Elsevier

Applied Surface Science

Volume 474, 30 April 2019, Pages 85-90
Applied Surface Science

Full Length Article
CuO-Cu2O nanocomposite layer for light-harvesting enhancement in ZnO dye-sensitized solar cells

https://doi.org/10.1016/j.apsusc.2018.05.037Get rights and content

Highlights

  • Mixed phases of copper oxide were synthesized by additive-microwave technique.

  • Larger size of CuO-Cu2O on ZnO layer in DSSC led to light-harvesting enhancement.

  • CuO-Cu2O can expand photon absorption range, leading to enhance light absorption.

Abstract

This work presents a novel nanocomposite layer on ZnO photoelectrodes prepared from mixed phases of copper oxide nanoparticles, CuO and Cu2O. The nanoparticles were rapidly synthesized by additive-microwave heating technique in a few minutes. Then, the nanoparticles were prepared with different concentrations of 2, 4, 6, and 8 mM followed by coating on a ZnO based layer in the DSSCs. The ZnO DSSCs with a copper oxide layer were then fabricated and measured under a solar simulator. The highest power conversion efficiency 2.31% and the highest current density 7.33 mA/cm2 were achieved at an optimum 6 mM of the nanoparticle layer. The architecture layer of the CuO-Cu2O nanoparticles led to light-harvesting enhancements in the ZnO DSSCs. This can be explained by the larger homogenous size for improving light scattering. The photovoltaic properties of the CuO-Cu2O improved a wide absorbance in the visible light region compared to a pure ZnO layer.

Graphical abstract

In this work, a novel nanocomposite layer was prepared from mixed phases of copper oxide nanoparticles, CuO and Cu2O, rapidly synthesized by additive-microwave heating technique. The obtained nanoparticles were then prepared with different concentrations of 2, 4, 6, and 8 mM followed by coating on a ZnO based layer in the DSSCs. The ZnO DSSCs with a copper oxide layer at the optimum 6 mM measured under a solar simulator showed the highest power conversion efficiency 2.31% and the highest current density 7.33 mA/cm2. The architecture layer of the CuO-Cu2O nanoparticles led to light-harvesting enhancements in the ZnO DSSCs.

  1. Download : Download high-res image (107KB)
  2. Download : Download full-size image

Introduction

The solar energy is a huge primary source of sustainable power needed for world development. Dye sensitized solar cells (DSSCs) are devices that can change light power from the sun to electrical power at a low cost and high performance. Using ZnO in DSSCs gives a high mobility to electrons because ZnO is a semiconductor with a wide band gap energy of about 3.3 eV. This is similar to that of TiO2 (the first semiconductor based DSSC). ZnO can collect electrons from excited state in conduction band before injection into the FTO glass [1], [2], [3]. However, pure ZnO in DSSCs gives a lower electric conductivity than TiO2 [2]. Thus, many researchers try to improve the ZnO DSSCs efficiency [4], [5], [6], [7]. In order to achieve higher photo conversion efficiency, efforts have been made for high charge carrier mobility with less recombination loss, more dye molecule absorption with a larger surface area and more light scattering to capture sunlight more efficiently [8], [9]. To overcome the task, cupric oxide (CuO) and cuprous oxide (Cu2O) are used as copper oxide layers in photoelectrodes [10], [11], [12], [13]. The copper oxides are attractive p-type semiconductors because of their optical and electrical properties. In addition, the use of the copper oxides is extremely attractive not only due to their non-toxic, low-cost and abundant starting materials, but also due to their cheap and simple synthesis [14]. CuO is a monoclinic with a lattice constant of a = 4.6837 Å, b = 3.4226 Å and c = 5.1288 Å. CuO is an antiferromagnetic material with indirect band gap energy of 1.2 eV and high absorption coefficients at above 1.4 eV [15]. Moreover, CuO is also believed to have copper vacancies as acceptors are responsible for hole conduction [16]. Whereas, Cu2O is a simple cubic with a lattice constant of a = 4.2696 Å having a Cu atom in a unit cell coordinated by two oxygen atoms [17]. The band gap energy of Cu2O is about 2.1 eV, which is stable for photovoltaic conversion [18]. Furthermore, Cu2O is a natural p-type semiconductor owning good carrier. Moreover, both of CuO and Cu2O can be easily prepared in several shapes and sizes [19], [20], [21], [22]. These lead to synthesis and surface modification of copper oxide layers to be used in the DSSCs. Previous studies confirm that coating layer with larger nanoparticles onto smaller nanoparticle size of a semiconducting layer in the DSSCs can enhance light scattering [23], [24], [25], [26]. Furthermore, using a spherical shape and a relatively larger average size on top of a semiconductor nanoparticles-based underlayer can support a higher concentration of the incident light more effectively [27]. Therefore, coating spherical shape and larger size of copper oxide in ZnO based DSSCs is expected to improve the DSSCs efficiency.

In this work the mixed phases of CuO and Cu2O nanoparticles with larger homogeneous size compared with ZnO were prepared by additive-microwave heating under normal atmosphere in a very short time. The mixed phases of the copper oxide nanoparticles were then used as a composite layer in photoelectrodes in order to improve ultraviolet–visible (UV–vis) spectral absorbance and light scattering efficiency in ZnO based DSSCs.

Section snippets

Synthesis of CuO-Cu2O nanoparticles

The mixed phases of CuO and Cu2O nanoparticles were synthesized by the additive-microwave heating technique under normal atmosphere for a few minutes. In a typical process, 1 g of copper powders (99.5%, MW: 63.55 g/mol, 40 μm of particle size) was used as a precursor and placed in a quartz rod, having a diameter and a length of about 2.8 and 10 cm, respectively. Then, 0.3 mL of ethanol was added into the precursor. The sample was heated by a microwave oven (SHARP model) in normal atmosphere, at

Characteristics of CuO-Cu2O nanoparticles

Fig. 1 shows the morphology of the copper oxide nanoparticles prepared by ultra-fast microwave-assisted technique. After 2 min of microwave heat, Cu powders with addition of ethanol gave spherical particles with a homogeneous size of about 100 nm. The nanoparticles were composed of mixed-phase CuO and Cu2O as shown in RAMAN spectrum in Fig. 2. The RAMAN peaks of CuO were at 296 and 630 cm1, whereas the peaks at 146, 219 and 414 cm1 belong to the Cu2O phase.

The mixed phases of CuO and Cu2O

Conclusions

Mixed phases of copper oxide nanoparticles, CuO and Cu2O, were simply and rapidly synthesized by using only copper powder with ethanol addition and heating under microwave radiation in a few minutes, in normal atmosphere. The CuO-Cu2O nanoparticles showed high homogeneous size of about 100 nm in spherical shape. Moreover, the enhanced efficiency of ZnO based DSSCs was achieved by coating CuO-Cu2O nanoparticles on ZnO photoelectrodes. The DSSC with a CuO-Cu2O layer of 6 mM exhibited the highest

Acknowledgements

The author (Karakade Kaewyai) would like to thank Ministry of Science and Technology (Thailand) for financial support. In addition, we would like to special thanks to Ms. Cynthia Bail for grammatical revise and rectification.

References (32)

  • Y. Jiang et al.

    Based on Cu(II) silicotungstate modified photoanode with long electron lifetime and enhanced performance in dye sensitized solar cells

    J. Power Sources

    (2015)
  • K.E. Lee et al.

    Enhanced surface hydroxylation of nanocrystalline anatase films improves photocurrent output and electron lifetime in dye sensitized solar cell photoanodes

    Electrochim. Acta

    (2012)
  • Y. Chergui et al.

    Comparative study of dye-sensitized solar cell based on ZnO and TiO2 nanostructures

  • E. Guillén, L.M. Peter, J.A. Anta, Electron transport and recombination in ZnO-based dye-sensitized solar cells, J....
  • B.E. Hardin et al.

    The renaissance of dye-sensitized solar cells

    Nat. Photon.

    (2012)
  • M. Karlsson, Materials development for solid-state dye-sensitized solar cells, Faculty of Science and Technology,...
  • Cited by (28)

    View all citing articles on Scopus
    View full text