Effects of atomic layer deposited HfO2 compact layer on the performance of dye-sensitized solar cells

The performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of photogenerated electrons from the photoelectrode back into liquid electrolyte. An atomic layer deposited (ALD) hafnium oxide (HfO₂) ultra thin compact layer was grown on the surface of the transparent conducting oxide (TCO) and its effects on the DSSC performance were studied with dark and illuminated current-voltage and electrochemical impedance spectroscopy measurements. It was found that this compact layer was effectively blocking the back-reaction of electrons from TCO to the liquid electrolyte, resulting in the overall photoconversion efficiency being enhanced by 66% compared to a DSSC with a conventional sol-gel processed TiO₂ compact layer. Reasons for the improved photovoltaic performance were attributed to passivation of the TCO surface, better electronic quality of the compact layer material and the higher compactness, shown by atomic force microscopic images, obtained from gas-based deposition methods. Also, an increased short-circuit current density suggests that the interfacial resistance for the injection of electrons from the porous nanoparticle network to TCO was reduced. Further, the theory of electron recombination at the TCO/compact layer/electrolyte interface was developed and used to explain the improved DSSC performance with an ALD HfO₂ compact layer.