Efficiency Enhancement in Dye-Sensitized Solar Cells with ZnO and TiO2 Blocking Layers

Dye-sensitized solar cells (DSSCs) have great potential for solar generation owing to their low cost and ease of fabrication compared with silicon-based photovoltaic devices. One of the major issues with TiO2-based DSSCs is the recombination loss at the substrate–electrolyte interface due to the mesoporous nature of the TiO2 film. It was proposed earlier that introduction of a blocking layer at the substrate–TiO2 interface could reduce this recombination loss by preventing direct contact of the substrate and electrolyte. In this study, ZnO blocking layers (ZBLs) with different thicknesses were spin-coated onto fluorine-doped tin oxide (FTO) glass and the influence studied in comparison with a conventional TiO2 blocking layer (TBL) prepared using the same technique. The ZBL functions as an energy barrier at the FTO–electrolyte interface to prevent backtransfer of electrons to the electrolyte from the FTO. The blocking effect of the ZBL was verified by an enhancement of the fill factor (FF) and open-circuit photovoltage (Voc) of the DSSC, leading to an improvement in the power conversion efficiency (PCE) from 3.86% to 4.34% for the ZBL with optimum thickness of 120 nm. The suppression of the dark current density revealed a reduction of charge recombination with increasing ZBL thickness. Further increase in the ZBL thickness resulted in reduced cell performance due to a drastic decrease of the short-circuit current density (Jsc). The PCE was slightly improved to 4.36% by replacing the ZBL with a 100-nm TBL. These results suggest that the facile and low-cost sol–gel spin-coating technique is a feasible method for formation of a ZBL or TBL to reduce the recombination loss in DSSCs.