Organic Electronics for Electrochromic Materials and Devices. Hong Meng

      Source: Kraft [25].

      In addition, when insightfully considering the injected/ejected charge Q_d, we can find it in fact to consist of three part: faradaic charge Q_F associated with doping/de‐doping, capacitive charge Q_C due to the capacitive nature of the ECD, and parasitic charge Q_P associated with electrolyte/impurity reactions. Among them, the faradaic charge is the source of redox activity leading to chromic change actually. Therefore, Fabretto et al. reported a new technique for measuring CE by extracting the faradaic charge from the total charge and calculated the only faradaic charge‐based CE value [29]. As we discussed, the total charge flow is simply the addition of the three individual charge flows and is given by

      where the parasitic current was a small component (approximately <2%) compared with the other two and therefore can be ignored. Then the time–evolution total current flow can be described as following:

      where n is the number of electrons transferred per molecule, F is the Faraday constant (96 500 C/mol), A is the electrode area (cm²), C₀ is the concentration of species in the bulk solution (mol/cm³), D is the apparent diffusion coefficient (cm²/s), t is time in seconds, I₀ is the maximum current flow at t = 0, R is the cell resistance, and C is the double layer capacitance. Then fitting the experimental data to this equation and substituting the constant k, at last, a plot of the time–evolution faradaic current will be obtained, and the corresponding faradic‐corrected CEs can be calculated. Usually, the faradic‐corrected CEs are larger than the uncorrected results, because the total charge ingress/egress (i.e. Q_d) is larger than the faradic charge (i.e. QF).

      1.3.4 Optical Memory

The optical or EC memory (also called open‐circuit memory) of an EC material can be defined as the propensity of the material to retain its redox/colored state upon removing the external bias. Usually, the memory effect are often observed in film‐state EC materials such as conjugated polymers, which well adhered onto the electrode, and hence restrict the movement of the electrons. In contrast, some solution‐based ECDs (e.g. viologens) will exhibit a self‐erasing effect, which means the colored state disappeared rapidly in the absence of applied voltage because the electrons diffuse freely in this type of device. The memory effect is useful for the energy‐saving devices and also can be applied for data storage. Figure 1.9 shows an optical memory test. The short‐term memory was investigated by applying a potential pulse for 2 seconds prior to forming the open‐circuit state for 100 seconds; the transmittance change at 423 nm was monitored simultaneously (Figure 1.9a). Then a long‐term memory is also studied by applying a potential for two seconds and removing the bias for one hour (Figure 1.9b). The EC conjugated polymers remain in the initial transmittance contrast well in the absence of an applied voltage, which exhibits a good optical memory.

Figure 1.9 Open‐circuit memory tests of PBOTT‐BTD spray coated on an ITO‐coated glass slide in 0.1 M TBAPF6/ACN at 423 nm: (a) short‐ and (b) long‐term performance.

      Source: Li et al. [21].

      1.3.5 Stability

In most cases of laboratory study, researchers record the number of redox cycles that an EC material stand without significant loss in the performance as the electrochemical stability, irreversible oxidation or reduction at extreme potentials, side reactions with water or oxygen, and heat release in the system during switches may cause the degradation of electrochemical stability. Usually, the charge density Q_d recorded under electrochemical cycling is up to 10⁴–10⁶, as shown in Figure 1.10a. The charge density of a Ti‐doped V₂O₅ EC film haven't changed through 2 × 10⁶ cycles; meanwhile the transmittance change at a certain wavelength during continuous cycling is also important to describe the stability of an EC material. Such as shown in Figure 1.10b, the transmittance of the ECD remains stable through 200 000 cycles. Actually, the CE change after numerous cycles also can be used to evaluate the long‐term stability of EC materials, because it contains information of both transmittance and charge density.

Figure 1.10 Charge density (a) and transmittance (b) variation curves of ECD with the cycle number K : 1000.

      Source: Wei et al. [30].

However, if we consider the real application of ECD in building windows, there are more strict conditions for durability and reliability. For instance, a lifetime over 20 years with more than 10⁶ switching cycles is necessary. Extreme weather conditions such as temperatures below −20 °C and above +40 °C are huge challenge for both EC materials and electrolytes, as well as other degradation factors such as high solar irradiation levels, fast temperature changes, uneven temperature distribution and additional stresses, rain, humidity, mechanical shock, and drying. Therefore, in 1998, Carl M. Lampert proposed a standard test guideline for industry application of EC [31], as shown in Figure 1.11. Recently, the International Organization for Standardization (ISO) also has launched an international standard: Glass in building – Electrochromic glazings – accelerated aging test and requirements (ISO 18543) for EC use in buildings.

Figure 1.11 Recommended testing guidelines for EC windows for exterior architectural applications.

      Source: Lampert et al. [31].

1.4 Conclusion

In this chapter, a broad overview of electrochromism, EC materials, device structure, development history, and key parameters of electrochromism have been introduced briefly. More detailed descriptions of each area will be discussed in Chapters 2–15.