Yuxi Tian

Thermally Activated Exciton Dissociation and Recombination Control the Carrier Dynamics in Organometal Halide Perovskite

Solar cells based on organometal halide perovskites have seen rapidly increasing efficiencies, now exceeding 15%. Despite this progress, there is still limited knowledge on the fundamental photophysics. Here we use microwave photoconductance and photoluminescence measurements to investigate the temperature dependence of the carrier generation, mobility, and recombination in (CH3NH3)PbI3. At temperatures maintaining the tetragonal crystal phase of the perovskite, we find an exciton binding energy of about 32 meV, leading to a temperature-dependent yield of highly mobile (6.2 cm(2)/(V s) at 300 K) charge carriers. At higher laser intensities, second-order recombination with a rate constant of γ = 13 × 10(-10) cm(3) s(-1) becomes apparent. Reducing the temperature results in increasing charge carrier mobilities following a T(-1.6) dependence, which we attribute to a reduction in phonon scattering (Σμ = 16 cm(2)/(V s) at 165 K). Despite the fact that Σμ increases, γ diminishes with a factor six, implying that charge recombination in (CH3NH3)PbI3 is temperature activated. The results underline the importance of the perovskite crystal structure, the exciton binding energy, and the activation energy for recombination as key factors in optimizing new perovskite materials.

Collective fluorescence blinking in linear J-aggregates assisted by long-distance exciton migration.

Fluorescence blinking corresponding to collective quenching of up to 100 dye monomers is reported for individual J-aggregates of a perylene bisimide (PBI) dye. This implies an exciton diffusion length up to 70 nm in these one-dimensional assemblies. The number of quenched monomers was directly measured by comparing the fluorescence brightness of the J-aggregates with that of noncoupled PBI molecules. This brightness analysis technique is useful for unraveling photophysical parameters of any individual fluorescent nanosystem.

Giant Photoluminescence Blinking of Perovskite Nanocrystals Reveals Single-Trap Control of Luminescence.

Fluorescence super-resolution microscopy showed correlated fluctuations of photoluminescence intensity and spatial localization of individual perovskite (CH3NH3PbI3) nanocrystals of size ∼200 × 30 × 30 nm(3). The photoluminescence blinking amplitude caused by a single quencher was a hundred thousand times larger than that of a typical dye molecule at the same excitation power density. The quencher is proposed to be a chemical or structural defect that traps free charges leading to nonradiative recombination. These trapping sites can be activated and deactivated by light.

Organization of Bacteriochlorophylls in Individual Chlorosomes from Chlorobaculum tepidum Studied by 2-Dimensional Polarization Fluorescence Microscopy

Chlorosomes are the largest and most efficient natural light-harvesting systems and contain supramolecular assemblies of bacteriochlorophylls that are organized without proteins. Despite a recent structure determination for chlorosomes from Chlorobaculum tepidum (Ganapathy Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 8525), the issue of a possible large structural disorder is still discussed controversially. We have studied individual chlorosomes prepared under very carefully controlled growth condition by a novel 2-dimensional polarization single molecule imaging technique giving polarization information for both fluorescence excitation and emission simultaneously. Contrary to the existing literature data, the polarization degree or modulation depth (M) for both excitation (absorption) and emission (fluorescence) showed extremely narrow distributions. The fluorescence was always highly polarized with M ≈ 0.77, independent of the excitation wavelength. Moreover, the fluorescence spectra of individual chlorosomes were identical within the error limits. These results lead us to conclude that all chlorosomes possess the same type of internal organization in terms of the arrangement of the bacteriochlorophyll c transition dipole moments and their total excitonic transition dipole possess a cylindrical symmetry in agreement with the previously suggested concentric multitubular chlorophyll aggregate organization (Ganapathy Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 8525).

Artifacts in Absorption Measurements of Organometal Halide Perovskite Materials: What Are the Real Spectra?

Organometal halide (OMH) perovskites have attracted lots of attention over the last several years due to their very promising performance as the materials for solar cells and light-emitting devices. Photophysical processes in these hybrid organic-inorganic semiconductors are still heavily debated. To know precise absorption spectra is absolutely necessary for quantitative understanding of the fundamental properties of OMH perovskites. We show that to measure the absorption of perovskite materials correctly is a difficult task which could be easily overlooked by the community. Many of the published absorption spectra exhibit a characteristic step-like featureless shape due to light scattering, high optical density of individual perovskite crystals and poor coverage of the substrate. We show how to recognize these artifacts, to avoid them, and to use absorption spectra of films for estimation of the surface coverage ratio.

High-yield production of quasi-aligned carbon nanotubes by catalytic decomposition of benzene

High-yield production of quasi-aligned carbon nanotube (CNT) bundles is presented. Benzene was used as the precursor for this catalytic process at a moderate temperature of 650°C over γ-Al2O3-supported Fe–Ni alloy catalysts with different mole ratios. The maximum yield of high-quality CNTs per hour is even greater than 350% for the optimized catalyst. The products were characterized by thermal gravimetry/differential scanning calorimetry and high-resolution transmission electron microscopy. The results indicated that the products are well graphitized with tube diameters of about 20 nm. Three typical morphologies of the end parts of the CNTs were identified as the open end capped with an Fe–Ni catalyst, the open end without catalyst and the closed end, respectively. A hexagonal carbon-cluster-based growth for this synthesis is suggested.

Reorganization of perylene bisimide J-aggregates: from delocalized collective to localized individual excitations

Water-induced reorganization of individual one-dimensional J-aggregates of perylene bisimide (PBI) dyes was observed by fluorescence microscopy. Fluorescence spectra and decay kinetics of individual J-aggregates immobilized on glass surfaces were measured under a dry nitrogen atmosphere and under humid conditions. The fluorescence properties of PBI J-aggregates arisen from collective excitons under dry nitrogen atmosphere were changed to those of non-interacting dye monomers when water vapor was introduced into the environment (sample chamber). Time-dependent changes of the fluorescence spectra and lifetimes upon exposure to water vapor suggest an initial coordination of water molecules at defect sites leading to the formation of H-type dimer units that act as exciton quenchers, and a subsequent slower disintegration of the hydrogen-bonded J-aggregate into monomers that lack resonance coupling. Our present studies resulted in a direct demonstration of how drastically the optical properties of molecular ensembles and characteristics of their excited states can be changed by delicate reorganization of dye molecules at nanometre scales.

Quantitative measurement of fluorescence brightness of single molecules

Single-molecule fluorescence spectroscopy and imaging probe many characteristics of the fluorescence from individual molecules like relative intensity, polarization, lifetime and spectrum. However, such an important and fundamental parameter as absolute fluorescence intensity (or in other words fluorescence brightness), which is proportional to the absorption cross section and fluorescence quantum yield, has not yet been sufficiently exploited in the field. One reason for that is the difficulty of absolute fluorescence brightness measurements. In the present work a detailed description of fluorescence brightness measurements of single molecules is given. We discuss several important factors like the power density and polarization of excitation light, the substrates and the local environment. It is shown that the fluorescence brightness of a single molecule indeed can be measured with sufficient accuracy and used as a powerful parameter for characterization of materials at single molecule/particle level. The brightness of a single object can give similar information as the fluorescence quantum yield that is crucial for understanding the photophysical properties for individual multi-chromophoric systems in inhomogeneous environments.

Photo-induced fluorescence quenching in conjugated polymers dispersed in solid matrices at low concentration

When isolated conjugated polymer (CP) chains are studied by single molecule spectroscopy, excitation power density in the range of 10–1000 W cm−2 is normally used. We show that at such excitation power densities the fluorescence ability of CPs is significantly reduced. A new methodological approach allowed us to measure the fluorescence quantum yield (QY) of thin matrix polymer films doped with fluorophores at very low concentration using fluorescence microscopy. Fluorescence QYs of different conjugated polymers (P3HT, MEH-PPV, PFBV and cyclodextrin-coated PFBV-Rtx) and a reference perylene diimide dye dispersed in the PMMA matrix were measured as a function of the excitation power density that ranged from ∼10−4 to 100 W cm−2. Already at an excitation power of 0.1 W cm−2 (the power density of the sunlight at the Earth) a detectable reduction of the fluorescence QY was observed for most of the polymers. The origin of the QY reduction is exciton annihilation by photo-generated triplet and/or change-transfer states. Insulation by cyclodextrin was found to decrease significantly the effect of non-emissive quenching states.

Molecular Weight Determination by Counting Molecules

Molecular weight (MW) is one of the most important characteristics of macromolecules. Sometimes, MW cannot be measured correctly by conventional methods like gel permeation chromatography (GPC) due to, for example, aggregation. We propose using single-molecule spectroscopy to measure the average MW simply by counting individual fluorescent molecules embedded in a thin matrix film at known mass concentration. We tested the method on dye molecules, a labeled protein, and the conjugated polymer MEH-PPV. We showed that GPC with polystyrene calibration overestimates the MW of large MEH-PPV molecules by 40 times due to chain aggregation and stiffness. This is a crucial observation for understanding correlations between the conjugated polymer length, photophysics and performances of devices. The method can measure the MW of fluorescent molecules, biological objects, and nanoparticles at ultimately low concentrations and does not need any reference; it is conformation-independent and has no limitations regarding the detected MW range.