Wei Teng Neo

Conjugated polymer-based electrochromics: materials, device fabrication and application prospects

The research on electrochromic conjugated polymers (ECPs) and their devices has gone a long way. They can now perform with a multitude of color variations, high optical contrasts, rapid switching speeds and low power consumption. This review traces the progress of color control, synthetic methods, and the development of building blocks for ECPs. The advances in the integration and optimization of electrochromic devices (ECDs) in terms of their constituting components are presented. The potential of ECDs in numerous applications through the illustration of several prototypes is then highlighted. Finally, the challenges associated with the prospective commercialization of the said devices are discussed.

Modulating high-energy visible light absorption to attain neutral-state black electrochromic polymers

Neutral-state black diketopyrrolopyrrole (DPP)-based conjugated polymers with the colour channels a* from −3 to 0 and b* from −2 to −7 are achieved by incorporating an additional donor segment – dialkoxythiophene (DalkOT), which leads to the enhancement in blue-wavelength absorption. The polymers display black-to-transmissive grey electrochromic switching with good redox stability.

Synthesis of Ultrahighly Electron-Deficient Pyrrolo[3,4-d]pyridazine-5,7-dione by Inverse Electron Demand Diels–Alder Reaction and Its Application as Electrochromic Materials

A new electron acceptor 6-alkylpyrrolo[3,4-d]pyridazine-5,7-dione (PPD) with a very low LUMO level has been synthesized via a challenging inverse electron demand Diels-Alder reaction between thiophene and furan-decorated tetrazine substrates and an electron-deficient 1-alkyl-1H-pyrrole-2,5-dione unit. The PPD monomer has been incorporated into a series of donor-acceptor-type conjugated polymers as electrochromic materials with good optical contrast, fast switching speed, and high coloration efficiency.

Pyrrolophthalazine dione (PPD)-based donor–acceptor polymers as high performance electrochromic materials

A novel pyrrolophthalazine dione (PPD) acceptor was sophisticatedly designed and synthesized via an inverse electron demand Diels–Alder reaction, followed by aromatization with N-bromosuccinimide. The PPD moiety was incorporated into donor–acceptor type conjugated polymers through Stille coupling polymerization to yield a series of polymers P1–P3 with different linking groups, namely, thiophene, thieno[3,2-b]thiophene, and bithiophene. The polymers reveal excellent solubility in common organic solvents. Due to their low bandgaps of around 1.72–1.78 eV, the polymers show broad absorption that covers most of the visible region. All the polymers reveal electrochromism, switching between dark blue and transmissive sky blue states. In particular, P2 exhibits exceptional electrochromic properties with high optical contrasts of up to 34 and 71% in the visible and NIR regions, high coloration efficiencies of 471 and 651 cm2 C−1 respectively, as well as reasonable switching speeds and good long-term stability.

Solution-processable blue-to-transmissive electrochromic benzotriazole-containing conjugated polymers

A series of donor–acceptor type electrochromic (EC) conjugated polymers P1–P4, poly(4,7-bis(3′,4′-ethylenedioxythiophen-2′-yl)-2-alkylbenzotriazole-5′,5′-diyl-alt-(3,4-dialkoxy-thiophen-2,5-diyl), was synthesized via a Stille coupling reaction. The polymers were characterized by gel permeation chromatography (GPC), nuclear magnetic resonance spectroscopy (NMR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The electrochemical, spectroelectrochemical and EC properties of the polymers were studied. The polymers displayed improved solubility in common organic solvents. Upon oxidation, the polymers switched from a saturated blue to a transmissive light blue state. In particular, P4 exhibited exceptional optical contrasts of 46% and 72% in the visible and near-infrared regions, respectively. Furthermore, the effects of the side chains on the EC properties were examined and the results revealed that polymers with a long alkyl chain in the benzotriazole moiety had faster switching speed both in the visible and near infrared regions in the polymer film state. The absorption–transmission ECDs were fabricated, revealing that under ambient conditions and without any encapsulation of ECDs, the polymers exhibited good optical contrasts and coloration efficiencies with improved stability.

Ultrahigh electron-deficient pyrrolo-acenaphtho-pyridazine-dione based donor–acceptor conjugated polymers for electrochromic applications

Novel electron acceptors 2-alkyl-6,9-di(thiophen-2-yl)-1H-pyridazino[4′,5′:2,3]indeno[6,7,1-def]isoquinoline-1,3(2H)-dione and 2-alkyl-6,9-di(furan-2-yl)-1H-pyridazino[4′,5′:2,3]indeno[6,7,1-def]isoquinoline-1,3(2H)-dione derived from pyrrolo-acenaphtho-pyridazine-diones (PAPD) with a very low-lying lowest unoccupied molecular orbital (LUMO) level have been synthesized via a regioselective inverse electron demand Diels–Alder reaction between thiophene- and furan-substituted 1,2,4,5-tetrazine and an electron-deficient compound 2-(2-alkyl)-1H-indeno[6,7,1-def]isoquinoline-1,3(2H)-dione. The chemical structures of two PAPD monomers were confirmed by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry as well as single-crystal X-ray structural analysis. The time-dependent density functional theory (TD-DFT) calculations were performed to show that PAPD series monomers have a LUMO energy level down to −3.42 eV, much lower than the popular electron acceptors such as benzotriazole, benzothiadiazole and its fluorinated derivatives (−2.19–−2.98 eV). The PAPD based monomer was incorporated into a series of donor–acceptor type conjugated polymers comprising 3,3-bis((dodecyloxy)methyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine and thieno[3,2-b]thiophene as co-monomers through Stille coupling polymerization to give electrochromic conjugated polymers P1–P5 with high number-average molecular weights in the range of 42 000–67 000 (g mol−1). The polymers showed optical bandgaps between 1.90–1.99 eV. Electrochromic devices displayed reversible color changes between purple/red in the neutral state and greyish blue/grey in the oxidized state with an outstanding redox stability of less than 1% decrease in contrast after 800 cycles using polymer P3 as an example, and high optical contrasts of up to 80% at 1500 nm in the near infrared region.

Effects of fluorination on the electrochromic performance of benzothiadiazole-based donor–acceptor copolymers

A series of thiophene and benzothiadiazole-based copolymers (PDAT-DTBT) is synthesized through Stille coupling polymerization with two mono- and di-fluorinated benzothiadiazole analogues: PDAT-DTBT-F (1F) and PDAT-DTBT-2F (2F). The introduction of fluorine atoms onto the conjugated polymer backbone is found to have a pronounced effect on the optical, electrochemical and morphological properties, which in turn, influences the electrochromic performance of the fabricated absorption/transmission type devices greatly. All the polymers switch reversibly between the colored neutral states (green/blue) to transmissive oxidized states. Systematic enhancement in the reduction process to sub-second speeds (<0.5 s), high coloration efficiency (748 cm2 C−1) and substantially improved ambient stability are observed upon fluorination of electron acceptors. Long-term stability testing of the PDAT-DTBT-F electrochromic device is carried out for up to 10 000 repeated redox cycles between the applied potentials of +1.6 and −1.6 V, without the observation of significant degradation.

Solution-processable low-bandgap 3-fluorothieno[3,4-b]thiophene-2-carboxylate-based conjugated polymers for electrochromic applications

In this paper, a series of low-bandgap donor–acceptor (D–A) conjugated polymers with 3-fluorothieno[3,4-b]thiophene-2-carboxylate (FTT) as an acceptor and ethylenedioxythiophene (EDOT) (P1), acyclic dioxythiophene (AcDOT) (P2) or propylenedioxythiophene (ProDOT) (P3) as donors were synthesized via Stille polymerization. The resultant polymers have good solubility in organic solvents. The polymers were characterized by gel permeation chromatography (GPC), nuclear magnetic resonance spectroscopy (NMR) and thermogravimetric analysis (TGA). Their electrochemical, morphological and electrochromic (EC) properties were investigated, and their absorption-transmission type electrochromic devices (ECDs) were fabricated and characterized. In their neutral states, the polymers displayed deep magenta (P1) to blue (P2, P3) hues, and upon electrochemical oxidation, they revealed grey tones with good optical contrasts (19–37 and 57–58% in visible and near-infrared (NIR) regions, respectively), good coloration efficiencies (158–380 and 279–378 cm2 C−1 for visible and NIR regions, respectively) and reasonable redox stability (retaining 64–80% original optical contrast after 1000 cycles) under ambient conditions and without any encapsulation of the ECDs.

Electrochromic π-Conjugated Copolymers Derived from Azulene, Fluorene, and Dialkyloxybenzothiadiazole

A series of random π-conjugated copolymers P1, P2, and P3 were synthesised from 1,3-dibromoazulene, 4,7-dibromo-5,6-bis(dodecyloxy)benzo-2,1,3-thiadiazole, and 9,9-dioctylfluorene-2,7-bis(trimethyleneborate) via Suzuki coupling reactions. The copolymers P1–3 had molecular weights in the range of 17000–30900 g mol–1 with polydispersity indexes of 1.45–2.03. Thermal analysis showed that the polymers P1–3 had good thermal stability with decomposition temperatures ranging from 341 – 363°C both in air and in nitrogen. Photoluminescence studies showed that polymer P1 and P2 are weakly fluorescent with low quantum yields of 0.013 and 0.0029 for P1 borne with 30 % azulene and P2 borne with 50 % azulene in the polymer backbone, respectively. P3 borne with 70 % azulene resulted in complete quenching of fluorescence. The electrochemical band gaps for P1–3 are very close to their corresponding optical band gaps. Electrochromic study showed that three polymer thin films displayed the same colour change from yellowish green at the neutral and electrochemically-reduced state to greyish brown at the electrochemically-oxidised state. In particular, electrochromic contrasts of 17 % and 13 % for P2 and P3, respectively, were recorded in the near infrared region.

Enhancing the electrochromic performance of conjugated polymers using thermal nanoimprint lithography

Thermal nanoimprint lithography was utilized as an efficient method to directly create periodic nanostructures on the conjugated polymer films that were synthesized from 2,5-bis(2-octyldodecyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione, 3,3-bis((tetradecyloxy)methyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine and thieno[3,2-b]thiophene monomers. This type of nanopatterned polymer film with 275 nm features was achieved by the use of a direct, top-down thermal nanoimprinting method, and then fabricated into absorption/transmission type electrochromic devices. The nanopatterned devices displayed considerably better overall electrochromic performance than their non-nanostructured pristine counterpart as evidenced by the improvement of up to 20% in switching speed, coloration efficiency as well as enhanced colored-to-transmissive optical contrast. The improvement in electrochromic performance could be ascribed to the increased exposed polymer surface area induced upon nano-structuring of the conjugated polymer films, which was demonstrated by the fact that the increase in the imprint depth from 100 to 130 nm led to a further improvement in the electrochromic performance such as shorter switching speeds and higher coloration efficiency. In addition, unlike the pristine un-patterned polymer film which displays an identical hue regardless of the viewing angle, the nanoimprinted films gave different hues under different viewing angles because of the ability of the nanogratings to diffract ambient white light, potentially opening up the path to many interesting applications.