One-step fabrication of a transparent and conductive TiO_x/Ag nanowire hybrid thin film with high robustness

Abstract

A TiO_x/Ag nanowire (NW) hybrid thin film with a sheet resistance of 10.4 Ω □⁻¹ and an average transmission of 83% has been successfully fabricated by a one-step solution deposition approach. The adhesion, smoothness, and stability can be significantly improved after incorporating amorphous TiO_x into Ag NW networks.

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A transparent electrode is an essential component in various photoelectric devices. To date, the most commonly used transparent and conductive thin films are indium-tin-oxide (ITO) and fluorine-doped tin oxide (FTO) because of their high conductivity and transmittance.^1–6 These high-quality transparent and conductive oxide thin films are usually deposited by some expensive vacuum-based approaches, such as sputtering and chemical vapor deposition methods, which will lead to a high manufacturing cost.^1–10 Recently, solution-processed and low-cost Ag NW networks have been drawing a lot of attention as transparent electrodes,^11–31 and they have been successfully used in the light-emitting diodes,^24–26 thin film solar cells^27–30 and touch-sensitive control panels³¹ owing to their low processing cost and high flexibility. So far, the optical and electrical properties (i.e. transmittance and sheet resistance) of Ag NW networks have been comparable to or even better than those of commercialize ITO thin films.^11–19 However, the pristine Ag NW networks exhibit very bad adhesion that cannot pass the tape adhesion test, which severely limits their applications. Thus, enhancing the adhesion and robustness of Ag NW networks is highly desirable. Several recent reports have demonstrated that the incorporation of some organic or inorganic components into Ag NW networks can significantly improve the adhesion and robustness of Ag NW networks.^{20–24,27–30} Among these organic or inorganic components, polycarbonate,²⁰ poly(3,4-ethylenedioxythiophene) (PEDOT),²⁷ ITO nanoparticles,²⁸ ZnO,²⁹ TiO₂,³⁰ and SnO_x (ref. 22) have been extensively investigated. In previous reports, these organic or inorganic/Ag NW hybrid networks were usually fabricated by a two-step approach. Generally, these organic or inorganic components and Ag NWs are immiscible; therefore, the organic or inorganic/Ag NW hybrid networks cannot be directly fabricated by a one-step approach. Compared with a two-step approach, one-step solution deposition approach is more facile and cost-effective.

To develop a one-step solution-based process to deposit inorganic/Ag NW hybrid networks, the compatibility issue between inorganic component solution and Ag NW dispersion is needed to be resolved. As known, polyvinylpyrrolidone (PVP)-capped Ag NWs can be well dispersed in water, ethanol and isopropanol. In this paper, amorphous TiO_x was chosen as the inorganic component to fabricate robust TiO_x/Ag nanowire hybrid thin film. TiO_x sol–gel solution was first prepared according to a previously reported method,³² and was then mixed with Ag NW dispersion, as shown in Fig. 1a and b. TiO_x sol–gel solution can be diluted with water, ethanol, and 2-methoxyethanol, making it well compatible with Ag NW/ethanol dispersion. No aggregation was observed in the mixed TiO_x/Ag NW dispersion. Such stable TiO_x/Ag NW dispersion enables us to directly deposit TiO_x/Ag nanowire hybrid thin film by a one-step approach. As-deposited TiO_x/Ag nanowire thin film was sintered on a pre-heated hotplate to form a dense and robust TiO_x/Ag NW thin film. It was found that incorporating of Ag NWs into the matrix of TiO_x can significantly improve the adhesion, smoothness, and stability of TiO_x/Ag NW networks.

Fig. 1 Digital photographs of TiO_x sol–gel solution (a) and mixed TiO_x/Ag NW dispersion (b).

Amorphous and dense TiO_x thin film was formed after sintering at high temperatures (see Fig. S1†). It was found that the sintering temperature has a profound effect on the transmittance and sheet resistance of TiO_x/Ag NW hybrid networks. Fig. 2a shows four UV-vis transmittance spectra of TiO_x/Ag NW hybrid networks which were sintered at different temperatures for 2 min. TiO₂ has an indirect band gap of 3.2 eV, and it does not absorb visible light;³³ thus TiO_x/Ag NW hybrid networks are highly transparent in the wavelength range of 400–800 nm. Our results revealed that the highest transmittance of 83% was achieved for the sample sintered at 260 °C. When the sintering temperature is over 320 °C, it was observed that Ag NWs were melted and destroyed, which is consistent with that of previously reported Ag NW networks.³⁴ In addition, the effect of sintering temperature on the sheet resistance was investigated, and the data are shown in Fig. 2b. The sheet resistance of TiO_x/Ag NW hybrid networks shows a dramatically decrease from 375 Ω □⁻¹ to 10.5 Ω □⁻¹ when the sintering temperature increases from 180 °C to 200 °C, whereas the sheet resistance keeps almost unchanged when further elevating the sintering temperature to 260 °C. According to the analysis of the influence of the sintering temperature on the transmittance and sheet resistance, the optimum sintering temperature was chosen as 260 °C. Finally, we found that the transmittance and sheet resistance of TiO_x/Ag NW hybrid networks are mainly influenced by the sintering temperature rather than by the sintering time.

Fig. 2 Effects of sintering temperature on transmittance (a) and sheet resistance (b) of transparent and conductive TiO_x/Ag NW hybrid thin films.

The transmittance and sheet resistance of TiO_x/Ag NW hybrid thin films are strongly related on the density of Ag NWs. Thus, the optical and electrical properties of TiO_x/Ag NW hybrid thin films can be tuned by varying the concentration of TiO_x/Ag NW dispersion or changing the spin speed of the spin coater. In this paper, we adopted a multi-step spin-coating method to change the density of Ag NWs, instead of changing the concentration and spin speed of TiO_x/Ag NW dispersion, which leads to the formation of TiO_x/Ag NW hybrid thin films with different transmittance and sheet resistance. By a single-step spin-coating process, as-fabricated TiO_x/Ag NW hybrid thin film exhibits a sheet resistance of 39 Ω □⁻¹ and transmittance of 87% in the range 400–900 nm, as shown in Fig. 3. Both the sheet resistance and transmittance gradually decrease with increasing of spin-coating cycles. After 4 spin-coating/sintering cycles, TiO_x/Ag NW hybrid thin film exhibits a transmittance of 64% and a sheet resistance of 5 Ω □⁻¹. Such semi-transparent TiO_x/Ag NW hybrid thin film is not suitable for the application in the photoelectric devices as the transparent electrode, as shown in the inset of Fig. 3. However, these TiO_x/Ag NW hybrid thin films with an extremely low resistance should be of great interest in anti-static coating. Note that it is difficult to fabricate low-resistance TiO_x/Ag NW hybrid thin films by a single-step spin-coating process because an extremely high-concentration Ag NWs dispersion and a low spin speed are required, which will result in the formation of a low-quality TiO_x/Ag NW hybrid thin film.

Fig. 3 Transparent and conductive TiO_x/Ag NW hybrid thin films with different transmittance and sheet resistance fabricated by a multi-step spin-coating method (inset: photograph of TiO_x/Ag NW hybrid thin films).

The morphology of TiO_x/Ag NW hybrid thin film was characterized by scanning electron microscopy (SEM). Fig. 4a and b display top-view SEM images of TiO_x/Ag NW hybrid thin film at different magnifications. A ∼7 nm white thin layer was observed on the surface of Ag NWs due to high-resistance TiO_x charging, as shown in Fig. 4b, revealing that Ag NWs were capped by a thin TiO_x layer. Cross-sectional SEM observation (Fig. 4c) shows that the Ag NW networks are completely buried in the TiO_x matrix, and have an average thickness of 228 nm. It can clearly be seen that TiO_x/Ag NW hybrid network is a continuous film. The incorporation of Ag NWs into the TiO_x matrix is expected to improve the robustness and stability as well as decrease the roughness of Ag NW networks. After incorporating TiO_x, Ag NW network exhibits high robustness, which can pass the tape adhesion test. Note that the pristine Ag NW networks fail to pass the tape peeling test, as shown in Fig. 4d. Additionally, the long term stability of Ag NW network is of great importance in their practical applications. As known, bulk Ag can be slowly oxidized by air, and the oxidation of Ag NW networks will become more prominent due to their high surface area. In our case, Ag NWs were fully covered by a dense and thin TiO_x layer, which can effectively prevent oxidation of Ag NWs. After 56 days of air storage, the sheet resistance of TiO_x/Ag NW hybrid thin film shows only a slight increase, as presented in Fig. S2.† As expected, the root mean square surface roughness of TiO_x/Ag NW hybrid thin film shows a significant decrease from 55 nm to 22 nm with respect to pristine Ag NW network, as determined by profiler meter. Furthermore, we found that the sheet resistance will increase with increasing TiO_x content, whereas the surface roughness will increase with decreasing the ratio of TiO_x/Ag NWs. Therefore, in our experiments, the optimum concentrations of Ag NWs and TiO_x sol–gel solution are found to be 5 mg mL⁻¹ and 0.25 mmol mL⁻¹, respectively (see Fig. S3†).

Fig. 4 Top-view SEM images (a and b) and cross-sectional SEM image (c) of transparent and conductive TiO_x/Ag NW hybrid thin films; (d) tape peeling tests of pristine Ag NW thin film and TiO_x/Ag NW hybrid thin film.

In conclusion, we demonstrated a facile one-step approach for depositing highly robust TiO_x/Ag NW hybrid thin films. TiO_x sol–gel solution and Ag NW dispersion exhibit good compatibility, which enables the formation of high quality TiO_x/Ag NW hybrid thin films. The adhesion, smoothness, and stability can be significantly improved after incorporating amorphous TiO_x into Ag NW networks.

A sheet resistance of 10.5 Ω □⁻¹ and an average transmission of 83% have been successfully achieved for TiO_x/Ag NW hybrid thin film by a one-step solution deposition approach. These robust and transparent TiO_x/Ag NW hybrid electrodes should be have a high potential in thin film solar cells and light-emitting diodes.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 91333108; 51302258; 51202241).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details; XRD pattern; the stability of sheet resistance of transparent and conductive TiO_x/Ag NW hybrid thin film. See DOI: 10.1039/c5ra27812k

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