**1.2 Cholinergic system**

The cholinergic system consists of organized nerve fibers responsible for projecting nerve impulses, also known as action potentials through acetylcholine. The release of acetylcholine stimulates the nerve impulse during transmission. Memory, selective attention, and emotional processing are among the cognitive activities connected with the cholinergic system.


**6. What should be the convergence criteria?**

In contrast to the simple analytical functions that we have used to illustrate the operation of the various Minimization methods, in 'real' molecular modeling applications it is rarely possible to identify the 'exact' location of minima and saddle points. We can only ever hope to find an approximation to the true minimum or saddle point. Unless instructed otherwise, most Minimization methods would keep going forever, moving ever closer to the minimum. It is therefore necessary to have some means to decide when the Minimization calculation is sufficiently close to the minimum and so can be terminated. Any calculation is of course limited by the precision with which numbers can be stored on the computer, but in most instances it is usual to stop well before this limit is reached. A simple strategy is to monitor the energy from one iteration to the next and to stop when the difference in energy between successive steps falls below a specified threshold. An alternative is to monitor the change in coordinates and to stop when the difference between successive configurations is sufficiently small. A third method is to calculate the root-meansquare gradient. This is obtained by adding the squares of the gradients of the energy with respect to the coordinates, dividing by the number of coordinates and taking the square root. It is also useful to monitor the maximum value of the gradient to ensure that the Minimization has properly relaxed all the degrees of freedom and has not left a large amount of strain in one or two coordinates [23].


*2.3. Breakage*

To get the equation for the pure breakage, put β(*x*, *x*) = 0 in Equation (1). Hence, the breakage process can be written as the following linear differential equation

$$\frac{\partial u(\mathbf{x},t)}{\partial t} = \int\_{\mathbf{x}}^{\infty} b(\mathbf{x},y)S(y)u(y,t)dy - S(\mathbf{x})u(\mathbf{x},t). \tag{33}$$

So, integrating Equation (33) over the cell *xi*−12 , *xi*+ 12 , can be obtained

$$\frac{d\underline{u\_i}}{dt} = B\_i - D\_{i\nu} \tag{34}$$

where

$$B\_{i} = \frac{1}{\Delta x\_{i}} \int\_{x\_{i-\frac{1}{2}}}^{x\_{i+\frac{1}{2}}} \int\_{x}^{x\_{L\_{1}+\frac{1}{2}}} b(\mathbf{x}, \boldsymbol{\varepsilon}) \mathbb{S}(\boldsymbol{\varepsilon}) u(\boldsymbol{\varepsilon}, t) d\boldsymbol{\varepsilon} d\mathbf{x} \tag{35}$$

and

$$D\_i = \frac{1}{\Delta x\_i} \int\_{\mathcal{X}\_{i-\frac{1}{2}}}^{\mathcal{X}\_{i+\frac{1}{2}}} S(\mathbf{x}) u(\mathbf{x}, t) d\mathbf{x},\tag{36}$$

where *L*1 is the number of discrete size classes.

Take the approximation of the initial data as

$$u(\mathbf{x},0) = \frac{1}{\Delta \mathbf{x}\_i} \int\_{\mathbf{x}\_{i-\frac{1}{2}}}^{\mathbf{x}\_{i+\frac{1}{2}}} u\_0(\mathbf{x})d\mathbf{x}.\tag{37}$$

*Processes* **2019**, *7*, 535

Proceeding in the same way as Kumar et al. [43], the discretized form with two weight functions can be obtained as

$$\frac{du\_i}{dt} = \frac{1}{\Delta x\_i} \sum\_{k=i}^{L\_1} \varphi\_k^b S\_k u\_k \Delta x\_k B\_{ik} - \psi\_i^d S\_i u\_i. \tag{38}$$

where the weighted parameters for birth and death ϕ*bi*and ψ*di*are given by

$$\varphi\_i^b = \frac{\mathbf{x}\_i(\mathfrak{d}(\mathbf{x}\_i) - 1)}{\sum\_{k=1}^{i-1} (\mathbf{x}\_i - \mathbf{x}\_k)B\_{ki}} \tag{39}$$

and

$$\psi\_{i}^{d} = \frac{\varphi\_{i}^{b}}{x\_{i}} \sum\_{k=1}^{i} x\_{k} B\_{ki} \quad \text{i} = 2, 3, 4, \dots, L\_{1} \; . \tag{40}$$

Take ϕ*b*1 = 0, ψ*d*1 = 0 and

$$B\_{ik} = \int\_{x\_{i-\frac{1}{2}}}^{p\_k^t} b(x, x\_k) dx \tag{41}$$

with

$$p\_k^i = \begin{cases} \mathbf{x}\_{i\nu} & k=i\\ \mathbf{x}\_{i+\frac{1}{2}\nu} & k \neq i \end{cases} \tag{42}$$

and

$$\mathcal{S}(\mathbf{x}\_{i}) = \int\_{0}^{\mathbf{x}\_{i}} b(\mathbf{x}\_{i}|\mathbf{x}\_{i})d\mathbf{x}.\tag{43}$$

Multiplying Δ*xi* on both sides of Equation (38), one obtains

$$\frac{d\Delta \mathbf{x}\_{i}\boldsymbol{u}\_{i}}{dt} = \sum\_{k=i}^{L\_{1}} \boldsymbol{\varphi}\_{k}^{b} \mathbf{S}\_{k} \boldsymbol{\mu}\_{k} \Delta \mathbf{x}\_{k} \mathbf{B}\_{ik} - \boldsymbol{\psi}\_{i}^{d} \mathbf{S}\_{i} \boldsymbol{u}\_{i} \Delta \mathbf{x}\_{i}. \tag{44}$$

Take *gi* = Δ*xiui*. Then, Equation (44) can be rewritten as

$$\frac{d\mathcal{g}\_i}{dt} = \sum\_{k=i}^{L\_1} \varphi\_k^b S\_k g\_k B\_{ik} - \psi\_i^d S\_i \mathcal{g}\_i. \tag{45}$$

To get the particle number at some certain time point τ, the time span [0, τ] is divided into *K* subintervals [*t*, *t* + Δ*t*], where *t*1 = 0, *tk* = τ and Δ*t* is the time step.

The particle number *g*(*t* + Δ*t*) at time point *t* + Δ*t* using transformation matrix *T*(*t*, Δ*t*) is given by

$$
\mathcal{g}(t + \Delta t) = \mathcal{g}(t) \cdot T(t, \Delta t),
\tag{46}
$$

where *g*(*t*) is a 1 × *L*1 row matrix (vectors) with elements denoted by *gi*(*t*), and *T*(*t*, Δ*t*) is a *L*1 × *L*1 matrix whose elements are denoted by *Tij*(*t*, Δ*t*). The elements of the transformation matrix can be written with the help of the discretized form of Equation (33) as follows

$$T\_{ij}(t, \Delta t) = \begin{cases} 1 + \left(\boldsymbol{\varrho}\_{\boldsymbol{i}}^{b} \boldsymbol{B}\_{\boldsymbol{i}\boldsymbol{i}} - \boldsymbol{\psi}\_{\boldsymbol{i}}^{d}\right) \mathbf{S}\_{i} \Delta t, & \boldsymbol{i} = \boldsymbol{j} \\\ \boldsymbol{S}\_{i} \boldsymbol{B}\_{\boldsymbol{j}i} \boldsymbol{\varrho}\_{\boldsymbol{i}}^{b} \Delta t, & \boldsymbol{i} > \boldsymbol{j} \\\ \boldsymbol{0}, & \boldsymbol{i} < \boldsymbol{j} \end{cases} \tag{47}$$

The scheme (Equation (46)) gives the first-order accuracy. To get a higher order accuracy,

$$
\lg(t + \Delta t) = \lg(t) \cdot T(t, \Delta t),
\tag{48}
$$

is proposed using the Runge–Kutta method. Here

$$\widetilde{T}(t,\Delta t) = \left[T(t,\Delta t) \cdot T\left(t + \Delta t, \frac{\Delta t}{2}\right) + \left(I - T\left(t, \frac{\Delta t}{2}\right)\right)\right].\tag{49}$$

*I* is the identity matrix. The method given by Equations (48) and (49) gives the second order accuracy.

Consider a positive initial data *gi*(0) for all *i*. However, the solution *gi*(*t*) may become negative. In order to ensure positivity, a condition for the time step must be defined:

$$
\Delta t \le \min\_i \left| \frac{2g\_i(t)}{Q(t)} \right|, \tag{50}
$$

where

$$\begin{aligned} Q(t) &= \mathcal{W}\_1(\mathcal{g}(t)) - \mathcal{W}\_2(\mathcal{g}(t)) + \mathcal{W}\_1(\mathcal{g}^\*(t)) - \mathcal{W}\_2(\mathcal{g}^\*(t)), \\ &\quad W\_1(\mathcal{g}(t)) = \sum\_{k=i}^{L\_1} \mathcal{q}\_k^b S\_k \mathcal{g}\_k(t) B\_{ik} - \psi\_i^d S\_i \mathcal{g}\_i(t), \\ &\quad \mathcal{W}\_2(\mathcal{g}(t)) = \psi\_i^d S\_i \mathcal{g}\_i(t), \\ &\quad \mathcal{g}\_i^\*(t) = \mathcal{g}\_i(t) + \Delta t \mathcal{W}\_1(\mathcal{g}(t)) - \mathcal{W}\_2(\mathcal{g}(t)). \end{aligned} \tag{51}$$


The morphology of the as‐deposited heat‐treated and CdCl2 ‐treated samples CdTe‐20 and CdTe‐63 show the change in the shape and size of the CdTe grains as in **Figure 6**. The average grain size of the as‐deposited sample is under 01.7 µm and 0.86 µm in the heat‐treated sample respectively while some of the bigger grains divide into smaller grains and reorient themselves, which results into an entirely different microstructure. The SEM images support the XRD results.

**Figure 5.** X‐ray diffraction spectrum for CdTe samples.

**Figure 6.** SEM micrographs of CdTe‐20 and CdTe‐63 samples.

### **5.3. Optical analysis of CdS thin film by chemical bath deposition**

The CdS layer has good transmittance especially above 550 nm wavelength region as shown in **Figure 7**. Transmission starts after 300 nm by CBD process. In the region of 600–800 nm, the transmission is more than 70% in samples CdS 21 and CdS/CBD 23. This is the characteristic which shows that CdS is good window layer for the thin film solar cells of many kinds.

The band gap can be obtained by extrapolating (α*hv*)2 versus the incident photon energy (*hv*) plot. CdS 23 and CdS 24 samples fabricated by chemical bath deposition results showed a slight decrease in band gap values as well. The band gap values extracted from these plots are 2.36 eV and 2.33 eV.

### **5.4. Structural analysis of CdS by chemical bath deposition (CBD)**

20 30 40 50 60 70 80

**C[220]**

**C[311]**

**Figure 5.** X‐ray diffraction spectrum for CdTe samples.

**CdTe-20**

**CdTe-63**

**Figure 6.**  The data analysis provided lattice constant as 6.50 Å for as‐deposited sample. The lattice parameter can be calculated by using the relation for only cubic structure.

**Figure 7.** Transmittance vs. wavelength and energy band gap of CdS samples.

$$a = d \left( h^2 + k^{2+} l^2 \right)^{1/2} \tag{6}$$

where *h*, *k*, and *l* are Miller indices, for hexagonal structure formula for "*d*'' is

$$1/d^2 = 4/\Im\{\left(h^2 + hk + k^2\right) + \frac{l^2}{c^2}}\tag{7}$$

By using the Scherer formula, crystalline size can be calculated

$$\text{Crystalline size} = \frac{0.9\lambda}{\beta \cos \theta} \tag{8}$$

*λ* is X‐rays wavelength, 0.9 is a constant shape factor, *β* is full width half maxima in radi‐ ans, *θ* is Bragg's angle [25]. Peaks relating to CdS can be identified by using standard card JCPDS‐00‐041‐1049. The *d*‐spacing values were compared with standard values of ASTM cards to find the structure. It was observed in this experimental work that CdS thin films had a mixed structure of H (0 0 2) and C (1 1 1) for both techniques. So CdS has a polycrystalline behavior [20–22, 30–39]. Preferred orientation is (0 0 2) and structure is hexagonal as shown in **Figure 8**.

#### **5.5. Optical analysis of CdS thin film by close sublimation technique (CSS)**

The CdS deposited by close spaced sublimation have transmission for light above 550 nm then it again decreases for infrared region as shown in **Figure 9**. So if we consider only visible spectrum as for the solar cell point of view then it can be observed that from 600 to 800 nm, the transmission is more than 60% which shows that CdS is a good and efficient window layer for the visible spectrum or solar spectrum exclusively. The energy band gap is also showing the 2.44 and 2.38 eV for samples CdS 13 and CdS 318, respectively.

**Figure 8.** X‐ray diffraction spectrum for CdTe samples.

Effects of CdCl2 Treatment on Physical Properties of CdTe/CdS Thin Film Solar Cell http://dx.doi.org/10.5772/67191 401

**Figure 9.** Transmittance vs. wavelength and energy band gap of CdS samples.

*a* = *d* (*h*2 + *k*2 + *l*

400 Modern Technologies for Creating the Thin-film Systems and Coatings

1/*d*2 = 4/3((*h*2 + *hk* + *k*2 ) +*l*

By using the Scherer formula, crystalline size can be calculated Crystalline size = \_\_\_\_\_\_ 0.9*λ*

where *h*, *k*, and *l* are Miller indices, for hexagonal structure formula for "*d*'' is

**5.5. Optical analysis of CdS thin film by close sublimation technique (CSS)**

the 2.44 and 2.38 eV for samples CdS 13 and CdS 318, respectively.

H(002)

**Figure 8.** X‐ray diffraction spectrum for CdTe samples.

0

50

100

**Counts**

150

200

*λ* is X‐rays wavelength, 0.9 is a constant shape factor, *β* is full width half maxima in radi‐ ans, *θ* is Bragg's angle [25]. Peaks relating to CdS can be identified by using standard card JCPDS‐00‐041‐1049. The *d*‐spacing values were compared with standard values of ASTM cards to find the structure. It was observed in this experimental work that CdS thin films had a mixed structure of H (0 0 2) and C (1 1 1) for both techniques. So CdS has a polycrystalline behavior [20–22, 30–39]. Preferred orientation is (0 0 2) and structure is hexagonal as shown in **Figure 8**.

The CdS deposited by close spaced sublimation have transmission for light above 550 nm then it again decreases for infrared region as shown in **Figure 9**. So if we consider only visible spectrum as for the solar cell point of view then it can be observed that from 600 to 800 nm, the transmission is more than 60% which shows that CdS is a good and efficient window layer for the visible spectrum or solar spectrum exclusively. The energy band gap is also showing

20 30 40 50 60 70 80

C(311)

**CdS23 CBD**

**2**θ

H(110)

2)1/2 (6)

*β* cos*θ* (8)

*c* 2 (7)

2 \_\_

#### **5.6. Structural analysis of CdS by close sublimation technique (CSS)**

The XRD spectra were taken scanning the values of 2*θ* from 20 to 80° as shown in **Figure 10**. In CdS (CSS) high intensity planes were grown during the film growth due to high tempera‐ ture. In CdS (CSS) strong peaks C (1 1 1), and H(0 0 2) were for two samples 317 and 318 as observed, which proved the polycrystalline behavior of CdS thin films. The height of peak/ intensity for CdS (CBD) was only up to 200, 150, and 400, first strong peak was H (0 0 2) for two samples and C (1 1 1) for third sample at 2*θ* value of (26.49, 25.19, 26.74) degree of angle. In our research work for both CBD and CSS techniques, first hexagonal strong peak was dom‐ inant. The size of grains was different for different fabrication techniques this may be due to film thickness, temperature or different nucleation of CdS for different deposition methods.

#### **5.7. SEM analysis of CdS (CBD) and CdS (CSS)**

Surface morphology was studied by using scanning electron microscopy (SEM), and grain size for CdS (CSS) was measured to be 300–400 nm as shown in **Figure 11**.

**Figure 10.** X‐ray diffraction spectrum for CdS samples.

The surface of CBD films was slightly nonuniform but CdS (CSS) structure was fine with bet‐ ter crystallinity. SEM analysis showed that CBD‐CdS thin films grain size approximately was found to be (50–100) nm and CSS‐CdS films grain size was approximately (200–300) nm. It was also observed that at high source temperature, grain size was bigger in the CSS technique. It is reported in the literature that even less crystalline and non‐uniform films of CBD process gave high‐efficiency as compared to CSS [5, 40–42].

**Figure 11.** SEM micrographs of CBD‐CdS and CSS‐CdS samples.

The transmission spectra of ITO coated thin films and ITO/CdS films shows more than 90% light is passes in the visible as well as near infrared regions shown in **Figure 12**. Since a CdS thin film (100–200 nm) is used for solar cells, the CdS film is well suited as the TCS for CdTe solar cells.

**Figure 12.** Transmission as a function of wavelength of ITO substrates and ITO/CdS thin films.

### **6. Conclusions**

The surface of CBD films was slightly nonuniform but CdS (CSS) structure was fine with bet‐ ter crystallinity. SEM analysis showed that CBD‐CdS thin films grain size approximately was found to be (50–100) nm and CSS‐CdS films grain size was approximately (200–300) nm. It was also observed that at high source temperature, grain size was bigger in the CSS technique. It is reported in the literature that even less crystalline and non‐uniform films of CBD process

**0**

**500**

**1000**

**counts**

**1500**

**2000**

H(0 0 2)

**20 30 40 50 60**

O(7 2 2)

H(1 0 2)

**CdS 318**

**C(3 1 1)**

**2** θ

gave high‐efficiency as compared to CSS [5, 40–42].

**Figure 11.**  X‐ray diffraction spectrum for CdS samples.

**0**

**500**

**1000**

**1500**

**counts**

**2000**

**2500**

**3000**

C(1 1 1)

An ITO/CdTe/CdS/glass solar cell was successfully prepared using CSS and CBD techniques. Optical analysis of ITO demonstrated transmittance of over 85% for whole wavelength. The effects of CdCl2 immersion and heating on the optical, structural, and morphological prop‐ erties of CdTe and CdS surfaces were studied with an effort to promote recrystallization, reorientation, and progressive increase in grain size. Structural properties investigated by using XRD exhibited shift in 2*θ* diffraction angles of peaks, change in crystalline size, change in intensity of preferred orientation, and change in the total number of peaks, which indi‐ cates toward recrystallization and reorientation. The optical results after CdCl2 heat‐treat‐ ment showed a decrease in transmittance in the case of CdTe thin film and increase in the case of CdS thin film. Similarly, band gap values were also observed to decrease after the CdCl2 heat‐treatment. Surface morphology of CdTe thin films was affected by CdCl2 heat‐treatment as well. The SEM micrographs assisted in investigating the changes. A grain size of CdTe as‐deposited sample was found to improve after the CdCl2 heat‐treatment. Similarly, a grain size of CdS as‐deposited sample was found to improve after the CdCl2 heat‐treatment. In conclusion, the significant improvement to the CdTe/CdS films solar cell performance can be achieved when annealed at 400°C in the presence of CdCl2 on the free surfaces of CdTe and CdS. These results can be used in turn to improve the short circuit current and open circuit voltage of solar cells.

### **Author details**

Nazar Abbas Shah1 \*, Zamran Rabeel2 , Murrawat Abbas3 and Waqar Adil Syed4

\*Address all correspondence to<EMAIL_ADDRESS>
1 Thin Films Technology Research Laboratory, Department of Physics, COMSATS Institute of Information Technology, Islamabad, Pakistan

2 Higher Education Department, Punjab, Lahore, Pakistan

3 Federal Urdu University of Arts & Science, Islamabad, Pakistan

4 Islamic International University, Islamabad, Pakistan

### **References**

[10] Menezes, CA, ''Low Resistivity CdTe‐Te Films by a Combined Hot‐Wall‐Flash Evaporation Technique.'' ECS Journal of Solid‐State Science and Technology, 127, 155 (1980).

**Author details**

Nazar Abbas Shah1

**References**

1982.

cm2

Florence, 2001.

JJAP.19.703.

\*, Zamran Rabeel2

404 Modern Technologies for Creating the Thin-film Systems and Coatings

Information Technology, Islamabad, Pakistan

Conference, p. 1138, 1982.

\*Address all correspondence to<EMAIL_ADDRESS>
2 Higher Education Department, Punjab, Lahore, Pakistan

4 Islamic International University, Islamabad, Pakistan

3 Federal Urdu University of Arts & Science, Islamabad, Pakistan

, Murrawat Abbas3

1 Thin Films Technology Research Laboratory, Department of Physics, COMSATS Institute of

[1] Werthen, JG, Anthony, TC, Fahrenbruch, AL, Bube, RH, 16th IEEE Photovoltaic Specialists

[2] Tyan, YS, Perez‐Albuerne, EA, 16th IEEE Photovoltaic Specialists Conference, p. 794,

[3] Chakrabarti, R, Dutta, J, Maity, AB, Chaudhuri, S, Pal, AK, "Photoconductivity of CdTe

[4] Touskova, J, Kindl, D, Tousek, J, ''Preparation and Characterization of CdS/CdTe Thin Film Solar Cells.'' Thin Solid Films, 293, 272 (1997). doi:10.1016/S0040‐6090(96)09113‐4. [5] Matsumoto, H, Kuribayashi, K, Komatsu, Y, Nakano, A, Uda, H, Ikegami, S, ''30 × 30

[6] Wu, X, Keane, JC, Dhere, RG, Dehart, C, Albin, DS, Duda, A, Gessert, TA, Asher, SD, Levi, H, Sheldon, P, "Seventeenth European Photovoltaic Solar Energy Conference." Proceedings from the International Conference, Vol. 1, p. 995. WIP Munich and ETA‐

[7] Abbas Shah, N, Ali, A, Ali, Z, Maqsood, A, Aqili, AKS, ''Properties of Te‐Rich Cadmium Telluride Thin FilmsFabricated by Closed Space Sublimation Technique.'' Journal of

[8] Nakayama, N, Matsumoto, H, Nakano, A, Ikegami, S, Uda, H, Yamashita, T, ''Ceramic Thin Film CdTe Solar Cell.'' Journal of Applied Physics, 19, 703 (1980). doi:10.1143/

[9] Khan, MA, Shah, NA, Ali, A, Basharat, M, Hannan, MA, Maqsood, A, ''Fabrication and Characterization of Cd Enriched CdTe Thin Films by Close Spaced Sublimation.''

CdS/CdTe Single Substrate Module Prepared by Screen Printing Method.'' Japan.

Films." Thin Solid Films, 32, 288 (1996). doi:10.1016/S0040‐6090(96)08801‐3.

Journal of Applied Physics, 22, 891 (1983). doi:10.1143/JJAP.22.891.

Crystal. Growth, 284, 477 (2005). doi:10.1016/j.jcrysgro. 08.005 2005.

Journal of Coatings Technology and Research 6, 251–256 (2009).

and Waqar Adil Syed4

[41] Dluzewski, P, ``TEM Characterization of MBE Grown CdTe/ZnTe Axial Nanowires'', Journal of Microscopy, 237, 337–340, (2010).

[24] Bonnet, D, "Manufacturing of CSS CdTe solar cells." Thin Solid Films, 361, 547 (2000). [25] Birkmire, RW, "Recent progress and critical issues in thin film polycrystalline solar cells and modules." IEEE Photovoltaics: 26th Conference. Proceedings. pp. 295–300, 1997.

[27] Laks, DB, Van de Walle, CG, Neumark, GF, Pantelides, ST, "Role of native defects in wide‐band‐gap semiconductors." Physical Review Letters, 66, 648–651 (1991).

[28] Chegaar, M, Ouennoughi, Z, Guechi, F, Langueur, H, ``Determination of Solar Cells Parameters under Illuminated Conditions'', Journal of Electronic Devices, 2, 17–21 (2003).

[29] Salinger, J, ``Measurement of Solar Cell Parameters with Dark Forward I‐V Characteristics'',

[30] Green, MA, "Solar Cell Fill Factors: General Graph and Empirical Expressions", Journal

[32] Shah, NA, Adil, WA, Atta, MA, ``Characterization of II–VI Semiconductor Thin Films Fabricated by Close Spaced Sublimation'', Nanoscience and Nanoletters, 1, 62–65 (2009).

[33] Shah, NA, Ali, A, Maqsood, A, Characterization of CdTe Thin Films Fabricated by Close Spaced Sublimation Technique and a Study of Cu Doping by Ion Exchange Process''.

[34] Kaydanov, VI, Ohno, TR, Studies of Basic Electronic Properties of CdTe‐Based Solar Cells and Their Evolution during Processing and Stress, Final Technical Report 16

[35] Fthenakis, VM, ``Life Cycle Impact Analysis of Cadmium in CdTe PV Production''.

[36] Kumar, V, ``Characterization of Large Area Cadmium Telluride Films and Solar Cells Deposited on Moving Substrates by Close Spaced Sublimation'', MS thesis, University

[37] Ferekides, CS, "Thin Films and Solar Cells of Cadmium Telluride and Cadmium Zinc

[38] Khrypunova, G, Romeo, A, Kurdesauc, F, Bätzner, DL, Tiwarie, AN, ``Recent Developments in Evaporated CdTe Solar Cells'', Solar Energy Materials and Solar Cells,

[39] Alvin Compaan, D, Gupta, A, Lee, S, Wang, S, Drayton, J, ``High Efficiency, Magnetron

[40] Feuillet, M, Charleux, G, Mariette, M, Pub, H, ``Atomic Layer Epitaxy of CdTe and

October 2001—31 August 2005 Colorado School of Mines Golden, Colorado.

Elsevier Renewable & Sustainable Energy Reviews, 8, 303–334 (2004).

Telluride", Ph.D. Dissertation, University of South Florida, USA (1991).

Sputtered CdS/CdTe solar Cells, Solar Energy, 77, 815–822 (2004).

MnTe Hartmann'', Journal of Applied Physics,79, 3035 (1996).

Method'', Journal of Coatings Technology and Research, 7, 105–110 (2010).

Journal of Non‐Crystalline Solids Elsevier, 355, 1474 (2009).

Treated CdTe Thin Films Deposited by CSS

[26] Chopra, KL, "Thin Film Phenomena." 1969 (New York: McGraw‐Hill).

Journal of Acta Polytechnica, 46, 25‐27 (2006).

406 Modern Technologies for Creating the Thin-film Systems and Coatings

of Solid‐State Electronics, 24, 788–789 (1981).

[31] Shah, NA, Ali, A, Hussain, S, ``CdCl2

of south Florida, USA (2003).

90, 664–677 (2006).

[42] Romeo, N, Bosio, A, Canevari, V, Podesta, A, ``Recent Progress on CdTe/CdS Thin Film Solar Cells'', Solar Energy, 77, 665–994 (2004).

### **Silver-Based Low-Emissivity Coating Technology for Energy-Saving Window Applications Silver-Based Low-Emissivity Coating Technology for Energy-Saving Window Applications**

Guowen Ding and César Clavero Guowen Ding and César Clavero

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67085

#### **Abstract**

Low-emissivity (low-E) technology is a unique and cost-effective solution to save energy in buildings for different climates. Its development combines advances in materials science, vacuum deposition, and optical design. In this chapter, we will review the fundamentals of energy saving window coatings, the history of its application, and the materials used. The current low-E coating technologies are overviewed, especially silverbased low-E technologies, which comprise more than 90% of the overall low-E market today. Further, the advanced understanding of generating high-quality silver thin films is discussed, which is at the heart of silver-based low-E product technology development. How the silver thin film electrical, optical, and emissivity properties are influenced by their microstructure, thickness, and by the materials on neighboring layers will be discussed from a theoretical and an experimental perspective.

**Keywords:** low-E, emissivity, silver, coating, glass, window, optical, materials

### **1. Introduction to low-E applications**

The growing awareness of global warming has intensified efforts to make buildings and vehicles more energy efficient. "Building heating, ventilation, and air conditioning (HVAC) accounted for 14% of primary energy consumption in the United States in 2013" [1]. Windows are often considered the least energy-efficient component in a building. Efficiency upgrades that improve the energy efficiency of windows are among the most promising and costeffective energy technology options available now. A National Academy of Sciences study concluded that, "by an order of magnitude, the largest apparent benefits [of the technologies examined] were realized as avoided energy costs in the buildings sector in energy efficiency"[2].

and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Modern coating technologies provide architectural window coatings with adequate illumination levels in building interiors, while controlling energy transfer to save either cooling or heating energy. In 2003, 95% typical new windows in U.S. were double-glazed windows, and 50% have a low-E coating [3].Thus, energy saving windows are very popular in developed countries.

### **1.1. Why low-E coating windows are needed**

Typical commercial buildings waste 30% of the energy they consume, mostly by heat and cooling loss through the building envelope (windows, doors, roofs, etc.) [4]. Losses through windows alone are estimated to cost U.S. consumers roughly \$40 billion each year [2]. Radiation losses occur through the window glass and represent about 60% of the total heat loss in a standard window [4]. How can the heat transfer through windows be effectively controlled? One cost-effective solution today is through low-emissivity (low-E) coatings.

### **1.2. What is low-emissivity coating?**

To understand low-emissivity coatings, let us first define emissivity. Emissivity is the ratio of heat emitted from a given material compared to that from a blackbody, from zero to one. A blackbody would have an emissivity of 100% and a perfect reflector would have zero value. The emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. The typical common materials emissivity is listed below in **Table 1**.

When the emissivity of a window coating is low, the window coating is called low-E coating. The standard varies for different countries. Pyrolytic low-E coating for single-pane glass normally can achieve around 20% emissivity, while silver-based sputter coatings can achieve 8–2% emissivity, which represents currently 90% of the low-E market. These two types of low-E coatings will be discussed in more detail in the later sections.

**Table 1.** Emissivity of common materials [39].

### **1.3. How can low-E coating windows save energy?**

Let us first review a few common terminologies in the low-E field according to Ref. [5]:

**Insulating glass unit (IGU)** commonly consists of double or triple panes of glass separated by a vacuum or gas-filled space and sealed together at the edge to reduce the heat transfer of the buildings. The common insulating filled gas are air, argon, or krypton. There are four surfaces for double-paned IGUs, commonly labeled as surface 1–4 from exterior to interior, as shown in **Figure 1** [5].

**Figure 1.** The structure of IGU.

Modern coating technologies provide architectural window coatings with adequate illumination levels in building interiors, while controlling energy transfer to save either cooling or heating energy. In 2003, 95% typical new windows in U.S. were double-glazed windows, and 50% have a low-E coating [3].Thus, energy saving windows are very popular in developed countries.

Typical commercial buildings waste 30% of the energy they consume, mostly by heat and cooling loss through the building envelope (windows, doors, roofs, etc.) [4]. Losses through windows alone are estimated to cost U.S. consumers roughly \$40 billion each year [2]. Radiation losses occur through the window glass and represent about 60% of the total heat loss in a standard window [4]. How can the heat transfer through windows be effectively controlled?

To understand low-emissivity coatings, let us first define emissivity. Emissivity is the ratio of heat emitted from a given material compared to that from a blackbody, from zero to one. A blackbody would have an emissivity of 100% and a perfect reflector would have zero value. The emissivity of the surface of a material is its effectiveness in emitting energy as thermal

When the emissivity of a window coating is low, the window coating is called low-E coating. The standard varies for different countries. Pyrolytic low-E coating for single-pane glass normally can achieve around 20% emissivity, while silver-based sputter coatings can achieve 8–2% emissivity, which represents currently 90% of the low-E market. These two types of

One cost-effective solution today is through low-emissivity (low-E) coatings.

radiation. The typical common materials emissivity is listed below in **Table 1**.

low-E coatings will be discussed in more detail in the later sections.

**Materials surface\*\* Thermal emissivity**

Aluminum foil 0.03 Asphalt 0.88 Brick 0.9 Concrete, rough 0.91 Glass, smooth (uncoated) 0.91 Limestone 0.92 Marble, Polished or white 0.89–0.92 Marble, Smooth 0.56 Paper, roofing or white 0.88–0.86 Plaster, rough 0.89 Silver, polished 0.02

**1.1. Why low-E coating windows are needed**

410 Modern Technologies for Creating the Thin-film Systems and Coatings

**1.2. What is low-emissivity coating?**

**Table 1.** Emissivity of common materials [39].

**Visible light transmittance (Tvis)** is the transmitted percentage of visible light (380–780 nm) through glass, (or insulating glass unit, IGU).

**Solar heat gain coefficient (SHGC)** is the percentage of the solar energy passing through the window over the incident solar energy (including direct solar transmittance and indirect reradiation).

**U-factor** is a measure of air-to-air heat transmission (loss or gain) in indoor and outdoor temperatures of a 1-m high glazing due to the thermal conductance and the difference. It is an overall coefficient of heat transfer, the lower the U-factor, the better the insulating properties of the windows.

Second, let us explain how low-E coating could save energy, which is discussed in two climate conditions:

concept is needed, and it is called LSG (light-to-Solar gain ratio). We desire LSG to be as high as possible and current highest LSG window product in the market is around 2.4.

In the spectra level, the ideal low-E coating spectra for cold and hot climates were illustrated from reference [6] as in **Figure 2**.

**Figure 2.** The idealized low-E coating spectra for hot or cold climates.

#### **1.4. Examples of low-E windows in saving energy cost**

The energy savings by installing low-E window are solid. Guardian Industries demonstrated two examples at the 2012 AIA National Convention and Design Exposition [7]: Two similar buildings were located in Chicago and Miami, respectively. The floor area of the six stories buildings were 120,000 square feet for both, with floor-to-floor height of 12 feet; slab on grade foundation, strip-type windows, window area 20,000 square feet, R-13 wall insulation, natural gas for heat, and electric power A/C. Several Guardian window products were installed as listed below (the IGU configuration were 6 mm glass/12 mm space/6 mm glass, and coating was at #2 surface) (**Table 2**).

Silver-Based Low-Emissivity Coating Technology for Energy-Saving Window Applications http://dx.doi.org/10.5772/67085 413

**Table 2.** Several typical glass product and their basic parameters [7] .

concept is needed, and it is called LSG (light-to-Solar gain ratio). We desire LSG to be as high as possible and current highest LSG window product in the market is around

In the spectra level, the ideal low-E coating spectra for cold and hot climates were illustrated

The energy savings by installing low-E window are solid. Guardian Industries demonstrated two examples at the 2012 AIA National Convention and Design Exposition [7]: Two similar buildings were located in Chicago and Miami, respectively. The floor area of the six stories buildings were 120,000 square feet for both, with floor-to-floor height of 12 feet; slab on grade foundation, strip-type windows, window area 20,000 square feet, R-13 wall insulation, natural gas for heat, and electric power A/C. Several Guardian window products were installed as listed below (the IGU configuration were 6 mm glass/12 mm space/6 mm glass, and coating

2.4.

from reference [6] as in **Figure 2**.

412 Modern Technologies for Creating the Thin-film Systems and Coatings

**1.4. Examples of low-E windows in saving energy cost**

**Figure 2.** The idealized low-E coating spectra for hot or cold climates.

was at #2 surface) (**Table 2**).

The annual cost for cooling and heating was calculated in comparison to clear glass (without low-E coating) as shown below. The savings purely from heating and cooling could be \$8000 annually in the Chicago building, and double in the Miami building, as shown in **Figure 3**. The reason for the reduction of heating and cooling cost is not only due to the low-E window preventing the heat transfer to make the room warm at night and cool at day time, but also due to a significant reduction of the solar IR heat through the windows.

**Figure 3.** Comparison of annual heating and cooling saving cost for a building in Chicago and Miami [7].

In addition, the cooling and heating system capacity could be reduced too, so that one-time savings from reduction in HVAC system cooling capacity was calculated in comparison to a noncoating unit, as shown in **Figure 4**. That cost reduction could be \$30,000 for the buildings.

Considering a 10-year period, the cost saving is very attractive: \$100,000 in Chicago, and double that amount in Miami (\$200,000). The exact number with different low-E product is showed in **Figure 5**, in comparison with clear glass windows (without low-E coatings).

**Figure 4.** One-time savings from reduction in HVAC system cooling capacity (compared to a noncoating unit) [7].

**Figure 5.** Ten-year savings including heating/cooling cost saving and one-time saving from reduction in HVAC system cooling capacity (compared to a noncoating unit) [7].

### **2. Low-E coating technology**

#### **2.1. Brief history of low-E coating technology**

Low-E coating technology's roots can be traced back more than 100 years ago, from Drude, Hagen, and Rubens's theories. The first glass coating that was able to selectively reflect radiation can be traced back to 1958, when Holland and Siddall [8] demonstrated a gold coating on glass with high-transmittance and high-heat reflections. In the 1960s, a product called "Stop Ray" was first released in the market for solar control glass to reduce the cooling cost of the building. Later, a product called "Infrastop" was also introduced to the market. In the 1970s and 1980s, another type of low-E glass appeared, called "K-Glass," which demonstrated high environmental and chemical durability, and also reduced the solar IR heat transfer. In the 1980s, the silver coating breakthrough was demonstrated, with higher transmittance, much lower emissivity, and friendly color. Quickly, silver coating low-E became the dominant low-E product in the market. Today, silver coatings can be further developed with multiple silver blocks, known as double silver and triple silver products. A brief history of low-E is covered in **Table 3** [9, 10].

#### **2.2. What materials can be used for low-E window coating**

Around the 1900s, the German physicist Paul Drude explained the optical behavior of free electrons in a solid based on the kinetic theory of free electrons in a metal. This theory is still widely used in literature today. In the early twentieth century, physicists Hagen and Rubens found that the heat emission from bulk metals described by their emissivity, *ε*, correlates strongly with their conductivity, *σ*, i.e., with the concentration of free electrons [11]. Based on the Drude model, they derived a formula to connect conductivity *σ* and emissivity:

$$
\varepsilon = \sqrt{\frac{8\varepsilon\_0\omega}{\sigma}}\tag{1}
$$

Thus, the higher the conductivity, the lower the emissivity. In the low-E industry, there are a few practical estimations on the relationship of emissivity and sheet resistivity, such as *ε* = 0.0106 R*□*, where R*□* is the film sheet resistivity [11]. This is the reason why film-sheet resistivity is a very important parameter in qualifying low-E products, and it is easily measured by a four-point probe. In addition, Ref. [12] found a way to estimate the optical properties of silver (refractive index) at near IR by resistivity measurements [12].

There are two types transparent low-E coatings in today's market: (1) semiconductive coatings, e.g., ITO (indium tin oxide) and FTO (fluorine-doped tin oxide) and (2) metallic coating. Some common low-E materials are listed in **Table 4** for comparison.

### **2.3. Major industry Low-E window coating technologies**

**2. Low-E coating technology**

cooling capacity (compared to a noncoating unit) [7].

414 Modern Technologies for Creating the Thin-film Systems and Coatings

**2.1. Brief history of low-E coating technology**

Low-E coating technology's roots can be traced back more than 100 years ago, from Drude, Hagen, and Rubens's theories. The first glass coating that was able to selectively reflect radiation can be traced back to 1958, when Holland and Siddall [8] demonstrated a gold coating on glass with high-transmittance and high-heat reflections. In the 1960s, a product called "Stop Ray" was first released in the market for solar control glass to reduce the cooling cost of the

**Figure 5.** Ten-year savings including heating/cooling cost saving and one-time saving from reduction in HVAC system

**Figure 4.** One-time savings from reduction in HVAC system cooling capacity (compared to a noncoating unit) [7].

Although there are many thin film coating methods available for glass coatings, such as solgel, PECVD, ALD, and E-beam evaporation, there are only two major low-E window coating technologies in today's market: sputtered coating and pyrolytic coating, which provide costeffective, high durablility, excellent uniformity on jumbo glass (3 m or more wide glass). The two technologies are discussed in the following:

**1.** Chemical vapor deposition (CVD) coating, or called pyrolytic coating, is a low-E coating technology that appeared in the market in the 1970s. This method deposits films directly on the hot glass while it is still on the float line, so it is also called online low-E. The layout is shown in **Figure 6**. The CVD process is chosen right after the float/tin bath on the production line with around 600°C. Because the substrate glass moves about 1 ft/s as it travels down the float line, only 1–1.5 s are available for the coating to form. The precursor compounds (both gases and liquids) are vaporized in a reactor that spreads the resulting gas mixture uniformly over an advancing, newly formed glass ribbon. Chemical reactions occur in the gas above the glass and on the growing surface of the deposited film. Temperature

**Table 3.** Brief history of low-E technology, data obtained from [9, 10]. control is easier because the large thermal mass of the system also keeps the temperature relatively uniform across the ribbon. Strongly adhered coatings maintain their integrity when the product is bent and tempered [13].

**Table 4.** The common low-E materials comparison, data obtained from Ref. [11] .

**1950s**

> Notes

Market for

First solar

control glass

to reduce

the cooling

cost of the

building

Firm: Film stack

Trade

"Stop ray"

"Infrastop"

Thermoplus

Thermoplus

k-Glass

Iplus

neutral

name

New

High

High IR

High IR

High

Transmission

Highly

Aging

Higher

Better

Better

resistant

transmission,

solar gain

solar gain

better neutral

control,

control,

higher

higher

LSG, >1.7

LSG >2.2

durable

form

40==>60%, U:

chemical/

and color

mechanical

neutral

color

resistance

better than

greenish/

pinkish

color by

Au/Cu

coating

1.3W/m 2 k,

Jumbo glass

transmittance

low-e coating

reflection,

low thermal

reflection,

low thermal

emissivity,

emissivity,

good

good

transmittance

Deposition

Sputter/

Sputter

Evaporation

Pyrolysis /

Sputter

Pyrolysis/

Sputter

Sputter

Sputter

Sputter

APCVD

APCVD

evaporation

method

**Table 3.**

Brief history of low-E technology, data obtained from [9, 10].

transmittance

features

transmittance/

high heat

reflection

**BiOx/Au/BiOx**

**BiOx/Au**

**ZnS/Au/ZnS**

**Tin oxide**

**BiOx/Au,**

**Tin oxide**

**BiOx/**

**Silver-based**

**Oxide /**

**3 cycles of**

416 Modern Technologies for Creating the Thin-film Systems and Coatings

**Ag/oxide**

**Oxide/Ag/**

**/Ag/**

**oxide**

**oxide**

Solarban

glass

**PbOx/**

**coating**

**Ag/PbOx/**

**BiOx**

**IGU filled Ar**

BOC Edwards

Flaverbel

Heraeus /

Philips (NL)

Flashglass /

Pilkington

Interpane

Interpane

Cardinal,

PPG

etc.

DELOG

DETAG

heat reflection

glass

**1960s**

**1960s**

**1970s** Heat

1st

insulating

energy

glass units of

crisis

building

**1974**

**1975–1980s**

**1980s**

**1981**

**1988** Asymmetrical

Double

Tripple

silver layer

silver

silver

system

**1990s**

**2006**

**Figure 6.** Schematic of a float bath production line used to deposit fluorine-doped tin oxide. Simplified from Ref. [13].

The typical deposition materials by pyrolytic method is fluorine-doped tin oxide (FTO). Their extinction coefficient *k* is very small (0.01 at 550 nm), so that the typical thickness is of µm scale, whose transmittance of >80% is acceptable. The refractive index is typically around 2. This kind of coating shows extremely good environmental and chemical resistance. Thus, sometimes it is called hard-low-E coating.

**2.** Today, more than 90% of the low-E window coatings are manufactured by sputtered coaters. Worldwide, billions of square meters per year of glass is coated by sputtering method and this amount is increasing steadily. Metallic low-E coating is conveniently manufactured by a sputtering method, because it provides a cost-effective solution and excellent coating uniformity; in addition, it also provides rich products with variety of choices on color, transmittance, solar heat gain, etc. The sputtering coater provides improved emissivity (below 0.08), therefore better heat radiation control, better solar heat control, and better optical performance. In addition, the price for current low-E coating products is very affordable, below \$1/ft2 for single silver coating on 3 mm soda-lime glass.

Sputtering coater could process many different types of materials including:

The typical metal sputter deposition coater layout is shown in **Figure 7**.

**Figure 7.** Schematic of a continuous-batch sputter-coating reactor, simplified from Ref. [13].

The sputtering coater is typically independent of the glass production, so it is also called offline low-E coating. The coater line starts with a glass washer, and an entrance chamber to pump down to vacuum condition. The glass travels through each chamber for different layers of material deposition, finally passing through the exit chamber to the ambient condition. Being an offline process, sputtering process allows for a high level of flexibility, such as flexible layer system, the scale of production, etc. Also, it is generally regarded to be environmentally safe, without waste products. Thus, the sputter coating is the most used technology in the low-E coating industry today.

### **3. Silver-based low-E coating technology overview**

Among the sputtered low-E coating products, silver-based low-E is dominant. There are three major categories of silver-based low-E products: single-silver, double-silver, and triple-silver products [14]. The structures are similar, with single-silver structure being the simplest one. A typical structure is as follows (**Figure 8**).

**Figure 8.** Typical single-silver coating layer structure.

It has been reported that the electrical/optical properties of Ag thin film strongly depend on its microstructure, such as crystallite size [15], grain size [16], grain boundaries [17], and surface roughness [18, 19], and also on the microstructure of the dielectric under-layers [20–22]. Thus, the R&D direction for the low-E industry has been focused on how to improve silver-thin film microstructure for better optical and thermal performance. Generating high-quality silver thin films is at the center of the technological development of silver-based low-E products. This will be further discussed in Section 3. The base and top layers are typically transparent dielectric materials layers, which are critical to the visible optical performance. Seed and blocker layers are very important to the emissivity properties, and they will be discussed in the following sections.

#### **3.1. Seed layer**

**2.** Today, more than 90% of the low-E window coatings are manufactured by sputtered coaters. Worldwide, billions of square meters per year of glass is coated by sputtering method and this amount is increasing steadily. Metallic low-E coating is conveniently manufactured by a sputtering method, because it provides a cost-effective solution and excellent coating uniformity; in addition, it also provides rich products with variety of choices on color, transmittance, solar heat gain, etc. The sputtering coater provides improved emissivity (below 0.08), therefore better heat radiation control, better solar heat control, and better optical performance. In addition, the price for current low-E coating products is

• *Metal materials*: most metal could be deposited by sputtering method. The most widely used metallic low-E coatings are silver or gold. The extinction coefficient *k* is very high for such metals, such as 3.5 for sliver and 2.6 for gold at wavelength of 550 nm, so only thin

• *Semiconductor*: such as indium-doped tin oxide (ITO), aluminum-doped zinc oxide (AZO), etc. • *Dielectric materials*: such as bismuth oxide, tin oxide, zinc oxide, titanium oxide, silicon

The sputtering coater is typically independent of the glass production, so it is also called offline low-E coating. The coater line starts with a glass washer, and an entrance chamber to pump down to vacuum condition. The glass travels through each chamber for different layers of material deposition, finally passing through the exit chamber to the ambient condition. Being an offline process, sputtering process allows for a high level of flexibility, such as flexible layer system, the scale of production, etc. Also, it is generally regarded to be environmentally safe, without waste products. Thus, the sputter coating is the most used technology in the low-E coating industry today.

Among the sputtered low-E coating products, silver-based low-E is dominant. There are three major categories of silver-based low-E products: single-silver, double-silver, and triple-silver products [14]. The structures are similar, with single-silver structure being the simplest one.

Sputtering coater could process many different types of materials including:

films such as 20 nm are acceptable for good transmittance.

**3. Silver-based low-E coating technology overview**

A typical structure is as follows (**Figure 8**).

The typical metal sputter deposition coater layout is shown in **Figure 7**.

**Figure 7.** Schematic of a continuous-batch sputter-coating reactor, simplified from Ref. [13].

for single silver coating on 3 mm soda-lime glass.

very affordable, below \$1/ft2

418 Modern Technologies for Creating the Thin-film Systems and Coatings

oxide, silicon nitride, etc

Using a seed layer is a common deposition technique to promote thin-film microstructure, and to enhance the thin-film properties, such as optical and mechanical properties. It is often reported that the ZnO seed layer can enhance silver thin film crystallite size and grain size, so that its resistivity and absorption is greatly reduced [22, 23]. Arbab et al. [23] have demonstrated that ZnO seed layer is better than other oxides, such as zinc stannate, as shown in **Figures 9** and **10**. The well-crystallized Ag (111) atop of ZnO (002) basal plane induced lower resistivity than that of polycrystalline Ag atop of zinc stannate. ZnO is a material that crystallizes very easily at room temperature even at very thin thickness such as 5 nm. The ZnO lattice sites are at the corners and center of a hexagon. Three silver atoms at alternate fourfold hollow sites form the unit cell of a (111) plane of silver, with 2.6% lattice mismatch between the ZnO and Ag layers [23], thus, the crystallized ZnO lattice promote the silver growing at (111) direction.

**Figure 9.** Well-crystallized Ag (111) atop of ZnO (002) basal plane induced lower resistivity than that of polycrystalline Ag atop of zinc stannate.

**Figure 10.** Ag (111) growth atop of ZnO (002).

#### **3.2. Blocker layer**

The blocker layer is extremely important in silver-based low-E coating. Treichel et al. [24] gave a good description of blocker layer functions by noting that the typical top layer is comprised by oxide materials, such as SnO2 , TiO2 , or ZnO. Without a blocker layer, the deposition of the top layer takes place directly on top of the unprotected silver film. In such a case, the silver layer meets a highly reactive sputter process, with the presence of oxygen radicals. The silver crystal lattice will be damaged and the silver atoms will agglomerate, until the top layer forms close to the surface, preventing further reacting species from reaching the silver [24]. Since the quality of the low-E coating is mainly determined by the crystallized silver, an additional barrier, the blocker layer, becomes necessary for high-quality low-E coating. In the study shown below, the metal titanium was chosen. In low-E applications, emissivity and resistivity is in nearly linear relationship. When the blocker is too thin, no close protecting layer formed yet, the emissivity/ resistivity (from silver) is high, which is called region 1. As the blocker thickness is increased, the emissivity/resistivity reaches a minimum, called region 2. Further increasing the thickness, called region 3, leads to emissivity/resistivity flat or increasing slightly, as shown in **Figure 11**.

**Figure 11.** The resistivity of single-silver stack is dependent on the blocker thickness.

The optimized blocker layer consists of two portions: a metallic portion close to the silver layer and a region with higher oxidization ratio close to the top layer. The typical blocker layer thickness is below 10 nm [24].

### **3.3. Double silver and triple silver**

**3.2. Blocker layer**

by oxide materials, such as SnO2

**Figure 10.** Ag (111) growth atop of ZnO (002).

420 Modern Technologies for Creating the Thin-film Systems and Coatings

The blocker layer is extremely important in silver-based low-E coating. Treichel et al. [24] gave a good description of blocker layer functions by noting that the typical top layer is comprised

top layer takes place directly on top of the unprotected silver film. In such a case, the silver layer meets a highly reactive sputter process, with the presence of oxygen radicals. The silver crystal lattice will be damaged and the silver atoms will agglomerate, until the top layer forms close to the surface, preventing further reacting species from reaching the silver [24]. Since the quality of the low-E coating is mainly determined by the crystallized silver, an additional barrier, the blocker layer, becomes necessary for high-quality low-E coating. In the study shown below, the metal titanium was chosen. In low-E applications, emissivity and resistivity is in nearly linear relationship. When the blocker is too thin, no close protecting layer formed yet, the emissivity/ resistivity (from silver) is high, which is called region 1. As the blocker thickness is increased, the emissivity/resistivity reaches a minimum, called region 2. Further increasing the thickness, called region 3, leads to emissivity/resistivity flat or increasing slightly, as shown in **Figure 11**.

, or ZnO. Without a blocker layer, the deposition of the

, TiO2

**Figure 11.** The resistivity of single-silver stack is dependent on the blocker thickness.

The coating stack in **Figure 8** can now be used as a building block for multi-low-E stacks. Introducing a sequence of two blocks in **Figure 8** leads to double silver stacks, which will enhance low-E coating with higher selectivity between IR and visible. The typical structure is illustrated in **Figure 12**.

**Figure 12.** The double silver stack layout from PPG products with the data obtained from Ref. [25].

In this case, the dielectric material is zinc stannate, the seed layer is zinc oxide, and the blocker layer is titanium. The zinc stannate layer in the middle has a thickness roughly equal to the total thickness of the base and the top layer of the single silver. The typical double low-E coating glass spectra is shown in **Figure 13**, with transmittance around 70%, and reflectance from film and glass sides below 10% in the visible region. However, the transmittance is near zero at the IR region (*λ* > 1000 nm), and the reflectance for IR is very high >90% (for *λ* > 1000 nm).

**Figure 13.** The typical double silver spectra of transmittance and reflectance from film side and glass side [26].

Further, if the three blocks of single silver of **Figure 8** were put together [13], it would make a triple silver stack which is shown in **Figure 14**. The selectivity of IR and visiblity is the best in today's market, however, the cost is higher.

**Figure 14.** A typical triple silver stack.


**Author details**

M. Basavaraju\* and B.S. Gunashree Department of Studies and Research in Microbiology, PG Centre, Mangalore University, Karnataka, India

\*Address all correspondence to<EMAIL_ADDRESS>
© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.