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

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.