Elsevier

Applied Surface Science

Volume 334, 15 April 2015, Pages 145-150
Applied Surface Science

Structural and photovoltaic characteristics of hierarchical ZnO nanostructures electrodes

https://doi.org/10.1016/j.apsusc.2014.08.156Get rights and content

Highlights

  • Hierarchically ZnO nanostructures electrodes were grown using hot plate magnetic stirring at different growth reaction temperature.

  • We have investigated the effect of working temperature of 160°, 170°, 180°, and 190° on the growth mechanism of nanospheres and on the power conversion efficiency of DSSCs.

  • ZnO nanospheres with perfect aggregation show superior power conversion efficiency of 1.24% which is about 83% higher than nanoparticles DSSC.

  • An obvious vogue is that the overall power conversion efficiency decreases as the degree of the spherical aggregation is gradually destroyed.

Abstract

Structural and photovoltaic characteristics of hierarchical ZnO nanostructures solar cell have been studied in relation to growth reaction temperature. It is found that the hierarchical ZnO nanostructures network to act not only as large surface area substrates but also as a transport medium for electrons injected from the dye molecules. The incident photon-to-current conversion efficiency is decreased by increasing the growth reaction temperature of ZnO electrodes. The best conversion efficiency of a 0.25 cm2 cell is measured to be 1.24% under 100 mW cm−2 irradiation.

Introduction

The longstanding record efficiency of present state-of-the-art dye-sensitized solar cells (DSSCs) is 12–15% [1], [2] which is however far lower than the theoretically predicted one, 20–33% [3], [4], [5]. For the up-scaling of DSSCs, there are three rooms that may be used to improve the conversion efficiency [6]. One way for boosting the efficiency of DSSCs is to develop new dyes with higher molar extinction coefficient and broader spectral response than the existing dyes that break the 15% efficiency limit is an unmet challenge and important focus of current research [7], [8], [9]. The second promising way to ameliorate the efficiency of DSSC is to improve the open-circuit voltage (VOC). In general, the VOC is determined by the difference between the redox potential of the electrolyte and the Fermi level of the oxide semiconductor. Therefore, it is necessary to find electrolytes with a redox couple more closely matched to the energy of the oxidized dye, i.e. to increase the VOC. Law et al. [6] estimated that, if there is an increase of 300 mV in VOC would mean a 35% improvement in device efficiency. The third way for increasing the efficiency of DSSCs is the optical enhancement effect which can be accomplished either by introducing the light scatters into the photoelectrode film [10], [11], [12], [13], [14], [15], [16] or by utilizing the hierarchical structures of ZnO aggregates which is an ideal system for providing a huge internal surface area for dye adsorption and promoting the light scattering [17], [18], [19]. However, to the best of our knowledge, few attempts have been made to study the effect of growth reaction temperature on the structure and morphology of ZnO nanospheres and on the performance of DSSCs. Therefore, it is urgent to investigate the effect of working temperature on the structure and morphology of nanospheres and on the power conversion efficiency of DSSCs. Insight into the factor that limit DSSC performance is gained by fabricating ZnO aggregates with different morphologies at different growth reaction temperature of 160, 170, 180 and 190 °C.

In this study, we fabricated four kinds of hierarchical ZnO nanostructures at different growth reaction temperature with different degrees of aggregation for a comparison of their DSSC performance. It has been manifested that the growth reaction temperature has pronounced effect on the morphology of ZnO nanomaterials and on the DSSCs performance.

Section snippets

Synthesis of hierarchical ZnO nanostructures

The precursors utilized for the preparation of hierarchical ZnO nanostructures, are diethylene glycol (C4H10O3, 99.5%) (DEG) and zinc acetate dihydrate (Zn (CH3COO)2·2H2O, 99.5%) (ZAD). All chemicals in this study were of reagent grade and used as received without further purification. A 100 mL DEG solution was prepared by mixing 0.09 M of ZAD in a conical flask through successive stirring and heating. The synthesis of hierarchical ZnO nanostructures was divided into three processes: solution

Structural characterization of hierarchical ZnO nanostructures

Fig. 2 shows the X-ray diffraction (XRD) pattern of four hierarchical ZnO nanomaterial. All the samples exhibit similar XRD patterns, the number of peaks, noted as N, considered in the following is only 9, i.e. nine diffraction peaks of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), and (2 0 1) with more dominant intensity of the (1 0 1) peak over other peaks corresponding to the pure ZnO crystalline phase [30]. Moreover, no characteristic peaks of other impurities such as Zn and Zn (OH)2

Conclusions

In conclusion, we report structural and photovoltaic characteristics of hierarchical nanostructures ZnO solar cell in relation to growth reaction temperature. Short circuit current density and overall power conversion efficiency were found to be dependent on the growth reaction temperature of ZnO electrodes. This suggests hierarchical ZnO nanostructures network to act not only as a large surface area substrates but also as a transport medium for electrons injected from the dye molecules. The

Acknowledgments

The authors acknowledge the support from HEC, PK, for startup research grant of 0.5 Million and National Natural Science Foundation of China under Grant No. 50942001 and 50975301, and the Third Stage of “211” Innovative Talent Training Project (No. S-09109) of Chongqing University.

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