Nanostructured hybrid ZnO thin films for energy conversion
© Moya et al; licensee Springer. 2011
Received: 5 November 2010
Accepted: 16 May 2011
Published: 16 May 2011
We report on hybrid films based on ZnO/organic dye prepared by electrodeposition using tetrasulfonated copper phthalocyanines (TS-CuPc) and Eosin-Y (EoY). Both the morphology and porosity of hybrid ZnO films are highly dependent on the type of dyes used in the synthesis. High photosensitivity was observed for ZnO/EoY films, while a very weak photoresponse was obtained for ZnO/TS-CuPc films. Despite a higher absorption coefficient of TS-CuPc than EoY, in ZnO/EoY hybrid films, the excited photoelectrons between the EoY levels can be extracted through ZnO, and the porosity of ZnO/EoY can also be controlled.
Replacing the conventional TiO2 nanocrystalline (nc) electrode with organic/inorganic hybrid nanostructures will undoubtedly improve overall performance of dye-sensitized solar cells (DSSC). Inorganic nc-ZnO is a promising candidate for such hybrid structures, due to its unique properties, such as high conductivity, wide bandgap (3.2 eV), and high excitonic binding energy (60 mV) . In addition, the conduction band edge position of ZnO (-4.3 eV) is similar to that of TiO2 (-4.5 eV). Furthermore, ZnO can easily be electrochemically deposited (ECD) at low temperature, and hybrid nanostructures with ZnO can also be co-deposited with dyes such as Eosin-Y (EoY). It has been reported that the dye molecules are strongly attached to the ZnO matrix, filling the voids by means of sulfonic or carboxyl groups . In this article, we report on a study of different hybrids of ZnO nanostructures obtained by changing the dye in the deposition electrolyte. While different aspects of pure ZnO have been reported extensively by the other authors [3, 4], this article focuses on the growth of various ZnO porous structures with dyes and their use in energy conversion.
The ECD of nc-ZnO was carried out in a three-electrode cell consisting of a cathode [substrate, indium oxide (ITO)-coated glass with a ~10 Ω/□ sheet resistance], a Pt counter electrode, and an Ag/AgCl reference (+222 mV vs normal hydrogen electrode). Three ECD baths were prepared, each solution containing a mixture of 0.1 M KCl (Merck) and 5 × 10-3 M ZnCl2 (Merck). The first electrolyte bath contained 1 × 10-4 M of EoY (Sigma Aldrich, Spain), while the other baths had two different concentrations of tetrasulfonated copper phthalocyanines (TS-CuPc) (1 × 10-4 M and 3 × 10-5 M). The optimum EoY concentration for ZnO/EoY was derived during a previous study of EoY concentrations . The electrolytes were purged with O2 at a volume flow rate of 200 mL/min, while stirring by means of a magnetic bar stirrer to facilitate the oxygen diffusion. Before the deposition process, the ITO glass was ultrasonically cleaned in acetone and subsequently with ethanol for 15 min, and then rinsed with deionized water. The total deposition time for ZnO/Ts-CuPc (approx. 1 μm thick), was 600-800 s, while for ZnO/EoY films, it was 200-500 s. The potentiostatic deposition was carried out by applying a potential of -0.9 V to the substrate (1.17 cm2), using an Autolab potentiostat. The bath temperature was set at 70°C controlled by a thermostat. After deposition, the ZnO/EoY films were immersed in a dilute aqueous NaOH solution (pH 10.5) for 40 min to desorb the loaded EoY molecules, while the ZnO/TS-CuPc films were immersed for 24 h. The films were dried in air for 1 h at 150°C. Desorbed thin films were re-sensitised with the relevant dye concentrations for characterization.
The morphology of the ZnO/organic hybrid films was studied using a JEOL-JSM6300 scanning electron microscope (SEM) operating at 10 kV. The structural characterization was carried out by high-resolution X-ray diffraction (XRD) using a Rigaku Ultima IV diffractometer in θ-2θ mode with a copper anticathode (CuKα, 1.54 Å). Optical transmittance measurements were performed by means of an Ocean Optics DT-MINI-2-GS deuterium-halogen lamp in association with a 500-mm spectrometer coupled to a backthinned CCD detector optimized for the UV-Vis range. To determine the surface topography, an atomic force microscope (AFM) Multimode Veeco was used, where the scanning was carried out in tapping mode using a silicon cantilever. The scanning frequency was set at 0.5 Hz, and the image size was 5 × 5 μm2. The photoelectrochemical study was performed in a conventional three-electrode arrangement in a glass cell, consisting of the deposited thin film as the working electrode, illuminated from the glass/ITO side, a Pt counter electrode, and an Ag/AgCl reference electrode in 0.1 M KCl electrolyte. The photocurrent was measured using a potentiostat/galvanostat and recorded. The illumination time of the electrode was controlled using an automatic mechanical shutter, with an adjusted illumination time of 10 s, for which a controller box had been designed. The shutter required approximately 10 ms to reach a completely open (or closed) position. All the measurements were performed at 0.05 V bias where the dark current was negligible.
AFM analysis of nanostructured ZnO films
Surface morphology with SEM
Desorption of dye can be easily followed using the naked eye because the color of the films varies according to the dye concentration. Furthermore, desorption of dye in the films can also be assessed by energy dispersive X-ray analysis through the Cu/Zn or Br/Zn ratios. For the ZnO/Ts-CuPc hybrid films with a concentration 5 × 10-3 M, the ratio Cu/Zn was 0.008%, whereas in desorbed thin films, it was 0.002%. This shows that only 25% of the dye in the hybrid film is maintained after desorption.
Relative peak intensities of three XRD spectra shown in Figure 4
Relative peak intensity
ZnO/EoY photoresponse showed an overshoot at the incidence of light, characteristic of a slow regeneration reaction  of the oxidized dye molecule following fast injection to ZnO, which is usually caused by non-exposure of dyes due to masking by the electrolyte. This non-exposure of dyes to light leads to a steady-state concentration of oxidized dye suggesting the presence of a positively charged interface, an increased recombination of charge carriers, and hence a small photocurrent. When the light is switched off, this recombination is directly observed in a small cathodic spike, suggesting the discharge of the positive surface charge by the electrode . For ZnO/TS-CuPc, the characteristics are similar, but the initial overshoot, as well as the cathodic current, as smaller in magnitude, are due possibly to deep traps in the ZnO. The overshoot can also be caused by the absorption of a fraction of light on the film surface. These observations confirm poor hybridization of TS-CuPc with ZnO, which results in poor efficiency in the generation of electrons. However, EoY appears to show formation of strong hybrid mesoporous structures with the ZnO.
Changes in the morphology of the ECD ZnO were studied using two sensitizer dyes, EoY and TS-CuPc. Ts-CuPc particles absorbed by the ZnO were found to be irregularly shaped. On the other hand, in ZnO/EoY hybrid films, the EY4-/Zn2+ were observed forming clusters in which the clear edges of the particles were absent, showing a granular and compact morphology with few cracks on the surface of the hybrid film. The crystallinity improved when dyes of ZnO were used, while maintaining the preferred orientation of ZnO in the direction (002). ZnO/TS-CuPc showed better crystallinity despite a large porous structure in comparison with ZnO/EoY, as shown by the average crystalline sizes. The photocurrent response indicates that a high proportion of dye molecules are being excited by the electrons acting as sensitizers. However, the absorbance peak of two dyes (524 and 690 nm), proves that the two dyes could be used together to improve DSSC efficiency.
atomic force microscope
dye-sensitized solar cells
full width at half maximum
scanning electron microscope
tetrasulfonated copper phthalocyanines
This study was supported by the Spanish Government through MCINN grant MAT2009-14625-C03-03, the Generalitat Valenciana through programme PROMETEO/2009/063, and the Fundação para a Ciência e a Tecnologia (Portugal). Thanks are due to Susie Pannell for critically reading the manuscript and providing invaluable suggestions.
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