(394g) Comparing Electron Recombination Via Interfacial Modifications in Dye-Sensitized Solar Cells

Authors: 
Li, L., University of Florida
Chen, S., University of Florida
Xu, C., University of Florida at Gainesville
Zhao, Y., University of Florida
Ziegler, K. J., University of Florida

Establishing a blocking layer between the interfaces of the photoanode is an effective approach to improve the performance of dye-sensitized solar cells (DSSCs). However, researchers have disagreed about the respective role of these blocking layers on electron transport processes at the transparent conductive oxide (TCO) or TiO2 interface. In this work, HfO2 blocking layers were deposited via atomic layer deposition (ALD) onto tin-doped indium oxide (ITO) and TiO2. In both cases, addition of the blocking layer increased cell performance to an efficiency greater than 7%, corresponding to about a 10% improvement. The improved performance for a HfO2 layer inserted between the ITO/TiO2 interface is associated with an energy barrier that reduces electron recombination. If the blocking layer is too thick, cell efficiency degrades due to decreased tunneling probability of electrons. HfO2 blocking layers between the TiO2/dye interface showed more complex behavior and were more sensitive to the number of ALD cycles. For thin blocking layers on TiO2, the improved device performance is attributed to the passivation of surface states in TiO2, which immobilize electrons and impede interfacial electron transport. A distinct transition in dark current and electron lifetime were observed after about 2 ALD cycles. These changes to performance for thicker HfO2 layers on TiO2 indicate the formation of an energy barrier that reduces electron recombination but significantly impedes cell performance due to changes in the series resistance of the cell.