Hole-conductor-free perovskite solar cells with carbon counter electrodes based on ZnO nanorod arrays

A one dimensional nanostructure array has been considered as a successful charge transport material for perovskite solar cells (PSCs), because of its large internal surface area, superior charge collection efficiency and fast charge transport. Herein we demonstrate a ZnO nanorod (NR) array as the electron collector in a holeconductor- free PSC with a carbon counter electrode (CE). A relatively low initial power conversion efficiency (PCE) of 5.6% was achieved using a 1 mu m long ZnO NR array as an electron collector. However, by introduction of a thin TiO2 coating layer on the surface of ZnO via TiCl4 treatment, the PCE of the cell has been improved to the highest value of 8.24%. It is revealed that the performance enhancement of the ZnO/TiO2 NR based PSCs is largely attributed to the larger surface area, reduced electron combination, and superior electron transport properties.