Organic Electronics for Electrochromic Materials and Devices. Hong Meng
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[29] polyester and PEO [30], polyvinyl alcohol (PVA), PAN, PMMA, poly(ethyl methacrylate) (PEMA) and poly(2‐ethoxyethyl methacrylate) (PEO EMA), [31] poly vinyl chloride (PVC), poly(vinyl sulfones) (PVS), and PVDF [32].
2.3.1.1 PEO‐/PEG‐Based Electrolytes
PEO (with a molecular mass above 20 000 g/mol) or PEG (with a molecular mass below 20 000 g/mol) simply refers to an oligomer or polymer of ethylene oxide. PEO‐based electrolyte is the earliest and most widely studied matrix for the formation of PE. Since Armand proposed a PE based on polyethylene oxide (PEO) for lithium batteries in 1978, research in this field has been extensively carried out [33]. Polyethylene oxide (PEO) has a polar ether group and significant segment mobility. As a wide range of salt polymers, especially high lithium ion stable polymers, it has excellent compatibility [34]. But because of high crystallinity, low melting point, limited operating temperature range, low hydroxyl ion migration number, poor interface characteristics, and other shortcomings, it fails to achieve the desired effect. In PEO‐based electrolytes, the most common method of increasing ion dissociation is the use of low lattice energy salts and to add fillers, which may prevent formation of polymer crystals, resulting in fast ion transportation via an interaction between the fillers and electrolyte [35]. PEO–H3PO4 and (PEO)8–LiClO4 were studied for electrochromism in devices [36]. However, the low Li+ conductivity inhibits its applicability. Later, PEO–LiSO3CF3 and PEO–LiN (SO2CF3)2 were reported to reach higher ion conductivities [37, 38]. PEO used as electrolyte in the gel form with an ionic conductivity of 2 mS/cm was reported in WO3 film ECD, which exhibited superior EC performance and memory characteristics [39]. In addition, PEO‐based GPE plasticized with ethylene carbonate/propylene carbonate or N‐butyl‐3methylpyridinium trifluoromethane sulfonylimide (PTFSI) has been studied by Desai et al. for the ECD [40]. Ionic conductivity can be significantly enhanced and the phase separation of the PEO and plasticizer was inhibited. Yang et al. studied the performance of the EC device prepared using poly(2,5‐dimethoxyaniline) (PDMA) and tungsten oxide (WO3) as electrode materials and PEO‐LiClO4 as GPE plasticized with polycarbonate [41]. Similarly various other hybrid electrolytes based upon PEO were successfully applied in EC applications [42].
2.3.1.2 PMMA‐Based Polymer Electrolytes
Due to high degree of crystallization, low ionic conductivity at ambient temperature exists in PEO‐based PE [43]. PMMA along with PVDF PEs have recently caused great attention in EC application. PMMA including amorphous phase and flexible backbone could increase ionic conductivity. As was reported, PMMA can provide a high transparency, excellent environmental stability, good gelatinizing properties, high solvent retention ability, and excellent compatibility with the liquid electrolytes [44, 45]. Moreover, PMMA shows good interfacial stability toward electrodes and high solvation ability to form complexation between polymer and salt. Highly ionic conductive PMMA exists as a gel and is mainly used for electrochromism [45]. Anderson et al. majored on ECDs with films of W oxide and vanadium pentoxide and an intervening layer of a PPG–PMMA–LiClO4 [46, 47]. Reynold and coworkers first reported a polymeric ECD using poly(3,4‐proplenedioxythiophene) (PProDOTMe2) and poly[3,6‐bis(2‐(3,4‐ethylenedioxy)‐thienyl)‐N‐methylcarbazole] (PBEDOT‐N‐MeCz) as the cathodically and anodically coloring polymers, respectively [48]. In this device, the EC films were electrosynthesized in poly (3,4‐ethylenedioxythiophene) (PEDOT)‐PSS electrodes at the oxidation potential of the monomer, and the electrolytes were prepared by using PMMA as the novel polymer matrix. The GPE having composition of 70 : 20 : 7 : 3 (ACN: PC: PMMA: TBAPF6 [tetrabutylammonium hexafluorophosphate]) was applied in the ECDs [48]. For the durability of the device, GPE was evaporated at the edges during the sealing process. The ECD exhibited a large transmittance change (Δ%T) of 51% at the wavelength of 540 nm and only 5% contrast loss after 32 000 switches. Beaupre et al. reported a flexible EC cell by using PMMA‐LiClO4 electrolyte, which was plasticized with propylene carbonate to form a highly transparent and conductive gel. Sonmez et al. has explored a highly transparent and conductive gel with LiClO4 plasticized with PC, having composition of PC: PMMA (MW: 350.000): LiClO4 (70 : 20 : 7 : 3), which was applied in EC device [49]. ACN was also added as a high vapor pressure solvent to make the gel ingredients blend easily. Oral et al. uses similar electrolyte of LiClO4: ACN: PMMA: PC in the ratio of 3 : 70 : 7 : 20 to study the EC properties [49]. Tung et al. studied the EC properties of the device of PEDOT‐Prussian Blue (PB) using PMMA as GPE [50]. The device exhibited high coloration efficiency and good long‐term cycling stability. Recently, Yang et al. studied a new type of GPE composed of free‐standing aramid nanofibers, which was used to fabricate all‐solid‐state near‐infrared (NIR) ECDs for NIR sheltering applications [51]. This new type of GPE showed excellent mechanical and heat endurance compared with currently available GPEs. Kim et al. presented a novel ECD‐based photonic device, which can modulate IR light intensity in a planar optical waveguide ECD by using PMMA gel electrolyte consisting of 5% (w/w) PMMA, 4% (w/w) phenothiazine, 0.1 m LiClO4, and 11.25 ≈ 10−3 m ferrocene [52]. The results confirm a new approach to consider ECD‐based optical modulators for the development of planar photonic‐integrated circuits and systems.
2.3.1.3 PVDF‐Based Polymer Electrolytes
PVDF is another popular host material for electrolytes and has recently been widely used in ECDs. PVDF offers many advantages, such as good thermomechanical properties, fairly high permittivity, high hydrophobicity, thermal and chemical stabilities, and chemical resistance. As a semi‐crystalline polymer, the PVDF crystalline phase provides thermal stability, while the amorphous phase provides the flexibility required for ECDs. PVDF can be soluble in high boiling point and commercial solvents, such as N‐methyl‐2‐pyrrolidone (NMP) and dimethylformamide (DMF). However, comparing with lithium salt, PVDF, which is expected to have a low donor number (DN), is insoluble and cannot be used effectively in polymer salt complexes. For EC application, PVDF‐based PEs have been identified as interesting candidates and their study is in progress. As was reported by Fabrettos group, an ECD was prepared by using PVDF for studying the coloration efficiency [53]. P(VDF‐TrFE) as GPE was also studied as potential PE in ECD with polyaniline as EC materials [1]. The gel electrolyte device reached an average ionic conductivity of 2.84 × 10−5 S/cm and shows stable and reversible light modulation up to 65% in gel state. It was found that the gel‐state device was affected by the number of free ions, while the movement of ions in the electrolyte bulk and the modulation of light in the semisolid device are indicated by the electrolyte/EC material interface.
The ionic conductivity of PVDF PEs can be enhanced by incorporating substantial amounts of plasticizers or combining with IL. Jia et al. have studied 1‐butyl‐3‐methylimidazolium hexafluorophosphate‐loaded SCCO2‐treated electrospun P(VDF‐HFP) membrane as an electrolyte in EC device [54].
Recently, Reynold's group reported paper‐based ECDs consisting of PEDOT:PSS electrodes and [EMI][TFSI]/PVDF‐HFP ion gel electrolyte layer (Figures 2.2 and 2.3) [55]. The ECDs incorporating an ion gel electrolyte were demonstrated where a magenta‐to‐colorless device achieves a color contrast (ΔE*) of 56, attributing to a highly color‐neutral bleached state of the extracellular protein (ECP) (a* = −0.5, b* = 2.9). It was found that the gel‐state device was affected by the number of free ions, while the movement of ions in the electrolyte bulk and the modulation of light in the semisolid device were indicated by the electrolyte/EC material interface.