Exciton Annihilation Processes Research Articles

Quantum efficiency harmonic analysis of exciton annihilation in organic light emitting diodes

Various exciton annihilation processes are known to impact the efficiency roll-off of organic light emitting diodes (OLEDs); however, isolating and quantifying their contribution in the presence of other factors such as changing charge balance continue to be a challenge for routine device characterization. Here, we analyze OLED electroluminescence resulting from a sinusoidal dither superimposed on the device bias and show that nonlinearity between recombination current and light output arising from annihilation mixes the quantum efficiency measured at different dither harmonics in a manner that depends uniquely on the type and magnitude of the annihilation process. We derive a series of analytical relations involving the DC and first harmonic external quantum efficiency that enable annihilation rates to be quantified through linear regression independent of changing charge balance and evaluate them for prototypical fluorescent and phosphorescent OLEDs based on the emitters 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran and platinum octaethylporphyrin, respectively. We go on to show that, in most cases, it is sufficient to calculate the needed quantum efficiency harmonics directly from derivatives of the DC light versus current curve, thus enabling this analysis to be conducted solely from standard light-current-voltage measurement data.

Studying of photoluminescence characteristics of CdTe/ZnS QDs manipulated by TiO2 inverse opal photonic crystals

The photoluminescence (PL) characteristics of CdTe/ZnS quantum dots (QDs) infiltrated in TiO2 inverse opal photonic crystals (PCs) are studied in detail. The PL dynamics of QDs show that the PCs could accelerate the PL relaxation rate of QDs as the PL peak of QDs is overlapped with the photonic stop band of PCs. Besides, the PCs could decrease the activation energy of QDs due to its porous structure and suppress the exciton annihilation process of QDs at high excitation intensity, owing to the light scattering effect. The final results are beneficial for people in further understanding the role of inverse opal PCs on manipulating the PL characteristics of QDs.

Anisotropic exciton relaxation in nanostructured metal (Zn and F(16)Zn)-phthalocyanine.

We report ultrafast excited state dynamics of zinc phthalocyanine and zinc hexadecafluoro phthalocyanine thin films which have nanorod-like structures. Excitons in the singlet states undergo multi-exponential relaxation processes to the ground state and the singlet lifetime within a few tens of picoseconds is attributed to the diffusion-limited exciton annihilation process. Diffusive migration of the singlet excitons shows the anisotropic lifetimes depending on the polarization of probe beam. Similar anisotropy is observed in the X-ray diffraction data which exhibits long-range alignment of molecular columns along the long axis of nanorod, whereas disordered arrangement in lateral direction to the axis of nanorod.

Color-tunable and stable-efficiency white organic light-emitting diode fabricated with fluorescent-phosphorescent emission layers

White organic light emitting diodes (OLEDs) were fabricated for color-tunable lighting applications. Fluorescent and phosphorescent hybrid emission layers (EMLs) were used to enhance the luminance and stability of the devices, which have blue-EML/CBP interlayer/green-EML/phosphorescent-sensitized-EML/red-EML structures. The influence of the composition and structure of the EMLs on the electroluminescence properties of the devices were investigated from the viewpoint of their emission spectra. The possible exciton harvesting, diffusion, transport, and annihilation processes occurring in the EMLs were also evaluated. A maximum luminance intensity of 7400cd/m2 and a highly stable current efficiency of 3.2cd/A were obtained. Good color tunability was achieved for the white OLEDs; the chromatic coordinates linearly shifted from pure white (0.300, 0.398) to cold white (0.261, 0.367) when the applied voltage was varied from 10 to 14V.

Exciton delocalization and dynamics in helical π-stacks of self-assembled perylene bisimides

Whilst the excitonic properties of J-aggregates have been investigated in great detail, those of H-aggregates have not been systematically investigated yet. In this regard, we have explored the exciton dynamics and excited-species formation processes in columnar H-aggregates of planar PBI dyes that are stacked in a helical fashion by various spectroscopic techniques such as time correlated single-photon counting and femtosecond pump–probe measurements with anisotropy changes. The outcome of this study is that photogenerated excitons in helically stacked PBI dyes experience complicated relaxation processes that involve excited-state interactions such as exciton delocalization and excimer formation. To scrutinize the exciton dynamics in the helically stacked aggregates, we have also included distorted bay-substituted PBI dyes as reference molecules that exhibit either no or only relatively small-sized dimeric aggregate structures. The comparative study revealed that the excited-state interactions in the large-sized helically stacked aggregates extend beyond two PBI units, leading to a final excimer (here, excimer means not only an “excited dimer” but an “excited multimer”) trap state within ∼50 ps. Although in competition with this relaxation path into the excimeric trap state, exciton diffusion has been revealed by exciton–exciton annihilation processes, occurring at high excitation power. Whilst the excimer formation process interrupts the direct observation of exciton diffusion in these columnar PBI aggregates, the exciton migration distance could be estimated by the incorporation of non-fluorescent PBI quencher molecules. From this analysis we can conclude that the exciton diffusion can reach a length of about 10 monomer units. Although this value appears to be shorter than those values observed for J-aggregates, this result shows that columnar PBI stacks might still be useful for optoelectronic applications if the relaxation process leading to excimer traps is prevented, e.g. by structural modifications of the molecules.

Exciton–Exciton Annihilation in Copper-phthalocyanine Single-Crystal Nanowires

Femtosecond one-color pump–probe spectroscopy was applied to study exciton dynamics in single-crystal α-phase copper-phthalocyanine (CuPc) nanowires grown on an opaque silicon substrate. The transient reflectance kinetics measured at different pump fluences exhibit a remarkable intensity-dependent decay behavior which accelerates significantly with increasing pump pulse intensity. All the kinetic decays can be satisfactorily described using a biexponential decay function with lifetimes of 22 and 204 ps, but the corresponding relative amplitudes depend on the pump intensity. The accelerated decay behavior observed at high pump intensities arises from a nonlinear exciton–exciton annihilation process. Detailed data analysis further shows that, as found for other metal-phthalocyanine polymorphs, the exciton–exciton annihilation in the CuPc nanowires is one-dimensional (1D) diffusion-limited, which possibly involves intrachain exciton diffusion along 1D molecular stacks.

Improving charge separation of solar cells by the co-sensitization of CdS quantum dots and dye

A co-sensitizer of CdS quantum dots plus N3 dye was developed to improve the conversion efficiency of solar cells. The photoanode of the solar cells was composed of ZnO nanowires coated with the CdS quantum dots and N3 dye successively; such a constructor can not only overcome the photocorrosion of CdS quantum dots, but also decrease the dissolution of ZnO nanowires, especially, improving the charge separation and electron injection. The open-circuit voltage of co-sensitized solar cells is also improved due to an enhanced retardation of charge recombination. The conversion efficiency of solar cells is enhanced from 0.5% of N3 sensitizer and 0.4% of CdS quantum dots to 1.9% of co-sensitizer; the significant improvement is attributed to the efficient hole transfer from CdS to N3 dye, which eliminates the exciton annihilation process and allows a leisurely electron injection into ZnO nanowires. Our result indicates that the co-sensitization is an essential step for highly efficient solar cell operation.

Excited-State Relaxation Process of Free-Base and Oxovanadium Naphthalocyanine in Near-Infrared Region

Excited-state relaxation process of free-base and oxovanadium naphthalocyanine (H(2)Nc and VONc) in solutions and in polymer films was studied by transient absorption measurements. In polymer films only H-aggregate was observed with H(2)Nc with increasing its weight fraction, whereas VONc formed both H- and J-type aggregates. The transient absorption of singlet excited-state of H(2)Nc and VONc in toluene solution decayed with time constant of 250 +/- 30 and 12 +/- 2 ps, respectively. The relaxation from singlet excited state of H(2)Nc and VONc in toluene solution is governed by the IC and ISC, respectively. The central metal ion, VO(2+), acceralated the ISC by the spin-orbital coupling due to unpaired electron. The excited-state relaxation in polymer films differed from that in toluene solution, which originates from the exciton-exciton annihilation process. The excitation power dependence of the excited-state dynamics and weight ratio dependence of absorption spectrum suggests that the aggregation of VONcs contributes to faster decay from the singlet excited-state. The excition-exciton annihilation occurs more efficiently in VONc system compared with H(2)Nc system. The dipole-dipole interaction depending on the aggregated structure controls excited-state relaxation processes in polymer films. This is mainly due to the differences in the transition dipole moment between H- and J-type aggregates. The ultrafast deactivation of VONc in polymer films can be applied to all-optical ultrafast modulation in the optical telecommunication wavelength region.

Single-photon emission from a single nanoparticle consisting of a single conjugated polymer chain

Single-photon behavior in the emission from a single nanoparticle consisting of a single poly[2-methoxy-5-(2′-ethylhexyloxy)- p-phenylene vinylene] (MEH-PPV, molecular weight: 2,600,000 amu) chain has been investigated. Photon correlation measurements of a number of single nanoparticles revealed that the probability of single-photon emission from the single nanoparticles clearly increases, compared with that of single chains embedded in host polymer matrices, poly(methyl methacrylate) (PMMA) and polystyrene (PS). The result suggested that even a single MEH-PPV chain with high molecular weight, which does not exhibit single-photon emission in PMMA and PS, is forced to exhibit single-photon emission by adopting “compact” chain conformation like nanoparticle. Efficient exciton migration and exciton annihilation processes in the compact chain conformation most probably result in the efficient single-photon emission from the single nanoparticle. The present results indicate that multi-quantum systems consisting of a large number of chromophores, such as the MEH-PPV, can be made to behave as single-photon sources by appropriately controlling their size.

Ultrafast Charge Separation at CdS Quantum Dot/Rhodamine B Molecule Interface

Ultrafast dissociation of excitons in CdS quantum dots via electron transfer to adsorbed rhodamine B (RhB) molecules was demonstrated. The rate of electron transfer can be controlled by the number of adsorbates attached on the nanoparticle and transfer time as fast as 12 picoseconds was observed. The rapid and controllable charge separation in this model quantum dot−adsorbate complex provides a potential approach for separating multiple excitons before the exciton−exciton annihilation process.

On the photoconductivity of layered molecular complex of fullerene C60 with saturated amine TMPDA

AbstractIt was revealed that the photoconductivity of layered molecular complex of fullerene C60 with saturated amine TMPDA: TMPDA · C60 is caused by intermolecular electronic processes in the fullerene layers. The intermediate magneto‐sensitive stage of photogenerating free charge carriers was found to be due to the effect of magnetic field on the rate constant of the triplet charge transfer exciton annihilation process. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast excitation relaxation in titanylphthalocyanine thin film

Abstract Ultrafast excitation relaxation in the whole Q band of titanylphthalocyanine amorphous thin film fabricated by physical jet deposition was investigated by femtosecond time-resolved pump–probe technique. The measured relaxation dynamics was found to be strongly dependent on the wavelength of the laser beam and consists of three quite different processes: an ultrafast process with a lifetime of 0.5–5 ps, a fast and a long-lived processes with lifetimes of about 5–10 ps and longer than 100 ps, respectively. The initial ultrafast decay appearing to be excitation intensity dependent is suggested to represent a bimolecular exciton–exciton annihilation process with a t −1/2 time dependence of the excited-state population, assigned to a one-dimensional exciton diffusion. The exciton–exciton annihilation is observed in the pump intensity as low as 0.27 GW/cm 2 .

Rapid Exciton Migration and Fluorescent Energy Transfer in Helical Polyisocyanides with Regularly Arranged Porphyrin Pendants

Photophysical properties of a series of polyisocyanides possessing porphyrin pendants (porphyrin polymers) were investigated by steady state and transient spectroscopic methods. The B (Soret)-band of the free base porphyrin polymers showed splitting indicating substantial exciton coupling in the porphyrin polymers due to a face-to-face stacking conformation. On the other hand, the B-band of the zinc porphyrin polymers split into four peaks indicating deviation from the strict face-to-face stacking conformation. The fluorescence quantum yields of the porphyrin polymers were small compared with those of the corresponding monomers and dimers. Furthermore, the fluorescence decay profiles of the porphyrin polymers deviated from single-exponential decay due to the structural heterogeneity of the polymers. It was revealed that the porphyrin polymers showed transient absorption changes attributable to the exciton−exciton annihilation accompanying structural change with the rate constant of (2−4) × 1010 s-1 regardless of the length of the polymer chain. The quantum yields of the intersystem crossing of the polymers were smaller than those of monomers due to the efficient exciton−exciton annihilation process in the singlet excited states. In the block copolymers of zinc and free base porphyrins, energy transfer processes from zinc to free base porphyrin moieties were confirmed by time-resolved fluorescence spectral and transient absorption spectral measurements. It became clear that the energy transfer rates were almost independent of the sequence of the block copolymers, suggesting that the rate-determining step of the energy transfer in the present block copolymers is the process at the interface of zinc and free base porphyrins because of the sufficiently fast exciton migration in each block.

Exciton exciton annihilation dynamics in chromophore complexes. II. Intensity dependent transient absorption of the LH2 antenna system.

Using the multiexciton density matrix theory of excitation energy transfer in chromophore complexes developed in a foregoing paper [J. Chem. Phys. 118, 746 (2003)], the computation of ultrafast transient absorption spectra is presented. Beside static disorder and standard mechanisms of excitation energy dissipation the theory incorporates exciton exciton annihilation (EEA) processes. To elucidate signatures of EEA in intensity dependent transient absorption data the approach is applied to the B850 ring of the LH2 found in rhodobacter sphaeroides. As main indications for two-exciton population and resulting EEA we found (i) a weakening of the dominant single-exciton bleaching structure in the transient absorption, and (ii) an intermediate suppression of long-wavelength and short-wavelength shoulders around the bleaching structure. The suppression is caused by stimulated emission from the two-exciton to the one-exciton state and the return of the shoulders follows from a depletion of two-exciton population according to EEA. The EEA-signature survives as a short-wavelength shoulder in the transient absorption if orientational and energetic disorder are taken into account. Therefore, the observation of the EEA-signatures should be possible when doing frequency resolved transient absorption experiments with a sufficiently strongly varying pump-pulse intensity.

Effects of strong excitation induced disorder on pump–probe spectra of molecular aggregates

We have studied the effects of induced off-diagonal disorder on pump–probe spectra in J-aggregates. Changes of the disorder, i.e. randomness in excite energy of site molecules and inter-molecular coupling within a meso-aggregates, is considered by introducing different random Hamiltonian matrices for two ensembles of aggregates. The theory allows to interpret the intensity-dependent spectra on the J-aggregates as contributed from the combined action of exciton–exciton annihilation and subsequent dynamic disordering processes. The possibility of the transient stage of aggregate ensembles exhibiting lasing due to the excess off-diagonal disorder is demonstrated.

Exciton exciton annihilation dynamics in chromophore complexes. I. Multiexciton density matrix formulation

The multiexciton (MX) description of excitation energy transfer in chromophore complexes and biological light harvesting antenna systems is extended to the incorporation of exciton exciton annihilation (EEA) processes. To achieve a complete microscopic description the approach is based on intrachromophore internal conversion processes leading to nonradiative transitions from higher to lower lying exciton manifolds. Besides an inclusion of EEA the MX density matrix theory which has been utilized for a description of excitation energy transfer also accounts for a coupling to low-frequency vibrational modes and the radiation field. Concentrating on transitions from the two to the single-exciton manifold exact and approximate expressions for the EEA rate are derived. In part II of this paper the approach is applied to the LH2 antenna putting emphasis on the EEA induced change of transient absorption spectra.

Ultrafast optical nonlinearity of titanylphthalocyanine films

The optical properties of titanylphthalocyanine (TiOPc) thin film fabricated by physical jet deposition technique were investigated by using absorption, femtosecond (fs) optical Kerr effect (OKE) and fs pump–probe techniques. The third-order nonlinear optical susceptibility of TiOPc film was measured to be 3×10 −10 esu. The OKE response of TiOPc films shows a decay process with subpicosecond time constant, demonstrating that the excited state of TiOPc molecule has little contribution to the photo-induced birefringence. From the fs pump–probe experiment, a complicated decay behavior with three decay components was observed, which originates from the contribution of exciton–exciton annihilation process, the enhanced intersystem crossing process and the relaxation of the triplet state.

Mirrorless optical bistability of an ultrathin glassy film built up of oriented J-aggregates: Effects of two-exciton states and exciton–exciton annihilation

We theoretically analyze the optical response of an ultrathin film with accounting for the effects of creating two exciton states as well as their annihilation in an individual aggregate. We show that these two processes do not violate mirrorless optical bistability of the film found in the framework of the ground-to-one exciton state transitions but, contrary to that, act towards improving the conditions for bistability to occur. In particular, the killing action of inhomogeneous broadening of the one-exciton absorption spectrum may be considerably reduced due to the exciton–exciton annihilation process giving hope for creating a unit for an all-optical device based on such systems.

Photoluminescence Excitation Study of Lo-Phonon Assisted Excitonic Transitions in Gan

AbstractWe report the results of a photoluminescence excitation (PLE) study on excitonic transitions in GaN. PLE measurements were carried out as a function of temperature to investigate exciton formation, thermalization, and annihilation processes in GaN. The PLE spectra detected at free-exciton resonance were found to exhibit a large number of oscillatory emission structures with an energy spacing of LO-phonon (92 meV), and such emission persisted up to 400K. The observation suggests that a phonon-assisted indirect exciton formation process, which simultaneously generates a free-exciton and an LO-phonon, is very significant and efficient in GaN, and the lifetime of the free-exciton is longer than the relaxation time of LO-phonon emission but much shorter than that of acoustic-phonon emission. The appearance of the broad spectral features at the PLE spectrum detected at free-exciton resonance, similar to those measured at the boundexciton resonance, is attributed to the higher order transition processes which involve acousticphonon scattering and thermal repopulation of free-excitons. In addition, we observed up to the 12th LO-phonon emission in PLE spectrum around 120 K, indicating the existence of the excitons with a large kinetic energy in GaN.

Nonexponential relaxation in solid C 60 via time-dependent singlet exciton annihilation

We present measurements and detailed modeling of the dynamics of solid C 60 following photoexcitation. Optical pulses 60 fs in duration centered at 620 nm are used to excite the lowest optical band, and the relaxation of the photoexcitations is measured using a time-resolved pump-probe technique. The response is shown quantitatively to follow a time-dependent singlet exciton annihilation process consistent with long-range Förster annihilation. This result is supported by measurements of the dependence of the relaxation on excitation density and probe wavelength, by comparison of the response in thin film, crystalline, and solution samples, and by numerical modeling.