Dynamical Localization Limiting the Coherent Transport Range of Excitons in Organic Crystals

Exciton or energy transport in organic crystals is commonly described by a series of incoherent hoppings. This picture is no longer valid if the transport range is on the order of the exciton coherent (or delocalization) size. However, coherent effects are often neglected because the exciton wave function generally localizes to a few molecules within an ultrafast time scale (<1 ps) after photoexcitation. Here, by using time-resolved photoemission spectroscopy and nanometer-thick zinc phthalocyanine crystals, we are able to observe a transition from the coherent to incoherent transport regime while the exciton coherent size is decreasing as a function of time. During the transition, a distinct phonon mode is excited, which suggests that the electron–vibrational interaction localizes the exciton and reduces its coherent size. It is anticipated that the coherent transport range can be increased by controlling the electron–vibrational coupling. An enhanced coherent transport range can be advantageous in applications such as organic photovoltaics

A Multidimensional View of Charge Transfer Excitons at Organic Donor–Acceptor Interfaces

How tightly bound charge transfer (CT) excitons dissociate at organic donor–acceptor interfaces has been a long-standing question in the organic photovoltaics community. Recently, it has been proposed that exciton delocalization reduces the exciton binding energy and promotes exciton dissociation. In order to understand this mechanism, it is critical to resolve the evolution of the exciton’s binding energy and coherent size with femtosecond time resolution. However, because the coherent size is just a few nanometers, it presents a major experimental challenge to capture the CT process simultaneously in the energy, spatial, and temporal domains. In this work, the challenge is overcome by using time-resolved photoemission spectroscopy. The spatial size and electronic energy of a manifold of CT states are resolved at the zinc phthalocyanine (ZnPc)–fullerene (C₆₀) donor–acceptor interface. It is found that CT at the interface first populates delocalized CT excitons with a coherent size of 4 nm. Then, this delocalized CT exciton relaxes in energy to produce CT states with delocalization sizes in the range of 1–3 nm. While the CT process from ZnPc to C₆₀ occurs in about 150 fs after photoexcitation, the localization and energy relaxation occur in 2 ps. The multidimensional view on how CT excitons evolve in time, space, and energy provides key information to understand the exciton dissociation mechanism and to design nanostructures for effective charge separation

Observation of an Ultrafast Exciton Hopping Channel in Organic Semiconducting Crystals

One of the major challenges in using organic semiconductors for photovoltaics is their extremely short exciton diffusion length. Recently, a number of studies have shown that the exciton transport range within the first few picoseconds after photoexcitation can be comparable to the exciton’s diffusion length over its entire lifetime. The origin of this fast transport channel is often attributed to the large spatial coherent size of the exciton right after photoexcitation. Here we observe an ultrafast exciton hopping channel in titanyl phthalocyanine crystals even though the exciton coherent size is a few times smaller than the transport range. This channel operates only within the first few picoseconds after photoexcitation and has a hopping rate that is an order of magnitude faster than the typical Förster resonance energy transfer rate. Resonant Raman spectroscopy shows that the optically excited exciton is strongly mixed with the macrocycle vibrational mode of the phthalocyanine molecules. A hypothesis involving vibronic coherence is proposed to explain the observed transport behavior

Charge Transfer Exciton and Spin Flipping at Organic–Transition-Metal Dichalcogenide Interfaces

Two-dimensional transition-metal dichalcogenides (TMD) can be combined with other materials such as organic small molecules to form hybrid van der Waals heterostructures. Because of different properties possessed by these two materials, the hybrid interface can exhibit properties that cannot be found in either of the materials. In this work, the zinc phthalocyanine (ZnPc)–molybdenum disulfide (MoS₂) interface is used as a model system to study the charge transfer at these interfaces. It is found that the optically excited singlet exciton in ZnPc transfers its electron to MoS₂ in 80 fs after photoexcitation to form a charge transfer exciton. However, back electron transfer occurs on the time scale of ∼1–100 ps, which results in the formation of a triplet exciton in the ZnPc layer. This relatively fast singlet–triplet transition is feasible because of the large singlet–triplet splitting in organic materials and the strong spin–orbit coupling in TMD crystals. The back electron transfer would reduce the yield of free carrier generation at the heterojunction if it is not avoided. On the other hand, the spin-selective back electron transfer could be used to manipulate electron spin in hybrid electronic devices

Ultrafast Imaging of Carrier Transport across Grain Boundaries in Hybrid Perovskite Thin Films

For optoelectronic devices based on polycrystalline semiconducting thin films, carrier transport across grain boundaries is an important process in defining efficiency. Here we employ transient absorption microscopy (TAM) to directly measure carrier transport within and across the boundaries in hybrid organic–inorganic perovskite thin films for solar cell applications with 50 nm spatial precision and 300 fs temporal resolution. By selectively imaging sub-bandgap states, our results show that lateral carrier transport is slowed down by these states at the grain boundaries. However, the long carrier lifetimes allow for efficient transport across the grain boundaries. The carrier diffusion constant is reduced by about a factor of 2 for micron-sized grain samples by the grain boundaries. For grain sizes on the order of ∼200 nm, carrier transport over multiple grains has been observed within a time window of 5 ns. These observations explain both the shortened photoluminescence lifetimes at the boundaries as well as the seemingly benign nature of the grain boundaries in carrier generation

Designing the Interface of Carbon Nanotube/Biomaterials for High-Performance Ultra-Broadband Photodetection

Inorganic/biomolecule nanohybrids can combine superior electronic and optical properties of inorganic nanostructures and biomolecules for optoelectronics with performance far surpassing that achievable in conventional materials. The key toward a high-performance inorganic/biomolecule nanohybrid is to design their interface based on the electronic structures of the constituents. A major challenge is the lack of knowledge of most biomolecules due to their complex structures and composition. Here, we first calculated the electronic structure and optical properties of one of the cytochrome c (Cyt c) macromolecules (PDB ID: 1HRC) using ab initio OLCAO method, which was followed by experimental confirmation using ultraviolet photoemission spectroscopy. For the first time, the highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of Cyt c, a well-known electron transport chain in biological systems, were obtained. On the basis of the result, pairing the Cyt c with semiconductor single-wall carbon nanotubes (s-SWCNT) was predicted to have a favorable band alignment and built-in electrical field for exciton dissociation and charge transfer across the s-SWCNT/Cyt c heterojunction interface. Excitingly, photodetectors based on the s-SWCNT/Cyt c heterojunction nanohybrids demonstrated extraordinary ultra-broadband (visible light to infrared) responsivity (46–188 A W^–1) and figure-of-merit detectivity <i>D</i>* (1–6 × 10¹⁰ cm Hz^1/2 W^–1). Moreover, these devices can be fabricated on transparent flexible substrates by a low-lost nonvacuum method and are stable in air. These results suggest that the s-SWCNT/biomolecule nanohybrids may be promising for the development of CNT-based ultra-broadband photodetectors

Compound binding affinities for MLKL, RIPK1 and RIPK3, differential thermal shift values and Necroptosis Inhibition.

<p>Compound binding affinities for MLKL, RIPK1 and RIPK3, differential thermal shift values and Necroptosis Inhibition.</p

Definition of the terms used to calculate degree of rescue or viability, based on presence or absence of compound and TNF.

<p>Definition of the terms used to calculate degree of rescue or viability, based on presence or absence of compound and TNF.</p

Chemical structure of MLKL binders described in text.

<p>Chemical structure of MLKL binders described in text.</p