Elsevier

Solar Energy

Volume 92, June 2013, Pages 41-46
Solar Energy

Incorporation of multiwalled carbon nanotubes into TiO2 nanowires for enhancing photovoltaic performance of dye-sensitized solar cells via highly efficient electron transfer

https://doi.org/10.1016/j.solener.2013.02.031Get rights and content

Highlights

  • MWCNT-TiO2 composite NWs are formed via electrospinning and calcination processes.

  • MWCNT content is varied in the MWCNT-TiO2 composite NW-based DSSC photoelectrodes.

  • MWCNT incorporation in TiO2 matrix NWs improves the photovoltaic properties of DSSCs.

  • MWCNTs act as efficient charge transfer medium in TiO2 matrix NWs.

Abstract

Multiwalled carbon nanotube (MWCNT)-embedded TiO2 nanowires (NWs) were synthesized by a combination of electrospinning and calcination processes. We examined the effect of the MWCNT mass fraction in the MWCNT-TiO2 composite NW-based photoelectrode on the photovoltaic properties of the resulting dye-sensitized solar cells (DSSCs). The MWCNT (5 wt%)-TiO2 composite NW-based DSSC fabricated in this study showed a significantly improved short circuit current density and power conversion efficiency (PCE) compared to pure TiO2 NW (i.e., MWCNT 0 wt%)-based DSSCs (the values increased from 2.91 ± 0.15 to 10.72 ± 0.21 mA/cm2 and from 1.44 ± 0.10% to 5.03 ± 0.35%, respectively). This improvement was due to an increase in rapid electron transfer and suppression in charge recombination caused by MWCNTs embedded in the TiO2 matrix NWs. These specially designed MWCNT-TiO2 composite NW-based photoelectrodes have great potential as an effective charge transfer medium to inherently enhance the photovoltaic performance of DSSCs.

Introduction

Dye-sensitized solar cells (DSSCs) are very promising third-generation solar cells because of their inexpensive and relatively simple atmospheric manufacturing processes and their potentially high power conversion efficiency (PCE) (O’Regan and Grätzel, 1991). Generally, conventional DSSCs have a solid TiO2 nanoparticle (NP)-accumulated photoelectrode sensitized with dye molecules, a liquid electrolyte containing an iodide/triiodide redox couple and a Pt-coated FTO glass cathode. Solid TiO2 NPs with a relatively high specific surface area can absorb a large amount of dye molecules and rapidly transport photoinduced electrons (Chou et al., 2012). However, fast recombination and slow diffusion of electrons in the interfacial grain boundaries among TiO2 NPs causes a decrease in the PCE of DSSCs.

Various methods have been investigated to achieve efficient electron diffusion and transport by employing TiO2 nanotube/nanowire arrays, which enable channeled electron transfer so that the loss of photoinduced electrons among solid TiO2 NPs can be significantly reduced (Law et al., 2005, Klimov, 2006, Mor et al., 2006, Zhao et al., 2010, Hwang et al., 2011, Zou et al., 2012). However, controlling the nanostructures of TiO2 to enhance the electron transfer can also alter various properties of the resulting DSSCs, such as charge separation/recombination and the specific surface area of TiO2 nanostructure-based photoelectrodes. As an alternative approach, nanocomposites have been suggested to enhance the electron transfer in DSSC photoelectrodes. In particular, multiwalled carbon nanotubes (MWCNTs) are regarded as promising components of nanocomposite-based DSSC photoelectrodes (Cai et al., 2012, Chang et al., 2012, Chen et al., 2012, Jang et al., 2004, Kim et al., 2006, Zhu et al., 2009). Since MWCNTs have relatively high electrical conductivity and a one-dimensional structure, they can act as an efficient charge transfer medium. However, a simple mixture of MWCNTs and TiO2 NPs does not appreciably improve the photovoltaic properties of DSSCs; this is presumably because MWCNTs dispersed on the surface of TiO2 NPs suppress the adsorption of dye molecules and simultaneously enhance charge recombination (Brown et al., 2008, Dang et al., 2011, Du et al., 2013, Jung et al., 2002, Peining et al., 2012). Thus, methods for effectively incorporating MWCNTs into DSSC photoelectrodes should be developed. This paper introduces the fabrication of MWCNT-TiO2 composite nanowires (NWs) and discusses the effects of embedding MWCNTs in TiO2 matrix NWs on the photovoltaic performance of DSSCs.

Section snippets

Synthesis of MWCNT-TiO2 composite NWs

Fig. 1a presents the schematic of the experimental setup for the fabrication of MWCNT-TiO2 composite NWs. Briefly, a precursor solution composed of 2.88 g of titanium (IV) isopropoxide (TTIP, Sigma Aldrich), 16 g of N,N-dimethylformamide (DMF, Sigma Aldrich), 2.7 g of polyvinylpyrrolidone (PVP, Mw: 1,300,000, Sigma Aldrich), and various amounts of MWCNTs was sonicated for 30 min. The aforementioned precursor solution was then dispensed at a rate of 3 mL/h by using a precision syringe pump (Model No.

Results and discussion

MWCNT-TiO2 composite NWs were formed using electrospinning and subsequent calcination processes. The PVP templates in the MWCNT-TiO2 composite NWs were confirmed via SEM analysis to play a key role in securing NW structures for the as-prepared PVP–MWCNT-TiO2 composite NWs (see Fig. 2a and b). The average diameter of the as-prepared PVP–MWCNT-TiO2 composite NWs was approximately ∼243 ± 20 nm, as shown in Fig. 2a. However, after removal of the PVP templates by calcination, the diameter of the

Conclusions

We described a simple and viable electrospinning method for the synthesis of MWCNT-TiO2 composite NWs. The method facilitates the formation of TiO2 NWs embedded with MWCNTs that can rapidly transport photogenerated electrons through TiO2 matrix NWs and simultaneously suppress charge recombination between the electron and the dye or the redox couple when used in DSSC photoelectrodes. The photovoltaic performance of DSSCs improved significantly when the amount of MWCNTs in the MWCNT-TiO2

Acknowledgement

This study was supported by the National Research Foundation of Korea (NRF) funded by the Korean government (MEST) (2011-0013114).

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