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

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.