Enhancing Photovoltaic Efficiency of Quantum Dots Sensitized Solar Cell By (001) Oriented Anatase TiO2 Nanosheets

Recently, anatase TiO2 nanocrystals/nanosheets with large fractions of the exposed high energy (001) facet have been synthesized by different reaction systems following the first report by Yang et al^[1]. The (001) crystal facets of the anatase TiO₂ are postulated to exhibit unique surface structure characteristics with enhanced functionalities beneficial for water splitting, solar cells, photocatalysis, lithium-ion batteries and sensors^[2]. In this study, we report for the first time the application of anatase TiO₂nanosheets (NSs) on the photoanodes of quantum dots sensitized solar cells (QDSSCs).

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 ^[3]. 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 (J_sc) of 14.4 mA/cm², higher open circuit voltage (V_oc) of 0.56 V and higher conversion efficiency (η) of 4.37 % compared with the P25-based photoanodes (J_sc=11.75 mA/cm², V_oc=0.52 V, η=2.86%) at the same film thickness. The higher J_sc 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 (R_k) and effective electron life time (t_eff) 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.

Reference

[1] 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.

[2] (a). F. Amano, O. O. Prieto-Mahaney, Y. Terada, T. Yasumoto, T. Shibayama, B. Ohtani, Chemistry of Materials 2009, 21, 2601-2603; (b). J. G. Yu, J. J. Fan, K. L. Lv, Nanoscale 2010, 2, 2144-2149.

[3] X. G. Han, Q. Kuang, M. S. Jin, Z. X. Xie, L. S. Zheng, J. Am. Chem. Soc. 2009, 131, 3152-+.