Au coated amorphous indium zinc oxide (a-IZO) bilayer and its application as counter electrode for dye-sensitized solar cell
Introduction
Owing to the demand in flexible and invisible electronics, there is an increasing interest in amorphous transparent conducting oxides (a-TCOs) since grain boundaries in polycrystalline TCOs have been idenfied as major deviation sources of device characteristics [1]. Unlike the most popularly studied amorphous ITO (indium tin oxide) that is easily crystalized at a low temperature, amorphous IZO has a much higher crystallization temperature than ITO. Therefore, amorphous IZO is more favorable than amorphous ITO as a conducting film on a flexible substrate because there are no grains and grain boundaries in an amorphous film, and there is much less chance to generate internal stress and severe film cracking [2]. Although amorphous IZO has these great advantages, it also has a big drawback of low conductivity especially when amorphous IZO is deposited at a low processing temperature. Unfortunately, a low processing temperature cannot be avoided for succeeding in electronics on a flexible substrate whose deterioration temperature is generally low.
To solve the low conductivity problem of a-IZO, various methods have been proposed and tried to increase conductivity and the figure of merit (FOM) of a-IZO, including the optimization of processing parameters, doping, and adding a thin metal layer to form TCO-metal-TCO (TMT) or metal-TCO (MT) structure. Although the improvement of conductivity by forming a TMT structure has been widely reported, the studies on a MT structure are much less [3], [4], [5], [6], [7] and systematic work is necessary. The processing steps for a MT structure are less, so the production cost will be less for MT structure process comparing with TMT structure process. There are advantages in achieving high performance with a bilayered material system [8], [9], [10]. However, the conduction mechanism of MT structures needs to be explored. Besides, we need to achieve optimum conditions for obtaining the best figure of merit (FOM), an important parameter on the applicability degree of a transparent conducting film.
When considering applications of TCO films in dye sensitized solar cells (DSSC), a MT structure should be more preferred and applicable since the counter electrode of DSSCs is generally metal/TCO. Compared with the most extensively used Pt as the metal for DSSC, Au has a conductivity (4.3 × 105 Scm−1) that is much higher than Pt (0.9 × 105 Scm−1). In addition, Au is catalytic, and has an excellent corrosion resistance [11]. Therefore, it is worth to examine the possibility in using Au coated IZO as counter electrode for DSSC. To our knowledge, an understanding of the electrical and optical behavior of an Au/a-IZO bilayer system is lacking and DSSC with a counter electrode of Au/a-IZO bilayer has not been reported yet.
In this paper, Au coated amorphous indium zinc oxide (a-IZO) bilayer is prepared at room temperature and systematically studied. Unexpectedly high conductivity of 1.2 × 105 Scm−1 and mobility of 65.6 cm2V−1s−1 are achieved for Au/a-IZO bilayers prepared under certain conditions. An optimum FOM value is obtained. The optimum Au/a-IZO MT bilayer film is successfully applied in DSSC.
Section snippets
Material and methods
Borosilicate glass substrates were sequentially cleaned in ultrasonic baths of acetone, ethanol and deionized water and then dried with blowing N2 before introducing into the reaction chamber. IZO films were then deposited by radio-frequency sputtering of IZO target (90 wt% In2O3–10 wt% ZnO) on the borosilicate glass substrates at room temperature. The base pressure of the reaction chamber was kept at 4 × 10−7 Torr and the working pressure of Ar gas (99.999%) was 1.1 × 10−2 Torr. The IZO layer
Results and discussion
Fig. 1 shows XRD patterns in the 2θ range of 20°–70° for different Au/a-IZO bilayer samples. There are no XRD peaks for the reference sample Au0, suggesting the amorphous state IZO, in agreement with the literature for IZO samples deposited at similar conditions [14]. Samples Au1 and Au2 also do not show any XRD peaks, indicating that the Au layer are too thin to be detected by XRD. Later, Au existence in both Au1 and Au2 can be demonstrated through the variation of the electrical and optical
Conclusion
In this paper, transparent conductive Au/a-IZO bilayers are processed at room temperature and systematically studied. Very high conductivity of 1.2 × 105 Scm−1 and mobility of 65.6 cm2V−1s−1 are achieved. An optimum FOM value dependent on conductivity and transmittance is obtained; based on those analyses, the optimum Au coated a-IZO sample is successfully applied as counter electrode of DSSC (dye sensitized solar cells). The DSSC exhibits light-to-electricity conversion under light
Acknowledgements
Funding supports from Singapore MoE T2 grants R-284-000-125-112 and R-284-000-113-112, the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (No. CUG140613), the Natural Science Foundation of Hubei Province of China (No. 2014CFB259) and the National Natural Science Foundation of China (No. 61404116) are appreciated.
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