Tõnu Pullerits

Organometal Halide Perovskite Solar Cell Materials Rationalized: Ultrafast Charge Generation, High and Microsecond-Long Balanced Mobilities, and Slow Recombination

Organometal halide perovskite-based solar cells have recently been reported to be highly efficient, giving an overall power conversion efficiency of up to 15%. However, much of the fundamental photophysical properties underlying this performance has remained unknown. Here, we apply photoluminescence, transient absorption, time-resolved terahertz and microwave conductivity measurements to determine the time scales of generation and recombination of charge carriers as well as their transport properties in solution-processed CH3NH3PbI3 perovskite materials. We found that electron-hole pairs are generated almost instantaneously after photoexcitation and dissociate in 2 ps forming highly mobile charges (25 cm(2) V(-1) s(-1)) in the neat perovskite and in perovskite/alumina blends; almost balanced electron and hole mobilities remain very high up to the microsecond time scale. When the perovskite is introduced into a TiO2 mesoporous structure, electron injection from perovskite to the metal oxide is efficient in less than a picosecond, but the lower intrinsic electron mobility of TiO2 leads to unbalanced charge transport. Microwave conductivity measurements showed that the decay of mobile charges is very slow in CH3NH3PbI3, lasting up to tens of microseconds. These results unravel the remarkable intrinsic properties of CH3NH3PbI3 perovskite material if used as light absorber and charge transport layer. Moreover, finding a metal oxide with higher electron mobility may further increase the performance of this class of solar cells.

Origin of Long-Lived Coherences in Light-Harvesting Complexes

A vibronic exciton model is applied to explain the long-lived oscillatory features in the two-dimensional (2D) electronic spectra of the Fenna–Matthews–Olson (FMO) complex. Using experimentally determined parameters and uncorrelated site energy fluctuations, the model predicts oscillations with dephasing times of 1.3 ps at 77 K, which is in a good agreement with the experimental results. These long-lived oscillations originate from the coherent superposition of vibronic exciton states with dominant contributions from vibrational excitations on the same pigment. The oscillations obtain a large amplitude due to excitonic intensity borrowing, which gives transitions with strong vibronic character a significant intensity despite the small Huang–Rhys factor. Purely electronic coherences are found to decay on a 200 fs time scale.

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.

Spectroscopic units in conjugated polymers: A quantum chemically founded concept?

In conjugated polymers the concept of spectroscopic units belonging to different spatial segments of the chain, which are responsible for the spectroscopic properties of the polymer, has been used to explain the spectral heterogeneity and the excitation migration by (Förster type) hopping transfer. In the present work we study the possible mechanism of segmentation of polythiophene into spectroscopic units by using quantum-chemical methods (ZINDO). We found that static geometric defects such as kinks or torsions do not result in a significant localization of the excited states to a certain segment. Hence, we propose that a dynamic localization of excitation due to the interaction between the nuclear and electronic degrees of freedom is responsible for the formation of the spectroscopic units.

Excitonic coupling in polythiophenes: Comparison of different calculation methods

In conjugated polymers the optical excitation energy transfer is usually described as Forster-type hopping between so-called spectroscopic units. In the simplest approach using the point-dipole approximation the transfer rate is calculated based on the interaction between the transition dipoles of two spectroscopic units. In the present work we compare this approach with three others: The line-dipole approximation, the Coulomb integral between the transition densities, and a quantum-chemical calculation of the interacting dimer as entity. The latter two approaches are based on the semiempirical method ZINDO. The line-dipole approximation is an attractive compromise between computational effort and precision for calculations of the excitonic coupling in extended conjugated polymers.

B800→B850 Energy Transfer Mechanism in Bacterial LH2 Complexes Investigated by B800 Pigment Exchange

Femtosecond transient absorption measurements were performed on native and a series of reconstituted LH2 complexes from Rhodopseudomonas acidophila 10050 at room temperature. The reconstituted complexes contain chemically modified tetrapyrrole pigments in place of the native bacteriochlorophyll a-B800 molecules. The spectral characteristics of the modified pigments vary significantly, such that within the B800 binding sites the B800 Q(y) absorption maximum can be shifted incrementally from 800 to 670 nm. As the spectral overlap between the B800 and B850 Q(y) bands decreases, the rate of energy transfer (as determined by the time-dependent bleaching of the B850 absorption band) also decreases; the measured time constants range from 0.9 ps (bacteriochlorophyll a in the B800 sites, Q(y) absorption maximum at 800 nm) to 8.3 ps (chlorophyll a in the B800 sites, Q(y) absorption maximum at 670 nm). This correlation between energy transfer rate and spectral blue-shift of the B800 absorption band is in qualitative agreement with the trend predicted from Förster spectral overlap calculations, although the experimentally determined rates are approximately 5 times faster than those predicted by simulations. This discrepancy is attributed to an underestimation of the electronic coupling between the B800 and B850 molecules.

Electron Transfer in Quantum-Dot-Sensitized ZnO Nanowires: Ultrafast Time-Resolved Absorption and Terahertz Study

Photoinduced electron injection dynamics from CdSe quantum dots to ZnO nanowires is studied by transient absorption and time-resolved terahertz spectroscopy measurements. Ultrafast electron transfer from the CdSe quantum dots to ZnO is proven to be efficient already on a picoseconds time scale (τ = 3-12 ps). The measured kinetics was found to have a two-component character, whose origin is discussed in detail. The obtained results suggest that electrons are injected into ZnO via an intermediate charge transfer state.

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.

Picosecond dynamics of directed excitation transfer in spectrally heterogeneous light-harvesting antenna of purple bacteria

Picosecond spectrally resolved fluorescence kinetics measurements, together with model simulations of the obtained data, based on coupled rate equations, have been employed to determine the rates and the pathways of heterogeneous excitation transfer in Rhodobacter sphaeroides and Chromatium minutissimum . The presence of a directed excitation flow from the short-wavelength bacteriochlorophyll forms to the long-wavelength one and from the latter to reaction centres has been revealed. As a result, the overall excitation trapping time in the bacteria investigated has been found to be about 60 ps both at 77 K and at room temperature, i.e. the same as in Rhodospirillum rubrum , although the number of antenna bacteriochlorophyll molecules per reaction centre is several times larger. A comparison of the experimental and theoretical kinetic data shows that, besides obvious spectral heterogeneity of the bacteriochlorophyll antenna represented by well-resolved B800, B850 and B875 spectra forms, an intrinsic spectral inhomogeneiry of these forms is likely to play an essential role in the excitation transfer. The obtained picture of the mutual arrangement of different complexes in membranes is similar to the one suggested earlier, except that the presence of at least two typesof B800-850 complexes, the ones closely associated with B875 and the more remote ones,has been discovered. The excitation transfer to B875 has been shown to take about 10 and 50 ps for the first and the second type of B850 molecules, respectively. The intracomplex B800 → B850 transfer time is an order of magnitude smaller, about 1 ps. These three time constants seem to be practically independent of the reaction centre state and the temperature in the 300 K-77 K interval. At high excitation intensities (more than 1 · 1010 photons per pulse) a shortening of the long-wavelength fluorescence decay time and a short-wavelength shiftof the corresponding band maximum have been observed. Both effects are due to the annihilation of singlet and triplet excitations.

Conformational Disorder and Energy Migration in MEH-PPV with Partially Broken Conjugation.

In order to obtain a better understanding of the role of conformational disorder in the photophysics of conjugated polymers the ultrafast transient absorption anisotropy of partially deconjugated MEH-PPV has been measured. These data have been compared to the corresponding kinetics of Monte Carlo–simulated polymer chains, and estimates of the energy hopping time and energy migration distances for the polymers have been obtained. We find that the energy migration in the investigated MEH-PPV is approximately 3 times faster than in previously studied polythiophenes. We attribute this to a more disordered chain conformation in MEH-PPV.