Enhancing Photovoltaic Efficiency of Quantum Dots Sensitized Solar Cell By (001) Oriented Anatase TiO2 Nanosheets
The anatase TiO2 NSs with highly exposed (001) facets, as shown in Fig. 1, were prepared using hydrofluoric acid (HF) as a capping agent under hydrothermal condition . The figure shows the prepared TiO2 NSs are composed of well-defined nanosheets with length of 60-80 nm and thickness of 10nm. The crystalline TiO2 NSs were investigated, and compared with the commercial Degussa P25 TiO2 nanoparticles (NPs) used as the photoanodes of the QDSSCs. The SEM pictures of our TiO2 NSs show a significant high percentage of exposed (001) facets, approximately 70%. On the other hand, the commercial TiO2 NPs have a low percentage of (001) facets, less than 10%, with dominant (101) facets over 90%. The TiO2-NSs-based QDSSCs were characterized by measuring the current-voltage behavior under the simulated sunlight illumination, and compared with photoanodes made of the TiO2 P25. The I-V characteristics curve and incident photon-to-electron conversion efficiency (IPCE) spectrum of these QDSSCs are illustrated in Fig. 2, and their photovoltaic parameters were tabulated in Table 1. The TiO2-NSs- based photoanodes exhibit a higher short circuit current (Jsc) of 14.4 mA/cm2, higher open circuit voltage (Voc) of 0.56 V and higher conversion efficiency (η) of 4.37 % compared with the P25-based photoanodes (Jsc=11.75 mA/cm2, Voc=0.52 V, η=2.86%) at the same film thickness. The higher Jsc of NSs is attributed to the increased in loading of quantum dots, favored by the more reactive anatase (001) surface that improved the nucleation of quantum dots. Furthermore, the higher IPCE from 350 nm to 600 nm wavelength strongly suggests that our NSs with better crystalline structure indeed pave a direct route for electrons to transmit.
Finally, the electrochemical impedance spectroscopy (EIS) measurements were carried out under illumination at open voltage. Fig. 3 demonstrates the Nyquist plots for the NSs-based and P25-based QDSSCs. The fitting data are all listed in Table 2. The charge transfer resistance (Rk) and effective electron life time (teff) of NSs are significantly better than that of the P25 electrode, which agree with the results of I-V and IPCE measurement.
In summary, 2-D anatase TiO2 NSs were prepared via a facile hydrothermal process, and applied to QDSSCs. The NSs-based photoanodes have shown to have an overall energy conversion efficiency of 4.37%, which is higher than that of P25-based photoanodes (2.86%) by 53%. This study provides a novel nanostructure photoanode design to improve the performance of QDSSCs.
 H. G. Yang, G. Liu, S. Z. Qiao, C. H. Sun, Y. G. Jin, S. C. Smith, J. Zou, H. M. Cheng, G. Q. Lu, J. Am. Chem. Soc. 2009, 131, 4078-4083.
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