Preparation of highly transparent conductive aluminum-doped zinc oxide thin films using a low-temperature aqueous solution process for thin-film solar cells applications

Solar Energy Materials and Solar Cells 203 (2019) 110161

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Preparation of highly transparent conductive aluminum-doped zinc oxide thin films using a low-temperature aqueous solution process for thin-film solar cells applications

T

Rongyue Liua,∗,1, Ying Chenc,1, Shuyu Dingb, Yuanbo Lia, Yong Tiana a

School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore Center of Intelligent Antenna and Radio Systems, Department of Electrical and Computer Engineering, University of Waterloo, Waterloo, Canada c Shenzhen Academy of Aerospace Technology, Harbin Institute of Technology Shenzhen, High-Tech Industry Park, Nanshan, Shenzhen, 518057, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: AZO thin films Aqueous solution process Ultraviolet exposure Photoelectric properties Cu2ZnSnS4 thin film solar cells

In this work, we developed a facile route to fabricate highly transparent conductive AZO thin films by using an aqueous solution process followed by an ultraviolet (UV) exposure technique at an annealing temperature as low as 200 °C, where the aluminum citrate complex was used as the Al doping source. Various of deposition parameters in the AZO thin films fabrication process were studied and optimized. The mechanisms on UV exposure and heat treatment to the conductivity improvement and conductivity stability of the AZO films were analysed. Results showed that the AZO thin films under the best parameters conditions demonstrated an optical transmittance higher than 85% in the visible spectra region and a lowest electrical resistivity of 4.8 × 10−3 Ω cm, while exhibited densely oriented columnar grains and uniform surface morphology as well as uniform composition distribution. The UV exposure could remove carbon species from the surface of the AZO thin films to reduce oxygen-related defects and release free carriers at the boundaries and interfaces, thereby improving the conductivity of the AZO thin films. The simultaneous treatment of the AZO thin film by ultraviolet exposure and heat treatment could remove carbon species at a deeper thickness, thereby improving the conductive stability of the AZO thin films. Kesterite Cu2ZnSnS4 thin film solar cells incorporating the optimal AZO thin films as top electrodes demonstrated a best power conversion efficiency (PCE) of 7.15%, which was comparable to the PCE value obtained by using the sputtering deposited AZO thin films as top electrodes.

1. Introduction Solution-processed transparent conducting doped ZnO thin films (e.g., Al, Ga, B, F, or Cl doping) with high transparency and high conductivity have arisen great interest in optoelectronic devices such as thin film solar cells, liquid crystal display, touch screens, light-emitting diodes and sensors because they can provide optoelectronic properties comparable to conventionally vacuum deposited AZO thin films [1–13]. Various of solution methods have been developed to prepare the AZO thin films such as sol–gel [14–18], electrochemical deposition [19–21], spray pyrolysis [22–24], chemical bath deposition [25,26], nanoparticle dispersions inks [27–29], successive ionic layer adsorption and reaction [30,31]. However, the as-prepared AZO thin films deposited using these methods suffer from high porosity, low crystallinity and high concentration of carbon impurities, resulting in poor electrical properties [32,33]. An annealing in vacuum at temperature higher than

300 °C combined with reactive atmospheres (N2/H2) is required to increase the concentration of oxygen vacancies and reduce the density of electron traps at the grain boundaries to improve the electrical properties of the AZO thin films [34,35]. This process limits its applications in low-temperature optoelectronic devices, such as copper indium gallium selenide (CIGS) thin film solar cells and copper zinc tin sulfide (CZTS) thin film solar cells, because the absorber/buffer p-n junction degrades irreversibly once heated above 250 °C [3,36]. In addition to thin film solar cells, it is also not suitable for the fabrication of optoelectronic devices on the flexible substrates such as polyimide (PI) and polyethylene terephthalate (PET), which have glass transition temperature below 300 °C [37]. Therefore, there is an urgent need to develop a new processing method and material structures to overcome these problems as mentioned above for the solution-processed transparent conducting AZO thin films. Recently, Jaebum Joo et al. used hydrothermal method to

∗

Corresponding author. E-mail addresses: [email protected], [email protected] (R. Liu). 1 The two authors contributed equally to this paper. https://doi.org/10.1016/j.solmat.2019.110161 Received 30 May 2019; Received in revised form 17 August 2019; Accepted 1 September 2019 Available online 05 September 2019 0927-0248/ © 2019 Elsevier B.V. All rights reserved.

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in the grown aqueous solution without hydroxide precipitation [41]. Moreover, the complex ion was stable and mainly existed in the grown aqueous solution in the form of a cation, which would no longer inhibit AZO thin films crystal growth [45]. In addition, the addition of right amount of citric to the grown aqueous solution could slow down the AZO thin films growth, thereby leading to a higher incorporated Al concentration. We then treated the AZO thin films by UV exposure and heat treatment simultaneously to improve its conductivity stability. Finally, the mechanisms on UV exposure and heat treatment to the conductivity improvement and conductivity stability of the AZO films were analysed. The optimal AZO thin films with an optical transmittance higher than 85% in the visible spectra region and an electrical resistivity of 4.8 × 10−3 Ω cm (sheet resistivity 36 Ω/sq) were demonstrated. In order to demonstrate the potential application of AZO thin films, kesterite Cu2ZnSnS4 (CZTS) thin film solar cells incorporating them as top electrodes were fabricated and characterized.

synthesize ZnO nanowires where the morphologies and the functional properties of the ZnO nanowires could be rationally controlled by means of the competitive and face-selective electrostatic adsorption of non-zinc complex ions under alkaline conditions [38]. The crystal growth was related to the decreasing thermodynamic stability of the zinc complex leading to controlled supersaturation and the retrograde solubility of ZnO upon increased temperature [34,35,39]. Based on this mechanism, Miyake et al. first made an attempt to synthesize highly transparent conducting AZO thin films using the chemical bath method [40]. The growing aqueous solution consisted of zinc nitrate hexahydrate, aluminum nitrate and ammonia water where the pH of the solution was 10.85 and the aluminum nitrate was used as Al doping source. The AZO thin films after annealing at 300 °C in air had a carrier concentration of 1019–1020 cm−3, a mobility of 0.7–7 cm2/V·s and an electrical resistivity of 3 × 10−2 Ω cm (sheet resistivity, 53 Ω/sq). Hagendorfer's group used the same method to prepare the highly transparent conducting AZO thin films, but used UV exposure instead of annealing and employed metal Al foil as the Al doping source [34,41,42]. The advantage of the method modification was that the heat treatment temperature reduced, and the Al concentration incorporated AZO thin films increased. In addition, UV exposure could remove more carbon impurities in the AZO thin films compared to lowtemperature annealing. This was because that the carbon impurities were more sensitive to UV exposure. Finally, the AZO thin films with excellent photoelectric performances was demonstrated showing an optical transmittance higher than 85%, an electrical resistivity of 5 × 10−3 Ω cm (sheet resistivity, 40 ± 5 Ω/sq), a carrier concentration of 1.2 × 1020 cm−3 and a electrical mobility of 10.9 cm2 V−1 s−1. Edinger’ group insisted on the preparation of the highly transparent conducting AZO thin films using metal aluminium salt as the doping source, but used UV exposure to treat the AZO thin films [43]. A comparative study of the ZnO films doping with Al, Ga and In was performed. Results showed that the carrier density was the lowest for undoped ZnO thin films and increased in the order: In-, Al- and Gadoped ZnO, while the mobility was just the opposite. The In-doped ZnO (IZO) thin films exhibited the lowest electrical resistivity with a value of 1.7 × 10−2 Ω cm, and the Ga-doped ZnO (GZO) thin films demonstrated the highest stability of the conductivity upon UV exposure. However, there still exists some challenging problems for all the AZO thin films mentioned above in practical applications. For the metal Al foil as the Al doping source, it offered little flexibility concerning pH and solution chemistry as a slow but sufficient corrosion rate always needed to be maintained in the grown aqueous solution [41]. Moreover, the Al concentration incorporated in the AZO thin films was uneven. For the metal Al salt as the Al doping source, it could well avoid the shortcomings of the above methods. But, electrical resistivity of the AZO thin films below 10−2 Ω cm could not be obtained using constant Al concentrations in the grown aqueous solution due to its relatively low Al doping concentration. In addition, the AZO thin film prepared by any of the above methods exhibited unstable conductivity. This was because that the UV exposure time for the AZO thin films was relatively short and there was no additional substrate heating, resulting in the carbon compounds are not removed effectively in the bulk AZO thin films due to limited UV penetration depth and limited out-diffusion of carbon species [44]. Thus, an increase in electrical resistivity of the AZO thin films was observed when they were stored without any protective coverage layer at ambient conditions, because residual carbon impurities might act as preferred absorption sites for oxygen species to form a high-resistance barrier. In order to solve the above problems, we had made some modifications in the preparation of highly transparent conductive AZO thin films by the aqueous solution deposition process combined with the UV exposure technique. We first reacted the Al(NO3)3·9H2O with citric acid to form the aluminum citrate complex, and used it as Al doping source. The merit was that the citric acid was an effective complexing agent of Al ions which could greatly increase the concentration of metal Al salt

2. Experimental section 2.1. Preparation of ZnO seed layer and AZO thin films Zinc oxide (ZnO) nanopowder (< 100 nm, CAS no. 1314-13-2), ammonia water (NH3·H2O) (ACS, 28%), aluminum nitrate nonahydrate (Al(NO3)3·9H2O), ammonium citrate (C6H5O7(NH4)3) were purchased from Sigma Aldrich and used without further purification unless stated otherwise. For the ZnO seed layer deposition, the ZnO nanopowder were firstly suspended in 18 MΩ cm deionised water, and then a large amount of NH3·H2O (28 wt%) was added to the ZnO suspension aqueous solution until the pH reached a value > 11.0. The ZnO content is excessive compared to ammonia. Then, the solution was stirred in a sealed container at room temperature for more than a day. Then, the above solution was filtered into PE bottles by using a filter membrane with the pore diameter of 0.45 μm and stored at room temperature (25 °C) until usage. The filter membrane was purchased from China National Pharmaceutical Group Chemical Reagent Co., Ltd (sample model: Φ35*0.45μ | 200s/box; sample number: 92411202). For the alkaline saturated ZnO aqueous solution, the ZnO existed in the form of [Zn(NH3)4]2+ complex ions where the NH3·H2O was used as the complexing agent. The concentration of ZnO in the aqueous solution, which was controlled by the added ammonia concentration and measured by inductively coupled plasma mass spectrometry (ICP-MS), was 50 mmol/ L. Then, the ZnO seed layer was deposited by spin-coating the filtered aqueous solution on a ultra-white glass substrate (30 mm × 30 mm) at 3000 rpm for 30 s, followed by annealing at 120 °C in air for 2 min on a hot plate. The coating and drying process was repeated two times to ensure homogeneous coverage of ZnO seed layer on the substrates, with the final layer annealed at 150 °C for 5 min. The AZO thin films deposition was based on the previous literatures [34,40,43], but some improvements had been made. In a typical procedure, the alkaline saturated ZnO aqueous solution was prepared according the method as mentioned above. The concentration of ZnO in the aqueous solution was related to the amount of ammonia added. The ammonium citrate was dissolved in 18 MΩ cm deionised water at a concentration of 1 mmol/L. Designing the ZnO concentration and ammonium citrate concentration required in the aqueous solution was to prepare the AZO deposition solution. The concentration of ammonium citrate in the AZO deposition solution was controlled according to the volume which it is added. The Al doping source solution was prepared by mixing citric acid and Al(NO3)3·9H2O in the deionised water with a molar ratio of 2:1 and the concentration was tunable. Then, the glass substrate deposited with ZnO seed layer was inserted into the above AZO deposition solution which was heated at temperature from 60 °C to 90 °C exceeding 15 min by a water bath. The temperature we measured was the temperature of the water bath. Next, the Al doping source solution was added to the above mixed solution. The total volume of the AZO deposition solution was 300 ml. Timely, a peristaltic pump was used to 2

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groups absorbed on the surface and grain boundaries of AZO thin films, thereby improving the electrical properties of the AZO thin films. Before obtaining dense and uniform AZO thin films, various of deposition parameters were studied and optimized, including deposition temperature, deposition time, ZnO concentration in the grown aqueous solution, ammonium citrate surface additive concentration, doping approach of Al doping source, Al doping source concentration, etc. Fig. 1b showed the SEM morphologies of the undoped ZnO film under optimal conditions, where the deposition temperature, deposition time, pH of the aqueous solution, ZnO concentration and ammonium citrate surface additive concentration in the aqueous solution were 80 °C, 60 min, 11.2, 38 mmol/L and 2 mmol/L, respectively. Further SEM morphologies at varied parameters and the relevant discussions were shown in the supplementary information (Fig. S1). It was observed that the undoped ZnO film exhibited dense and uniform surface morphologies and showed a thick layer with columnar grains at the top and a thin layer with disordered grains at the bottom. Moreover, the columnar grains of the undoped ZnO film grow preferentially along the caxis. The thickness of the undoped ZnO film was 2.6 μm. The mechanism of the undoped ZnO film growth in the chemical bath solution was discussed below. For the alkaline supersaturated ZnO aqueous solution, the ZnO existed in the form of [Zn(NH3)4]2+ complex ions. The formation process was shown as follow:

transport the same concentration of Al doping source solution to the reaction bath to adjust the concentration of Al ions in the aqueous solution to keep its concentration stable. The addition speed of the Al doping source solution was summarized by practical experiments. Here, the changes of concentrations of ZnO and ammonium citrate in the reaction solution were not monitored in real time. We used the initial ZnO and ammonium citrate concentrations as statistics. After several tens of minutes (40–100 min) of deposition, dense and uniform AZO could be obtained, which were then carefully rinsed with deionised water and ultrasonicated in ethanol to remove surface precipitates, and dried in a nitrogen stream. Finally, the AZO thin films were treated by UV exposure combined with heat treatment simultaneously, where a 250 W UV-handheld apparatus with an iron doped Hg lamp was used. The distance between the samples and UV lamp was 5 cm and the UV lamp had a spectrum range between 185 nm and 254 nm. 2.2. Fabrication of CZTS thin film solar cells CZTS thin film solar cells were fabricated according to our previous literatures [46–48]. The process was achieved by sequentially stacking 1000-nm-thick Mo layer (DC sputtering deposition) as back contact on the glass substrate, 1000-nm-thick CZTS layer (sol-gel spin-coating deposition) as light absorber, 70-nm-thick CdS layer (chemical bath deposition) and 50-nm-thick i-ZnO layer (RF sputtering deposition) as buffer layer, and ~500-nm-thickness AZO thin films (chemical bath deposition in this work) as top contact to replace the traditional ITO electrodes. The effective active area of the device was 0.45 cm2.

ZnO + 4NH3 + H2 O → [Zn (NH3) 4]2 + + 2OH−

(1)

When the supersaturated ZnO aqueous solution was heated, the solubility of ZnO was lowered so that the ZnO would precipitate and grow. In the early stage of the ZnO growth, it preferentially nucleated and grew on the ZnO seed layer and exhibited random orientation. This was because the ZnO seed layer was amorphous or showed random plane orientation. As the ZnO grain continued to grow, it showed a preferential growth orientation along the c-axis. The reason could be attributed to the face-selective electrostatic interaction during the ZnO grain growth [38]. The schematic diagram of the ZnO grain growth mechanism was shown in Fig. 1c. The negative (002) top surface plane and the positive (100) sidewalls exhibited opposite charges. In the alkaline saturated ZnO aqueous solution containing no other metal ions, the [Zn(NH3)4]2+ complex ions were the main species and showed positive charge. Thus, it was easily absorbed on the negative (002) top surface plane and promoted the ZnO grain growth along this surface, while the ZnO grain growth along the positive (100) sidewalls was inhibited. Therefore, the ZnO grain grew preferentially along the c-axis and exhibited columnar grains in the later stage. Fig. 1d showed the XRD pattern of the undoped ZnO films deposited on a glass substrate. An XRD pattern of ZnO powders was shown for comparison. It was observed that the ZnO films showed one intense diffraction peak at 34.46° assignable to (002) reflection of hexagonal ZnO, which further demonstrated that the ZnO films showed a preferential growth orientation along the c-axis. Metal doping (e.g., Al, Ga, or In doping) was an effective approach to obtain highly conductive ZnO films. Here, we used metal Al doping due to its low cost and environmental friendliness. We first used metal Al foil as the Al doping source. The deposition process was based on the previously reported method [34]. The principle was that the metal Al foil was etched in the alkaline saturated ZnO aqueous solution, and gradually released the Al ions into the grown aqueous solution, thereby forming the Al ions doping. The changes in SEM morphologies, optical transmittance and electrical properties of the AZO thin films with the increase of metal Al foil impregnation area were shown in Fig. 2. For the morphologies change, dense and uniform columnar grains were obtained when the impregnation area of metal Al foil was below 6 cm2. The thickness of the AZO thin films gradually decreased as the impregnation area of metal Al foil increased. This was because that the higher Al ion concentration (existent as [Al(OH)4]− species) in the aqueous solution suppressed the columnar grains growth of the AZO thin films. When the impregnation area of metal Al foil was beyond

2.3. Characterization of AZO thin films and CZTS thin film solar cells Scanning electron microscope (SEM, Hitachi S4100) images were taken to study morphology features of the films, and the attached energy dispersive spectrometer (EDS) analyzer was used to study the composition distribution of the films. The crystallinity of the films was estimated by using X-ray diffraction (XRD, D/Max-IIIC) equipped with a Cu-Kα source of wavelength λ = 1.54060 Å and operated at 40 kV and 20 mA. The compositional depth profiles of the films were measured by time-of-flight secondary ion mass spectrometry (TOF-SIMS). Every sample was deeply profiled at least twice at different locations to ensure reproducibility. X-ray Photoelectron Spectroscopy (XPS) measurement was performed on a SPECS HSA-3500 to determine the valence state of each element in the samples. The optical properties of the films were tested at room temperature by using a UV–vis spectrophotometer (Shimadzu, UV3600) equipped with an integrating sphere in wavelength ranging from 300 nm to 1700 nm, and a blank glass substrate was used as reference. The electrical properties of the films were measured by using the Hall effect method and the sheet resistance of the films was measured by using the four-point probe method. Currentvoltage (J-V) curves of the CZTS solar cells were measured under simulated AM 1.5 Global spectrum and 100 mW cm−2 illumination by using a Keithley 2400 source meter and a solar simulator. External quantum efficiency (EQE) curves of the CZTS thin film solar cells were measured by using a monochromatic illumination (Oriel, LOT-2100) equipped with a 150 W xenon light source and the calibration of the incident light was performed with a monocrystalline silicon diode. 3. Results and discussions Fig. 1a showed the schematic diagram of the preparation of AZO thin films. Firstly, the ZnO seed layer was formed by spin-coating deposition of alkaline saturated ZnO aqueous solution on a cleaning glass substrate followed by low-temperature annealing at 150 °C. Subsequently, it acted as a nucleation center to grow the AZO thin films in the alkaline supersaturated ZnO aqueous solution containing Al ions. The as-prepared AZO thin films were then treated by UV exposure combined with low-temperature annealing to remove carbon-based functional 3

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Fig. 1. a, schematic diagram of the preparation of AZO thin films; b, SEM morphologies of undoped ZnO films at optimal parameters; c, schematic diagram of the growth mechanism of undoped ZnO films; d, XRD pattern of undoped ZnO thin films in comparison with that of ZnO powders.

6 cm2, the columnar grains of the AZO thin films disappeared and were replaced by the nanoparticles. For the optical transmittance change, all the AZO thin films with columnar grains showing optical transmittance higher than 85% in the visible spectrum region were observed, while those values gradually decreased in the near-infrared spectrum region with increasing metal Al foil impregnation area. The transmittance spectrums of the AZO thin films without columnar grains were not plotted due to their low optical transmittance and irregular change. The decrease in optical transmittance in the near-infrared spectrum region for the AZO thin films could be attributed to the increase of free carrier electron absorption [34]. Moreover, an obvious interference phenomenon was observed in the optical transmission spectrum for all the AZO thin films, demonstrating the smooth surface morphologies [49]. For the electrical properties test, the carrier concentration of the AZO thin films gradually increased with increasing metal Al foil impregnation area, while the mobility was just the opposite. When the impregnation area of metal Al foil was 6 cm2, the AZO thin films showed the lowest electrical resistivity of 6.0 × 10−3 Ω cm (sheet resistivity, 40 Ω/sq), with a carrier concentration of 1.02 × 1020 cm−3 and a mobility of 10.2 cm2/V s. These results were consistent with the results reported in the previous literature [34]. However, using metal Al foil as the Al

doping source had some drawback. On the one hand, it offered little flexibility concerning pH and solution chemistry as a slow but sufficient corrosion rate always needed to be maintained in the grown aqueous solution [41]. On the other hand, the Al doping concentration in the AZO films was uneven and showed a linear increase towards the surface, which was evidenced by the composition depth distribution measurement (in the lower right corner of Fig. 2, where the sputtering rate was 1.57 nm s−1, the sputtering time was 720 s, and the depth was 1130 nm). This was detrimental to the conduction uniformity of the AZO thin films. We then prepared the AZO thin films using aluminum salt (Al (NO3)3·9H2O) as the Al doping source, and kept the concentration of aluminum salt in the grown aqueous solution constant, thereby obtaining the AZO thin films with uniform Al doping concentration. The deposition process was based on the previously reported method [40]. The changes in morphologies, optical transmittance and electrical properties of the AZO thin films with increasing Al salt concentration were shown in Fig. 3. For the morphologies change, dense and uniform columnar grains were obtained at low Al salt concentrations. The thickness of the AZO thin films sharply decreased with increasing Al salt concentrations. This was also due to the higher Al ion concentration 4

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Fig. 2. The changes in SEM morphologies, optical transmittance and electrical properties of the AZO thin films with the increase of metal Al foil impregnation area (in the lower right corner: composition depth distribution of the AZO thin films prepared by using the metal foil as the doping source).

(existent as [Al(OH)4]− species) in the aqueous solution which suppressed the columnar grains growth of the AZO thin films. When the Al salt concentration was beyond 2 mmol/L, the columnar grains of the AZO thin films began to disappear and were replaced by the nanoparticles. For the transmittance spectrum measurement, the change in optical transmittance was similar to that of the AZO thin films prepared by using metal Al foil as the Al doping source. Clearly, all the AZO thin films with columnar grains showed optical transmittance higher than 85% in the visible spectrum region, and these values gradually decreased in the near-infrared spectrum region with the increase of Al salt

concentrations. For the electrical properties test, the carrier concentration of the AZO thin films gradually increased with increasing Al salt concentration, while the mobility was just the opposite. When the Al salt concentration in the aqueous solution was 2 mmol/L, the AZO thin films showed the lowest electrical resistivity of 1.9 × 10−2 Ω cm (sheet resistivity, 120 Ω/sq), with a carrier concentration of 4.20 × 1019 cm−3 and a mobility of 7.9 cm2/V s. Further increasing Al salt concentration, the conductivity of the AZO thin films deteriorated. This meant that electrical resistivity of the AZO thin films lower than 10−2 Ω cm could not be obtained by using a constant Al salt

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Fig. 3. The changes in SEM morphologies, optical transmittance and electrical properties of the AZO thin films with the increase of Al salt concentration (in the lower right corner: composition depth distribution of the AZO thin films prepared by using the Al salt as the doping source).

(NO3)3·9H2O with citric acid to form the aluminum citrate complex, and used it as the Al doping source. The merits were shown as follows: (1) the citric acid was an effective complexing agent of Al ions which could greatly increase the concentration of Al salt in the grown aqueous solution (e.g., 100 mM) without hydroxide precipitation [41,45]. This was due to the coexistence of citrate inhibited the hydrolysis of the aluminum citrate complex even under alkaline conditions. (2) the complex was stable and mainly existed in the grown aqueous solution in the form of a cation, such as [Al·LH3]3+, [Al·LH2]2+ and [Al·LH1]1+ (LH3 represented citric acid, LH2 and LH1 represented deprotonated

concentration doping. This because that the Al doping concentration in the AZO thin films was too low. These results were consistent with the results reported in the previous literature [40,43]. The composition depth profile of the optimal AZO thin films was also shown in the lower right corner of Fig. 3, demonstrating the uniform Al doping concentration in the AZO thin films. The sputtering rate was 1.57 nm s−1, the sputtering time was 786 s, and the thickness was 1234 nm. In order to improve the Al doping concentration in the AZO thin films to improve its electrical properties, some modifications to the metal Al salt as the doping source were made. We first reacted Al 6

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Fig. 4. The changes in SEM morphologies, optical transmittance and electrical properties of the AZO thin films with the increase of aluminum citrate complex concentration.

with the increase of aluminum citrate complex concentration. As can be seen, all the AZO thin films with dense and uniform columnar grains were obtained, and the thickness of the AZO thin films decreased with increasing aluminum citrate complex concentration. The decrease in film thickness was because that the higher citrate ions (existent as [C6H5O7]− species) in the aqueous solution suppressed columnar grains growth of the AZO thin films. Moreover, some white nanoparticle impurities appeared on the surface of the AZO thin films for high aluminum citrate complex concentration, which was also attributed to increased concentration of the citrate in the aqueous solution which affected the AZO thin film crystal growth. The changes in optical

citric acid) [45]. This meant that the crystal growth of the AZO thin films affected by high concentration of [Al(OH)4]− species was eliminated, and dense and uniform AZO thin films with columnar grains could be obtained. (3) the addition of right amount of citric acid to the grown aqueous solution could slow down the AZO films growth, thereby leading to a higher incorporated Al concentration. We then used a peristaltic pump to transfer the Al doping source into the grown aqueous solution so that the Al ions concentration in the aqueous solution could be kept constant to obtain a uniform Al concentration profile in the AZO thin films. Fig. 4 showed the change in morphologies, optical transmittance and electrical properties of the AZO thin films 7

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transmittance were similar to those of the AZO thin films prepared by using metal Al foil or Al salt as the Al doping source. Clearly, all the AZO thin films showed optical transmittance higher than 85% in the visible spectrum region, and those values gradually decreased in the near-infrared spectrum region with increasing aluminum citrate complex concentration. The obvious interference phenomenon in the transmission spectrum for all the AZO thin films indicated the smooth surface morphologies. The optical energy band gap of the AZO thin films were measured by performing a plot of [αhυ]2 against hυ via extrapolating the straight line portion where the value of the Y-axis was equal to 0. As can be seen, the lowest value of the Eg were calculated to be 3.39, and gradually increased with increasing aluminum citrate complex concentration, indicating the increase of carrier concentration in the AZO thin films [34,50]. This could be proved by the electrical properties test where the carrier concentration of the AZO thin films gradually increased with increasing aluminum citrate complex concentration (in Fig. 4). While, the carrier mobility was just the opposite. When the aluminum citrate complex concentration was 3 mmol/L, the AZO thin films showed the lowest electrical resistivity of 4.8 × 10−3 Ω cm (sheet resistivity, 36 Ω/sq), with a carrier concentration of 1.09 × 1020 cm−3 and a mobility of 11.9 cm2/V s. Further increasing the aluminum citrate complex concentration, the electrical conductivity of the AZO thin films deteriorated. This could be attributed to the fact that more citric anions are doped into the AZO thin films during its deposition, resulting in more carbon impurities which could form more carrier scattering centers and reduce carrier mobility, thereby worsening the electrical properties of the AZO thin films. Fig. 5a and b showed the transmittance spectrum and the energy band gap of the best AZO thin films prepared from the metal Al foil doping, the Al salt doping and the Al citrate complex doping. As can be seen, all the AZO thin films exhibited comparable optical transmittance values in the visible spectrum region. In the near-infrared spectrum region, the optical transmittance values decreased in an order: Al salt doped AZO, metal Al foil doped AZO and Al citrate complex doped AZO. The AZO thin films prepared form the Al citrate complex doping

showing the lowest optical transmittance values might be related to that it had largest free carrier electron absorption resulting from its high carrier concentration, which had been evidenced by the previous electrical properties test. The AZO thin films prepared form the Al citrate complex doping showed the largest Eg value in Fig. 5b, which further proved the above analysis. The reason was attributed that the AZO thin films prepared from the Al citrate complex doping had higher Al doping concentration than those of the AZO thin films prepared from the metal Al foil or Al salt doping. This was evidenced by the ICP-MS test where the results were also shown in Fig. 5a. The EDS mapping images of Zn, O and Al elements on the surface of the AZO thin films and the composition depth profile of the AZO thin films were shown in Fig. 5c and d. For the composition depth test, the sputtering rate was 1.57 nm s−1, the sputtering time was 1208 s, and the thickness was 1896 nm. As can be seen, all the elements showed uniform distribution. The electrical stability of the AZO thin films stored in air ambient (25 °C, humidity 80%) was evaluated as shown in Fig. 6. The change in sheet resistivity of the AZO thin film without heat treatment with the increase of UV exposure time and storage time was shown in Fig. 6a. It was observed that the sheet resistivity of the AZO thin film first sharply decreased with the increase of UV exposure time and then gradually stabilized, and remained unchanged when the UV exposure time exceeded 25 min. The mechanism of the decrease in sheet resistivity of the AZO thin films after UV exposure will be discussed later. With increasing storage time, the sheet resistivity of the AZO thin films gradually increased. This was because that the carbon impurities on the surface of the AZO thin film was not completely removed after UV exposure, so that the water or oxygen in the ambient atmosphere were re-absorbed to the surface of the AZO thin films to form a high resistivity barrier, thereby increasing its sheet resistivity. Then, heat treatment at temperature from 80 °C to 200 °C was carried out for 60 min to treat the AZO thin films (see in the right side of Fig. 6a). However, it showed no effect. In turn, we first used heat treatment to treat the AZO film followed by UV exposure. The change in sheet resistivity of the AZO thin film after heat treatment with the increase of Fig. 5. a and b A comparison in optical transmittance and optical energy band gap of the AZO thin films prepared by using the metal Al foil, Al salt and Al citrate complex as the Al doping source; c, EDS mapping images of Zn, O and Al elements on the surface of the AZO thin films; d, composition depth distribution of the AZO thin films prepared by using the Al citrate complex as the Al doping source.

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Fig. 6. Electrical stability of the optimal AZO thin films stored in air ambient (25 °C, humidity 80%) at varied parameters.

time were shown in Fig. 6c. It was seen that the change trend of the resistivity of the AZO thin films at varied temperature was also similar to that of the AZO thin films without heat treatment and UV exposure, but showed a more faster rate of decline in sheet resistivity with the increase of UV exposure time. When the UV exposure time exceeded 10 min, the sheet resistivity of the AZO thin films remained unchanged showing a value of 36 Ω/sq. With increasing storage time, the AZO thin films exhibited excellent conductive stability at higher heat treatment temperature. When the heat treatment temperature was 200 °C, only a slight increase in sheet resistivity of the AZO thin films was observed. This indicated that the simultaneous treatment of the AZO thin film by ultraviolet exposure and heat treatment was able to improve its conductive stability. The mechanism will be discussed later. Further improvement of the heat treatment temperature might result in better results. However, high temperature heat treatment larger than 200 °C

UV exposure time and storage time was shown in Fig. 6b. It was seen that the change trend of the resistivity of the AZO thin films with heat treatment was similar to that of the AZO thin films without heat treatment, but exhibited a faster rate of decline in sheet resistivity with the increase of UV exposure time. This might be because that the heat treatment contributed to the decomposition of hydroxides or oxygen species on the surface of the AZO thin films, which sped up the removal of carbon species from the AZO thin films by UV exposure. The sheet resistivity of the AZO thin films remained unchanged when the UV exposure time exceeded 20 min. However, the conductivity of the AZO thin films was still unstable and its sheet resistivity gradually increased with the increase of storage time as shown on the right in Fig. 6b. We then treated the AZO thin films by UV exposure and heat treatment simultaneously. The changes in sheet resistivity of the AZO thin films at varied temperature with the increase of UV exposure time and storage 9

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Fig. 7. XPS survey of the optimal AZO thin films and High resolution XPS spectrum of Zn 2p (b), Al 2p (c), O 1s for the optimal AZO thin films.

was detrimental to the fabrication of thin film solar cells and other flexible electronics. Thus, heat treatment temperature beyond 200 °C was not carried out. XPS spectra of the optimal AZO thin films were performed to study the atomic compositions and chemical binding states as shown in Fig. 7. It was observed that the peaks assigned to the Zn, O and Al elements were obtained, indicating the existence of the Zn, O and Al in the deposited thin films. The Zn2p, Al2p and O1s spectra zoomed in X-axis were shown in Fig. 7 (b)–(d). As can be seen, the Zn2p spectra showed two peaks at 1020.8 eV (2p3/2) and 1043.8 eV (2p1/2) with a splitting energy of 23.0 eV, which was the stander status of Zn2+. The O1s spectra showed a strong peak at 531.2 eV, indicating that the chemical binding states of O element was negative divalent. The Al2p showed a weak peak at 73.9 eV, indicating the formation of trivalent Al ions. These results demonstrated the formation of AZO thin films. In order to investigate the roles of UV exposure and heat treatment on the electrical properties of the AZO thin films, asymmetric peaks of the O1s and C1s spectra before and after UV exposure combined with heat treatment were deconvoluted by using Lorentzian-Gaussian fitting as shown in Fig. 8. For the O1s spectra, three peaks OL, OM and OH centered at 530.15, 531.39 and 532.15 eV were observed. The lower binding energy peak OL (530.15 eV) was attributed to the lattice oxygen atoms fully coordinated with the metal ions; the medium binding energy peak OM (531.39 eV) was related with the oxygen deficient regions of metal oxide; the higher binding energy peak OH (532.15 eV) was associated with the chemical absorbed carbon or oxygen species on the surface of the AZO films, such as H2O and CxHyOz [51,52]. The relative strength (%) of OL, OM and OH components was defined as follows [53,54]:

Relative strength (%) of OX =

OX × 100% OL + OM + OH

increase of OL could be attributed to the thermal decomposition of metal compounds such as metal-O-C or metal-OH or (OH)xZn(CO3)y, which in turn formed a more stable metal oxide. The formation and thermal decomposition mechanisms of these metal compounds can refer to the reported literature [55]. The decrease of OM indicated that the concentration of oxygen vacancy reduced, and the carrier concentration would also decrease accordingly, which would be detrimental to the electrical properties of the AZO thin films. However, the AZO thin films showed an increase in carrier concentration and a decrease in electrical resistivity as shown in Table 1. This was contrary to that of O1s spectral analysis. This might be because that UV exposure reduced oxygen-related defects associated with electronic traps in the AZO thin films, resulting in the release of free carriers at the boundaries and interfaces, although the oxygen vacancy concentrations decreased. In addition, the reduction of the above metal compounds with high resistance also played an important role. The decrease of OH indicated that the chemical absorbed CxHyOz species on the surface and grain boundaries of the AZO thin film reduced. This was very of significance because that the reduction of these species could reduce carrier scattering centers arising from the carbon impurities. In Fig. 8c, the relative strength (%) of OL and OM for the AZO thin films after UV exposure and heat treatment further slightly increased, while that of OH further decreased, implying that more chemical absorbed oxygen species in the AZO thin films were removed by UV exposure and heat treatment simultaneously. For the C1s spectra in Fig. 8d, three fitting peaks centered at 284.7, 285.8 and 288.4 eV were observed, which were assigned to be the C-C or C-H, C-OH and O-C]O bands. The relative strength (%) of the three fitting peaks for the AZO thin films after UV exposure without additional heating conditions (in Fig. 8e) sharply decreased, indicating that the carbon impurities in the AZO thin films was removed, but not completely. The reason was due to limited UV penetration depth and limited out-diffusion of carbon species [44]. When the AZO thin films were treated by UV exposure and heat treatment simultaneously, the relative strength (%) of the three fitting peaks assigned to the carbon species almost disappeared, demonstrating more the carbon impurities in the AZO thin films was removed (in Fig. 8f). This was consistent with the O1s spectral analysis. Fig. 9 showed the depth distribution of the carbon composition of

(2)

The relative strength (%) of OL, OM and OH components was calculated to be 14.16, 68.28 and 17.56% for the AZO thin films before UV exposure and heat treatment (in Fig. 8a), while those were 58.40, 41.47 and 0.13% for the AZO thin films after UV exposure without additional heating conditions (in Fig. 8b). It was observed that the relative strength of OL increased, while those of OM and OH decreased. The 10

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Fig. 8. Asymmetric peaks of the O1s and C1s spectra deconvoluted by using Lorentzian-Gaussian fitting: (a) and (d) before UV exposure and heat treatment, (b) and (e) after UV exposure without heat treatment, (c) and (f) simultaneous treatment by UV exposure and heat treatment. Table 1 Electrical properties of the AZO thin films at varied parameters: a, before UV exposure and heat treatment; b, UV exposure without heat treatment; c, simultaneous treatment by UV exposure and annealing. Sample

Carrier concentration ( × 1020 cm−3)

Carrier mobility (cm2/V s)

Electrical resistivity ( × 10−3 Ω cm)

a b c

0.21 1.01 1.09

14.9 8.3 11.9

0.204 7.42 4.81

the AZO thin films under varied conditions: (1) before UV exposure and heat treatment, (2) UV exposure without heat treatment, (3) first heat treatment and then UV exposure, (4) UV exposure and heat treatment simultaneously. Before UV exposure and heat treatment, it was seen that the carbon concentration was uniformly distributed across the whole film thickness of the AZO layer. After UV exposure without heat treatment, the carbon concentration towards the surface of the AZO thin film decreased only at a shallower thickness. By first heat treatment and then UV exposure, the change in carbon concentration was similar to that treated by UV exposure without heat treatment. In contrast, the carbon concentration towards the surface of the AZO thin film treated by UV exposure and heat treatment simultaneously decreased at a deeper thickness. This might be the reason that the AZO thin films treated by UV exposure and heat treatment simultaneously exhibited excellent conductive stability. This was because that the removal of more carbon impurities on the surface of the AZO thin film at a deeper thickness reduced the chance that water and oxygen in the ambient atmosphere were re-adsorbed to the site of carbon species, thereby improving the conductive stability of the AZO thin films.

Fig. 9. Carbon composition depth profiles of the AZO thin films treated at varied parameters.

Kesterite CZTS thin film solar cells involving the aqueous solution deposited AZO thin films as top electrodes were fabricated according to a structure of glass/Mo/CZTS/CdS/i-ZnO/AZO/Ni:Al without antireflection layer (Fig. 10a). Fig. 10b showed the cross-sectional SEM image of the best CZTS thin film solar cell device. It was clearly observed that dense AZO thin films uniformly covered on the adjacent layers, and there were no voids or gaps between them, indicating excellent electrical contact. The J-V curves of the CZTS thin film solar cells were measured under simulated solar light illumination (AM 1.5, 11

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Fig. 10. (a) and (b) Schematic diagram and cross-sectional SEM image of CZTS thin film solar cell device involving the aqueous solution deposited AZO thin films as top electrodes; (c) J-V curve and device parameters of CZTS thin film solar cell using the aqueous solution deposited AZO thin films as top electrodes (the red line in Fig. 10 (c) is the J-V curve of CZTS thin film solar cell using the sputtering deposited AZO thin films as top electrodes as reference); (d) EQE curve of CZTS thin film solar cell device involving the aqueous solution deposited AZO thin films as top electrodes. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

curves and device parameters were also shown in Fig. 10c and insert table. Except for the top AZO layer, all the other layers in the CZTS devices were fabricated under identical conditions. The CZTS thin film solar cell device showed a best PCE of 7.25%, with a VOC of 669 mV, a JSC of 18.18 mA cm−2 and a FF of 59.5%. By comparing the device parameters, it was observed that the CZTS solar cell using the aqueous solution deposited AZO thin films as top electrodes showed comparable values in PCE and VOC, and only slightly difference in JSC and FF. The slightly difference in JSC and FF was attributed to the different series resistance (see in Fig. 10c insert table) which might be arising from the different in sheet resistance of the AZO thin films (aqueous solution deposited AZO films: 36 Ω·sq−1, sputtering deposited AZO films: 20 Ω·sq−1). Fig. 10d showed the EQE curve of the CZTS thin film solar cells device using the aqueous solution deposited AZO thin films as top electrodes. It was observed that the CZTS solar cells exhibited high EQE values in the wavelength ranging from 400 nm to 950 nm, indicating excellent carrier collection efficiency. These results illustrated that the AZO thin films fabricated by the aqueous solution method in this work could meet photoelectric requirements for thin film solar cells applications. The stability of the best device was also studied. The experimental conditions were as follows: air ambient, 25 °C, humidity 80%, storage time 80 days. Fig. 11 showed the changes in PCE of the best CZTS cell using the aqueous solution deposited AZO thin films as top contact as a function of the days and the interval was five days. There were almost no changes in PCE with increasing the days indicating that the device was stable.

Fig. 11. Changes in PCE of the best CZTS cell using the aqueous solution deposited AZO thin films as top contact as a function of the days.

100 mW cm−2), demonstrating a best power conversion efficiency (PCE) of 7.15%, with an open circuit voltage (VOC) of 668 mV, a short current density (JSC) of 18.08 mA cm−2 and a fill factor (FF) of 59.2%. The other cells fabricated under identical conditions achieved the PCE ranging from 6.62% to 7.15% (Supporting Information, Table S1). The J-V curve of the best CZTS solar cell were shown in Fig. 10c, and the corresponding device parameters were shown in Fig. 10c insert table. For comparison, the CZTS thin film solar cells using the sputtering deposited AZO thin films as top electrodes were fabricated, and the J-V

4. Conclusion In summary, highly transparent conductive AZO thin films were 12

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successfully fabricated by using an aqueous solution process followed by an ultraviolet (UV) exposure technique at an annealing temperature as low as 200 °C, where the aluminum citrate complex was used as the Al doping source. Through rational control over the deposition parameters, dense AZO thin films with columnar grains showing a preferential growth orientation along the c-axis were obtained, and exhibited smooth surface morphology and homogeneous composition distribution. Compared with the metal Al foil doping and the Al salt doping, the AZO thin films prepared from the Al citrate complex doping exhibited higher Al doping concentration and as a result, showing a better electrical properties. The best AZO thin films demonstrated an optical transmittance higher than 85% in the visible spectra region and a lowest electrical resistivity of 4.8 × 10−3 Ω cm. XPS spectrum analysis showed that UV exposure could remove carbon species from the surface of the AZO thin films to reduce oxygen-related defects and release free carriers at the boundaries and interfaces, thereby improving the conductivity of the AZO thin films. The composition depth analysis showed that the simultaneous treatment of the AZO film by ultraviolet exposure and heat treatment could remove carbon species at a deeper thickness and as a result, improving the conductive stability of the AZO thin films. Kesterite Cu2ZnSnS4 thin film solar cells incorporating the optimal AZO thin films as top electrodes demonstrated a best power conversion efficiency (PCE) of 7.15%, which was comparable to the PCE value obtained by using the sputtering deposited AZO thin films as top electrodes. We believe that these AZO thin films can be widely used in thin film solar cells and other low-temperature photoelectric device owing to their superior performance, low material costs and low-temperature processing.

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Acknowledgements [22]

The authors greatly acknowledge the financial support by the National Research Foundation of Singapore (Program Grant No. NRFCRP13-2014-02).

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Appendix A. Supplementary data [25]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solmat.2019.110161.

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