Reactive Sputter Deposition of WO₃/Ag/WO₃ Film for Indium Tin Oxide (ITO)-Free Electrochromic Devices

Abstract

Functioning both as electrochromic (EC) and transparent-conductive (TC) coatings, WO3/Ag/WO3 (WAW) trilayer film shows promising potential application for ITO-free electrochromic devices. Reports on thermal-evaporated WAW films revealed that these bifunctional WAW films have distinct EC characteristics; however, their poor adhesive property leads to rapid degradation of coloring-bleaching cycling. Here, we show that WAW film with improved EC durability can be prepared by reactive sputtering using metal targets. We find that, by introducing an ultrathin tungsten (W) sacrificial layer before the deposition of external WO3, the oxidation of silver, which leads to film insulation and apparent optical haze, can be effectively avoided. We also find that the luminous transmittance and sheet resistance were sensitive to the thicknesses of tungsten and silver layers. The optimized structure for TC coating was obtained to be WO3 (45 nm)/Ag (10 nm)/W (2 nm)/WO3 (45 nm) with a sheet resistance of 16.3 Ω/□ and a luminous transmittance of 73.7%. Such film exhibits compelling EC performance with decent luminous transmittance modulation ΔTlum of 29.5%, fast switching time (6.6 s for coloring and 15.9 s for bleaching time), and long-term cycling stability (2000 cycles) with an applied potential of ±1.2 V. Thicker external WO3 layer (45/10/2/100 nm) leads to larger modulation with maximum ΔTlum of 46.4%, but at the cost of significantly increasing the sheet resistance. The strategy of introducing ultrathin metal sacrificial layer to avoid silver oxidation could be extended to fabricating other oxide-Ag-oxide transparent electrodes via low-cost reactive sputtering.

The Effect of Metal in MoOx/Metal/MoOx Anode on Open Circuit Voltage in Organic Photovoltaic Cells

“…As known, the MS‐sputtered metal films have higher density and stronger adhesion to the substrate than the TE films . Besides, as seen from the S q values in Table , the electrodes with MS metals on glass are, in general, smoother than fully TE electrodes.…”

“…Short circuit current J sc in the cells with a TE anode correlates well with the value of sheet resistance R sq measured on the blank trilayer electrodes (Table ). There are at least three factors that cause the J sc value to fluctuate when the anode metal is being changed: i) resistive losses, basically associated with the series resistance of composite electrode; ii) optical losses, basically associated with the different transparencies of the MoO x /metal/MoO x electrodes through which the device is illuminated; or iii) unintentional doping of the SubPc layer by the mobile metal atoms released from the electrode . In our case J sc is the reciprocal function of R sq (Table ).…”

“…In this series, chromium, nickel, silver, aluminum, and tantalum were deposited on glass by the MS technique and then covered with a thermally evaporated MoO x overlayer, in the same way as done for the reference cells. TE of the MoO x seed layer on glass was omitted here because of the unexpectedly large sheet resistance of such trilayer systems. The archetypal cells with MS metals in the bilayer anode were similarly fabricated and tested, and their performance was compared to that of the cells with the fully TE anodes.…”

Direct Regulation of O/W Stoichiometric Ratio and Microstructure in Tungsten Oxide Electrochromic Films by Ar Pressure Using Oxide Target Sputtering

“…The transmittance of films was also altered according applied voltage. The optical modulation is an important EC performance parameter and can be defined aswhere T b and T c represent the transmittance in the bleached and colored states, respectively …”

“…The coloration efficiency ( η ) is usually used to assess the EC device performance and also defined bywhere ΔOD is the variation in optical density at a given wavelength (here, we use the transmittance at 550 nm), calculated by the logarithm of the transmittance ratio between T b and T c , and Q is the injected charge density, corresponding to the ratio of the inserted charge over the device area . According to this formula, the coloration efficiencies of the tungsten oxide films sputtered at Ar gas pressures of 0.8, 1.2, 1.6, and 2.0 Pa were calculated as 55.3, 51.3, 38.6, and 36.5 cm 2 C −1 , respectively.…”