Cocktail effect of Fe2O3 and TiO2 semiconductors for a high performance dye-sensitized solar cell

doi:10.1016/j.apsusc.2010.09.066

Applied Surface Science

Volume 257, Issue 6, 1 January 2011, Pages 2164-2169

https://doi.org/10.1016/j.apsusc.2010.09.066 Get rights and content

Abstract

The bi-semiconductors of TiO₂ and Fe₂O₃ were used as a photoelectrode material in a high performance dye-sensitized solar cell due to cocktail effects from the two conduction bands. The size of the semiconductors was reduced by using a paint shaker to enlarge the contact area of the semiconductor with the dye or electrolyte. The fill factor and the efficiency of the prepared dye-sensitized solar cell were improved by over 16% and 300%, respectively; these parameters were measured from a current–voltage curve that was based on the effects of the Fe₂O₃ co-semiconductor and the size reduction. A mechanism is suggested wherein the conduction band of Fe₂O₃ works to prohibit the trapping effects of electrons in the conduction band of TiO₂. This result is attributed to the prevention of electron recombination between electrons in the TiO₂ conduction band with dye or electrolytes. The mechanism is suggested based on impedance results, which indicate improved electron transport at the interface of the TiO₂/dye/electrolyte.

Introduction

Recently, future alternative energy devices are very attractive due to the depletion of global petroleum. Among various promising alternative energies, such as wind, hydrogen, geothermal and nuclear, studies that focus on high performance dye-sensitized solar cell (DSSC) have accelerated due to many advantages: low cost, easy fabrication, accessible renewable energy production and high efficiency to convert solar energy into electricity [1], [2], [3], [4]. However, the industrial application of DSSCs still suffers due to their insufficient efficiency. Therefore, investigations on improving the efficiency of DSSCs have been critical [5], [6], [7].

As an initial reaction in a DSSC, the absorption of solar light by dye molecules excites electrons from the highest occupied molecule orbital (HOMO) to the lowest unoccupied molecule orbital (LUMO) in the dye molecules. The excited electrons are diffused into the conduction band (CB) of TiO₂. These electrons travel through the working electrode to a counter electrode. The oxidized dye is restored by electron donation via a redox reaction in the electrolyte (usually 3I⁻ → I₃⁻ + 2 e⁻) [8], [9], [10], [11]. In this process, some electrons in the CB of TiO₂ travel back to either the electrolyte or the HOMO in the dye molecules, which results in a loss of efficiency in the DSSC [12], [13], [14]. Therefore, one method to generate high efficiency DSSCs is to decrease the recombination of electrons in the dye or electrolyte to enhance electron movement towards the counter electrode and through the working electrode [15], [16], [17]. Another method is to increase the contact area of the TiO₂ semiconductor with the dye to maximize electron flow from the LUMO of the dye to the TiO₂ CB [18], [19], [20]. Even though there is some research on these methods, the proper mechanism has not yet been suggested.

In this study, the two methods listed above were considered to improve the efficiency of the DSSCs. To prevent electron recombination in the HOMO of the dye and the electrolyte, the effect of a Fe₂O₃ co-semiconductor was investigated. In addition, the effect of an increase in the contact area between the TiO₂ and the dye was investigated with size-reduced TiO₂. A mechanism that explains the high performance of the DSSCs was suggested by focusing on the electron transport at the TiO₂/dye/electrolyte interface, which was measured by impedance spectroscopy.

Section snippets

Preparation of TiO₂ and Fe₂O₃ complex

Commercial TiO₂ (anatase, 99.7%, 5 g, Aldrich Co.) and Fe₂O₃ (1.6 g, formula weight (Fw): 159.69, ≥99% Aldrich Co.) were mixed with distilled water (25 ml) and then stirred at room temperature for 48 h. The resultant mixture was thermally treated at 600 °C for 1 h with a heating rate of 5 °C/min and 30 ml/min of nitrogen gas flow to remove impurities, which creates a better interface between the two semiconductors. The prepared samples are denoted NT (neat titania) and BS (bi-semiconductors). The

Effects of the size reduction on the surface morphology and size density distribution of the NT, BS and SBS samples

The effects of the Fe₂O₃ co-semiconductor and size reduction (via a paint shaker) on the surface morphology of the NT, BS and SBS samples were investigated by analyzing the FE-SEM images in Fig. 3. The average particle size of the NT sample was 45 ± 11 nm, as shown in Fig. 3(a). The NT sample showed oval or spherical shapes. The particle size of the BS sample increased up to 60 ± 13 nm, which was caused by the Fe₂O₃ co-semiconductor and the thermal treatment, as shown in Fig. 1(b). The Fe₂O₃

Suggested mechanism for the prepared DSSC's improved efficiency

The suggested mechanism for the DSSC is depicted based on the effects of the Fe₂O₃ co-semiconductor and the size reduction. In Fig. 7(a), the proposed DSSC mechanism for the NT sample is presented. The electrons are excited by solar energy from the HOMO to the LUMO, which is indicated as (1). These excited electrons diffuse into the CB of the TiO₂, which is indicated as (2). Then, the electrons travel through the CB of the TiO₂ layer towards the FTO of the working electrode, which is indicated

Conclusions

A high performance DSSC was obtained due to the effects of the Fe₂O₃ co-semiconductor and the size reduction. The slightly low CB of Fe₂O₃ prevented electron trapping effects in the CB of TiO₂. The phenomenon was beneficial to prohibit the recombination of electrons in the CB of TiO₂ with those in the dye or the electrolyte. The size reduction effect of the bi-semiconductor improved electron charge transfer at the semiconductor/dye/electrolyte interface. The supporting mechanism was suggested

Acknowledgement

The authors thank Mr. Tae-Sung Bae at KBSI (Korea Basic Science Institute) in Jeonju center for providing the FE-SEM images.

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Cocktail effect of Fe2O3 and TiO2 semiconductors for a high performance dye-sensitized solar cell

Abstract

Introduction

Section snippets

Preparation of TiO2 and Fe2O3 complex

Effects of the size reduction on the surface morphology and size density distribution of the NT, BS and SBS samples

Suggested mechanism for the prepared DSSC's improved efficiency

Conclusions

Acknowledgement

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

J. Ind. Eng. Chem.

Electrochim. Acta

J. Colloid Interface Sci.

Electrochem. Commun.

Sol. Energy Mater. Sol. Cells

J. Photochem. Photobiol. A: Chem.

Chin. Chem. Lett.

J. Power Sources

Electrochim. Acta

Org. Electron.

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Powder Technol.

Cocktail effect of Fe₂O₃ and TiO₂ semiconductors for a high performance dye-sensitized solar cell

Preparation of TiO₂ and Fe₂O₃ complex