Interface Structure of MoO3 on Organic Semiconductors

Lifetime enhancement in QDLEDs via an electron-blocking hole transport layer

This study investigates the impact of an engineered hole transport layer (HTL) on the stability of electroluminescent quantum dot light-emitting devices (QDLEDs). The 9-Phenyl-3,6-bis(9-phenyl-9Hcarbazol-3-yl)-9H-carbazole (Tris-PCz) HTL, which possesses a shallower lowest unoccupied molecular orbital (LUMO) energy level compared to the widely used 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) HTL, is employed to confine electron overflow toward the HTL. Utilizing the Tris-PCz HTL results in a 20× improvement in the electroluminescence half-life (LT50) of QDLEDs compared with conventional QDLEDs using the CBP HTL. Electric and optoelectronic analyses reveal that the migration of excess electrons toward the HTL is impeded by the up-shifted LUMO level of Tris-PCz, contributing to prolonged operational device stability. Furthermore, the augmented electric field at the QD/Tris-PCz interface, due to accumulated electrons, expedites hole injection rates, leading to better charge injection balance and the confinement of the exciton recombination zone within the QD and thus the device stability enhancement. This study highlights the significant influence of the HTL on QDLED stability and represents one of the longest LT50 for a QDLED based on the conventional core/shell QD structure.

Injection-limited and space charge-limited currents in organic semiconductor devices with nanopatterned metal electrodes

Charge injection at metal-organic interfaces often limits the electric current in organic light-emitting diodes without additional injection layers. Integrated nanopatterned electrodes may provide a way to overcome this current injection limit by local field enhancements leading to locally space charge-limited currents. We compare electrical characteristics of planar and nanopatterned hole-only devices based on the charge transport material NPB with different thicknesses in order to investigate the nanopattern's effect on the current limitation mechanism. Integration of a periodic nanograting into the metal electrode yields a current increase of about 1.5-4 times, depending on thickness and operating voltage. To verify the experimental results, we implement a finite element simulation model that solves the coupled Poisson and drift-diffusion equations in a weak form. It includes space charges, drift and diffusion currents, nonlinear mobility, and charge injection at the boundaries. We find in experiment and simulation that the planar devices exhibit injection-limited currents, whereas the currents in the nanopatterned devices are dominated by space charge effects, overcoming the planar injection limit. The simulations show space charge accumulations at the corners of the nanopattern, confirming the idea of locally space charge-limited currents.

Damage-Free Charge Transfer Doping of 2D Transition Metal Dichalcogenide Channels by van der Waals Stamping of MoO3 and LiF

To dope 2D semiconductor channels, charge-transfer doping has generally been done by thermal deposition of inorganic or organic thin-film layers on top of the 2D channel in bottom-gate field-effect transistors (FETs). The doping effects are reproducible in most cases. However, such thermal deposition will damage the surface of 2D channels due to the kinetic energy of depositing atoms, causing hysteresis or certain degradation. Here, a more desirable charge-transfer doping process is suggested. A damage-free charge-transfer doping is conducted for 2D MoTe₂ (or MoS₂ ) channels using a polydimethylsiloxane stamp. MoO₃ or LiF is initially deposited on the stamp as a doping medium. Hysteresis-minimized transfer characteristics are achieved from stamp-doped FETs, while other devices with direct thermal deposition-doped channels show large hysteresis. The stamping method seems to induce a van der Waals-like damage-free interface between the channel and doping media. The stamp-induced doping is also well applied for a MoTe₂ -based complementary inverter because MoO₃ - and LiF-doping by separate stamps effectively modifies two ambipolar MoTe₂ channels to p- and n-type, respectively.

Characterizing and Improving the Thermal Stability of Organic Photovoltaics Based on Halogen-Rich Non-Fullerene Acceptors

The thermal stability of inverted, halogen-rich non-fullerene acceptor (NFA)-based organic photovoltaics with MoO_x as the hole transporting layer is studied at temperatures up to 80 °C. Over time, the power conversion efficiency shows a "check-mark" shaped thermal aging pattern, featuring an early decrease, followed by a long-term recovery. A high Cl concentration at the bulk heterojunction (BHJ)/MoO_x interface in the thermally aged device is found using energy dispersive X-ray spectroscopy. X-ray photoelectron spectroscopy shows that the MoO_x is chlorinated after thermal aging. With bulk quantum efficiency analysis, we propose an explanation to the check-mark shaped pattern. Inserting a thin C₇₀ layer between the BHJ and MoO_x suppresses the thermal degradation mechanisms, resulting in three orders of magnitude increase in device lifetime at 80 °C.

De Novo Simulation of Charge Transport through Organic Single-Carrier Devices

In amorphous organic semiconductor devices, electrons and holes are transported through layers of small organic molecules or polymers. The overall performance of the device depends both on the material and the device configuration. Measuring a single device configuration requires a large effort of synthesizing the molecules and fabricating the device, rendering the search for promising materials in the vast molecular space both nontrivial and time-consuming. This effort could be greatly reduced by computing the device characteristics from the first principles. Here, we compute transport characteristics of unipolar single-layer devices of prototypical hole- and electron-transporting materials, N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) and 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) using a first-principles multiscale approach that requires only the molecular constituents and the device geometry. This approach of generating a digital twin of the entire device can be extended to multilayer stacks and enables the computer design of materials and devices to facilitate systematic improvement of organic light-emitting diode (OLED) devices.

Non-fullerene acceptor organic photovoltaics with intrinsic operational lifetimes over 30 years

Organic photovoltaic cells (OPVs) have the potential of becoming a productive renewable energy technology if the requirements of low cost, high efficiency and prolonged lifetime are simultaneously fulfilled. So far, the remaining unfulfilled promise of this technology is its inadequate operational lifetime. Here, we demonstrate that the instability of NFA solar cells arises primarily from chemical changes at organic/inorganic interfaces bounding the bulk heterojunction active region. Encapsulated devices stabilized by additional protective buffer layers as well as the integration of a simple solution processed ultraviolet filtering layer, maintain 94% of their initial efficiency under simulated, 1 sun intensity, AM1.5 G irradiation for 1900 hours at 55 °C. Accelerated aging is also induced by exposure of light illumination intensities up to 27 suns, and operation temperatures as high as 65 °C. An extrapolated intrinsic lifetime of > 5.6 × 104 h is obtained, which is equivalent to 30 years outdoor exposure.

Computing Charging and Polarization Energies of Small Organic Molecules Embedded into Amorphous Materials with Quantum Accuracy

The ionization potential, electron affinity, and cation/anion polarization energies (IP, EA, P⁽⁺⁾, P^(-)) of organic molecules determine injection barriers, charge carriers balance, doping efficiency, and light outcoupling in organic electronics devices, such as organic light-emitting diodes (OLEDs). Computing IP and EA of isolated molecules is a common task for quantum chemistry methods. However, once molecules are embedded in an amorphous organic matrix, IP and EA values change, and accurate predictions become challenging. Here, we present a revised quantum embedding method [Friederich et al. J. Chem. Theory Comput. 2014, 10 (9), 3720-3725] that accurately predicts the dielectric permittivity and ionization potentials in three test materials, NPB, TCTA, and C60, and allows straightforward interpretation of their nature. The method paves the way toward reliable virtual screening of amorphous organic semiconductors with targeted IP/EA, polarization energies, and relative dielectric permittivity.

Thin Thermally Evaporated Organic Hole Transport Layers for Reduced Optical Losses in Substrate-Configuration Perovskite Solar Cells

Parasitic optical absorption is one of the root causes of the moderate efficiency of metal halide perovskite solar cells (PSCs) with an opaque substrate configuration. Here, we investigate the reduction of these optical losses by using thin (7-10 nm), undoped, thermally evaporated 2,2',7,7'-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9'-spirobifluorene (spiro-OMeTAD), N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine) (NPB), and tris(4-carbazoyl-9-ylphenyl)amine) (TCTA) hole transport layers (HTLs). Of these, NPB is found to offer the best compromise between efficiency and stability. In semitransparent n-i-p configuration PSCs with an indium tin oxide bottom and a MoO₃/thin-Au/ZnS dielectric-metal-dielectric top electrode, NPB gives 14.9% and 10.7% efficiency for bottom and top illumination, respectively. The corresponding substrate-configuration PSC fabricated on an Au bottom electrode has 13.1% efficiency. Compared to a 14.0% efficient PSC with a thick spin-coated doped spiro-OMeTAD layer, the cell with NPB provides an improved short-circuit current density but has slightly lower open-circuit voltage and fill factor. Detailed analysis of the optical losses in the opaque devices demonstrates that evaporated NPB offers negligible parasitic absorption compared to solution-processed spiro-OMeTAD. The optical losses that remain are due to absorption and reflection of the transparent top electrode.

Molybdenum Oxide Supported on Ti3AlC2 is an Active Reverse Water-Gas Shift Catalyst

MAX phases are layered ternary carbides or nitrides that are attractive for catalysis applications due to their unusual set of properties. They show high thermal stability like ceramics, but they are also tough, ductile, and good conductors of heat and electricity like metals. Here, we study the potential of the Ti₃AlC₂ MAX phase as a support for molybdenum oxide for the reverse water-gas shift (RWGS) reaction, comparing this new catalyst to more traditional materials. The catalyst showed higher turnover frequency values than MoO₃/TiO₂ and MoO₃/Al₂O₃ catalysts, due to the outstanding electronic properties of the Ti₃AlC₂ support. We observed a charge transfer effect from the electronically rich Ti₃AlC₂ MAX phase to the catalyst surface, which in turn enhances the reducibility of MoO₃ species during reaction. The redox properties of the MoO₃/Ti₃AlC₂ catalyst improve its RWGS intrinsic activity compared to TiO₂- and Al₂O₃-based catalysts.

Influence of Electrostatic Interactions During the Resorcinol-Formaldehyde Polymerization on the Characteristics of Mo-Doped Carbon Gels

The resorcinol (R)-formaldehyde (F) polymerization was carried out in different experimental conditions to obtain RF/Mo doped carbon xerogels with different morphology, porosity and nature and dispersion of metal. Attractive or repulsive electrostatic interactions were forced in the starting aqueous solution of RF-monomers using different synthesis conditions, namely, combinations of cationic or anionic surfactants, Mo-precursors and pH values. The results showed that when both cationic surfactant and Mo-precursor were used at neutral pH, attractive interactions with the anionic RF-macromolecules are favored during polymerization and the final carbon xerogel exhibited the most developed porosity and the strongest Mo-organic phase interaction, leading to deeper Mo-phase reduction during carbonization and the formation of highly-dispersed crystalline nanoparticles of Mo2C. On the contrary, the use of both anionic surfactant and Mo-precursor leads to repulsive interactions, which generates less porous carbon gels with a Mo-phase formed by large MoO3 platelet structures and low Mo-surface contents. RF/Mo-doped gels with intermediate properties were obtained by combining cationic and anionic surfactants, metal precursors or both. After carbonization, the obtained materials would be suitable to be used directly as catalysts with different physicochemical properties and active phases.

Investigation of the buried planar interfaces in multi-layered inverted organic solar cells using x-ray reflectivity and impedance spectroscopy

The hole and electron extracting interlayers in the organic solar cells (OSCs) play an important role in high performing devices. The present work focuses on an investigation of Zinc oxide/bulk heterojunction (ZnO/BHJ) and BHJ/MoO _x (Molybdenum oxide) buried planar interfaces in inverted OSC devices using the optical contrast in various layers along with the electrical measurements. The x-ray reflectivity (XRR) analysis demonstrates the formation of additional intermixing layers at the interfaces of ZnO/BHJ and BHJ/MoO _x . Our results indicate infusion of PC71BM into ZnO layer up to ~4 nm which smoothen the ZnO/BHJ interface. In contrast, thermally evaporated MoO _x molecules diffuse into PTB7-Th dominant upper layers of BHJ active layer resulting in an intermixed layer at the interface of MoO _x /BHJ. The high recombination resistance (~5 kΩ cm²) and electron lifetime (~70 μs), obtained from the impedance spectroscopy (IS), support such vertical segregation of PTB7-Th and PC71BM in the active layer. The OSC devices, processed in ambient condition, exhibit high power conversion efficiency of 6.4%. We consider our results have great significance to understand the structure of buried planar interfaces at interlayers and their correlation with the electrical parameters representing various interfacial mechanisms of OSCs.

Influence of Moisture on the Energy-Level Alignment at the MoO3/Organic Interfaces

MoO₃ is widely used in polymer-based organic solar cells as an anode buffer layer because of its high workfunction and formation of a strong dipole at the MoO₃/polymer interface facilitating charge transfer across the MoO₃/polymer interface. In the present work, we show that exposure of the MoO₃/polymer interface to moisture attracts water molecules to the interface via diffusion. Because of their own strong dipole, water molecules counter the dipole at the MoO₃/polymer interface. As a consequence, the charge transfer across the MoO₃/polymer will reduce and affect the charge transport across the interface. The outcome of this work thus suggests that it is critical to keep the MoO₃/polymer interface moisture-free, which requires special precautions in device fabrications. The composition of the MoO₃/P3HT:PC₆₁BM interface is analyzed with X-ray photoelectron spectroscopy and the depth profiling technique, neutral impact collision ion scattering spectroscopy. The results show that the concentration of oxygen increases upon exposure but leaves the oxidation state of Mo unchanged. The valence electron spectroscopy technique shows that the dipole across the MoO₃/P3HT:PC₆₁BM interface decreases even for short-time exposure to atmosphere because of the diffusion of water molecules to the interface. The far-ranging consequences for organic electronic devices are discussed.

Ion Migration Studies in Exfoliated 2D Molybdenum Oxide via Ionic Liquid Gating for Neuromorphic Device Applications

The formation of an electric double layer in ionic liquid (IL) can electrostatically induce charge carriers and/or intercalate ions in and out of the lattice which can trigger a large change of the electronic, optical, and magnetic properties of materials and even modify the crystal structure. We present a systematic study of ionic liquid gating of exfoliated 2D molybdenum trioxide (MoO₃) devices and correlate the resultant electrical properties to the electrochemical doping via ion migration during the IL biasing process. A nearly 9 orders of magnitude modulation of the MoO₃ conductivity is obtained for the two types of ionic liquids that are investigated. In addition, notably rapid on/off switching was realized through a lithium-containing ionic liquid whereas much slower modulation was induced via oxygen extraction/intercalation. Time of flight-secondary ion mass spectrometry confirms the Li intercalation. Density functional theory (DFT) calculations have been carried out to examine the underlying metallization mechanism. Results of short-pulse tests show the potential of these MoO₃ devices as neuromorphic computing elements due to their synaptic plasticity.

Tweaking the Electronic and Optical Properties of α-MoO3 by Sulphur and Selenium Doping - a Density Functional Theory Study

First-principles calculations were carried out to understand how anionic isovalent-atom doping affects the electronic structures and optical properties of α-MoO₃. The effects of the sulphur and selenium doping at the three unique oxygen sites (O_t, O_a, and O_t) of α-MoO₃ were examined. We found that the valence p orbitals of Sulphur/Selenium dopant atoms give rise to impurity bands above the valence band maximum in the band structure of α-MoO₃. The number of impurity bands in the doped material depends on the specific doping sites and the local chemical environment of the dopants in MoO₃. The impurity bands give rise to the enhanced optical absorptions of the S- and Se-doped MoO₃ in the visible and infrared regions. At low local doping concentration, the effects of the dopant sites on the electronic structure of the material are additive, so increasing the doping concentration will enhance the optical absorption properties of the material in the visible and infrared regions. Further increasing the doping concentration will result in a larger gap between the maximum edge of impurity bands and the conduction band minimum, and will undermine the optical absorption in the visible and infrared region. Such effects are caused by the local geometry change at the high local doping concentration with the dopants displaced from the original O sites, so the resulting impurity bands are no long the superpositions of the impurity bands of each individual on-site dopant atom. Switching from S-doping to Se-doping decreases the gap between the maximum edge of the impurity bands and conduction band minimum, and leads to the optical absorption edge red-shifting further into the visible and infrared regions.

Universal strategy for Ohmic hole injection into organic semiconductors with high ionization energies

Barrier-free (Ohmic) contacts are a key requirement for efficient organic optoelectronic devices, such as organic light-emitting diodes, solar cells, and field-effect transistors. Here, we propose a simple and robust way of forming an Ohmic hole contact on organic semiconductors with a high ionization energy (IE). The injected hole current from high-work-function metal-oxide electrodes is improved by more than an order of magnitude by using an interlayer for which the sole requirement is that it has a higher IE than the organic semiconductor. Insertion of the interlayer results in electrostatic decoupling of the electrode from the semiconductor and realignment of the Fermi level with the IE of the organic semiconductor. The Ohmic-contact formation is illustrated for a number of material combinations and solves the problem of hole injection into organic semiconductors with a high IE of up to 6 eV.

Facile Solution Synthesis of Tungsten Trioxide Doped with Nanocrystalline Molybdenum Trioxide for Electrochromic Devices

A facile, highly efficient approach to obtain molybdenum trioxide (MoO3)-doped tungsten trioxide (WO3) is reported. An annealing process was used to transform ammonium tetrathiotungstate [(NH4)2WS4] to WO3 in the presence of oxygen. Ammonium tetrathiomolybdate [(NH4)2MoS4] was used as a dopant to improve the film for use in an electrochromic (EC) cell. (NH4)2MoS4 at different concentrations (10, 20, 30, and 40 mM) was added to the (NH4)2WS4 precursor by sonication and the samples were annealed at 500 °C in air. Raman, X-ray diffraction, and X-ray photoelectron spectroscopy measurements confirmed that the (NH4)2WS4 precursor decomposed to WO3 and the (NH4)2MoS4-(NH4)2WS4 precursor was transformed to MoO3-doped WO3 after annealing at 500 °C. It is shown that the MoO3-doped WO3 film is more uniform and porous than pure WO3, confirming the doping quality and the privileges of the proposed method. The optimal MoO3-doped WO3 used as an EC layer exhibited a high coloration efficiency of 128.1 cm2/C, which is larger than that of pure WO3 (74.5 cm2/C). Therefore, MoO3-doped WO3 synthesized by the reported method is a promising candidate for high-efficiency and low-cost smart windows.

Organic/Organic Heterointerface Engineering to Boost Carrier Injection in OLEDs

We investigate dynamic formation of nanosheet charge accumulations by heterointerface engineering in double injection layer (DIL) based organic light emitting diodes (OLEDs). Our experimental results show that the device performance is considerably improved for the DIL device as the result of heterointerface injection layer (HIIL) formation, in comparison to reference devices, namely, the current density is doubled and even quadrupled and the turn-on voltage is favorably halved, to 3.7 V, which is promising for simple small-molecule OLEDs. The simulation reveals the (i) formation of dynamic p-type doping (DPD) region which treats the quasi Fermi level at the organic/electrode interface, and (ii) formation of dynamic dipole layer (DDL) and the associated electric field at the organic/organic interface which accelerates the ejection of the carriers and their transference to the successive layer. HIIL formation proposes alternate scenarios for device design. For instance, no prerequisite for plasma treatment of transparent anode electrode, our freedom in varying the thicknesses of the organic layers between 10 nm and 60 nm for the first layer and between 6 nm and 24 nm for the second layer. The implications of the present work give insight into the dynamic phenomena in OLEDs and facilitates the development of their inexpensive fabrication for lighting applications.

Epitaxial growth and metallicity of rutile MoO2thin film

We demonstrate an unconventional epitaxial relation between monoclinic MoO₂film and hexagonal Al₂O₃. Clear Drude absorption mode and other direct optical transitions are unveiled by optical spectroscopies.