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 effi ciencies by poor fi ll factors (FFs). This work explores the origins of
this limitation. Transient absorption spectroscopy identifi es 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 effi ciency, η INJ , under applied load is found to play an equally
important role. Both, the dye regeneration yield, Φ REG , and η 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 effi ciencies, η LH , under applied load. The
peak light-harvesting effi ciency 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 effi ciency 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.