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Organic Electronics for Electrochromic Materials and Devices. Hong MengЧитать онлайн книгу.

Organic Electronics for Electrochromic Materials and Devices - Hong Meng


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lamda Over integral Subscript lamda Subscript min Baseline Superscript lamda Subscript max Baseline Baseline upper S left-parenthesis lamda right-parenthesis upper P left-parenthesis lamda right-parenthesis normal d lamda EndFraction"/>

      1.3.2 Switching Time

      In the context of electrochromism, the switching time (t) can be defined as the time needed for EC materials to switch from one redox state to the other. It is generally followed by a square wave potential step method coupled with optical spectroscopy. Switching time depends on several parameters, such as the ability of the electrolyte to conduct ions as well as the ease of intercalation and deintercalation of ions across the EC active layer, the electrical resistances of electrolytes, and the transparent conducting films. Usually the liquid electrolyte has a lower resistance than the solid electrolyte; therefore the half device and the liquid electrolyte ECD will exhibit a rapid switching than solid ECD. Meanwhile, the large area ECD such as the large smart windows will show a lower switching compared with the small laboratory samples due to the larger electrical resistances of transparent conducting films. However, fast switching is not required in all applications, such as the switchable window technologies; the obvious color change process will increase the fun of user experience. Conversely, the sub‐second magnitude rapid switching is particularly desired for display applications.

Graphs depict the switching time of EC materials. (a) Electrochemical switching time. (b) Optical switching time.

      Source: Li et al. [21]. © 2018, Royal Society of Chemistry

      (b) Optical switching time.

      Source: Hsiao et al. [27].

upper Delta upper T upper M left-parenthesis t right-parenthesis equals upper Delta upper T normal upper M Subscript max Baseline left-parenthesis 1 minus normal e Superscript negative t slash tau Baseline right-parenthesis

      where ΔTmax represents the full‐switch contrast obtained for long pulse lengths and τ is the time constant. If switching time t is equal to τ, 63.2% of the maximum transmittance change is reached. At a time of 2.3τ, 90% ΔTMmax is switched, identically 95% and 99% of ΔTMmax corresponding to 3τ and 4.6τ.

      Therefore, for new EC materials, the same chronoabsorptometric responses [28] should be measured and fitted to the aforementioned function. From the fittings, the max values of ΔTMmax (the contrast corresponding to a full switch), the time constant τ, and the corresponding regression coefficient r2 will be obtained. Afterwards the switching time t90% or t95% will be easily calculated. This method allows an easy direct comparison between different reported values.

      1.3.3 Coloration Efficiency

      Coloration efficiency (CE) plays a fundamental role in the evaluation of the efficiency of charge utilization during the EC processes. It relates the optical absorbance change of an EC material at a given wavelength (ΔA) to the density of injected/ejected electrochemical charge necessary to induce a full switch (Qd). The higher CE value indicates a large transmittance change with a small amount of charge, which makes more effective use of the injected charge. CE value can be calculated using the following equation:

upper C upper E equals StartFraction upper Delta upper A Over upper Q Subscript d Baseline EndFraction equals StartStartFraction log left-parenthesis StartFraction upper T Subscript o x Baseline Over upper T Subscript neut Baseline EndFraction right-parenthesis OverOver upper Q Subscript d Baseline EndEndFraction Graph depicts the calculation of Qd.

      Source: Hsiao et al. [27].

Graph depicts the different types of CE value of the same EC materials.
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