Comparative study of structural and electro-optical properties of ZnO:Ga films grown by steered cathodic arc plasma evaporation and sputtering on plastic and their application on polymer-based organic solar cells
Introduction
Transparent conductive oxide (TCO) films, which can combine the properties of electronic conductivity with transparency in the visible light region, are crucial materials for many optical devices, such as solar cells, organic light emitting diodes and touch sensors. Flexible electronics have been a great interest in the fabrication of devices on plastic substrates, such as polyethylene terephthalate (PET), in order to achieve the advantages of light weight, low thickness and high flexibility [1]. Polymer-based organic solar cells have recently attracted great interest with regard to developing low-cost, large-area, and flexible photovoltaic devices [2]. Among numerous photoactive donor/acceptor composites, the blend of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) has been intensively investigated in recent years, because the devices based on these active layers have been shown to have a good power conversion efficiency [3], [4].
For applications in flexible electronics, in addition to good electro-optical properties, TCO films should possess other key features, such as easy processing, low deposition temperature, low residual stress/stain, and low cost. Indium tin oxide (ITO) is currently to mostly commonly used TCO. Although ITO films have excellent optical transmission and low sheet resistance, they suffer from several drawbacks, including a high deposition temperature and the lack of indium availability. A substrate temperature ≥ 300 °C is usually required to obtain well-crystallized ITO films with a low resistivity (~ 5 × 10− 4 Ω·cm) and high optical transmittance (~ 90%) in the visible region when using a conventional sputter deposition process [5]. ZnO-based films doping with group III B elements (such as B, Al, Ga, and In) were actively investigated as an alternative material to high cost ITO due to their excellent electro-optical properties, non-toxicity, relatively low cost, natural abundance of source elements, stability under the plasma of reducing species [6] and moisture resistance [7]. GZO films have high doping stability because the ionic radius of Ga3 + (r = 0.1225 nm) is the same as that of Zn2 + (r = 0.1225 nm), and the difference in electronegativity (EN) between Zn2 + (EN = 0.99) and Ga3 + (EN = 1.13) is the smallest in the group III elements [8]. GZO is thus the main candidate for replacing ITO in industrial applications.
The magnetron sputtering (MS) process has been extensively used in ZnO-based film deposition technology. However, the main drawback of conventional MS deposition processes are the high kinetic energy negative ions (e.g., O–) which are generated in the cathode sheath and accelerated through the full cathode potential [9], [10]. The typical kinetic energy of several hundred eV depends on the cathode voltage, and such high kinetic energy ions lead to damage and high residual stress in the films [11], resulting in nonuniform distribution and deterioration of electro-optical properties [12]. In general, in order to obtain a high growth rate in conventional MS deposition the cathode voltage should be increased, but a higher voltage would accelerate the negative ions, causing more damage and residual stress in the resulting films. Therefore, growing GZO thin films with high crystallinity and good performance at both a high growth rate and low temperature simultaneously is very difficult using the conventional MS process. To solve this problem, steered cathodic arc plasma evaporation (steered CAPE) has been used to prepare high-quality GZO films. In contrast to MS methods, the arc plasma can be created by the cathode spots with an extremely high areal power density (on the order of 1013 W/m2) in CAPE technique, leading to a high deposition rate, high degree of ionization, ion kinetic energies in the range of 10–100 eV, and multiple charge states [13]. The CAPE technique is a high current density and low voltage process, in which a cloud of positive ions near the cathode surface establish a potential barrier of a few tens of volts [14], providing an appropriate kinetic energy of ions to enhance the thin film properties, which are similar to those obtained with ion beam assisted deposition.
The present study prepared GZO films with a low resistivity (on the order of 10− 4 Ω·cm) and high optical transmittance (> 88%) on PET substrates at low temperature via steered CAPE at a low substrate temperature of 90 °C. The comparative analysis of the microstructure, residual stress, surface morphology, electrical and optical properties, chemical states, and doping efficiency of the films produced by the steered CAPE and MS processes was performed, and the effect of thickness on the CAPE-grown GZO films was investigated in detail. We have also produced and examined polymer solar cells using both steered CAPE-grown and MS-grown GZO electrodes on a PET substrate.
Section snippets
Thin film growth
GZO films were deposited on PET using steered CAPE with a sintered ceramic GZO (3 wt% Ga2O3) circular target (diameter = 8.9 cm). A low resistivity (3 × 10− 4 Ω cm) circular ceramic GZO target was soldered to a Cu backing plate to improve electrical conductivity, thermal conduction and mechanical strength. To prevent overheating and obtain uniform erosion of the target, the steered arc process was achieved by using circular permanent magnets behind the circular cathode (magnetic field at center of
Crystal structure and residual stress
Fig. 1 shows the XRD patterns of GZO films with various thicknesses deposited on PET substrate at 90 °C by steered CAPE, and an XRD pattern of the GZO film deposited by a MS deposition at the same substrate temperature is also shown for comparison. In addition to the intense PET substrate peak at about 25.9°, the patterns exhibit a single (0002) peak of the hexagonal wurtzite (WZ) structure and absent of any secondary phases, such as spinel ZnGa2O4. These GZO films have a c-axis preferred
Conclusion
In summary, this work reports that high quality c-axis preferred orientation GZO films were successfully grown on the PET substrate at a low temperature of 90 °C using the steered CAPE process. The GZO films grown by steered CAPE exhibited higher crystallinity and lower internal stress than those grown by MS. TEM images show that the film grown by MS had a higher stacking fault density, smaller crystalline domains with size (~ 5 nm) and local lattice distortion, such as dislocations in the
Acknowledgments
This study was supported by the Bureau of Energy, Ministry of Economic Affairs, and by the Research Center for Energy Technology and Strategy (D103-23002) at National Cheng Kung University in Taiwan. The use of the facilities at the Industrial Technology Research Institute in Taiwan is also greatly appreciated.
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