High Work Function Metal Research Articles

Perylene diimides: a thickness-insensitive cathode interlayer for high performance polymer solar cells

With the power conversion efficiency of polymer solar cells (PSCs) approaching the milestone value of 10%, their instability associated with a low work function metal cathode, particularly in the presence of oxygen and moisture, becomes a critical issue for real applications. To alleviate the air-sensitive problem, two easy-accessible solution-processed, environmentally friendly organic small-molecule cathode interlayers, with perylene diimides (PDI) as the core and amino (PDIN) or amino N-oxide (PDINO) as the terminal substituent, are explored. Benefitting from the extended planar structure of the PDI units, the two interlayer materials show high conductivities of ∼10−5 S cm−1, which make them capable of functioning efficiently in a wide thickness range of 6 to 25 nm. This is the first time that thickness-insensitive small-molecule-based cathode interlayers are reported. It is also found that the work function tuning effect of the two PDI-based interlayers allows high work function metals (such as Au and Ag) to act as the cathode. With the conventional device structure with PTB7 as a donor and PC70BM as an acceptor, the PDINO-based devices exhibit an efficiency of 8.24% with Al as the top electrode and 8.16% with Ag as the top electrode, much higher than that of the corresponding Ca/Al-based device (6.98%). The high efficiency of 8.35% is also achieved in the device with PTB7-Th as the donor. The success of the two PDI-interlayers indicates that π-delocalized planar structures with high electron affinities could be particularly useful in developing high-performance organic interlayer materials.

Spectral and Injection Level Dependence of Recombination Lifetimes in Silicon Measured by Impedance Spectroscopy

The effect of photo-generated carrier concentration on recombination lifetime is studied on silicon wafers using the impedance spectroscopy technique. The induced p <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^+$</tex></formula> -p-n structures are created by depositing semitransparent layers of high (Pd) and low (Al) work function (w.r.t silicon) metal layers on crystalline silicon (c-Si) wafers. The measurements are carried out by illuminating the device from the p <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^+$</tex></formula> -p side. The impedance data are analyzed by assuming a network consisting of a number of RC circuits, which, in turn, give recombination times (τ) controlled by defects in bulk and at the interfaces of the device. The maximum value of the effective recombination lifetime (τ) is found to be 12.3 ± 0.4 μs at an intensity of ∼100 mW/cm <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^2$</tex></formula> , which matches closely with the maximum value 11.4 ± 1.0 μs obtained under forward bias (+ 0.4 V) conditions in the same sample in the dark. These values match well with the effective minority carrier lifetime obtained using the microwave photoconductive decay technique on the same silicon wafer prior to device fabrication.

A facile solution-processed alumina film as an efficient electron-injection layer for inverted organic light-emitting diodes

Inverted organic light-emitting diodes (IOLEDs) can effectively improve device stability because they concentrate the air-stable anode and high work function (WF) metal oxide hole-injection layer (HIL) at the top of the devices. In this work, we report a facile solution-processed ultra-thin alumina film used as an electron-injection layer in IOLEDs and present significantly improved device performance. We achieved a high current efficiency of 5.12 cd A−1 at 10 mA cm−2 and the best current efficiency approaching 5.5 cd A−1 at 40 mA cm−2 without doping of an emission layer (EML) for a single Alq3-based green fluorescent IOLED, and a high current efficiency for a green phosphorescent IOLED as well. Furthermore, the extrapolated 50% decay lifetime (t50) shows that our Alq3-based green fluorescent IOLED is about 5 times more stable than the conventional OLED.

Tunable Electronic Properties of Two-Dimensional Transition Metal Dichalcogenide Alloys: A First-Principles Prediction.

We investigated the composition-dependent electronic properties of two-dimensional transition-metal dichalcogenide alloys (WxMo1-xS2) based on first-principles calculations by applying the supercell method and effective band structure approximation. It was found that hole effective mass decreases linearly with increasing W composition, and electron effective mass of alloys is always larger than that of their binary constituents. The different behaviors of electrons and holes in alloys are attributed to the fact that metal d-orbitals have different contributions to conduction bands of MoS2 and WS2 but almost identical contributions to valence bands. We examined the conduction polarity of WxMo1-xS2 monolayer alloys with four metal electrode materials (Au, Ag, Cu, and Pd). It suggests the main carrier type for transport in transistors could change from electrons to holes as W composition increases if high work function metal contacts were used. The tunable electronic properties of two-dimensional transition-metal dichalcogenide alloys make them attractive for electronic and optoelectronic applications.

Characterization of current transport in ferroelectric polymer devices

We report the charge injection characteristics in poly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE), as a function of electrode material in metal/ferroelectric/metal device structures. Symmetric and asymmetric devices with Al, Ag, Au and Pt electrodes were fabricated to determine the dominant carrier type, injection current density, and to propose transport mechanisms in the ferroelectric polymer. Higher work function metals such as Pt are found to inject less charges compared to lower work function metals, implying n-type conduction behavior for P(VDF-TrFE) with electrons as the dominant injected carrier. Two distinct charge transport regimes were identified in the P(VDF-TrFE) devices; a Schottky-limited conduction regime for low to intermediate fields (E<20MV/m), and a space-charge limited conduction (SCLC) regime for high fields (20<E<120MV/m). Implication of these results for degradation in P(VDF-TrFE) memory performance are discussed.

Electrical Contacts to Three-Dimensional Arrays of Carbon Nanotubes

We use numerical simulations to investigate the properties of metal contacts to three-dimensional arrays of carbon nanotubes (CNTs). For undoped arrays top-contacted with high or low work function metals, electrostatic screening is very strong, resulting in a small Schottky barrier for current injection in the top layer and large Schottky barriers for current injection in the deeper layers. As a consequence, the majority of the current flows through the top layer of the array. Our simulations show that doping of the CNT array can alleviate this problem, even without direct contact to each tube in the array; however, we find that the charge transfer length is unusually long in arrays and increases with the number of CNT layers under the contact. We also show that a bottom gate can modulate the contact resistance, but only very weakly. These results are important for the design of electronic and optoelectronic devices based on CNT arrays, because they suggest that increasing the thickness of the array does little to improve the device performance unless the film is strongly doped at the contacts and the contact is long, or unless each tube in the array is directly contacted by the metal.

Computational comparison of conventional and inverted organic photovoltaic performance parameters with varying metal electrode surface workfunction

Inverted polymer-based bulk-heterojunction organic photovoltaic (BHJ-OPV) device designs have enabled a breakthrough in operational lifetime through use of stable electrode materials. To date, there have not been systematic performance parameter comparisons between conventional and inverted devices that consider a range of different metal electrodes and presence of native metal oxides at the organic-metal interface. Here, we systematically compute optical and electronic performance parameters for both conventional and inverted BHJ-OPV devices for 15 different electrode types covering a range of workfunctions. We quantitatively demonstrate that (1) high-workfunction metal electrodes (Au, Pd, Ni) are ideal for high-efficiency inverted device performance; and (2) native metal oxide on metal electrodes (e.g., CuO/Cu, Ag2O/Ag, NiO/Ni), which dramatically reduce conventional device efficiencies, can result in highly efficient inverted BHJ-OPV devices (efficiency of up to 6.7% for the P3HT:PCBM system). This work is an important advance over prior studies as it predicts the electrode materials and configurations that can lead to both high efficiency and high stability BHJ-OPV devices.

The metal interlayer in the charge generation layer of tandem organic light-emitting diodes

This work studies the interface in the charge generation layer (CGL), consisting of aluminum (Al) doped in poly(ethylene glycol) dimethyl ether as an n-type layer and 2, 3, 5, 6-tetrafluoro-7, 7, 8, 8-tetracyanoquinodimethane (F4-TCNQ) doped in N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4-4′-diamine as an p-type layer, in tandem organic light-emitting diodes (OLEDs). Introducing a thin high work function metal interlayer (e.g., Ag or Au) effectively improves the transport and inhibits the accumulation of charges in the CGL, which markedly reduces the operating voltage and enhances the efficiency of tandem OLEDs. We attribute that the high density of surface states on metal clusters (interlayer) reduce the junction barrier to facilitate the transport of carriers through CGL. Experimental results show enhancements of tandem OLEDs by an additional metal interlayer as follows: luminous efficiency increases from 37.2 to 51.4 cd A−1, the light turn-on voltage decreases from 9.2 to 6.6 V, and luminescence at 10 mA cm−2 increases from 3712 to 5211 cd m−2.

Threshold Tuning of III-V Nanowire Transistors via Metal Clusters Decoration

Recently, III-V semiconductor nanowires are widely explored as field effect transistor (FET) materials for the next generation high speed electronics. Further tuning the working mode of the nanowire FET is essential in the large scale device design and fabrication, which is still a significant challenging in the current moment. In this study, decoration of metal nanoparticles with different work functions can effectively tune the threshold (VTH) of III-V nanowire FETs. Specifically, high work function metals such as Au and Pt can shift VTH to the positive side while low work function metals such as Al can tune VTH to the negative side, for the typical InAs, InP and InGaAs NWFETs, by depleting electrons from and donating electrons to the III-V nanowire channels. Furthermore, the fabrication of inverters using the depletion and enhancement mode InAs NWFETs show the great potency of the metal decoration method in low-power and high performance electronic devices.

Hard x-ray photoelectron spectroscopy and electrical characterization study of the surface potential in metal/Al2O3/GaAs(100) metal-oxide-semiconductor structures

Hard x-ray photoelectron spectroscopy (HAXPES) has been used to study metal-oxide-semiconductor (MOS) structures fabricated with both high (Ni) and low (Al) work-function metals on 8-nm thick Al${}_{2}$O${}_{3}$ dielectric layers, deposited on sulfur passivated $n$- and $p$-doped GaAs substrates. A binding energy difference of 0.6 eV was measured between the GaAs core levels of the $n$- and $p$-doped substrates in the absence of gate metals, indicating different Fermi level positions in the band gap. Subsequent photoemission measurements made on the MOS structures with the different work-function metals displayed very limited change in the GaAs core level binding energies, indicating that the movement of the Fermi level at the Al${}_{2}$O${}_{3}$/GaAs interface is restricted. Using a combination of HAXPES measurements and theoretical calculations, the Fermi level positions in the band gap have been determined to be in the range of 0.4--0.75 eV and 0.8--1.11 eV above the valence band maximum for $p$- and $n$-type GaAs, respectively. Analysis of capacitance voltage (C-V) measurements on identically prepared samples yield very similar Fermi level positions at zero applied gate bias. The C-V analysis also indicates a higher interface defect density ($D$${}_{it}$) in the upper half of the GaAs bandgap.

P.119: High‐Performance Polymer Light‐Emitting Diodes with a Conjugated Polyelectrolyte

AbstractWe demonstrated red, yellow, green and blue polymer light‐emitting diodes (PLEDs) with high performances using a conjugated polyelectrolyte electron injection layer and high work function metal (Ag). PLEDs using a conjugated polyelectrolyte layer show improved performances in terms of the luminance and efficiency, as compared to the devices without this layer. Furthermore, it is providing a new possibility of all solution‐processed PLEDs with Ag electrodes.

Origins of Effective Work Function Roll-Off Behavior for High-κ Last Replacement Metal Gate Stacks

Origins of effective work function (EWF) roll-off behavior accompanied by equivalent oxide thickness (EOT) scaling for high- κ last replacement metal gate (RMG) stacks are investigated using a low-temperature interfacial layer (IL) scavenging technique. The EWF-EOT roll-off is driven by a high work function metal and the trend is linear and reversible by means of IL scavenging and regrowth reactions. These findings are consistent with the oxygen vacancy model, indicating that the same mechanism that plagued gate-first devices emerges as the IL thickness is scaled <; 4 A (EOT 8 A) for RMG stacks.

A comparative study of the structural and electrical properties of n-type InGaN epilayer grown by MBE and commercially

Abstract This work reports the growth of n-In 0.27 Ga 0.73 N/GaN/AlN epitaxial layer on Si(1 1 1) substrate by using plasma-assisted molecular beam epitaxy (MBE) and commercially obtained n-In 0.08 Ga 0.92 N/AlN. As-grown and commercial thin films were characterized by using field emission scanning electron microscopy, atomic force microscopy, and high-resolution X-ray diffraction. A high work function metal (Pt) was deposited as metal contact on the thin films, and the electrical characteristics of the films pre- and post-annealed at 500 °C were studied under 3 V. Results show that the electrical characteristics of post-annealed thin films are better than those of pre-annealed thin films.

Metal Catalyzed Porous n-type GaN Layers: Low Resistivity Ohmic Contacting and Single-Step MgO/GaN Diode Formation

Porous GaN crystals have been successfully grown and electrically contacted simultaneously on Pt- and Au-coated silicon substrates as porous crystals and as porous layers. By the direct reaction of metallic Ga and NH3 gas through chemical vapor deposition, intermetallic metal-Ga alloys form at the GaN-metal interface, allowing vapoursolid- solid seeding and subsequent growth of porous GaN. Currentvoltage and capacitance-voltage measurements confirm that the intermetallic seed layers prevent interface oxidation and give a highquality reduced workfunction contact that allows exceptionally low contact resistivity. Additionally, the simultaneous formation of a lower workfunction intermetallic permits ohmic electron transport to n-type GaN grown using high workfunction metals that best catalyze the formation of porous GaN layers and may be employed to seed and ohmically contact a range of III-N compounds and alloys for broadband absorption and emission. Additionally, we show how a porous GaN rectifying diode can be formed by oxidatively crystallizing Mg typically employed for p-doping GaN, as a layer formed under porous structure resulting in a high-k polycrystalline MgO dielectric.

Synthesis and optoelectronic properties of amino-functionalized carbazole-based conjugated polymers

A series of alcohol soluble amino-functionalized carbazole-based copolymers were synthesized via Suzuki coupling reaction. The pendent amino groups endow them high solubility in polar solvents, as well as efficient electron injection capability from high work-function metals. The relationships between the photophysical and electrochemical properties and the polymer backbone structure were systematically investigated. These alcohol-soluble carbazole-based copolymers were used as cathode interlayers between the high work-function metal Al cathode and P-PPV emissive layer in polymer light-emitting diodes with device structure of ITO/PEDOT:PSS/P-PPV/interlayer/Al. The resulting devices exhibited improved performance due to the better electron injection/transporting ability of the designed copolymers from Al cathode to the light-emitting layer.

Generalized mechanism of the resistance switching in binary-oxide-based resistive random-access memories

We report that V${}_{\mathrm{O}}$ cohesion-isolation transition caused by carrier injection/removal is a generalized resistance switching mechanism of binary-oxide-based resistive random-access memories (ReRAMs). We propose universal guiding principles by which ReRAM with unipolar and bipolar operations can be designed by controlling electrode work functions. We found by first-principles calculations that structural phase transition with V${}_{\mathrm{O}}$ cohesion-isolation is the physical origin of the resistance switching mechanism of binary-oxide-based ReRAM. Based on our theory, we can propose a guiding principle toward bipolar switching ReRAM with stable high work function metal electrodes.

Improved Performance of Photosensitive Field-Effect Transistors Based on Palladium Phthalocyanine by Utilizing Al as Source and Drain Electrodes

In conventional photosensitive organic field-effect transistors (FETs) (OFETs) (photo-OFETs) based on p-type organic semiconductors, high work function metals such as Au are generally used as source/drain electrodes, whose photosensitivity is generally low. We report on the performance improvements of photo-OFETs based on palladium phthalocyanine (PdPc) by using Al as source and drain electrodes. It is concluded that the dark currents of the photo-OFET based on PdPc with Al as source and drain electrodes (denoted as Al-PdPc device) are about only one-thousandth of that of the photo-OFET based on PdPc with Au as source and drain electrodes (denoted as Au-PdPc device). This tremendous decrease of dark current results in about a three-order-of-magnitude increase for photosensitivity at the gate and drain voltages of -50 V. The enormous decrease of dark current is ascribed to the Schottky contacts between the Al source/drain electrodes and PdPc. In addition, the Al-PdPc devices show also larger photoresponsivity compared with the Au-PdPc devices.

Current Characteristics of AlGaN/AlN Ultraviolet Photodetector with Metal-Semiconductor-Metal Structure

An AlGaN/AlN ultraviolet photodetector with metal-semiconductor-metal structure is fabricated on n type 4H-SiC substrate, which is conventionally epitaxial by metal-organic chemical vapor deposition (MOCVD). The MSM structure is composed of two interdigitated fingers usually formed by Schottky contact which deposited metal with high work function metal by e-beam metallization and thermal evaporator on high resistance layers. This type of MSM has potential advantages, including ultra low dark current because of its rectifying contacts. The current Characteristics are revealed in this paper. A reference sample of AlGaN/GaN heterostructure with two-dimensional electron gas (2DEG) is also fabricated for comparision.

Amine group functionalized fullerene derivatives as cathode buffer layers for high performance polymer solar cells

An easy-accessible amine group functionalized fullerene derivative, DMAPA-C60, is explored as a cathode buffer layer (CBL) in polymer solar cells (PSCs) for our presently tested three different material systems, namely P3HT:PCBM, PBDTTT-C:PC70BM and PBDTTT-C-T:PC70BM. The power conversion efficiencies of the three systems with DMAPA-C60 as the CBL reach 3.88%, 6.29% and 7.42%, respectively, which are much higher than those of the corresponding PSCs with the Al-only cathode and even slightly higher than those of the corresponding Ca/Al devices of these systems. The DMAPA-C60 CBL also allows high work function metals (Ag, Cu, and Au) as cathodes.

Reduced Workfunction Intermetallic Seed Layers Allow Growth of Porous n-GaN and Low Resistivity, Ohmic Electron Transport

Porous GaN crystals have been successfully grown and electrically contacted simultaneously on Pt- and Au-coated silicon substrates as porous crystals and as porous layers. By the direct reaction of metallic Ga and NH(3) gas through chemical vapor deposition, intermetallic metal-Ga alloys form at the GaN-metal interface, allowing vapor-solid-solid seeding and subsequent growth of porous GaN. Current-voltage and capacitance-voltage measurements confirm that the intermetallic seed layers prevent interface oxidation and give a high-quality reduced workfunction contact that allows exceptionally low contact resistivities. Additionally, the simultaneous formation of a lower workfunction intermetallic permits ohmic electron transport to n-type GaN grown using high workfunction metals that best catalyze the formation of porous GaN layers and may be employed to seed and ohmically contact a range of III-N compounds and alloys for broadband absorption and emission.