Direct identification of interfacial degradation in blue OLEDs using nanoscale chemical depth profiling.

Understanding the degradation mechanism of organic light-emitting diodes (OLED) is essential to improve device performance and stability. OLED failure, if not process-related, arises mostly from chemical instability. However, the challenges of sampling from nanoscale organic layers and interfaces with enough analytical information has hampered identification of degradation products and mechanisms. Here, we present a high-resolution diagnostic method of OLED degradation using an Orbitrap mass spectrometer equipped with a gas cluster ion beam to gently desorb nanometre levels of materials, providing unambiguous molecular information with 7-nm depth resolution. We chemically depth profile and analyse blue phosphorescent and thermally-activated delayed fluorescent (TADF) OLED devices at different degradation levels. For OLED devices with short operational lifetimes, dominant chemical degradation mainly relate to oxygen loss of molecules that occur at the interface between emission and electron transport layers (EML/ETL) where exciton distribution is maximised, confirmed by emission zone measurements. We also show approximately one order of magnitude increase in lifetime of devices with slightly modified host materials, which present minimal EML/ETL interfacial degradation and show the method can provide insight for future material and device architecture development.

Effect of trifluoroacetic acid on InP/ZnSe/ZnS quantum dots: mimicking the surface trap and their effects on the photophysical properties.

Understanding the precise effects of defects on the photophysical properties of quantum dots (QDs) is essential to their development with near-unity luminescence. Because of the complicated nature of defects in QDs, the origins and detailed roles of the defects still remain rarely understood. In this regard, we used detailed chemical analysis to investigate the effect of surface defects on the optical properties of InP/ZnSe/ZnS QDs by introducing shell defects through controlled trifluoroacetic acid (TFA) etching. TFA treatment on the InP/ZnSe/ZnS QDs partially removed the ZnS shell as well as ligands and reduced the quantum yield by generating energetically deep surface traps. The surface defects of QDs by TFA cause charged trap sites inducing an Auger recombination process with a rate of ca. 200 ps. Based on these results, we proposed possible trap-assisted non-radiative decay pathways between the band-edge state and surface deep traps in InP/ZnSe/ZnS QDs.

Pt cocatalyst morphology on semiconductor nanorod photocatalysts enhances charge trapping and water reduction.

In photocatalysis, metal-semiconductor hybrid structures have been proposed for ideal photocatalytic systems. In this study, we investigate the effect of morphology and surface nature of Pt cocatalysts on photocatalytic hydrogen evolution activity in Pt-tipped CdSe nanorods. Three distinct morphologies of Pt cocatalysts were synthesized and employed as visible light photocatalysts. The rough tips exhibit the highest activity, followed by the round and cubic tips. Kinetic investigations using transient absorption spectroscopy reveal that the cubic tips exhibit lower charge-separated states feasible for reacting with water and water reduction rates due to their defectless surface facets. In contrast, the rough tips show a similar charge-separation value but a two-fold higher surface reaction rate than the round tips, resulting in a significant enhancement of hydrogen evolution. These findings highlight the importance of rational design on metal cocatalysts in addition to the main semiconductor bodies for maximizing photocatalytic activities.

Probing twisted intramolecular charge transfer of pyrene derivatives as organic emitters in OLEDs.

Intramolecular charge transfer (ICT) plays a critical role in determining the photophysical properties of organic molecules, including their luminescence efficiencies. Twisted intramolecular charge transfer (TICT) is a process in which structural change accompanies ICT. Herein, we used time-resolved spectroscopy to study TICT in pyrene derivatives that are promising blue organic light emitting diode (OLED) emitter candidates; these derivatives show strong solvent-dependent charge-transfer (CT) behavior with unique fluorescence properties, increased fluorescence intensity in polar solvent. Slight structural changes that do not affect excited state dynamics were observed in nonpolar solvents, while polar solvents were found to affect excited state dynamics and CT characteristics, which affect their unusual fluorescence behavior. The TICT behavior of these pyrene derivatives can be modulated through structural modification. Our study provides valuable guidelines for the control of optical properties, including the luminescence efficiencies of OLED emitters that show TICT characteristics.

Increasing the Energy Gap between Band-Edge and Trap States Slows Down Picosecond Carrier Trapping in Highly Luminescent InP/ZnSe/ZnS Quantum Dots.

Non-toxic InP-based nanocrystals have been developed for promising candidates for commercial optoelectronic applications and they still require further improvement on photophysical properties, compared to Cd-based quantum dots (QDs), for better device efficiency and long-term stability. It is, therefore, essential to understand the precise mechanism of carrier trapping even in the state-of-the-art InP-based QD with near-unity luminescence. Here, it is shown that using time-resolved spectroscopic measurements of systematically size-controlled InP/ZnSe/ZnS core/shell/shell QDs with the quantum yield close to one, carrier trapping decreases with increasing the energy difference between band-edge and trap states, indicating that the process follows the energy gap law, well known in molecular photochemistry for nonradiative internal conversion between two electronic states. Similar to the molecular view of the energy gap law, it is found that the energy gap between the band-edge and trap states is closely associated with ZnSe phonons that assist carrier trapping into defects in highly luminescent InP/ZnSe/ZnS QDs. These findings represent a striking departure from the generally accepted view of carrier trapping mechanism in QDs in the Marcus normal region, providing a step forward understanding how excitons in nanocrystals interact with traps, and offering valuable guidance for making highly efficient and stable InP-based QDs.

Degradation of blue-phosphorescent organic light-emitting devices involves exciton-induced generation of polaron pair within emitting layers.

Degradation of organic materials is responsible for the short operation lifetimes of organic light-emitting devices, but the mechanism by which such degradation is initiated has yet to be fully established. Here we report a new mechanism for degradation of emitting layers in blue-phosphorescent devices. We investigate binary mixtures of a wide bandgap host and a series of novel Ir(III) complex dopants having N-heterocyclocarbenic ligands. Our mechanistic study reveals the charge-neutral generation of polaron pairs (radical ion pairs) by electron transfer from the dopant to host excitons. Annihilation of the radical ion pair occurs by charge recombination, with such annihilation competing with bond scission. Device lifetime correlates linearly with the rate constant for the annihilation of the radical ion pair. Our findings demonstrate the importance of controlling exciton-induced electron transfer, and provide novel strategies to design materials for long-lifetime blue electrophosphorescence devices.

Strong enhancement of electrical conductivity in two-dimensional micrometer-sized RuO2 nanosheets for flexible transparent electrodes.

The enhancement in electrical transport properties of exfoliated individual RuO2 NSs was systemically investigated for their application in flexible electronics and optoelectronics. Decoration of Ag NPs on the surface of the RuO2 NSs provides donor electrons and dramatically increases the electrical conductivity of the monolayer RuO2 NSs by up to 3700%. The n-type doping behavior was confirmed via Hall measurement analysis of the doped RuO2 NSs. The layer number- and temperature-dependence of the conductivity were also investigated. Moreover, carrier concentration and mobility were obtained from Hall measurements, indicating that the undoped RuO2 NSs had ambipolar transport and semi-metallic characteristics. Moreover, the Ag-doped RuO2 NS multilayer films on polycarbonate substrates were demonstrated by the Langmuir-Blodgett assembly methods, showing one-third reduction in the sheet resistance and extraordinarily high bending stability that the change in the resistance was less than 1% over 50 000 cycles.

Ultrafast photocarrier dynamics in nanocrystalline ZnOxNy thin films.

We examined the ultrafast dynamics of photocarriers in nanocrystalline ZnOxNy thin films as a function of compositional variation using femtosecond differential transmittance spectroscopy. The relaxation dynamics of photogenerated carriers and electronic structures are strongly dependent on nitrogen concentration. Photocarriers of ZnOxNy films relax on two different time scales. Ultrafast relaxation over several picoseconds is observed for all chemical compositions. However, ZnO and oxygen-rich phases show slow relaxation (longer than several nanoseconds), whereas photocarriers of films with high nitrogen concentrations relax completely on subnanosecond time scales. These relaxation features may provide a persistent photocurrent-free and prompt photoresponsivity for ZnOxNy with high nitrogen concentrations, as opposed to ZnO for display applications.

Heterogeneous stacking of nanodot monolayers by dry pick-and-place transfer and its applications in quantum dot light-emitting diodes.

Layered assembly structures composed of nanomaterials, such as nanocrystals, have attracted considerable attention as promising candidates for new functional devices whose optical, electromagnetic and electronic behaviours are determined by the spatial arrangement of component elements. However, difficulties in handling each constituent layer in a material-specific manner limit the 3D integration of disparate nanomaterials into the appropriate heterogeneous electronics. Here we report a pick-and-place transfer method that enables the transfer of large-area nanodot assemblies. This solvent-free transfer utilizes a lifting layer and allows for the reliable transfer of a quantum dot (QD) monolayer, enabling layer-by-layer design. With the controlled multistacking of different bandgap QD layers, we are able to probe the interlayer energy transfer among different QD monolayers. By controlling the emission spectrum through such designed monolayer stacking, we have achieved white emission with stable optoelectronic properties, the closest to pure white among the QD light-emitting diodes reported so far.

Quantum-dot-sensitized solar cell with unprecedentedly high photocurrent.

The reported photocurrent density (J(SC)) of PbS quantum dot (QD)-sensitized solar cell was less than 19 mA/cm(2) despite the capability to generate 38 mA/cm(2), which results from inefficient electron injection and fast charge recombination. Here, we report on a PbS:Hg QD-sensitized solar cell with an unprecedentedly high J(SC) of 30 mA/cm(2). By Hg(2+) doping into PbS, J(SC) is almost doubled with improved stability. Femtosecond transient study confirms that the improved J(SC) is due to enhanced electron injection and suppressed charge recombination. EXAFS reveals that Pb-S bond is reinforced and structural disorder is reduced by interstitially incorporated Hg(2+), which is responsible for the enhanced electron injection, suppressed recombination and stability. Thanks to the extremely high J(SC), power conversion efficiency of 5.6% is demonstrated at one sun illumination.

Photochemical generation of a new, highly fluorescent compound from non-fluorescent resveratrol.

UV irradiation of trans-resveratrol leads to its photochemical transformation to a new, highly fluorescent compound, whose chemical structure was unambiguously identified. The new compound has large values of fluorescence quantum yield, Stokes' shift, and two-photon absorption cross section, which make it suitable for bio-imaging, multi-color labeling, and two-photon microscopy.

Interactions of tyrosine in Leu-enkephalin at a membrane-water interface: an ultrafast two-dimensional infrared study combined with density functional calculations and molecular dynamics simulations.

The interactions of neuropeptides and membranes play an important role in peptide hormone function. Our current understanding of peptide-membrane interactions remains limited due to the paucity of experimental techniques capable of probing such interactions. In this work, we study the nature of opioid peptide-membrane interactions using ultrafast two-dimensional infrared (2D IR) spectroscopy. The high temporal resolution of 2D IR is particularly suited for studying highly flexible opioid peptides. We investigate the location of the tyrosine (Tyr) side chain of leucine-enkephalin (Lenk) in lipid bilayer membranes by measuring spectral diffusion of the phenolic ring vibrational mode in three different systems: Lenk in lipid bilayer membranes (bicelles), Lenk in deuterated water, and p-cresol in deuterated water. Frequency-frequency correlation functions obtained from waiting-time-dependent 2D IR spectra reveal an ultrafast decaying component with an approximately 1 ps time constant that is common for all three systems. On the basis of density functional theory calculations and molecular dynamics simulations, this spectral diffusion component is attributed to hydrogen-bond dynamics of the phenolic hydroxyl group interacting with bulk water. Unlike p-cresol in water, both Lenk systems exhibit static spectral inhomogeneity, which can be attributed to conformational distributions of Lenk that do not interconvert within 4 ps. Our results suggest that the Tyr side chain of Lenk in bicelles is located at the water-abundant region at the membrane-water interface and not embedded into the hydrophobic core.

Conformations of N-acetyl-L-prolinamide by two-dimensional infrared spectroscopy.

Femtosecond two-dimensional infrared (2D IR) spectroscopy has been applied to study the conformations of a model dipeptide, N-acetyl-L-prolinamide (AcProNH2) in deuterated chloroform (CDCl3). Spectral features in the amide-I and -II regions are obtained by rephasing (R), nonrephasing (NR), and reverse photon echo (RPE) pulse sequences with two polarization conditions. The 2D spectra obtained by the RPE and NR sequences with (0, 0, 0, 0) polarization reveal new spectral features associated with the multiple conformers of AcProNH2 that are difficult to discern using R sequence and linear-IR spectroscopy. The high resolving power of the RPE sequence comes from destructive interference between the positive and negative peaks of nearby vibrators, similar to the NR sequence. The RPE response functions that are useful for 2D spectral simulations are evaluated, including the effects of vibrational frequency correlations. The 2D spectra obtained with (45, -45, 90, 0) polarization exhibit clear cross-peak patterns in the off-diagonal region for the R and RPE sequences but in the diagonal region for the NR sequence. These patterns, free from strong diagonal contributions, are crucial for structure determination. DFT calculations, normal-mode analysis, Hessian matrix reconstruction, and vibrational exciton Hamiltonian diagonalization yield molecular parameters needed for quantitative simulations of 2D spectra: angles between transition dipoles, coupling constants, and off-diagonal anharmonicities of the amide-I and -II modes are obtained for solvated trans-C7 and cis structures and for gas-phase trans conformers in the region of phi = -120 degrees to 0 degrees and psi = -100 degrees to 180 degrees in the Ramachandran space. Systematic simulations based on a 4:1 population ratio of the solvated trans-C7 and cis structures reproduce well the 2D spectral features obtained at both polarization conditions. However, better agreement between the experimental and simulated cross-peak patterns can be reached if the dihedral angles of the major trans conformer are close to (phi, psi) = (-80 degrees , 100 degrees ). Our results suggest that the major conformer of AcProNH2 in CDCl3 deviates from the gas-phase global minimum, the trans-C7 form, to an extended intermediate between the C7 and polyproline-II structure. These results are discussed in relationship with earlier findings obtained by NMR, transient IR studies, and MD simulations.