Abstract
NiO electrodes are widely applied in p-type dye-sensitized solar cells (DSSCs) and photoelectrochemical cells, but due to excessive charge recombination, the efficiencies of these devices are still too low for commercial applications. To understand which factors induce charge recombination, we studied electrodes with a varying number of NiO layers in benchmark P1 p-DSSCs. We obtained the most efficient DSSCs with four layers of NiO (0.134%), and further insights into this optimum were obtained via dye loading studies and in operando photoelectrochemical immittance spectroscopy. These results revealed that more NiO layers led to an increasing light harvesting efficiency (η (LH)), but a decreasing hole collection efficiency (η (CC)), giving rise to the maximum efficiency at four NiO layers. The decreasing η (CC) with more NiO layers is caused by longer hole collection times, which ultimately limits the overall efficiency. Notably, the recombination rates were independent of the number of NiO layers, and similar to those observed in the more efficient n-type DSSC analogues, but hole collection was an order of magnitude slower. Therefore, with more NiO layers, the beneficial increase in η (LH) can no longer counteract the decrease in η (CC) due to slow hole collection, resulting in the overall efficiency of the solar cells to maximize at four NiO layers.