Role of Thick‐Lithium Fluoride Layer in Energy Level Alignment at Organic/Metal Interface: Unifying Effect on High Metallic Work Functions

The function of ≈3‐nm thick lithium fluoride (LiF) buffer layers in combination with high work function metal contacts such as coinage metals and ferromagnetic metals for use in organic electronics and spintronics is investigated. The energy level alignment at the organic/LiF/metal interface is systematically studied using photoelectron spectroscopy and the integer charge transfer model. The thick‐LiF buffer layer is found to pin the Fermi level to ≈3.8 eV, regardless of the work function of the initial metal due to energy level bending in the LiF layer caused by depletion of defect states. At 3‐nm thickness, the LiF buffer layer provides full coverage, and the organic semiconductor adlayers are found to physisorb with the consequence that the energy level alignment at the organic/LiF interface follows the integer charge transfer model's predictions.     

Charge transfer through amino groups-small molecules interface improving the performance of electroluminescent devices

A carboxylic group functioned charge transporting was synthesized and self-assembled on an indium tin oxide (ITO) anode. A typical electroluminescent device [modified ITO/TPD (50 nm)/Alq₃ (60 nm)/LiF (2 nm)/(120 nm)] was fabricated to investigate the effect of the amino groups-small molecules interface on the characteristics of the device. The increase in the surface work function of ITO is expected to facilitate the hole injection from the ITO anode to the Hole Transport Layer (HTL) in electroluminescence. The modified electroluminescent device could endure a higher current and showed a much higher luminance than the nonmodified one. For the produced electroluminescent devices, the I-V characteristics, optical characterization and quantum yields were performed. The external quantum efficiency of the modified electroluminescent device is improved as the result of the presence of the amino groups-small molecules interface.     

Work function and valence band structure of tin-doped indium oxide thin films for OLEDs

Transparent conducting ITO thin film has been widely used as anode material in OLEDs due to its good optical transparency, low electrical resistivity, ease of patterning, high work function and efficient hole injection properties. The interface between ITO and organic layer in OLED device is thus important and can influence the electrical and luminescent properties. In this report, ITO substrates were treated with 20% H₃PO₄ solution. The corresponding changes in crystalline morphology were studied by X-ray diffraction. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed at ∼10⁻⁹ Torr to study the work function and the valence band structure of ITO substrates. It was found that work function became slightly lower after the treatment, probably caused by the formation of metal complex compounds and metal hydroxides. The binding energy of In 3d_5/2 shifted from 444.6 to 445.3 eV. This shifting was referred to the formation of In-OH bonding. It would be easier to provide electron by In-OH bonding than by In-O-In or Sn-O-Sn when photons reached ITO surface. The interface between ITO and CuPc was improved through polar surface and less aggregation. In addition, the OLED devices exhibited improved performance in both external quantum efficiency and luminescence efficiency.     

Demonstration of a Broadband Photodetector Based on a Two-Dimensional Metal–Organic Framework

Metal–organic frameworks (MOFs) are emerging as an appealing class of highly tailorable electrically conducting materials with potential applications in optoelectronics. Yet, the realization of their proof-of-concept devices remains a daunting challenge, attributed to their poor electrical properties. Following recent work on a semiconducting Fe₃(THT)₂(NH₄)₃ (THT: 2,3,6,7,10,11-triphenylenehexathiol) 2D MOF with record-high mobility and band-like charge transport, here, an Fe₃(THT)₂(NH₄)₃ MOF-based photodetector operating in photoconductive mode capable of detecting a broad wavelength range from UV to NIR (400–1575 nm) is demonstrated. The narrow IR bandgap of the active layer (≈0.45 eV) constrains the performance of the photodetector at room temperature by band-to-band thermal excitation of charge carriers. At 77 K, the device performance is significantly improved; two orders of magnitude higher voltage responsivity, lower noise equivalent power, and higher specific detectivity of 7 × 10⁸ cm Hz^1/2 W⁻¹ are achieved under 785 nm excitation. These figures of merit are retained over the analyzed spectral region (400–1575 nm) and are commensurate to those obtained with the first demonstrations of graphene- and black-phosphorus-based photodetectors. This work demonstrates the feasibility of integrating conjugated MOFs as an active element into broadband photodetectors, thus bridging the gap between materials' synthesis and technological applications.     

A New Route to Low Resistance Contacts for Performance‐Enhanced Organic Electronic Devices

The barrier to charge carrier injection across the semiconductor/electrode interface is a key parameter in the performance of organic transistors and optoelectronic devices, and the work function of the electrode material plays an important role in determining the size of this barrier. We present a new, chemical route for making metal surfaces with low work functions, by functionalizing gold surfaces with self‐assembled monolayers of n,n‐dialkyl dithiocarbamates. Ultraviolet photoemission spectroscopy measurements show that work functions of 3.2 eV ± 0.1 eV can be achieved using this surface modification. Electronic structure calculations reveal that this low work function is a result of the packing‐density, polarization along the N‐C bond, and charge rearrangement associated with chemisorption. We demonstrate that electrodes functionalized with these monolayers significantly improve the performance of organic thin‐film transistors and can potentially be employed in charge selective contacts for organic photovoltaics.     

Charge injection engineering of ambipolar field-effect transistors for high-performance organic complementary circuits

Ambipolar π-conjugated polymers may provide inexpensive large-area manufacturing of complementary integrated circuits (CICs) without requiring micro-patterning of the individual p- and n-channel semiconductors. However, current-generation ambipolar semiconductor-based CICs suffer from higher static power consumption, low operation frequencies, and degraded noise margins compared to complementary logics based on unipolar p- and n-channel organic field-effect transistors (OFETs). Here, we demonstrate a simple methodology to control charge injection and transport in ambipolar OFETs via engineering of the electrical contacts. Solution-processed caesium (Cs) salts, as electron-injection and hole-blocking layers at the interface between semiconductors and charge injection electrodes, significantly decrease the gold (Au) work function (～4.1 eV) compared to that of a pristine Au electrode (～4.7 eV). By controlling the electrode surface chemistry, excellent p-channel (hole mobility ～0.1-0.6 cm(2)/(Vs)) and n-channel (electron mobility ～0.1-0.3 cm(2)/(Vs)) OFET characteristics with the same semiconductor are demonstrated. Most importantly, in these OFETs the counterpart charge carrier currents are highly suppressed for depletion mode operation (I(off) < 70 nA when I(on) > 0.1-0.2 mA). Thus, high-performance, truly complementary inverters (high gain >50 and high noise margin >75% of ideal value) and ring oscillators (oscillation frequency ～12 kHz) based on a solution-processed ambipolar polymer are demonstrated.     

Suppression of Time‐Dependent Donor/Acceptor Interface Degradation by Redistributing Donor Charge Density

Poor operation stability is a major hurdle for the wide application of organic photovoltaic (OPV) devices. While most attention is given to environmental threats to device stability, we herein show evidence from X‐ray photoemission spectroscopy (XPS) of an intrinsic time‐dependent chemical reaction at a donor/acceptor interface. Albeit with impressive device performance from boron subphthalocyanine chloride (SubPc)/fullerene (C₆₀) interface, the forming boride bonds at its interface hinders the interfacial exciton dissociation and leads to device deterioration. Due to the high electron affinity of molybdenum oxide (MoO₃) film, the incorporation of MoO₃ layer under the SubPc film has strong electron‐drawing property and leads to charge‐transfer complex (CTC) formation at the MoO₃/SubPc interface. The resulting charge redistribution in SubPc molecules effectively suppresses the further interfacial reaction at SubPc/C₆₀ junction. Our results provide insight for new degradation mechanisms of OPV devices and corresponding stability control via charge redistribution in the donor film.     

Dominant conduction mechanism and the effects of device temperature on electrical characteristics of Al/ZnPc/n-Si structures

Aluminum/Zinc Phthalocyanine/n-Si metal semiconductor contact with organic interfacial layer has been developed and characterized by Current-Voltage-Temperature (I-V-T) measurements for the study of its junction and charge transport properties. The junction parameters, such as diode ideality factor (n), barrier height (φ_b) and series resistance (R_S), of the device were found to shift with device temperature. The diode ideality factor was found to increase with the device temperature up to 323 K. However, a decreasing trend in the value of n was observed beyond this temperature. The barrier height and series resistance were found to increase and decrease, respectively with increasing device temperature. The peak of interface state energy distribution curves was shifted, in terms of Ess-Ev value, from 0.622 eV to 0.827 eV with 52 meV activation energy of the charge carriers. The data analysis implies that the Fermi level of the organic interfacial layer shifts as function of device temperature. In terms of dominant conduction mechanism, the I-V-T data analysis confirms the relationship log (IV⁴) ∝V^1/2 with the device temperature in the range of 313-343 K and the Poole-Frenkel type is found to be the dominant conduction mechanism for the hybrid device.     

The Importance of Quantifying the Composition of the Amorphous Intermixed Phase in Organic Solar Cells

The relation of phase morphology and solid-state microstructure with organic photovoltaic (OPV) device performance has intensely been investigated over the last twenty years. While it has been established that a combination of donor:acceptor intermixing and presence of relatively phase-pure donor and acceptor domains is needed to get an optimum compromise between charge generation and charge transport/charge extraction, a quantitative picture of how much intermixing is needed is still lacking. This is mainly due to the difficulty in quantitatively analyzing the intermixed phase, which generally is amorphous. Here, fast scanning calorimetry, which allows measurement of device-relevant thin films (<200 nm thickness), is exploited to deduce the precise composition of the intermixed phase in bulk-heterojunction structures. The power of fast scanning calorimetry is illustrated by considering two polymer:fullerene model systems. Somewhat surprisingly, it is found that a relatively small fraction (<15 wt%) of an acceptor in the intermixed amorphous phase leads to efficient charge generation. In contrast, charge transport can only be sustained in blends with a significant amount of the acceptor in the intermixed phase (in this case: ≈58 wt%). This example shows that fast scanning calorimetry is an important tool for establishing a complete compositional characterization of organic bulk heterojunctions. Hence, it will be critical in advancing quantitative morphology–function models that allow for the rational design of these devices, and in delivering insights in, for example, solar cell degradation mechanisms via phase separation, especially for more complex high-performing systems such as nonfullerene acceptor:polymer bulk heterojunctions.     

Effect of interface structure on the Ru on HfO<Subscript>2</Subscript> work function

We report an ab initio study of the effect of the metal-high-k dielectric (Ru–HfO₂) interface structure on the effective work function of the metal. Depending on the structure of Ru deposited on the HfO₂ substrate we find a variation of ~0.4 eV in the effective work function of the metal. The interfacial structures determine the extent of charge transfer from the metal to the dielectric and hence, affect the nature of the interface dipole. Consequently, variability in interface structure may result in differences in the Schottky barrier height and thus affect the electrode work function.     

Formation and electron properties of the interface between organic (TPD) and inorganic (ZnO) materials

The initial stage of formation of an organic coating of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) on a crystalline (ZnO) surface in the course of thermal deposition in high vacuum has been studied using the low-energy total current spectroscopy technique. A change in the work function and the density of unoccupied electron states in an energy interval of 0–20 eV above the vacuum level was traced as the organic coating thickness increased up to 8 nm. The energy positions of the bands of unoccupied electron states in the TPD film have been determined, including π* band (7–9 eV above the Fermi level), σ₁* band (10–12 eV), σ₂* band (14–16 and 18–20 eV).     

Large Modulation of Charge Carrier Mobility in Doped Nanoporous Organic Transistors

Molecular doping of organic electronics has shown promise to sensitively modulate important device metrics. One critical challenge is the disruption of structure order upon doping of highly crystalline organic semiconductors, which significantly reduces the charge carrier mobility. This paper demonstrates a new method to achieve large modulation of charge carrier mobility via channel doping without disrupting the molecular ordering. Central to the method is the introduction of nanopores into the organic semiconductor thin films via a simple and robust templated meniscus‐guided coating method. Using this method, the charge carrier mobility of C₈‐benzothieno[3,2‐b]benzothiophene transistors is boosted by almost sevenfold. This paper further demonstrates enhanced electron transport by close to an order of magnitude in a diketopyrrolopyrrole‐based donor–acceptor polymer. Combining spectroscopic measurements, density functional theory calculations, and electrical characterizations, the doping mechanism is identified as partial‐charge‐transfer induced trap filling. The nanopores serve to enhance the dopant/organic semiconductor charge transfer reaction by exposing the π‐electrons to the pore wall.     

Photoelectrochemical studies on colloidal copper (I) oxide/modified with some organic semiconductors: Incentive for use of nanoparticle systems

Colloidal Cu₂O solutions were used to explore photonic activities at the semiconductor/electrolyte interface. Fluorescence spectroscopic studies were performed on Cu₂O colloidal particles modified with some conjugated organic monomers such as 2-amino-phenyl pyrrole (2-APPy), tri-phenyl amine (TPA), or 2-thionyl pyrrole (2-Th-Py) to investigate the quantum absorbance efficiency at this inorganic/organic interface (IOI). Our study shows that colloidal p-type Cu₂O possesses a bandgap with direct transition of ≈ 2·2 eV and indirect transition of 1·85 eV. The recorded rates of charge injection into colloidal Cu₂O, k _ct, were 2·31 × 10⁹ s⁻¹, 5·05 × 10⁸ s⁻¹, and 7·22 × 10⁸ s⁻¹ for 2-APPy, TPA and 2-Th-Py, respectively. The studied systems show more stability in colloidal form than in thin solid form. Results were interpreted using the optical and electrical parameters of the organic monomer such as ionization potential (IP), electron affinity (EA) and energy bandgap (Eg), and the barrier height at the IOI interface. Stability of the colloidal system is attributed to the physical dimensions of the photoactive system. The nano-colloidal particle offers a condition where its size is less than √Dt.     

Al and Ti/Al contacts on n-GaN

Al(60 nm) and Ti(40 nm)/Al(160 nm) metal layers have been deposited by thermal evaporation onto n-GaN epitaxial layers grown by metal organic chemical vapour deposition (MOCVD) on a c-plane sapphire substrate. The samples have been annealed at 300, 400, 700 or 900 °C for 10 min in vacuum. The microstructural and electrical properties of the contacts have been investigated by electron microscopy, X-ray diffraction and by current-voltage measurements. As-deposited Al and Ti/Al contacts were rectifying with Schottky barrier heights below 0.35 eV and 0.38 eV, respectively. After heat treatment at 300 °C and 400 °C both contacts exhibited linear current-voltage characteristics. After annealing at 700 °C Al contacts became rectifying with a barrier height of 0.42 eV, while Ti/Al contacts remained nearly linear at the same temperature. The electrical characteristics and XRD analysis indicated that the upper metal in Ti/Al contact diffused in the Ti layer already during deposition. Cross-sectional transmission electron microscopy revealed that in the case of Ti/Al contacts, the continuity of the Ti layers ceased when annealing above 700 °C. X-ray diffractions showed, that a Ti₂N interface phase formed in Ti/Al contacts at 700 and 900 °C, and an AlN interface phase developed in the same contact at 900 °C.     

Organic field-effect transistors using perylene

Organic thin-film field-effect transistors using organic semiconductor, perylene are fabricated, and electrical measurements are performed. The field-effect mobility of the device using perylene shows only p-type behavior while the electron and hole mobilities of its single crystal form are 5.5 and 0.5 cm²/V s, respectively. Stacked layers of perlyene (a layer fabricated with low deposition rate followed by another layer with high deposition rate) are formed for the active layer. Furthermore, hexadecafluorocopperphthalocyanine (F₁₆CuPc) and pentacene buffer layers are also used to modify the interface. For all of these devices, perylene layers acts as p-type. Electron trapping at grain boundaries and interface is thought to be a crucial factor. Hole mobility of 3.9×10⁻⁴ cm²/V s is obtained for the perylene film field-effect transistor device.     

Real-time optical modelling and investigation of inorganic nano-layer growth onto flexible polymeric substrates

A major factor for the achievement of the desirable performance, efficiency and lifetime of flexible organic electronic devices is the optimization of the encapsulation layers that protect the device active layers by atmospheric gas molecule permeation. The active layers consisted of small molecule and/or polymer organic semiconductors as well as the organic conductors need to be encapsulated into a transparent medium that will provide the necessary protection and maintain their charge generation and transport characteristics. The encapsulation layers are generally consisted of inorganic thin films (silicon oxide—SiO_x and aluminium oxide—AlO_x) deposited onto the polymeric substrates, such as PolyEthylene Terephthalate (PET). In this work, in situ and real-time Spectroscopic Ellipsometry in the ultraviolet spectral region has been implemented in order to investigate the growth of inorganic SiO_x and AlO_x nano-layers onto PET flexible polymeric substrates as well as the PET/inorganic interface effects during the early stages of growth. The analysis of the pseudodielectric function <?(ω)> that was measured in real-time in very short time scales (in the order of hundreds of ms) has provided detailed information on the time evolution of the thickness and deposition rate of the inorganic nano-layers during their growth process as well as on their optical and electronic properties. This work proposes a methodology for using real-time optical monitoring technique with the aim to tailor and control the functionality of these materials for application in flexible electronic devices.     

Charge‐Trapping‐Induced Non‐Ideal Behaviors in Organic Field‐Effect Transistors

Organic field‐effect transistors (OFETs) with impressively high hole mobilities over 10 cm² V^?1 s^?1 and electron mobilities over 1 cm² V^?1 s^?1 have been reported in the past few years. However, significant non‐ideal electrical characteristics, e.g., voltage‐dependent mobilities, have been widely observed in both small‐molecule and polymer systems. This issue makes the accurate evaluation of the electrical performance impossible and also limits the practical applications of OFETs. Here, a semiconductor‐unrelated, charge‐trapping‐induced non‐ideality in OFETs is reported, and a revised model for the non‐ideal transfer characteristics is provided. The trapping process can be directly observed using scanning Kelvin probe microscopy. It is found that such trapping‐induced non‐ideality exists in OFETs with different types of charge carriers (p‐type or n‐type), different types of dielectric materials (inorganic and organic) that contain different functional groups (? OH, ? NH₂, ? COOH, etc.). As fas as it is known, this is the first report for the non‐ideal transport behaviors in OFETs caused by semiconductor‐independent charge trapping. This work reveals the significant role of dielectric charge trapping in the non‐ideal transistor characteristics and also provides guidelines for device engineering toward ideal OFETs.     

Manipulation of Charge Transport by Metallic V13O16 Decorated on Bismuth Vanadate Photoelectrochemical Catalyst

Conductive metal oxides represent a new category of functional material with vital importance for many modern applications. The present work introduces a new conductive metal oxide V₁₃O₁₆, which is synthesized via a simplified photoelectrochemical procedure and decorated onto the semiconducting photocatalyst BiVO₄ in controlled mass percentages ranging from 25% to 37%. Owing to its excellent conductivity and good compatibility with oxide materials, the metallic V₁₃O₁₆‐decorated BiVO₄ hybrid catalyst shows a high photocurrent density of 2.2 ± 0.2 mA cm^?2 at 1.23 V versus reversible hydrogen electrode (RHE). Both experimental characterization and density functional theory calculations indicate that the superior photocurrent derives from enhanced charge separation and transfer, resulting from ohmic contact at the interface of mixed phases and superior electrical conductivity from V₁₃O₁₆. A Co–Pi coating on BiVO₄–V₁₃O₁₆ further increases the photocurrent to 5.0 ± 0.5 mA cm^?2 at 1.23 V versus RHE, which is among the highest reported for BiVO₄‐based photoelectrodes. Surface photovoltage and transient photocurrent measurements suggest a charge‐transfer model in which photocurrents are enhanced by improved surface passivation, although the barrier at the Co–Pi/electrolyte interface limits the charge transfer.     

Charge transport and light emission in bilayer organic field-effect transistors

Recently there has been some major interest in the charge transport and light emission properties of organic field-effect transistors (OFETs). Different device structures have been proposed and they can be divided into two broad categories consisting of either a single layer or a bilayer. In the case of the single-layer OFETs, efficient light emission has not been observed while the performance of the bilayer OFETs appear to be more promising (for instance: recent work on a bilayer OFET has shown distinct ambipolar characteristics as well as limited light emission). In this work, we examined the electroluminescence intensities of bilayer OFETs reported in the open literature and attempted to identify the transport and recombination mechanisms. As observed, light emission in these devices appeared to be linked to a narrow region at the interface acting as a light-emitting source. To understand the recombination mechanisms, we computed the spatial charge distributions under various biasing conditions and correlated the results to the reported electroluminescence intensity data. Our overall results re-affirmed the significance of the light-emitting interface layer and the fact that device operation critically depended on the alignment of the energy levels at the respective interface.