Effects of Pre‐reducing Sb‐Doped SnO2 Electrodes in Viologen‐Anchored TiO2 Nanostructure‐Based Electrochromic Devices

In this paper, we investigate the effects of pre‐reducing Sb‐doped SnO₂ (ATO) electrodes in viologen‐anchored TiO₂ (VTO) nanostructure–based electrochromic devices. We find that by pre‐reducing an ATO electrode, the operating voltage of a VTO nanostructure–based electrochromic device can be lowered; consequently, such a device can be operated more stably with less hysteresis. Further, we find that a pre‐reduction of the ATO electrode does not affect the coloration efficiency of such a device. The aforementioned effects of a pre‐reduction are attributed to the fact that a pre‐reduced ATO electrode is more compatible with a VTO nanostructure–based electrochromic device than a non‐pre‐reduced ATO electrode, because of the initial oxidized state of the other electrode of the device, that is, a VTO nanostructure–based electrode. The oxidation state of a pre‐reduced ATO electrode plays a very important role in the operation of a VTO nanostructure–based electrochromic device because it strongly influences charge movement during electrochromic switching.     

A novel 1,1′-bis[4-(5,6-dimethyl-1H-benzimidazole-1-yl)butyl]-4,4′-bipyridinium dibromide (viologen) for a high contrast electrochromic device

A new electrochromic viologen, 1,1′-bis-[4-(5,6-dimethyl-1H-benzimidazole-1-yl)-butyl]-4,4′-bipyridinium dibromide (IBV) was synthesized by di-quaternization of 4,4′-bipyridyl using 1-(4-bromobutyl)-5,6-dimethyl-1H-benzimidazole. X-ray photoelectron spectroscopy confirmed the formation of the IBV (viologen) salt as distinct signals due to quaternary nitrogen and neutral nitrogen, and ionic-bonded bromide were identified. An electrochromic device encompassing a dicyanamide ionic liquid based gel polymeric electrolyte with high ionic conductivity, a thermal decomposition temperature above 200 °C, and a stable voltage window of ～4 V with the IBV viologen dissolved therein, was constructed. IBV is a cathodically coloring organic electrochrome and the device underwent reversible transitions between transparent and deep blue hues; the color change was accompanied by an excellent optical contrast (30.5% at 605 nm), a remarkably high coloration efficiency of 725 cm² C^?1 at 605 nm and switching times of 2–3 s. Electrochemical impedance spectroscopy revealed an unusually low charge transfer resistance at the IBV salt/gel interface, which promotes charge propagation and is responsible for the intense coloration of the reduced radical cation state. The device was subjected to repetitive switching between the colored and bleached states and was found to incur almost no loss in redox activity, up to 1000 cycles, thus ratifying its suitability for electrochromic window/display applications.     

Enhanced electrochromic write–erase efficiency of a device with a novel viologen: 1,1′-bis(2-(1H-indol-3-yl)ethyl)-4,4′-bipyridinium diperchlorate

A novel cathodically coloring viologen electrochrome: 1,1′-bis(2-(1H-indol-3-yl)ethyl)-4,4′-bipyridinium diperchlorate (IEV), comprising of a 4,4′-bipyridyl core, sandwiched between two indole moieties, was synthesized using 3-(2-bromoethyl)-indole. An electrochromic device (ECD) was assembled using an electrolyte containing an imidazolium imide ionic liquid and characterized by a large electrical conductivity, thermal stability upto 150 °C, and an electrochemical potential stability range of ∼3.6 V. The IEV viologen was dissolved in the electrolyte and Prussian blue was used as the anodic electrochrome. The indole moieties of the IEV²⁺ salt owing to their electron donating tendency can act like bleaching agents and bleach the viologen faster (IEV⁺ → IEV²⁺) and this hypothesis was used for the improved write–erase efficiency of the device. The device switched between colorless and dark violet–blue hues under applied potentials of ±1.5 V. A large transmission modulation (52%, λ = 605 nm), a high electrochromic coloring efficiency of 533 cm² C⁻¹ at 605 nm and switching times of ∼2 s and good stability during 2000 cycles were reported herein. The electrochemical activity of the ECD improved when it was maintained at an elevated temperature of 70 °C, with no sign of thermal degradation. Furthermore, we also present the ability of this new viologen to function as an excellent redox mediator as we achieved an 86% enhancement in the power conversion efficiency of a solution processed solar cell, by its’ addition in the electrolyte. Our studies demonstrate this new viologen to be a highly versatile electroactive material which can be useful for both electrochromics and photovoltaics.     

Multistage Coloring Electrochromic Device Based on TiO2 Nanotube Arrays Modified with WO3 Nanoparticles

WO₃ nanoparticles loaded in TiO₂ nanotube arrays, fabricated by a chemical bath deposition (CBD) technique in combination with a pyrolysis process, is uniform and the diameter can be easily adjusted by the deposition times. The resultant hybrid nanotubes array shows a multistage coloring electrochromic response at different potential bias. The formation of a 3‐dimensional WO₃/TiO₂ junction promotes unidirectional charge transport due to the one‐dimensional features of the tubes, which leads to the significant positive‐shift onset potential of the cathodic reaction (ion insertion) and the highly increased proton storage capacity. Compared to non‐decorated nanotube arrays, the enhanced electrochromic properties of longer lifetime, higher contrast ratio (bleaching time/coloration time), and improved tailored electrochromic behavior could be achieved using the composite films.     

Highly contrasted and stable electrochromic device based on well-matched viologen and triphenylamine

An electrochromic device (ECD) can change color absorption when subjected to an appropriate voltage. Such a device includes three components: a working electrode, a counter electrode and an electrolyte. Compatibility of these three components is important for ECD’s stability. In this study, two novel compatible electrochromic materials, cathodic 1-(9-hexyl-9H-carbazole)-1′-(propylphosphonic acid)-4,4′-bipyridilium dichloride and anodic (4-(diphenylamino)phenyl)methylphosphonic acid were designed, synthesized and fabricated into electrochromic electrodes using a chemisorption method. We characterized the electrochromic performance of these two electrodes, including the degree of color change, color changing voltage and charge capacity; the results indicated that they matched each other very well. An electrochromic device fabricated using these two electrodes, as expected, exhibited rapid, vivid color changes and proved highly stable for up to 100,000 cycles.     

Characterization of P-channel power trench MOSFETs with polycrystalline silicon germanium gate electrode for faster switching

Electrical switching characteristics using polycrystalline silicon–germanium (poly-Si_l?xGe_x) gate for P-channel power trench MOSFETs was investigated. Switching time reduction of over 22% was observed when the boron-doped poly-Si gate was replaced with a similarly boron-doped poly-SiGe gate on the P-channel power MOSFETs. The fall time (T_f) on MOSFETs with poly-SiGe gate, was found to be ～11 ns lesser than the poly-Si gate MOSFET which is ～60% improvement in switching performance. However, all the switching improvement was observed during the fall times (T_f). The reason could be the higher series resistance in the switching test circuit masking any reduction in the rise times (T_r). Faster switching is achieved due to a lower gate resistance (R_g) offered by the poly-SiGe gate electrode as compared to poly-silicon (pSi) material. The pSi gate resistance was found to be 6.25 Ω compared to 3.75 Ω on the poly-SiGe gate measured on the same device. Lower gate resistance (R_g) also means less power is lost during switching thereby less heat is generated in the device. A very uniform boron doping profile was achieved with-in the pSiGe gate electrode, which is critical for uniform die turn on and better thermal response for the power trench MOSFET. pSiGe thin film optimization, properties and device characteristics are discussed in details in the following sections.     

Fully solution-processed and multilayer blue organic light-emitting diodes based on efficient small molecule emissive layer and intergrated interlayer optimization

Efficient and fully solution-processed blue organic light-emitting diodes (OLEDs) based on fluorescent small-molecule and methanol/water soluble conjugated polymer as electron-injection material are reported. The emitting layer is 3,6-bis(9,9,9′,9′-tetrakis (6-(9H-carbazol-9-yl)hexyl)-9H,9′H-[2,2′-bifluoren]-7-yl)dib-nzo[b, d]thiophene 5, 5-dioxide (OCSoC) with a blue-fluorescent small-molecule, and a methanol/water soluble polymer poly[(9,9-bis(30-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)] (PFN) acted as electron-injection layer (EIL). All the organic layers are spin-coated from solution. The multilayer device structure with emitting layer/electron-injection layer is achieved by solution-processed method without the dissolution problem between layers. The performances of the devices show that the maximum luminous efficiency of the multilayer device is increased about 43%, compared to the single-layer device. PFN acting as the EIL material plays a key role in the improvement of the device performance when used in solution-processed small-molecule OLEDs.     

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.     

Cerium-Modified Mesoporous Antimony Doped Tin Oxide as Intercalation-Free Charge Storage Layers for Electrochromic Devices

Charge storage layers are an important component of electrochromic devices, which are expected to exhibit high storage capacity and transparency as well as fast electron transfer rates. However, these layers often rely on the (de)intercalation of ions into the crystal lattice of the material and therefore require optimization to be compatible with non-intercalating electrolytes. In this report, the post-modification of mesoporous antimony-doped tin oxide (ATO) nanoparticle layers with a redox-active cerium compound is described. In particular, the switching of the Ce³⁺/Ce⁴⁺ couple on the conductive nanoparticle scaffold is demonstrated using tetrabutylammonium perchlorate as a non-intercalating electrolyte. Remarkably, high storage capacities of up to 27 mC cm⁻² and transmittance values of ≈90% are achieved. Variation of the antimony doping concentration revealed that nanoparticle layers doped with 15% Sb exhibit the highest capacity, which can be attributed to increased conductivity in the potential range where the Ce³⁺ ions are oxidized. Finally, the cerium-modified ATO films show promising performance as charge storage layers in an electrochromic device with a viologen-anchored ATO layer as the electrochromic working electrode. Switching times of ≈0.4 s highlight the fast electron transfer capability of the cerium-decorated ATO layer, even when a non-intercalating electrolyte is used.     

Highly Stable Ag–Au Core–Shell Nanowire Network for ITO-Free Flexible Organic Electrochromic Device

Solid and flexible electrochromic (EC) devices require a delicate design of every component to meet the stringent requirements for transparency, flexibility, and deformation stability. However, the electrode technology in flexible EC devices stagnates, wherein brittle indium tin oxide (ITO) is the primary material. Meanwhile, the inflexibility of metal oxide usually used in an active layer and the leakage issue of liquid electrolyte further negatively affect EC device performance and lifetime. Herein, a novel and fully ITO-free flexible organic EC device is developed by using Ag–Au core–shell nanowire (Ag–Au NW) networks, EC polymer and LiBF₄/propylene carbonate/poly(methyl methacrylate) as electrodes, active layer, and solid electrolyte, respectively. The Ag–Au NW electrode integrated with a conjugated EC polymer together display excellent stability in harsh environments due to the tight encapsulation by the Au shell, and high area capacitance of 3.0 mF cm⁻² and specific capacitance of 23.2 F g⁻¹ at current density of 0.5 mA cm⁻². The device shows high EC performance with reversible transmittance modulation in the visible region (40.2% at 550 nm) and near-infrared region ( − 68.2% at 1600 nm). Moreover, the device presents excellent flexibility ( > 1000 bending cycles at the bending radius of 5 mm) and fast switching time (5.9 s).     

Quenching in single emissive white phosphorescent organic light-emitting devices

To explore energy loss by diffusive triplet excitons in single emissive white phosphorescent organic light-emitting devices, the authors investigated collisional quenching between the electron transport materials 4,7-diphenyl-1,10-phenanthroline (Bphen), 2′,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), or 1,3,5-tri(3-pyrid-3-yl-phenyl)benzene (TmPyPB) and the blue phosphorescent material, 3,5-difluoro-2-(2-pyridyl)-phenyl-(2-carboxypyridyl) Iridium III (FIrpic) spectroscopically in solution. The luminous efficiency and the external quantum efficiency (EQE) of an emissive white phosphorescent organic light-emitting device, in which TmPyPB acted as the electron transport material, was found to be greater than those of devices prepared using Bphen or TPBi due to the lack of collisional quenching. In addition, it was found that to prevent triplet exciton loss, an ETL material should have a low bimolecular quenching rate constant k_q of less than 1.458 × 10⁷ s⁻¹ M⁻¹, which is the k_q of TmPyPB.     

Silicon-based carbazole and oxadiazole hybrid as a bipolar host material for phosphorescent organic light-emitting diodes

A silicon-based bipolar compound, 2-(4-((4-(9H-carbazol-9-yl)phenyl)dimethylsilyl)phenyl)-5-phenyl-1,3,4-oxadiazole (COHS), was designed and prepared as a host material for phosphorescent organic light-emitting diodes (OLEDs). The conjugated analogue of COHS, 2-(4′-(9H-carbazol-9-yl)biphenyl-4-yl)-5-phenyl-1,3,4-oxadiazole (COH), was also prepared to investigate their structure–property relationships. Thermal-, photophysical- and electrochemical properties as well as their single-crystal X-ray structures were studied for COHS and COH. The central silicon atom in COHS successfully disconnected the electronic communication between the carbazole and oxadiazole groups, resulting in relatively high triplet energy of ca. 2.71 eV, which were capable of hosting green phosphorescent emitters. DFT calculations were conducted to investigate the electronic structures of COHS and COH, and the results showed good correlation to experimental results. Finally, COHS and COH were used as a bipolar host material for a green phosphorescence organic light-emitting diode (PHOLED) devices with Ir(ppy)₃ (tris[2-phenylpyridinato-C²,N]iridium(III)) as a dopant. The resulting device with COHS (device I) showed higher performance than the device with COH (device II), exhibiting high efficiencies and low-efficiency roll-off. Device I achieved maximum external quantum efficiencies (EQE) of 15.8%, whereas device II exhibited a relatively lower EQE of 13.0%.     

Ambient Stable and Efficient Monolithic Tandem Perovskite/PbS Quantum Dots Solar Cells via Surface Passivation and Light Management Strategies

Here, highly efficient and stable monolithic (2-terminal (2T)) perovskite/PbS quantum dots (QDs) tandem solar cells are reported, where the perovskite solar cell (PSC) acts as the front cell and the PbS QDs device with a narrow bandgap acts as the back cell. Specifically, ZnO nanowires (NWs) passivated by SnO₂ are employed as an electron transporting layer for PSC front cell, leading to a single cell PSC with maximum power conversion efficiency (PCE) of 22.15%, which is the most efficient NWs-based PSCs in the literature. By surface passivation of PbS QDs by CdCl₂, QD devices with an improved open-circuit voltage and a PCE of 8.46% (bandgap of QDs: 0.92 eV) are achieved. After proper optimization, 2T and 4T tandem devices with stabilized PCEs of 17.1% and 21.1% are achieved, respectively, where the 2T tandem device shows the highest efficiency reported in the literature for this design. Interestingly, the 2T tandem cell shows excellent operational stability over 500 h under continuous illumination with only 6% PCE loss. More importantly, this device without any packaging depicts impressive ambient stability (almost no change) after 70 days in an environment with controlled 65% relative humidity, thanks to the superior air stability of the PbS QDs.     

Electrochemical synthesis and electrochromic application of a novel polymer based on carbazole

The compound containing carbazole and thiophene, named as B1 was synthesized with 4-(9H-carbazol-9-yl) phenol and 3,4-dibromo thiophene. Additionally, the electrochemical polymer of B1 was synthesized and coated onto an ITO-glass surface via electrochemical oxidative polymerization. The electrochemical synthesis of the polymer was performed both in 0.05 M LiClO₄ supporting electrolyte in AN/BF₃EtE (1:1, v/v) and an AN/LiClO₄ solvent/electrolyte solution. The compounds were characterized by FT-IR and NMR techniques. The spectroelectrochemical and electrochromic properties of this polymer were also investigated for two electrolyte solution systems. The switching ability of this polymer was measured as the percent transmittance (%T) at its point of maximum contrast. According to the electrochromic measurements, the synthesized polymer had a blue color when it was oxidized, and also when it was reduced, it had a transparent color. Additionally, redox stability measurements indicates that the polymer had a high stability and it could be used to produce new polymeric electrochromic devices and also, it was a good candidate for electrochromic devices (ECDs) applications.     

Blue light-emitting OLED using new spiro[fluorene-7,9′-benzofluorene] host and dopant materials

A new spiro-type compound, 2-(10-biphenylanthracene)-spiro[fluorene-7,9′-benzofluorene] (BH-3B) containing anthracene moiety was prepared for the blue host material. Also new dopant materials, 2-[4′-(phenyl-4-vinylbenzeneamine)phenyl-spiro[fluorene-7,9′-benzofluorene] (BH-3BD) and 4-[2-naphthyl-4′(phenyl-4-vinylbenzeneamine)]phenyl (BD-1N) were successfully synthesized and a blue OLEDs were made from them. The structure of the device was as follows; ITO/DNTPD/α-NPD/Host:5% dopant/Alq₃/Al-LiF. Among all of the devices, the device obtained from BH-3B host doped with 5% BH-3BD showed the best electroluminescence characteristics. The emission peak of EL is at 456 nm and the CIE value is (0.15, 0.14). The brightness of the device is up to 5407 cd/m² at 10 V with the maximum EL efficiency of 3.4 cd/A.     

Efficient low-molecule phosphorescent organic light-emitting diodes fabricated by wet-processing

We demonstrate high efficiency electrophosphorescence in organic light-emitting devices employing a phosphorescent dye doped into a low-molecule material. Methoxy-substituted 1,3,5-tris[4-(diphenylamino)phenyl]benzene (TDAPB) was selected as the host material for the phosphorescent dopant fac-tris(2-phenylpyridine) iridium(III) [Ir(ppy)₃], and organic films were fabricated by spin-coating. A peak external quantum efficiency of 8.2% (29 cd/A), luminous power efficiency of 17.3 lm/W, and luminance of 33,000 cd/m² were achieved at 9.4 V with a 90 nm-thick emitting layer. Emission from the host TDAPB material was not observed in the electroluminescence (EL) and photoluminescence (PL) spectra. The decrease in efficiencies at a high current is analyzed using the triplet–triplet annihilation model. The high performance for the simple device structure in this study is attributed to excellent film forming properties of the material and efficient energy transfer from the host to dopants.     

Synthesis of the novel thienoisoindigo-based donor-acceptor type conjugated polymers and the stable switching performance of purple-to-transparent as the electrochromic materials

Three soluble conjugated polymers named PTINS-1, PTINS-2, and PTINS-3 were synthesized via Stille coupling reaction by employing different molar feed ratios of the donor units to the acceptor units. (E)-2,2′-dibromo-4,4′-bis(2-ethylhexyl)-[6,6′-bithieno [3,2-b]pyrrolylidene]-5,5′(4H, 4H′)-dione (TIIG, M2) was used as the acceptor unit, and 2,5-bis (trimethylstannyl)thieno [3,2-b] thiophene (M1) and tris(thienothiophene) (TTT, M3) were selected as the donor units. Electrochemical research demonstrates that three polymers could switch reversibly and stably between purple neutral states with different saturation and transparent oxidized states. Amid the three polymers, PTINS-1 exhibits the best comprehensive performance including the satisfying optical contrast of 41.28% in the visible region with the response time of coloring and bleaching in only 0.47 s and 0.93 s, respectively, and the optical contrasts up to 81.98% with the coloring and bleaching time in only 0.56 s and 1.18 s at the 1590 nm. Furthermore, the other two polymers display beyond 70% of the transmittance change in the near-infrared region (NIR), which endows three polymers with commendable prospects in NIR electrochromic applications, and deserves more attention and penetrating research.     

A novel high-quality electrochromic material from 3,4-ethylenedioxythiophene bis-substituted fluorene

Poly(2,7-bis (2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)-9H-fluorene) (P(EDOT-FE)), a novel electrochromic material obtained from 3,4-ethylenedioxythiophene (EDOT) bis-substituted fluorene (FE) in CH₂Cl₂ solution, and its applications in electrochromic devices (ECD) are discussed. The external EDOT units will not only function as donor groups but also lower the oxidation potential. Fluorescent spectral studies indicate that P(EDOT-FE) with high fluorescence quantum yields and photochemical stability is a novel green-light-emitter. P(EDOT-FE) is switched between brown in the reduced state and blue in the oxidized state. ECD based on P(EDOT-FE) and poly(3,4-ethylenedioxythiophene) (PEDOT) was also fabricated and showed a good electrochromic performance. The ECD constructed by P(EDOT-FE) and PEDOT has good optical contrast (36% at 625 nm), high coloration efficiency (784 cm² C⁻¹), fast response time (0.5 s at 625 nm), better optical memory and long-term stability. Clear change from dark red (neutral) to dark blue color (oxidized) of ECD is demonstrated with robust cycle life. These results provide an avenue for applications of PEDOT family in electrochromic devices.     

Ultrathin Polyoxometalate Coating as the Redox Shuttle for Acid‐Free Electrochromic Polymer Capacitive Windows

Polyoxometalate is explored as a redox shuttle for non‐electrochromic charge‐balancing material of the conjugated polymer (CP)‐based electrochromic window (ECW). H₃PW₁₂O₄₀ is electrochemically deposited on a TiO₂ nanoparticle film that is coated on a fluorine doped tin oxide glass. The content of PW₁₂ is increased as the TiO₂ film is thicker. The PW₁₂‐coated TiO₂ film (PWTNF) is applied as a counter electrode (CE) of a CP‐based ECW in an ionic liquid electrolyte, to increase the potential distribution at the CP‐coated working electrode, thereby decreasing the operation voltage of the ECW. This could be ascribed to the function of PW₁₂ as an electron acceptor and redox shuttle that allows effective charge transport from/to the CE and nearby TiO₂ nanoparticles. For the optimized ECW with PWTNF as a CE, the maximum optical contrast is obtained at the applied voltage of 1.5 to ?2.0 V (68%), along with long cyclability (2500 cycles), bistability (> 2.35 h) and UV stability (> 60 h). The ECW with PWTNF exhibits reproducible capacitive properties, along with high figure of merit for color change with a high power output, in acid‐free electrolyte for the first time.     

Bright green organic light-emitting devices having a composite electron transport layer

A bright green organic light-emitting device employing a co-deposited Al-Alq₃ layer has been fabricated. The device structure is glass/indium tin oxide (ITO)/ N, N′-diphenyl-N, N′- (3-methylphenyl)-1, 1′-biphenyl-4, 4′-diamine (TPD)/tris(8-quinolinolato) aluminum (Alq₃)/ Al-Alq₃/Al. In this device, Al-Alq₃ is used as electron transport layer (ETL). The device shows an operation voltage of 6.1 V at 20 mA/cm². At optimal condition, the brightness of a device at 20 mA/cm² is 2195 cd/m² achieved a luminance efficiency of 5.64lm/W. The result proves that the composite Al-Alq₃ layer is suitable for the ETL of organic light-emitting devices (OLEDs).