Semitransparent Electrode Research Articles

Synergetic light trapping effects in organic solar cells with a patterned semi-transparent electrode.

The dielectric/ultra-thin metal/dielectric structure has been widely used for semi-transparent electrodes in organic solar cells (OSCs) due to its potential replacement of the transparent conductive oxide indium tin oxide. Here, we introduce the dual light trapping structures, i.e. the nanopatterned MoO3/Ag/MoO3 (MAM) as the anode and a short-pitched metallic grating as the cathode, to cooperatively improve the OSC performance. The optical and electrical properties of the OSCs have been investigated via solving the coupled Maxwell and semiconductor equations by the finite-difference method. The results indicate that the optical light absorption of the active layer and the electrical carrier collection have been significantly enhanced after the adoption of the proposed dual light trapping structures. We have shown that the optical and electrical improvements are attributed to the synergetic effects of surface plasmon resonance of the grating patterned cathode and scattering effect of the nanopatterned MAM anode. Our results have further revealed that the short-pitched metal grating can induce considerable field confinement due to the mode coupling and hybridization of the surface plasmons in-between the adjacent short-distance metal strips. With the optimized structural parameters of the dual light trapping structure, the power conversion efficiency (PCE) of the OSCs has been substantially enhanced by 39% in comparison with a flat cell. Besides the efficiency improvement, the OSCs with the proposed dual light harvesting structures reveal an alleviated angular dependence of electrical properties as the light is beyond the normal incident angle. Our results contribute to the further development of ITO-free OSCs and are promising for semi-transparent optoelectronics.

Evaluation of the potential distribution in a multiple stacked Si quantum dots structure by hard X-ray photoelectron spectroscopy

Six-fold stacked Si quantum dots (Si-QDs) structures with ultrathin SiO2 interlayers were formed on ultrathin SiO2 layer/n-Si substrates by low-pressure chemical vapor deposition using a SiH4 gas and their vertical electric potential distributions were evaluated using hard X-ray photoelectron spectroscopy under a DC bias application to the semitransparent electrodes formed on the stacked Si-QDs structure. The Si1s photoelectron spectra due to Si–Si bonding units can be deconvoluted into seven components corresponding to the six Si-QDs layers and Si substrate. Obviously, the energy shift between the components for two adjacent dots layers becomes larger towards the upper side of the stacked dots structure. This result indicates that the electric field concentrates on the upper side and is consistent with the proposed model that can explain the ballistic electron emission characteristics from multiple stacked Si-QDs structures.

Optoelectric charging-discharging of Ge nanocrystals in floating gate memory

Photo-induced effects on charging and discharging of nanocrystals (NCs) in capacitor memories with Ge NCs in an HfO2 matrix as a floating gate layer are studied. The sequence of layers HfO2/Ge-HfO2/HfO2 was deposited on a p-Si substrate using magnetron sputtering. Well separated Ge NCs are obtained after rapid thermal annealing at 600 °C. The optoelectric capacitor memories were fabricated with a semi-transparent electrode on top of the structure and an Al electrode on the back side of the Si substrate. Light illumination effects on hysteresis curves were investigated using different operation modes. The hysteresis window increases by increasing the light exposure time. The spectral dependence of the hysteresis window shows the maximum contribution of the light in the wavelength range of 950–1000 nm, corresponding to both contributions from the Si substrate and from Ge NCs. The stored information about the electrical and optical pulses is also investigated in the regime of the flat band potential measurements (retention measurements). It is shown that in our memory structure, the photo-effect on the memory retention corresponds to a tunnelling transfer of negative charges from the Si substrate to Ge NCs, up to a mean value of 1.6 electrons per NC.

Semitransparent ternary nonfullerene polymer solar cells exhibiting 9.40% efficiency and 24.6% average visible transmittance

Efficient semitransparent ternary polymer solar cells (PSCs) are fabricated with narrow band gap material PTB7-Th as donor, two narrow band gap nonfullerene materials BDTThIT-4F and IEICO-4F as one alloyed acceptor. Firstly, the content of IEICO-4F in acceptors is optimized in opaque PSCs with 100 nm Ag as electrode, power conversion efficiency (PCE) of the optimized opaque PSCs arrives to 12.03% with 50 wt% IEICO-4F in the alloyed acceptor. The 1 nm Au/10 nm Ag is designed to replace 100 nm Ag as semitransparent electrode. The semitransparent electrode exhibits high transmittance in visible light range and low transmittance in long wavelength range, which is beneficial to improve photon harvesting of active layers prepared with narrow band gap materials. The semitransparent ternary PSCs exhibit a PCE of 9.40% with 24.6% average visible transmittance from 370 nm to 740 nm, which should be among the highest values for semitransparent PSCs. This work indicates that ternary strategy may have great potential in preparing efficient semitransparent PSCs with two narrow band gap nonfullerene materials.

Impact of selective reflectance in a microcavity-based organic light-emitting diode

A practical correlation between selective reflectance of the semitransparent electrode and the color coordinates of light emitting from the organic light-emitting diode (OLED) has been presented in this paper. OLED technology has a great potential to substitute other technologies for lighting and display applications. For full-color displays, use of color filters is the most acceptable technique to convert a white light into primary colors (red, green, and blue). In a microcavity design, it is possible to control the emission color of an OLED by incorporating an active layer in between two metal mirrors. A careful design of a microcavity makes possible the realization of mixed colors in a single OLED structure. In this work, a metal microcavity has been designed with an organic stack, sandwiched between two metal mirrors, a fully reflective mirror-aluminum (Al) and semitransparent metal mirror-silver (Ag). The Commission International de l’Eclairage (CIE) color coordinates of a blue fluorescent (460 nm) OLED was tuned from (1.85, 1.54) to (2.36, 2.63) by selective reflectance.

Thickness-dependent photoelectrochemical properties of a semitransparent Co3O4 photocathode

Co3O4 has been widely studied as a catalyst when coupled with a photoactive material during hydrogen production using water splitting. Here, we demonstrate a photoactive spinel Co3O4 electrode grown by the Kirkendall diffusion thermal oxidation of Co nanoparticles. The thickness-dependent structural, physical, optical, and electrical properties of Co3O4 samples are comprehensively studied. Our analysis shows that two bandgaps of 1.5 eV and 2.1 eV coexist with p-type conductivity in porous and semitransparent Co3O4 samples, which exhibit light-induced photocurrent in photoelectrochemical cells (PEC) containing the alkaline electrolyte. The thickness-dependent properties of Co3O4 related to its use as a working electrode in PEC cells are extensively studied and show potential for the application in water oxidation and reduction processes. To demonstrate the stability, an alkaline cell was composed for the water splitting system by using two Co3O4 photoelectrodes. The oxygen gas generation rate was obtained to be 7.17 mL·h−1 cm−1. Meanwhile, hydrogen gas generation rate was almost twice of 14.35 mL·h−1·cm−1 indicating the stoichiometric ratio of 1:2. We propose that a semitransparent Co3O4 photoactive electrode is a prospective candidate for use in PEC cells via heterojunctions for hydrogen generation.

Screen-printing of flexible semi-transparent electrodes and devices based on silver nanowire networks

Silver nanowire networks have demonstrated significant potential as semi-transparent electrodes for various applications. However, for their widespread utilisation in devices, upscaled coating technologies such as screen-printing need to be explored and related to this, the formulation of suitable inks is indispensable. This work contributes to this effort by the synthesis of Ag-NW based formulations. The rheological characteristics that are essential for screen-printing are obtained by the addition of hydrophobically modified cellulose. The electrical and optical characteristics of screen-printed features on PET are compared by a Van der Pauw method and UV–vis spectroscopy. Despite the presence of the cellulose additive, the screen-printed electrodes exhibit a transmittance from 92.8% to 57.3% and a sheet resistance down to 27 Ohm sq−1. Based on the percolation theory in composites, a mathematical expression is presented, which allows the in-depth analysis of the resulting opto-electrical properties. The application potential of the nanowire-containing formulations is finally demonstrated by screen-printing functional, flexible electroluminescent devices.

Low-cost semi-transparent copper sulfide electrode for indium-tin-oxide-free perovskite solar cells

Development of transparent electronics has been hindered by the lack of stable p-type transparent conductive electrodes. Herein we report the fabrication of p-type copper sulfide thin films (CuxS) used for the first time as semitransparent electrode used as an example in inverted-planar perovskite solar cells (PSCs). The resulting hole-transporting-material-free and indium-tin-oxide-free (ITO-free) device showed an unprecedented 5.9% efficiency (PCE). Morphological, structural and optoelectronic analysis of the CuxS films were accomplished to correlate the photovoltaic behavior of the obtained PSCs. In particular, covellite (CuS) and digenite (Cu1.8S) crystal phases were identified in the resulting CuxS film. The p-type CuxS semitransparent electrode exhibited high 1094 S cm−1 electrical conductivity comparable to state-of-the-art transparent conductive electrodes (TCE). Although the CuxS electrode exhibits a relatively low bandgap around 2.7 eV, the spectral response of the devices demonstrates that the related optical loss happens just in the blue and UV region of the visible spectrum leading to a reduction of the photocurrent below 10%. On the other hand, surface photovoltage analysis of the grown perovskite indicates that the CuxS electrode induces an optimal band alignment in the perovskite surface while trap states were favored by using CuxS regarding ITO. These results suggest that photovoltaic parameters of CuxS-based PSCs can be still improved by a better control of perovskite growing. Finally, material cost per square meter (US$/m2) was estimated for CuxS electrodes and an impressive reduction of 77.6% was observed compared to commercial ITO. These results highlight the potential application of CuxS film as low-cost, non-toxic and scalable p-type semitransparent electrode in PSCs and other optoelectronic devices.

The Effect of Metal in MoOx/Metal/MoOx Anode on Open Circuit Voltage in Organic Photovoltaic Cells

Archetypal photovoltaic cells are fabricated in order to elucidate the effect of a metal embedded in a semitransparent trilayer electrode on photovoltage. A series of electrodes are formed by thermal deposition of MoOx, metal and MoOx layers, in said succession (MoOx = substoichometric molybdenum trioxide, metal = Au, Cu, Ag, Al, In, Mg, Yb). The parameters of the Schottky‐type cells consisting of thus made bottom anode, a subphthalocyanine boron chloride (SubPc) photoabsorber and a bilayer LiF/Al top cathode are measured and compared to those of the reference cell with an ITO/MoOx anode. Photovoltage does not correlate with the work function of a metal in the anode, while the photocurrent is inversely proportional to the anode sheet resistance. The highest efficiency is achieved with the silver interlayer, but the reference cell with ITO outperforms any cell with a trilayer anode. Then, comparison is made with a series of cells, in which the metal (Cr, Ni, Ag, Al, and Ta) in anode is deposited by a dc magnetron sputtering. In these cells, the photovoltage rises monotonically with the metal work function. Magnetron sputtered nickel in the anode provides the best cell performance, closely approaching that of the reference cell.

61‐2: Weakening Micro‐Cavity Effects in White Top‐Emitting WOLEDs with Semitransparent Metal Top Electrode

Top emitting white organic light emitting diodes (WOLEDs) hold great promise for high brightness and high resolution active matrix OLED displays. One key challenge of high‐performance top emitting WOLEDs is the availability of top electrodes with superior optical, electrical properties. Herein, we demonstrate the improvement of both electrical and optical properties of widely used metal electrode, which is achieved by co‐depositing a proper amount of magnesium (Mg) with silver (Ag) deposition with optimized deposition rate of both Mg and Ag. Corresponding WOLEDs are fabricated using this optimized 80Å thick Mg:Ag electrode, which show a 14.73% enhanced current efficiency at 10mA/cm2 compared to their 120Å thick Mg:Ag electrode counterparts. Also, the 80Å thick Mg:Ag based WOLEDs show close‐to‐identical emission spectra at different view angles and color shift.

Semitransparent carbon microelectrodes for opto- and electrophysiology

Transparent microelectrodes can enable electrophysiology with simultaneous optical stimulation or imaging directly at the electrode site. Owing to their high surface area, chemical and mechanical stability and biocompatibility, carbon nanomaterials are promising candidates for such electrodes, but the challenge of balancing transparency and impedance must be solved. In this work, carbon nanostructures were deposited on transparent substrates by low temperature thermal chemical vapor deposition. Scanning electron microscopy (SEM) and Raman spectroscopy confirmed the presence of carbon. The nanostructures were deposited on 30 µm-diameter microelectrodes with indium tin oxide leads. Optical transmittance measurements demonstrated up to 40% transparency of carbon layers. Impedance spectroscopy confirmed impedance values in line with standard opaque microelectrodes. Electrophysiology with cultured chicken ventricular myocytes using these semitransparent electrodes matched conventional microelectrode array performance with signals up to 3 mV and noise of 20 µV peak-to-peak. Ultrastructure of the cell–electrode interface revealed by focused ion beam cross-sectioning and SEM imaging showed stability of the carbon electrodes after cell culture. Altogether, a robust fabrication procedure for semitransparent carbon electrodes was demonstrated, and first results showed their suitability for simultaneous electrophysiology and optical investigation.

Efficient Bifacial Semitransparent Perovskite Solar Cells Using Ag/V2O5 as Transparent Anodes.

Bifacial semitransparent inverted planar structured perovskite solar cells (PSCs) based on Cs0.05FA0.3MA0.7PbI2.51Br0.54 using an Ag thin film electrode and V2O5 optical coupling layer are investigated theoretically and experimentally. It is shown that the introduction of the cesium (Cs) ions in the perovskite could obviously improve the device performance and stability. When only the bare Ag film electrode is used, the PSCs show a bifacial performance with the power conversion efficiency (PCE) of 14.62% illuminated from the indium tin oxide (ITO) side and 5.45% from the Ag film side. By introducing a V2O5 optical coupling layer, the PCE is enhanced to 8.91% illuminated from the Ag film side, which is 63% improvement compared with the bare Ag film electrode, whereas the PCE illuminated from the ITO side remains almost unchanged. Moreover, when a back-reflector is employed, the PCE of device could be further improved to 15.39% by illumination from the ITO side and 12.44% by illumination from the Ag side. The devices also show superior semitransparent properties and exhibit negligible photocurrent hysteresis, irrespective of the side from which the light is illuminated. In short, the Ag/V2O5 double layer is a promising semitransparent electrode due to its low cost and simple preparation process, which also point to a new direction for the bifacial PSCs and tandem solar cells.

Squaraine Dye for a Visibly Transparent All-Organic Optical Upconversion Device with Sensitivity at 1000 nm.

Efficient light detection in the near-infrared (NIR) wavelength region is central to emerging applications such as medical imaging and machine vision. An organic upconverter (OUC) consists of a NIR-sensitive organic photodetector (OPD) and an visible organic light-emitting diode (OLED), connected in series. The device converts NIR light directly to visible light, allowing imaging of a NIR scene in the visible. Here, we present an OUC composed of a NIR-selective squaraine dye-based OPD and a fluorescent OLED. The OPD has a peak sensitivity at 980 nm and an internal photon-to-current conversion efficiency of ∼100%. The OUC conversion efficiency (0.27%) of NIR to visible light is close to the expected maximum. The materials of the OUC multilayer stack absorb very little light in the visible wavelength range. In combination with an optimized semitransparent metal top electrode, this enabled the fabrication of transparent OUCs with an average visible transmittance of 65% and a peak transmittance of 80% at 620 nm. Visibly transparent OUCs are interesting for window-integrated electronic circuits or imaging systems that allow for the simultaneous detection of directly transmitted visible and NIR upconverted light.

Enhanced device performances of a new inverted top-emitting OLEDs with relatively thick Ag electrode.

To improve the device performances of top-emitting organic light emitting diodes (TEOLEDs), we developed a new inverted TEOLEDs structure with silver (Ag) metal as a semi-transparent top electrode. Especially, we found that the use of relatively thick Ag electrode without using any carrier injection layer is beneficial to realize highly efficient device performances. Also, we could insert very thick overlying hole transport layer (HTL) on the emitting layer (EML) which could be very helpful to suppress the surface plasmon polariton (SPP) coupling if it is applied to the common bottom-emission OLEDs (BEOLEDs). As a result, we could realize noteworthy high current efficiency of approximately ~188.1 cd/A in our new inverted TEOLEDs with 25 nm thick Ag electrode.

2D Single-Crystalline Copper Nanoplates as a Conductive Filler for Electronic Ink Applications.

Large-scale 2D single-crystalline copper nanoplates (Cu NPLs) are synthesized by a simple hydrothermal method. The combination of a mild reductant, stabilizer, and shape modifier allows the dimensional control of the Cu nanocrystals from 1D nanowires (NWs) to 2D nanoplates. High-resolution transmission electron microscopy (HR-TEM) reveals that the prepared Cu NPLs have a single-crystalline structure. From the X-ray photoelectron spectroscopy (XPS) analysis, it is found that iodine plays an important role in the modification of the copper nanocrystals through the formation of an adlayer on the basal plane of the nanoplates. Cu NPLs with an average edge length of 10 μm are successfully synthesized, and these Cu NPLs are the largest copper 2D crystals synthesized by a solution-based process so far. The application of the metallic 2D crystals as a semitransparent electrode proves their feasibility as a conductive filler, exhibiting very low sheet resistance (0.4 Ω ▫-1 ) compared to Cu NWs and a transmittance near 75%. The efficient charge transport is due to the increased contact area between each Cu NPL, i.e., so-called plane contact (2D electrical contact). In addition, this type of contact enhances the current-carrying capability of the Cu NPL electrodes, implying that the large-size Cu NPLs are promising conductive fillers for printable electrode applications.

Enahnced Al0.11Ga0.89N/Cu2O Solar Cells by a Semi Transparent Electrode

Polycrystalline Cu2O thin films are prepared on c‐sapphire substrates and n‐type templates by radio frequency sputtering at a temperature of 650 K employing a metallic target, an argon/oxygen gas mixture as well as a biased semi transparent electrode placed between the target and the substrate. Introducing that grid, we demonstrate the possibility to tune the cuprous oxide thin film properties, especially the morphology, by controlling the electron temperature in the plasma as well as the beneficial effect of a suspected reduction of interface state densities at photovoltaic heterostructures.

UV‐Sensing Semitransparent Organic Field‐Effect Transistors with Wide Bandgap Small Molecular Channel and Polymeric Gate‐Insulating Layers

AbstractThis study demonstrates UV‐sensing semitransparent organic field‐effect transistors (OFETs) with wide bandgap small molecular channel and polymeric gate‐insulating layers. N,N′‐di(1‐naphthyl)‐N,N′‐diphenyl‐(1,1′‐biphenyl)‐4,4′‐diamine (NPB) is employed as the wide bandgap channel layer, while poly(methyl methacrylate) is introduced as the wide bandgap gate‐insulating layer. The performance of OFETs is optimized by NPB thickness control and thermal treatment. Results show that the best device performance (on/off ratio = 4.7 × 106 and hole mobility = 4.2 × 10−5 cm2 V−1 s−1) is achieved by thermal treatment of the 130 nm thick NPB layers at 70 °C for 30 min leading to a noticeably changed surface morphology in the NPB layers. The optimized OFETs exhibit excellent operation stability without hysteresis, while those with semitransparent silver electrodes deliver quite a good transparency. The semitransparent OFETs can sensitively detect a UV light with high stability even though no photoresponse is measured under a visible light.

ITO with embedded silver grids as transparent conductive electrodes for large area organic solar cells

In this work, development of semi-transparent electrodes for efficient large area organic solar cells (OSCs) has been demonstrated. Electron beam evaporated silver grids were embedded in commercially available ITO coatings on glass, through a standard negative photolithography process, in order to improve the conductivity of planar ITO substrates. The fabricated electrodes with embedded line and square patterned Ag grids reduced the sheet resistance of ITO by 25% and 40%, respectively, showing optical transmittance drops of less than 6% within the complete visible light spectrum for both patterns. Solution processed bulk heterojunction OSCs based on PTB7:[70]PCBM were fabricated on top of these electrodes with cell areas of 4.38 cm2, and the performance of these OSCs was compared to reference cells fabricated on pure ITO electrodes. The Fill Factor (FF) of the large-scale OSCs fabricated on ITO with embedded Ag grids was enhanced by 18% for the line grids pattern and 30% for the square grids pattern compared to that of the reference OSCs. The increase in the FF was directly correlated to the decrease in the series resistance of the OSCs. The maximum power conversion efficiency (PCE) of the OSCs was measured to be 4.34%, which is 23% higher than the PCE of the reference OSCs. As the presented method does not involve high temperature processing, it could be considered a general approach for development of large area organic electronics on solvent resistant, flexible substrates.

Growth of aligned polypyrrole acicular nanorods and their application as Pt‐free semitransparent counter electrode in dye‐sensitized solar cell

Polypyrrole films on fluorine doped tin oxide (FTO)‐coated glass substrate were prepared in situ by placing FTO/glass substrates where pyrrole was polymerized by methyl orange‐ferric chloride complex. The atomic force microscopy image indicated growth of acicular nanorods of polypyrrole. These films exhibited catalytic activity towards I3−/I− redox couple and have been investigated for counter electrode application in dye‐sensitized solar cell (DSSC). The fabricated DSSC with N719 dye/TiO2 as photoanode, and PPy/FTO as counter electrode shows ~1.7% efficiency.

Cr/ITO semi-transparent n-type electrode for high-efficiency AlGaN/InGaN-based near ultraviolet light-emitting diodes

We investigated the electrical performance of near ultraviolet (NUV) (390 nm) light-emitting diodes (LEDs) fabricated with various semi-transparent Cr/ITO n-type contacts. It was shown that after annealing at 400 °C, Cr/ITO (10 nm/40 nm) contact was ohmic with a specific contact resistance of 9.8 × 10−4 Ωcm2. NUV AlGaN-based LEDs fabricated with different Cr/ITO (6–12 nm/40 nm) electrodes exhibited forward-bias voltages of 3.27–3.30 V at an injection current of 20 mA, which are similar to that of reference LED with Cr/Ni/Au (20 nm/25 nm/200 nm) electrode (3.29 V). The LEDs with the Cr/ITO electrodes gave series resistances of 10.69–11.98 Ω, while the series resistance is 10.84 Ohm for the reference LED. The transmittance of the Cr/ITO samples significantly improved when annealed at 400 °C. The transmittance (25.8–45.2% at 390 nm) of the annealed samples decreased with increasing Cr layer thickness. The LEDs with the Cr/ITO electrodes exhibited higher light output power than reference LED (with Cr/Ni/Au electrode). In particular, the LED with the Cr/ITO (12 nm/40 nm) electrode showed 9.3% higher light output power at 100 mA than reference LED. Based on the X-ray photoemission spectroscopy (XPS) and electrical results, the ohmic formation mechanism is described and discussed.