Low Work Function Electrode Research Articles

Cationic nitrogen-doped graphene coated silver as low-work function electrode for inverted perovskite solar cells

Interfacial layers (ILs) located between the cathode and electron transport layer (ETL) in inverted perovskite solar cells are commonly required to achieve high-performance devices. Therefore, it is essential to develop excellent IL materials to improve efficiency and stability. This paper introduces the graphene-based IL material, namely cationic nitrogen-doped graphene (CNG), and evaluates its performance on a methylammonium lead iodide (MAPbI3)-type inverted perovskite solar cells. The device with a CNG IL achieved a power conversion efficiency of 13.5%, which is higher than a state-of-the-art reference using bathocuproine IL. Mechanistic studies demonstrated that the CNG IL can (1) efficiently collect electrons from the lowest unoccupied molecular orbital of ETL by lowering the work function of the silver cathode, (2) improve the conductivity of the silver electrode for better electron transfer, and (3) smooth out the interface contact between ETL and cathode to reduce defects in the device. As a result, the CNG IL enhanced the inverted perovskite solar cells performance by simultaneously increasing the open-circuit voltage, short-circuit current density, and fill factor. Moreover, the unencapsulated CNG IL-applied device demonstrated good long-term stability, with 96% efficiency retained over 1000 h in nitrogen atmosphere at room temperature.

Laminated high-performance semi-transparent perovsktie solar cells: Enabled by sticky polyethyleminine as glue

Electric glue plays a key role in laminating top electrodes for organic and perovskite thin-film solar cells. However, there has been seldom reported for chemically reactive low–work function electrode yet. In this paper, polyethyleneimine (PEI) was developed as transparent top electrode adhesive, surface modifier for air-stable low–work function, and barrier to suppress the chemical corrosion of metal electrode in perovskite solar cells. The lamination was finished under room temperature with a pressure as low as 8 MPa, which is a very mild condition for device fabrication. The solar cells with transparent top electrode laminated for 10min had the best performances, in which the average value of PEI thickness between silver nanowires and the adjacent layer was found to play a key role in adjusting the performance of solar cells.

A metal chelation strategy suppressing chemical reduction between PEDOT and polyethylenimine for a printable low-work function electrode in organic solar cells

Zinc ion chelation on PEIE can effectively suppress the chemical reaction between PEIE and PEDOT:PSS. PEDOT:PSS/PEI-Zn is an efficient electron-collecting electrode for organic solar cells.

Interfacing Low‐Temperature Atomic Layer Deposited TiO2 Electron Transport Layers with Metal Electrodes

AbstractLow‐temperature atomic layer deposited (ALD) TiO2 is an electron transport layer that is relatively unreactive with mixed halide perovskite semiconductors. This amorphous, ALD‐grown metal oxide is typically interposed between the perovskite layer and a low work function metal electrode, prompting interest in the electronic and chemical structure of the resulting TiO2/metal interface. Here, the interfaces between TiO2 and two common metal low work function electrodes, polycrystalline thin films of silver and aluminum are studied. It is found that a Schottky barrier forms between Ag and low‐temperature ALD‐grown TiO2, consistent with the observed solar cell current–voltage characteristics, and confirmed by in situ measurement of Ag deposition‐induced core level shifts in X‐ray photoelectron spectra and barrier height extraction from temperature‐dependent current–voltage data. On the other hand, Al reacts chemically with the surface of TiO2 during its deposition. The resulting interface acts as an Ohmic contact, due to the apparent oxygen vacancy doping of the underlying ALD‐TiO2. These findings provide lessons for metal electrode selection in future halide perovskite device structures.

Interface engineering in efficient vacuum deposited perovskite solar cells

We studied the effect of the charge transport layers in p-i-n perovskite solar cells using vacuum deposited methylammonium lead iodide thin-film absorbers. While solution-processed perovskite films are frequently deposited directly on PEDOT:PSS leading to good solar cell performances, in some cases even to very good Voc values, we show that in devices employing vacuum deposited MAPbI3 perovskites, the removal of the polyTPD electron blocker substantially reduces the photovoltaic behavior. This is indicative of rather different charge transport properties in the vacuum deposited MAPbI3 perovskites compared to those prepared from solution. On the other hand, we investigated the use of ionic interlayers as a possible alternative to low work function electrodes, whose reactivity towards air and moisture compromises the device stability. Two different electron extraction materials were evaluated as interlayers between the fullerene electron transport layer and a silver electrode, in particular a perylenediimide derivative and a conjugated polyelectrolyte. By studying the photovoltaic response and the electroluminescence properties of planar diodes using the ionic films and comparing them with devices employing barium, we found that such ionic interlayers can successfully replace the use of reactive electrodes, since they facilitate the electron extraction while reducing the non-radiative recombination at the electron transport interface.

Lanthanum Hexaboride As Novel Interlayer for Improving the Thermal Stability of P3HT:PCBM Organic Solar Cells.

For efficient organic photovoltaic (OPV) solar cells, a low work function electrode is necessary to enhance the built-in voltage of the active layer, thereby improving the overall efficiency. Calcium is often used for this purpose in the laboratory; however, its development on a larger scale is impaired by its high reactivity with oxygen and water and the resulting low stability of solar cells under operation. The influence of a novel interlayer, lanthanum hexaboride (LaB6), on the electronic properties of OPV is studied in this work. Similarly to calcium, when LaB6 is used as an interlayer, it enhances the built-in voltage in the device, leading to a higher fill factor (FF) and optimal open circuit voltage (V(oc)). As a result, optimized LaB6-based devices present significantly improved power conversion efficiencies. More importantly, while calcium/aluminum (Ca/Al) and aluminum (Al) cathodes lose their capacity to enhance the internal electrical field during thermal aging, the LaB6/aluminum (LaB6/Al) electrode remains stable. This remarkable effect results in a highly stable V(oc) and flat-band potential during aging.

Modification on ITO to Fabricate Low Work Function Electrode in Inverted Organic Photovoltaics

The PCE has a great relationship with the work function of conductors in organic and printed electronics devices. And organic photovoltaics require an electrode with a work function (WF) that is low enough to either facilitate the transport of electrons in and out of various optoelectronic devices or collect electrons from the lowest unoccupied molecular orbital (LUMO) of a given organic semiconductor. In inverted organic photovoltaics, the ITO is normally used as cathode to collect electrons .By using PDDA deposition, the surface work function of ITO can be decreased by 0.3 eV, which is able to improve the electrons transport and the PCE in OPV, as it has been proved that the surface electronic potential of ITO is very sensitive to the presence of self-assembled molecular layers.

Low work function thin film growth for high efficiency thermionic energy converter: Coupled Kelvin probe and photoemission study of potassium oxide

Recent researches in thermal energy harvesting have revealed the remarkable efficiency of thermionic energy converters comprising very low work function electrodes. From room temperature and above, this kind of converter could supply low power devices such as autonomous sensor networks. In this type of thermoelectric converters, current injection is mainly governed by a mechanism of thermionic emission at the hot electrode which explains the interest for low work function coating materials. In particular, alkali metal oxides have been identified as excellent candidates for coating converter electrodes. This paper is devoted to the synthesis and characterization of potassium peroxide K2O2 onto silicon surfaces. To determine optimal synthesis conditions of K2O2, we present diagrams showing the different oxides as a function of temperature and oxygen pressure from which phase stability characteristics can be determined. From the experimental standpoint, we present results on the synthesis of potassium oxide under ultra high vacuum and controlled temperature. The resulting surface is characterized in situ by means of photoemission spectroscopy (PES) and contact potential difference (CPD) measurements. A work function of 1.35 eV is measured which and the expected efficiency of the corresponding converter is discussed. It is generally assumed that the decrease of the work function in the alkali/oxygen/silicon system, is attributed to the creation of a surface dipole resulting from a charge transfer between the alkali metal and oxygen.

Electronic structure of low work function electrodes modified by C16H33SH

Organic and printed electronics technologies require electrodes with low work functions to facilitate the transport of electrons in and out of various optoelectronic devices. We show that the surface modifier of 1-hexadecanethiol reduces the work function of conductors using in situ ultraviolet photoemission spectroscopy, and we combine experimental and theoretical methods to investigate the origin of the work function changes. The interfacial electronic structures of pentacene/1-hexadecanethiol/Au were investigated via in situ ultraviolet photoemission spectroscopy and X-ray photoemission spectroscopy in order to understand the change in the carrier injection barrier and chemical reactions upon surface modification. Theoretical calculations using density functional theory were also performed to understand the charge distribution of 1-hexadecanethiol, which affects the reduction of the work function. The 1-hexadecanethiol surface modifier is processed in air from solution, providing an appealing alternative to chemically-reactive low-work-function metals.

Replacing the metal oxide layer with a polymer surface modifier for high-performance inverted polymer solar cells

Replacing ZnO with PEIE, both as a surface modifier for the low work function electrode and as an electron selective layer, enhances the performance and air stability of inverted polymer solar cells by improving electron transport, wettability between the active layer and the cathode, and maximizing light absorption within the active layer without light interference.

An Indium‐Free Low Work Function Window Electrode for Organic Photovoltaics Which Improves with In‐Situ Oxidation

A low-cost window electrode for organic photovoltaics that simultaneously removes the requirement for conducting oxide and conventional low work function electrodes and functions as a sink for oxygen/water in the heart of the device. Remarkably the functionality of this electrode, which is based on a 7.8 nm nanostructured Cu:Al film, improves upon in situ oxidation as demonstrated in bulk heterojunction organic photovoltaics.

A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics

Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low-work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.

Effect of Eosin-Y Coating Temperature on the Performance of Inverted Bulk Heterojunction Organic Solar Cells

Inverted type bulk heterojunction organic solar cell based on ZnO nanorod arrays have been used to overcome the degradation problem of conjugated polymer and low work function electrode. ZnO nanorods and Eosin-Y are widely used to increase the charge mobility and light absorbance, respectively. The effects of Eosin-Y coating temperature (30 – 50 oC) on the performance of organic solar cells based on a blend of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) as donor and (6,6)-phenyl-C61 butyric acid methyl ester (PC61BM) as acceptor with a structure of fluorine-doped tin oxide (FTO)/Eosin-Y coated ZnO nanorod arrays/MEHPPV:PCBM/Ag were investigated. By using FTO as a substrate, sol-gel and spin coating method was used to deposit ZnO seed layer on FTO surface. The ZnO nanorods were then grown on the ZnO seed layer by chemical bath deposition method and coated with dye by immersing the substrates into Eosin-Y solution at different temperatures. The result shows that absorption of Eosin-Y coated ZnO nanorods increased with dye coating temperature. The optimum power conversion efficiency of η = 1.24 × 10-2 % was achieved at dye coating temperature 50 oC under 100 mW/cm2 simulated AM 1.5 G sunlight.

A layered structure at the surface of P3HT/PCBM blends

In an organic photovoltaic device, a layer consisting of two organic compounds, one functioning as an electron acceptor and the other as an electron donor, is sandwiched between two electrodes. Sufficient concentration of the electron acceptor is required at the interface to the low work function electrode as well as sufficient concentration of the electron donor at the interface to the high work function electrode. Frequently used compounds are poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM). It is reported in the literature that blends of these materials show a gradient throughout the polymer layer within devices due to phase separation. The direction of the gradient is dependent on the surface energy of the substrates onto which the polymer solutions are spincoated. This is assumed to be detrimental in the usual device structure of photovoltaic devices. In this report the surface of the blend was studied with a combination of a depth profiling technique sensitive for elemental composition and two electron spectroscopy techniques differing in their surface sensitivity. After spin coating, a thin layer of PCBM with a thickness of approximately a single monolayer was found just below the blend surface. The implication for organic photovoltaic devices is discussed.

Planar heterojunction organic photovoltaic diodes via a novel stamp transferprocess

Solution-processed, planar heterojunction organic photovoltaic diodes offer severalpotential advantages over bulk heterojunction structures in relation to electrodeselectivity, reduced dark currents and suitability for fundamental studies. They have,however, received less interest in recent years, in large part due to fabricationdifficulties encountered for sequential solution deposition steps. In this study, a novelstamp transfer technique that allows ready fabrication of planar heterojunctionsfrom a variety of solution-processed organic materials is applied to constructbilayer heterojunctions from poly(3-hexylthiophene) (P3HT) and [6,6]-phenylC61 butyric acid methyl ester (PCBM). We show that whilst ‘as made’ planar heterojunctionsyield relatively poor photocurrent generation (compared to equivalent bulk heterojunctiondevices), thermal annealing improves their performance via creation of a diffuse mixedP3HT:PCBM interface layer. Good device performance with the anticipated low darkcurrent is then achieved. Spectroscopic ellipsometry allows us to monitor the changes in theinterface layer that result from annealing. We also model the external quantum efficiencyspectra and show that they are consistent with the ellipsometry data. Furthermore, it isshown that good device performance is strongly dependent on the P3HT and PCBM layerordering with respect to the electrodes, confirming the important role of electrodeselectivity. Melting of ‘incorrectly’ ordered planar heterojunction devices (with donor nextto the high work function and acceptor next to the low work function electrode) leads tothe formation of bulk heterojunction devices, thereby recovering much of the desiredperformance.

Charge conduction mechanism and photovoltaic properties of 1,2-diazoamino diphenyl ethane (DDE) based schottky device

The dark electrical, impedance and photovoltaic properties of 1,2-diazoamino diphenyl ethane (DDE) based schottky device were investigated. It is found that DDE behaves as an n-type organic semiconductor which forms the schottky barrier with ITO (high work function electrode) and ohmic contact with Al (low work function electrode) having ITO/DDE/Al configuration. The J– V characteristics of device reveal that the conduction mechanism for biasing voltage between 0.6 and 1.2 V can be described by modified Schockley equation. For the higher voltage (beyond 1.2 V), the current transport is due to the space charge limited current (SCLC) in the presence of exponentially distributed traps, however for voltage below 0.6 V, the conduction mechanism is determined on the basis of schottkey emission and Poole–Frenkel emission. The formation of Schottky barrier at ITO–DDE interface is also evidenced by the impedance analysis and comparison of photoaction spectra of the device with optical absorption spectra of DDE thin layer. The dependence of ac conductivity on temperature yields low activation energy in low temperature region which indicates the dominance of hopping conduction process in DDE thin film layer. It is also found that the slower relaxation process occurs at the interface, which limits the carrier injection across the ITO–DDE junction. The observation of high interfacial resistance and low electron mobility in DDE leads to the conclusion that most of the electron–hole recombination occurs at the ITO–DDE junction, thus increasing the diode ideality factor. The photogeneration mechanism of the charge carriers in the device has also been described in detail. The analysis of dark space charge limited current at high voltage region and exponential relation of the photocurrent with illumination intensity attributes an exponential trap distribution in the band gap of the film.

Role of Si/film interface in photovoltaic devices based on aromatic molecular films

Sandwich structures based on cast films of poly(3-dodecylthiophene) (PDDT) and multilayer Langmuir–Blodgett (LB) corbathiene films are studied under ambient conditions. The two types of the structures demonstrate similar photovoltaic properties. Photovoltage and photoconductivity are particularly distinguishable when n-Si substrate is used in the structures. Two photovoltaic components are observed: caused by light absorption in the film and caused by light absorption in n-Si substrate. Donor–acceptor interaction at film/n-Si interface superposed on rigid-band approach is used to suggest mechanisms of the phenomena observed. The mechanisms may be used in order to provide a low work function electrode for organic light emitting diodes.

A New Alternative for the Low-Workfunction Electrode in Organic Devices

ABSTRACTThe application of ele ctroactive organic materials in devices (light emitting diodes, photovoltaic cells) often requires electrodes with a low workfunction. Commonly, aluminum or alkaline earth metals are used, which usually exhibit strong interaction with the organic material, such as the formation of new covalent bonds or doping. This results in a strong modification of the electronic structure of the organic/metal interface, and in most cases does not yield the energy level alignment expected for the unreacted interface. As a n alternative to the above-mentioned metals we propose the use of samarium, with a workfunction of 2.7 eV, for the following reason: we have studied the interface formation between Sm and p -sexiphenyl (6P, which exhibits intense blue electroluminescence), with ultraviolet photoelectron spectroscopy. Sm was deposited stepwise onto thin films of 6P in ultrahigh vacuum, and the photoelectron spectra were recorded after each step. We did not find any indication for a strong interaction between the two materials. Metallic Sm is formed instantaneously and the valence electronic structure of 6P remains unchanged upon the metal deposition. The weak interaction at this interface allows one to determine the energy level alignment between a metal and an organic material in a direct manner from the photoelectron spectra, without the need for making any assumptions on the workfunction or ionization potential.

Photoemission spectroscopy of LiF coated Al and Pt electrodes

Thin lithium fluoride (LiF) interlayers between the low work function electrode and the electron transport layer in organic light emitting diodes (OLED) result in improved device performance. We investigated the electronic structure of LiF coated Al and Pt electrodes by x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). Thin LiF films were grown in several steps onto Ar+ sputtered Al and Pt foils. After each growth step the surfaces were characterized in situ by XPS and UPS measurements. After evaluating band bending, work function and valence band offset for both samples, their band lineups were determined. Our measurements indicate that despite the insulating character of LiF in both samples, band bending is present in the LiF layer. The difference in band bending between the samples allows the conclusion that the driving force for the development of the band bending results from the contact potential between the metal and the LiF overlayer. The band bending is most likely caused by a redistribution of charged Frenkel or Schottky type defects within the LiF layer. The work function of both samples after LiF deposition was dramatically lowered compared to the values obtained on the clean sputtered metal surfaces.

Electrical and photoelectrical properties of poly(phenyl azomethine furane) thin films devices

Poly(phenyl azomethine furane) (PPAF), a conjugated system of poly-Schiff base was synthesised by condensation of p-aminobenzene with furane-2-aldehyde. Oxidative polymerization was conducted at 0–5 °C temperature, and the polymer formed was found to be soluble in common organic solvent. Optical properties were studied in the UV-visible-IR region. Electrical and photo electrical properties of Al/PPAF/ITO and Ag/PPAF/ITO sandwich devices were investigated by measuring the steady state photocurrent resulting from illumination. The device Al/PPAF/ITO shows a Schottky barrier at the Al-PPAF interface and ohmic contact at ITO-PPAF interface. The device Ag/PPAF/ITO shows ohmic contact at both interfaces. This behaviour has been explained in terms of P-type conductivity of the PPAF and the formation of a Schottky barrier with the low work function electrode and ohmic contact with the higher work function electrode. Under low forward bias voltage (ITO positive) ohmic conduction is observed while at higher voltage there is space charge-limited conduction (SCLC). The dependence of the photo short-circuit current on illumination intensity was discussed in detail. The C-V characteristics of both devices have also been analysed.