Anode Interfacial Layer Research Articles

Barrierless hole injection through sub-bandgap occupied states in organic light emitting diodes using substoichiometric MoOx anode interfacial layer

In this letter, highly efficient hole injection was demonstrated in hole only devices based on organic semiconductors with different highest occupied molecular orbital level and transport properties. The barrierless hole injection was achieved by using a substoichiometric MoOx thin film (consisting of 65% Mo+6 and 35% Mo+5) as a higly effective anode interfacial layer. The current in these devices was found to be space charge limited, achieved due to the formation of highly efficient anode ohmic contact via the excellent band alignment through occupied gap states at the ITO/MoOx and MoOx/organic semiconductor modified interface. Quite remarkably, the efficiency of hole injection was found to be almost independent of the MoOx thickness, which is indicative of perfect band alignment at the anode interface.

Phase-controllable copper oxides for an efficient anode interfacial layer in organic light-emitting diodes

We investigated phase-controllable copper oxide (CuOx) as a hole injection layer for improving the electrical and optical properties of organic light emitting diodes (OLEDs). The phase of CuOx is changed from CuO to Cu2O by elevating the deposition rate during thermal evaporation, and a non-stoichiometric film mixing with two phases occurs in the intermediate condition. In non-stoichiometric films, unbonded oxygen atoms with a large density of gap states are induced near the Fermi level, acting as a hole transport path. Thus, holes can be more easily injected into the highest occupied molecular orbital states of α-NPD, lowering the hole injection barrier significantly. Films of non-stoichiometric CuOx are inserted at the interface of α-NPD with an Ag electrode, serving as the hole injection layer. Significantly improved current densities and luminance were achieved, reducing the operation voltage at 1 mA cm−2 from 6.1 V to 3.2 V.

The diffusions and associated interfacial layer formation between thin film electrolyte and cermet anode in IT-SOFC

Dense 20 at.% gadolinium-doped (20GDC) thin film electrolyte supported by Ni–20GDC cermet anode has been fabricated by electrophoretic deposition technique. The microstructure and spatial distribution of constituent elements of this half-cell sample have been characterized by a combination of various techniques. The energy-dispersive X-ray spectroscopy (EDX) investigations, operated in scanning transmission electron microscopy (STEM) mode, illustrate that not only diffusion of metallic Ni from the anode into the electrolyte but also simultaneous counter-diffusion of Ce and Gd rare-earth cations from the electrolyte to the anode. Such mutual diffusion mainly occurs not only at the electrolyte–anode interface, but also occurs at the anode where Ni grains meet GDC grains. STEM high-angle annular dark-field (HAADF) imaging detects grain boundary segregation phenomena and suggests that grain boundaries can provide another possible pathway for diffusing atoms/ions. These investigations help explain the formation of an electrolyte–anode interfacial layer in terms of the mutual diffusion and diffusion along grain boundaries, and raise awareness of such diffusions that take place even during the process of cell sample preparation.

Effect of solution processed graphene oxide/nickel oxide bi-layer on cell performance of bulk-heterojunction organic photovoltaic

We report the solution processed graphene oxide (GO), NiO x and GO/NiO x bi-layer used as an anode interfacial layer in organic bulk-heterojunction solar cells. The bulk-heterojunction solar cells using GO, NiO x and GO/NiO x bi-layer exhibited the conversion efficiency of 2.33%, 3.10% and 3.48%, respectively. The cell efficiency is correlated with the matching of energy levels between ITO, hole transport layer and P3HT and thus a well-matched stack layer of ITO/GO/NiO x /P3HT:PCBM/LiF/Al shows the best cell efficiency of 3.48% with the J SC of 8.71 mA/cm 2, V OC of 0.602 V and FF of 66.44%.

Efficient, Air‐Stable Bulk Heterojunction Polymer Solar Cells Using MoOx as the Anode Interfacial Layer

The use of molybdenum oxide as the anode interfacial layer in conventional bulk heterojunction polymer solar cells leads to an improved power conversion efficiency and also dramatically increases the device stability. This indicates that the engineering of improved anode interface materials is an important method by which to fabricate efficient and stable polymer solar cells.

Reduction of Tungsten Oxide: A Path Towards Dual Functionality Utilization for Efficient Anode and Cathode Interfacial Layers in Organic Light‐Emitting Diodes

AbstractHere, we report on the dual functionality of tungsten oxide for application as an efficient electron and hole injection/transport layer in organic light‐emitting diodes (OLEDs). We demonstrate hybrid polymer light‐emitting diodes (Hy‐PLEDs), based on a polyfluorene copolymer, by inserting a very thin layer of a partially reduced tungsten oxide, WO2.5, at the polymer/Al cathode interface to serve as an electron injection and transport layer. Significantly improved current densities, luminances, and luminous efficiencies were achieved, primarily as a result of improved electron injection at the interface with Al and transport to the lowest unoccupied molecular orbital (LUMO) of the polymer, with a corresponding lowering of the device driving voltage. Using a combination of optical absorption, ultraviolet spectoscopy, X‐ray photoelectron spectroscopy, and photovoltaic open circuit voltage measurements, we demonstrate that partial reduction of the WO3 to WO2.5 results in the appearance of new gap states just below the conduction band edge in the previously forbidden gap. The new gap states are proposed to act as a reservoir of donor electrons for enhanced injection and transport to the polymer LUMO and decrease the effective cathode workfunction. Moreover, when a thin tungsten oxide film in its fully oxidized state (WO3) is inserted at the ITO anode/polymer interface, further improvement in device characteristics was achieved. Since both fully oxidized and partially reduced tungsten oxide layers can be deposited in the same chamber with well controlled morphology, this work paves the way for the facile fabrication of efficient and stable Hy‐OLEDs with excellent reproducibility.

Polysilane based ultraviolet light-emitting diodes with improved turn-on voltage, stability and color purity

We demonstrate ultraviolet organic light-emitting diodes (OLEDs) with improved stability, low turn-on voltage and color purity by changing the cathode and annealing temperature of the polymer film. The electron injection process, which limits the electroluminescent performance of organic devices, has been enhanced tremendously by inserting a layer of LiF with appropriate thickness between the cathode and a poly( n-butylphenylsilane) (PS-4) layer, whose device structure is ITO/PEDOT:PSS/polysilane (PS-4)/LiF/Al. Devices with a LiF (6 Å) have the turn-on voltage of 4 V, which is lower than 7 V of devices made with Ca/Al layer. By inserting LiF as the anode interfacial layer, there is increase in the injection of electrons from Al (cathode) side due to tunneling effect and also act as hole blocking layer which enhance the recombination of electron and hole in the emissive layer. PS-4 is spin coated and annealed in vacuum for 1 h at different temperatures (90–120 °C). EL Spectra from these devices consists of white emission along with the UV peak. White emission is significantly suppressed when PS-4 is annealed at higher temperature and threshold voltage is lowest at 110 °C annealing temperature.

All-spin-coating vacuum-free processed semi-transparent inverted polymer solar cells with PEDOT:PSS anode and PAH-D interfacial layer

Semi-transparent inverted polymer solar cells (PSCs) were fabricated by all-spin-coating vacuum-free process. An interfacial layer of poly(allylamine hydrochloride) and dextran (PAH-D) was introduced to modify the surface wettability of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) film for depositing aqueous-solution-processed poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (CLEVIOS™ PH500) (PH500) anode as well as enhancing the adhesion and bonding strength between the active layer (P3HT:PCBM) and conducting polymer electrode (PH500). The photovoltaic performance of the semi-transparent PSCs under top and bottom illumination was investigated and the highest PCE of about 1.86% was achieved under simulated AM 1.5 illumination (100 mW/cm 2) from the bottom side with a reflective white paper back side. The semi-transparent Cell A have the potential for use as energy-generating color window glasses as well as the subcell of tandem solar cells. The all-spin-coating vacuum-free processing provides a possibility of high-throughput all-solution-processed fabrication of PSCs by roll-to-roll processing.

Consequences of Anode Interfacial Layer Deletion. HCl-Treated ITO in P3HT:PCBM-Based Bulk-Heterojunction Organic Photovoltaic Devices

In studies to simplify the fabrication of bulk-heterojunction organic photovoltaic (OPV) devices, it was found that when glass/tin-doped indium oxide (ITO) substrates are treated with dilute aqueous HCl solutions, followed by UV ozone (UVO), and then used to fabricate devices of the structure glass/ITO/P3HT:PCBM/LiF/Al, device performance is greatly enhanced. Light-to-power conversion efficiency (Eff) increases from 2.4% for control devices in which the ITO surface is treated only with UVO to 3.8% with the HCl + UVO treatment--effectively matching the performance of an identical device having a PEDOT:PSS anode interfacial layer. The enhancement originates from increases in V(OC) from 463 to 554 mV and FF from 49% to 66%. The modified-ITO device also exhibits a 4x enhancement in thermal stability versus an identical device containing a PEDOT:PSS anode interfacial layer. To understand the origins of these effects, the ITO surface is analyzed as a function of treatment by ultraviolet photoelectron spectroscopy work function measurements, X-ray photoelectron spectroscopic composition analysis, and atomic force microscopic topography and conductivity imaging. Additionally, a diode-based device model is employed to further understand the effects of ITO surface treatment on device performance.

Role of tungsten oxide in inverted polymer solar cells

Tungsten oxide (WO3) was inserted as an anode interfacial layer between the photoactive layer and top electrode in inverted polymer solar cells (PSCs) with nanocrystalline titanium dioxide as an electron selective layer. The device with WO3 exhibited a remarkable improvement in power conversion efficiency compared with that without WO3, which indicated that WO3 efficiently prevented the recombination of charge carriers at the organic/top electrode interface. The dependence of the device performances on WO3 film thickness and different top metal electrodes was investigated. Transparent inverted PSCs with thermally evaporable Ag/WO3 as a transparent anode were also investigated when introducing a WO3 buffer layer.

High-efficiency hole extraction/electron-blocking layer to replace poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) in bulk-heterojunction polymer solar cells

An anode interfacial layer is reported for bulk-heterojunction (BHJ) polymer solar cells to replace the commonly used poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). A poly[9,9-dioctylfluorene-co-N-[4-(3-methylpropyl)]-diphenylamine] (TFB)+4,4′-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPDSi2) blend is crosslinked, forming robust ∼10nm thick films covalently bound to indium tin oxide, which transport holes while blocking misdirected electrons. The thermal stability and photovoltaic performance metrics of TFB:TPDSi2-modified BHJ cells are significantly greater than those of cells fabricated in parallel with PEDOT:PSS or with no interfacial layer. For a poly[2-methoxy-5-(3′,7′-dimethyloctyloxyl]-1,4-phenylene vinylene: methanofullerene[6 6]-phenyl C61-butyric acid methyl ester cell, Voc=0.89V, Jsc=4.62mA∕cm2, FF=54.4%, and ηp=2.23%.

Characteristics of Polymer Light Emitting Diodes with the LiF Anode Interfacial Layer

Electrical and electroluminescent characteristics of poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV)-based polymer light emitting diodes (PLEDs) with the LiF anode interfacial layer were investigated. Current–voltage measurement and impedance analysis showed that LiF insertion affected the electrical properties of PLEDs notably. The maximum luminance efficiency of the PLED device with a LiF anode interfacial layer of 1-nm-thick was 3.0 lm/W, which is higher than 1.97 lm/W of the PLED device without a LiF layer. By inserting LiF as the anode interfacial layer, excess injection of holes from indium tin oxide anode can be reduced and hence the balance between electrons and holes can be increased in the emitting layer to improve device efficiency.