Fabrication of a novel low-cost triple layer system (TaZO/Ag/TaZO) with an enhanced quality factor for transparent electrode applications

Abstract

A triple layer system (TaZO/Ag/TaZO), consisting of tantalum doped zinc oxide (TaZO) as the top and bottom layers and metallic silver (Ag) as the intermediate layer, was deposited onto glass substrates. The top and bottom layers were deposited using an inexpensive home-made automated jet nebulizer spray technique (AJNSP) and the intermediate Ag layer was deposited using a thermal evaporation technique. Three different sets of samples with top and bottom layers having thickness values of around 50, 75 and 100 nm, respectively, were prepared, keeping the thickness of the middle layer constant (15 nm). The influence of thickness on structural, electrical, optical and photoluminescence properties along with surface morphology of the deposited triple layer systems was studied. A structural study revealed that the deposited films have the hexagonal wurtzite structure of ZnO, and an X-ray photoelectron spectroscopic study confirmed the presence of the expected elements in the system. Optical studies showed that the overall thickness of the films influences only marginally the transmittance of the films. The decrease in sheet resistance with the increase in thickness is explained on the basis of a grain boundary scattering mechanism. Among all the films examined, the multilayer film with the 75 + 15 + 75 nm thickness exhibited the best quality factor (8.98 × 10⁻³ (Ω sq⁻¹)⁻¹), and may be considered as a potential candidate for transparent electrode applications. To the best of our knowledge, this is the first report in which a low-cost chemical technique has been employed to fabricate a triple layer system.

---

1. Introduction

Among all the transparent conducting oxide (TCO) materials, indium tin oxide (ITO) films have been extensively studied and widely used in various applications, as they have high transparency, low resistivity and wide band gaps. However, ITO films are unstable and degrade at high temperatures and they are also expensive and toxic.^1–3 These factors create a need for an alternative inexpensive, nontoxic and long-lasting TCO material with low resistance and high transmittance. Therefore extensive studies are in progress on alternate thin films that can exhibit high electrical conductivity as well as high transparency in the visible region, and high reflectivity in the infrared region.

Low resistivity and high optical transparency are reciprocally exclusive properties. Transparency of a film is high if the thickness is low, whereas the resistivity is low if the thickness is high. In other words, at one extreme there are films of low thickness having discontinuous islands of noticeable transparency but poor conductivity, while at the other extreme there are films that are continuous and thick, possessing high conductivity but low transparency.

It is believed that, instead of attempting to develop new TCO materials with electrical and optical properties superior to those of the already available TCOs, it is better to explore appropriate modifications in the design of the existing TCOs to obtain improved electrical and optical properties over a broader spectral range. Systems with an enormous variety of enhanced properties can be obtained with the use of multilayer films through proper optimization of processing parameters.⁴ For these reasons, multilayers consisting of a combination of different semi-conducting materials and metals are used to achieve high transparent conducting properties.

In this juncture, multilayer structures are proposed, to deposit practically usable, highly transparent conductive oxide films. Multilayer (TCO/metal/TCO) structures are considered as a fascinating alternative to single layer transparent electrodes due to their enhanced electrical characteristics. For transparent electrode applications, the expected transparent conducting properties can be achieved through a triple layer system with a noticeable reduction in the overall thickness compared with single layer counterparts.

In TCO/metal/TCO structures, the main constituent for the sheet resistance is the intermediate metal layer due to its excellent electrical properties. The resistivity of a multilayer film decreases with increasing metal layer thickness. However, the transparency decreases when the film thickness increases and hence fine tuning of thickness offers better transmittance. By sandwiching a metal layer between two layers of very thin ITO or ZnO film, transmittance can be increased further.

Among all metals, silver (Ag) has the lowest resistivity (for bulk Ag, resistivity is 1.6 μΩ cm at 20 °C). Moreover, it has a low refractive index and high extinction co-efficient. For the intermediate metal layer, Ag is the best choice due to its chemical stability, high conductivity, and the ability to realize continuous, ultra-thin layers in addition to the above said properties.^5,6 Even though ITO/Ag/ITO films have been examined broadly, the rising cost of indium has forced the substitution of ITO by other metal oxides.

ZnO is a potential alternative to ITO owing to its abundancy in nature, non-toxic nature and low cost. Furthermore, it has excellent electrical and optical properties, and thermal stability.^7–10 However, for realistic applications, bare ZnO films are inferior to indium or tin based films. Therefore, doped ZnO films have gained the materials scientists' attention for the replacement of ITO films, as they can exhibit low electrical resistivity and high optical transmission in the visible spectrum range when compared with pristine ZnO.^10–12

Many elements (Al, In, Sn, Ga, Ta, Mo, Zr, Mn) have been doped into ZnO and their influences on the transparent conducting properties of ZnO have been investigated.^10–17 Among them, ZnO with Ta doping (TaZO) seems to be very scarce, but it is a promising material due to its noticeable transport characteristics.¹⁴

So far, triple layer systems (TCO/metal/TCO) have been deposited using different expensive techniques, such as ion beam sputtering, electron beam evaporation and magnetron sputtering.^18–22 However, there are no reports on the deposition of triple layer films using an automated jet nebulizer spray technique (AJNSP). The advantages of this technique are its low material consumption with better spray control and soft carrier gas flow, so that very thin layers of pinhole free depositions of uniform thickness can easily be achieved. Moreover, it is a simple and inexpensive technique, operated with simple solution flow control and temperature monitoring units, which requires no vacuum for deposition.^23,24 Hence, in the present study, this inexpensive nebulizer spray technique is employed for the first time to deposit the top and bottom layers of a triple layer system (TaZO/Ag/TaZO). The intermediate Ag layer is deposited using a thermal evaporation technique.

The deposited films are characterized for their structural, electrical, optical, photoluminescence and surface morphological properties. By optimizing the individual as well as the overall thicknesses of the multilayer structure, a transparent electrode having a high quality factor and high transparency with reduced sheet resistance can be produced.

2. Materials and methods

Zn(CH₃COO)₂·2H₂O (0.1 M) (Sigma-Aldrich, 99.999%, USA) and TaCl₅ (0.001 M/1 at%) (Sigma-Aldrich, 99.99%, Germany) were used as the host and dopant precursors. A mixture of double deionised water, acetone and acetic acid in the ratio of 7 : 2 : 1 was used as the solvent.

A spray tube of 10 mm diameter was kept at a distance of 10 cm above the surface of the substrate. Thoroughly cleaned glass substrates were used in the deposition of the films. The schematic diagram of the AJNSP unit is shown in Fig. 1. Double filtered compressed dry air (pressure 20 kg cm⁻³) was allowed to pass through the jet nebulizer. This optimum pressure was enough for the venturi jet effect to take place at the nozzle. As a result, the precursor solution was converted into fine fumes, which reached the ultrasonically cleaned pre-heated substrate which was kept at an elevated temperature of 400 °C.

Fig. 1 Schematic diagram of the automated jet nebulizer spray technique.

The sprayed solution pyrolytically decomposed on reaching the hot surface of the substrate, resulting in the formation of a thin film. With the help of a timer, the spray time and spray interval were varied and optimized (each 10 s). A pair of magnetic sensors determined the horizontal distance (area of coating) to be covered by the jet nebulizer, and a double acting cylindrical control unit enabled the continuous reciprocation of the coating head. The covered area of deposition was 2.5 × 7.5 cm². The spray rate was calculated as 0.816 mL min⁻¹. Many trials were attempted to decide the thickness of the top and bottom layers. Finally the thicknesses of the top and bottom layers were fixed at 50, 75 and 100 nm. These values are relatively low when compared with those values generally used for single layer transparent electrodes. These smaller thicknesses cause a remarkable reduction in the material cost in addition to enhancing the transparency.

For the deposition of the metal layer, a thermal evaporation technique was employed. The intermediate metal layer was prepared by evaporating Ag (Sigma-Aldrich, 99.9% purity, 1 g) using a molybdenum boat as the source container. The vacuum chamber was maintained at 4 × 10⁻⁵ mbar. The systematically cleaned substrates were placed inside the chamber and the substrate temperature was maintained at 100 °C. The source to substrate distance was maintained at 20 cm. The boat was heated to the temperature of 970 °C and maintained at this temperature until the Ag was evaporated completely. After evaporation, the coated glass substrates were carefully removed and used.

The film thicknesses (t) were measured by a stylus probe technique. Different thicknesses of the intermediate metal layer were tested to improve the transport properties and optical transmittance, simultaneously. The film deposited with a thickness of 15 nm was found to possess excellent opto-electrical properties. An increase in the thickness of the middle layer resulted in a diminished transparency. Thus the thickness of the middle layer was optimized as 15 nm. The schematic representation of the deposited triple layer film is shown in Fig. 2. Markings A, B, C, D, E and F are electrical contacts.

Fig. 2 Schematic representation of deposited TaZO/Ag/TaZO triple layer films.

A structural analysis of the deposited films was carried out using X-ray powder diffraction (XRD) (PANalytical-PW 340/60 X'pert PRO) using Cu-Kα radiation (λ = 1.5406 Å). Spectrofluorometry (JobinYvon FLUROLOG-FL3-11) with a xenon lamp (450 W) as the excitation source of wavelength 325 nm, and UV-vis-NIR double-beam spectrophotometry (PerkinElmer Lambda 35 model) were used to study the defect levels and optical properties of the films, respectively. The surface morphology of the films was analyzed using field emission scanning electron microscopy (FESEM) (HITACHI S-3000H) and the elemental analyses were made using energy dispersive X-ray analysis (EDX) (Model: JEOL-JSM 6390 with attachment INCA-Penta FETX3 OXFORD). The chemical state analysis was done using X-ray Photoelectron Spectrometry (XPS) (Thermo Scientific™ K-Alpha™ ⁺) with an Al Kα (146.7 eV) X-ray source. The electrical properties of the films were studied using a linear four-point probe (Van der Pauw method) and Hall probe (ECOPIA HMS-3000). The sheet resistance of the triple layer film with optimal properties (75 + 15 + 75 nm) was measured both in the dark and in UV light injected conditions. The “Scotch-tape” was applied to the coated film surface and the film was pressed by forceps. Then, the tape was slowly pulled off and the coated layers were inspected for adherence.

3. Results and discussion

3.1 Structural studies

The XRD patterns of the films deposited with three different top and bottom layer thicknesses (50, 75 and 100 nm) and a constant intermediate metal layer thickness (15 nm) are shown in Fig. 3a–c. It is seen that all the deposited films are polycrystalline in nature and have the hexagonal wurtzite structure of ZnO (JCPDS card no. 36-1451).²⁵ The XRD patterns of the double layer (Ag/TaZO) and triple layer (TaZO/Ag/TaZO) films in all the three cases have a peak around 2θ = 38° corresponding to the (111) plane of Ag²¹ and confirming the presence of the middle layer material.

Fig. 3 (a–c) XRD patterns of TaZO, TaZO/Ag and TaZO/Ag/TaZO films with three different bottom and top layer thicknesses.

The prominent peaks seen in the patterns (Fig. 3a) are associated with the lattice planes of Miller indices (100), (002), (101), (102), (110) and (103). The order of the first three strongest peaks is (101), (002) and (100), irrespective of the thickness as well as the number of layers of the films, indicating that the growth patterns/orientations of the majority of the crystallites are not altered appreciably by the addition of over layers. The absence of any extra peaks which are not related to either ZnO or Ag in the patterns reveals that the films are free from any secondary phases or large size precipitations.

The strongest peaks of all the triple layer systems have increased intensities compared to their single and double layer counterparts. The increased intensity of all the peaks implies that the crystallinity is increased with the support of the over layers. Thus it reveals that the bottom layer of the triple layer structure acts as the buffer layer in improving the crystallinity of the double and triple layer structures.

The crystallite size (D) of the deposited films was calculated using Scherrer's formula D = 0.94λ/β cos θ, where λ is the wavelength of the X-ray used (1.5406 × 10⁻¹⁰ m), β is the full-width at half-maximum (FWHM) and θ is the diffraction angle.²⁶ The calculated values are shown in Table 1. Keen observation of the crystallite sizes (D) of the films reveals that they follow the order: single layer < double layer < triple layer. This order is true for all the three thickness values. These results suggest that the bottom layer supports the growth of the grains better than does the bare glass substrate. Moreover, we believe that when the film thickness increases, it favours the merging of scattered islands into larger grains.²⁷

Table 1 Electrical parameters of TaZO/Ag/TaZO triple layer films

Thickness (t) (nm)	Sample code	Grain size (D) (nm)	Sheet resistance (Rsh) (Ω sq^−1)
50	TaZO	40	1900
50 + 15	TaZO/Ag	44	74
50 + 15 + 50	TaZO/Ag/TaZO	51	69
75	TaZO	52	1398
75 + 15	TaZO/Ag	55	49
75 + 15 + 75	TaZO/Ag/TaZO	69	32
100	TaZO	74	893
100 + 15	TaZO/Ag	86	42
100 + 15 + 100	TaZO/Ag/TaZO	92	31

---

3.2 Electrical studies

The measured sheet resistances (R_sh) of the deposited films are given in Table 1. To measure the resistance of the multilayer films, the layers were considered as parallel films. A drastic reduction in the sheet resistance values was observed for the double layer systems compared to the corresponding single layer films. For example in the case of a single layer having a thickness of 50 nm, the sheet resistance is 1900 Ω sq⁻¹, but the R_sh reduces to 74 Ω sq⁻¹ for the corresponding double layer system which is a 25-fold reduction. When all three layers are present, the values of the sheet resistance reduce further and reach 69, 32 and 31 Ω sq⁻¹ for the films having thickness proportions of 50 + 15 + 50, 75 + 15 + 75 and 100 + 15 + 100 nm, respectively.

The main reason for this reduction in sheet resistance value is the difference in the work function between Ag and ZnO.^28,29 It is known that when an ohmic contact is formed between the interfaces of two layers of different work functions, an accumulation of carriers in the layer having higher work function ensues. The amount of the charge carrier injection is determined by the difference in the work functions.

When the two layers are joined together (Fig. 4), there is a rearrangement in the Fermi level of the system owing to the transfer of carriers from the region of the greater number of free electrons to the region of the fewer ones. Thus, there is a band bending at the junction leading to the gathering of charges at the interface. The barrier for the flow of electrons becomes negligible resulting in a free flow of charges from the Ag layer to the ZnO layer. From these factors, it is understood that the intermediate metal layer determines the resistivity of the multilayer system.

Fig. 4 Schematic energy gap diagram of separate and interfaced Ag and ZnO:Ta layers.

It is observed from Table 1 that, the sheet resistance of single layer films decreases when thickness is increased, as expected. It is well known that the thickness of thin films plays an important role in increasing the flow of free carrier charges across the film.

Fig. 5 shows the variation in sheet resistance, carrier concentration and mobility with thickness of all three triple layers. The figure shows that the carrier concentration and mobility increase with thickness. This variation can be explained on the basis of the size of the grains and the grain boundary scattering effect,³⁰ as discussed in Section 3.6.

Fig. 5 Variation in sheet resistance, carrier concentration and mobility of TaZO/Ag/TaZO films with different overall thicknesses.

The sheet resistance values of the film with thickness 75 + 15 + 75 nm were measured in the dark and in UV light injected conditions as 57 and 28 Ω sq⁻¹, respectively. The sheet resistance value reduces slightly (32 to 28) under UV light injected conditions and increases considerably from 32 to 59 Ω sq⁻¹ in darkness.

3.3 Optical studies

Fig. 6a–c summarises the transmittance spectra of the triple layer structures deposited with different thicknesses. From Fig. 6a, it can be seen that the transmittance value of the base layer (50 nm thick) is around 94% whereas the transmittance value reduces to 87% for the double layer (50 + 15 nm). As Ag is a very good reflector, it plays the role of a reflecting mirror for the incoming light and as a consequence, the transmittance value of the double layer films is less than that of the single layer, as expected.³¹ Moreover, the increased total thickness may be another reason for this decrease in the transmittance. The transmittance value of the triple layer system (50 + 15 + 50 nm) is found to be 91%. Even though the overall thickness of the triple layer system is higher, the transmittance is found to be improved compared to that of the double layer films. This is because of the fact that the topmost TaZO layer of the triple layer structure acts as an anti-reflective coating and thereby suppresses the reflection from the middle layer (Ag). In other words, the deposition of the oxide layer over the metal layer improves the transmittance of the system. This discussion is also valid for the other two thicknesses (75 and 100 nm).

Fig. 6 (a–c) Transmittance spectra of TaZO/Ag/TaZO films.

It is interesting to note the transmittance values in the IR range. For double and triple layer cases, the transmittance value decreases in the near IR region which may be due to the increasing carrier concentration in these samples. Generally, the transmittance value in the IR range gives an indication of the carrier concentration in the final product.³² This result supports the discussion of the increase in carrier concentration given in Section 3.2.

The optical energy gap values of the deposited triple layer films are calculated with the help of the plots of dT/dλ (where T is transmittance) as a function of energy, as shown in Fig. 7a–c. The calculated energy values are given in Table 2.

Fig. 7 (a–c) dT/dλ vs. energy plots of TaZO/Ag/TaZO films.

Table 2 Optical parameters of TaZO/Ag/TaZO triple layer films

Thickness (t) (nm)	Transmittance (T) %	Figure of merit (ϕ) 10^−3 (Ω sq^−1)^−1	Energy gap (Eg) (eV)
50	94	0.28	3.27
50 + 15	83	2.10	3.29
50 + 15 + 50	90	5.05	3.30
75	92	0.31	3.29
75 + 15	81	2.48	3.30
75 + 15 + 75	88	8.98	3.32
100	89	0.35	3.37
100 + 15	80	2.56	3.38
100 + 15 + 100	85	6.15	3.39

---

The energy gap values of the double and triple layer films are found to be higher than those of their single layer counterparts. This result may be explained on the basis of the Moss–Burstein effect.³³ The widening of the energy gap is due to the increased strength of carrier concentration and the consequent filling of lower levels of the conduction band. This result is consistent with the electrical studies related to the carrier concentration, as discussed in Section 3.2.

To determine the optimum thickness of a multilayer film, the figure of merit value of all the samples was calculated using the following formula suggested by Haacke:³⁴

where T is the transmittance and R_sh is the sheet resistance of the film. The calculated values are given in Table 2. Among the triple layer films, that with the 75 + 15 + 75 nm thickness possesses the highest figure of merit followed by the 100 + 15 + 100 nm thickness film. In the minimum thickness case (50 + 15 + 50 nm), although transmittance is high owing to the minimum thickness, the sheet resistance value is not sufficiently low to give a better figure of merit value. The reverse is true in the case of the film with the highest thickness (100 + 15 + 100 nm). It has high sheet resistance but relatively low transmittance. The film with the 75 + 15 + 75 nm thickness possesses a sheet resistance value which is as low as that possessed by the highest thickness film and a transmittance that is as high as that possessed by the lowest thickness film. Hence, based on the highest obtained figure of merit value (8.98 × 10⁻³ (Ω sq⁻¹)⁻¹), the film with the 75 + 15 + 75 nm thickness can be considered as a potential candidate for photovoltaic applications.

3.4 XPS studies

The surface of the TaZO/Ag/TaZO multilayer film (75 + 15 + 75 nm) was investigated by XPS and the corresponding results are shown in Fig. 8a–d. The survey spectrum of the film is shown in Fig. 8a, which reveals the presence of Zn, O and Ta. The binding energies in the spectrum were calibrated by taking C 1s (284.8 eV) as the reference. Fig. 8b shows the results of peak fitting for the Zn 2p spectrum. The spectrum could be fitted into two peaks centred at 1045.6 (FWHM = 2.03 eV) and 1022.6 eV (FWHM = 2.02 eV), which are ascribed to Zn 2p_1/2 and Zn 2p_3/2, respectively. The binding energy difference between the peaks is 23 eV. This result is in accordance with values already reported in the literature.³⁵

Fig. 8 (a) XPS survey spectrum of a TaZO/Ag/TaZO triple layer film. XPS spectra of (b) Zn 2p, (c) O 1s and (d) Ta 4f states.

The O 1s profile was fitted with the Gaussian fitting function. The spectrum of O 1s was decomposed into two components as seen in Fig. 8c. The peaks at 531.1 eV and 532.9 eV can be associated with the lattice oxygen of ZnO and chemisorbed oxygen, respectively.³⁶ The high binding energy component of oxygen (532.9 eV) is usually attributed to the presence of loosely bound oxygen on the surface of a film, belonging to molecular water, carboxylate or carbonate type species. The binding energy component found around 531 eV is associated with O²⁻ ions in the oxygen deficient regions of the ZnO matrix.³⁷ By establishing the variation in this peak area, variations in the concentration of oxygen vacancies can be deduced.

The Ta 4f spectrum has two peaks at 26.8 eV (4f_7/2) and 28.7 eV (4f_5/2), with an energy splitting of 1.9 eV (Fig. 8d). These peak values are consistent with the reported values for the binding energy of the 5+ valence state of tantalum. Generally, the presence of Ta in the Ta⁵⁺ chemical state is confirmed by the presence of the peak around 27 eV.³⁸ The stoichiometry of the elements Zn, Ta and O on the surface of the film were also calculated and found to be 48.63% : 0.46% : 50.90%. These calculated values agree well with the EDX results.

3.5 Photoluminescence (PL) studies

The PL spectra of the TaZO/Ag/TaZO triple layer structures deposited with three different thicknesses are shown in Fig. 9. The spectra show peaks representing an ultraviolet emission around 400 nm, a violet emission around 420 nm, blue emissions around 450, 465 and 480 nm, and also a green emission around 530 nm.

Fig. 9 Room temperature PL spectra of TaZO/Ag/TaZO films.

The intense peak positioned around 400 nm represents the near band edge emission of the wide band gap ZnO.³⁹ It represents the radiative recombination between the electrons in the conduction band with the holes in the valence band or with closer defect levels. The peak observed around 420 nm is attributed to the transition of electrons from interstitial zinc (Zn_i) to the valence band.⁴⁰ The weak peak around 450 nm is attributed to the zinc vacancy (V_Zn) defect level.⁴¹ The band around 465 nm is related to the singly ionised oxygen vacancies (V⁺_O).⁴² Generally, these oxygen vacancies play a crucial role in the electrical properties of ZnO thin films. In our study, the peak around 465 nm provides evidence of the presence of oxygen vacancies, which also supports the obtained minimum sheet resistance, discussed in Section 3.2. The transition of electrons from interstitial zinc (Zn_i) to zinc vacancies (V_Zn) is represented by the peak around 480 nm (ref. 40) and the weak peak around 530 nm is associated with the presence of oxygen vacancies (V_O).⁴³ Even though the above mentioned various defect level peaks are observed in all three cases, the peaks representing zinc vacancy and oxygen vacancy defects are more obvious in the medium and highest thickness films (75 and 100 nm), when compared with the lowest thickness film (50 nm).

3.6 Surface morphology and compositional studies

The surface morphological images of the single, double and triple layer films are shown in Fig. 10a–i. In the case of single layer films, when the thickness is minimum (Fig. 10a), the surface of the film consists of spherical grains with some voids here and there. When the thickness is increased, the size of the grains increases without any remarkable changes in the shape. However, for the film with the highest thickness, the shape as well as the size of the grains changes (Fig. 10g). Here the surface shows polygon shaped grains along with some linear grains with well-defined grain boundaries.

Fig. 10 (a–i) FESEM images of TaZO/Ag/TaZO films.

The middle metal layers of all the three cases exhibit some irregular agglomerated grains on the surface, but otherwise all the three films appear similar. Thus the middle layer does not greatly influence the morphology of the system (Fig. 10b, e and h).

Fig. 10c, f and i reveals that all the triple layer films consist of closely packed grains. It is obvious from the images that the shapes of the grains in all the three top layers resemble their bottom layer counterparts. When the thickness is increased, the grain size increases (Fig. 10f) which may be due to Ostwald ripening.^44,45 A few grains with hexagonal shape can also be seen which are indicated in the image with red colour markings (Fig. 10i). The grain size is highest for the film with the highest thickness (100 nm). This leads to a decrease in the grain boundary scattering of free carrier charges, thereby causing an increase in mobility. This observed increase in grain size supports strongly the enhanced electrical properties of the film discussed in Section 3.2.

The presence of elements in the deposited films is confirmed by EDX. The EDX spectrum of the film (75 + 15 + 75 nm) confirms the presence of all the anticipated elements, viz., Zn, O, Ta and Ag (Fig. 11). The atomic and weight percentages of the elements obtained from the EDX analysis are given as the inset in Fig. 11. From the composition values, it is clear that some of the metal dopant (Ta) did incorporate into the ZnO crystal lattice. The profile value is in accordance with the amount of dopant in the starting solution.

Fig. 11 EDX spectrum of TaZO/Ag/TaZO film.

Mapping images of the film are shown in Fig. 12, and give information about the position of the elements in the deposited films and the proportions of the elements on the surface under study.

Fig. 12 Compositional mapping of TaZO/Ag/TaZO film.

3.7 Adherence, stability and effect of humidity tests

In order to test the adherence of the deposited films, the scotch tape test was carried out for all the deposited films. The adherence of the TaZO/Ag/TaZO multilayer on the glass substrate was found to be good. The stacks were found to withstand the scotch tape test. In addition to the scotch tape test, the TaZO/Ag/TaZO multilayer withstood rubbing with cotton and scratching with a finger nail. On the other hand, it could not withstand scratching with a sharp metal object.

The stabilities of the bare Ag layer and the TaZO/Ag/TaZO multilayer were also tested. Both the layers were kept at atmospheric conditions and transmittance was calculated at regular intervals of 10 days. It was found that, in the case of bare Ag films, some signs of staining and some white spots were seen on the layer. However, the transmittance of the multilayer film was found to be almost the same as that of the initial sample even after 180 days (Fig. 13) and no white dots were found on the multilayer samples. This result reveals that the TaZO layer has good durability.

Fig. 13 Sheet resistance and transmittance values of the TaZO/Ag/TaZO film over a range of relative humidity values.

The influence of relative humidity on the electrical and optical properties of the deposited layers was also tested. The multilayer coatings show almost the same performance regarding electrical sheet resistance and optical transmittance over a range of humidity values (72–98%) as shown in Fig. 13. The test results reveal that the performance, stability and the life time of the multilayer system are not affected by the moisture in the air.

4. Conclusion

Uniformly coated, good quality, highly durable and well adhered TaZO/Ag/TaZO multilayer films which are not appreciably affected by humidity were successfully deposited using an inexpensive technique for the first time. The multilayer films possess reduced sheet resistance with good transmittance which cannot be achieved through single layer structures of the same material. The calculated electrical and optical parameters show that the thickness of the film plays a vital role. Among the deposited films, the one with the thickness of 75 + 15 + 75 nm has the highest quality factor value of 8.98 × 10⁻³ (Ω sq⁻¹)⁻¹. Even though the highest quality factor obtained in this study is not higher than the highest value reported for triple layers fabricated using sophisticated physical methods, this result is important because in this study, the bottom and top layers were prepared by a low-cost simple chemical technique. Moreover, the quality factor can be improved through suitable modifications in the process parameters such as the thicknesses of the layers, spray rate, spray time and deposition temperature. Hence, it is concluded that the films that are deposited using a low-cost chemical technique can be considered as a better alternative to the single layer electrodes of solar cells. In addition, these results will be very helpful in improving both the understanding of problems related to the balance between resistance and transmittance and the design of photovoltaic devices through cost-effective techniques in the future.

Acknowledgements

The author K. Ravichandran gratefully acknowledges the Council for Scientific and Industrial Research (CSIR), New Delhi, India for financial support through the research scheme (No. 03(1321)/14/EMR-II). The author K. Subha gratefully acknowledges the University Grants Commission, New Delhi, India for financial assistance through the Minor Research Project (MRP-5688/15(SERO/UGC)).

References

1. Y. T. Park, A. Y. Ham, Y. H. Yang and J. C. Grunlan, RSC Adv., 2011, 1, 662.
2. H. K. Kim and J. W. Lim, Nanoscale Res. Lett., 2012, 7, 67.
3. Z. L. Pei, C. Sun, M. H. Tan, J. Q. Xiao, D. H. Guan, R. F. Huang and L. S. Wen, J. Appl. Phys., 2001, 90, 3432.
4. M. Suchea, S. Christoulakis, N. Katsarakis, T. Kitsopoulos and G. Kiriakidis, Thin Solid Films, 2007, 515, 6562.
5. R. K. Gupta, K. Ghosh and P. K. Kahol, Phys. E, 2010, 42, 2000.
6. G. Ding, C. Clavero, D. Schweigert and M. Le, AIP Adv., 2015, 5, 117234.
7. K. Saravanan, R. Krishnan, S. H. Hsieh, H. T. Wang, Y. F. Wang, W. F. Pong, K. Asokan, D. K. Avasthi and D. Kanjilal, RSC Adv., 2015, 5, 40813.
8. Z. Zulkifli, M. Subramanian, T. Tsuchiya, M. S. Rosmi, P. Ghosh, G. Kalita and M. Tanemura, RSC Adv., 2014, 4, 64763.
9. A. Z. Barasheed, S. R. S. Kumar and H. N. Alshareef, J. Mater. Chem. C, 2013, 1, 4122.
10. S. Chen, N. Noor, I. P. Parkin and R. Binions, J. Mater. Chem. A, 2014, 2, 17174.
11. V. Gokulakrishnan, S. Parthiban, K. Jeganathan and K. Ramamurthi, Appl. Surf. Sci., 2011, 257, 9068.
12. R. K. Gupta, K. Ghosh and P. K. Kahol, Appl. Surf. Sci., 2009, 256, 1538.
13. R. Eisele, N. J. Blumenstein, J. Baier, S. Walheim, T. Schimmel and J. Bill, CrystEngComm, 2014, 16, 1560.
14. K. Ravichandran, K. Subha, N. Dineshbabu and A. Manivasaham, J. Alloys Compd., 2016, 656, 332.
15. S. Yang and Y. Zhang, J. Magn. Magn. Mater., 2013, 334, 52.
16. N. Chahmat, T. Souier, A. Mokri, M. Bououdina, M. S. Aida and M. Ghers, J. Alloys Compd., 2014, 593, 148.
17. S. Ghosh, M. Saha and S. K. De, Nanoscale, 2014, 6, 7039.
18. X. Liu, X. Cai, J. Qiao, J. Mao and N. Jiang, Thin Solid Films, 2003, 441, 200.
19. L. Gong, J. Lu and Z. Ye, Sol. Energy Mater. Sol. Cells, 2011, 95, 1826.
20. T. Dimopoulos, G. Z. Radnoczi, B. Pecz and H. Bruckl, Thin Solid Films, 2010, 519, 1470.
21. S. Song, T. Yang, Y. Xin, L. Jiang, Y. Li, Z. Pang, M. Lv and S. Han, Curr. Appl. Phys., 2010, 10, 452.
22. D. R. Sahu, S. Y. Lin and J. L. Huang, Thin Solid Films, 2008, 516, 4728.
23. M. Girish, R. Sivakumar, C. Sanjeeviraja and Y. Kuroki, Optik, 2015, 126, 2074.
24. E. Young, R. M. Smith, B. L. Sharp and J. R. Bone, J. Chromatogr. A, 2012, 1236, 16.
25. K. Ravichandran, R. Mohan, B. Sakthivel, S. Varadharajaperumal, P. Devendran, T. Alagesan and K. Pandian, Appl. Surf. Sci., 2014, 321, 310.
26. N. J. Begum, R. Mohan and K. Ravichandran, Superlattices Microstruct., 2013, 53, 89.
27. G. Jeffers, M. A. Dubson and P. M. Duxbury, J. Appl. Phys., 1994, 75, 5016.
28. K. Sivaramakrishnan, N. D. Theodore, J. F. Moulder and T. L. Alford, J. Appl. Phys., 2009, 106, 063510.
29. A. Indluru and T. L. Alford, J. Appl. Phys., 2009, 105, 123528.
30. X. Yu, J. Ma, F. Ji, Y. Wang, C. Cheng and H. Ma, Appl. Surf. Sci., 2005, 245, 310.
31. X. Sun, R. Hong, H. Hou, Z. Fan and J. Shao, Chin. Opt. Lett., 2006, 4, 366.
32. C. M. Muiva, T. S. Sathiaraj and K. Maabong, Ceram. Int., 2011, 37, 555.
33. X. l. Chen, J. m. Liu, J. Ni, Y. Zhao and X. d. Zhang, Appl. Surf. Sci., 2015, 328, 193.
34. Z. Chen, W. Li, R. Li, Y. Zhang, G. Xu and H. Cheng, Langmuir, 2013, 29, 13836.
35. X. Deng, K. Yao, K. Sun, W. X. Li, J. Lee and C. Matranga, J. Phys. Chem. C, 2013, 117, 11211.
36. E. A. Dalchiele, P. Giorgi, R. E. Marotti, F. Martin, J. R. R. Barrado, R. Ayouci and D. Leinen, Sol. Energy Mater. Sol. Cells, 2001, 70, 245.
37. J. Z. Kong, A. D. Li, X. Y. Li, H. F. Zhai, W. Q. Zhang, Y. P. Gong, H. Li and D. Wu, J. Solid State Chem., 2010, 183, 1359.
38. E. Atanassova, T. Dimitrova and J. Koprinarova, Appl. Surf. Sci., 1995, 84, 193.
39. Z. H. Wang, D. Y. Geng, Z. Han and Z. D. Zhang, Mater. Lett., 2009, 63, 2533.
40. N. S. Han, H. S. Shim, J. H. Seo, S. Y. Kim, S. M. Park and J. K. Song, J. Appl. Phys., 2010, 107, 084306.
41. A. K. Srivastava and J. Kumar, Sci. Technol. Adv. Mater., 2013, 14, 065002.
42. X. H. Guo, J. QiMa and H. G. Ge, J. Phys. Chem. Solids, 2013, 74, 784.
43. S. C. Liu and J. J. Wu, Mater. Res. Soc. Symp. Proc., 2002, 703, 49.
44. T. Ghosh, P. Karmakar and B. Satpati, RSC Adv., 2015, 5, 94380.
45. H. Wang, L. Xin, H. Wang, X. Yu, Y. Liu, X. Zhou and B. Li, RSC Adv., 2013, 3, 6538.

---