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 This picture is no longer valid if the transport range is on the order of the exciton coherent (or delocalization) size.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

Harvesting Singlet Fission for Solar Energy Conversion: One- versus Two-Electron Transfer from the Quantum Mechanical Superposition

Singlet fission, the creation of two triplet excitons from one singlet exciton, is being explored to increase the efficiency of solar cells and photo detectors based on organic semiconductors, such as pentacene and tetracene. A key question is how to extract multiple electron–hole pairs from multiple excitons. Recent experiments in our laboratory on the pentacene/C₆₀ system (Chan, W.-L.; et al. <i>Science</i> <b>2011</b>, <i>334</i>, 1543–1547) provided preliminary evidence for the extraction of two electrons from the multiexciton (ME) state resulting from singlet fission. The efficiency of multielectron transfer is expected to depend critically on other dynamic processes available to the singlet (S₁) and the ME, but little is known about these competing channels. Here we apply time-resolved photoemission spectroscopy to the tetracene/C₆₀ interface to probe one- and two-electron transfer from S₁ and ME states, respectively. Unlike ultrafast (∼100 fs) singlet fission in pentacene where two-electron transfer from the multiexciton state resulting from singlet fission dominates, the relatively slow (∼7 ps) singlet fission in tetracene allows both one- and two-electron transfer from the S₁ and the ME states that are in a quantum mechanical superposition. We show evidence for the formation of two distinct charge transfer states due to electron transfer from photoexcited tetracene to the lowest unoccupied molecular orbital (LUMO) and the LUMO+1 levels in C₆₀, respectively. Kinetic analysis shows that ∼60% of the S₁ ⇔ ME quantum superposition transfers one electron through the S₁ state to C₆₀ while ∼40% undergoes two-electron transfer through the ME state. We discuss design principles at donor/acceptor interfaces for optimal multiple carrier extraction from singlet fission for solar energy conversion

How the Number of Layers and Relative Position Modulate the Interlayer Electron Transfer in π‑Stacked 2D Materials

Understanding the interfacial electron transfer (IET) between 2D layers is central to technological applications. We present a first-principles study of the IET between a zinc phthalocyanine film and few-layer graphene by using our recent method for the calculation of electronic coupling in periodic systems. The ultimate goal is the development of a predictive in silico approach for designing new 2D materials. We find IET to be critically dependent on the number of layers and their stacking orientation. In agreement with experiment, IET to single-layer graphene is shown to be faster than that to double-layer graphene due to interference effects between layers. We predict that additional graphene layers increase the number of IET pathways, eventually leading to a faster rate. These results shed new light on the subtle interplay between structure and IET, which may lead to more effective “bottom up” design strategies for these materials

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

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

Graphene Field-Effect Transistor as a High-Throughput Platform to Probe Charge Separation at Donor–Acceptor Interfaces

In organic and low-dimensional materials, electrons and holes are bound together to form excitons. Effective exciton dissociation at interfaces is essential for applications such as photovoltaics and photosensing. Here, we present an interface-sensitive, time-resolved method that utilizes graphene field effect transistor as an electric-field sensor to measure the charge separation dynamics and yield at donor–acceptor interfaces. Compared to other interface-sensitive spectroscopy techniques, our method has a much reduced measurement time and can be easily adapted to different material interfaces. Hence, it can be used as a high throughput screening tool to evaluate the charge separation efficiency in a large number of systems. By using zinc phthalocyanine/fullerene interface, we demonstrate how this method can be used to quantify the charge separation dynamics and yield at a typical organic donor–acceptor interface

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

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

Distribution of (A) normalized Put, (B) normalized Spd, (C) normalized Spm values in PCa, BPH and HC.

<p>The black bar in the figures indicates the mean value of each subset while the error bar indicates the corresponding SEM.</p

Column statistics of normalized polyamine contents (μmol/g of creatinine) in different subsets.

<p>Column statistics of normalized polyamine contents (μmol/g of creatinine) in different subsets.</p