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, ηINJ, under applied load is found to play an equally important role. Both, the dye regeneration yield, ΦREG, and ηINJ 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, ηLH, 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. NiO-based p-type dye-sensitized solar cells (DSCs) are limited in their efficiency by their inherently low fill factor. The origins of this limitation are investigated, following a systematic approach investigating changes in the light-harvesting efficiency, injection efficiency, dye regeneration yield, and charge collection efficiency. Overcoming the identified limitations will result in improved p-type DSCs and higher tandem DSC efficiencies. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim