State‐of‐the‐Art Neutral Tint Multichromophoric Polymers for High‐Contrast See‐Through Electrochromic Devices

Two new multichromophoric electrochromic polymers featuring a conjugated EDOT/ProDOT copolymer backbone (PXDOT) and a reversible Weitz‐type redox active small molecule electrochrome (WTE) tethered to the conjugated chain are reported here. The careful design of the WTEs provides a highly reversible redox behavior with a colorless red switching that complements the colorless blue switching of the conjugated backbone. Subtractive color mixing successfully provides high performing solution processable polymeric layers with colorless neutral tint switchable limiting states for application in see‐through electrochromic devices. Design, synthesis, comprehensive chemical and spectroelectrochemical characterization as well as the preparation of a proof‐of‐concept device are discussed.     

Development and Manufacture of Polymer‐Based Electrochromic Devices

The field of organic electrochromics is reviewed here, with particular focus on how the “electrochromic” as a functional material can be brought from the current level of accurate laboratory synthesis and characterization to the device and application level through a number of suited roll‐to‐roll methods compatible with upscaling and manufacture. The successful approaches to operational devices are presented in detail, as well as areas where future research would have a high impact and accelerate the development such as highly conducting and transparent substrates, electrolytes adapted for multilayer application and morphologically stable conjugated polymers.     

On‐Substrate Preparation of an Electroactive Conjugated Polyazomethine from Solution‐Processable Monomers and its Application in Electrochromic Devices

An electroactive polyazomethine is prepared from a solution processable 2,5‐diaminothiophene derivative and 4,4′‐triphenylamine dialdehyde by spray‐coating the monomers on substrates, including indium tin oxide (ITO) coated glass and native glass slides. The conjugated polymer was rapidly formed in situ by heating the substrates at 120 °C for 30 min in an acid saturated atmosphere. The resulting immobilized polymer is easily purified by rinsing the substrate with dichloromethane. The on‐substrate polymerization is tolerant towards large stoichiometry imbalances of the comonomers, unlike solution step‐growth polymerization. The resulting polyazomethine is electroactive and it can be switched reversibly between its neutral and oxidized states both electrochemically and chemically without degradation. A transmissive electrochromic device is fabricated from the immobilized polyazomethine on an ITO electrode. The resulting device is successfully cycled between its oxidized (dark blue) and neutral (cyan/light green) states with applied biases of +3.2 and ‐1.5 V under ambient conditions without significant color fatigue or polymer degradation. The coloration efficiency of the oxidized state at 690 nm is 102 cm² C^?1.     

Flexible Multi‐Colored Electrochromic and Volatile Polymer Memory Devices Derived from Starburst Triarylamine‐Based Electroactive Polyimide

Flexible multi‐colored electrochromic and volatile memory devices are fabricated from a solution‐processable electroactive aromatic polyimide with starburst triarylamine unit. The polyimide prepared by the chemical imidization was highly soluble in many organic solvents and showed useful levels of thermal stability associated with high glass‐transition temperatures. The polyimide with strong electron‐donating capability possesses static random access memory behavior and longer retention time than other 6FDA‐based polyimides. The differences of the highest‐occupied and lowest unoccupied molecular orbital levels among these polyimides with different electron‐donating moieties are investigated and the effect on the memory behavior is demonstrated. The polymer film shows reversible electrochemical oxidation and electrochromism with high contrast ratio both in the visible range and near‐infrared region, which also exhibits high coloration efficiency, low switching time, and the outstanding stability for long‐term electrochromic operation. The highly stable electrochromism and interesting volatile memory performance are promising properties for the practical flexible electronics applications in the future.     

High Performance Roll‐to‐Roll Produced Fullerene‐Free Organic Photovoltaic Devices via Temperature‐Controlled Slot Die Coating

Solution‐processed organic photovoltaics (OPVs) have continued to show their potential as a low‐cost power generation technology; however, there has been a significant gap between device efficiencies fabricated with lab‐scale techniques—i.e., spin coating—and scalable deposition methods. Herein, temperature‐controlled slot die deposition is developed for the photoactive layer of OPVs. The influence of solution and substrate temperatures on photoactive films and their effects on power conversion efficiency (PCE) in slot die coated OPVs using a 3D printer‐based slot die coater are studied on the basis of device performance, molecular structure, film morphology, and carrier transport behavior. These studies clearly demonstrate that both substrate and solution temperatures during slot die coating can influence device performance, and the combination of hot substrate (120 °C) and hot solution (90 °C) conditions result in mechanically robust films with PCE values up to 10.0% using this scalable deposition method in air. The efficiency is close to that of state‐of‐the‐art devices fabricated by spin coating. The deposition condition is translated to roll‐to‐roll processing without further modification and results in flexible OPVs with PCE values above 7%. The results underscore the promising potential of temperature‐controlled slot die coating for roll‐to‐roll manufacturing of high performance OPVs.     

Paper‐Based Electrochromic Devices Enabled by Nanocellulose‐Coated Substrates

As environmental considerations for both the processing and disposal of electronic devices become increasingly important, the ability to replace plastic and glass substrates with bioderived and biodegradable materials remains a major technological goal. Here, the use of cellulose nanofiber‐coated paper is explored as an environmentally benign substrate for preparing low‐resistance (460 Ω sq^?1), colorless (a* = ?2.3, b* = ?2.7) printed poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) electrodes. The PEDOT:PSS/paper electrodes support the reversible oxidation of three electrochromic polymers (ECPs) (cyan, magenta, and yellow), affording the possibility for fully printed, color displays on paper. Lateral electrochromic devices (ECDs) incorporating an ion gel electrolyte are demonstrated where a magenta‐to‐colorless device achieves a color contrast (ΔE*) of 56 owing to a highly color‐neutral bleached state of the ECP (a* = ?0.5, b* = 2.9). Black‐to‐colorless devices achieve ΔE* = 29 and are able to retain 86% of their color contrast after 9000 switches. The switching times of these lateral devices are quantified through colorimetric image analysis which shows comparable performance for devices constructed on paper as devices using ITO/glass electrodes (10 Ω sq^?1). The paper ECDs are then combusted in air leaving 3% of the initial mass at 600 °C, highlighting this approach as a promising route toward disposable displays.     

Roll‐to‐Roll Printed Silver Nanowire Semitransparent Electrodes for Fully Ambient Solution‐Processed Tandem Polymer Solar Cells

Silver nanowires (AgNWs) and zinc oxide (ZnO) are deposited on flexible substrates using fast roll‐to‐roll (R2R) processing. The AgNW film on polyethylene terephthalate (PET) shows >80% uniform optical transmission in the range of 550–900 nm. This electrode is compared to the previously reported and currently widely produced indium‐tin‐oxide (ITO) replacement comprising polyethylene terephthalate (PET)|silver grid|poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)|ZnO known as Flextrode. The AgNW/ZnO electrode shows higher transmission than Flextrode above 490 nm in the electromagnetic spectrum reaching up to 40% increased transmission at 750 nm in comparison to Flextrode. The functionality of AgNW electrodes is demonstrated in single and tandem polymer solar cells and compared with parallel devices on traditional Flextrode. All layers, apart from the semitransparent electrodes which are large‐scale R2R produced, are fabricated in ambient conditions on a laboratory roll‐coater using printing and coating methods which are directly transferrable to large‐scale R2R processing upon availability of materials. In a single cell structure, Flextrode is preferable with active layers based on poly‐3‐hexylthiophene(P3HT):phenyl‐C61‐butyric acid methylester (PCBM) and donor polymers of similar absorption characteristics while AgNW/ZnO electrodes are more compatible with low band gap polymer‐based single cells. In tandem devices, AgNW/ZnO is more preferable resulting in up to 80% improvement in PCE compared to parallel devices on Flextrode.     

A Unique Processable Green Polymer with a Transmissive Oxidized State for Realization of Potential RGB‐Based Electrochromic Device Applications

Realization of commercial RGB‐based polymer electrochromic‐device applications can only be achieved by processable materials that possess three complementary colors in the reduced state and are transparent in the oxidized state. This report highlights the synthesis of the first processable green polymer with a transmissive oxidized state. The polymer revealed superior optical contrast in the visible region with fast switching times and robust stability. Hence, this material is the outstanding candidate for completion of RGB color space through commercial polymeric electrochromics.     

Anodized Aluminum Oxide/Polydimethylsiloxane Hybrid Mold for Roll‐to‐Roll Nanoimprinting

Two types (hard and soft) of the molds are widely used in nanoimprint lithography for a high throughput over a large area, and high‐resolution parallel patterning. Although hard molds have proven excellent resolutions and can be used at high temperatures, cracks often occur in the mold in addition to the requirement of high imprinting pressure. On the other hand, though soft molds can operate at lower pressures, they give poor pattern resolution. Here, a novel hybrid mold of anodized aluminum oxide (AAO) template attached to a flexible polydimethylsiloxane (PDMS) plate is introduced. Due to the flexible nature of PDMS, various polymer nanostructures are obtained on flat and curved substrates without crack formation on the AAO mold surface. Furthermore, the hybrid mold is successfully used for roll‐to‐roll imprinting for the fabrication of high density array of various shaped polymeric nanostructures over a large area.     

Humidity‐Tolerant Roll‐to‐Roll Fabrication of Perovskite Solar Cells via Polymer‐Additive‐Assisted Hot Slot Die Deposition

Heating‐assisted deposition is an industry‐friendly scalable deposition method. This manufacturing method is employed together with slot die coating to fabricate perovskite solar cells via a roll‐to‐roll process. The feasibility of the method is demonstrated after initial testing on a rigid substrate using a benchtop slot die coater in air. The fabricated solar cells exhibit power conversion efficiencies (PCEs) up to 14.7%. A nonelectroactive polymer additive is used with the perovskite formulation and found to improve its humidity tolerance significantly. These deposition parameters are also used in the roll‐to‐roll setup. The perovskite layer and other solution‐processed layers are slot die‐coated, and the fabricated device shows PCEs up to 11.7%, which is the highest efficiency obtained from a fully roll‐to‐roll processed perovskite solar cell to date.     

A Solution‐Processable High‐Coloration‐Efficiency Low‐Switching‐Voltage Electrochromic Polymer Based on Polycyclopentadithiophene

A solution‐processable electrochromic (EC) polymer, poly(4,4‐dioctyl‐cyclopenta[2,1‐b:3,4‐b′]‐dithiophene) (PDOCPDT) is prepared by means of chemical oxidative polymerization of the corresponding monomer. The UV‐vis spectrum of the spin‐coated PDOCPDT film displays an absorption maximum of 580 nm. Although the polymer is deep blue in its neutral state, it turns to transparent bluish after being oxidized. PDOCPDT film (thickness 120 nm) exhibits high coloration efficiency (CE)—as high as 932 C cm^–2 at 580 nm, low response time (0.75 s), high optical density (0.75 at 580 nm), and high‐level stability for long term switch (it switches repetitively 1000 times with less than 8 % contrast loss). The electrochemical stability and redox potentials of PDOCPDT films are independent of film thickness (50–180 nm) and active area (up to 2 cm × 2 cm). Nevertheless, optical contrast increases as the film thickness increases, although the CE and response time changes irregularly with the film thickness. The good EC properties combined with the easy film fabrication process make PDOCPDT a notable candidate for application in EC devices. (ECDs) A simple transmissive‐type ECD with good CE using PDOCPDT film as an active layer is also demonstrated.     

Synergistic Roll‐to‐Roll Transfer and Doping of CVD‐Graphene Using Parylene for Ambient‐Stable and Ultra‐Lightweight Photovoltaics

A roll‐to‐roll (R2R) transfer technique is employed to improve the electrical properties of transferred graphene on flexible substrates using parylene as an interfacial layer. A layer of parylene is deposited on graphene/copper (Cu) foils grown by chemical vapor deposition and are laminated onto ethylene vinyl acetate (EVA)/poly(ethylene terephthalate). Then, the samples are delaminated from the Cu using an electrochemical transfer process, resulting in flexible and conductive substrates with sheet resistances of below 300 Ω sq^?1, which is significantly better (fourfold) than the sample transferred by R2R without parylene (1200 Ω sq^?1). The characterization results indicate that parylene C and D dope graphene due to the presence of chlorine atoms in their structure, resulting in higher carrier density and thus lower sheet resistance. Density functional theory calculations reveal that the binding energy between parylene and graphene is stronger than that of EVA and graphene, which may lead to less tear in graphene during the R2R transfer. Finally, organic solar cells are fabricated on the ultrathin and flexible parylene/graphene substrates and an ultra‐lightweight device is achieved with a power conversion efficiency of 5.86%. Additionally, the device shows a high power per weight of 6.46 W g^?1 with superior air stability.     

Roll‐to‐roll gravure printing of organic photovoltaic modules—insulation of processing defects by an interfacial layer

Gravure printing as direct patterning roll‐to‐roll (R2R) production technology can revolutionize the design of thin‐film organic photovoltaic (OPV) devices by allowing feasible manufacturing of arbitrary‐shaped modules. This makes a distinction to coating methods, such as slot die coating, in which the pattern is limited to continuous stripes. Here, we analyze the thin‐film formation and its influence on OPV module performance as the gravure printing of hole transport and photoactive layers are transferred from laboratory to R2R pilot production environment. Insertion of a 0.8‐nm layer of lithium fluoride (LiF) as an interfacial layer between the active layer and the electron contact provided insulation against the detrimental pinholes formed in the R2R printing process. Using this device configuration, we produced well‐performing R2R‐printed monolithic modules with a mean efficiency of 1.7%. In comparison, reference modules with an efficiency of 2.2% were fabricated using laboratory‐scale bench top sheet‐level process. Surface energy and tension measurements together with optical microscopy were used to analyze the printability of the materials. The pinhole insulation was investigated in detail by processing R2R‐printed OPV modules with different interfacial layer materials and performing electrical measurements under dark and AM1.5 illumination conditions. Furthermore, we analyzed the LiF distribution using X‐ray photoelectron spectroscopy. The insulating nature of the LiF layer to improve module performance was confirmed by manufacturing lithographically artificial pinholes in device structures. The results show the possibility to loosen the production environment constraints and the feasibility of fabricating well‐performing thin‐film devices by R2R gravure printing. Copyright © 2014 John Wiley & Sons, Ltd.     

Engineering Efficiency Roll‐Off in Organic Light‐Emitting Devices

Previous studies have identified triplet‐triplet annihilation and triplet‐polaron quenching as the exciton density‐dependent mechanisms which give rise to the efficiency roll‐off observed in phosphorescent organic light‐emitting devices (OLEDs). In this work, these quenching processes are independently probed, and the impact of the exciton recombination zone width on the severity of quenching in various OLED architectures is examined directly. It is found that in devices employing a graded‐emissive layer (G‐EML) architecture the efficiency roll‐off is due to both triplet‐triplet annihilation and triplet‐polaron quenching, while in devices which employ a conventional double‐emissive layer (D‐EML) architecture, the roll‐off is dominated by triplet‐triplet annihilation. Overall, the efficiency roll‐off in G‐EML devices is found to be much less severe than in the D‐EML device. This result is well accounted for by the larger exciton recombination zone measured in G‐EML devices, which serves to reduce exciton density‐driven loss pathways at high excitation levels. Indeed, a predictive model of the device efficiency based on the quantitatively measured quenching parameters shows the role a large exciton recombination zone plays in mitigating the roll‐off.     

Solution‐Processable,High‐Performance Flexible Electroluminescent Devices Based on High‐k Nanodielectrics

Flexible alternating‐current electroluminescent (ACEL) devices have attracted considerable attention for their ability to produce uniform light emission under bent conditions and have enormous potential for applications in back lighting panels, decorative lighting in automobiles, and panel displays. Nevertheless, flexible ACEL devices generally require a high operating bias, which precludes their implementation in low power devices. Herein, solution‐processed La‐doped barium titanate (BTO:La) nanocuboids (≈150 nm) are presented as high dielectric constant (high‐k) nanodielectrics, which can enhance the dielectric constant of an ACEL device from 2.6 to 21 (at 1 kHz), enabling the fabrication of high‐performance flexible ACEL devices with a lower operating voltage as well as higher brightness (≈57.54 cd m^?2 at 240 V, 1 kHz) than devices using undoped BTO nanodielectrics (≈14.3 cd m^?2 at 240 V, 1 kHz). Furthermore, a uniform brightness across the whole panel surface of the flexible ACEL devices and excellent device reliability are achieved via the use of uniform networks of crossaligned silver nanowires as highly conductive and flexible electrodes. The results offer experimental validation of high‐brightness flexible ACELs using solution‐processed BTO:La nanodielectrics, which constitutes an important milestone toward the implementation of high‐k nanodielectrics in flexible displays.     

Electrochromic Effect in Titanium Carbide MXene Thin Films Produced by Dip‐Coating

MXenes, a large family of 2D transition metal carbides and nitrides, have shown potential in energy storage and optoelectronic applications. Here, the optoelectronic and pseudocapacitive properties of titanium carbide (Ti₃C₂T_x) are combined to create a MXene electrochromic device, with a visible absorption peak shift from 770 to 670 nm and a 12% reversible change in transmittance with a switching rate of <1 s when cycled in an acidic electrolyte under applied potentials of less than 1 V. By probing the electrochromic effect in different electrolytes, it is shown that acidic electrolytes (H₃PO₄ and H₂SO₄) lead to larger absorption peak shifts and a higher change of transmittance than the neutral electrolyte (MgSO₄) (Δλ is 100 nm vs 35 nm and ΔT_{770 nm} is ≈12% vs ≈3%, respectively), hinting at the surface redox mechanism involved. Further investigation of the mechanism by in situ X‐ray diffraction and Raman spectroscopy reveals that the reversible shift of the absorption peak is attributed to protonation/deprotonation of oxide‐like surface functionalities. As a proof of concept, it is shown that Ti₃C₂T_x MXene, dip‐coated on a glass substrate, functions as both transparent conductive coating and active material in an electrochromic device, opening avenues for further research into optoelectronic and photonic applications of MXenes.