Electrical Control of Valley Polarized Charged Exciton Species in Monolayer WS.

Excitons are key to the optoelectronic applications of van der Waals semiconductors, with the potential for versatile on-demand tuning of properties. Yet, their electrical manipulation remains challenging due to inherent charge neutrality and the additional loss channels induced by electrical doping. We demonstrate the dynamic electrical control of valley polarization in charged excitonic states of monolayer tungsten disulfide, achieving up to a 6-fold increase in the degree of circular polarization under off-resonant excitation.

Constructing custom thermodynamics using deep learning.

One of the most exciting applications of artificial intelligence is automated scientific discovery based on previously amassed data, coupled with restrictions provided by known physical principles, including symmetries and conservation laws. Such automated hypothesis creation and verification can assist scientists in studying complex phenomena, where traditional physical intuition may fail. Here we develop a platform based on a generalized Onsager principle to learn macroscopic dynamical descriptions of arbitrary stochastic dissipative systems directly from observations of their microscopic trajectories.

Automated electrokinetic stretcher for manipulating nanomaterials.

In this work, we present an automated platform for trapping and stretching individual micro- and nanoscale objects in solution using electrokinetic forces. The platform can trap objects at the stagnation point of a planar elongational electrokinetic field for long time scales, as demonstrated by the trapping of <100 nm polystyrene beads and DNA molecules for minutes, with a standard deviation in displacement from the trap center <1 μm. This capability enables the stretching of deformable nanoscale objects in a high-throughput fashion, as illustrated by the stretching of more than 400 DNA molecules within ∼4 hours.

Defect Passivation Using a Phosphonic Acid Surface Modifier for Efficient RP Perovskite Blue-Light-Emitting Diodes.

Defect management strategies are vital for enhancing the performance of perovskite-based optoelectronic devices, such as perovskite-based light-emitting diodes (PeLEDs). As additives can fucntion both as acrystallization modifier and/or defect passivator, a thorough study on the roles of additives is essential, especially for blue emissive Pe-LEDs, where the emission is strictly controlled by the -domain distribution of the Ruddlesden-Popper (RP, LAPbX, where L refers to a bulky cation, while A and X are monovalent cation, and halide anion, respectively) perovskite films. Of the various additives that are available, octyl phosphonic acid (OPA) is of immense interest because of its ability to bind with uncoordinated Pb ( notorious for nonradiative recombination) and therefore passivates them.

Gate-Defined Quantum Confinement in CVD 2D WS.

Temperature-dependent transport measurements are performed on the same set of chemical vapor deposition (CVD)-grown WS single- and bilayer devices before and after atomic layer deposition (ALD) of HfO . This isolates the influence of HfO deposition on low-temperature carrier transport and shows that carrier mobility is not charge impurity limited as commonly thought, but due to another important but commonly overlooked factor: interface roughness. This finding is corroborated by circular dichroic photoluminescence spectroscopy, X-ray photoemission spectroscopy, cross-sectional scanning transmission electron microscopy, carrier-transport modeling, and density functional modeling.

Quantum Transport in Two-Dimensional WS with High-Efficiency Carrier Injection through Indium Alloy Contacts.

Two-dimensional transition metal dichalcogenides (TMDCs) have properties attractive for optoelectronic and quantum applications. A crucial element for devices is the metal-semiconductor interface. However, high contact resistances have hindered progress.

Carrier control in 2D transition metal dichalcogenides with AlO dielectric.

We report transport measurements of dual gated MoS and WSe devices using atomic layer deposition grown AlO as gate dielectrics. We are able to achieve current pinch-off using independent split gates and observe current steps suggesting possible carrier confinement. We also investigated the impact of gate geometry and used electrostatic potential simulations to explain the observed device physics.

Identifying Fundamental Limitations in Halide Perovskite Solar Cells.

The temperature dependence of the principal photovoltaic parameters of perovskite photovoltaics is studied. The recombination activation energy is in good agreement with the perovskite's bandgap energy, thereby placing an upper bound on the open-circuit voltage. The photocurrent increases moderately with temperature and remains high at low temperature, reinforcing that the cells are not hindered by insufficient thermally activated mobility or carrier trapping by deep defects.

Optimal Shell Thickness of Metal@Insulator Nanoparticles for Net Enhancement of Photogenerated Polarons in P3HT Films.

Embedding metal nanoparticles in the active layer of organic solar cells has been explored as a route for improving charge carrier generation, with localized field enhancement as a proposed mechanism. However, embedded metal nanoparticles can also act as charge recombination sites. To suppress such recombination, the metal nanoparticles are commonly coated with a thin insulating shell.

Enhancing the performance of solution-processed bulk-heterojunction solar cells using hydrogen-bonding-induced self-organization of small molecules.

Small-molecule solar-cell performance is highly sensitive to the crystallinity and intermolecular connectivity of the molecules. In order to enhance the crystallinity for the solution-processed small molecule, it is possible to make use of carboxylic acid end-functional groups to drive hydrogen-bonding-induced π-π stacking of conjugated molecules. Herein, we report the synthesis and characterization of quarterthiophenes with carboxylic acid as end groups.

Device physics and operation of lateral bulk heterojunction devices.

Measurements of lateral bulk heterojunction (BHJ) devices have recently been reported as a means to characterize charge transport and recombination properties within organic photovoltaic (OPV) materials. These structures allow for the direct measurement of the lateral extents of the space charge regions, potential and electric field profiles, current versus voltage characteristics, and other physical and chemical properties. This article describes numerical simulations that show three different transport regimes present within lateral BHJ devices and two different experimental methods, which verify those findings.

Scanning photocurrent microscopy of lateral organic bulk heterojunctions.

Scanning confocal photocurrent microscopy has been used to characterize carrier collection efficiency in lateral bulk heterojunction devices. By analyzing the photocurrent mappings within these devices, the lateral extents of the space charge regions has been measured and reported. Modulation via white light bias or increased voltage bias is also shown to increase the size of the space charge regions.

Poly(2,5-bis(2-octyldodecyl)-3,6-di(furan-2-yl)-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione-co-thieno[3,2-b]thiophene): a high performance polymer semiconductor for both organic thin film transistors and organic photovoltaics.

A new diketopyrrolopyrrole (DPP)-containing donor-acceptor polymer, poly(2,5-bis(2-octyldodecyl)-3,6-di(furan-2-yl)-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-dione-co-thieno[3,2-b]thiophene) (PDBF-co-TT), is synthesized and studied as a semiconductor in organic thin film transistors (OTFTs) and organic photovoltaics (OPVs). High hole mobility of up to 0.53 cm(2) V(-1) s(-1) in bottom-gate, top-contact OTFT devices is achieved owing to the ordered polymer chain packing and favoured chain orientation, strong intermolecular interactions, as well as uniform film morphology of PDBF-co-TT.