1. Introduction

Transparent conductive oxide (TCO) thin films have a wide range of applications in the photovoltaic (PV) industry. As a transparent electrode material, TCO has to provide high light transparency, low electrical resistance, and strong capability of light scattering. Currently fluorine-doped tin oxide (SnO$_{2}$:F or FTO) films are widely used as TCO films in the PV industry. However, as an alternative approach, aluminum doped zinc oxide (ZnO:Al or AZO) film, which can be produced on large areas in a controllable manner by magnetron sputtering, is considered as a substitute for use in amorphous silicon solar cells. Compared to well established FTO films by chemical vapor deposition (CVD), AZO films maintain good stability in ambient hydrogen plasma^{[1, 2]}, lower resistivity, less absorption, and improved light scattering thanks to the fact that a precise control of the morphology can be achieved from the wet chemical etching process^[3-5]. In particular, in a-Si:H/$\mu $c-Si:H tandem thin film solar cells, AZO has a better performance in increasing the short circuit current, reducing silicon thickness, and lowering down the production cost of solar modules^{[6, 7]}.

So far, among various deposition processes, magnetron sputtering is one of the most promising methods for ZnO deposition owing to the inherent ease with which the deposition conditions can be controlled well^[8]. In this study, AZO thin films were prepared by a DC sputtering system with ZnO:Al$_{2}$O$_{3}$ ceramic targets, and the correlations between the sputtering processes and properties of AZO thin films were established. AZO thin films with high transmittance, high light scattering, and low electrical resistivity were obtained through the optimization of sputtering processes. As a result, the AZO thin films were incorporated into a-Si:H/$\mu $c-Si:H thin film solar cells, and an initial conversion efficiency up to 12.5% has been achieved in preliminary experiments.

2. Experiments

AZO thin films were deposited by a high vacuum magnetron sputtering system on 3.2 mm thick, 140 $\times$ 140 mm$^{2}$ sized low iron glass substrates using ceramic targets with an Al$_{2}$O$_{3}$-concentration of 1 wt%. The deposition pressure was adjusted through Ar gas flow; the substrate temperature was pre-calibrated on 3.2 mm thick glass with a thermal sensor and controlled by heater temperature during film deposition, and the film thickness was controlled by sputtering power and deposition time. Then, the as-deposited AZO thin films were textured by wet-chemical etching with diluted hydrochloric acid (HCl). The surface morphology was controlled by etchant concentration, etching time, and temperature. Immediately after the chemical etching process the samples were cleaned by D.I. water. Finally, the glass substrates with textured AZO were used to fabricate Si thin film solar cells. The a-Si:H/$\mu $c-Si:H tandem junctions were prepared using a plasma-enhanced chemical vapor deposition (PECVD) system, followed by AZO/Ag back electrodes that were formed by sputtering with a shadow mask to define a small cell area around 0.5 cm$^{2}$.

Electrical resistivity of the AZO films was tested by a four-point probe meter (RTS-9). Hall mobility and carrier concentration, were measured by a Hall meter (Ecopia, HMS-2000). Transmittance and haze of the films were measured using a UV/VIS spectrometer (Perkin Elmer, Lambda 750) in the wavelength range of 300-1100 nm. Film thickness was scanned by a surface profiler (KLA, P-16+). Surface morphology was inspected by an atomic force microscopy (AFM) (Agilent, 5400 SPM). The crystal structure of the thin films was investigated using X-ray diffraction (XRD, Bruker advance D8) with Cu K$\alpha$ radiation ($\lambda$ $=$ 0.1540 nm). Current-voltage (IV) and quantum-efficiency (QE) characteristics of solar cells were tested at standard testing conditions (AM1.5, 100 mW/cm$^{2}$, 25 ℃).

3. Results and discussion

3.1 Structure of AZO thin films

Figure 1 shows the typical XRD pattern of the AZO thin film with a thickness of $\sim $860 nm. It can be found that the film has a hexagonal wurtzite crystal structure (P63mc). In this work, all the deposited films have prominent $\langle$002$\rangle$ orientation as is observed from the XRD spectra, indicating a preferential $c$-axis orientation perpendicular to the substrate^[9].

3.2 Effects of the deposition processes

AZO thin films were sputtered with a similar thickness around 860 nm with the deposition temperature varied as other sputtering conditions are fixed. Then the electrical and optical properties were analyzed. The dependence of carrier concentration ($n)$, Hall mobility ($\mu$), and electrical resistivity ($\rho$) on deposition temperature is shown in Fig. 2(a). It is observed that carrier concentration lies in a narrow range of (4-5) $\times$ 10$^{20}$ cm$^{-3}$, showing a weak dependence on deposition temperature. The Hall mobility shows an initial sharp rise and then a gradual decrease as the deposition temperature further increases. Co-determined by carrier concentration and mobility, electrical resistivity shows a minimum value at an intermediate temperature. The above data can be interpreted as follows: (1) a higher deposition temperature can enhance film crystallization, increase grain sizes, and reduce defects, resulting in better Hall mobility due to less carrier scattering and trapping; (2) A too high deposition temperature leads to excess increase of Al precipitation at grain boundaries in the form of Al$_{2}$O$_{3}$, which causes variation of grain orientation, increases scattering, and thus reduces carrier mobility^{[10, 11]}. Both factors work together and co-determine that minimal electrical resistivity occurs at the intermediate temperature.

Figure 2(b) shows the optical transmittance in the UV-VIS range of AZO films deposited at different deposition temperatures, with sputtering pressure and deposition power fixed. By increasing the deposition temperature, the transmittance increases in the short wavelength range (340-450 nm), and then reaches saturation at an intermediate temperature. As mentioned above, temperature increase leads to higher carrier concentration and wider optical bandgap, which shortens the 'cutoff' wavelength of light absorption, leading to a higher transmittance in the short wavelength range^{[10, 11]}.

Effects of sputtering pressure on AZO properties were plotted in Fig. 3. As shown in Fig. 3(a), sputtering pressure plays a negligible role in carrier concentration in the whole pressure range examined in this study. However, Hall mobility has a decreasing trend with increase of sputtering pressure, especially in the higher pressure range. Accordingly, electrical resistivity basically remains at a low level of 4 $\times$ 10$^{-4}$$\Omega$$\cdot$cm at the lower pressure range, but evidently increases after that. This may be explained by less collision among sputtered atoms and argon ions, and thus less energy loss at a low sputtering pressure. Higher energy of sputtering species helps improve the crystallization of the AZO film, resulting in less carrier scattering and thus higher mobility. At a higher sputtering pressure, a higher probability of collision between argon ions and sputtered atoms results in more energy loss when the atoms reach the substrate surface, which degrades the crystallization of the AZO film, leading to lower mobility and higher resistivity^{[12, 13]}.

Sputtering pressure shows very minor effects on transmittance of AZO films as illustrated in Fig. 3(b). As mentioned earlier, AZO optical properties are considerably dominated by Al concentration, which is not sensitive to sputtering pressure. Thus, sputtering pressure influences electrical resistivity instead of film transmittance, unlike the case in which sputtering temperature influences both properties.

Dependence of carrier concentration, Hall mobility, and electrical resistivity on sputtering power is shown in Fig. 4(a). It is observed that as sputtering power increases, carrier concentration and Hall mobility increase initially and then saturate. Accordingly, electrical resistivity decreases first and then increases, with a minimum value at a higher sputtering power value. At lower sputtering power, the sputtering atoms might not have sufficient energy to break the Al-O bonds in Al$_{2}$O$_{3}$. Thus, zinc atoms and oxygen vacancies play a dominant role in electrical conductivity. At higher power, more Al-O bonds can be broken up, and Al plays a key role in electrical conductivity. The high energy of sputtering atoms may also improve the crystallization of AZO films, making grains bigger and reducing defect numbers. All these contribute to the electrical conductivity as illustrated in Fig. 4. A slight increase of electrical resistivity at high power may be explained as that sputtering species with very high energy can damage the growing surface, which leads to deterioration of crystal structures and AZO properties^{[14, 15]}.

Figure 4(b) shows spectral transmittance of AZO films at different sputtering power. AZO transmittance increases in the short wavelength range (340-450 nm) as sputtering power increases. This may be attributed to a higher carrier concentration and wider optical bandgap at a higher sputtering power according to the Burstein-Moss effect^[14-16]. When carrier concentration increases, the lower levels in the conduction band are occupied by electrons, resulting in an increase in the Fermi level and then the optical band gap widens. The optical transmittance in the short wavelength range (340-450 nm) of the film deposited at higher power is larger than that of AZO films deposited at lower power, suggesting that higher power is more helpful for the effective substitution of Zn$^{2+}$ ions by Al$^{3+}$ ions releasing extra electrons into the conduction band.

It should be noted that, although film thickness is targeted at 860 nm, the compactness of the AZO films varies among different processes; and actual deposition thickness also has minor variation, therefore the optical thickness of the AZO films seems to be slightly different, as shown in Figs. 2(b), 3(b) and 4(b).

Considering the electrical and optical performance of AZO thin films for tandem thin film Si solar cell applications, the preferred ranges of sputtering process parameters from this study are advised as follows: temperature 250-350 ℃, pressure 0.25-0.68 Pa, and power 400-500 W.

3.3 Effects of texturing processes

Suitable surface morphology and haze ratio of textured AZO films can enhance light scattering and absorption inside the cell. The rough surface structure can reduce directional reflection, increase internal reflection effects, enhance the effective absorption of solar energy, and consequently improve the power efficiency of the solar cells. To create the desired textures, the etching processes were varied as follows: the as-deposited AZO films were etched for 10, 20, 30, 40, 60 and 80 s, in 0.3% hydrochloric acid solution (8.1 mL HCl/1 L D.I. water). Then, the etchant concentration was varied to find an appropriate etching rate. Additionally, the etching temperature range of 25-48 ℃ was also examined.

Figure 5 shows the haze ratio obviously increases along with the etching time varying from 10 to 80 s. A longer etching time leads to more craters, a larger etching depth, and significantly higher haze ratio. At a constant average etching rate, however, long etch time causes craters to reach the substrate and thus increases resistance significantly (not shown here). The AFM micrographs of the textured AZO films for different etching times also show a similar tendency. The etch depth and lateral diameter increase linearly with the etching time. Also, the surface of films receiving a long etching time (more than 40 s) is full of deep craters approaching the substrate.

With the increase of the concentration of etchant (HCl %), a dominant rise in film haze ratio and etching rate is observed in Fig. 6. AFM images show that a higher concentration has a larger etching depth. When the concentration further increases, excessive corrosion occurs, more craters touch the substrate and result in resistance increase (not shown here).

Meanwhile, the etching temperature was also investigated. Figure 7 illustrates the effects of various etching temperatures on the AZO film haze ratio and etching rate. Raising the solution temperature could accelerate the chemical reaction rate, thereby increase the etching rate. As shown in Fig. 7, the higher the etching temperature, the faster the reaction. AFM images reveal a higher temperature leads to a faster etching rate and larger etching depth, and hence the film haze obviously increases along with the etching temperature. But when temperature further increases, excessive corrosion occurs and the craters could be deep enough to reach the substrate, which could result in deterioration of resistance (not shown here).

The above results reveal that the wet etching rate could be controlled by changing the etching solution concentration and etch temperature. Then at a certain etching rate, an appropriate surface morphology can be obtained through controlling the etching time. Taking into account etch rate, surface morphology and haze ratio, the AZO film etching process is suggested as 20 s etching in 0.5% HCl at 33 ℃.

3.4 Solar cell results

Based on the above results and taking into account requirements for thin film solar cell applications, AZO films were further optimized and applied to a-Si:H/$\mu $c-Si:H tandem junction thin film solar cells. The AZO film was first deposited on a 3.2 mm low iron glass substrate, then surface-textured by wet-chemical etching with diluted hydrochloric acid prior to the deposition of the solar cell layers. Figure 8(a) shows the optical and electrical properties of the optimized AZO film. The average transmittance is over 81% in the whole wavelength range of 380-1100 nm, and the sheet resistance is around 11 $\Omega $/$\square $. Compared with the normal FTO, AZO was found to have a higher haze ratio (average over the 380-760 nm wavelength range) of 41.3% as shown in the AFM image of the textured AZO in Fig. 8(b). It shows large feature sizes that are particularly important in the long wavelength range for light trapping and play a critical role in a-Si:H/$\mu$c-Si:H tandem junction solar cells.

Tandem junction a-Si:H/$\mu $c-Si:H thin film solar cells were fabricated on the aforementioned AZO coated glass substrates. The $I$-$V$ characteristic of a typical cell is shown in Fig. 9. The initial conversion efficiency reaches 12.5% and a stable conversion efficiency of 10.3% could be expected according to reported literature^[17], which is comparable to the results based on high quality FTO-and previously reported AZO-based solar cells. Figure 10 shows the spectral response (QE) of a typical tandem junction cell under red and blue light. The current density of the top cell and bottom cell are 11.8 mA/cm$^{2}$ and 11.4 mA/cm$^{2}$, respectively, indicating a good current match between subcells, with higher performance yet to be expected from further improvements.

4. Conclusions

In this paper, electrical and optical properties of AZO films, including Hall mobility, carrier concentration, electrical resistivity, and light transmittance in the VIS-NIR spectral range, were comprehensively characterized and analyzed by varying sputtering conditions, including chamber pressure, substrate temperature, and sputtering power. The correlations between sputtering processes and AZO thin film properties were given: low electrical resistivity is achieved at low pressure and high substrate temperature, and high light transmittance, especially in the 340-450 nm short wavelength range, is evidently improved with an increase of substrate temperature and sputtering power. Based on the optimization of sputtering processes, textured AZO thin films are found to have good properties with high transmittance above 81% over the 380-1100 nm range, low sheet resistance of 11 $\Omega $/$\square$ and a high haze ratio of 41.3%. An initial conversion efficiency of up to 12.5% has been achieved on small-area cells in preliminary experiments that incorporate the AZO films into a-Si:H/$\mu$c-Si:H tandem junction thin film solar cells.

Acknowledgement: The authors would like to acknowledge Mr. Zhao Xin and Dr. Zhang Lin for their extensive technical assistance and Ms. Zhu Qiaokun, Ms. Liu Ying and Mr. Li Weijie for their valuable optical and electrical characterization.