Synthesis of ZnO and Fe2O3 nanoparticles by sol–gel method and their application in dye-sensitized solar cells

https://doi.org/10.1016/j.mssp.2011.09.007Get rights and content

Abstract

ZnO and Fe2O3 nanoparticles have been formed in a silica matrix, through the sol–gel method and were used as a photoanode to fabricate dye-sensitized solar cells (DSCs). The obtained oxides were characterized by X-ray diffraction, infrared spectroscopy, scanning electron microscope and UV–visible absorption spectroscopy. The results indicate that ZnO and Fe2O3 prepared by this method may be used as photoanodes in photo-electro-chemical energy conversion systems. DSSCs have been built using eosin Y as photosensitizer and their photocurrent, open-circuit voltage, fill factor and efficiency have been measured under direct sunlight illumination (1000 Wcm−2). A ZnO-film solar cell had the best performance with an open-circuit voltage of Voc=0.7 V and short-circuit current density of Isc=490 μA/cm2. This was attributed to high optical gap energy and transparency of ZnO compared to Fe2O3. The effects of annealing temperature and concentration of Fe2O3 on conversion efficiency of the Fe2O3 based solar cell were also studied.

Introduction

Solar energy has the potential to fulfill an important part of the sustainable energy demand of future generations [1]. In recent years, Dye-sensitized nano-crystalline TiO2 solar cell developed by Grätzel and his co-workers has attracted much attention due to its low cost and easy of fabrication [2]. It is composed of a wide bandgap semiconductor electrode such as TiO2 coated on a conducting glass substrate sensitized by R(dcbipy)2(NCS)2, a counter electrode and an I/I3 redox electrolyte [3]. It has yielded electrical energy conversion efficiency of ∼10% [4]. The performance of such solar cell depends on both the dye and the oxide. Many studies were focused upon nanocrystalline semiconductor oxides owing to their larger effective surface area available for adsorption of dye molecules, which in turn helps in appreciable absorption of incident light for its conversion into electrical energy. Among them ZnO and Fe2O3 are semiconducting materials which have many physical properties that make them suitable for thin film applications. They are inexpensive, good electrical conductivity, excellent substrate adherence and chemical stability [5], [6]. These properties have made ZnO and Fe2O3 attractive materials for many applications in photoelectrochemistry, energy storage, catalysis and sensors [5], [6]. Also, the method of preparation of the semiconductor electrode plays an important role in the performance of such solar cells. Many techniques can be used to prepare ZnO and Fe2O3 electrodes such as spray pyrolysis, chemical vapor deposition and sol–gel method [6], [7]. Among those the sol–gel technology is of great scientific interest. It has several advantages, such as low cost as no special instruments are required [8] and the possibility to control the properties of the film through the solution composition by adjusting different parameters such as, sol concentration, spin speed and annealing temperature [6]. On the other hand, the search for alternate sensitizers has been continued to improve the nanocrystalline cell efficiency. Although little attention has been paid to organic dyes because they have been considered to be less stable and efficient than metal complexes, some Grätzel-type cells using organic dyes have been reported lately, such as B(RhB)ZnO, Rh6G/SnO2, perylene/SnO2, nature dye/TiO2 and coumarin/TiO2 [9]. Compared with metal complex, organic dyes have several advantages such as a wider variety, higher absorption coefficient, lower cost and easier handling. Thus, the eosin Y was used as photosensitizer in this study. The formula structure of eosin Y is shown in Fig. 1.

This study reports sol–gel synthesis of composite thin films containing ZnO and Fe2O3 nanoparticles dispersed in SiO2 matrix and used as photoanode in dye-sensitized solar cells. The effect of drying temperatures of sol and concentration of oxide on the photoelectrochemical properties of eosin Y dye sensitized these electrodes were carried out by current–voltage measurements.

Section snippets

Sol preparation

The acid-catalyzed sol–gel synthesis was used to prepare iron oxide and zinc oxide dispersed in silica matrix. The chemical used for preparing sols were tetraethylorthosilicate Si(OC2H5)4 (TEOS, Fluka), iron chloride, zinc chloride (Aldrich) and HCl as a catalyst. The initial solution contained 7 ml of TEOS, 7 ml of ethyl alcohol, 8 ml of water and 1 ml of HCl serving as catalyst. A solution of ferric chloride was added to the initial solution under magnetic stirring for 3 h at 60 °C to obtain the

Characterization

X-ray diffraction pattern of the sol–gel-synthesized ZnO and Fe2O3 are shown in Fig. 4, Fig. 5. An amorphous pattern without any detectable crystalline phase was revealed for both ZnO and Fe2O3 annealed at 100 °C. After baking the Fe2O3 film at 500 °C and increasing its content from 10% to 15% in silica matrix, the characteristic diffraction lines of γ-Fe2O3 (maghemite phase) was grown and it seems to be the only phase evidenced by XRD, Fig. 5. This was attributed to the stabilizing effect of

Conclusions

Dye-sensitized nano-particle ZnO and Fe2O3 solar cells are prepared using ZnO and Fe2O3 films deposited by the sol–gel spin coating method. DSCs were fabricated using eosin Y as photosensitizer and their efficiency were recorded under white light illumination. A DSC fabricated with ZnO electrode showed the highest conversion efficiency with Isc=490 μA/cm2, Voc=0.7 V, FF=0.62 and η=0.02%. This was attributed to the high transparency of ZnO film compared to Fe2O3 film. The low values of power

Acknowledgment

The author is very grateful to the International Research Center for cooperation and support.

References (18)

There are more references available in the full text version of this article.

Cited by (0)

1

Present address: Chemistry Department, College of Science for Girls, Dammam University, Dammam, Kingdom of Saudi Arabia.

View full text