Energy Transfer In Films Research Articles

Providing Time to Transfer: Longer Lifetimes Lead to Improved Energy Transfer in Films of Semiconducting Carbon Nanotubes.

The performance of photovoltaic devices made using semiconducting carbon nanotubes is limited by the transverse exciton diffusion length, which is ultimately set by intertube energy transfer. In this paper, we study whether extending the exciton lifetime improves energy transfer, by allowing more time for exciton transfer between carbon nanotubes, and thereby device performance. To do so, we prepare nanotubes by either shear-force mixing or ultrasonication, leading to different lengths and defect densities. We create thin films that mix (6,5) and (7,5) nanotubes and quantify the relative amounts of energy transfer in them using two-dimensional white-light (2DWL) spectroscopy and photoluminescence excitation (PLE) spectroscopy. Cross-peaks appearing in 2DWL spectra and quenching of the (6,5) PLE signal upon mixing both quantify energy transfer from (6,5) to (7,5). In both spectroscopies, energy transfer between shear-force mixed tubes is ∼20% more efficient. The cross-peaks in 2DWL spectra grow in at the same rate regardless of the processing method with the all shear-force mixed sample ultimately reaching a larger cross-peak amplitude. Shear-force mixing methods instead of sonication have improved external quantum efficiency in carbon nanotube devices by 30%. The spectroscopic results observed here link energy transfer to exciton diffusion and correlate to device performance.

Imaging and Manipulating Energy Transfer Among Quantum Dots at Individual Dot Resolution

Many processes of interest in quantum dots involve charge or energy transfer from one dot to another. Energy transfer in films of quantum dots as well as between linked quantum dots has been demonstrated by luminescence shift, and the ultrafast time-dependence of energy transfer processes has been resolved. Bandgap variation among dots (energy disorder) and dot separation are known to play an important role in how energy diffuses. Thus, it would be very useful if energy transfer could be visualized directly on a dot-by-dot basis among small clusters or within films of quantum dots. To that effect, we report single molecule optical absorption detected by scanning tunneling microscopy (SMA-STM) to image energy pooling from donor into acceptor dots on a dot-by-dot basis. We show that we can manipulate groups of quantum dots by pruning away the dominant acceptor dot, and switching the energy transfer path to a different acceptor dot. Our experimental data agrees well with a simple Monte Carlo lattice model of energy transfer, similar to models in the literature, in which excitation energy is transferred preferentially from dots with a larger bandgap to dots with a smaller bandgap.

Interplay of localized pyrene chromophores and π-conjugation in novel poly(2,7-pyrene) ladder polymers.

We present a detailed spectroscopic study, along with the synthesis, of conjugated, ladder-type 2,7-linked poly(pyrene)s. We observe a delocalization of the first singlet excited state along the polymer backbone, i.e., across the 2,7 linkage in the pyrene moiety, in contrast to earlier studies on conjugated 2,7-linked poly(pyrene)s without ladder structure. The electronic signature of the pyrene unit is, however, manifested in an increased lifetime and reduced oscillator strength as well as a modified vibronic progression in absorption of the singlet state compared to a ladder-type poly(para-phenylene) (MeLPPP). Furthermore, the reduced oscillator strength and increased lifetime slow down Förster-type energy transfer in films, where this transfer occurs to sites with increasing inter-chain coupling of H-type nature.

Enhanced Emission of Nanocrystal Solids Featuring Slowly Diffusive Excitons

Solution processing of semiconductor nanocrystal (NC) solids represents an attractive platform for the development of next-generation optoelectronic devices. In search of enhanced light-emitting performance, NC solids are typically designed to have large interparticle gaps that minimize exciton diffusion to dissociative sites. This strategy, however, reduces electrical coupling between nanoparticles in a film, making the injection of charges inefficient. Here, we demonstrate that bright emission from nanocrystal solids can be achieved without compromising their electrical conductivity. Our study shows that solids featuring a low absorption-emission spectral overlap (J) exhibit an intrinsically slower exciton diffusion to recombination centers, promoting longer exciton lifetimes. As a result, enhanced emission is achieved despite a strong electronic coupling. The observed phenomenon was found consistent with a decreased resonant energy transfer in films exhibiting a reduced J value. The inverse correlation...

Suppressing the energy transfer in polymer blends films upon addition of a co-solvent

Energy transfer in conjugated polymer blends is an important process in optoelectronics, with its efficiency very sensitive to the details of phase separation in films. Modification of the relative composition of the blend has been the usual strategy to tune the energy transfer efficiency. In this letter we report on a new approach to switch-off that process: the use of a co-solvent. In particular, we find that the addition of 1,8-diiodooctane to xylene solutions of poly(9,9-dioctylfluorene): poly(9,9-dioctylfluorene-alt-benzothiadiazole) (PFO:F8BT), blends, 99:1 by weight, suppresses the energy transfer in films. This leads to a change from green light-emitting diodes into whitish emitting diodes. This energy transfer suppression is attributed to a stronger phase segregation, with the concomitant formation of larger domains of the two polymers, thereby increasing the distance between energy donor (PFO) and acceptor (F8BT) sites.

Singlet and Triplet Exciton Harvesting in the Thin Films of Colloidal Quantum Dots Interfacing Phosphorescent Small Organic Molecules

Efficient nonradiative energy transfer is reported in an inorganic/organic thin film that consists of a CdSe/ZnS core/shell colloidal quantum dot (QD) layer interfaced with a phosphorescent small organic molecule (FIrpic) codoped fluorescent host (TCTA) layer. The nonradiative energy transfer in these thin films is revealed to have a cascaded energy transfer nature: first from the fluorescent host TCTA to phosphorescent FIrpic and then to QDs. The nonradiative energy transfer in these films enables very efficient singlet and triplet state harvesting by the QDs with a concomitant fluorescence enhancement factor up to 2.5-fold, while overall nonradiative energy transfer efficiency is as high as 95%. The experimental results are successfully supported by the theoretical energy transfer model developed here, which considers exciton diffusion assisted Förster-type near-field dipole–dipole coupling within the hybrid films.

Directed Energy Transfer in Films of CdSe Quantum Dots: Beyond the Point Dipole Approximation

Understanding of Förster resonance energy transfer (FRET) in thin films composed of quantum dots (QDs) is of fundamental and technological significance in optimal design of QD based optoelectronic devices. The separation between QDs in the densely packed films is usually smaller than the size of QDs, so that the simple point-dipole approximation, widely used in the conventional approach, can no longer offer quantitative description of the FRET dynamics in such systems. Here, we report the investigations of the FRET dynamics in densely packed films composed of multisized CdSe QDs using ultrafast transient absorption spectroscopy and theoretical modeling. Pairwise interdot transfer time was determined in the range of 1.5 to 2 ns by spectral analyses which enable separation of the FRET contribution from intrinsic exciton decay. A rational model is suggested by taking into account the distribution of the electronic transition densities in the dots and using the film morphology revealed by AFM images. The FRET dynamics predicted by the model are in good quantitative agreement with experimental observations without adjustable parameters. Finally, we use our theoretical model to calculate dynamics of directed energy transfer in ordered multilayer QD films, which we also observe experimentally. The Monte Carlo simulations reveal that three ideal QD monolayers can provide exciton funneling efficiency above 80% from the most distant layer. Thereby, utilization of directed energy transfer can significantly improve light harvesting efficiency of QD devices.

Electronic excitation energy transfer in poly-N-epoxypropyl-3,6-dibromocarbazole

The mechanisms of electronic excitation energy transfer in films of poly-N-epoxypropyl-3,6-dibromocarbazole doped with pyrene, rubrene, and bis[2-(2′-benzothienyl)-pyridinato-N,C3′](acetylacetonate)iridium(III) (Btp2Ir(acac)) have been studied in the range T = 5–295 K. It has been established that the mechanisms of transfer depend on the relative position of the S1 and T1 levels of the base and dopant: if the S1 level of the dopant lies between the S1 and T1 levels of the base and its T1 level is below than that of the base (pyrene molecules), only the triplet-triplet transfer takes place, whereas if the S1 level of the dopant lies below the T1 level of the base (pyrene and Btp2Ir(acac) molecules), the triplet-singlet transfer is observed in addition to the triplet-triplet transfer. It has been assumed that the spin-forbidden triplet-singlet energy transfer is allowed because of the strong spinorbit interaction induced by two heavy bromine atoms in chromophores of the polymer. This transfer proceeds through the long-range dipole-dipole interaction at low temperatures and the electron exchange interaction in the vicinity of room temperature.

Effect of Hard Polymer Filler Particles on Polymer Diffusion in a Low-Tg Latex Film

The intercelluar diffusion of poly(butyl methacrylate) (PBMA; glass transition temperature (Tg) = 21 °C) at 60 °C was monitored by fluorescence energy transfer in films formed from blends of PBMA l...

Excitation energy transfer in films of Poly(N-vinylcarbazole)

Time-resolved fluorescence of films of poly(N-vinylcarbazole) was measured at 100 and 200 K. The spectra consist of three components, one of the carbazolyl group and the other two belonging to excimers.Decay curves of the components document the slowing down of the diffusion energy migration processes in the polymer at low temperatures.

Enhanced Luminescence of Eu(III) by La(III), Gd(III), Tb(III) and Y(III) in Langmuir-Blodgett Films of Mixed Rare Earth Complexes

Abstract Four kinds of LB films containing 0.5%Eu(TTA)3Phen + 99.5%RE(TTA)3Phen (in molar percentage; RE = La(III), Gd(III), Tb(III) and Y(III)) mixed with arachidic acid (AA) in molar ratio of 1:1 were fabricated. The luminescent enhancement effect of RE(TTA)3Phen on Eu(III) was studied. The influence of rare earth ions on the efficiency of the intermolecular energy transfer in films was discussed. When the complexes of Tb(III) and Gd(III) coexist with Eu(TTA)3Phen, the efficiency of energy transfer is higher than that of the complexes of La(III) and Y(III).

Luminescence of poly(n-vinyl-carbazole) films at 77 K. II. Kinetic model of exciton trapping and annihilation

Energy transfer in films of PVCA at low temperatures is due to singlet and triplet excitons. The range of the excitons is limited by excimer-forming sites. Delayed fluorescence is caused by triplet exciton-triplet excimer annihilation. These processes are described quantitatively by an exciton hopping model tailor made for energy transfer in PVCA. It predicts triplet excimer-forming sites of the order of 10 −5 mole/mole basic unit for the cationic polymer and 10 −4 for the radicalic one if a radiative phosphorescence lifetime of 50 s is assumed. The model correctly describes the dependence of fluorescence, phosphorescence and delayed fluorescence intensities on excitation intensity reported in part I of this work.

Investigation of triplet—triplet energy transfer in films of poly-n-vinylcarbazole employing chemiluminescence techniques

Chemiluminescent techniques are employed to investigate energy transfer and exciton in polyvinylcarbazole films at 34°C. Tetramethyl-1,2-dioxetane is used as a source of chemiexcited acetone triplets and 1,4-dibromonaphthalene and 1,10-phenanthroline are used as triplet acceptors which phosphoresce under the conditions of measurement. A study of the relationship of acceptor chemiluminescence intensity to acceptor concentration leads to the conclusion that extensive energy transfer by triplet excitons can occur in polyvinylcarbazole films.