ITO‐Free,Small‐Molecule Organic Solar Cells on Spray‐Coated Copper‐Nanowire‐Based Transparent Electrodes

The great potential of solution‐processed metal nanowire networks utilized as a transparent electrode has attracted much attention in the last years. Typically, silver nanowires are applied, although their replacement by more abundant and cheaper materials is of interest. Here, a hydrazine‐free synthesis route for high aspect ratio copper nanowires is used to prepare conductive networks showing an enhanced electrode performance. The network deposition is done with a scalable spray‐coating process on glass and on polymer foils. By a pressing or an annealing step, highly conductive transparent electrodes are obtained, and they reveal transmittance‐resistance values similar to indium tin oxide (ITO) and networks made of silver nanowires. The application potential of the copper nanowire electrodes is demonstrated by integrating them into an evaporated small‐molecule organic solar cell with 3% efficiency.     

Highly Stable Transparent Conductive Silver Grid/PEDOT:PSS Electrodes for Integrated Bifunctional Flexible Electrochromic Supercapacitors

Silver grids are attractive for replacing indium tin oxide as flexible transparent conductors. This work aims to improve the electrochemical stability of silver‐based transparent conductors. A silver grid/PEDOT:PSS hybrid film with high conductivity and excellent stability is successfully fabricated. Its functionality for flexible electrochromic applications is demonstrated by coating one layer of WO₃ nanoparticles on the silver grid/PEDOT:PSS hybrid film. This hybrid structure presents a large optical modulation of 81.9% at 633 nm, fast switching, and high coloration efficiency (124.5 cm² C^?1). More importantly, an excellent electrochemical cycling stability (sustaining 79.1% of their initial transmittance modulation after 1000 cycles) and remarkable mechanical flexibility (optical modulation decay of only 7.5% after 1200 compressive bending cycles) is achieved. A novel smart supercapacitor is presented that functions as a regular energy‐storage device and simultaneously monitors the level of stored energy by a rapid and reversible color variation even at high current charge/discharge conditions. The film sustains an optical modulation of 87.7% and a specific capacitance of 67.2% at 10 A g^?1 compared to their initial value at a current density of 1 A g^?1. The high‐performance silver grid/PEDOT:PSS hybrid transparent films exhibit promising features for various emerging flexible electronics and optoelectronic devices.     

High Performance Flexible Transparent Electrode via One‐Step Multifunctional Treatment for Ag Nanonetwork Composites Semi‐Embedded in Low‐Temperature‐Processed Substrate for Highly Performed Organic Photovoltaics

For ideal flexible transparent electrodes, the features of good electrical/optical properties, low surface roughness, efficient charge transportation, robust electrical stability under simultaneously continuous operation bias, and mechanical bending are critical. Herein, a flexible transparent electrode fulfilling all these features is demonstrated by silver (Ag) nanonetwork composites semi‐embedded in low‐temperature‐processed colorless polyimide (cPI), which shows a figure of merit over 1000 (5.4 Ω sq^?1 sheet resistance and >94% diffused transmission at 550 nm wavelength), extremely smooth topography (<1 nm root‐mean‐square roughness and <3 nm peak‐to‐valley roughness), remarkable bending stability under continuous operation bias, and increased work function favoring the band alignment with typical charge transport layers for efficient devices. These characteristics are attributed to one‐step multifunctional chemical treatment on the composite of Ag nanowires and an example polymer of poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS). The strategic one‐step process simultaneously offers selective welding at nanowires cross junctions to form an Ag nanonetwork, and removing polyvinylpyrrolidone surfactant from Ag nanowires and PSS from PEDOT:PSS. The flexible electrode also favors the residue‐free cPI transfer for applications. Flexible organic solar cells (OSCs) made from the electrode achieve an averaged power conversion efficiency of 14.46% (best, 15.12%), which is the best flexible OSCs reported so far.     

Conductive,Solution‐Processed Dioxythiophene Copolymers for Thermoelectric and Transparent Electrode Applications

Conjugated polymers with high electrical conductivities are attractive for applications in capacitors, biosensors, organic thermoelectrics, and transparent electrodes. Here, a series of solution processable dioxythiophene copolymers based on 3,4‐propylenedioxythiophene (ProDOT) and 3,4‐ethylenedioxythiophene (EDOT) is investigated as thermoelectric and transparent electrode materials. Through structural manipulation of the polymer repeat unit, the conductivity of the polymers upon oxidative solution doping is tuned from 1 × 10^?3 to 3 S cm^?1, with a polymer consisting of a solubilizing alkylated ProDOT unit and an electron‐rich biEDOT unit (referred to as PE₂) showing the highest electrical conductivity. Optimization of the film casting method and screening of dopants result in AgPF₆‐doped PE₂ achieving a high electrical conductivity of over 250 S cm^?1 and a thermoelectric power factor of 7 μW m^?1 K^?2. Oxidized spray cast films of PE₂ are also assessed as a transparent electrode material for use with another electrochromic polymer. This bilayer shows reversible electrochemical switching from a colored charge‐neutral state to a highly transmissive color‐neutral, oxidized state. These results demonstrate that dioxythiophene‐based copolymers are a promising class of materials, with ProDOT–biEDOT serving as a soluble analog to the well‐studied PEDOT as a p‐type thermoelectric and electrode material.     

Extremely Flexible Transparent Conducting Electrodes for Organic Devices

Extremely flexible transparent conducting electrodes are developed using a combination of metal‐embedding architecture into plastic substrate and ultrathin transparent electrodes, which leads to highly transparent (optical transmittance ≈93% at a wavelength of 550 nm), highly conducting (sheet resistance ≈13 Ω □^?1), and extremely flexible (bending radius ≈ 200 μm) electrodes. The electrodes are used to fabricate flexible organic solar cells and organic light‐emitting diodes that exhibit performance similar or superior to that of devices fabricated on glass substrates. Moreover, the flexible devices do not show degradation in their performance even after being folded with a radius of ≈200 μm.     

Nanowire‐Based Three‐Dimensional Transparent Conducting Oxide Electrodes for Extremely Fast Charge Collection

A 3D transparent conducting oxide (3D‐TCO) has been fabricated by growing Sn‐doped indium oxide (ITO) nanowire arrays on glass substrates via a vapor transport method. The 3D TCO charge‐collection properties have been compared to those of conventional two‐dimensional TCO (2D‐TCO) thin films. For use as a photoelectrode in dye‐sensitized solar cells, ITO‐TiO₂ core‐shell nanowire arrays were prepared by depositing a 45 nm‐thick mesoporous TiO₂ shell layer consisting of ～6 nm anatase nanoparticles using TiCl₄ treatments. Dye‐sensitized solar cells fabricated using these ITO‐TiO₂ core‐shell nanowire arrays show extremely fast charge collection owing to the shorter electron paths across the 45 nm‐thick TiO₂ shell compared to the 2D TCO. Interestingly, the charge‐collection time does not increase with the overall electrode thickness, which is counterintuitive to conventional diffusion models. This result implies that, in principle, maximum light harvesting can be achieved without hindering the charge collection. The proposed new 3D TCO should also be attractive for other photovoltaic applications where the active layer thickness is limited by poor charge collection.     

Semi‐Transparent Tandem Organic Solar Cells with 90% Internal Quantum Efficiency

Semi‐transparent (ST) organic solar cells with potential application as power generating windows are studied. The main challenge is to find proper transparent electrodes with desired electrical and optical properties. In this work, this is addressed by employing an amphiphilic conjugated polymer PFPA‐1 modified ITO coated glass substrate as the ohmic electron‐collecting cathode and PEDOT:PSS PH1000 as the hole‐collecting anode. For active layers based on different donor polymers, considerably lower reflection and parasitic absorption are found in the ST solar cells as compared to solar cells in the standard geometry with an ITO/PEDOT:PSS anode and a LiF/Al cathode. The ST solar cells have remarkably high internal quantum efficiency at short circuit condition (～90%) and high transmittance (～50%). Hence, efficient ST tandem solar cells with enhanced power conversion efficiency (PCE) compared to a single ST solar cell can be constructed by connecting the stacked two ST sub‐cells in parallel. The total loss of photons by reflection, parasitic absorption and transmission in the ST tandem solar cell can be smaller than the loss in a standard solar cell based on the same active materials. We demonstrate this by stacking five separately prepared ST cells on top of each other, to obtain a higher photocurrent than in an optimized standard solar cell.     

High‐Performance Suspended Particle Devices Based on Copper‐Reduced Graphene Oxide Core–Shell Nanowire Electrodes

Smart windows are one of the key components of so‐called “green” buildings. These windows are based on an actively switchable electro‐optic material that is sandwiched between two transparent electrodes. Although great progress has been made in identifying the optimal materials for such active windows, there is still a great need to improve their key elements, especially the performance of the transparent electrodes. Here, a new suspended particle device (SPD), holding a great potential for smart window applications, which is built upon copper‐reduced graphene oxide (Cu‐rGO) core–shell nanowire (NW) films as a transparent conductive electrode is reported. With the wrapping of rGO, the Cu NW electrodes demonstrate both high optical transparency and electrical conductivity, as well as significantly improved stability under various testing conditions. The novel sandwich‐structured SPDs, based on these electrodes, show a large change in their optical transmittance (42%) between “on” and “off” states, impressively fast switching time and superior stability. These high performances are comparable to those of the SPDs based on indium tin oxide electrodes. These promising results pave the way for the electrodes to be an integral part of a variety of optoelectronic devices, including energy‐friendly and flexible electronics.     

High‐Performance Solution‐Processed Double‐Walled Carbon Nanotube Transparent Electrode for Perovskite Solar Cells

Double‐walled carbon nanotubes are between single‐walled carbon nanotubes and multiwalled carbon nanotubes. They are comparable to single‐walled carbon nanotubes with respect to the light optical density, but their mechanical stability and solubility are higher. Exploiting such advantages, solution‐processed transparent electrodes are demonstrated using double‐walled carbon nanotubes and their application to perovskite solar cells is also demonstrated. Perovskite solar cells which harvest clean solar power have attracted a lot of attention as a next‐generation renewable energy source. However, their eco‐friendliness, cost, and flexibility are limited by the use of transparent oxide conductors, which are inflexible, difficult to fabricate, and made up of expensive rare metals. Solution‐processed double‐walled carbon nanotubes can replace conventional transparent electrodes to resolve such issues. Perovskite solar cells using the double‐walled carbon nanotube transparent electrodes produce an operating power conversion efficiency of 17.2% without hysteresis. As the first solution‐processed electrode‐based perovskite solar cells, this work will pave the pathway to the large‐size, low‐cost, and eco‐friendly solar devices.     

Visibly‐Transparent Organic Solar Cells on Flexible Substrates with All‐Graphene Electrodes

Portable electronic devices have become increasingly widespread. Because these devices cannot always be tethered to a central grid, powering them will require low‐cost energy harvesting technologies. As a response to this anticipated demand, this study demonstrates transparent organic solar cells fabricated on flexible substrates, including plastic and paper, using graphene as both the anode and cathode. Optical transmittance of up to 69% at 550 nm is achieved by combining the highly transparent graphene electrodes with organic polymers that primarily absorb in the near‐IR and near‐UV regimes. To address the challenge of transferring graphene onto organic layers as the top electrode, this study develops a room temperature dry‐transfer technique using ethylene‐vinyl‐acetate as an adhesion‐promoting interlayer. The power conversion efficiency achieved for flexible devices with graphene anode and cathode devices is 2.8%–3.8% at for optical transmittance of 54%–61% across the visible regime. These results demonstrate the versatility of graphene in optoelectronic applications and it is important step toward developing a practical power source for distributed wireless electrical systems.     

Efficient Semi‐Transparent Organic Solar Cells with Good Transparency Color Perception and Rendering Properties

Window integrated photovoltaics for automotive and building applications are a promising market segment for organic solar modules. Besides semi‐transparency, window integrated applications require a reasonable transparency perception and good color rendering properties in order to be suitable for realistic scene illumination. Here, the transmitted light through semi‐transparent organic solar cells comprising the polymer/fullerene blend poly[(4,4'‐bis(2‐ethylhexyl)dithieno[3,2‐b:2',3'‐d]silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl]: [6,6]‐phenyl C₇₁‐butric acid methyl ester (PSBTBT:PC₇₀BM) as active layer and a sputtered aluminum doped zinc oxide cathode were found to exhibit a color neutral perception for the human eye and very good color rendering properties. Moreover, the electrical cell properties allow for efficient energy harvesting with an overall power conversion efficiency η ≈ 3%.     

Transparent Soil for Imaging the Rhizosphere

Understanding of soil processes is essential for addressing the global issues of food security, disease transmission and climate change. However, techniques for observing soil biology are lacking. We present a heterogeneous, porous, transparent substrate for in situ 3D imaging of living plants and root-associated microorganisms using particles of the transparent polymer, Nafion, and a solution with matching optical properties. Minerals and fluorescent dyes were adsorbed onto the Nafion particles for nutrient supply and imaging of pore size and geometry. Plant growth in transparent soil was similar to that in soil. We imaged colonization of lettuce roots by the human bacterial pathogen Escherichia coli O157:H7 showing micro-colony development. Micro-colonies may contribute to bacterial survival in soil. Transparent soil has applications in root biology, crop genetics and soil microbiology.     

Fabrication of Nano-engineered Transparent Conducting Oxides by Pulsed Laser Deposition

Nanosecond Pulsed Laser Deposition (PLD) in the presence of a background gas allows the deposition of metal oxides with tunable morphology, structure, density and stoichiometry by a proper control of the plasma plume expansion dynamics. Such versatility can be exploited to produce nanostructured films from compact and dense to nanoporous characterized by a hierarchical assembly of nano-sized clusters. In particular we describe the detailed methodology to fabricate two types of Al-doped ZnO (AZO) films as transparent electrodes in photovoltaic devices: 1) at low O₂ pressure, compact films with electrical conductivity and optical transparency close to the state of the art transparent conducting oxides (TCO) can be deposited at room temperature, to be compatible with thermally sensitive materials such as polymers used in organic photovoltaics (OPVs); 2) highly light scattering hierarchical structures resembling a forest of nano-trees are produced at higher pressures. Such structures show high Haze factor (>80%) and may be exploited to enhance the light trapping capability. The method here described for AZO films can be applied to other metal oxides relevant for technological applications such as TiO₂, Al₂O₃, WO₃ and Ag₄O₄.     

Nanowire Interconnects for Printed Large‐Area Semitransparent Organic Photovoltaic Modules

Semitransparent organic photovoltaic (OPV) cells promise applications in various transparent architectures where their opaque counterparts cannot contribute. Realizing practical applications of this technology requires the manufacturing of large‐area modules without significant performance loss compared to the lab‐scale devices. In this work, efficient semitransparent OPV modules based on ultrafast laser patterning on both glass and flexible substrates are reported. Solution‐processed metallic silver nanowires (AgNWs) are used as transparent top electrodes. The efficient low‐ohmic contact of the interconnects between the top AgNWs and the bottom electrode in combination with high‐precision laser beam positioning system allow to fabricate semitransparent modules with high electrical fill factor of ≈63% and a remarkable geometric fill factor exceeding 95%, respectively. These results represent an important progress toward upscaling of high‐performance OPV modules with reduced production costs.     

A Transparent Planar Concentrator Using Aggregates of gem‐Pyrene Ethenes

The luminescence properties of pyrene ethenes, both as monomer and aggregate species, are found to depend on the regioisomer structure. Systematic shifts in absorption, emission, and excitation spectra of the gem‐pyrene ethenes, both in solution and in rigid polymer hosts, are consistent with weakly interacting H‐aggregate formation. This aggregation leads to excimer‐like emission with Stokes shifts greater than 1 eV. Planar concentrators fabricated from gem‐pyrene diphenylethenes show comparable performance to previously reported inorganic phosphors. The UV absorption and emission properties of the planar concentrator devices exhibit potential for transparent solar concentrators or visible–blind photodetector applications. This is the first demonstration of exploiting the unusual photophysics of molecular aggregates in planar concentrators.     

3D Nanowire Arrayed Cu Current Collector toward Homogeneous Alloying Anode Deposition for Enhanced Sodium Storage

Alloying electrodes are regarded as promising anodes for lithium/sodium storage thanks to their multielectron reaction capacity, moderate voltage plateau, and high electrical conductivity. However, huge volume change upon cycling, especially for sodium storage, usually causes the loss of electrical connection between active components and their delaminations from traditional current collectors, thus leading to rapid capacity decay. Herein, a unique 3D current collector is assembled from 1D nanowire arrays anchored on 3D porous Cu foams for constructing core‐shelled Cu@Sb nanowires as advanced sodium‐ion battery (SIB) anodes. The so‐formed hierarchical 3D anode with interconnected 3D micrometer sized pores and abundant voids between nanowires not only effectively accommodates the structural strains during repeated cycling but also ensures the structural integrity and contributes to a uniform ion/electron scattered distribution throughout the whole surface. When employed as anodes for SIBs, the obtained electrode shows a high capacity of 605.3 mAh g^?1 at 330 mA g^?1, and demonstrates a high capacity retention of 84.8% even at a high current density of 3300 mA g^?1. The 3D nanowire arrayed Cu current collector in this work can offer a promising strategy for designing and building advanced alloy anodes for lithium/sodium storage.     

Robust Sub‐Monolayers of Co3O4 Nano‐Islands: A Highly Transparent Morphology for Efficient Water Oxidation Catalysis

The scalable synthesis of highly transparent and robust sub‐monolayers of Co₃O₄ nano‐islands, which efficiently catalyze water oxidation, is reported. Rapid aerosol deposition of Co₃O₄ nanoparticles and thermally induced self‐organization lead to an ultra‐fine nano‐island morphology with more than 94% light transmission at a wavelength of 500 nm. These transparent sub‐monolayers demonstrate a remarkable mass‐weighted water oxidation activity of 2070–2350 A g_Co3O4^?1 and per‐metal turnover frequency of 0.38–0.62 s^?1 at an overpotential of 400 mV in 1 m NaOH aqueous solution. This mixed valent cobalt oxide structure exhibits excellent long‐term electrochemical and mechanical stability preserving the initial catalytic activity over more than 12 h of constant current electrolysis and 1000 consecutive voltammetric cycles. The potential of the Co₃O₄ nano‐islands for photoelectrochemical water splitting has been demonstrated by incorporation of co‐catalysts in GaN nanowire photoanodes. The Co₃O₄‐GaN photoanodes reveal significantly reduced onset overpotentials, improved photoresponse and photostability compared to the bare GaN ones. These findings provide a highly performing catalyst structure and a scalable synthesis method for the engineering of efficient photoanodes for integrated solar water‐splitting cells.     

High‐Performance,Transparent, Dye‐Sensitized Solar Cells for See‐Through Photovoltaic Windows

Dye‐sensitized solar cells (DSCs) have attracted great interest as one of the most promising photovoltaic technologies, and transparent DSCs show potential applications as photovoltaic windows. However, the competition between light absorption for photocurrent generation and light transmittance for obtaining high transparency limits the performance of transparent DSCs. Here, transparent DSCs exhibiting a high light transmittance of 60.3% and high energy conversion efficiency (3.66%) are reported. The strategy is to create a cocktail system composed of ultraviolet and near‐infrared dye sensitizers that selectively and efficiently harvest light in the invisible or low‐eye‐sensitivity region while transmitting light in high‐eye‐sensitivity regions. This new design provides a reasonable approach for realizing high efficiency and transparency DSCs that have potential applications as photovoltaic windows.     

A Pair of NiCo2O4 and V2O5 Nanowires Directly Grown on Carbon Fabric for Highly Bendable Lithium‐Ion Batteries

Mechanically bendable and flexible functionalities are urgently required for next‐generation battery systems that will be included in soft and wearable electronics, active sportswear, and origami‐based deployable space structures. However, it is very difficult to synthesize anode and cathode electrodes that have high energy density and structural reliability under large bending deformation. Here, vanadium oxide (V₂O₅) and nickel cobalt oxide (NiCo₂O₄) nanowire‐carbon fabric electrodes for highly flexible and bendable lithium ion batteries are reported. The vanadium oxide and nickel cobalt oxide nanowires were directly grown on plasma‐treated carbon fabric and were used as cathode and anode electrodes in a full cell lithium ion battery. Most importantly, a pre‐lithiation process was added to the nickel cobalt oxide nanowire anode to facilitate the construction of a full cell using symmetrically‐architectured nanowire‐carbon fabric electrodes. The highly bendable full cell based on poly(ethylene oxide) polymer electrolyte and room temperature ionic liquid shows high energy density of 364.2 Wh kg^?1 at power density of 240 W kg^?1, without significant performance degradation even under large bending deformations. These results show that vanadium oxide and lithiated nickel cobalt oxide nanowire‐carbon fabrics are a good combination for binder‐free electrodes in highly flexible lithium‐ion batteries.     

Transparent Cuprous Oxide Photocathode Enabling a Stacked Tandem Cell for Unbiased Water Splitting

Photoelectrochemical water splitting represents an attractive method of capturing and storing the immense energy of sunlight in the form of hydrogen, a clean chemical fuel. Given the large energetic demand of water electrolysis, and the defined spectrum of photons available from incident sunlight, a two absorber tandem device is required to achieve high efficiencies. The two absorbers should be of different and complementary bandgaps, connected in series to achieve the necessary voltage, and arranged in an optical stack configuration to maximize the utilization of sunlight. This latter requirement demands a top device that is responsive to high‐energy photons but also transparent to lower‐energy photons, which pass through to illuminate the bottom absorber. Here, cuprous oxide (Cu₂O) is employed as a top absorber component, and the factors influencing the balance between transparency and efficiency toward operation in a tandem configuration are studied. Photocathodes based on Cu₂O electrodeposited onto conducting glass substrates treated with thin, discontinuous layers of gold achieve reasonable sub‐bandgap transmittance while retaining performances comparable to their opaque counterparts. This new high‐performance transparent photocathode is demonstrated in tandem with a hybrid perovskite photovoltaic cell, resulting in a full device capable of standalone sunlight‐driven water splitting.