Ivan G. Scheblykin

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.

Dimensionality and temperature dependence of the radiative lifetime of J-aggregates with Davydov splitting of the exciton band

Abstract The temperature dependence of the radiative exciton lifetime (τrad) of J-aggregates of 3,3′-bis(sulfopropyl)-5,5′-dichloro-9-ethylthiacarbocyanine (THIATS) characterized by a Davydov splitting of the exciton band has been determined over the temperature range from 4.2 to 130 K. The Davydov splitting of the exciton band was taken into account during the calculation of the coherent length (Nc) from the values of τrad. The temperature dependence of τrad(T) in units of (Nc)−1 was compared to that for PIC aggregates. In contrast to the observations for the PIC J-aggregates [Potma and Wiersma, J. Chem. Phys., 108 (1998) 4894] the τrad(T) of J-aggregates of THIATS can be rationalized within the framework of a 1-D exciton model.

NON-COHERENT EXCITON MIGRATION IN J-AGGREGATES OF THE DYE THIATS : EXCITON-EXCITON ANNIHILATION AND FLUORESCENCE DEPOLARIZATION

Abstract Exciton–exciton annihilation (EEA) in J-aggregates of 3,3′-disulfopropyl-5,5′-dichloro-9-ethylthiacarbocyanine in solution at room temperature and at 77 K was related to incoherent exciton migration within large aggregate domains. Inside a domain, the rate constant for EEA was estimated to be at least 1.5 times smaller than that of exciton decay. The experimental results were incompatible with one-dimensional incoherent exciton transport. The intensity dependence of the fluorescence anisotropy decay was assigned to EEA. At 77 K and in the absence of EEA, the average angle of the transition dipole changed less than 25° during exciton migration over a domain.

Electroluminescence and optical properties of poly(phenylenevinylene)/J-aggregate composites

A composite material of poly(phenylenevinylene) (PPV) doped by dye aggregates was prepared. A very efficient and temperature activated energy transfer (ET) from the PPV to the dye aggregates was attributed to F . orster ET accompanied by exciton diffusion. A clear complementary relation between the photoluminescence (PL) and electroluminscence (EL) images was observed for thin (15 nm) PPV-based OLEDs. So-called ‘‘black spots’’ in EL become bright ones when the photoluminescence of the same area was excited. This effect was attributed to the presence of an insulating layer between the polymer and aluminium. r 2001 Elsevier Science B.V. All rights reserved.

Reorientation of transition dipoles during exciton relaxation in J-aggregates probed by flourescence anisotropy

Abstract A negative fluorescence anisotropy is observed in J-aggregates of two thiacarbocyanine dyes in water-ethylene glycol low-temperature glass upon excitation outside the J-band. The fluoresence anisotropy gradually increases from −0.16 upon non-resonant excitation at 532 nm to +0.37 upon excitation at 610 nm (near the maximum of the J-band absorption). The fluorescence anisotropy is found to vary non-monotonically as a function of the emission wavelength within the J-band of the fluorescence. The dependence of the anisotropy on excitation frequency can be explained by a chain model with two molecules per unit cell. The transition dipole moment of each molecule is estimated to make an angle of 65–70° with the chain direction.

Relaxation and trapping of excitons in J-aggregates of a thiacarbocynine dye

Abstract Exciton relaxation processes in J-aggregates of 3,3′,9-triethyl-5,5′-dichlorothiacarbocyanine iodide (TDC) dye have been studied by means of site-selective, steady-state and time-resolved spectroscopy. We found that TDC forms three different types of J-aggregates (J1,J2, and J3) in frozen solutions. Fluorescence polarization measurements showed that exciton diffusion between randomly oriented segments of aggregates plays a minor role in exciton relaxation. Optical properties of J3-aggregate fluorescent states are well described by the theoretical model of barrierless self-trapping of an excitation in a strictly one-dimensional discrete lattice. Upon optical excitation J3-aggregates also show thermally activated transformation to J2-aggregates. The height of the related potential barrier is approximately equal to 30 cm−1. Hence, two different exciton relaxation processes are proposed to take place in the system under study: barrierless self-trapping in J3-aggregates and thermally activated J3 → J2 photorearrangement.