Before studying the ZnO as a spin-coating layer to im-prove the performance of TiO2 DSCs, it is worth pre-senting 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) to-gether 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 wave-length) and α the optical absorption coefficient deter-mined by the approximate relation T = exp(−α·d) which ignores the film reflectance (d is the film thickness). The plots of Figure 1(b) are based on the assumption of di-rect electron transitions in ZnO and TiO2 semiconductors where the relation αhν ~ (hν − Eg)1/2 holds. The parame-ter Eg called “optical gap” can be experimentally deter-mined 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).