Self-assembled polymer layers of linear polyethylenimine for enhancing electrochromic cycling stability

Abstract

A self-assembled polymer of linear polyethylenimine (LPEI) on indium tin oxide (ITO) is found to stabilize the titanium-doped vanadium oxide film. After modification by LPEI, the oxide film shows good cycling stability, and sustains over 1500 cycles in a threeelectrode system, a remarkable improvement compared to that without LPEI modification.Transition metal oxides such as tungsten, vanadium, titanium and molybdenum oxides are among the most studied electrochromic materials. 1-3 Out of these transition metal oxides, vanadium oxide shows anodic and cathodic coloration, and its electrochromism is attributed to a reversible reduction/oxidation process accompanying the intercalation/deintercalation of cations and electrons. Vanadium oxide has been studied extensively as a counter electrode in electrochromic devices; 4-7 however, reports on its intrinsic electrochromic properties are very limited as compared with other metal oxides due to some disadvantages of low conductivity, narrow optical modulation, and inferior cycling stability. Furthermore, thermal treatment is always required for the fabrication of vanadium oxide lms, especially for those synthesized from aqueous solutions, in order to ensure that the lms rmly adhere to the substrates. Nevertheless, the thermal treatment somewhat deteriorates the electrochromic properties, especially the switching speed. 8,9 Hence, fabrication of stable lms, which do not require thermal post-treatment, is highly desired for electrochromism.Typically, a self-assembled monolayer can incorporate a wide range of functional groups both in the alkyl chain and at the chain terminal. 10-15 Accordingly, a variety of surfaces with specic interactions can be produced with ne chemical control. Polymers are also used to modify the surface properties, especially in the layer-by-layer (LbL) assembly system. 16,17 Herein, a layer of linear polyethylenimine (LPEI) was introduced for increasing the interfacial interaction between vanadiumbased oxide lm and ITO substrate to enhance the lm stability. Modication of the ITO surface by LPEI polymer offers a few advantages: (1) it does not block electron transfer at the electrode surface and (2) it improves the adhesion between the metal oxide and the ITO substrate due to the electrostatic and hydrogen-bonding interactions. Therefore, the modication of the ITO surface by the LPEI polymer enhances the stability of the as-prepared lm that therefore eliminates the need for thermal treatment. This is likely to pave the way for fabricating electrochromic materials on thermally sensitive substrates, such as polymers.For comparison, poly(diallyldimethylammonium chloride) (PDDA) was also selected to modify the ITO substrate. Aer modication by LPEI and PDDA, a positively charged surface on ITO was obtained. The modied ITO was used as the working electrode for the electrochemical deposition of titanium doped vanadium oxide lms (see the ESI † for the detailed procedure). The introduction of a titanium component into the vanadium oxide lm was ...

Recent Advances in Flexible Electrochromic Devices: Prerequisites, Challenges, and Prospects

“…In general, EC materials can be classified into three different categories. The first category comprises transition metal oxides, for example, the most commonly adopted are tungsten oxide (WO 3 ), nickel oxide (NiO), vanadium pentoxide (V 2 O 5 ), and the polymers, such as polyaniline, 3,4‐propylendioxythiophene (ProDOT), and poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) …”

“…The modified EC film showed good cycling stability for 1500 cycles with retention of 63.3 % of the initial contrast. The adhesion of the EC layer to the underlying substrates could be enhanced by surface functionalization, which may also be applicable to stretchable devices . Optimization of the EC layer thickness can also improve the adhesion stability .…”

Green synthesis of nanobelt-membrane hybrid structured vanadium oxide with high electrochromic contrast

“…The surface treatment was carried out following the procedure outlined in our previous work. 39 Briey, ITO/glass was washed thoroughly in ethanol, then water, and dried via purging with nitrogen gas before immersing in 1 mg L À1 aqueous linear polyethylenimine(LPEI, Mw 25 000) solution for 20 min with pH of 2.5 adjusted by 1 M HCl.…”

“…23, 27, 39 and 40) in the visible range of light spectrum. Besides, the long switching time, sometimes up to hundreds of seconds of pristine vanadium pentoxide, 41 and the poor stability manifested by a severe peeling within cycles 25,39 limit its practicality as a material for commercial applications.…”

Foldable Electrochromics Enabled by Nanopaper Transfer Method

“…For inorganic metal oxide materials, the color switching process is always coupled with the intercalation and de‐intercalation process of ions from electrolyte into/out of the inorganic lattice structures. During these processes, the electrochromic materials would expand and undergo structure deformation which often results in the peel‐off of the active films . It has been proven that soft paper surface that can be shaped into a wrinkled structure is very effective in alleviating the intercalation stress and it is very useful to enhance electrochemical properties .…”