Dominating Energy Losses in NiO p-Type Dye-Sensitized Solar Cells

Nickel oxide based p-type dye-sensitized solar cells (DSCs) are limited in their efficiencies by poor fill factors (FFs). This work explores the origins of this limitation. Transient absorption spectroscopy identifies fast recombination between the injected hole and the dye anion under applied load as one of the predominant reasons for the poor FF of NiO-based DSCs. A reduced hole injection efficiency, (INJ), under applied load is found to play an equally important role. Both, the dye regeneration yield, phi(REG), and (INJ) decrease by approximately 40%-50% when moving from short- to open-circuit conditions. Spectroelectrochemical measurements reveal that the electrochromic properties of NiO are a further limiting factor for the device performance leading to variable light-harvesting efficiencies, (LH), under applied load. The peak light-harvesting efficiency decreases from 63% at short circuit to 57% at 600 mV reducing the FF of NiO DSCs by 5%. This effect is expected to be more pronounced for future devices with higher operating voltages. Incident, photon-to-electron conversion efficiency front-back analysis at applied bias is utilized to characterize the interfacial charge recombination. It is found that the recombination between the injected hole and the redox mediator has a surprisingly small effect on the FF.