ZnO/TiO2 core–shell photoelectrodes for dye-sensitized solar cells by screen printing and room temperature ALD

Abstract

Mesoporous photoelectrodes for dye-sensitized solar cells (DSSCs) in a ZnO/TiO2 core–shell structure have been fabricated at low temperature by combination of screen printing of ZnO nanoparticles and room temperature atomic layer deposition (ALD) of TiO2. ALD nicely achieved a uniform coating of the individual ZnO particles with a thin skin of TiO2 of uniform thickness. The overall mesoporous structure was unchanged, so that a high surface area for dye adsorption and pores for penetration of the electrolyte solution were preserved. DSSCs employing the core–shell electrode sensitized with D149 indoline dye achieved a higher open circuit voltage (V OC) than that with ZnO, but accompanied with a decrease of short circuit photocurrent density (J SC) due to decreased efficiency of electron injection. Increase of the recombination resistance to extend the electron lifetime by the TiO2 shell has been confirmed by electrochemical impedance spectroscopy (EIS). X-ray diffraction analysis has revealed that the ALD TiO2 is amorphous and can only be converted to crystalline anatase TiO2 by annealing at 450 °C. The crystallization recovered current but lost the advantage of the voltage increase. EIS has indicated a high trap density introduced by the amorphous ALD TiO2, which could be reduced by crystallization, but accompanied with an increased rate of recombination.