Dependence of power conversion properties of the hole-conductor-free mesoscopic perovskite solar cells on the thickness of carbon film

Effect of film thickness of mesoporous carbon on power conversion properties of the hole-conductor-free mesoscopic perovskite solar cells is studied by varying the volume of carbon paste. It is observed that, with increment of the film thickness, perovskite crystallites become smaller; As for the performance parameters, opencircuit voltage (V-OC) drops slowly; while short-circuit current density (J(SC)) and fill factor (FF) rise at first, and then decrease after the film thickness surpasses about 22 mu m. The slight decrease in open-circuit voltage (V-OC) is due to heavier recombination brought by smaller crystallite size of perovskite. The upgraded short-circuit current density is due to accelerated charge-extracting rate across the interface between perovskite and mesoporous carbon, as revealed by the transient photocurrent (TPC) decay measurement. As for the improved fill factor, it is ascribed to the increased conductivity of the mesoporous carbon film. Besides, shelf-stability of the devices is tested. Stability up to 112 days (relative humidity of 40-70%, no encapsulation) are observed for these carbon based mesoporous perovskite solar cells, even though the mesoporous carbon is as thin as similar to 4 mu m. The sound stability shows that the TiO2 nanoparticle-bonding carbon film could provide effective barrier against H2O/O-2.