1. Introduction
The ZnO sol-gel spin-coating method is a combination of two techniques applied to produce thin nanostructured ZnO films: The sol-gel technique consists of the preparation, under defined conditions, of a ZnO solution from the mixture of a ZnO precursor [Zn(CH3COO)2·2H2O: zinc acetate dehydrate] and a solvent (2-methoxyethanol) with the addition of a stabilizing substance (ethanolamine). The clear transparent colloidal solution thus obtained, containing suspended ZnO nanoparticles, is deposited on the surface of a substrate by the spin-coating technique [1-4]. Nanostructured ZnO films prepared by this method have been used as photo-electrodes in DSSC fabrication [5,6], but low efficiencies were recorded compared to TiO2-based DSSCs. However, the deposition of a ZnO layer on the surface of a nanoporous TiO2 film to improve the DSSC photovoltaic properties is an active issue in the context related to surface treatment of TiO2 photo-electrodes by thin metal oxide films [7-11]. Kao et al. [12] employed the sol-gel spin-coating method to prepare TiO2 and ZnO spin-coated TiO2 electrodes for DSSCs. Multilayer coating was applied to TiO2 to reach the required thickness and a monolayer of ZnO was applied to spin-coat the TiO2 film. All the photovoltaic parameters were increased upon ZnO spin-coating, resulting in the promotion of the power conversion efficiency η from 2.5% to 3.25%. In this paper, we use the sol-gel technique to prepare ZnO solutions of 5 different precursor concentrations (0.1, 0.2, 0.3 and 0.4 M). An amount of 5 drops from each solution is used to deposit a ZnO layer on the TiO2 photo-electrode surface by spincoating. The measured efficiency of the associated DSSC shows a variation with the ZnO precursor concentration, exhibiting a maximum at 0.1 M, beyond which it shows a sharp decrease to 0%. It is explained that this optimal amount of the deposited ZnO forms the thinnest layer that can create the required energy barrier to reduce the rate of back-recombination of electrons to dye molecules and electrolyte species, enhancing the photocurrent without affecting the dye-adsorption efficiency of the TiO2 film.
2. Experiment
Titanium dioxide (TiO2) was coated on F-doped SnO2 (FTO) glass using the classical screen printing technique. The TiO2 film was then dried and sintered at 450˚C for 1 hour to obtain the bare TiO2 photo-electrode to be used in this work for the preparation of the DSSCs. The sol-gel spin-coating method is employed to deposit thin layers of ZnO on the surface of TiO2 photo-electrodes. The precursor (Zn(CH3COO)2·2H2O) is mixed with the solvent (2-methoxyethanol) and the stabilizer (monoethanolamine) is added drop wise until satisfying a molar ratio of 1 with the precursor. The solution is heated at 60˚C under continuous magnetic stirring for one hour. The prepared solution containing suspended ZnO nanoparticles is then kept firmly enclosed in a clean flask before spin-coating. The spin-coater (SPIN-150, SPS) is adjusted to run at a speed of 3000 rpm for 30 s to produce reasonable thin films. The ZnO amount is monitored by the number of ZnO sol drops injected through the needle of a “smart” dispenser by carefully advancing the plunger into the barrel. The ZnO-coated TiO2 photoelectrode is annealed at 400 K for one hour. Rutheniumbased dye (C26H20O10N6S2Ru) known as N3 (Solaronix) is used to sensitize the bare and ZnO-coated TiO2 photoelectrodes by immersing in 3 × 10–4 M ethanolic solution for 24 hours. Pt-coated FTO-glass (Solaronix) is assembled as the counter-electrode against the dye-loaded photoelectrode using two paper clips. The electrolyte solution (Solaronix) containing Iodide/Triodide Redox couple is injected through a capillary channel originally drilled across the counter-electrode. The I-V characteristics of the DSSCs are measured using a solar simulator (Solar-Light) and an electrometer (Keithley 2400). The latter is computer-controlled to acquire and plot the I-V data, while AM1.5-filtered light from the 300 W-Xenon lamp of the solar simulator shines the DSSC at a power density Pi = 100 mW/cm2. The transmittance and absorbance spectra of the different films are measured in the visible range by means of a UV/VIS/NIR spectrophotometer (Jasco V-570).
3. Results and Discussions
Before studying the ZnO as a spin-coating layer to improve the performance of TiO2 DSCs, it is worth presenting first some optical characteristics of this ZnO layer, with the aim to identify the coating material prior to its application. The transmittance spectra of five ZnO films with different ZnO precursor concentrations (0.1, 0.2, 0.3, 0.4 and 0.5 M) are shown in Figure 1(a) together with the transmittance spectrum of the TiO2 film for comparison. The ZnO layers were deposited by the sol-gel spin-coating technique on FTO-glass substrates using 5 sol drops for each layer, while the Solaronix TiO2 film was screen-printed on FTO-glass substrate. The transmission wavelength threshold of the ZnO (~300 nm) is lower than that of the TiO2 (~350 nm), suggesting a wider energy gap for the ZnO, which can be the physical reason for the observed higher ZnO transmittance.
In Figure 1(b), we present the plots (αhν)2 versus hν for the curves of Figure 1(a), with hν = hc/λ the photon energy (c = 3 × 108 m/s is the light velocity, h = 6.66 × 10–34 Js is the Plank constant and λ is the light wavelength) and α the optical absorption coefficient determined by the approximate relation T = exp(−α·d) which
(a)
(b)
Figure 1. (a) Transmittance spectra for five ZnO films spincoated on FTO-glass at different ZnO precursor concentrations (0.1, 0.2, 0.3, 0.4 and 0.5 M). (b) (α·hν)2 versus hν curves plotted from the transmittance spectra of Figure 1 (a).
ignores the film reflectance (d is the film thickness). The plots of Figure 1(b) are based on the assumption of direct electron transitions in ZnO and TiO2 semiconductors where the relation αhν ~ (hν − Eg)1/2 holds. The parameter Eg called “optical gap” can be experimentally determined by extrapolating the line portion of the plot (αhν)2 versus hν to zero absorption coefficient. Thus, average optical gap values of 3.8 eV and 3.4 eV for the ZnO and TiO2 films are respectively determined from the curves of Figure 1(b).
Figure 2 shows the I-V characteristics of DSSCs with ZnO spin-coated TiO2 photo-electrodes at five different precursor concentrations (0.1, 0.2, 0.3, 0.4 and 0.5 M). The I-V characteristic of a DSSC with bare TiO2 photoelectrode is included for comparison. In Table 1, we present the PV parameters extracted from the I-V curves of Figure 2. At the smallest precursor concentration 0.1 M, there is a clear enhancement of the short circuit photocurrent JSC from 5.73 to 7.48 mA/cm2, while small shifts