Work Function Of Metal Electrode Research Articles

Simulation of the Sb2Se3 solar cell with a hole transport layer

A model of the Sb2Se3 solar cell with a hole transport layer (HTL) has been investigated by solar cell capacitance simulator (SCAPS). The influence of different HTLs on device performance has been firstly analyzed, and CuO has been found to be the best HTL. Then, Sb2Se3 thickness, CuO thickness, the doping concentration of CuO, the hole mobility of CuO, the defect density of Sb2Se3 layer, the defect density at the CdS/Sb2Se3 interface, and the work function of metal electrode on device performance have been systematically studied. The optimum thicknesses of Sb2Se3 and CuO are 300 nm and 20 nm, respectively. To achieve ideal performance, the doping concentration of CuO should be more than 1019 cm−3, and its hole mobility should be over 1 cm2V−1 s−1. The defect densities in the Sb2Se3 layer and at the CdS/ Sb2Se3 interface play a critical role on device performance, both of which should be as low as 1013 cm−3 and 1014 cm−2, respectively. In addition, the work function of the metal electrode should be more than 4.8 eV to avoid formation of Schottky junction at the metal electrode interface. After optimization, a best efficiency of 23.18% can be achieved. Our simulation results provide valuable information to further improve the efficiency of Sb2Se3 solar cells in practice.

High voltage vacuum-processed perovskite solar cells with organic semiconducting interlayers.

In perovskite solar cells, the choice of appropriate transport layers and electrodes is of great importance to guarantee efficient charge transport and collection, minimizing recombination losses. The possibility to sequentially process multiple layers by vacuum methods offers a tool to explore the effects of different materials and their combinations on the performance of optoelectronic devices. In this work, the effect of introducing interlayers and altering the electrode work function has been evaluated in fully vacuum-deposited perovskite solar cells. We compared the performance of solar cells employing common electron buffer layers such as bathocuproine (BCP), with other injection materials used in organic light-emitting diodes, such as lithium quinolate (Liq), as well as their combination. Additionally, high voltage solar cells were obtained using low work function metal electrodes, although with compromised stability. Solar cells with enhanced photovoltage and stability under continuous operation were obtained using BCP and BCP/Liq interlayers, resulting in an efficiency of approximately 19%, which is remarkable for simple methylammonium lead iodide absorbers.

Negative Fermi-Level Pinning Effect of Metal/n-GaAs(001) Junction Induced by a Graphene Interlayer.

It is demonstrated that the electric dipole layer due to the overlapping of electron wave functions at the metal/graphene contact results in a negative Fermi-level pinning effect on the region of the GaAs surface with low interface-trap density in the metal/graphene/n-GaAs(001) junction. The graphene interlayer plays the role of a diffusion barrier, preventing the atomic intermixing at the interface and preserving the low interface-trap density region. The negative Fermi-level pinning effect is supported by the decrease of the Schottky barrier with the increase of the metal work function. Our work shows that the graphene interlayer can invert the effective work function of the metal between high and low, making it possible to form both Schottky and Ohmic-like contacts with identical (particularly high work function) metal electrodes on a semiconductor substrate possessing low surface-state density.

Ultrahigh Gauge Factor in Graphene/MoS2 Heterojunction Field Effect Transistor with Variable Schottky Barrier.

Piezoelectricity of transition metal dichalcogenides (TMDs) under mechanical strain has been theoretically and experimentally studied. Powerful strain sensors using Schottky barrier variation in TMD/metal junctions as a result of the strain-induced lattice distortion and associated ion-charge polarization were demonstrated. However, the nearly fixed work function of metal electrodes limits the variation range of a Schottky barrier. We demonstrate a highly sensitive strain sensor using a variable Schottky barrier in a MoS2/graphene heterostructure field effect transistor (FET). The low density of states near the Dirac point in graphene allows large modulation of the graphene Fermi level and corresponding Schottky barrier in a MoS2/graphene junction by strain-induced polarized charges of MoS2. Our theoretical simulations and temperature-dependent electrical measurements show that the Schottky barrier change is maximized by placing the Fermi level of the graphene at the charge neutral (Dirac) point by applying gate voltage. As a result, the maximum Schottky barrier change (ΔΦSB) and corresponding current change ratio under 0.17% strain reach 118 meV and 978, respectively, resulting in an ultrahigh gauge factor of 575 294, which is approximately 500 times higher than that of metal/TMD junction strain sensors (1160) and 140 times higher than the conventional strain sensors (4036). The ultrahigh sensitivity of graphene/MoS2 heterostructure FETs can be developed for next-generation electronic and mechanical-electronic devices.

Influence of organic material and sample parameters on the surface potential in Kelvin probe measurements

Scanning Kelvin probe is a method for material surface studies. It is used to determine the work function of metals. In the case of organic semiconductors, the measured surface potential is considered to be the Fermi level of the material which has been shown in some cases. But in most papers, the surface potential dependence on the metal electrode or film thickness was observed. Material properties and their influence on the measured surface potential and its relation to the Fermi level previously have not been systematically studied. In this work, the surface potential was measured for different materials—metal, organic dielectric material, and organic semiconductors. In most of the cases, the obtained surface potential was dependent on the metal electrode work function. This dependence decreased with the increase in electrical conductivity of the material. Several materials were chosen for studies where sample thickness was varied. Results showed that for most of the studied semiconductors the sample thickness of around 1.5–2 µm was required to obtain surface potential values which do not depend on the electrode work function.

Energy Level Bending of Organic‐Inorganic Halide Perovskite by Interfacial Dipole

Cesium carbonate (Cs2CO3) is a widely used interfacial modification material in organic solar cells (OSCs), organic light‐emitting diode (OLED) and so on, which can: 1) lower the work function of metal electrode; 2) induce n‐doping effect; and 3) alter the energy level of semiconductor materials under the action of dipole. Cs2CO3 is selected in this study to modify the interface of perovskite solar cells (PSCs) and therefore improves the device performance. Further studies indicate that this performance improvement is due to the diffusion of Cs2CO3 through [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) to form dipoles with perovskite to bend its energy level, resulting in an increased electron collection efficiency. This proposed mechanism for the interfacial modification of PSCs by bending the perovskite energy level through interfacial dipoles deepens the understanding of the role of interfacial layer.

Toward development of high-performance perovskite solar cells based on CH3NH3GeI3 using computational approach

We reported numerical simulations of device performances made of methylammonium germanium halide (CH3NH3GeI3)-based perovskite solar cells. The main goal here is to seek for an efficient method to improve the device efficiency of alternative lead-free perovskite based on germanium solar cells by using various organic and inorganic hole transport materials. For that aspiration, the effect of several parameters on the solar cell performance were investigated such as thicknesses of perovskite, HTM, defect density, hole mobility, and metal electrode work function on the charge collection. The device simulation revealed that the optimum thickness of CH3NH3GeI3 absorber is found around 600 nm. Furthermore, Ge-based perovskite solar cells with Cu2O and D-PBTTT-14 as HTM exhibited a remarkable overall power conversion efficiency reaching 21%. The defect density reduction is a critical factor to improve the solar cell performance and should be controlled under the order of ∼1015 cm3. Further simulations were performed to study the effect of operating temperature on the performance. Our simulation results advocate for a viable route to design hole-transporting materials for highly efficient and stable perovskite solar cells with low cost.

Phosphonium conjugated polyelectrolytes as interface materials for efficient polymer solar cells

A series of cationic phosphonium-based conjugated polyelectrolytes (CPCPs, named as PFP-F, PFP-Cl, PFP-Br, PFP-I) with different counteranion were developed as individual cathode interface material and ZnO dopant for polymer solar cells (PSCs). The CPCPs are soluble in environmentally friendly polar solvents, which is beneficial to fabricating multilayer organic optoelectronic devices. Besides, the CPCPs show better thermal stability than traditional quaternary ammonium salt-based conjugated polyelectrolytes. The CPCPs perform efficiently as electrode modifiers to lower the work function of metal electrode and possess capability to n-dope PC71BM, both of which contribute to the improved performance of polymer solar cells. Significant enhancement in power conversion efficiency from 6.3% to >9.0% were achieved by employing a thin layer of CPCP between active layer and Al electrode in polymer solar cells with conventional device structure. Moreover, the CPCPs can be used as dopants to ZnO. The formed ZnO:CPCP hybrids can improve the interface contact between bottom contacts and active layer in inverted solar cells, resulting in improved performance.

Baking soda: an ultra-cheap and air stable electron injection layer for organic electronic devices

To realize high performance printed organic optoelectronic devices, the development of air-stable, cheap, and solution-processable electron injection layers (EILs) is urgently required to avoid the use of a low work-function metal electrode with poor air stability.

Orientation-Dependent Work-Function Modification Using Substituted Pyrene-Based Acceptors.

The adsorption of molecular acceptors is a viable method for tuning the work function of metal electrodes. This, in turn, enables adjusting charge injection barriers between the electrode and organic semiconductors. Here, we demonstrate the potential of pyrene-tetraone (PyT) and its derivatives dibromopyrene-tetraone (Br-PyT) and dinitropyrene-tetraone (NO2-PyT) for modifying the electronic properties of Au(111) and Ag(111) surfaces. The systems are investigated by complementary theoretical and experimental approaches, including photoelectron spectroscopy, the X-ray standing wave technique, and density functional theory simulations. For some of the investigated interfaces the trends expected for Fermi-level pinning are observed, i.e., an increase of the metal work function along with increasing molecular electron affinity and the same work function for Au and Ag with monolayer acceptor coverage. Substantial deviations are, however, found for Br-PyT/Ag(111) and NO2-PyT/Ag(111), where in the latter case an adsorption-induced work function increase of as much as 1.6 eV is observed. This behavior is explained as arising from a face-on to edge-on reorientation of molecules in the monolayer. Our calculations show that for an edge-on orientation much larger work-function changes can be expected despite the prevalence of Fermi-level pinning. This is primarily ascribed to a change of the electron affinity of the adsorbate layer that results from a change of the molecular orientation. This work provides a comprehensive understanding of how changing the molecular electron affinity as well as the adsorbate structure impacts the electronic properties of electrodes.

Impact of metal electrode work function of CH3NH3PbI3/p-Si planar heterojunction perovskite solar cells

The use of p-type silicon (p-Si) as hole transport materials (HTM) has been suggested to enhance the stability of perovskite-based solar cell (PSC). We design and simulate the CH3NH3PbI3/p-Si planar heterojunction perovskite solar cell and the power conversion efficiency exceeding 18%. In the solar cell, the metal electrode work function of perovskite solar cells is a decisive factor for built-in voltage (Vbi). The device simulation revealed that the Vbi and VOC increases when rear electrode work function (фM−rear) decreasing from −4.3eV to −4.7eV, the Vbi almost saturate to the plateau and the performance is optimal when фM−rear=−4.8 to −5.2eV. However, the work function of the front electrode (фM−front) also has a significant effect on the performance of p-Si perovskite solar cell. When фM−front decreasing from −3.4 to −4.0eV, a higher performance can be obtained and the Vbi almost saturate to the plateau. The results showed the importance of the фM matching to maintain Vbi, which is useful guideline for the design of the p-Si perovskite solar cell.

Gradual Controlling the Work Function of Metal Electrodes by Water Dissolved Mixed Interlayers for Ambipolar Polymer Field-Effect Transistors

Gradual Controlling the Work Function of Metal Electrodes by Water Dissolved Mixed Interlayers for Ambipolar Polymer Field-Effect Transistors

Tunable Schottky Barrier at MoSe2/Metal Interfaces with a Buffer Layer

Transition-metal dichalcogenide monolayers have gained significant attention because of their excellent physical properties and promising applications as a channel material in the next-generation transistors. In this work, we focus on contacts at the surface of various metals and single-layer MoSe2. Partial Fermi level pinning is demonstrated by the first-principle calculations, which indicates modulation of the electron Schottky barrier. Upon inserting a VS2 layer between MoSe2 layer and metal electrodes, all the n-type contacts at MoSe2/metal interfaces turn into p-type, and the hole Schottky barrier can be tuned effectively by varying metal electrodes. The high work function of the VS2 layer exerts significant influence on the band realignment of MoSe2, making all the n-type contacts at MoSe2/metal interfaces become p-type contacts at MoSe2/VS2–metal interfaces. Variation of the Schottky barriers and band alignments with the work function of metal electrodes demonstrated a partial Fermi level pinning a...

Effect of Structure, Temperature, and Metal Work Function on Performance of Organometallic Perovskite Solar Cells

The impact of hole transport materials (HTMs) on the performance of methylammonium lead halide (CH3NH3PbI3)-based perovskite solar cells has been investigated using computational analysis. The main objective is to replace the HTM with the aim of enhancing the lifetime and decreasing the overall cost of the device. As the CH3NH3PbI3 absorber layer shows an absorption coefficient as high as 105/cm, all photons with incident energy larger the material bandgap are absorbed within only a 400-nm-thick layer. Also, all the electronic and optical properties of such an absorber layer are suitable for use in photovoltaic (PV) devices. Hence, the effects of the HTM thickness, operating temperature, incident light spectrum, and metal electrode work function on the charge collection were studied numerically. For a cell with Cu2O as HTM, efficiency exceeding 25% is predicted for a 350-nm-thick absorber layer. Also, a fully optimized device architecture without HTM shows the possibility of fabricating a perovskite solar cell with PV efficiency exceeding 15%. We expect considerable minimization of the energy loss in this structure due to charge transfer across the heterojunction. Moreover, the effect of temperature on perovskite solar cells and potential electrodes with different work functions has been investigated. Our results are believed to help open an experimental avenue to achieve optimum results for perovskite solar cells with various structures.

The Potential of Zero Total Charge As a Descriptor of Catalytic Activity of Ru@Pt Core-Shell Catalysts

Ru@Pt core-shell catalytic particles consist of a thin Pt shell covering a Ru core, and may be supported on carbon to provide catalysts with properties intermediate between those of Pt and RuPt alloy particles of similar size [1]. Whereas carbon-supported Pt typically oxidizes adsorbed CO in stripping experiments with a peak potential around 0.8 V vs. a reversible hydrogen reference electrode (RHE) in 0.5 mol dm-3 HClO4(aq) the current for the core-shell catalysts peaks around 0.6 V. The value for the alloy is around 0.5 V. However, although the catalytic properties for for example electrooxidation of CO may not be quite as good as those of the alloy, the core-shell particles display superior stability. These core-shell catalysts should therefore be of interest in applications such as direct methanol fuel cells [2] or as CO-tolerant catalysts in PEM fuel cells. The potential of total zero charge (pztc), the potential corresponding to zero electrode charge (including both free charges and bonded charged species), may be assessed through the CO-displacement technique by Climent et al. [3] and shown by Mayrhofer et al. [4] to correlate with catalytic acticity. The method is based on CO displacing more weakly adsorbed cations and anions at the electrode or catalyst surface. The pztc has been found to correlate with the work function of metal electrodes, as has also the enthalpy of adsorption of CO and O2 [5]. This suggests that it should be possible to infer the enthalpy of adsorption for these species directly from the pztc. Moreover, scaling relations established by density-functional theory (DFT) calculations [6] imply that it is possible to estimate adsorption enthalpies for a number of other adsorbates relevant for for example the methanol oxidation reaction (MOR) or the oxygen reduction reaction (ORR), if the adsorption enthalpies of CO and oxygen are known. With a knowledge of adsorption enthalpies for a given reaction mechanism involving reactions of the type A + 1/2 H2 → AH the free energy of the steps may be inferred as explained by Rossmeisl and co-workers [7,8]. Through this sequence of correlations we have been able to infer the rate-determining step and rank the catalytic activity from the pztc. The pztc measurements thus predict that the onset potentials for the ORR should increase in the order RuPt < Ru@Pt < Pt, whereas that of CO-stripping and the MOR should decrease in the same order, RuPt > Ru@Pt > Pt, in close agreement with experimental results, as shown in the the figure; the figure shows the dependence of the specific activity of the ORR and the apparent rate constant, the exponent of -FEp/RT [9] where Epis of the peak potential and the other symbols take their usual meaning, for CO-stripping to the potential of zero total charge vs. RHE. From left to right are PtRu, Ru@Pt, and Pt, respectively. Moreover, this combination of CO-displacement measurements and DFT results appears to predict the right order of magnitude differences in the onset potential between the catalysts as well. Akcnowledgement This work was funded by NTNU.

Performance investigation of InAs based dual electrode tunnel FET on the analog/RF platform

In this paper for the first time, InAs based doping-less Tunnel FET is proposed and investigated. This paper also demonstrates and discusses the impact of gate stacking (SiO2 + HfO2) with equivalent oxide thickness EOT = 0.8 for analog/RF performance. The charge plasma technique is used to form source/drain region on an intrinsic InAs body by selecting proper work function of metal electrode. The paper compares different combinations of gate stacking (SiO2 and HfO2) on the basis of different analog and RF parameters such as transconductance (gm), transconductance to drive current ratio (gm/ID), output conductance (gd), intrinsic gain (AV), total gate capacitance (Cgg) and unity-gain cutoff frequency (fT). The proposed device produces an ON state current of ION ∼6 mA along with ION/IOFF ∼1012, point subthreshold slope (SS ∼ 1.9 mV/dec), average subthreshold slope (AV-SS ∼ 14.2 mV/dec) and cut-off frequency in Terahertz. The focus of this work is to eliminate the fabrication issues and providing the enhanced performance compared to doped device.

High frequency performance of dual metal gate vertical tunnel field effect transistor based on work function engineering

A novel dual metal gate doping-less vertical tunnel field effect transistor (D-VTFET) on silicon body, using work function engineering is proposed. The proposed structure does not required impurity doping for formation of the drain and the source regions. In this concern, source and drain regions are formed by selecting appropriate work-function of metal electrode. The source and drain regions are not formed by conventional ways of ion implantation or diffusion. Hence, proposed structure is immune greatly to the process variation, issues of doping control and random dopant fluctuations which are serious problems in ultrathin silicon devices. For further improvement in ON state current and analogue/RF figures of merit dual work function of single gate material is considered. The electrical characteristics of the proposed device with the D-VTFET are simulated and compared.

A comparison of copper indium sulfide-polymer nanocomposite solar cells in inverted and regular device architecture

In this study, the influence of device architecture (regular versus inverted) and different interlayers on performance and lifetime of polymer-nanocrystal hybrid solar cells is explored. The absorber layers of the investigated solar cells consist of the low band gap conjugated polymer poly[[9-(1-octylnonyl)-9H-carbazole-2,7-diyl]-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) and copper indium sulfide (CIS) nanocrystals. These are prepared directly in the polymer matrix via an in situ approach using copper and indium xanthates, which are converted to CIS nanocrystals by mild thermal annealing. Different interlayers – poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and inorganic metal oxides (V2O5, MoO3) – are used as hole extraction layers between absorber and anode. All three layers were successfully applied in solar cells in regular architecture. Using an inverted device design, a PEDOT:PSS interlayer and Ag electrodes, open circuit voltages of 550mV could be obtained, which are comparable to values obtained with solar cells having easily oxidizing low work function metal electrodes. However, inverted solar cells with a V2O5 hole extraction layer could not be obtained due to possible incompatibility of the V2O5 coating process with the underlying absorber layer. In the case of MoO3 interlayers, the introduction of additional layers led to working inverted solar cells.

Metal Evaporation-Induced Degradation of Fullerene Acceptors in Polymer/Fullerene Solar Cells.

Surface-sensitive NEXAFS spectroscopy is used to probe the interaction between low work function metal electrodes and fullerene derivatives in organic solar cells. Evaporation of either Ca or Al electrodes onto films of the fullerene derivatives (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) and indene-C60 bisadduct (ICBA) leads to a dramatic change in the observed NEXAFS spectrum. The observed changes cannot be explained only in terms of interfacial electronic doping or charge transfer, but rather point to the formation of new chemical bonds that destroy the extensive electron delocalization on the C60 cage. A combination of ex situ and in situ ultrahigh vacuum measurements indicates that metal evaporation results in a change in the electronic structure of PCBM that then facilitates chemical degradation and oxidation in the presence of oxygen. To investigate the effect of this chemical interaction on device performance, a unique transfer method to laminate the Al electrode to the top of polymer blend is used, in which case, the chemical degradation of the fullerene is not observed. Device performance of P3HT/PCBM blend solar cells in which the top metal electrode has either been thermally evaporated or transferred is then compared. These results highlight that chemical, as well as electronic, interactions between metals and organic semiconductors must be considered.

Performance analysis of different top metal electrodes in inverted polymer solar cells

Abstract In recent years, since the introduction of bulk heterojunction concept in an organic solar cell, the efficiency is increased to ∼6%. The benefits of bulk heterojunction (BHJ) organic solar cells are drawing interest for applications in next-generation solar cells. In this study, we analyze the optimal top electrode for practical polymer solar cells (PSC) fabrication by utilizing the optical properties of the electrode material and study their performance of PSC devices. The high work function metal electrodes having enhanced optical properties such as aluminum (Al), silver (Ag) and gold (Au) are considered in fabricating inverted PSC devices. It comprises of a blended polymer of poly(3-hexylthiophene) (P3HT) and phenyl-C 61 -butyric acid methyl ester (PCBM). Among the key photovoltaic parameters, the open circuit voltages ( V oc ) were strongly affected by the optical properties of the top electrode . From the experimental evidence, the increase in V oc of the Al and Ag electrode devices were found to be approximately 11.32% and 26.42% respectively. These values were significantly higher when compared to the Au electrode, as the parasitic absorption of incident photons were below 600 nm. Ag electrodes have excellent stability to ambient exposure which is comparable to Au. These investigations directed to conclude that, Ag is the optimal top electrode material for use in inverted devices. Also, in this study we investigated the incorporation of monodispersed ZnO as an n-type buffer layer to effectively transport electrons from active layer to the cathode electrode.