Candidate: Jonathan Atkinson
Title: Silver Nanowire Networks in Electrochromic Devices
Date: April 14, 2023
Time: 10:00 AM
Place: EIT 3151-53
Supervisor(s): Goldthorpe, Irene (Adjunct)
Abstract:
Transparent electrodes are a necessary component of electrochromic devices. These electrodes are most commonly made from indium tin oxide (ITO), but this material is far from ideal. ITO is expensive, brittle and thus unsuitable for flexible applications, and has low transparency in the near infrared (NIR) region which limits the solar heat gain of smart windows in the winter. Random networks of silver nanowires are a promising alternative material to replace ITO since they are cheaper, simpler to deposit, have deposition temperatures compatible with plastic substrates and are much more mechanically flexible. This thesis researches if and how silver nanowires can improve the performance of electrochromic devices.
Two outstanding problems with silver nanowire electrodes that hinder their widespread use is tackled. First, their low lifetime due to silver corrosion and their surface roughness. In chapter 2 a passivation strategy that meets the requirements for electrochromic devices is explored. Unlike many nanowire electrode passivation materials used in the literature, the use of a non-conductive passivation layer is researched here which allows the use of transparent polymers. Of the candidates tested, polyurethane (PU) was found to perform the best with electrode resistance only increasing 1.8X after six months. PU is cheap and easy to deposit, 96% transparent across the visible and NIR regions, increases the mechanical flexibility of nanowire electrodes, and improves nanowire adhesion and surface roughness. It is shown through simulation and electrostatic force microscopy measurements that despite the non-conductive nature of the passivation material, the magnitude of the electric field above the electrode is quite uniform, decreasing by at most a factor of 0.5 above nanowire gaps compared to directly above a nanowire. Electrochromic displays made with PU-passivated nanowire electrodes have a uniform colour switch across the device area.
In Chapter 3, the PU-passivated nanowire electrodes are integrated into mechanically flexible PEDOT:PSS based electrochromic displays. Compared to the same devices that used ITO electrodes, these devices have improved colour changing properties and a longer lifetime. Most noteworthy is their far superior mechanical properties. After 50 bending cycles, the devices with nanowire electrodes had little change in performance whereas devices with ITO no longer worked. Also, the capability of making these displays not only flexible, but recyclable as well is demonstrated by successfully printing working nanowire/PEDOT:PSS electrochromic devices on biodegradable paper in place of plastic substrates.
One issue with PEDOT:PSS as an electrochromic material is its low conductivity, causing PEDOT:PSS based electrochromic devices to have slow switching speeds and elevated operating voltages. In Chapter 4, it is shown that sheet resistance can be lowered from 280 Ω/sq to 34 Ω/sq by mixing silver nanowires into the PEDOT:PSS electrochromic layer. The longest, thinnest nanowires at a concentration of 3.0 mg/ml led to the best performance. The addition of nanowires into the electrochromic layer more than halved switching times and lowered the turn-on voltage from 1.5 to 1.1 V. Furthermore, the increased conductivity allows the device to operate well without transparent electrodes thereby reducing cost and complexity. Passivation of the silver nanowires in solution before mixing with PEDOT:PSS was attempted using two small molecules, MuA and MBI. However, the performance of electochromic devices using these passivated nanowires was poor due to nanowire clumping. Alternative suggestions for passivating silver nanowire surfaces in solution are given.
One advantage of silver nanowire electrodes that has received little attention in the literature is their high transparency in the NIR region, which is highly desirable for some applications including smart windows. In Chapter 5 it is shown that for electrodes that are 96% transparent in the visible, ones made from ITO are only 35% transparent at a wavelength of 2500 nm, while those made from silver nanowires maintain a transparency as a high as 94%. Experiments and modelling show that to minimize the transparency drop from the visible to the NIR, the nanowires should be sparse and larger in diameter. This is found to be attributed to both the larger average spacing between nanowires in such networks and the lower absorption losses of larger diameter nanowires in the NIR.
In Chapter 6, silver nanowire electrodes are integrated into tungsten oxide/nickel oxide based electrochromic smart windows and compared to the same windows with ITO electrodes through both experiments and modelling. Windows using nanowire electrodes are shown to have higher NIR transparency, leading to a higher solar heat gain coefficient. This would allow more heat to enter a building in the winter, thereby improving energy efficiencies in cold climates.