2013: Nano Energy
Yichuan Ling, Jason K. Cooper, Yi Yang, Gongming Wang, Linda Munoz, Hanyu Wang, Jin Z. Zhang,Yat Li
We report a simple and yet powerful method to improve the performance of TiO2-based N3 dye-sensitized solar cells (DSSCs) by hydrogen-treatment of TiO2 nanostructures as photoelectrodes. The solar conversion efficiency of DSSC based on TiO2 rutile nanowires was increased from 0.28% to 0.45% after the nanowire electrode was annealed at 350 °C in a pure hydrogen atmosphere. The enhanced conversion efficiency was attributed to improved charge transport as a result of increased electron density by three orders of magnitude upon hydrogenation. While the conversion efficiency was improved by 61%, the overall efficiency was still low, possibly due to the limited loading of N3 dye molecules on TiO2 nanowires. To improve dye loading, a similar study of hydrogen-treated Degussa P25 nanoparticles (H-P25) electrodes was conducted in which the conversion efficiency was enhanced by 13% compared to untreated P25. The DSSC based on H-P25 achieved a very high photocurrent, 20.81 mA/cm2, and solar conversion efficiency, 9.30%, under 1 sun illumination. The donor density of H-P25 was found to increase by 1.5 times compared to P25, consistent with the relatively small enhancement in overall conversion efficiency. To gain new physical insight into the dye sensitization process, ultrafast transient absorption (TA) spectroscopy was applied to probe the excited dynamics of N3 dye in ethanol solution as well as adsorbed on H-P25, P25 and ZrO2. The TA spectrum of H-P25 and P25 was dominated by N3+ generated following electron injection, which occurs in <150 fs. In addition, time dependent density function theory (TDDFT) calculations of N3 and N3+ provided further insight into the origin of TA spectra as well as the related dynamic processes. The results demonstrate that hydrogenation of TiO2 electrodes can be a low cost and effective way to enhance performance of DSSC by rationally introducing bandgap states that enhanced the donor density and thereby charge transport.