Semitransparent Top Electrode Research Articles

High transparency and performance slot‐die‐coated large‐area polymer solar module

AbstractWe fabricate polymer solar cells (PSCs) and modules by sheet‐to‐sheet slot‐die coating under air environment based on halogen‐free host solvent. How to upscale small‐area PSC to large‐area PSC module is demonstrated here. The opaque module exhibits the most superior performance among the slot‐die‐coated modules reported. The scaling of both the semitransparent top electrode and PSC module is a challenge due to the complex difficulties. In this study, the semitransparent MoO3/Ag/MoO3 top electrode is successfully upscaled to be in the large‐area semitransparent PSC module. Ag layer thickness as reflector in the tri‐layer electrode is used to tune the trade‐off between transmittance, conductivity, and the light absorption in the active layer. The matching combination of transmittance, absorption spectra of MoO3/Ag/MoO3 top electrode, and active layer is a main key of our approach greatly improving photovoltaic and optical performance. The power conversion efficiencies (PCEs) of the semitransparent PSC modules with Ag thickness of 25, 20, and 15 nm are 7.1%, 5.9%, and 4.4%, respectively. The corresponding average visible transmittance (AVT) values are 20.5%, 28.3%, and 36.1%, respectively. The transmittance values at 550 nm are ranging from 24% to 40%. According to the review of all the reported semitransparent modules (>10 cm2), our modules demonstrate the excellent trade‐off performance about two times better than the large‐area semitransparent PSC modules currently reported either based on the similar PCEs or AVTs.

Untwining polar contributions from light-polarization dependent photovoltaic response of LuMnO3-based ferroelectric capacitors

Narrow bandgap ferroelectrics are receiving a renewed interest for photovoltaic applications aiming at exploiting new functionalities arising from bulk photovoltaic effect (BPE). We report on the photovoltaic response of vertical capacitors of ferroelectric hexagonal LuMnO3 films sandwiched between semitransparent top electrodes (Pt, Co, Ti) and a common bottom (Pt) bottom electrode. Our results show that the presence of electrodes, other than their optical transmittance, crucially determines the imprint in the ferroelectric layer and ultimately the sensitivity of short circuit current density (Jsc) to the ferroelectric polarization direction. The use of ultrathin (7 nm) Pt top electrodes allowed to obtain a large Jsc (up to 100 mA/cm2) and an open circuit voltage of Voc ≈ 0.52 V, with a responsivity of 2 × 10−3 A/W. Yet, polarization back-switching due to imprint largely washes out the dependence of Jsc on the direction of the polarization and thus, at first sight, Jsc seemingly appears to be ruled by conventional photovoltaic response. However, a pioneering analysis of the light-polarization dependent photosensitivity, allowed to disentangle for the first time, a genuine contribution of BPE from a ubiquitous Fresnel-like contribution arising from interfaced optical media.

A solvent-compatible filter-transfer method of semi-transparent carbon-nanotube electrodes stacked with silver nanowires

ABSTRACT Low-density films of single-walled carbon nanotubes (SWNTs) can be used as a semi-transparent top electrode for all-solution-processed film devices; however, their semiconductor characteristics vary depending on the experimental factors in their dispersion into solvents, and the sublayers are damaged as a result of solvent incompatibility. In this study, we report a solvent-compatible filter-transfer method for SWNT films stacked with silver nanowires (AgNWs), and evaluate the semiconductor characteristics through the p/n heterojunction with a Si wafer (SWNT/Si). AgNWs and SWNTs were successively filtered through their aqueous dispersion solutions using a membrane filter. The stacked semi-transparent films (AgNW/SWNT films with controlled densities) were successfully transferred onto glass plates and Si wafers. The transmittance at 550 nm revealed a window between 60% and 80% with a narrow sheet resistance range between 11 and 23 Ω □−1. The power conversion efficiency (PCE) of SWNT/Si was improved to 11.2% in a junction area of 0.031 cm2 through the use of spin-coated Nafion resins; however, the accumulated resistance of SWNTs drastically reduced the PCE to 2% as the area increased to ≥0.5 cm2. AgNWs maintained the PCE within a range of 10.7% to 8.6% for an area ranging from 0.031 cm2 to 1.13 cm2. All of the photovoltaic parameters were dependent on the junction areas, suggesting that AgNWs function as an effective current-collector layer on the semiconductor layer of SWNTs without direct contact of AgNWs with the Si surface. In addition, we report a solvent-compatible experiment for transferring AgNW/SWNT films onto a solvent-sensitive perovskite material (CH3NH3PbI3).

Curved Mirror Arrays for Light Extraction in Top-Emitting Organic Light-Emitting Diodes.

The light outcoupling efficiency of a top-emitting organic light-emitting diode (OLED) is only about 20%, and the majority of the light is trapped in the waveguide modes and surface plasmon polariton (SPP) modes. Extracting the trapped modes can reduce the device power consumption and improve the operating lifetime. In this study, we demonstrate a top-emitting OLED structure with a dielectric spacer to suppress the SPP mode and with a patterned back mirror to extract the waveguide modes. We examine and compare several curved mirror arrays and conclude that a micromirror array (μMA) can efficiently extract the waveguide modes while minimizing the absorption loss. The optimized μMA device with a semi-transparent top electrode shows a 36% external quantum efficiency, 2 times higher than the referenced device. This optical design can be easily incorporated into a top-emitting device and has a great potential for displays and lighting applications.

Low-Temperature Solution-Processed All Organic Integration for Large-Area and Flexible High-Resolution Imaging

A facile blade-coating process is developed for large area deposition of uniform thick organic active layers in organic photodiodes (OPDs). Large-area semi-transparent top metal electrodes are thermally evaporated with an optimal deposition rate to achieve good balance between transparency and conductivity for top illumination integration structure with the organic thin-film transistor (OTFT) backplane. The maximum process temperature of the OPD is 85∘C, so that the performance of the OTFT underneath is not affected. Based on the developed integration structure and processes, an all-organic integrated flexible active-matrix imager is developed, having the largest size (130 mm ×130 mm), highest resolution (1536×1536 pixels, 300 ppi) and lowest process temperature (100∘C) reported so far for the OPD based imagers.

Wide Bandgap Polymer with Narrow Photon Harvesting in Visible Light Range Enables Efficient Semitransparent Organic Photovoltaics

AbstractWide bandgap polymer D18 with narrow photon harvesting in visible light range and small molecule N3 with near‐infrared photon harvesting are adopted for building semitransparent organic photovoltaics (OPVs). To find out the optimal D18:N3 weight ratio for semitransparent OPVs, series of opaque OPVs are built with a varied D18:N3 weight ratio. The power conversion efficiency (PCE) and fill factor can be maintained over 16% and 77% in the D18:N3 (0.7:1.6, wt/wt) based opaque OPVs, respectively. The average visible transmittance (AVT) of the corresponding blend films can be achieved over 50%, demonstrating the great potential in fabricating efficient semitransparent OPVs. The semitransparent OPVs based on D18:N3 (0.7:1.6, wt/wt) are fabricated by using 1 nm Au/(10, 15, 20 nm) Ag as cathode. The thickness of Ag layers is varied to balance the optical properties and electrical properties of semitransparent top electrode. The semitransparent OPVs with 10 nm Ag achieve the highest light utilization efficiency of 2.90% with a PCE of 12.91% and an AVT of 22.49%, which should be among the best performance of reported semitransparent OPVs. This work demonstrates that the wide bandgap polymer donor with narrow photon harvesting in visible light range has great potential in preparing efficient semitransparent OPVs.

Quantum Tunneling Induced Optical Rectification and Plasmon-Enhanced Photocurrent in Nanocavity Molecular Junctions.

Molecular junctions offer the opportunity for downscaling optoelectronic devices. Separating two electrodes with a single layer of molecules accesses the quantum-tunneling regime at low voltages (<1 V), where tunneling currents become highly sensitive to local nanometer-scale geometric features of the electrodes. These features generate asymmetries in the electrical response of the junction which combine with the incident oscillating optical fields to produce optical rectification and photocurrents. Maximizing photocurrents requires accurate control of the overall junction geometry and a large confined optical field in the optimal location. Plasmonic nanostructures such as metallic nanoparticles are prime candidates for this application, because their size and shape dictate a consistent junction geometry while strongly enhancing the optical field from incident light. Here we demonstrate a robust lithography-free molecular optoelectronic device geometry, where a metallic nanoparticle on a self-assembled molecular monolayer is sandwiched between planar bottom and semitransparent top electrodes, to create molecular junctions with reproducible morphology and electrical response. The well-defined geometry enables predictable and intense plasmonic localization, which we show creates optical-frequency voltages ∼ 30 mV in the molecular junction from 100 μW incident light, generating photocurrent by optical rectification (>10 μA/W) from only a few hundred molecules. Quantitative agreement is thus obtained between DC- and optical-frequency quantum-tunneling currents, predicted by a simple analytic equation. By measuring the degree of junction asymmetry for different molecular monolayers, we find that molecules with a large DC rectification ratio also boost zero-bias electrical asymmetry, making them good candidates for sensing and energy harvesting applications in combination with plasmonic nanomaterials.

Bifacial Color-Tunable Electroluminescent Devices.

Alternate current electroluminescent (ACEL) devices provide a range of interesting properties, such as facile large-area processability, mechanical flexibility, and outstanding resilience, when compared with other large-area light-emitting technologies. To widen the scope of possible applications for ACEL devices, color tunability and white light emission are desirable. Here, we introduce a novel three-terminal device architecture based on two monolithically stacked ACEL devices (e.g., orange and blue) that allows for color tunability via independent operation of the subdevices. The tandem devices comprise semitransparent bottom and top electrodes based on networks of silver nanowires, which endow the tandem ACEL device with bifacial Janus-type emission. We provide a detailed analysis of the sources of optical losses in single and tandem ACEL devices. Our novel device concept enables novel facets of applications for ACEL in signage and lighting.

Flexible Organic Photovoltaics with Colorful Semi-Transparent Metal/Dielectric/Metal Top Electrode

Organic solar cells (OSCs) have the advantages of a semi-transparent thin film, and of changing colors through controlling optical energy bandgap of organic semiconducting materials. In addition, the flexible OSCs can also be fabricated to be applied in multiple applications such as film-type or foldable solar cells. Here, we have fabricated flexible semi-transparent organic solar cells (STOSCs) with high transparency accompanying highly vivid red, green, and blue colors. To gain high transparency of STOSCs, we employed Fabry-Pérot etalon-type electrodes. An antimony oxide (Sb2O3) cavity layer was introduced between two thin silver layers. By adjusting the thickness of Sb2O3 from 45 nm to 100 nm, vivid colors of red, green, and blue semi-transparent top electrodes were achieved. Besides, selecting well-matched photoactive layers for each color, a less drop of short-circuit current density (Jsc) has been observed. Finally, we obtained high maximum transmittance of 22.52%, 30.10%, and 22.13%, with a power conversion efficiency of 9.16%, 9.85%, and 12.64% for red, green, and blue STOSCs, respectively. Likewise, a power conversion efficiency of 5.75% was achieved with a green-colored large area sub-module.

Optical outcoupling in monochromatic top-emitting organic light-emitting diodes with an Au/Ag semi-transparent electrode

Top-emitting organic light-emitting diodes can achieve high efficiencies due to the strong cavity effect resulting from the relatively thick semi-transparent metallic top electrode. The strong cavity resonance, however, simultaneously brings along negative side effects such as pronounced angular-dependent emission and spectral narrowing. In this work, through numerical simulations, we demonstrate that top-emitting organic light-emitting diodes using a thin Au(2 nm)/Ag(7 nm) top electrode can achieve light-outcoupling efficiency comparable to a thick silver electrode, while reducing spectral narrowing. This can be realized by tuning the organic capping layer thickness without affecting the electrical properties, which can be applied to diodes based on either intrinsic or efficiently doped charge transport layers.

A critical review on semitransparent organic solar cells

Semitransparent solar cells not only can generate electricity but also exhibit promising application in facades and roofs of building, as well as the coatings of vehicle. Ideal semitransparent solar cells should have a high utilization efficiency of ultraviolet and near infrared photons, and maintain an appropriate balance between absorption and transparency in visible light range. Organic semiconducting materials with adjustable optical band gap own great potential in preparing highly efficient semitransparent organic solar cells (OSCs). There are great challenges in achieving semitransparent electrodes with good conductivity and high transmittance in visible light range, as well as high reflectance in ultraviolet and near infrared range. In this critical review, we summarize the recent progress of semitransparent OSCs, concentrating on organic semiconducting materials, semitransparent top electrode and device engineering. Semitransparent OSCs have attracted more and more attention due to its great potential in energy source in door, electricity-generating windows, building-integrated photovoltaics and agricultural greenhouse. Semitransparent OSCs have attracted more and more attention due to its great potential in electricity-generating windows, building-integrated photovoltaics and agricultural greenhouse. This review comprehensively summarizes recent progresses of organic material, semitransparent top electrode and device engineering, the challenges on achieving semitransparent OSCs are highlighted to promote the development of this field. • Recent development of organic materials with strong ultraviolet or near infrared absorption were summarized. • The key parameters for evaluating the performance of semitransparent OSCs were discussed. • This review comprehensively summarized recent progress of transparent top electrodes. • The light management methods and device designs are firstly comprehensively reported.

Impermeable Charge Transport Layers Enable Aqueous Processing on Top of Perovskite Solar Cells

AbstractSeveral applications of perovskite solar cells (PSCs) demand a semitransparent top electrode to afford top‐illumination or see‐through devices. Transparent conductive oxides, such as indium tin oxide (ITO), typically require postdeposition annealing at elevated temperatures, which would thermally decompose the perovskite. In contrast, silver nanowires (AgNWs) in dispersions of water would be a very attractive alternative that can be deposited at ambient conditions. Water is environmentally friendly without safety concerns associated with alcohols, such as flammability. Due to the notorious moisture sensitivity of lead‐halide perovskites, aqueous processing of functional layers, such as electrodes, on top of a perovskite device stack is elusive. Here, impermeable electron transport layers (ETLs) are shown to enable the deposition of semitransparent AgNW electrodes from green aqueous dispersions on top of the perovskite cell without damage. The polyvinylpyrrolidone (PVP) capping agent of the AgNWs is found to cause a work–function shift and an energy barrier between the AgNWs and the adjacent ETL. Thus, a high carrier density (≈1018 cm−3) in the ETL is required to achieve well‐behaved J/V characteristics free of s‐shapes. Ultimately, semitransparent PSCs are demonstrated that provide an efficiency of 17.4%, which is the highest efficiency of semitransparent p‐i‐n perovskite solar cells with an AgNW top electrode.

A solution processed Ag-nanowires/C60 composite top electrode for efficient and translucent perovskite solar cells

All solution-processed semi-transparent perovskite solar cells (PSCs) with high efficiency were fabricated with Ag nanowire (AgNW) top electrode directly coated on the organic electron transport layer (ETL). A unique AgNW/ETL interface morphology was attained by the fabrication of the AgNW-C60 composite electrode through a sequential spin-coating method. The C60 filler spin-coated on the AgNW network engendered a high contact area and favourable energy level alignment at the interface between the ETL and AgNW. As a result, devices based on AgNW-C60 electrode far outperformed devices with pristine AgNW electrodes. By adjusting the thickness of the AgNW film, the AgNW-C60 devices attained a PCE of 11.02%. The incorporation of C60 into the AgNW electrode also led to remarkable stability improvement. The low-temperature solution-processed and semi-transparent AgNW-C60 top electrode is also applicable to other PSC structures with various interfacial layers, thus showing a promising path towards fully-printable, flexible and translucent photovoltaic devices.

The Use of an rGO Semi-transparent Organic Electrode in a ZnO Schottky Diode for UV Detection

Successful manufacture of a simple, low cost sprayed Schottky diode compatible with printed electronics using reduced graphene oxide (rGO) as a semi-transparent top electrode and zinc oxide (ZnO) as active layer has been reported in this paper. The rGO was synthesized using modified Hummers’ method and deposited by spray coating. The transmittance analyses of rGO films shows 62.3% transparency for ultraviolet (UV) light and by the Van der Pauw method shows sheet resistance of 19 kΩ/□. The rGO electrode provides an alternative to expensive metallic electrodes such as Au, Pt, Pd and Ag which are not ecofriendly. Furthermore, rGO performs Schottky contact with sprayed zinc oxide film resulting in a diode with the sequence of deposited layers Al/ZnO/rGO and low turn-on voltage that has current–voltage characteristics whose rectifying behavior is affected by the incidence of UV light resulting in a possibility to use that diode as photodetector for this kind of radiation.

Efficient semi-transparent perovskite solar cells with a novel indium zinc tin oxide top electrode grown by linear facing target sputtering

We propose that an indium zinc tin oxide electrode grown by linear facing target sputtering without additional thermal treatment can effectively act as an efficient semi-transparent top electrode for high-performance semi-transparent hybrid perovskite solar cells. The semi-transparent perovskite solar cells using an indium zinc tin oxide electrode as a novel top electrode achieves a power conversion efficiency of 12.85%, similar to that of an opaque electrode-based device of 13.48%. More importantly, by adjusting the perovskite thickness, a power conversion efficiency of 8.306% is obtained at an average visible transmittance of 33.9%, thus well supporting that the indium zinc tin oxide electrode can be considered to be an advanced and efficient semi-transparent top electrode for the fabrication of building integrated photovoltaics with high efficiency and good transparency.

Thin metal top electrode and interface engineering for efficient and air-stable semitransparent perovskite solar cells

To find promising device architecture for air-stable as well as efficient semitransparent perovskite solar cells (PeSCs), Ag & Cu semitransparent top electrode and interfacial engineering technologies are explored in PeSCs consisting with indium tin oxide (ITO)/hole transporting layer (HTL)/CH3NH3PbI3/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM)/Ag or Cu. Devices with NiOx HTL exhibits better performance than conventional poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) based devices and modification of PCBM with polyethylenimine ethoxylated (PEIE) also induces improved performance. Optimized semitransparent PeSC with Cu shows superior performance, achieving a power conversion efficiency (PCE) of 11.95% and average visible transmittance (AVT) of ∼20%, and stability to Ag based device.

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.

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.

Probing the surface potential of oxidized silicon by assessing terahertz emission

Using laser terahertz emission microscopy, we measured laser-excited terahertz (THz) emission from silicon wafers with silicon-oxide passivation layers, revealing a strong correlation between the THz waveform and the surface potential. The surface potential was electrically tuned by a semitransparent top electrode disc and evaluated by measuring capacitance–voltage characteristics. The waveform changed with external bias and inverted near the flatband voltage, and changes appeared in the peak amplitude were similar to the capacitance–voltage characteristics. These results indicate that by analyzing the waveform of laser-excited THz emission generated by laser terahertz emission microscopy, we could quantitatively measure and map the internal field of surface band bending in semiconductors.