2+ yields the QD–ligand adsorption constant, Ka (4.4 × 104 M–1) for V2+ ligands adsorbed in geometries conducive to electron transfer. The value of Ka, together with the measured rates of (i) formation of the V+• electron transfer product and (ii) recovery of the ground state bleach of the QD, enables determination of the intrinsic rate constant for charge separation, kCS,int ∼ 1.7 × 1010 s–1, the rate for a single QD–V2+ donor–acceptor pair. This analysis confirms previous reports that the number of ligands adsorbed to each QD is well-describe...
TL;DR: A homologous series of six novel oligothiophene-naphthalene diimide-based oligomer semiconductors with a donor-acceptor architecture was synthesized and used to explore a set of criteria for the design of non-fullerene electron acceptor materials for organic solar cells as mentioned in this paper.
Abstract: A homologous series of six novel oligothiophene–naphthalene diimide-based oligomer semiconductors with a donor–acceptor architecture, NDI-nTH (n = 1, 2, 3, 4) and NDI-nT (n = 2, 3), was synthesized and used to explore a set of criteria for the design of non-fullerene electron acceptor materials for organic solar cells. Thin films of the oligomer semiconductors had optical band gaps that varied from 2.1 eV in NDI-1TH and 1.6 eV in NDI-3TH to 1.4 eV in NDI-4TH, demonstrating good potential for light harvesting and exciton generation. The LUMO energy levels of the oligomer semiconductors were similar (ca. −4.0 eV), but the HOMO levels varied from −5.5 eV in NDI-3TH and NDI-4TH to −6.1 eV in NDI-1TH, showing that suitable energy band offsets necessary for efficient photoinduced charge transfer could be achieved with current donor polymers. Single-crystal X-ray structures of NDI-3TH and NDI-4TH showed a slipped face-to-face π-stacking with short intermolecular distances (0.321–0.326 nm), which enabled facile s...
TL;DR: The presence of a rapid singlet decay at all temperatures indicates that the initially created J-type singlet exciton decays to an intermediate that only produces free triplets (and delayed fluorescence) at high temperatures.
Abstract: The excited state dynamics of polycrystalline tetracene films are studied using femtosecond transient absorption in combination with picosecond fluorescence, continuing work reported in an earlier paper [J. J. Burdett, A. M. Muller, D. Gosztola, and C. J. Bardeen, J. Chem. Phys. 133, 144506 (2010)]. A study of the intensity dependence of the singlet state decay is conducted to understand the origins of the discrepancy between the broadband transient absorption and fluorescence experiments seen previously. High-sensitivity single channel transient absorption experiments allow us to compare the transient absorption dynamics to the fluorescence dynamics measured at identical laser fluences. At high excitation densities, an exciton-exciton annihilation rate constant of ~1 × 10(-8) cm(3) s(-1) leads to rapid singlet decays, but at excitation densities of 2 × 10(17) cm(-3) or less the kinetics of the transient absorption match those of the fluorescence. At these lower excitation densities, both measurements confirm that the initially excited singlet state relaxes with a decay time of 80 ± 3 ps, not 9.2 ps as claimed in the earlier paper. In order to investigate the origin of the singlet decay, the wavelength-resolved fluorescence dynamics were measured at 298 K, 77 K, and 4 K. A high-energy J-type emitting species undergo a rapid (~100 ps) decay at all temperatures, while at 77 K and 4 K additional species with H-type and J-type emission lineshapes have much longer lifetimes. A global analysis of the wavelength-dependent decays shows that the initial ~100 ps decay occurs to a dark state and not via energy transfer to lower energy bright states. Varying the excitation wavelength from 400 nm to 510 nm had no effect on the fast decay, suggesting that there is no energy threshold for the initial singlet relaxation. The presence of different emitting species at different temperatures means that earlier interpretations of the fluorescence behavior in terms of one singlet state that is short-lived due to singlet fission at high temperatures but long-lived at lower temperatures are probably too simplistic. The presence of a rapid singlet decay at all temperatures indicates that the initially created J-type singlet exciton decays to an intermediate that only produces free triplets (and delayed fluorescence) at high temperatures.
TL;DR: The study demonstrates that the key optoelectronic properties of composite heterostructures comprising electrically coupled metal and semiconductor domains are substantially different from those observed in systems with weak interdomain coupling.
Abstract: The nature of exciton-plasmon interactions in Au-tipped CdS nanorods has been investigated using femtosecond transient absorption spectroscopy. The study demonstrates that the key optoelectronic properties of composite heterostructures comprising electrically coupled metal and semiconductor domains are substantially different from those observed in systems with weak interdomain coupling. In particular, strongly coupled nanocomposites promote mixing of electronic states at semiconductor-metal domain interfaces, which causes a significant suppression of both plasmon and exciton excitations of carriers.
TL;DR: By performing surface-dependent measurements on colloidal CdSe QDs, this work finds that surface-induced charge trapping processes lead to false MER and MEG signals resulting in an inaccurate measurement of these processes.
Abstract: Multiple exciton recombination (MER) and multiple exciton generation (MEG) are two of the main processes for assessing the usefulness of quantum dots (QDs) in photovoltaic devices. Recent experiments, however, have shown that a firm understanding of both processes is far from well established. By performing surface-dependent measurements on colloidal CdSe QDs, we find that surface-induced charge trapping processes lead to false MER and MEG signals resulting in an inaccurate measurement of these processes. Our results show that surface-induced processes create a significant contribution to the observed discrepancies in both MER and MEG experiments. Spectral signatures in the transient absorption signals reveal the physical origin of these false signals.
TL;DR: The concept of an exciton-polariton expresses the nonperturbative coupling between the electromagnetic field and the optically induced matter polarization as discussed by the authors, which is a measure of a semiconductor's coupling to an optical field.
Abstract: The integrated absorption of an excitonic resonance is a measure of a semiconductor's coupling to an optical field. The concept of an exciton–polariton expresses the non-perturbative coupling between the electromagnetic field and the optically induced matter polarization. Ways to alter this coupling include confining the light in optical cavities and localizing the excitonic wavefunction in quantum wells and dots, which is illustrated by quantum strong coupling between a single dot and an optical nanocavity. Positioning quantum wells in periodic or quasiperiodic lattices with spacing close to a half wavelength results in pronounced modifications to the light transmission. Light–matter coupling can also be used to generate and interrogate an exciton population, for example by the recently developed technique of absorbing terahertz radiation.
TL;DR: A new class of colloidal nanocrystals composed of Cu-doped ZnSe cores overcoated with CdSe shells is developed, conclusively demonstrating that Cu impurities represent paramagnetic +2 species and serve as a source of permanent optically active holes.
Abstract: We have developed a new class of colloidal nanocrystals composed of Cu-doped ZnSe cores overcoated with CdSe shells. Via spectroscopic and magneto-optical studies, we conclusively demonstrate that Cu impurities represent paramagnetic +2 species and serve as a source of permanent optically active holes. This implies that the Fermi level is located below the Cu2+/Cu1+ state, that is, in the lower half of the forbidden gap, which is a signature of a p-doped material. It further suggests that the activation of optical emission due to the Cu level requires injection of only an electron without a need for a valence-band hole. This peculiar electron-only emission mechanism is confirmed by experiments in which the titration of the nanocrystals with hole-withdrawing molecules leads to enhancement of Cu-related photoluminescence while simultaneously suppressing the intrinsic, band-edge exciton emission. In addition to containing permanent optically active holes, these newly developed materials show unprecedented em...
TL;DR: The exciton lifetime is found to increase as the Cu(2)O thickness is increased, which leads to an increase in the electron-hole separation time and thus an increased in the hole (and so the hydroxyl radical) concentration leading to an observed enhanced rate of the dye degradation.
Abstract: Cu2O–Au nanoframes with different nanolayer thicknesses of Cu2O were prepared, and their photocatalytic properties in aqueous solutions were studied. Cu2O semiconductor excitation leads to electron–hole separation. In aqueous solution, the hole is known to oxidize water to produce hydroxyl radicals whose concentration (and that of the holes) can be monitored by the rate of the degradation of dissolved methylene blue dye. The exciton lifetime is determined by femtosecond techniques and is determined by electron–hole recombination which depends on the rates of a number of competing processes such as, electron or hole transfer to an acceptor such as a gold nanoframe and/or the electron or hole trapping processes at the Cu2O–Au nanoframe interface. We measured the exciton lifetime as a function of the average Cu2O–Au layer separation. A good correlation was found between the rate of the photocatalytic degradation of methylene blue and the exciton lifetime. The exciton lifetime is found to increase as the Cu2O...
TL;DR: In this article, the authors present a comprehensive study of the properties of the deeply bound excitons with particular focus on the ${Y}_{0}$ transition at 3.33 and 3.35 eV.
Abstract: ZnO single crystals, epilayers, and nanostructures often exhibit a variety of narrow emission lines in the spectral range between 3.33 and 3.35 eV which are commonly attributed to deeply bound excitons ($Y$ lines). In this work, we present a comprehensive study of the properties of the deeply bound excitons with particular focus on the ${Y}_{0}$ transition at 3.333 eV. The electronic and optical properties of these centers are compared to those of the shallow impurity related exciton binding centers ($I$ lines). In contrast to the shallow donors in ZnO, the deeply bound exciton complexes exhibit a large discrepancy between the thermal activation energy and localization energy of the excitons and cannot be described by an effective mass approach. The different properties between the shallow and deeply bound excitons are also reflected by an exceptionally small coupling of the deep centers to the lattice phonons and a small splitting between their two electron satellite transitions. Based on a multitude of different experimental results including magnetophotoluminescence, magnetoabsorption, excitation spectroscopy (PLE), time resolved photoluminescence (TRPL), and uniaxial pressure measurements, a qualitative defect model is developed which explains all $Y$ lines as radiative recombinations of excitons bound to extended structural defect complexes. These defect complexes introduce additional donor states in ZnO. Furthermore, the spatially localized character of the defect centers is visualized in contrast to the homogeneous distribution of shallow impurity centers by monochromatic cathodoluminescence imaging. A possible relation between the defect bound excitons and the green luminescence band in ZnO is discussed. The optical properties of the defect transitions are compared to similar luminescence lines related to defect and dislocation bound excitons in other II--VI and III--V semiconductors.
TL;DR: A detailed study of the exciton recombination dynamics in CdSe/CdS heterorods shows a clear size-dependent radiative lifetime, which can be linked to the different degree of electron wave function (de)localization.
Abstract: Colloidal semiconductor quantum structures allow controlling the strong confinement of charge carriers through material composition and geometry. Besides being a unique platform to study fundamental effects, these materials attracted considerable interest due to their potential in opto-electronic and quantum communication applications. Heteronanostructures like CdSe/CdS offer new prospects to tailor their optical properties as they take advantage of a small conduction band offset allowing tunability of the electron delocalization from type-I toward quasi-type-II. Here, we report on a detailed study of the exciton recombination dynamics in CdSe/CdS heterorods. We observed a clear size-dependent radiative lifetime, which can be linked to the different degree of electron wave function (de)localization. Moreover, by increasing the temperature from 70 to 300 K, we observed a considerable increase of the radiative lifetime, clearly demonstrating a reduction of the conduction band offset at higher temperatures. ...
TL;DR: In this article, an approach for measuring the Forster radius of energy transfer between electron donating and accepting materials commonly used in organic photovoltaic cells (OPVs) is presented. But the authors do not consider the role of the quenching material.
Abstract: This work demonstrates an approach for measuring the Forster radius of energy transfer between electron donating and accepting materials commonly used in organic photovoltaic cells (OPVs). While energy transfer processes are surprisingly common in OPVs, they are often incorrectly ignored in measurements of the exciton diffusion length and in models of device performance. Here, the efficiency of energy transfer between an emissive donor and an absorptive acceptor is investigated through complementary experimental and theoretical techniques. This is accomplished by spatially separating the donor and acceptor materials using a wide-energy-gap spacer layer to suppress direct charge transfer and tracking donor photoluminescence as a function of spacer layer thickness. Fitting experimental data obtained for a variety of donor materials allows for the extraction of Forster radii that are in good agreement with predicted values. The impact of donor–acceptor excitonic energy transfer on device performance and on measurements of the exciton diffusion length is also investigated using the archetypical small molecule donor material boron subphthalocyanine chloride (SubPc). An average exciton diffusion length of 7.7 nm is extracted from photoluminescence quenching experiments using SubPc. This value is independent of the quenching material when the role of energy transfer is properly modeled.
TL;DR: It is shown that the exciton mobility in this material is strongly anisotropic with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, along the b axis.
Abstract: We visualize exciton diffusion in rubrene single crystals using localized photoexcitation and spatially resolved detection of excitonic luminescence. We show that the exciton mobility in this material is strongly anisotropic with long-range diffusion by several micrometers associated only with the direction of molecular stacking in the crystal, along the b axis. We determine a triplet exciton diffusion length of 4.0 ± 0.4 μm from the spatial exponential decay of the photoluminescence that originates from singlet excitons formed by triplet-triplet fusion.